Metal borohydrides and derivatives
- synthesis, structure and properties -
Mark Paskevicius,a Lars H. Jepsen,a Pascal Schouwink,b Radovan Černý,b
Dorthe B. Ravnsbæk,c Yaroslav Filinchuk,d Martin Dornheim,e
Flemming Besenbacher,f Torben R. Jensena *
a Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO), and
Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark b Laboratory of Crystallography, DQMP, University of Geneva, 1211 Geneva, Switzerland c Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej
55, 5230 Odense M, Denmark d Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Place L.
Pasteur 1, B-1348 Louvain-la-Neuve, Belgium e Helmholtz-Zentrum Geesthacht, Department of Nanotechnology, Max-Planck-Straße 1, 21502
Geesthacht, Germany f Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, DK-
8000 Aarhus C, Denmark
* Corresponding author: [email protected]
Contents
Abstract 1. Introduction 2. Synthesis of metal borohydrides and derivatives 2.1 Solvent-based synthesis of monometallic borohydrides 2.2 Trends in the mechanochemical synthesis of metal borohydrides 2.3 Trends in the mechanochemical synthesis of metal borohydride-halides 2.4 Mechanochemical reactions, general considerations 2.5 Mechanical synthesis in different gas atmosphere 2.6 Single crystal growth of metal borohydrides 2.7 Synthesis of metal borohydrides with neutral molecules 3. Trends in structural chemistry of metal borohydrides 3.1 Monometallic borohydrides, the s-block – pronounced ionic bonding 3.2 Monometallic borohydrides with the d- and f-block 3.3 Strongly covalent molecular monometallic borohydrides 3.4 Bimetallic s-block borohydrides 3.5 Bimetallic d- and f-block borohydrides 3.6 Bimetallic p-block borohydrides 3.7 Tri-metallic borohydrides 3.8 General trends in the structural chemistry of metal borohydrides 3.9 Comparisons between metal borohydrides and metal oxides 4. Structure of anion substituted metal borohydrides 4.1 Metal borohydride halides - fully ordered structures 4.2 Metal borohydride halides - partly ordered structures 4.3 Metal borohydride halides - fully disordered structures 4.4 Metal borohydride halides forming solid solutions 4.5 General trends for halide substitution in metal borohydrides 4.6 Trends in series of bimetallic borohydride halides 4.7 Metal borohydride closo-boranes 4.8 Metal borohydride amides 5. Derivatives of metal borohydrides with neutral molecules 5.1 Ammine metal borohydrides 5.2 Hydrazine metal borohydride complexes 5.3 Ammonia borane metal borohydride complexes 5.4 Metal borohydride crystallizing with organic solvents 6. Thermal properties of metal borohydrides 6.1 Thermodynamics of hydrogen release and uptake 6.2 Thermolysis of monometallic borohydrides 6.3 Thermolysis of bi- and trimetallic borohydrides 6.4 Eutectic melting of bi- and trimetallic borohydrides 6.5 Trends in thermal decomposition for metal borohydrides 6.6 The released gas from metal borohydrides, stability of diborane 6.7 Additives to improve reaction kinetics 6.8 Reactive Hydride Composites 6.9 Hydrolysis – decomposition by reaction with water 6.10 Nanoconfinement 7. New properties and future perspectives of metal borohydrides 7.1 Porous metal borohydrides for gas adsorption 7.2 Metal borohydrides as ion conductors 7.3 Metal borohydrides with optical properties 7.4 Metal borohydrides with magnetic properties 7.5 Perspectives for design of novel metal borohydrides
7.6 CO2-recyling with metal borohydrides 7.7 A possible paradigm shift for solid state hydrogen storage 8. Conclusions 9. Acknowledgements 10. References Chem Soc Rev
REVIEW ARTICLE
Metal borohydrides and derivatives – synthesis, structure and properties Cite this: Chem. Soc. Rev., 2017, 46,1565 a a b b Mark Paskevicius, Lars H. Jepsen, Pascal Schouwink, Radovan Cˇerny´, Dorthe B. Ravnsbæk,c Yaroslav Filinchuk,d Martin Dornheim,e Flemming Besenbacherf and Torben R. Jensen*a
A wide variety of metal borohydrides, MBH4, have been discovered and characterized during the past decade, revealing an extremely rich chemistry including fascinating structural flexibility and a wide range of compositions and physical properties. Metal borohydrides receive increasing interest within the energy storage field due to their extremely high hydrogen density and possible uses in batteries as solid state ion conductors. Recently, new types of physical properties have been explored in lanthanide-bearing borohydrides related to solid state phosphors and magnetic refrigeration. Two major classes of metal borohydride derivatives have also been discovered: anion-substituted compounds where the complex borohydride anion, BH4 , is replaced by another anion, i.e. a halide or amide ion; and metal borohydrides
modified with neutral molecules, such as NH3,NH3BH3,N2H4, etc. Here,wereviewnewsyntheticstrategies along with structural, physical and chemical properties for metal borohydrides, revealing a number of new trends correlating composition, structure, bonding and thermal properties. These new trends provide Received 27th September 2016 general knowledge and may contribute to the design and discovery of new metal borohydrides with tailored DOI: 10.1039/c6cs00705h properties towards the rational design of novel functional materials. This review also demonstrates that there is still room for discovering new combinations of light elements including boron and hydrogen, leading to rsc.li/chem-soc-rev complex hydrides with extreme flexibility in composition, structure and properties.
1. Introduction and high hydrogen content. Thus, the focus of this review is on the compounds where boron and hydrogen form a negatively A wide variety of metal borohydrides have been discovered and charged BH4 anion, counterbalanced by metal cations. The characterized during the past few years, revealing an extremely boron–hydrogen bond in the complex anion, e.g. BH4 ,is rich chemistry, including fascinating structural flexibility and a covalent, whereas the coordination to metals in the solid state wide variety of physical properties to be explored for possible is more diverse, ranging from ionic to more covalent with well- future applications.1–7 Hydrogen and boron form a variety of defined directionality.8 Two major classes of metal borohydride
molecular compounds, boranes (BnHm), which were intensely derivatives have recently been discovered. The first class researched in the 1950’s as high energy rocket fuels. Since then, includes mixed-anion compounds, where the complex anion boranes have received significant academic interest, but metal BH4 is substituted by a halide anion or combined with a less borohydrides appear to have a larger potential for a variety of similar anion such as an amide or borate.9–11 The second class future applications due to their lower toxicity, lower volatility, includes metal borohydrides coordinated to neutral H-rich 6 molecules, such as NH3,NH3BH3,N2H4, etc. This review focuses a Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO), on new trends correlating composition, structure and properties and Department of Chemistry, Aarhus University, Langelandsgade 140, of series of compounds, aiming to provide general knowledge, DK-8000 Aarhus C, Denmark. E-mail: [email protected] which contributes to the rational design and discovery of new b Laboratory of Crystallography, DQMP, University of Geneva, 1211 Geneva, Switzerland metal borohydrides and derivatives with tailored properties. c Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, The first homoleptic metal borohydrides Al(BH4)3, Be(BH4)2 Campusvej 55, 5230 Odense M, Denmark and LiBH4 were discovered by Schlesinger, Brown and Burg and d Institute of Condensed Matter and Nanosciences, Universite´ catholique de Louvain, reported in 1940.12–14 Schlesinger and Brown discovered uranium Place L. Pasteur 1, B-1348 Louvain-la-Neuve, Belgium borohydride, U(BH4)4, while working on the Manhattan project e Helmholtz-Zentrum Geesthacht, Department of Nanotechnology, Max-Planck-Straße 1, 21502 Geesthacht, Germany during World War II after being assigned with the task of finding f Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and volatile uranium compounds for use in the diffusion separation 15 Astronomy, DK-8000 Aarhus C, Denmark of uranium isotopes. U(BH4)4 was found to be one of the most
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volatile uranium compounds known, second only to uranium Ti(BH4)3, the first genuine transition metal borohydride, was 20 hexafluoride. Another compound of great importance, NaBH4, characterized in the gas phase by Dain et al. in 1991, while the first was prepared for the first time by Brown and co-workers, during full structural characterization of a transition metal borohydride in 21 their research for the US Department of Defense, however, their the solid state, Mn(BH4)2, was presented in 2009. work remained ‘‘classified’’ until 1953.16 The first application During the past decade, metal borohydrides have received of a metal borohydride as a ‘hydrogen carrier’ dates back to significant interest as hydrogen storage materials initiated by 22–26 World War II, when the hydrolysis of a NaBH4 suspension was investigations of LiBH4. Renewable energy sources have been used for solid state storage of hydrogen for weather balloons. highly sought after due to the ever increasing human energy Later, in 1979, Herbert C. Brown was awarded the Nobel Prize consumption and made urgent by an increasing awareness of in chemistry for his development of boron containing com- possible climatic changes and healthcare problems associated pounds as reducing reagents in organic synthesis, which today with the combustion of extreme amounts of fossil fuels.27,28 still remains the most important use of metal borohydrides. Unfortunately, renewable energy is unevenly distributed both Over the decades many synthesis methods and new compounds over time and geographically, therefore it must be stored in an have been reported, as borohydrides have constantly remained efficient way.29 Renewable electricity may be stored directly in of interest in organic chemistry.17 Sodium borohydride was the lithium based batteries or converted to hydrogen by electrolysis.30,31 first member to be structurally investigated in 1947, followed by Hydrogen has the highest gravimetric energy content for any known K, Rb and Cs borohydrides in 1954.18,19 Titanium borohydride, compound, but is a gas at ambient conditions and is therefore
Mark Paskevicius is a Senior Lars H. Jepsen obtained his PhD in Lecturer in the Fuels and Energy chemistry and nanoscience from Technology Institute at Curtin Aarhus University in 2015 under University in Australia. He began the supervision of Prof. Torben R. this position in 2017 after a 3 year Jensen. Lars’ research was focused research fellowship at Aarhus on synthesis and characterization University in Denmark. His of boron and nitrogen based research is focussed on renewable complex metal hydrides, including energy storage. He is dedicated to crystal structure determination by developing new materials for combining powder X-ray diffrac- solid-state hydrogen storage to tion, DFT calculations and solid- store energy for automotive, state NMR spectroscopy. Today Mark Paskevicius stationary and concentrated solar Lars H. Jepsen Lars works as consultant at the thermal applications. He is also Danish Technological Institute leading research into new solid-state ion conductors for battery with the aim of supporting Danish and international companies applications, focussing on boron-rich materials. with forefront knowledge and development within materials chemistry.
Pascal Schouwink obtained his Radovan Cˇerny´ did his PhD in PhD in 2014 at the Laboratory of Solid State Physics at the Charles Crystallography of the Department University, Prague. In 1989 he of Condensed Matter Physics of the came to the University of Geneva University of Geneva, where he for a postdoctoral research, and explored the functional aspects was appointed Lecturer in 1995 of complex hydrides using and Associated Prof. in 2012. He diffraction and different spectro- is interested in the development of scopies. Following a post doc powder diffraction methodology, centered around solid state ionic especially ab initio structure conductors he joined the E´cole solution (computer program Fox), polytechnique fe´de´rale de studies of structural defects and Pascal Schouwink Lausanne (EPFL) in 2016, where Radovan Cˇerny´ structural chemistry of inorganic he is responsible for the X-ray compounds especially of metal diffraction facility of ISIC. His interests concentrate on in situ hydrides where his group solved more than 120 crystal structures. studies of energy materials using both X-ray and neutron Recently he turned his interest in the applied research on complex scattering, in particular on polycrystalline materials and thin films. hydrides from hydrogen storage to solid electrolytes.
1566 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article difficult to store in an efficient and compact way. This is a mono-, bi- and trimetallic borohydrides have been exposed to fundamental problem associated with the utilization of hydrogen intense research efforts to develop new synthesis strategies, as an energy carrier,32 which may be solved by the design of novel perform structural characterization, and investigate physical light weight metal hydrides, such as metal borohydrides.32–35 and chemical properties. In the following, we critically review Research over the past decade has revealed a range of new light the progress of metal borohydrides and their derivatives, which metal borohydrides possessing extremely high hydrogen densities, may form the basis of future technological applications. The which may be considered for possible energy storage applications. focus of this review is to discover new trends in synthesis, However, in general, these compounds suffer from poor thermo- structure and properties and provide perspectives for further dynamic and kinetic properties, i.e. they are often too thermally research that may act as inspiration for the development of stable and the hydrogen release and uptake reactions are too slow novel materials towards rational materials design. for practical applications.5,6,36 A new approach is to allow multiple hydrides, denoted as reactive hydride composites (RHC), to react during the release 2. Synthesis of metal borohydrides and of hydrogen to form a different dehydrogenated state. These derivatives composites change the hydrogen release pathway and may improve the thermodynamic and kinetic properties, enabling hydrogen A number of different synthetic approaches have been developed release and uptake under more favourable conditions.37–42 The to prepare new types of metal borohydrides, including anion concept aims to lower the overall reaction enthalpy by introducing substitution and the preparation of derivatives including neutral extra (endothermic) chemical reaction(s), i.e. by utilizing the molecules, such as ammonia, hydrazine etc. A range of different well-known Hess law from chemistry, also denoted as Medima chemical reactions can be undertaken in different media; and principles.43,44 of those, metathesis, i.e. double substitution reactions, and Composites of two or more metal borohydrides often induce addition reactions are the most common. In some cases, more co-melting or eutectic melting.45 Low temperature melting may complex or competing reactions are observed. Halide- and reduce the hydrogen release temperature, as observed for solvent-free mono-metallic borohydrides can be obtained using 51 xLiBH4–(1 x)Mg(BH4)2, or for preparation of molten metal solvent based methods, mechanochemical methods success- borohydrides systems for convenient nanoconfinement, e.g. in fully provide numerous bi- and tri-metallic borohydrides,52 46–48 the 0.725LiBH4–0.275KBH4 with Tm = 105 1C. Metal boro- whereas solid–gas reactions are important for the preparation hydrides with low melting points could allow for fast and of ammine metal borohydrides.53,54 Typically, air and moisture convenient re-fuelling of vehicles with similar technology to sensitive reagents are utilized, which need to be manipulated in that known today for liquid fossil fuels. a glove box or using standard vacuum and Schlenk techniques in dry Metal borohydrides are a multifunctional class of materials and inert atmosphere. In general, metal borohydrides, anhydrous that may also be used as fast ion conductors for new types of metal halides and many solvents are hygroscopic and readily adsorb batteries or for gas adsorption.3,49 They may also have optical, water. The hydrates of metal borohydrides containing metals with electronic and magnetic properties and can be used as reducing low electronegativity are often stable at RT but react with water at agents in organic synthetic chemistry.1,2,17,50 For this reason, elevated temperatures and evolve hydrogen. Metal borohydrides of
Dorthe B. Ravnsbæk is Associate Torben R. Jensen is Professor at Professor at Department of Department of Chemistry and Physics, Chemistry and Pharmacy iNANO, The Interdisciplinary Nano- at University of Southern Denmark. science center at Aarhus University. Her research focuses on synthesis His research interests focus on and structure–property relations in synthesis, structural, physical and novel materials for energy storage – chemical properties of new especially rechargeable batteries. inorganic materials and utilisation She obtained her PhD in Nanotech- of synchrotron X-ray radiation for nology from Aarhus University in materials characterization. He has 2011 based on her research in published 215 research papers synthesis and characterization of (45400 citations). Torben received Dorthe B. Ravnsbæk multiple novel metal borohydrides. Torben R. Jensen his PhD degree (1999) in materials Before joining University of South- chemistryatUniversityofSouthern ern Denmark she spent two years as post doc at Massachusetts Denmark, Odense, and a Doctor of Science degree (DSc in 2014) at Institute of Technology (MIT) specializing in studies of electrode Aarhus University. He was awarded a Steno research stipend (2002), a materials for Li-ion batteries especially with focus on operando Carlsberg research stipend (2005) by the Carlsberg Foundation and the diffraction studies of battery materials. Science of Hydrogen & Energy prize 2016.
This journal is © The Royal Society of Chemistry 2017 Chem. Soc. Rev., 2017, 46, 1565--1634 | 1567 Review Article Chem Soc Rev
metals with higher electronegativity may adsorb water and such as [Cat][Zn(BH4)3] or [Cat][Y(BH4)4], where [Cat] symbolizes + + react exothermically at RT, releasing hydrogen. Alkali metal a bulky cation, e.g. [Ph4P] or [nBu4N] . The method is also useful borohydrides are stable in basic aqueous solution and closo- for producing less stable bimetallic borohydrides, i.e. LiY(BH4)4, 62,63 boranes are stable in neutral and acidic solutions. This section NaY(BH4)4 or LiZn2(BH4)5. reveals general trends in chemical reactions for the synthesis of The use of dimethyl sulphide, S(CH3)2, as a co-solvent and different metal borohydrides and their derivatives under different extraction solvent has several advantages: (i) the reaction is physical conditions. Hydrogen release and uptake reactions in allowed to proceed faster as the products are being dissolved in metal borohydrides and synthesis of some of those from the S(CH3)2 as they form, (ii) weak solvent coordination allows for elements are discussed in Section 6. solvent removal at moderate conditions, (iii) the product dissolved in the solvent can readily be separated from the byproducts 2.1 Solvent-based synthesis of monometallic borohydrides (i.e. LiCl) by filtration. In this manner, the formation of ternary The first successful solvent-based synthesis of a metal boro- chlorides is avoided, e.g. Li- or NaMn-chlorides, or bimetallic hydride was accomplished by Wiberg and Bauer in 1950 by a borohydrides, previously reported to form mechanochemically 64,65 from MnCl2 and LiBH4 or NaBH4. reaction between diethylmagnesium, Mg(CH2CH3)2, and diborane, 55 Recently, yttrium and gadolinium borohydrides were prepared B2H6, in ether. Since diborane is challenging to handle, i.e. poisonous, self-igniting and explodes in contact with air or using dimethyl sulfide as a solvent, providing new solvates as moisture, a borane complex such as triethylamine borane, intermediates, M(BH4)3 S(CH3)2 (M = Y, La, Ce or Gd), which 66,67 transform to a-Y(BH4)3 or Gd(BH4)3 at T B 140 1C. Other (C2H5)3NBH3 or dimethylsulfide borane, (CH3)2SBH3, is safer and easier to handle. Solvent mediated addition reactions take halide-free rare-earth metal borohydrides, Eu(BH4)2 and Sm(BH4)2, place between a borane donor and a metal hydride or a metal are prepared by a metathesis reaction of trivalent metal chlorides alkyl compound. The obtained product is commonly a solvate and LiBH4 in ether, combined with solvent extraction using 68 that needs further treatment in order to produce a solvent-free dimethyl sulfide to remove the halide salts. Donor atoms in product.56,57 This approach has provided the first open-structured solvents typically have increasing strength of coordination in the 3,58 series S o N o O and are increasing difficult to remove from the metal borohydrides, g-M(BH4)2, M = Mg or Mn. Ca(BH4)2 can be synthesized in a similar manner from triethylamine borane metal borohydride products. 59 and CaH2. Thus, the electron deficient molecule BH3 can be 2.2 Trends in the mechanochemical synthesis of metal considered a Lewis acid, which reacts with the Lewis base H . borohydrides Therefore, the reaction is only possible if an ionic or polar covalent metal hydride can be utilised as a reactant. Mechanochemistry is an alternative synthesis approach to Metal borohydrides are also often synthesized via a quanti- bring chemicals together on the atomic scale. Investigations tative metathesis reaction, illustrated by the preparation of indicate that the induced pressure and reactant composition Al(BH ) from aluminum trichloride and lithium borohydride are of major importance in determining the obtained reaction 4 3 52 in an organic solvent (e.g. toluene).60 product. High-energy ball milling induces high pressure and mechanical stress between the reacting materials, which can
3LiBH4(s) + AlCl3(s) - Al(BH4)3(l) + 3LiCl(s) (2.1) facilitate chemical reactions, in contrast to traditional solid-state synthesis techniques, which are mainly ‘temperature-driven’, i.e. Metal borohydrides and halides often have different solubi- increasing cation diffusion in the solid state with increasing lities in solvents, allowing for salt-free products to be obtained temperature.69 Previously, ball milling was mainly aimed at the in high yield. A weakly coordinating solvent that only dissolves preparation of alloys and solid solutions of metals, i.e. products the metal borohydride allows for the removal of the metal with statistical distribution of atoms, but now also encompasses halide by solvent extraction. The reaction kinetics and yield the synthesis of ionic, ionic/covalent materials and also organic can be improved by using finely ground reactants with large materials, i.e. crystalline materials with ordered distribution of surface area. atoms. In some cases, materials can be obtained by mechano- A limitation of solvent-based methods is that the thermo- chemistry, which apparently cannot be obtained by other means.52 dynamically most stable compound is obtained, which is often a High energy mechanochemistry can be very fruitful in the monometallic borohydride, typically obtained as a polymorphic preparation of metal borohydrides where the ball trajectories pure product. Therefore, reaction mixtures containing two or more approach the center of the milling vial, which contrasts grinding metals tend to form mixtures of two or more monometallic mode where the balls follow the circumference of the vial. This borohydrides rather than the bi- or trimetallic borohydrides, which treatment maximizes the ‘intrinsic pressure’ at grain boundaries are obtainable through mechanochemical synthesis. Thus, very few and limits frictional heating of the sample. Sample heating may bimetallic solvates have been discovered, e.g. in diethyl ether, Et2O, be further reduced using pauses during the mechanochemical 61 70 LiMn(BH4)3 1/2(Et2O) and NaMn(BH4)3 1/2(Et2O). Recently, a new treatment. Continuous prolonged ball milling may lead to method was developed for the production of bimetallic metal decomposition of the sample, but may also produce new materials, borohydrides using bulky organic cations and weakly coordinating e.g. anion-substituted Na(BH4)1 xClx, or nano-sized vanadium anions.62,63 The metathesis reaction is carried out in organic boride.71–73 Mechanochemical synthesis can be performed under solvents, CH2Cl2, CHCl3 or toluene, and utilizes precursors a variety of different conditions, e.g. at cryogenic conditions, in
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a solvent, or in different gases at elevated pressures (the latter is Sodium scandium borohydride, NaSc(BH4)4, can also be often denoted reactive ball-milling when the gas takes part in the prepared mechanochemically and is assumed to follow the reaction).74–77 Hence, mechanochemistry is a versatile technique, metathesis reaction (2.8),80 which can be used for the preparation of a wide range of ScCl (s) + 4NaBH (s) - NaSc(BH ) (s) + 3NaCl(s) (2.8) materials.52 3 4 4 4 New bi- and trimetallic borohydrides can often be prepared However, reaction (2.8) is strongly coupled to the synthesis by mechanochemical methods. In cases where pure mono- of a ternary sodium scandium chloride, Na3ScCl6 via an addition metallic borohydrides are available as precursors (e.g. prepared reaction: by solvent-based methods) addition reactions may be utilized 78 ScCl (s) + 3NaCl(s) - Na ScCl (s) (2.9) as exemplified by the synthesis of Cs2LiY(BH4)6. A major 3 3 6 advantage is that a pure product may be obtained. Reaction (2.9) is much faster than the formation of the - 2CsBH4(s) + LiBH4(s) + Y(BH4)3(s) Cs2LiY(BH4)6(s) (2.2) borohydride, NaSc(BH4)4 (2.8) and these reactions, (2.8) and (2.9) are strongly coupled so they can be added to form an overall reaction
Monometallic borohydrides may be prepared via metathesis (2.10). Therefore, the optimal ratio of reactants ScCl3–NaBH4 to reactions, i.e. double substitution, usually between a metal halide synthesize NaSc(BH4)4 turns out to be 1 : 2. This mechanochemical and a metal borohydride. The reaction scheme (2.3) illustrates the synthesis leads to a maximum NaSc(BH4)4 yield of 22 wt% for a 21,79 synthesis of Mn(BH4)2 from LiBH4 and MnCl2. sample with composition (1 : 2) as suggested by reaction (2.10) and - not the ratio (1 : 4) suggested in reaction (2.8) MnCl2(s) + 2LiBH4(s) Mn(BH4)2(s) + 2LiCl(s) (2.3)
2ScCl3(s) + 4NaBH4(s) - NaSc(BH4)4(s) + Na3ScCl6(s) (2.10) The drawback is obviously the formation of a mixture of an unwanted metal halide along with the product. The system The above considerations are also valid for synthesis of
ZnCl2–MBH4 (M=LiorNa)canbeusedtoillustratethecomplexity KSc(BH4)4, but not for synthesis of LiSc(BH4)4 where no ternary of reaction products from mechanochemical synthesis, which may halide is formed.70,81 proceed via more complex chemical reactions during ball milling.70 There is a tendency for more complex chemistry and other chemical reactions to occur when the heavier alkali metals and 2ZnCl (s) + 5LiBH (s) - LiZn (BH ) (s) + 4LiCl(s) (2.4) 2 4 2 4 5 halides are used as reactants. Mechanochemical synthesis of
M2ZnX4, M = Li or Na, X = Cl or Br from stoichiometric mixtures 2ZnCl2(s) + 5NaBH4(s) - NaZn2(BH4)5(s) + 4NaCl(s) (2.5) of MX and ZnX2 reveals that reaction times and activation 82 ZnCl2(s) + 3NaBH4(s) - NaZn(BH4)3(s) + 2NaCl(s) (2.6) energy decrease as Li2ZnCl4 4 Na2ZnCl4 4 Na2ZnBr4. The formation of ternary halides can be avoided in the Reaction schemes (2.5) and (2.6) illustrate that small deviations synthesis of trimetallic borohydrides by varying the ratio in the composition of reactants, e.g. from (1 : 2.5) to (1 : 3), may lead between borohydrides and halides if at least two reactants are to significantly different reaction products, both in terms of available as mono-metallic borohydrides. While reaction (2.11) the stoichiometry and crystal structures, i.e. the structures of produces the ternary salt, Li2MCl4, the alternative reaction 83 NaZn(BH4)3 and NaZn2(BH4)5 are significantly different, which (2.12) avoids this byproduct formation. may suggest that the synthesis mechanism for these compounds - is also different. 15LiBH4 + 5MCl2 + 5ZnCl2 Li3MZn5(BH4)15 + 4LiCl
Further complications occur due to strong or weak coupling + 4Li2MCl4 M = Mg or Mn (2.11) between the chemical reactions that occur during mechano- 13LiBH + M(BH ) + 5ZnCl - Li MZn (BH ) + 10LiCl chemical synthesis. Besides the main reaction that forms the 4 4 2 2 3 5 4 15 (2.12) product, competing side reactions may also take place, e.g. producing bimetallic halides. Sodium chloride produced in This observation also suggests that the formation of bime- reaction (2.5) or (2.6) may further react with the reactant, zinc tallic halide salts can be considered a side reaction where the chloride: produced metal halide reacts with remaining reactants. In some - cases, such side reactions can be suppressed partly or fully by ZnCl2(s) + 2NaCl(s) Na2ZnCl4(s) (2.7) using an excess of metal borohydride reactants, e.g. LiBH4,which
Thus, the synthesis of NaZn2(BH4)5 only partly proceeds may shift the reaction equilibrium towards the bi- or trimetallic (reaction scheme (2.5)), due to the formation of a stable ternary borohydrides and the monometallic halide, e.g. LiCl. metal chloride, Na2ZnCl4, by a simultaneous and competing reaction (scheme (2.7)). The reaction schemes, (2.5) or (2.6) 2.3 Trends in the mechanochemical synthesis of metal and (2.7), are weakly coupled shown by the fact that the borohydride-halides reaction products contain varying amounts of NaCl, Na2ZnCl4, Metal borohydride halides can be prepared with either ordered NaZn2(BH4)5, and reactants. Thus, reaction schemes (2.5)–(2.7) or disordered crystal structures, where in the latter case solid cannot simply be added to provide a scheme for the total solutions occur. The system ZnCl2–KBH4 will be used to illustrate reaction. the complexity of the mechanochemical synthesis. Ball milling a
This journal is © The Royal Society of Chemistry 2017 Chem. Soc. Rev., 2017, 46, 1565--1634 | 1569 Review Article Chem Soc Rev
0 mixture of ZnCl2–KBH4 (1 : 1) leads to an addition reaction and a to form bimetallic halide salts, MxM Xx+n via addition reactions 84 single product, KZn(BH4)Cl2: (the nomenclature is explained in Table 1). Such reactions may occur when LiCl is present together with a heavier metal halide, - ZnCl2(s) + KBH4(s) KZn(BH4)Cl2(s) (2.13) 0 M Xn or LiCl may be incorporated into the product producing
ordered bimetallic borohydrides chlorides, LiCe(BH4)3Cl (except in In this case, there are significant differences between the thecaseofLi2CdCl4 and Li2MCl4,M=Mg,Mn).Arangeofrarer structures of the reactants and the product. The latter contains reactions can also be observed, which are considered analogous to a heteroleptic complex ion containing BH4 as a ligand, i.e. the general reactions, (1)–(4) or combinations of those. For example, [Zn(BH4)Cl2] where Zn coordinates to two chloride ions and mono- or bimetallic borohydride chlorides with crystallographically 2 84 two hydrogen atoms in Z -BH4. This clearly demonstrates that ball disordered structures i.e. NaY(BH4)1.58Cl2.42 can be formed either by milling may initiate complex chemical reactions involving bond substitution (eqn (4)) or complex (eqn (2)) reactions.87 52 breaking and bond formation. In contrast, three novel potassium 0 Generally, di- and tripositive metal halides, MBH4–M Cln, zinc borohydrides/chlorides are observed in ball-milled KBH4–ZnCl2 have very different reactivities and stabilities depending on their mixtures with molar ratios in the range 1.5 : 1 to 3 : 1, KZn(BH4)3, metal electronegativity and d-electron configuration. Therefore, 85 K2Zn(BH4)xCl4 x and K3Zn(BH4)xCl5 x. different milling times are required to successfully obtain the Mechanochemical treatment may also facilitate one solid to desired product. For example, synthesis of MSc(BH4)4 (M = Li, Na or dissolve into another, i.e. an addition reaction, which leads to K) occurs by ball-milling MBH4–ScCl3 (2 : 1) for 120–180 min, while formation of a solid solution, e.g. formation of a hexagonal solid the cadmium borohydrides, a-Cd(BH ) , b-Cd(BH ) , KCd(BH ) 4 2 4 2 4 3 solution h-Li(BH4)1 xBrx where the two anions, BH4 and Br ,are and K2Cd(BH4)4 are formed from MBH4–CdCl2 (M = Li, Na or K) statistically distributed on the same crystallographic position, after only 20–30 min mechanochemical treatment. Only the transi- 86 formed by mechanochemical treatment of LiBH4–LiBr. tion metals with d0,d5 or d10 electron configurations have sufficient In some cases, the crystallinity of the sample and also the stability to allow for mechanochemical preparation.8 However, overall sample composition may be modified by thermal treatment 0 0 2+ 4 2+ 6 transition metal borohydrides, M (BH4)2,M =Cr (d ), Fe (d ) of the sample. For example, the composition of NaY(BH ) Cl 2+ 7 4 2 2 and Co (d ), were recently prepared in solution at T o 30 1C, 1 87 changed to NaY(BH4)1.58Cl2.42 at T = 230 C. The rare-earth metals and stabilised by ammonia–metal coordination, i.e. formation of are yet another example: LiCe(BH4)3Cl and LiLa(BH4)3Cl were M0(NH ) (BH ) compounds.94 4 3 6 4 2 prepared by ball milling, whereas LiGd(BH4)3Cl only crystallized after heat treatment at T =1951C.88 A similar trend is observed for 2.5 Mechanical synthesis in different gas atmosphere the synthesis of metal borohydride amides e.g. Li2(BH4)(NH2), 89–91 Li4(BH4)(NH2)3,Na2(BH4)(NH2)andMg(BH4)(NH2), which are Solvent-free synthesis of monometallic borohydrides, e.g. all metal cation conductors. These compounds are formed through lithium borohydride, LiBH4, formed in an addition reaction ball milling (and annealing) of M(BH4)n–MNH2.Inthiscase,NH2 between lithium hydride, LiH, and diborane, B2H6 gas at T = 95 does not substitute for BH4 to form solid solutions as for Cl , 120 1C has recently been demonstrated. The formation of instead, new structures are formed with distinct sites for BH4 and LiBD4 from LiD and B2D6 was followed using in situ powder NH2 . neutron diffraction, which reveals that the nucleation of LiBD4 A detailed analysis of the unit cell volumes of LiBH4, KBH4 begins at T B 102 1C, i.e. with the formation of the hexagonal and LiK(BH4)2 indicates that the formation of the bimetallic polymorph, h-LiBD4. However, the reaction is incomplete and borohydride is facilitated by a more dense packing as compared to the yield is only B50% even at elevated temperatures (187 1C) 47 the reactants. Thus, formation of LiK(BH4)2 is pressure-induced and takes place during ball milling, but not by annealing. This is in Table 1 Generalization of chemical reactions observed during mechano- agreement with DFT calculations suggesting that LiK(BH4)2 should chemical synthesis of metal borohydrides from MBH –M0X reactant 92 4 n decompose to the reactants at ambient conditions. In contrast, mixtures along with selected examples. Nomenclature: X = halide anion, 0 the unit cell volumes of the solid solution Na1 xKxBH4 is larger M = alkali metal cation and M = di- or tri-positive metal than the sum of reactant volumes, (1 x)NaBH –xKBH .93 Hence, 4 4 Reaction type/no. Reaction the formation of Na1 xKxBH4 is temperature-induced and occurs 0 - 0 upon annealing, i.e. thermal expansion, but not during ball milling. Metathesis/1 nMBH4 +MXn M (BH4)n + nMX Example 2LiBH4 + MnCl2 - Mn(BH4)2 + 2LiCl 2.4 Mechanochemical reactions, general considerations - Addition/2 yMBH4 + xMXn M(BH4)yXx Table 1 highlights the range of different chemical reactions Example - that are typically observed during the synthesis of metal boro- 0.75NaBH4 + 0.25NaCl Na(BH4)0.75Cl0.25 hydrides by mechanochemistry. The literature suggests that 0 0 Metathesis- (x + ny)MBH4 + yM Xn - MxM y(BH4)x+ny +(ny)MX monometallic borohydrides are more likely to form when addition/3 Example - LiBH4 is used as the precursor, possibly driven by the formation 4LiBH4 + ScCl3 LiSc(BH4)4 + 3LiCl of the stable halide, LiCl. Furthermore, an excess of LiBH4 may 0 - 0 Metathesis- M Xn + nMBH4 MM (BH4)nX+(n 1)MX improve the crystallinity of reaction products. The heavier addition/4 Example - alkali metal borohydrides have a significantly greater tendency CeCl3 + 3LiBH4 LiCe(BH4)3Cl + 2LiCl
1570 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
with the product containing small amounts of Li2B12H12.A were noticed under the lid, explained by the low sublimation passivation layer of LiBH4 is suggested to form on the surface of temperature of Zr(BH4)4 (29 1C) because of its molecular the LiH grains retarding the process.96,97 Mechanochemical structure.109 CVD is an elegant method for single crystal growth treatment allows for the continuous removal of as-formed of metal borohydrides, which may be limited to volatile mole- borohydride from the surface layer of particles, facilitating a more cular compounds with low vapour pressures and sublimation complete reaction. Diborane gas is produced by heating a ball milled temperatures. 70,96 2.5LiBH4–ZnCl2 sample, i.e. decomposition of LiZn2(BH4)5, The importance of producing single crystal samples stems which is considered a convenient and relatively safe source of not only from the fact that physical properties such as ionic diborane as compared to pressurized gas bottles. This new gas– conductivity can be highly anisotropic110 and powder averaging solid mechanochemical synthesis method has also successfully can present problems to such characterizations, but also from provided other metal borohydrides, such as Mg(BH4)2,Ca(BH4)2 the necessity of providing accurate input for theoretical evalua- 97,98 and Y(BH4)3. Direct synthesis of a-Y(BH4)3 by ball milling tions of hydrogen storage systems. 98,99 YH3 in a B2H6 atmosphere gave yields above 75%, and ball 2.7 Synthesis of metal borohydrides with neutral molecules milling Y(BH4)3 in B2H6 has also led to the discovery of Y(B3H8)3 and Y2(B12H12)3, suggesting that higher boranes might also be Metal borohydrides form complexes with hydrogen-rich nitrogen- 100 formed during the decomposition of Y(BH4)3. These solid–gas based neutral molecules, which can be in the solid state (e.g. reactions can be considered as addition reactions analogous to ammonia borane, NH3BH3), liquid state (e.g. hydrazine, NH2NH2), the solvent-based reactions described above (see Section 2.1). or gas state (e.g. ammonia, NH3). These compounds are capable of Thus, this analogy suggest that the solvent-free solid–gas generating large amounts of hydrogen by combining the hydrogen reactions are only useful when Lewis base type ionic or atoms present in the two counterparts. Borohydride complexes polar covalent hydrides, such as LiH, providing an electron- with water have similar properties but usually have a lower 111 112 pair donating H ion, or BH4 , can be utilised as reactants to stability, where the hydrates of sodium, lithium, and form a borohydride or higher borane. The solid–gas reactions calcium113 are characterized. are promising for extending the series of solvent-free metal Ammonia borane metal borohydride complexes. Ammonia boranes. borane metal borohydride complexes are formed by addition
reactions between NH3BH3 and LiBH4, Mg(BH4)2, Ca(BH4)2 and
2.6 Single crystal growth of metal borohydrides Al(BH4)3. For the solids LiBH4,Mg(BH4)2,Ca(BH4)2 the complexes 114,115 Despite significant efforts, single crystal investigations of metal are prepared by ball-milling. Interestingly, the mixture borohydrides are very scarce. This is mainly due to the compli- a-Mg(BH4)2–NH3BH3 (1 : 2) converts completely into Mg(BH4)2- cations arising when attempting to grow metal borohydrides by (NH3BH3)2 upon ball milling, while Mg(BH4)2(NH3BH3)2 is not 115 classical solution chemistry. Solvents need to be carefully formed when starting from amorphous Mg(BH4)2 and NH3BH3. selected in order to avoid the formation of solvates. Unfortunately, This is counter-intuitive, since amorphous matter usually has non-coordinating apolar solvents often have low solubility limits. higher reactivity, suggesting that there is a structural relation- Notably, single crystal studies have been reported on mono-metallic ship allowing the borohydride to coordinate with neutral 101 102,103 104 105 ligands. Optimal contact between reactants is offered by liquid compounds such as LiBH4, NaBH4, KBH4, Be(BH4)2, Mg(BH ) ,106 and Al(BH ) ,107 where crystals can be grown from a Al(BH4)3 at ambient conditions to form single-crystals of 4 2 4 3 116 solution, by annealing the powder, or by recrystallizing from a Al(BH4)3 NH3BH3. melt. Techniques involving melting are useful for mono-metallic Hydrazine metal borohydrides. Complexes with hydrazine borohydrides but limited to bi- and trimetallic borohydrides that are reported for LiBH4,NaBH4 and Mg(BH4)2, e.g. LiBH4 NH2NH2. are sufficiently thermally stable (recall that a high number of These compounds were synthesized by placing the precursor metal these compounds are prepared mechanochemically, stabilized borohydride and hydrazine in different chambers of a sealed system 117 by high pressure rather than elevated temperature). Powder at room temperature. Due to the vapour pressure, hydrazine samples of framework-type metal borohydrides produced mechano- typically absorbs into the borohydride over the course of hours. chemically may, in some cases, be partly dissolved in a suitable Alternatively, the complexes are formed by ball milling solid–liquid 118 solvent and then recrystallized to larger crystals. Single crystals of mixtures of M(BH4)m–NH2NH2. Ammine metal borohydrides. Metal borohydrides readily KCa(BH4)3 were recently prepared from flux, which can be an eutectically melting mixture of more stable alkali and/or earth alkali react with ammonia gas in solid–gas reactions forming ammine metal borohydrides with sufficiently large thermal stability.108 metal borohydrides, M(BH4)m nNH3, except for the heavier Chemical vapour deposition (CVD) was reported as an alkali metal borohydrides: NaBH4,KBH4,RbBH4 and CsBH4.How- approach for the crystal growth of Zr(BH ) , which is formed by a ever, the first ammine metal borohydrides, e.g. Zn(BH4)2 4NH3,were 4 4 119 109 synthesized in the 1950’s bysolvent-basedmethods. metathesis reaction of ZrCl4 and LiBH4 during ball milling: Recent literature reveals that the NH3/BH4 ratio (n/m)of - ZrCl4(s) + 4LiBH4(s) Zr(BH4)4(s) + 4LiCl(s) (2.14) ammine metal borohydrides, M(BH4)m nNH3, can be efficiently tailored by combining solvent based methods, solid–gas reac- The ball milled sample was stored in a vial at 30 1C for 2–4 tions and mechanochemistry. Initially, a metal borohydride is weeks when transparent single-crystals with a rectangular shape prepared by a solvent-based method or by solvent extraction
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1571 Review Article Chem Soc Rev of a mechanochemically prepared metal borohydride metal covalent and complex bi- and trimetallic borohydrides (Tables 3 halide mixture, e.g. using a weakly coordinating solvent such and 4), including framework structures and anionic complexes. as dimethyl sulfide, S(CH3)2. Ammine metal borohydrides may 3.1 Monometallic borohydrides, the s-block – pronounced be prepared by passing dry NH3 gas through the solution of ionic bonding M(BH4)3 in S(CH3)2 or by a reaction between dry NH3 gas and solid M(BH4)3. Ammonia always coordinates to the metal by a The alkali metal borohydrides, MBH4, M = Li, Na, K, Rb and Cs, strongly exothermic reaction and cooling is usually needed in primarily exhibit ionic bonding and theoretical studies show order to obtain the metal borohydride containing the highest that there is almost no charge density (o5%) between the M+ 173–176 possible number of ammonia molecules for further synthesis, and BH4 ions in their structures. An experimental 53 e.g. M(BH4)3 7NH3, M = Y, Gd or Dy prepared at T B 5 1C. charge density investigation of o-LiBH4 reveals transfer of Synthesis of ammine metal borohydrides in more strongly 0.86 0.09 e (at 10 and 90 K) from Li to BH4 in accordance coordinating solvents, such as THF, is also possible, however, with the ionic structure description.175 the samples are contaminated by the formation of ammonia Lithium borohydride. Four polymorphs of lithium borohydride, 120 borane. LiBH4, have been described and the orthorhombic o-LiBH4 (Pnma) Mechanochemical treatment of the ammine metal borohydride structure at ambient conditions is unique.24 Single crystal XRD having the highest possible number of ammonia molecules and the investigations of o-LiBH4 at 225 K unambiguously reveal nearly corresponding metal borohydride in various ratios is a convenient ideal tetrahedral geometry of the BH4 group, in contrast to more 22,24 way to get new ammine metal borohydrides with varying NH3/BH4 distorted tetrahedra reported in earlier PXD studies. Powder 7 11 ratio. This approach is demonstrated for ammine manganese neutron diffraction of the triple isotopically substituted o- Li BD4 54 borohydride, Mn(BH4)2 nNH3 for n =1,2,3and6. at 3.5 K reveals boron–hydrogen bond lengths (B–H) of 1.208– 1.225 Å, and bond angles (H–B–H) in the range 107.2–111.71.177 - nMn(BH4)2 6NH3(s) + (6 n)Mn(BH4)2(s) 6Mn(BH4)2 nNH3(s) Orthorhombic o-LiBH4 transforms to a hexagonal wurtzite- (2.15) like polymorph h-LiBH4 (P63mc)atB110 1C. PND and PXD studies reveal large and anisotropic displacements of hydrogen atoms.177,178 The observed atomic displacement ellipsoids may 3. Trends in structural chemistry of indicate orientational disorder in addition to thermal vibrations metal borohydrides of the BH4 group. The large vibrational amplitudes of BH4 in the hexagonal polymorph agree with an apparent shortening of the The past decade has experienced a major increase in the number B–H bonds to B1.07 Å. An entropy contribution from this of experimental structural studies of novel metal borohydrides, disorder is considered to be a factor in stabilizing the hexagonal 121–123 101,178 in particular using powder X-ray or neutron diffraction. structure. Two high pressure polymorphs of LiBH4 are also Various equipment has been designed to enable this type of PXD observed at room temperature. One polymorph is observed at a characterization under high gas pressure and temperature.124,125 pressure of 1.2–18 GPa, which is pseudo-tetragonal (Ama2) with
The interest in crystal chemistry of metal borohydrides was Li tetrahedrally coordinated by four BH4 groups and the BH4 initiated in the late 1940’s by investigations into the most ionic groups in a nearly square-planar coordination by four Li 131 and stable monometallic borohydrides, namely MBH4,M=Na, atoms. A more complex ordering of the BH4 groups, showing K, Rb and Cs.18,19 Later studies focused on the more challenging repulsive interactions, was first suggested in the O2a 2c 179 molecular, volatile and covalent compounds, e.g. Al(BH4)3, supercell of the Ama2 structure theoretically and then shown 107,126,127 180 Zr(BH4)4 and Hf(BH4)4. In between these two extremes experimentally. At high pressure a cubic LiBH4 polymorph 131 we find numerous metal borohydrides with framework struc- forms, which is isostructural to cubic NaBH4. In general, tures, pronounced bonding directionality and clearly some the four polymorphs of lithium borohydride primarily show 3,21,106,128,129 degree of covalency, e.g. Mg(BH4)2 and Mn(BH4)2. coordination between lithium and the borohydride complex
The successful structural analysis of LiBH4 in 2002 was the anion by edge sharing, but h-LiBH4 and o-LiBH4 appear to have initiation of this renewed interest in metal borohydrides.24 face sharing for the shortest Li–B contact (2.37 Å). The structural
The first bimetallic borohydride, LiK(BH4)2 was discovered distortions in LiBH4 polymorphs have been analysed by group- and characterized in 2008.130 Since then, a rapidly increasing theoretical and crystal-chemical considerations revealing the 83 181 number of bimetallic and also trimetallic borohydrides have been layered structure of LiBH4 where the deformation of layers described, often containing complex anions. Some compounds fall define the structural stability of the observed polymorphs. within more than one of these categories, for instance, LiZn2(BH4)5 The heavier alkali metals. Available p-orbitals and the contains [Zn2(BH4)5] complex anions but can also be considered increasing sizes of heavier alkali metals facilitate the formation of as being built from interpenetrated frameworks.70 rock salt (NaCl, Fm3%m) type structures of alkali metal borohydrides,
Clearly, metal borohydrides exhibit a fascinating and surprisingly MBH4, M = Na, K, Rb and Cs. Theoretical studies suggest that versatile crystal chemistry, which the following section will electron density deformations of the heavier alkali metals may also review, highlighting general trends and relationships. Initially, contribute to the formation of NaCl-type structures.176 Additional mono-metallic borohydrides will be discussed (Table 2), starting studies at low temperature and high pressures have discovered from the simple cubic ionic structures, moving towards more several other structural polymorphs.18,19,133,135,182
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Table 2 Known experimental structures of monometallic borohydrides ordered by cation atomic number
Cation Polymorph rm (wt% H) Stability Crystal system Space group Ref. + Li o-LiBH4 18.5 RT Orthorhombic Pnma 24 h-LiBH4 4380 K Hexagonal P63mc 24 hp1-LiBH4 RT, 1.2–10 GPa Orthorhombic Ama2 131 % hp2-LiBH4 RT, 410 GPa Cubic Fm3m 131 2+ Be Be(BH4)2 20.8 RT Tetragonal I41/cd 105 + % Na a-NaBH4 10.7 RT Cubic Fm3m 19 lt-NaBH4 o190 K or RT, 46 GPa Tetragonal P42/nmc 132 hp-NaBH4 49 GPa Orthorhombic Pnma 133 2+ Mg a-Mg(BH4)2 14.9 RT Hexagonal P6122 106 and 129 b-Mg(BH4)2 RT, metastable, HT polymorph Orthorhombic Fddd 128 and 134 % g-Mg(BH4)2 RT, metastable Cubic Ia3d 3 d-Mg(BH4)2 RT, 42.1 GPa Tetragonal P42nm 3 z-Mg(BH4)2 B500 K Hexagonal P3112 58 3+ Al a-Al(BH4)3 16.9 150 K Monoclinic C2/c 107 b-Al(BH4)3 195 K Orthorhombic Pna21 107 + % K a-KBH4 7.5 RT Cubic Fm3m 19 and 104 b-KBH4 o70 K or RT, 3.8–6.8 GPa Tetragonal P42/nmc 135 and 136 P421c g-KBH4 RT, 46.8 GPa Orthorhombic Pnma 136 2+ Ca a-Ca(BH4)2 11.6 RT Orthorhombic F2dd 137 0 % a -Ca(BH4)2 4495 K Tetragonal I42d 137 % b-Ca(BH4)2 RT, metastable, HT polymorph Tetragonal P4 137 and 138 P42/m g-Ca(BH4)2 RT, metastable Orthorhombic Pbca 139 2+ Mn a-Mn(BH4)2 9.5 RT Hexagonal P3112 21 % g-Mn(BH4)2 RT, metastable Cubic Ia3d 58 d-Mn(BH4)2 RT, 1–8 GPa Tetragonal I41/acd 140 0 d -Mn(BH4)2 RT, 48 GPa Orthorhombic Fddd 140 + % Rb rt-RbBH4 4.0 RT Cubic Fm3m 19 hp1-RbBH4 RT, 3–12 GPa Tetragonal P4/nmm 141 hp2-RbBH4 RT, 12–20 GPa Orthorhombic C222 141 % hp3-RbBH4 RT, 420 GPa Tetragonal I42m 141 2+ Sr o-Sr(BH4)2 6.9 RT Orthorhombic Pbcn 142 t-Sr(BH4)2 4723 K Tetragonal P41212 143 3+ % Y a-Y(BH4)3 9.1 RT Cubic Pa3 144 % b-Y(BH4)3 RT, HT polymorph Cubic Fm3c 145 4+ % Zr Zr(BH4)4 10.7 o243 K Cubic P43m 109 and 127 2+ Cd a-Cd(BH4)2 5.7 RT Tetragonal P42nm 146 % b-Cd(BH4)2 4328 K Cubic Pn3m 146 + % Cs CsBH4 2.7 RT Cubic Fm3m 19 + Ba o1-Ba(BH4)2 4.8 RT Orthorhombic Pnnm 143 o2-Ba(BH4)2 4668 K Orthorhombic Pbcn 143 t-Ba(BH4)2 4718 K Tetragonal P41212 143 3+ % La La(BH4)3 6.6 RT Trigonal R3c 67 3+ % Ce Ce(BH4)3 6.6 RT Trigonal R3c 67 2+ Sm o-Sm(BH4)2 4.5 B500 K Orthorhombic Pbcn 68 and 147 3+ % Sm a-Sm(BH4)3 6.2 RT Cubic Pa3 147 % b-Sm(BH4)3 RT Cubic Fm3c 147 2+ B Eu o-Eu(BH4)2 4.4 430 K Orthorhombic Pbcn 68 t-Eu(BH4)2 4668 K Tetragonal P41212 143 % c-Eu(BH4)2 4668 K Cubic Fm3m 143 3+ % Gd Gd(BH4)3 6.0 RT Cubic Pa3 144 and 147 3+ % Tb Tb(BH4)3 5.9 RT Cubic Pa3 147 3+ % Dy Dy(BH4)3 5.8 RT Cubic Pa3 144 3+ % Ho a-Ho(BH4)3 5.8 RT Cubic Pa3 148 % b-Ho(BH4)3 RT Cubic Fm3c 148 3+ % Er a-Er(BH4)3 5.7 RT Cubic Pa3 147 % b-Er(BH4)3 RT Cubic Fm3c 147 2+ Yb a-Yb(BH4)2 4.0 RT Orthorhombic F2dd 149 % b-Yb(BH4)2 4523 K Tetragonal P4 149 and 150 g-Yb(BH4)2 473–573 K Orthorhombic Pbca 149 and 150 3+ % Yb a-Yb(BH4)3 5.6 RT Cubic Pa3 150 % b-Yb(BH4)3 RT, metastable Cubic Pm3m 150 4+ % Hf Hf(BH4)4 6.8 o263 K Cubic P43m 126 4+ Th Th(BH4)4 5.5 RT Tetragonal P41212 151 and 152 4+ U a-U(BH4)4 5.4 RT Tetragonal P41212 153 b-U(BH4)4 RT Orthorhombic Cmcm 154 4+ B Np Np(BH4)4 5.5 130 K Tetragonal P42/nmc 155
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1573 Review Article Chem Soc Rev
Table 3 Known experimental structures of bimetallic borohydrides ordered by cation atomic number of the first cation
Cations Polymorph rm (wt% H) Crystal system Space group Ref. + + Li K LiK(BH4)2 10.6 Orthorhombic Pnma 130 + + Li Rb m-LiRb(BH4)2 6.6 Monoclinic C2/m 156 o-LiRb(BH4)2 6.6 Orthorhombic Cmc21 156 h-Li2Rb(BH4)3 8.4 Hexagonal P6222 156 + + Li Cs o-LiCs(BH4)2 4.8 Orthorhombic Cmc21 156 h-Li2Cs(BH4)3 6.3 Hexagonal P6222 156 m-Li2Cs(BH4)3 6.3 Monoclinic Cc 156 m-LiCs2(BH4)3 3.8 Monoclinic C2/c 156 o-Li3Cs2(BH4)5 5.6 Orthorhombic Fddd 156 m-Li2Cs3(BH4)5 4.1 Monoclinic P21/c 156 m-Li3Cs(BH4)4 7.6 Monoclinic P21/c 156 + 3+ % Li Sc LiSc(BH4)4 14.5 Tetragonal P42c 81 + 3+ % Li Y LiY(BH4)4 10.4 Tetragonal P42c 63 + 3+ % Li Yb LiYb(BH4)4 6.7 Tetragonal P42c 150 + 3+ % Li Lu LiLu(BH4)4 6.7 Tetragonal P42c 147 + 2+ Li Zn LiZn2(BH4)5 9.5 Orthorhombic Cmca 70, 157 and 158 + 3+ % Li Al Li4Al3(BH4)13 17.4 Cubic P43n 159 and 160 3+ Al LiAl(BH4)4 17.3 Monoclinic P21/c 160 + 3+ Na Sc NaSc(BH4)4 12.7 Orthorhombic Cmcm 80 + 3+ Na Y NaY(BH4)4 9.4 Orthorhombic C2221 63 and 161 + 3+ Na La NaLa(BH4)4 7.3 Orthorhombic Pbcn 162 + 3+ Na Yb NaYb(BH4)4 6.3 Orthorhombic Cmcm 163 + 2+ Na Zn NaZn(BH4)3 9.1 Monoclinic P21/c 70 and 157 NaZn2(BH4)5 8.8 Monoclinic P21/c 70 and 157 + 3+ Na Al NaAl(BH4)4 14.7 Monoclinic C2/c 160 + 2+ K Mg K2Mg(BH4)4 10.0 Monoclinic P21/n 164 K3Mg(BH4)5 9.3 Tetragonal P42/mbc 164 I4/mcm + 2+ K Ca lt-KCa(BH4)3 9.8 Orthorhombic Pba21 ht-KCa(BH4)3 9.8 Orthorhombic Pna21 1 + 2+ K Sr KSr(BH4)3 7.1 Orthorhombic Pna21 165 + 3+ K Sc KSc(BH4)4 11.2 Orthorhombic Pnma 166 + 3+ K Y o-KY(BH4)4 8.6 Orthorhombic Cmcm 167 m-KY(BH4)4 8.6 Monoclinic C2/c 78 + 3+ K Ce K3Ce(BH4)6 7.0 Monoclinic P21/c 168 + 3+ K Gd KGd(BH4)4 6.3 Monoclinic P21/c 50 K2Gd(BH4)5 6.5 Monoclinic P21/m 50 K3Gd(BH4)6 6.7 Monoclinic P21/c 50 + 3+ K Ho KHo(BH4)4 6.1 Orthorhombic Cmcm 148 + 2+ % K Yb lt-KYb(BH4)3 4.7 Cubic P43m 1 ht-KYb(BH4)3 4.7 Orthorhombic Pmc21 1 + 3+ K Yb KYb(BH4)4 5.9 Orthorhombic Cmcm 163 + 2+ K Mn K2Mn(BH4)4 8.4 Monoclinic P21/c 164 KMn(BH4)3 8.7 Tetragonal P42/mbc 164 + 2+ K Zn KZn(BH4)3 8.1 Trigonal R385 K2Zn(BH4)4 7.9 Monoclinic P21/c 85 K3Zn(BH4)5 7.8 Tetragonal P42/ncm 85 + 2+ % K Cd KCd(BH4)3 6.2 Cubic Ia3 146 K2Cd(BH4)4 6.5 Monoclinic P21/c 146 + 3+ K Al KAl(BH4)4 12.9 Orthorhombic Fddd 169 + 2+ Rb Mg Rb3Mg(BH4)5 5.7 Tetragonal I4/mcm 170 + 2+ % Rb Ca lt-RbCa(BH4)3 7.1 Cubic Fm3c 1 ht1-RbCa(BH4)3 7.1 Orthorhombic C2221 1 ht2-RbCa(BH4)3 7.1 Tetragonal I4/mcm 1 + 2+ Rb Sr RbSr(BH4)3 5.6 Orthorhombic Pna21 1 and 165 + 3+ Rb Y o-RbY(BH4)4 6.9 Orthorhombic Pnma 78 m-RbY(BH4)4 6.9 Monoclinic P21/c 171 % Rb3Y(BH4)6 5.6 Cubic Fm3 1 and 78 + 3+ Rb Ce Rb3Ce(BH4)6 5.0 Monoclinic P21/c 1 + 2+ Rb Eu RbEu(BH4)3 4.3 Orthorhombic Pna21 1 + 3+ Rb Al RbAl(BH4)4 9.4 Orthorhombic Fddd 160 + 2+ Cs Mg CsMg(BH4)3 6.0 Monoclinic Cc 1 Cs3Mg(BH4)5 4.1 Tetragonal I4/mcm 170 + 2+ % Cs Ca lt-CsCa(BH4)3 5.6 Cubic Fm3c 1 % ht-CsCa(BH4)3 5.6 Cubic Pm3m 1 + 2+ Cs Sr CsSr(BH4)3 4.6 Orthorhombic P21212 1 and 165 + 3+ Cs Y CsY(BH4)4 5.7 Tetragonal I41/a 171 % Cs3Y(BH4)6 4.2 Cubic Fm3 1 and 78
1574 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
Table 3 (continued)
Cations Polymorph rm (wt% H) Crystal system Space group Ref. + 2+ Cs Eu CsEu(BH4)3 3.7 Tetragonal P4/mbm 1 + 3+ % Cs Gd Cs3Gd(BH4)6 3.8 Cubic Fm31 + 2+ Cs Mn CsMn(BH4)3 5.2 Monoclinic Cc 1 + 3+ Cs Al ht-CsAl(BH4)4 7.4 Tetragonal I41/amd 160 + 2+ Cs Pb CsPb(BH4)3 3.1 Tetragonal P4/mbm 1
Table 4 Known experimental structures of trimetallic borohydrides ordered by cation atomic number of the first cation
Cations Polymorph rm (wt% H) Crystal system Space group Ref. + + 2+ Li K Mg LiKMg(BH4)4 12.4 Orthorhombic Aba2 172 Li2K3Mg2(BH4)9 11.6 Monoclinic P2/c 172 + + 2+ Li K Mn LiKMn(BH4)4 10.1 Orthorhombic Aba2 172 + + 3+ % Li K La Li3K3La2(BH4)12 8.1 Cubic Ia3d 168 + + 3+ % Li K Ce Li3K3Ce2(BH4)12 8.1 Cubic Ia3d 168 + + 2+ Li Rb Mg LiRbMg(BH4)4 9.2 Orthorhombic Aba2 172 + + 2+ Li Rb Mn LiRbMn(BH4)4 7.8 Orthorhombic Aba2 172 + + 3+ % Li Rb Y Rb2LiY(BH4)6 6.8 Cubic Fm3 78 and 171 + + 3+ % Li Cs Y Cs2LiY(BH4)6 5.4 Cubic Fm3 78 and 171 + + 3+ % Li Cs Ce Cs2LiCe(BH4)6 4.8 Cubic Fm31 + + 3+ % Li Cs Gd Cs2LiGd(BH4)6 4.7 Cubic Fm31 + 2+ 2+ Li Mg Zn Li3MgZn5(BH4)15 10.2 Hexagonal P63/mcm 83 + 2+ 2+ Li Mn Zn Li3MnZn5(BH4)15 9.7 Hexagonal P63/mcm 83 + 2+ 2+ Rb Mg Ca RbMgxCa1 x(BH4)3 — Orthorhombic C2221 1
183–185 Alkaline earth metal borohydrides. The monometallic alkaline which are structurally related to polymorphs of TiO2. The 137 earth metal borohydrides show increasingly covalent and direc- orthorhombic polymorph, stable at RT, is a-Ca(BH4)2 (F2dd). tional bonding with decreasing ionic radius and increasing A second order phase transition at B220 1C was described as a electronegativity and charge density, in the series, Be 4 Mg 4 group–subgroup transition from F2dd to I42% d for the polymorphs 0 137 Ca 4 Sr 4 Ba. Indeed, the number of polymorphs (in parentheses) a-toa -Ca(BH4)2. At higher temperatures, the polymorphs a-and 0 of composition M(BH4)2 show decreasing trends in the series a -Ca(BH4)2 transform to b-Ca(BH4)2 with a more complex structure 138 % 137 Mg (7) 4 Ca (4) 4 Sr (3) 4 Ba (4). Porosity in the solid state is described by two similar models P42/m and P4, combining also observed in frameworks with the highest covalence, diffraction data and DFT optimization. The b-Ca(BH4)2 polymorph namely a- and g-Mg(BH4)2, with 6 and 30% of open space in may have some dynamic motion of the BH4 groups in the solid 9,101 the structures, respectively. state, e.g. similar to the high-temperature h-LiBH4 polymorph.
Strontium and barium borohydride. Sr(BH4)2 and Ba(BH4)2 The high-temperature b-Ca(BH4)2 polymorph is 4–6% denser than 142,143 0 are the most ionic of the alkaline earth metal borohydrides. a-anda -Ca(BH4)2, and at room temperature it slowly converts 186 Barium borohydride is synthesised as an orthorhombic polymorph back to orthorhombic a-Ca(BH4)2. Solvent based synthesis
(o1), which transforms to another orthorhombic polymorph (o2) methods provide another orthorhombic polymorph, g-Ca(BH4)2, 139 at T B 395 1C, which is isostructural to o-M(BH4)2, M = Sr, Sm, which irreversibly transforms to b-Ca(BH4)2 at T B 320 1C. The 68,142,143,147 Eu. o2-Ba(BH4)2 transforms to t- and c-Ba(BH4)2 poly- crystal structures of all four Ca(BH4)2 polymorphs contain calcium morphs at 445 and 460 1C. Similarly, o-M(BH4)2,M=Sr,Eualso cations coordinated by six borohydride anions forming octahedra, transform to t- and c-M(BH4)2 polymorphs upon heating. These which suggests ionic bonding. However, the existence of non- polymorphs are structurally related, o1- to CaCl2-type, o- and densely packed polymorphs points towards some degree of 143 o2- to a-PbO2,t-toht-ZrO2, and c- to CaF2. In fact, the CaCl2 directionality in the Ca–BH4 bonding. structure type is an orthorhombic deformation of the tetragonal Magnesium borohydride. The extreme structural flexibility rutile type (TiO2), which is based on hcp packing of BH4 anions, observed for metal borohydrides is highlighted in magnesium i.e. metal ions placed in octahedral positions. The a-PbO2 borohydride for which seven very different crystalline polymorphs 0 3,106,128,129,134,187–189 structure is then related to the rutile type by a well-known phase are known: a-, b-, b -, e-, z-, g-andd-Mg(BH4)2. transition which may be induced by applying pressure. The The a-Mg(BH4)2 polymorph crystallizes in a large hexagonal unit 190,191 t-polymorph is a superstructure (doubled c-axis) of the ht-ZrO2 cell a =10.354andc = 37.055 Å (originally reported in P61) type which is a deformation of cubic CaF2 structure type. The in the symmetry P6122, confirmed by DFT and single-crystal 106,192 orthorhombic structures of the a-PbO2-type likely form due diffraction. Analysis of the initial P61 models shows that to the similar cation sizes, Sm2+ (r =1.22Å),Eu2+ (r =1.25Å), the determination of H-atom locations from powder data posed Sr2+ (r =1.26Å),Ba2+ (r =1.42Å). the main problem for identification of the correct symmetry.
Calcium borohydride. Four distinct polymorphs of calcium Solid a-Mg(BH4)2 contains an unoccupied volume of 6.4% within 0 106 borohydride, denoted a-, a -, b- and g-Ca(BH4)2, are described, the structure. These voids are, in principle, large enough
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3 (37 Å ) to accommodate a small molecule, such as H2O. The porous g-Mg(BH4)2 transforms to an X-ray amorphous material a-polymorph transforms to an orthorhombic b-Mg(BH4)2 poly- at 0.4–0.9 GPa and then to crystalline d-Mg(BH4)2 at B2GPa.The morph at T 4 220 1C, which can be described in a super cell with polymorph d-Mg(BH4)2 has a tetragonal structure consisting of 106,134,190 Fddd symmetry, a = 37.072, b = 18.6476 and c = 10.9123 Å. two interpenetrated Mg(BH4)2 frameworks (Fig. 1). Each frame- B The high-temperature b-polymorph is 3% less dense and contains work resembles the cristobalite structure (a polymorph of SiO2), no unoccupied voids. whilst their doubly interpenetrated arrangement has a Cu2O Solvent based synthesis using triethylamine borane or dimethyl topology, which is typical for MOFs. This structural organization sulfide borane complexes provide two different crystalline is stable on compression up to B15 GPa, upon decompression magnesium borohydride solvates.3,56 The monoclinic solvate to 1 bar, and even on heating up to B100 1Catambient structure of Mg(BH4)2 1/2S(CH3)2 is a 3D framework containing pressure, where it then transforms to a-Mg(BH4)2. two Mg sites: one Mg is tetrahedrally coordinated to four BH4 All experimentally discovered Mg(BH4)2 polymorphs, and the groups, and the other to four BH4 groups and one S(CH3)2 lowest-energy theoretical structures, contain BH4 coordinated by 2 ligand forming a trigonal bipyramid. The S(CH3)2 ligand can be two Mg cations via opposite tetrahedral edges (Z )withMg–B–Mg extracted at moderate conditions (T o 100 1C), which provides angles deviating by 3–201 from linearity. The Mg BH4 inter- % the highly symmetric cubic structure of g-Mg(BH4)2 (Ia3d) (see action is highly directional and the Mg–BH4–Mg fragments can be Fig. 1), where a single Mg site is tetrahedrally coordinated to considered fundamental building units in all the structures. the BH4 groups. The structure has a 3D network of interpene- Magnesium atoms coordinate four BH4 tetrahedra in a strongly trated channels and g-Mg(BH4)2 is the first hydride to have a deformed tetrahedral environment. Interestingly, Mg atoms only large permanent porosity. The empty volume in the structure form a limited set of MgH8 polyhedra where only the less uniform amounts to B33%. The narrowest part of the pore channel is Johnson solids are found in the experimental structures.196 1 11 defined by a 5.8 Å distance between hydrogen atoms, but in Hand B spin–lattice relaxation NMR studies of the BH4 contrast, a point at (1/8, 1/8, 1/8) in the structure is 3.56 Å away reorientational motion support the highly anisotropic BH4 Mg 2 from the nearest H-atom, 4.12 Å from B and 4.82 Å from Mg. interaction in a-Mg(BH4)2. This contrasts the theoretically pre- 3 The framework topology of g-Mg(BH4)2 is isomorphic to both a dicted structures, which always contain MgH8 cubes. The pre- hypothetical zeolite type polymorph of SiO2 and to a porous dicted Mg(BH4)2 structures also tend to be either highly porous zinc imidazolate framework ZIF-72.3,193 This is illustrated by or dense, e.g. the low-density I4%m2(r =0.56gcm 3)andF222 3 the amorphisation of porous g-Mg(BH4)2 at RT over time and (r = 0.54 g cm ) structures, containing a single porous frame- recrystallization of g-Mg(BH4)2 upon heating at T =801C. work or a dense, doubly interpenetrated framework I41/amd Amorphisation of porous materials, e.g. metal organic frame- (r = 1.01 g cm 3). Although these predicted structures are works (MOF) is a well-known feature, but recrystallization of a topologically similar to g-andd-Mg(BH4)2 (Fig. 1), they have porous framework upon heating has possibly not been not yet been observed experimentally. observed for any other material.194 Both experiments and theoretical predictions suggest vast
Powder X-ray diffraction experiments conducted at elevated polymorphism of Mg(BH4)2. Moreover, all the experimentally pressures, using diamond anvil cells, reveal extreme collapse of observed polymorphs are stable at ambient conditions and in 195 the a-andg-Mg(BH4)2 structures upon compression, equating relatively wide temperature and pressure ranges indicating that B B to a 20% and 44% volume contraction, respectively. The the reconstruction of strongly bound Mg(BH4)2 coordination compression of g-Mg(BH4)2 displays the largest volume contrac- frameworks is kinetically hindered. This may be due to the high tion observed, or predicted, for a hydride material. a-Mg(BH4)2 stability of the linear Mg–BH4–Mg units, linking the MgH8 transforms to a new high pressure polymorph of magnesium nodes into various framework structures, similar to partly borohydride denoted d-Mg(BH4)2 at 1.1–1.6 GPa. Whereas highly covalently-bonded MOFs. The stability of Mg(BH4)2 polymorphs
1 1 Fig. 1 The crystal structure of (a) porous g-Mg(BH4)2, r = 0.550 g mL and (b) the high-pressure d-Mg(BH4)2, r = 0.987 g mL .
1576 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
may also be related to the relative stability of the MgH8 polyhedra. than a-Y(BH4)3 and contains relatively large unoccupied voids 3 Clearly, the BH4 complexes act as directional ligands, similar to of 39 Å , which are smaller in the low-temperature polymorph. organic ligands (‘‘linkers’’) with partly covalent coordination to The H H repulsion seems to be at the origin of the poly- metals in MOFs. The Mg–BH4 interaction is directional and partly morphic transition. The orientation of BH4 groups in b-Y(BH4)3 covalent and is essentially the reason for the MOF-like behaviour slightly expands the structure and maximizes the H Hcontacts, of Mg(BH4)2, i.e. the rich polymorphism and their relatively high whereas half of the BH4 complexes in a-Y(BH4)3 are flipped, stability, the large pressure-induced volume collapses, the amor- allowing for a more densely packed structure, but with shorter phisation under pressure and the surprising recrystallization of H H contacts (Fig. 2). This situation is somewhat similar to the porous structures from amorphous material. The unusual crystal disordered cubic alkali borohydrides, MBH4 (M = Na–Cs), where chemistry and high structural complexity of Mg(BH4)2 is the most the number of repulsive H H contacts is minimized via alocal 56 extreme case for metal borohydrides, but is typical for coordina- ordering of the BH4 groups at low temperatures. 2+ 5 tion polymers, such as MOFs. Manganese borohydride. Mn(BH4)2, (Mn ,d) was the first transition metal borohydride to be described, in 2009, denoted 3.2 Monometallic borohydrides with the d- and f-block a-Mn(BH4)2, and is one amongst a total of four manganese Stable metal borohydrides with the d-block metals can be borohydride polymorphs known to date.21,58,140 The polymorph, prepared for metals, which in a low oxidation state, have the a-Mn(BH4)2, was originally prepared mechanochemically from 0 5 10 8 200 d-electron configurations, d ,d or d . Scandium borohydride MnCl2 and LiBH4. A different manganese borohydride solvate has not yet been obtained in pure form, but a series of alkali can be obtained using dimethylsulfide as the solvent, Mn(BH4)2 scandium bimetallic borohydrides, MSc(BH4)4, M = Li, Na, K, 1/2S(CH3)2, which is isostructural to the magnesium analogue,
Rb, Cs are discussed in Section 3.5 of this review. Titanium Mg(BH4)2 1/2S(CH3)2. Gentle removal of the solvent provides 1 borohydride, Ti(BH4)3 with d configuration appears as an a porous polymorph, g-Mn(BH4)2 that is isostructural to the 58 exception to this trend, but it is a molecular borohydride (see magnesium analogue, g-Mg(BH4)2. The unit cell volume of 3 Section 3.3), with limited stability, decomposing at T o 0 1C. g-Mn(BH4)2 is B8.8% larger than that of the Mg-counterpart, 2+ 4 2+ 6 2+ Transition metal borohydrides, M(BH4)2,M=Cr (d ), Fe (d ) which corresponds well to the radius of Mn (r =0.80Å)being and Co2+ (d7) are stable in solution (T o 30 1C) but not yet in larger than that of Mg2+ (r = 0.71 Å). Furthermore, SR-PXD data58 the solid state at RT.94 indicate one or more unidentified compounds, which may be a 188 Yttrium borohydride. Y(BH4)3, has received significant inter- manganese analogue of e-Mg(BH4)2. Thus, the structural est as a hydrogen storage material due to the high gravimetric chemistry of manganese borohydride resembles that of the hydrogen content, rm(Y(BH4)3) = 9.07 wt% H2, and moderate magnesium analogue. decomposition temperature, T B 270 1C. Y(BH4)3 exists as The structure of a-Mn(BH4)2 contains two independent Mn two polymorphs, a- and b-Y(BH4)3, both stable at ambient atoms both surrounded by four BH4 units in a deformed tetrahedral 144,145,197,198 conditions. Mechanochemical treatment of LiBH4 coordination, whilst each BH4 is nearly linearly coordinated by two 2 and YCl3 yields a-Y(BH4)3, and in some cases small amounts Mn atoms via the opposite tetrahedral edges (Z ),exactlyasobserved of b-Y(BH4)3. Solvent-based methods may provide a solvate for magnesium borohydrides. The structure of a-Mn(BH4)2 is 58 Y(BH4)3S(CH3)2, which transforms to a-Y(BH4)3 during desolvation also isostructural to z-Mg(BH4)2. The structures of a-Mn(BH4)2 B 66 at T 140 1C. Transformation from a-tob-Y(BH4)3 is observed when heating in a narrow temperature interval, 190–200 1C, as decomposition begins at higher temperatures.145,197 The high- temperature polymorph is stable on cooling. % a-Y(BH4)3 crystallizes in a cubic unit cell a = 10.7445 Å (Pa3), whilst b-Y(BH4)3 crystallizes in a slightly larger cubic unit cell % 145,197,199 a = 11.0086 Å (Fm3c). The structure of b-Y(BH4)3 was initially suggested to be disordered primitive cubic, a = 5.4547 Å (Pm3m), based on SR-PXD data, but later defined as ordered in a supercell using powder neutron diffraction data.145 The major difference between the two powder diffraction patterns is that the strongest reflection in PND data (531), which is the only reflection defining the doubled lattice parameter, is practically Fig. 2 Fragment (1/8 of the unit cell shown by green lines) of the crystal invisible in PXD data.145,197 The cubic framework structures of structure of the low-temperature a-Y(BH4)3 polymorph (left) and the high- the two polymorphs are closely related, yttrium being octahedrally temperature b-Y(BH4)3 polymorph (right). Both structures are viewed coordinated to six BH4 anions. In b-Y(BH4)3, the octahedral coordi- along the same direction of their respective cubic unit cells. While in the low-temperature polymorph the (BH ) octahedron around yttrium is nation of Y is regular but is somewhat distorted in a-Y(BH4)3.The 4 6 deformed allowing for more efficient packing in the structure but with [Y(BH4)6] octahedra share corners and coordinate via Y–B–Y bridges shorter H H contacts (given in Å), the same octahedron in the high- Z2 where BH4 units share edges ( ) to yttrium, i.e. resulting in a temperature polymorph is regular with maximized H H contacts and a 145 12-fold coordination of yttrium to hydrogen. Furthermore, less dense structure. The polymorphic transition is induced by flipping of the high-temperature b-polymorph structure is 4.6% less dense three BH4 groups labelled with white numbers.
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and a-Mg(BH4)2 show similarity and both contain similar chiral also forms two polymorphs, a-Er(BH4)3 and b-Er(BH4)3, iso- layers, stacked along the c-axis, and rotated by 1201 by the 31 axis structural to yttrium borohydride analogues (Fig. 2). in a-Mn(BH4)2 and by 601 by the 61 axis in a-Mg(BH4)2.Existence However, the rare-earths gadolinium, terbium and dysprosium % of a solid solution MgxMn1 x(BH4)2 further illustrates the close each only form one polymorph (Pa3) Gd(BH4)3,Tb(BH4)3 and 65 144,147 similarity between manganese and magnesium borohydrides. Dy(BH4)3, which are isostructural to a-Y(BH4)3, but a high 203 Similar to a-Mg(BH4)2, a-Mn(BH4)2 also has some degree of temperature polymorph may also exist for Gd(BH4)3. porosity in the structure and contains isolated voids with an estimated volume of 21 Å3, occupying 6% of the structure volume. 3.3 Strongly covalent molecular monometallic borohydrides
At pressures above B1 GPa, a-Mn(BH4)2 transforms into the Several monometallic borohydrides appear to have prevailingly % high-pressure d-phase having either P4n2orI41/acd space covalent bonding and directionality in the M–BH4 coordination. 140 group symmetry, the latter shows the same ordering pattern The first example is beryllium borohydride, Be(BH4)2,knownin 105 for the BH4 group as suggested by Yao and Klug for high-pressure one very unique structural form. The tetragonal structure 179 LiBH4. d-Mn(BH4)2 is quenchable to ambient pressure and was investigated by single-crystal X-ray diffraction and reveals 1 shows a volumetric hydrogen density of 125 g H2 L at ambient trigonal-planar coordinated beryllium by three edge-sharing conditions but turns back into the a-phase on heating to 67–109 1C. BH4 groups, i.e. beryllium has an unusually high coordination At pressure above 8.6 GPa, a d0-phase with Fddd structure forms number of six hydrogen atoms in the nearest coordination reversibly. sphere. Two borohydride complexes are bridging beryllium with 2+ 10 2 Cadmium borohydride. Cd(BH4)2 (Cd ,d )isanotherrecently linear Be–B–Be geometry and edge sharing (Z ) coordination. characterized monometallic borohydride with low and high Another borohydride anion also coordinates to Be by edge sharing 2 temperature polymorphs, a-andb-Cd(BH4)2 and a first order (Z ) but is terminal with two dangling hydrogen atoms. The polymorphic transition at T B 55 1C.146 Both polymorphs consist structure is built from one-dimensional polymeric chains held of two identical interpenetrated three-dimensional frameworks together by weak interactions. This is illustrated by the fact that 2 built from Cd coordinated to four BH4 units by edge sharing (Z ). beryllium borohydride is reported to have a significant vapour
Each Cd atom is coordinated almost linearly by two BH4 com- pressure at RT and readily sublimes, which illustrates that there plexes. The [Cd(BH4)4] tetrahedra are slightly distorted in tetra- are weak intermolecular interactions and that the polymeric gonal a-Cd(BH4)2 whilst exhibiting ideal tetrahedral geometry in chains dissociate easily. In the gas phase, beryllium borohydride cubic b-Cd(BH4)2. The cubic b-Cd(BH4)2 has a Cu2O-antitype Be(BH4)2 may be a molecular compound, with face-sharing BH4 structure with B and Cd on the positions of Cu and O, respec- (Z3) and the same coordination of beryllium, CN(Be) = 6. Further- tively, while a-Cd(BH4)2 has a tetragonally distorted structure, more, the compound is unusually reactive and pyrophoric due to identical to the tetragonal high-pressure polymorph of magnesium the characteristic three-centre-two-electron bonds in the Be–H–B 3 204 borohydride d-Mg(BH4)2. The parent cubic structure has also contacts, similar to diborane, B2H6.Be(BH4)2 is also extremely been mentioned as a possible high-symmetry polymorph of poisonous like other beryllium compounds.105 The highest gravi- 106,191 Mg(BH4)2. metric hydrogen density of a metal borohydride is observed for
Monometallic rare-earth borohydrides. Synthesis of La(BH4)3 and Be(BH4)2, rm(Be(BH4)2)=20.8wt%H2 (Table 2), which also has
Ce(BH4)3 was initially conducted by mechanochemically facilitated the second-highest volumetric hydrogen density for metal boro- 201,202 1 metathesis reactions but without structural characterization. hydrides, rV(Be(BH4)2)=127gH2 L . A number of other monometallic rare-earth borohydrides are The most strongly polarizing cations form molecular mono- reported, which can be subdivided into two groups, those with metallic borohydride structures, e.g. Ti3+,Al3+ with electro- B 4+ 4+ B oxidation state (III), isostructural to polymorphs of Y(BH4)3 and negativity wp 1.5 and Zr and Hf with wp 1.3. The those with oxidation state (II), isostructural to Sr(BH4)2 or Ca(BH4)2. structures of both zirconium and hafnium borohydride are 68 The later consists of Sm(BH4)2, Eu(BH4)2 and Yb(BH4)2 as was isostructural molecular metal borohydrides and consist of mentioned in Section 3.1. Samarium forms monometallic isolated neutral M(BH4)4 units (Fig. 3) having low sublimation 109,126,127 borohydrides both in oxidation state (II) and (III). Mechano- temperatures, Tsub(Zr(BH4)4)=291C. The tetrahedral chemical treatment of SmCl3 and LiBH4 reveals Bragg peaks molecules of Zr and Hf borohydrides are analogous to the corresponding to two primitive cubic unit cells with dimen- methane molecule where each BH4 complex coordinates via sions a = 11.10 Å (Pa3)% and a = 5.64 Å (Pm3%m).147 The former is face sharing (Z3) to the metal with one dangling hydrogen atom isostructural to a-Y(BH4)3 and the latter is isostructural to the and the solids have primitive cubic unit cells. The experimental 197 initially reported structure for b-Y(BH4)3 from SR-PXD. structural data clearly supports previous assumptions that solid Ytterbium reveals a similar structural diversity to samarium, Zr(BH4)4 consists of discrete neutral molecules interacting forming borohydrides in both oxidation states (II) and (III), mainly by weak van der Waals and dispersion interactions. which are isostructural to known structure types.150,163 In the The shortest distance between neighbouring molecules in oxidation state (II), b-Yb(BH4)2 and g-Yb(BH4)2 are isostructural the solid is defined by the intermolecular H H distances of to b-Ca(BH4)2 and g-Ca(BH4)2, respectively. Mechanochemical 2.77 Å. Four hydrogen atoms point towards the center of the treatmentofYbCl3 and LiBH4 yields a-Yb(BH4)3 and b-Yb(BH4)3 that unit cell, being responsible for the intermolecular interactions 109 are isostructural to a-Y(BH4)3 and b-Y(BH4)3. Erbium borohydride in the crystal structure.
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considered as a consequence of the electronegativity difference between the two metals in the structures, which causes formation of partly covalent bonds between themoreelectronegativemetal
and the BH4 units. This contrasts bi-alkali metal borohydrides where the interaction is rather ionic. Series of bimetallic boro- hydrides have been discovered and are subdivided here in three groups, those based on s-block metals only, those based on a d- or f-block metals and typically an alkali metal and those based on a p-block metal and an alkali metal. This review focuses on series of compounds in order to aim at analyzing trends in structures and properties. Bi-alkali metal borohydrides. The first bimetallic borohydride
was discovered in 2008, lithium potassium borohydride, LiK(BH4)2, which has an ionic structure due to similarities in electronegativity of the two metals.130 Lithium and potassium coordinate to four and
seven BH4 tetrahedra, respectively. The structure can be regarded as built from anionic layers [Li(BH4)2] cut from an o-LiBH4 frame- Fig. 3 Illustration of the cubic crystal structure of Zr(BH4)4. workandintercalatedbyK+ cations. The decomposition tempera- ture for this compound is nearly an average of those for the two individual borohydrides. Aluminum borohydride. Aluminum borohydride is another Sodium potassium borohydride, NaK(BH ) , was initially molecular metal borohydride, consisting of discrete Al(BH ) units, 4 2 4 3 described as a stoichiometric compound,211 but was later shown that was characterized in 1997.107 Aluminum borohydride has a high to be a solid solution NaxK1 xBH4,0o x o 1, stable in the gravimetric hydrogen content, rm = 16.8 wt% H2, but is volatile and temperature range 200 o T o 450 1C. The solid solution, pyrophoric with a tendency to release diborane at RT and therefore Na K BH ,0o x o 1, is metastable at RT and phase-separates very challenging to handle and utilize in practical applications.205 x 1 x 4 into the monometallic borohydrides at RT within 24 h.93 Titanium borohydride. Titanium borohydride, Ti(BH ) is of 4 3 Lithium rubidium and lithium cesium borohydrides. The high interest as the only genuine transition metal borohydride phase diagram LiBH –ABH (A = Rb, Cs) has to date revealed with an open set of d-orbitals (d1 electron configuration), and 4 4 ten compounds Li A (BH ) (A = Rb, Cs), with i, j ranging despite many published reports on this compound the solid i j 4 i+j between 1 and 3. Amongst these, eight new structure types of state structure remains unknown.151,206 However, the gas phase homoleptic borohydrides are found. The Li–BH substructure structure has been characterized by electron diffraction,20 which 4 adopts various topologies as a function of the compound’s Li reveals monomeric Ti(BH ) species with BH face sharing to the 4 3 4 content, ranging from one-dimensional isolated chains to metal center (Z3)withanoverallC symmetry. The tridentate 3h three-dimensional networks. The coordination environment bonding in volatile Ti(BH ) has been confirmed by vibrational 4 3 around the Li-node is either triangular or tetrahedral in these spectroscopy20 and calculated structural optimizations.207,208 sublattices, and various compounds of this series contain the Though following a tridentate coordination-scheme in the gas rare connectivity of edge-sharing Li(BH ) -tetrahedra, following phase, in some molecular compounds such as [Li(Et O) ]- 4 4 2 2 the building principle of LiKMg(BH ) ,172 where a charged, [Ti (BH ) (PMe Ph) ],209 titanium centres are bridged by bidentate 4 4 2 4 5 2 4 layered, honeycomb-like substructure LiMg(BH ) is charge- BH (Z2), thus forming one dimensional chains of linked metal 4 4 4 balanced by the larger K+ counter-cation. Such substructures centres. These chains can be considered as dimers [Ti (BH ) ] 2 4 6 may suggest that, next to Mg2+,Li+ may have potential to being linked by Li, also via bidentate BH (Z2). Pronounced 4 participate in framework building with the tetrahydroborate aggregation of Ti(BH ) molecules has also been suggested to 4 3 anion, BH , as a linker, if the framework is stabilized by large occur in the solid state for titanium borohydride implying that 4 electropositive counter-cations. the predominant species is the dimer [Ti(BH ) ] .20 Titanium 4 3 2 Alkali and alkaline earth bimetallic borohydrides. No bimetallic borohydride rapidly decomposes at room temperature but has borohydrides between Li or Na and Mg have been observed. This recently been stabilized through incorporation into a metal– may be understood when considering the preferred coordination organic framework (MOF).210 polyhedra of the different cations. (4,4)-Connected tetrahedral Li–Mg frameworks cannot form without countercations, i.e. 3.4 Bimetallic s-block borohydrides other than as trimetallic borohydrides.172 Sodium cations are The majority of the recently discovered bimetallic borohydrides readily accommodated in octahedral coordination but do not consist of complex metal borohydride anions counter-balanced adopt higher coordination numbers, which explains why the by alkali cations. In all these structures the more electronegative structural analogues to K–Mg borohydrides do not exist for metal constitutes the central atom in the complex anion by sodium. coordination to a number of borohydride units, as for instance Two bimetallic potassium magnesium borohydrides have been 2 [Mg(BH4)4] . The formation of discrete complex anions can be discovered in the KBH4–Mg(BH4)2 system, namely K2Mg(BH4)4
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164 and K3Mg(BH4)5. The structure of K2Mg(BH4)4 is isostructural 3.5 Bimetallic d- and f-block borohydrides 2 to K2Mn(BH4)4 and built from distorted tetrahedral [Mg(BH4)4] 2 complex anions. The BH4 anions share an edge with Mg (Z ) Alkali metal scandium borohydrides. The structure of LiSc(BH4)4 similar to all polymorphs of Mg(BH4)2. Potassium ions form a K11 was the first bimetallic borohydride containing discrete anions to be cage (Edshammar polyhedron) around the complex anions. Struc- characterized in 2008, while two other members of the series, 80,81,166 tural similarities for the series of isostructural bimetallic boro- NaSc(BH4)4 and KSc(BH4)4, were described in 2010 (Fig. 5). hydrides K2M(BH4)4 (M = Mg, Mn, Zn, Cd) are discussed later in Mono-metallic scandium borohydride, Sc(BH4)3, has not yet been this section of the review. prepared nor structurally characterized. LiSc(BH4)4 is tetragonal % 81 The structure of K3Mg(BH4)5 is orthorhombic (P42bc) and (P42c), NaSc(BH4)4 is orthorhombic (Cmcm)withtheHT-CrVO4 has an intriguing nature that can be described as an intergrowth of structuretypeandKSc(BH4)4 is also orthorhombic (Pnma)butwitha 80,166 KBH4 and K2Mg(BH4)4. Interestingly, the BH4 complexes occur in BaSO4 type structure. Scandium–boron distances and B–Sc–B 2 two different environments, forming complex anions [Mg(BH4)4] angles in the tetrahedral geometry of [Sc(BH4)4] in NaSc(BH4)4 similar to those observed in the structure of K2Mg(BH4)4 and with a are relatively regular and resemble the almost ideal tetrahedral 81 second boron position only coordinating to potassium. This latter [Sc(BH4)4] anion, found by DFT optimization. The [Sc(BH4)4]
BH4 is octahedrally coordinated, strongly resembling the local anion is slightly distorted in KSc(BH4)4 and more deformed in environment of BH4 in KBH4, which crystallizes in the rock salt the structure of LiSc(BH4)4. The Sc–B distances (2.27–2.50 Å) are structure as described in Section 3.1. The second component, similar in all three known alkali metal scandium borohydrides building up the intergrowth of K3Mg(BH4)5, is a bicapped trigonal and compare well to those in the tetrahedral molecular com- 2 19,109,126,127 prism of K8, surrounding the composite anions of [Mg(BH4)4] pounds Zr(BH4)4 and Hf(BH4)4. In all three scandium- 164 and interconnected via common edges. based compounds, the [Sc(BH4)4] anion is located inside alkali
Alkali metal calcium borohydrides. The heavier alkali metal metal cages, i.e. tetragonal Li8 prisms (all prisms occupied) in borohydrides, MBH4, M = K, Rb and Cs, readily react with LiSc(BH4)4, slightly deformed trigonal Na6 prisms (each second calcium borohydride and form perovskite-type metal boro- prism occupied) in NaSc(BH4)4 and monocapped trigonal K7 1,193 hydrides (Fig. 4). The compounds all have the same com- prisms (all prisms occupied) in KSc(BH4)4. Note that the Li position MCa(BH4)3, M = K, Rb, Cs, and each exists in at least position is disordered along the c-axis in LiSc(BH4)4 and there- two polymorphs, see Fig. 4. Furthermore, a perovskite type solid fore the exact shape of the Li8 polyhedra cannot be determined. 1 solution is also described, RbCaxMg1 x(BH4)3. The primary building principle in NaSc(BH4)4 and KSc(BH4)4 + + Alkali metal strontium borohydrides. Alkali metal strontium consists of packing Na /K cations and [Sc(BH4)4] anions. The 2 borohydrides are also found to form perovskite-type compounds, BH4 complex coordinates to Sc by edge sharing (Z )forminga 165 MSr(BH4)3, M = K, Rb, Cs. The alkali-Ca and alkali-Sr boro- dominantly covalent bond whereas alkali metal ions tend to hydrides are the only two series where all three metal cations coordinate to the complex anions with a dominantly ionic (K, Rb, Cs) stabilize the perovskite structure type. Interestingly, coordination. the Ca-compounds reveal decreasing symmetry across their Alkali metal yttrium borohydrides. Discrete complex [Y(BH4)4] 1 high temperature polymorphic transitions. In contrast, the ions have been shown to exist in MY(BH4)4, produced by ball 167,171 Sr-perovskites have increasing symmetry and the ambient milling Y(BH4)3–3LiCl mixtures and MBH4 (M = K, Rb or Cs). 167 temperature Sr-polymorphs are lower in symmetry than their KY(BH4)4 is isostructural to NaSc(BH4)4 (Cmcm), whereas 171 Ca-analogues. This suggests that the Sr cation is too large to RbY(BH4)4 and CsY(BH4)4 form unique structures. RbY(BH4)4 stabilize the ideal cubic perovskite type structure, and that the crystallizes in monoclinic unit cell (P21/c)withaAgMnO4 structure
Ba-cation will come to lie outside the stability criteria of this type, which is a deformation of the BaSO4-type structure. There + structure type amongst borohydrides. are seven Rb cations located around each [Y(BH4)4] and seven
Fig. 4 Crystal structure of (a) the ordered room temperature polymorph of CsCa(BH4)3 and (b) of the modulated high temperature polymorph of RbCa(BH4)3. Ca (brown), Rb, Cs (blue), BH4 (green).
1580 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
Fig. 5 Tetrahedral complex anion [Sc(BH4)4] , and the packing inside Li8 square prisms, Na6 triangular prisms and K7 mono-capped triangular prisms of respective alkali scandium borohydrides.
+ [Y(BH4)4] anions around Rb forming slightly distorted mono- The more zinc-rich alkali zinc borohydrides, MZn2(BH4)5 capped trigonal prisms. CsY(BH4)4 is tetragonal (I41/a)witha (M = Li or Na) also exhibit similar general structural topologies + CaWO4-scheelite structure type. Cs is surrounded by eight BH4 consisting of two identical interpenetrated three-dimensional groups forming a distorted dodecahedron (snub disphenoid). While frameworks.70,158 This type of structural topology is common these structures were originally studied on chloride-containing for the coordination polymers involving organic ligands, such samples, hence the solution of chloride on borohydride sites cannot as MOFs,128 however observed for the first time in hydrides in be excluded, chlorine-free samples later allowed for additional this case. Alternatively, the structures of MZn2(BH4)5 (M = Li or 78 analysis. It was found that KY(BH4)4 also crystallizes in a Na) can be considered to be built from dinuclear complex ions, monoclinic (C2/c) structure with an octahedral coordination of [Zn2(BH4)5] and alkali metal ions. The MZn2(BH4)5 structures
Y, and RbY(BH4)4 crystallizes in the regular BaSO4 structure type. reveal strong and directional Zn–BH4 bonding by edge sharing 1 2 Double perovskites A3Y(BH4)6 (A = Rb, Cs) were also observed. (Z ) that defines the structural architecture. The Zn atoms in
Recently, also LiY(BH4)4 and NaY(BH4)4 have been prepared by both compounds have a triangular, nearly planar, coordination by thermal treatment and the two compounds were found to be three BH4 groups, similar to Zn in NaZn(BH4)3 and KZn(BH4)3.The 62,161 isostructural to LiSc(BH4)4 and NaSc(BH4)4, respectively. Li and Na atoms in LiZn2(BH4)5 and NaZn2(BH4)5 have a saddle-like
Alkali metal manganese borohydrides. A bimetallic potassium coordination to four BH4 units. All BH4 groups in the MZn2(BH4)5 manganese borohydride was synthesized in the KBH4–Mn(BH4)2 structures are almost linearly coordinated by two metal atoms, system, K2Mn(BH4)4, which is isostructural to K2Mg(BH4)4 and is either bridging the two zinc atoms in [Zn2(BH4)5] or coordinating indexed in a monoclinic cell as discussed above. Structural simila- to one Zn and one M atom. Furthermore, LiZn2(BD4)5 has been rities between the series of isostructural bimetallic borohydrides studied by PND, revising the orientation of one of the BD4 groups,
K2M(BH4)4 (M = Mg, Mn, Cd) are discussed later in this section. revealing that the coordination mode for the BD4 units to both 2 The decomposition of K2Mn(BH4)4 has been shown to generate Zn and Li corresponds to the cation D2B(Z ) coordination 158 157 KBH4 and a new compound KMn(BH4)3 identified as a perovskite scheme, which is supported by DFT calculations. 164 type metal borohydride. Later, the compound CsMn(BH4)3 (Cc) Alkali metal cadmium borohydrides. The potassium cadmium 1 was also noticed to have the perovskite type structure. borohydride, KCd(BH4)3, has an interesting 3D polymeric frame- n Alkali metal zinc borohydrides. Several alkali metal zinc work topology and contains a complex anion, [Cd(BH4)3]n built borohydrides have been characterized in which zinc coordi- from corner-sharing Cd(BH4)4 tetrahedra and Cd(BH4)6 octahedra. nates trigonally, in a planar fashion, to three BH4 units forming All six BH4 units in the [Cd(BH4)6] octahedra are coordinated complex discrete anions, as for example MZn(BH4)3 (M = Na or K) almost linearly by another Cd atom from the neighbouring 70,85 2 containing [Zn(BH4)3] . The Zn–BH4 (Z ) coordination is Cd(BH4)4 tetrahedra, whereas two of the four BH4 units within significantly more directional and covalent in contrast to the the [Cd(BH4)4] tetrahedra are terminal. The structure of KCd(BH4)3 alkali metal Na+ or K+ coordination. The trigonal coordination can also be described as a garnet, with K on dodecahedral and in [Zn(BH4)3] anions in KZn(BH4)3 is more regular than those octahedral sites, and Cd on octahedral and tetrahedral sites. In in NaZn(BH4)3. Interestingly, the Zn–B distance of 2.573 Å in contrast, the structure of K2Cd(BH4)4 is built from discrete 2 + KZn(BH4)3 is similar to the average of the three shortest Zn–B [Cd(BH4)4] anions counter-balanced by K cations. In the distances of 2.56 Å in NaZn(BH4)3. In both compounds the complex anion the Cd atom is coordinated to four BH4 units in a 146 [Zn(BH4)3] anions are located in Na8 or K8 cubes and the alkali distorted tetrahedral geometry. The structure of K2Cd(BH4)4 cations coordinate to six BH4 units in a trigonal prism. How- resembles that of MSc(BH4)4 (M = Li, Na and K) and K2M(BH4)4 ever, the coordination of the BH4 unit by the cations differs in these (M = Mg, Mn, Cd) as discussed below. two compounds, i.e. in NaZn(BH4)3 the BH4 units coordinate Electronic and size effects stabilizing tetrahedral complex either almost linearly to one Na and one Zn atom, triangular anions based on bivalent metals. The compounds containing m+ (4 m) planar to one Zn and two Na atoms or to one Zn and three Na tetrahedral complex anions of the type [M (BH4)4] (m =2,3) 3+ 3+ 3+ 3+ atoms in a tetrahedral geometry, while in KZn(BH4)3 only the are based either on a trivalent cation M such as Sc ,Y ,Al ,Ho , triangular coordination is observed. Yb3+,Lu3+ or on a bivalent cation such as Mg2+,Mn2+,Zn2+,Cd2+,
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1581 Review Article Chem Soc Rev and have already been discussed. Alkali metals often act as counter- manganese ion, r(Mn2+) = 0.80 Å is larger than magnesium cations in these compounds and stabilise the composite anions r(Mg2+) = 0.71 Å. This is attributed to electronic effects, the with a metal center. electronegativity of Mn (1.55) being closer to that of BH4
The compounds K2M(BH4)4 (M = Mg, Mn, Zn, Cd) all represent (group electronegativity 2.13) than Mg (1.31), thus driving 212 monoclinically distorted variants of b-K2SO4, i.e. MandBH4 covalent bonding within the complex anion. occupying the S- and O-positions, respectively and thus replacing Another compound containing an isolated tetrahedral 2 the sulphate tetrahedron by a complex anion [M(BH4)4] ,with complex anion is found in this system, K3Mg(BH4)5. Bidentate 2 2 consistently bidentate bonding scheme M H2B(Z ). DFT calcula- bonding is retained, while the isolation of [Mg(BH4)4] is 164 164 tions performed on K2M(BH4)4 (M = Mg), suggest high stability increased to 1.45 due to the larger coordination polyhedron of the complex anion. DFT structural optimizations reveal that K12. In this compound, an isolated BH4 not pertaining to the the inner B–H distances are longer than the outer ones, while the complex anion, is positionally and orientationally disordered.
H–B–H angle is smaller on the BH4 edge facing the Mg metal A repulsive interaction between this BH4 and the terminal ones 164 2 B centre. As discussed above, the coordination environment of of [Mg(BH4)4] takes place at higher temperatures, 87 1C, 2 2 [M(BH4)4] is best described as a pentacapped trigonal prism resulting in reorientation of [Mg(BH4)4] that in turn triggers a
(Edshammar polyhedron, K11), The idealized MK11 building symmetry change. Perovskite-type KMn(BH4)3 forms as a reac- block is undistorted in the compound b-K2SO4. tion product in the K–Mn system. In the perovskite, Mn is no Analysis of the relative interatomic distances K–B/M–B in longer tetrahedrally coordinated, most likely due to enhanced the above mentioned compounds provides some insight to the thermal vibration and thus simulated size increase of the metal structural differences caused by differences in metal Pauling centre. This phase only exists at high temperatures, which electronegativity and ionic radii.164 A general trend shows that ‘the supports this assumption. degree of isolation’ (i.e. the extent to which the crystal should be Alkali rare-earth bimetallic borohydrides. Significant atten- described on the basis of the complex anion rationalization rather tion has been devoted to the synthesis and characterization than a framework), as defined by the K-B/M-B ratio, decreases from of novel mono- and bimetallic rare-earth based metal boro-
1.41 in the Mn compound to 1.11 in the Cd compound. hydrides since the discovery of LiSc(BH4)4 and Y(BH4)3 in 81,144 Interestingly, the volume of the MK11 cage, as well as the 2008. Rare-earth metal (RE) borohydrides of all rare-earth M–H bonding mode, remain nearly identical, within errors, in metals (with the exception of promethium) have been structurally, 2 K2Mg(BH4)4 and K2Mn(BH4)4 while the volume of [M(BH4)4] physically and chemically characterized and many trends in varies significantly, i.e. 7.17 and 7.69 Å3 for Mn and Mg, properties appear to follow the size of the rare-earth metal ions, respectively (Table 5). This contrasts the fact that the divalent see Table 6. In bimetallic RE-bearing compounds based on Li, the smaller elements: scandium, ytterbium and lutetium crystal-
lize as LiRE(BH4)4, which are isostructural to LiSc(BH4)4,whereas Table 5 Comparison of the local environments of the complex tetra- 164 the larger elements from lanthanum to samarium all form hedral anions [M(BH4)4]2 with M = Mg, Mn, Zn and Cd LiRE(BH4)3Cl compounds, which are discussed in Section 4.1.
Compound Average M–B (Å) B–K/M–B (Å) V(MK11)/V(MB4) The moderately sized RE metals tend to crystallize with single cation structures similar to a-andb-Y(BH ) . The most rich K2Mg(BH4)4 2.4544(51) 1.41 36.61 4 3 K2Mg(BH4)4 2.5027(31) 1.38 34.33 variation of structure types is observed for the stoichiometry K2Zn(BH4)4 2.466(17) 1.33 31.85 ARE(BH ) , where A is an alkali metal. Small and medium size K Cd(BH ) 2.881(10) 1.11 24.02 4 4 2 4 4 RE form different structure types in this manner whilst the large KSc(BH4)4 2.3204(3) 1.64 31.56 K3Mg(BH4)5 2.4369(77) 1.45 38.23 V(MK12)/V(MB4) RE do not form structures with this stoichiometry, but instead prefer
Table 6 Relationship between the size of a trivalent rare-earth metal cation and the structure type of the corresponding binary and ternary alkali rare- earth metal borohydrides with the composition ABX4
213 RE ion Ion radius (Å) LiRE(BH4)4 NaRE(BH4)4 KRE(BH4)4 a-RE(BH4)3 b-RE(BH4)3 LiRE(BH4)3Cl Ref. 3+ % Sc 0.885 P42c (CuAlCl4) Cmcm (ht-CrVO4) Pnma (BaSO4) 80, 81 and 166 3+ % Lu 1.001 P42c (CuAlCl4) 147 3+ % % % Yb 1.008 P42c (CuAlCl4) Cmcm (ht-CrVO4) Cmcm (ht-CrVO4) Pa3 (ReO3) Fm3c (ReO3) 150 and 163 3+ % % Er 1.03 Pa3 (ReO3) Fm3c (ReO3) 147 3+ % % % Y 1.04 P42c (CuAlCl4) Cmcm (ht-CrVO4) Cmcm (ht-CrVO4) Pa3 (ReO3) Fm3c (ReO3) 62, 78, 144, 145, 167 and 197 C2/c (LaNbO4) 3+ % % Ho 1.041 Cmcm (ht-CrVO4) Pa3 (ReO3) Fm3c (ReO3) 148 3+ % Dy 1.052 Pa3 (ReO3) 144 3+ % Tb 1.063 Pa3 (ReO3) 147 3+ % % Gd 1.078 P21/c (LiMnF4) Pa3 (ReO3) I43m (spinel) 50, 88, 144 and 147 3+ % % % Sm 1.098 Pa3 (ReO3) Fm3c (ReO3) I43m (spinel) 147 Nd3+ 1.123 I43% m (spinel) 147 Pr3+ 1.13 I43% m (spinel) 147 3+ % % Ce 1.15 R3c (ReO3) I43m (spinel) 1, 4 and 168 3+ % % La 1.172 Pbcn R3c (ReO3) I43m (spinel) 88 and 162
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to crystallize as double-perovskites with the formula A3RE(BH4)6. a = 15.371, and c =8.586Å,(P63/mcm)fortheMg-compoundat
Different stoichiometries such as K2Gd(BH4)5 with square pyramidal RT. Interestingly, DFT optimization suggests locally ordered models and trigonal double pyramidal complex cations have also been in an ortho-hexagonal unit cell, which maximize Mg–Mg separation. observed. As for bivalent RE such as Sm2+,Eu2+ and Yb2+ they adopt Thus, the hexagonal structure observed by diffraction may be a the structures of alkali-earth borohydrides: the one of Sr(BH4)2 for superposition of three such orthorhombic structures in three orien- 2+ 2+ 2+ the larger Sm and Eu whilst that of Ca(BH4)2 forthesmallerYb . tations along the hexagonal c-axis. The framework in Li3MZn5(BH4)15 is of the mco-topological type, and contains channels built from face 3.6 Bimetallic p-block borohydrides sharing [M(BH4)6] octahedra with triangularly coordinated lithium Alkali p-block borohydrides. Mechanochemical treatment of occupying the faces, and consists of two interpenetrated frameworks
AlCl3–LiBH4 and AlCl3–NaBH4 produce chloride containing alkali of tfa-type (Fig. 6). The framework type is related to bimetallic aluminium borohydrides Li4Al3(BH4)B9ClB4 and NaAl(BH4)B1ClB3, LiZn2(BH4)5 with cation disorder (Li/Mg) on the Li-position. The 159,214 which are further discussed in Section 4.2. The chloride-free new borohydrides Li3MZn5(BH4)15 are potentially interesting as solid synthesis by the spontaneous reaction of aluminium borohydride state electrolytes, if the lithium mobility within the octahedral with alkali borohydrides results in different structures that are, channels is improved by disordering the site via heterovalent sub- however, based on the same tetrahedral complex anion [Al(BH4)4] stitution. The two compounds decompose via formation of the more with tridentate binding as seen in the chloride syntheses (with the stable mono- and bimetallic borohydrides.83 169,205 exception of lithium). The reaction between LiBH4 and Alkaline earth alkali trimetallic borohydrides. The five tri-
Al(BH4)3 produces a framework structure LiAl(BH4)4 with both metallic compounds ALiM(BH4)4 (A = K or Rb; M = Mg or Mn) cations tetrahedrally coordinated and resembling the framework and K3Li2Mg2(BH4)9 represent the first two-dimensional topologies of RT o-LiBH4. The sodium compound NaAl(BH4)4 crystallizes with amongst homoleptic borohydrides. The layered substructure is a monoclinic structure that is a deformation of the orthorhombic formed from negatively charged Li–Mg 4-connected networks type ht-CrVO4 also known for NaSc(BH4)4. Larger alkali metals K (Fig. 7), which can be rationalized as a honeycomb-like sheet of and Rb crystallize with the orthorhombic structure type of TbAsO4, edge- and corner-sharing tetrahedra. On their basis, the general 160 observed for potassium for the first time. The larger CsAl(BH4)4 chemistry of hypothetical, long sought-after, bimetallic Li–Mg crystallizes with the CaWO4-scheelite type known from CsY(BH4)4. borohydrides has been discussed and the results have been put
Cesium lead borohydride, CsPb(BH4)3, is hitherto the compound into context with the end-members Mg(BH4)2 and LiBH4.The containing the most electronegative metal, wP = 1.8, which crystal- crystal structures of these trimetallic compounds reveal interesting 1 lizes with a perovskite type structure. structural relationships to aluminosilicates such as KAlSi3O8 (feldspar) and may be exploited by using them as precursors in 3.7 Tri-metallic borohydrides an attempt to produce zeolite-like metal borohydrides, since Trimetallic borohydrides with cation disorder. The first zeolite contains three-dimensional negatively charged Al–Si–O trimetallic homoleptic borohydride compounds, Li3MZn5(BH4)15, networks, which could be rebuilt by their Li–Mg–BH4 analogues. M = Mg and Mn, were presented in 2013 and are also new cationic The double-sheets found in K3Li2Mg2(BH4)9 of type [LiMg(BH4)4] 83 solid solutions (see Table 4). The two compounds are isostructural are fragments similar to the structure of b-Mg(BH4)2 and could and were structurally investigated using in situ SR-PXD and PND, provide a new route to design nanoporous open-structured materi- which provides an average view of the structure as hexagonal, als with new host–guest adsorption properties.172
Fig. 6 Structural projections of hexagonal Li3MgZn5(BH4)15 along the chex-axis (left), and of orthorhombic LiZn2(BH4)5 along the aortho-axis (right).
A structural fragment with an ideal hexagonal symmetry in Li3MgZn5(BH4)15 and pseudo-hexagonal symmetry in LiZn2(BH4)5 is clearly visible and underlines the similarity of both structures built from similar frameworks.
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Table 7 Coordination number of metal and ligand in bimetallic com- pounds. The hapticity, Z, is given in the last column if determined by neutron diffraction or periodic DFT calculations
Borohydrides Hapticity, 0 0n+ 0 of metal, M r(M ) CN(M ) CN(BH4) Z BHx M Ba 1.38 6 3 — Sr 1.18 6 3 — Gd 0.94 5 2, 3, 4 2, 3 Y 0.90 6, 4 2 2, 3 Cd 0.78 4 2, 4 — Sc 0.75 4 2, 3, 4 3 Zn 0.60 3 2, 3, 4 2
Fig. 7 Honeycomb-like [LiMg(BH ) ] sheet making up the structure of structural similarities of metal borohydrides and metal oxides, 4 4 193 ALiMg(BH4)4, with the larger alkali-cation providing the connectivity which has been reported recently. This similarity may be due between adjacent sheets along the crystallographic b-axis. A (blue), Li to the fact that the tetrahydridoborate, BH4 , and the oxide, 2 (lavender), Mg (brown), BH4 (green). O , anions are isoelectronic, both carrying 10 electrons. How- ever, the main reason is probably due to the fact that both borohydrides and oxides are ionocovalent compounds follow- Double perovskite and garnet trimetallic borohydrides. Double ing similar rules on structure stability such as those dictated perovskites LiA M(BH ) have been observed for large alkali metals 2 4 6 by the Pauling rules. The structural similarities are notable for A = Rb, Cs and M = Y, Ce, Gd while the smaller alkali metal A = K 168 b- and g-Mg(BH4)2 where frameworks built from corner-sharing stabilizes the garnet-type structure Li3K3M2(BH4)12 for M = La, Ce. 188 [Mg(BH4)4] tetrahedra are related to SiO2 structure types. Another polymorph, d-Mg(BH ) , was described as a Cu O-type 3.8 General trends in the structural chemistry of metal 4 2 2 framework, i.e. as two interpenetrated b-cristobalite-type frame- borohydrides works (cristobalite is a polymorph of SiO2). Analogous simi- Metals with increasing Pauling electronegativity have increasing 0 larities between the polymorphs of a/a -, b- and g-Ca(BH4)2 and degrees of directionality in the M–BH4 coordination, in particular the TiO2 polymorphs anatase, rutile and brookite, respectively, alkaline earth and d-block metals, which mostly form framework were also described185 and are illustrated in Fig. 8. The simi- structures. These structures may have unexpected structural larities between borohydride and oxide structures reveal that the topologies and contain interpenetrated frameworks or some degree homoleptic solid metal borohydrides are, like oxides, structures with of porosity. These metal borohydrides often exist as several poly- partly ionic and partly covalent bonding having different degrees morphs (metastable in a wide T range), with the extreme case of of electron transfer between a metal cation and borohydride Mg(BH4)2 showing at least seven polymorphs. These observations anion. Consequently, the Pauling rules215 can rationalize the highlight the structural flexibility mainly assigned to directionality crystal structures of borohydrides.216 and some degree of covalence in the M–BH4 interaction. Structural differences between metal borohydrides and oxides. The majority of the bimetallic compounds containing an There are differences between the coordination of borohydride, alkali metal and an alkaline earth metal, or a d-block metal, 2 BH4 , and oxide, O , anions to metal cations, resulting in tend to form structures built from discrete composite complex structural differences between metal borohydrides and oxides. anions formed by a metal and BH4, which is due to a significant The negative charge of the oxide O2 has twice the charge of the difference in Pauling electronegativity. In all cases, the more borohydride BH4 , which allows for more structural flexibility in electronegative metal coordinates relatively strongly to BH4 oxides, e.g. incorporation of metal ions with mixed oxidation states. with a dominantly covalent interaction, while the less electro- This can lead to charge-ordering related phenomena such as negative metal, usually an alkali, acts as a charge-balancing ion. magnetism or ferroelectricity. The oxide anion also contains electron The number of BH4 complexes in the first coordination lone pairs, which decrease the sphericity in case of coordination sphere of metals, in the structures of metal borohydrides, numbers lower than four. In contrast, the borohydride anion has a appears to correlate with the ionic radius of the metal, M. As more pronounced non-spherical tetrahedral shape. Therefore, metal expected, the number of BH4 complexes increases with the borohydride crystal structures often have lower symmetry when increasing cationic radius of the metal and the number varies compared to the symmetry of the related oxides. 193 from three to thirteen in o-RbY(BH4)4. In general, the M–BH4 Furthermore, the borohydride anion is rather large r(BH4 )= distances also increase with increasing cationic radii of the 2.03 Å,19,217 being nearly twice as large as the radius of oxide metal. The number of BH4 units coordinating to the same anion r(O2 ) = 1.24 or 1.28 Å for tetrahedral or octahedral metal vary as shown in Table 7. coordination, respectively. As a consequence, the charge density of the oxide ion is more than eight times larger than that of 3.9 Comparisons between metal borohydrides and metal oxides the borohydride ion. Secondly, the Pauling electronegativity is Structural relationship between metal borohydrides and 3.44 for oxygen and only 2.13 (group electronegativity)212 for the metal oxides. This section provides numerous examples of borohydride anion. This illustrates that the BH4 anion is more
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Fig. 8 Stacking of octahedral layers in structures of a-Ca(BH4)2 (left) and anatase-TiO2 (right) showing their similarity (hydrogen atoms are omitted for clarity). Octahedral chains in two layers are shown in red and light red. One octahedron from a third layer is shown in green. reducing than O2 . Therefore, stable oxides are often formed by is also likely at the origin of the pressure-induced phase metal ions with a high oxidation state, small radius and, thus, transitions in Mg(BH4)2 and Mn(BH4)2. These compounds high charge density. This section shows that the opposite is valid change their structure with pressure from a more porous a-to for the metal borohydrides, which are often formed by the larger more dense d-ord0-polymorphs, which are also deformation metal ions with lower oxidation state and lower charge density, variants of anti-Cu2O-type. 6+ 0 d d e.g. potassium tetraoxoferrate(VI), K2FeO4 (Fe ,d), is stable at The repulsive H H interactions are naturally expected 2+ 6 RT whereas Fe(BH4)2 (Fe ,d) exists in solution at T o 30 1C, to be more pronounced for higher partial hydrogen charges, d. 94 175 or stabilized with ammonia at RT as [Fe(NH3)6](BH4)2. Struc- Indeed, d varies from 0.86 in o-LiBH4 (and even closer to tures of metal oxides are often considered as a close packing 1 for heavier alkali metals) to values much closer to zero for of anions with cations filling octahedral or tetrahedral sites. more covalent borohydrides such as Mg(BH4)2, where only a
This structural view holds less convincingly for the metal boro- partial charge transfer occurs from the metal cation to the BH4 hydrides, which often have a structural packing deviating signifi- anion. The limiting case is Cd(BH4)2, as Cd is the most cantly from the close packing of anions. This may be ascribed electropositive metal forming a borohydride at ambient conditions to higher ‘softness’ and polarizability, which introduces more without stabilization by alkali metal ions. The deformations of directionality and covalence in the M–BH4 bonding compared to the Cu2O-antitype Mg(BH4)2 and Cd(BH4)2 with pressure and metal oxides.218 temperature3,146 is determined by H H interactions driven by Role of H H interactions. The repulsive H H inter- different values of d, as well as by different radii of the metal action between neighbouring borohydride groups is another atoms. Thus, the strength of the repulsive H H interactions structure-determining parameter. The Switendick limit of 2.1 Å depend on the charge transfer from the counter ion and this for a minimal H H distance known from metallic hydrides also influences the stability of some polymorphs. seems to hold approximately for borohydrides. An illustrative example of the role of the repulsive H H interaction is the
RT to HT polymorphic transition from a-tob-Y(BH4)3. While the 4. Structure of anion substituted metal RT polymorph allows for more dense packing, three H H borohydrides distances around each Y are close to Switendick limit of 2.1 Å. The HT b-polymorph is less dense, and by reorientation of three Metal borohydrides have diverse compositions and rich and out of six borohydrides in the Y coordination, the shortest H H fascinating structural chemistry, as discussed in the previous distance becomes 2.6 Å (Fig. 2). Interestingly, DFT calculations section of this review. The metal borohydrides have been suggest the HT phase to be the ground state, indicating the mainly prepared mechanochemically during the past decade, overestimation of the H H repulsion in the calculation. In which often produced a variety of metal borohydride halides.
ReO3, the oxide equivalent of Y(BH4)3 is indeed the equivalent to Halide substitution in metal borohydrides is a valuable b-Y(BH4)3, and the oxide equivalent to the RT a-Y(BH4)3 is approach for tailoring physical and chemical properties. Metal obtained by applying the pressure. The flipping of three boro- borohydride halides have either fully ordered, partly ordered or hydride groups also straightens the Y–BH4–Y bridges in the disordered structures (Fig. 9 and Table 8), which is the major HT-polymorph, which allows maximizing the H Hdistances. focus of this section. Another class of anion substituted metal A similar behaviour was observed for a-andb-Cd(BH4)2 with a borohydrides contains both the borohydride, BH4 , and the tetrahedral coordination of Cd. While the RT-polymorph is a amide, NH2 , anions. These boron–nitrogen compounds are distorted anti-Cu2O-type structure, the HT-polymorph crystal- also investigated intensively due to their hydrogen storage and lizes in the regular cubic anti-Cu2O type. Whether the flipping ion conductivity properties. In cases where metal borohydrides of BH4 groups is at the origin of this phase transformation are contaminated by oxygen containing impurities, e.g. water remains controversial, since the hydrogen positions have not or solvents, they may partly decompose to metal borohydride d d 3 2 been reliably determined. The repulsive H H interaction borates, BO3 . Recently, borohydride closo-boranes, i.e. B12H12 ,
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Fig. 9 Crystal structure of (a) fully ordered Sr(BH4)Cl, (b) partly disordered Y(BH4)2 xCl2+x, and (c) fully disordered K2Zn(BH4)xCl4 x. Sr, Na, K (red), Zn, Y (blue) Cl (gray) and B (green).
Table 8 Overview of halide substitution in metal borohydrides crystal system at RT (Pnma) with the structure type of Sr(OH) .142 Cation Composition Space group Ref. 2 Potassium zinc borohydride chloride. The first mixed-anion Fully ordered structures 84 mixed-cation borohydride, KZn(BH4)Cl2, characterized in 2010, Sr Sr(BH4)Cl Pnma 142 K, Zn KZn(BH4)Cl2 P21/m 84 has a structure with crystallographic sites fully occupied by BH4 or % Li, La LiLa(BH4)3Cl I43m 88 chloride anions and consists of isolated heteroleptic [Zn(BH )Cl ] % 4 2 Li, Ce LiCe(BH4)3Cl I43m 4 + % anions charge balanced by K cations. The [Zn(BH4)Cl2] anion is Li, Pr LiPr(BH4)3Cl I43m 219 % Li, Nd LiNd(BH4)3Cl I43m 219 built from Zn in a trigonal-planar orientation to two Cl atoms and % Li, Sm LiSm(BH4)3Cl I43m 147 one BH unit, similar to zinc in the structures of NaZn(BH ) and % 4 4 3 Li, Gd LiGd(BH4)3Cl I43m 88 KZn(BH4)3.TheBH4 complex coordinates to Zn by edge sharing 2 Partly ordered structures (Z ), which then has coordination number four, CN(Zn) = 4. Ba o-Ba((BH4)1 xClx)2 Pnma 220 The potassium atom has an unusual coordination number of % h-Ba((BH4)1 xClx)2 P62m 220 eight, with a distorted square antiprismatic coordination, formed Na, Y NaY(BH4)2 xCl2+x P2/c 87 and 167 Li, Al Li4Al3((BH4)1 xClx)13 P21/c 159 by five Cl atoms and three BH4 groups. In agreement with the Na, Al NaAl(BH4)xCl4 x Pmn21 214 high coordination number of K, the K–Cl (3.159–3.66 Å) and K–B
(3.274–3.781 Å) distances in KZn(BH4)Cl2 are slightly longer than in Fully disordered structures 229 230 + K, Zn K2Zn(BH4)xCl4 x P21/n 85 KCl (3.130 Å) and KBH4 (3.364 Å), where the K ion has an K3Zn(BH4)xCl5 x P42/ncm 85 octahedral environment coordinated by six Cl or BH ,respec- % 4 Li, Yb LiYb(BH4)4 xClx P42c 81 and 150 tively. This coordination difference underlines that the structure is Yb Yb(BH4)2 xClx Pbca 150 not directly related to that of KBH4 and cannot be considered a Solid solutions simple composite of the starting materials. Li o-Li(BH4)1 xClx (RT) Pnma 9 Lithium rare-earth metal borohydride chlorides. A series of h-Li(BH4)1 xClx (HT) P63mc 9 o-Li(BH4)1 xBrx (RT) Pnma 86 alkali rare-earth metal borohydride chlorides with fully anion- h-Li(BH4)1 xBrx (HT) P63mc 86 ordered structures, LiRE(BH4)3Cl, RE = La, Ce, Pr, Nd, Sm or Gd h-Li(BH4)1 xIx P63mc 221 % crystallize in cubic unit cells with space group symmetry Na Na(BH4)1 xClx Fm3m 222 % % 4,88,147,219 NaB(H1 xFx)4 Fm3m 223 I43m. A combination of X-ray diffraction (SR-PXD), % K K(BH4)1 xClx Fm3m 166 neutron diffraction (PND) and density functional theory (DFT) % KB(H1 xFx)4 Fm3m 224 optimization of structural models reveals that the structure con- Mg b-Mg((BH4)1 xClx)2 Fddd 225 4 b-Mg((BH4)1 xBrx)2 Fddd 225 tains isolated tetranuclear anionic clusters [RE4Cl4(BH4)12] with a Ca b-Ca((BH4)1 xClx)2 P42/m 226 and 227 distorted cubane-like RE Cl core, known from coordination % 4 4 tri-Ca((BH4)1 xIx)2 P3m1 228 231 4 chemistry. The complex anions [RE4Cl4(BH4)12] are charge o-Ca((BH4)1 xIx)2 Pnnm 228 + 4 tet-Ca((BH4)1 xIx)2 I4mm 228 balanced by discrete Li ions. Each rare-earth metal atom, RE, b-Ca((BH4)1 xIx)2 P42/m 228 coordinates to three chloride ions and three borohydride groups forming an octahedron, but the BH4 tetrahedra coordinates to the large rare-earth ions by face sharing (Z3). Li+ ions are disordered, were discovered to host fast ion conduction. These new classes of occupying 2/3 of the 12d Wyckoff site, leading to fast lithium ion materials, metal borohydride amides, borates and closo-boranes are conduction,4 further discussed in Section 7. DFT calculations discussed in the last part of this section. indicate that the structure of LiCe(BH4)3Cl is stabilized by entropy, in accordance with the disorder on Li+ positions.4,88 This 4.1 Metal borohydride halides – fully ordered structures complicated structure type, containing both light and heavy
Strontium borohydride chloride, Sr(BH4)Cl, is a stoichiometric, atoms, could not be resolved by any two of the three methods, ordered compound, which crystallizes in the orthorhombic SR-PXD, PND and DFT calculations, and it was only possible to
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4,88 solvewiththecombineduseofallthreemethods. Alternatively, polyhedra as observed for the MgWO4 and NaY(BH4)2Cl2 type the structure may be rationalized from the borohydride-oxide structures. similarity (Section 3.9) as derived from spinel. If the anions, Lithium aluminum borohydride chloride. The first example BH4 or Cl , are considered to be close packed then rare-earth of a bimetallic borohydride based on s- and p-block metals has 159 metals occupy the octahedral sites, and lithium atoms occupy the ideal composition Li4Al3(BH4)13. The structure is built from 3+ the tetrahedral sites (similar to those in spinel, but in a ratio cationic, [(BH4)Li4] and a anionic, [Al(BH4)4] , complexes. However, 3+ 1 : 1 instead of 2 : 1 as in spinels). these units are not isolated, i.e. the Li–B distances within [(BH4)Li4] and between the Li atom and B within [Al(BH4)4] are similar. Hence 4.2 Metal borohydride halides – partly ordered structures this structure can be describedasa3Dframework.Ontheother
Barium borohydride chloride. Mechanochemical treatment hand, the structure of Li4Al3(BH4)13 can be related to that of the anti- of LiBH4–BaCl2 in molar ratios 1 : 1 and 2 : 1 produces two new Cr3Si type, by rationalizing it as close packing of complex anions and barium borohydride chlorides with different structures.220 An cations. Detailed structural analysis reveals chloride substitution 3+ orthorhombic chloride-rich compound, o-Ba(BH4)0.16Cl1.84,is mainly in the anion-centered complex, [(BH4)Li4] ,providingLi–Cl obtained using the reactant ratio 1 : 1 and a borohydride-rich coordination similar to that observed in LiBH4–LiCl solid solutions 9,234 hexagonal compound, h-Ba(BH4)0.85Cl1.15, using molar ratio 2 : 1. (see Section 4.4.1). Rietveld refinements of SR-PXD data mea-
The structures of o-Ba(BH4)0.16Cl1.84 and h-Ba(BH4)0.85Cl1.15 were sured for AlCl3–LiBH4 1:4 and1:5 samplesrevealthepresenceof described on the basis of SR-PXD data using Rietveld refinement B27 mol% and B46 mol% chloride anions in the compositions and structural similarities to orthorhombic barium chloride, Li4Al3(BH4)B9.8ClB3.2 and Li4Al3(BH4)B7.5ClB5.5. 232 o-BaCl2 (Pnma) and the hexagonal high temperature barium Sodium aluminum borohydride chloride. NaAl(BH4)xCl4 x % 233 chloride polymorph, h-BaCl2 (P62m), respectively. Boron and with a partly ordered orthorhombic (Pmn21) structure is also B chlorine atoms occupy the two Cl positions in the BaCl2 poly- described. The highest yield of NaAl(BH4)xCl4 x, 86 mol% morphs with the overall occupancy of B and Cl constrained to together with NaAlCl4, B14 mol%, was obtained by mechan- 214 one. A third anion position in each structure, o-Ba(BH4)0.16Cl1.84 ochemical treatment of a AlCl3–NaBH4 1:1. The structure and h-Ba(BH4)0.85Cl1.15, can be regarded as BH4-free. consists of one Na, one Al, one Cl and two BH4 sites. There is no Sodium yttrium borohydride chloride. Mechanochemical indication of any borohydride substitution on the general synthesis produces a mixed-cation mixed-anion borohydride position occupied by Cl. The borohydride anions appear to 87,167 chloride with the ideal composition NaY(BH4)2Cl2. The prefer two sites with higher local symmetry on the mirror plane. compound crystallizes in a polymeric pseudo-orthorhombic However, Rietveld refinement indicates Cl-substitution on the unit cell (P2/c), only slightly distorted from the orthorhombic BH4 site. The structure contains complex tetrahedral anions of 87,167 metrics (with b =901). [Al(BH4)xCl4 x] , which form in a relatively narrow composition However, Rietveld refinements suggest some Cl-substitution range 1 o x o 1.43, counter balanced by Na+ cations. The highest on the BH4 site, resulting in a partly ordered structure with the borohydride content (x = 1.43) is obtained from a sample with 87 composition, NaY(BH4)2 xCl2+x with x =0.21atRT. Interestingly, composition AlCl3–NaBH4 1 : 3 with relative anion occupancies 159,214 both experimental and theoretical results reveal two anion sites BH4 /Cl 0.82/0.18 and 0.61/0.39 on each boron site. with different sizes in the structural model. The BH4 anions Contrary to the complex anions [M(BH4)4] (M = Mg, Sc, Mn, Cd), clearly have a preference for the larger of the two sites, whereas [Zn(BH4)3] and [Zn2(BH4)5] the complex anion [Al(BH4)4] appears the chloride anion prefers the smaller, due to the size difference of to be more difficult to stabilize (see the discussion in Section 3.5), the two anions, r(Cl ) = 1.81 and r(BH4 ) = 2.03 Å. Both the yttrium requiring a certain extent of partial substitution of BH4 units by the 214 and sodium atoms in NaY(BH4)2Cl2 have slightly distorted smaller Cl ligand. octahedral coordination environments consisting of two BH4 sites and four Cl atoms for yttrium (i.e. Y(BH4)2Cl4) and two Cl 4.3 Metal borohydride halides – fully disordered structures atoms and four BH4 sites for sodium atoms (i.e. Na(BH4)4Cl2). DFT optimization of the experimental structure and Raman Compounds with fully disordered anion positions in the structures spectroscopy suggests that the BH4 units coordinate by face- have statistical distributions of BH4 and the heavier halide anion sharing (Z3) towards yttrium and by a combination of edge and on all anionic sites. The section after this one, 4.4, is devoted to face-sharing to sodium.87 compounds, which may be classified as solid solutions of two The existence of one larger and one smaller anionic site compounds. is analogous to oxides. The structure of NaY(BH4)2Cl2 is related Potassium zinc borohydride chloride. A fully disordered to MgWO4, which again is a cation substitution variant of the monoclinic bimetallic borohydride chloride, K2Zn(BH4)xCl4 x a-PbO2 structure. The latter is the structure type observed for (P21/n) has been identified where all four anionic sites are partly 85 Sr(BH4)2. There is only one anion site in a-PbO2 but two in occupied by BH4 and Cl . The structure of K2Zn(BH4)xCl4 x 2 MgWO4 with different sizes due to the size difference of the consists of [Zn(BH4)xCl4 x] anions with Zn coordinating to 2+ 6+ cations, Mg and W . Interestingly, the cations’ coordination four BH4/Cl groups, forming a somewhat distorted tetrahedron. number increases from six to seven for the borohydride free The compound K2Zn(BH4)xCl4 x is isostructural to the series of compound NaYCl4. However, the triclinic structure of NaYCl4 bimetallic borohydrides K2M(BH4)4, M = Mg, Mn, Cd, discussed has similar packing and connectivity of the cation coordination in Section 3.5.
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In fact, K2Zn(BH4)xCl4 x is a monoclinic distortion of the h-Li(BH4)0.58Cl0.42, obtained using thermal treatment is stable upon high temperature K2ZnCl4 polymorph observed at T 4 315 1C cooling to RT, however slow segregation of LiCl is observed and 235 9,234,237 with a K2SO4 type structure (Pnma). Interestingly, the volume o-Li(BH4)0.91Cl0.09 is obtained after several months. The per formula unit (V/Z) for K2Zn(BH4)1.47Cl2.53 measured at RT is mechanism for anion substitution in the system LiBH4–LiCl is almost identical to that of the high temperature polymorph of described in reaction scheme (4.1), i.e. one solid, LiCl, dissolves
K2ZnCl4 at 315 1C. This indicates that the unit cell volume in another solid, LiBH4. expansion caused by the incorporation of BH is similar to the 4 xLiCl + (1 x)LiBH - Li(BH ) Cl (4.1) thermal expansion causing the polymorphic phase transformation 4 4 1 x x from the RT polymorph of K2ZnCl4 (Pna21) to the high temperature polymorph (Pnma). Lithium borohydride–lithium bromide. The mechanism for anion substitution in LiBH4–LiBr, facilitated by mechanochemical K3Zn(BH4)xCl5 x is formed after prolonged ball milling of 85 or thermal treatment, has been investigated by in situ SR-PXD and KBH4–ZnCl2 (2 : 1). The structure is not fully characterized but the formation of two solid solutions is observed.86 Mechanochemical may crystallize in the tetragonal space group P42/mbc and contains 2 treatment, or just hand mixing, of a sample of LiBH4–LiBr leads to the complex anion [Zn(BH4)xCl4 x] making it a substitutional formation of a hexagonal solid solution h-Li(BH4)1 xBrx with a unit and deformation variant of the tetragonal (I4/mcm)Cs3CoCl5 structure type. cell volume similar to b-LiBr. This indicates that the first step in the Lithium ytterbium borohydride chloride. The compound anion substitution process is a dissolution of small amounts of LiBH4 into a-LiBr (b-LiBr and h-LiBH4 are isostructural). Heating the LiYb(BH4)4 xClx canalsobeformed,whichcrystallizesinaprimitive 1 tetragonal structure with x B 1.0 (P42% c), which is isostructural to sample to elevated temperatures, T 4 112 C, also enables 81,150 dissolution of LiBr in h-LiBH4 forming a lithium borohydride rich LiSc(BH4)4. During heating, ytterbium is also reduced, similar to solid solution with unit cell volume similar to LiBH4.Twosolid samarium borohydrides, forming Yb(BH4)2 xClx.Thiscompound 1 exists with two different compositions depending on both tempera- solutions can be observed at T 4 112 Candthedissolutionprocess is further accelerated resulting in a single solid solution at elevated ture and synthesis conditions. Yb(BH4)2 xClx, x = 0.76, crystallizes in 86 % temperatures and full solubility in the system LiBH4–LiBr. Cooling a primitive tetragonal unit cell (P4), whilst Yb(BH4)2 xClx, x =0.3, crystallizes in a primitive orthorhombic unit cell (Pbca).150 such a solid solution of h-Li(BH4)1 xBrx should finally lead to crystallization of a-LiBr, o-Li(BH4)1 xBrx and/or a hexagonal solid
solution h-Li(BH4)1 xBrx. The composition of the solid solution 4.4 Metal borohydride halides forming solid solutions h-Li(BH4)1 xBrx appears to remain constant (x B 0.5) upon storage Compounds with statistically distributed anions in their structures at RT for several months, but a-LiBr and presumably amorphous are denoted fully disordered and may be classified as solid solutions o-LiBH4 are slowly segregated. Bromide substitution stabilizes of two compounds. This will be further discussed in the following the hexagonal structure of LiBH4 to RT as a solid solution with 86 section. Anion substitution is a method for tailoring the properties of composition h-Li(BH4)0.5Br0.5. 8 known metal borohydrides. Substitution of BH4 or H by anions with the same charge and comparable size may lead to formation of Lithium borohydride–lithium iodide. Solid solutions of Li(BH4)1 xIx 221 solid solutions (see Table 9). The first part of this subsection is have been obtained by reacting LiBH4 with a-LiI. The substitution process can be induced either by heat treatment devoted to investigations regarding substitution of the BH4 B 1 complex anion with the heavier halides, Cl ,Br or I .Fluorine at T 245 C or by mechanochemical treatment. In situ SR-PXD substitution has also been attempted, which is discussed in the end has demonstrated that anion substitution initially forms two of this section, see Section 4.4.2. solid solutions, which at elevated temperatures merge into one In some cases, full solubility between a metal borohydride containing up to 62 mol% LiI. Two solid solutions may occur and the halide salt can be observed, e.g. Na(BH ) Cl . General due to substitution of I for BH4 in LiBH4 and substitution of 4 1 x x rules for anion substitution are derived from the numerous BH4 for I in LiI. The structures of the two solid solutions are examples presented in the following section and are described identical to the hexagonal structures of h-LiBH4 and b-LiI. The in the end of this section. solid solutions of Li(BH4)1 xIx have a broader stability range 1 4.4.1. Metal borohydride halides, the s-block (from RT to the melting at 330 C) compared to that of both 221 Lithium borohydride–lithium chloride. Initially, anion substitution h-LiBH4 and b-LiI. was observed by in situ diffraction experiments during heating a Sodium borohydride–sodium chloride. The mechanism for 9 sample of LiBH4–LiCl 0.85 : 0.15. Diffraction data suggest that solid anion substitution, facilitated by annealing, was investigated LiCl dissolves into the structure of the solid hexagonal polymorph, by in situ SR-PXD during three consecutive cycles of heating to 1 h-LiBH4,atT 4 110 C, obtaining a maximum chlorine substitution 300 1C, annealing for 30 min and cooling to RT for a sample of 234 of 42 mol%, i.e. h-Li(BH4)0.58Cl0.42. The hexagonal solid solution, 222 NaBH4–NaCl (1 : 1), see Fig. 10. The unit cell volume of NaCl increases and decreases linearly due to thermal expansion and Table 9 Ionic radii of selected anions213,236 contraction, respectively (linear thermal expansion coefficients, a(NaCl) = 2.562 10 4 K 1 for T = 32–300 1C). No changes are Anion F H NH2 Cl Br BH4 I observed in the unit cell volume of NaCl before and after the
Radius (Å) 1.33 1.40 1.68 1.81 1.96 2.03 2.20 three annealing cycles, i.e. no dissolution of NaBH4 into NaCl
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observed as a continuous change in the position of KBH4 diffraction peaks toward higher 2y values, i.e., the unit cell of
KBH4 contracts upon heating. This suggests two simultaneous and coupled chemical reactions taking place. (a) The ternary
chloride K3ScCl6 reacts with KBH4 to form potassium chloride, KCl, and possibly also amorphous scandium borides, boron, and/or higher boranes and (b) simultaneously, KCl dissolves in
the remaining KBH4 and forms a solid solution K(BH4)1 xClx with increasing amounts of KCl, see reaction scheme (4.2). - (1 x)KBH4 + xKCl K(BH4)1 xClx (4.2)
Magnesium borohydride–magnesium halide. Anion substitution
in Mg(BH4)2 is observed after annealing a milled mixture of
a-Mg(BH4)2–MgX2 (X = Cl or Br). The substitution is identified Fig. 10 In situ SR-PXD data for BM NaBH4–NaCl (1 : 1) heated from from the shift of Bragg peaks from a-tob-Mg(BH4)2, i.e. anion RT–300 1C, kept at 300 1C for 30 min, cooled from 300–30 1C(DT/Dt = 225 substitution only occurs in the b-Mg(BH4)2 polymorph. Structure 10 1Cmin 1). This is repeated by three cycles. Symbols: NaCl, NaBH 4 refinement revealed that unit cell parameters decreased by about and J NaBO2 (l = 1.072 Å). Adapted with permission from ref. 222. Copyright 2011 Elsevier. 0.1–0.2 Å for b-Mg(BH4)2 in the mixtures, which corresponds to a volume contraction of the unit cell of about 2%. A combined experimental and theoretical approach for the Cl substitution in takes place during the short time of the experiment (only 90 min Mg(BH4)2 has shown that the enthalpy gain is more favourable for at 300 1C). the b-solid solution than for the a-solid solution.225 At T B 240 1C the unit cell volume of Na(BH4)1 xClx decreases rapidly and continues to do so for approximately 20 min. On this Calcium borohydride–calcium chloride and bromide. Halide time scale the unit cell parameter is found to decrease from substitution in Ca(BH4)2–CaCl2 and Ca(BH4)2–CaBr2 was also 226,227 6.2099–6.1295 Å, while the degree of substitution, x changes from investigated. Chlorine substitution was not observed after approximately 0.15 to 0.50. During the second and third annealing BM of the samples but readily occurs after the transition from a-tob-Ca(BH ) upon heating above B250 1C. Rietveld analysis periods at T =3001C the unit cell volume of Na(BH4)1 xClx decreases 4 2 linearly due to anion substitution. Furthermore, the fact that the of the obtained b-Ca(BH4)1 xClx phases indicates compositions from x = 0.3 to 0.7 depending on the amount of CaCl in the decrease in unit cell volume of Na(BH4)1 xClx is linear upon cooling 2 mixtures. No anion substitution with CaF or CaBr is observed.226 suggests that NaCl does not segregate from NaBH4 on the time 2 2 scale of the experiment and the final composition is found to be 222 Calcium borohydride–calcium iodide. Anion substitution in Na(BH4)0.43Cl0.57. the system Ca(BH4)2–CaI2 resulted in the formation of three Dissolution of 10 mol% NaCl into NaBH4,formingNa(BH4)0.9- new compounds, which are solid solutions with BH4 and I Cl , takes place during relatively mild ball milling, i.e. 60 2min 0.1 anions sharing the same positions in the structures.228 Sample with 2 min breaks to restrict sample heating (total BM time preparation methods have a significant influence on the reaction 120 min). A higher degree of dissolution of NaCl into NaBH is 4 pathway. Mechanochemical synthesis (ball milling) mediates a obtained by more harsh BM (e.g. 24 h) in some cases combined significant degree of dissolution of Ca(BH4)2 into CaI2 (B70 mol%) with sample heating by the reaction heat from decomposition yielding a CaI2-type trigonal solid solution, tri-Ca((BH4)1 xIx)2 of metal borohydrides. These investigations suggest full solubility B 71,72 (x 0.7). Upon heating, tri-Ca((BH4)0.7I0.3)2 transforms to a in the system NaBH4–NaCl, i.e. one solid solution. This is CaCl2-type orthorhombic structure, o-Ca((BH4)1 xIx)2 (x B 0.3), expected because NaCl and NaBH4 are both isomorphous. Inves- which is structurally related to b-Ca(BH4)2. Upon further heat- tigations reveal a linear correlation between the compositions of ing (at 335 1C), the sample partly transforms to a more iodide- the solid solutions and the unit cell volume, in agreement with B rich tetragonal solid solution, tet-Ca((BH4)1 xIx)2 (x 0.6), with Vegard’s law.71,72,222 Chlorine substitution for the larger iodide a novel structure type. This compound decomposes to CaHI anion also occurs readily and stoichiometrically during relatively and CaB6 (at T 4 345 1C). Apparently, minor amounts of CaI2 mild ball milling. also dissolve into b-Ca(BH4)2 forming a solid solution, which is
structurally similar to o-Ca((BH4)1 xIx)2, i.e. b-Ca((BH4)1 xIx)2 Potassium borohydride–potassium chloride. Anion substitution with x B 0.05.228 in potassium borohydride has been observed during the decom- 4.4.2 Fluoride substitution in metal borohydrides. The smaller 166 position of KSc(BH4)4. The mechanochemically produced halide ion, F (1.33 Å), has a similar size to the hydride ion, H sample originally contained the three compounds: KSc(BH4)4, (1.40 Å) and metal hydrides and fluorides share many properties, K3ScCl6, and KBH4. The amount of KBH4 increases during the e.g. the ionic compounds are often found to be isostructural. decomposition of KSc(BH4)4 in the temperature range 150 to 230 1C This suggests that hydrogen and fluorine may substitute for and further reacts with K3ScCl6 at T 4 230 1C. The reaction is each other in both ionic and covalently bonded compounds.
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The most significant difference between the two lies in the the structure providing shorter average bond lengths and there- electronegativity, which is 2.1 and 4.0 for hydrogen and fluorine, fore larger lattice energy. respectively. Therefore, fluorine substitution is expected to lead to (ii) Some polymorphs of metal borohydrides are more prone significant thermodynamic changes in metal hydrides. The first to perform anion substitution than others. This effect is illustrated fluorine-substituted complex hydride was prepared by hydrogena- by the high temperature polymorphs, e.g. h-LiBH4, b-Mg(BH4)2 and tion of the system NaF–Al–H2,whichyieldedthecomplexhydride b-Ca(BH4)2, which more readily dissolve solid metal halides into
Na3AlH6 xFx, i.e. fluoride substitution of the complex anion their structures. Investigations using diffraction and solid state 3 238,239 [AlH6 xFx] . The substitution of fluorine into NaMgH3, MAS NMR reveal that anion substitution is facilitated by structural forming NaMgH2F, has been shown to stabilize the structure dynamics and an orientational disorder, i.e. entropy effects, in and increase the enthalpy of hydrogen absorption/desorption, a particular related to the BH4 unit, which possibly make the BH4 desirable property for concentrating solar thermal energy storage tetrahedra appear more spherical and similar to halides.177,251 applications.240 (iii) In the case where the metal borohydride and the metal - Fluoride substitution in the BH4 complex anion, i.e. H F , halide are isostructural, anion substitution may occur in both is also possible. The system NaBH4–NaBF4 provided the first compounds, which can lead to two solid solutions. This effect is fluorine substituted borohydride, NaBH2.1F1.9, observed in a observed during moderate heat treatment of mechanochemically narrow temperature range of 200 to 215 1C by SR-PXD.223 treated mixtures of various borohydrides and halides, leading to two
However, the obtained materials MB(H1 xFx)4 appear to have solid solutions and indicating non-equilibrium conditions. Thus, relatively low thermal stability and the released hydrogen can mechanochemical treatment facilitates formation of the solid be contaminated with significant amounts of boronfluorides solution with volume contraction and moderate heat treatment and/or borane gases and may lead to the formation of signifi- may facilitate formation of the solid solution with larger volume, 223,241 cant amounts of unwanted M2B12H12. The system KBH4– as compared to the parent compound, due to thermal expansion. 11 19 KBF4 was investigated by solid state B and F NMR, which Such binary solid solutions tend to merge to a single solid solution appear more consistent with a statistical distribution of fluorine after prolonged heating, i.e. a solid solution exists in the full substituted [B(H,F)4] tetrahedra rather than a mix of discreet concentration range at higher temperatures (and can be quenched 224 BH4 and BF4 tetrahedra. to RT). This is observed for LiBH4–LiBr, LiBH4–LiI and NaBH4–NaCl
Hydrogen substitution by fluorine in orthorhombic o-LiBH4 systems, and notice that b-LiBr, b-LiI and h-LiBH4 and also NaBH4 has been investigated with quantum-mechanics calculations and NaCl are isostructural. aiming to describe thermodynamic properties of LiB(H,F)4 for Reactions during mechanochemical treatment appear to be hydrogen storage applications. Excess enthalpies of the mixed promoted by elevated pressure, i.e. by compression of materials, compounds were computed, suggesting that fluorine atoms therefore all anion substitutions obtained by BM appear to tend to agglomerate on the same boron atoms rather than be follow (i). In contrast, thermal treatment (annealing) tends to 242 homogeneously dispersed over all the available BH4 tetrahedra. facilitate increased mobility, e.g. of BH4 , and thermal expansion These data show that, at room temperature, the formation of solid of the structures. Therefore, initially rule (i) appears to be solutions between lithium borohydride and borofluoride is thermo- violated in cases where (iii) is valid, but prolonged heating will dynamically unfavoured, so the fluorine substitution destabilizes eventually lead to chemical equilibrium and appear to follow (i), the pure hydride.242 i.e. two solid solutions tend to merge to one. In the cases where A number of reactive hydride composites (further discussed anion substitution occurs at elevated temperature (ii) the high in Section 6.8) containing metal fluorides have been successfully temperature polymorph is often stabilised to lower temperatures. hydrogenated and in some cases cycled with hydrogen release and Here the dissolution of a metal halide at elevated temperatures 243,244 uptake, (CaH2/CaF2)–MgB2, Mg–LiBH4 composites catalysed appears to be a fast process but segregation of the metal halide at 245 246,247 248–250 by FeF3, LiH–LiF–MgB2 system, and Ca(BH4)2–MgF2. lower temperatures is often a slower process. A drawback is the high stability of both ionic metal fluorides MF Thus, in conclusion, anion substitution and anion disorder and molecular covalent borontrifluoride, BF3. is a well-known property of metal borohydrides that has been exploited to tailor hydrogen release properties, and more recently to 4.5 General trends for halide substitution in metal develop ionic conductors. Cation disorder, on the other hand, borohydrides has not been observed very often, with some examples being, 65,83 Metal borohydrides and the heavier halides. The above Mn1 xMgx(BH4)2, and Li3MZn5(BH4)15, M = Mg and Mn. Section 4.4 provides a range of examples of halide substitution Only very recently have some researchers started to consider in stable monometallic alkali and alkaline earth metal boro- the functional design of physical properties by means of cation hydrides and some trends in the structural chemistry of anion substitution, which in turn is the most dominant tool to tailor substitution are observed. Anion substitution by the heavier transition metal oxides. Cation disorder may occur for structures halides, i.e. Cl ,Br or I , of borohydrides: containing very large anions dominating structural organization,4 (i) The solid containing the smaller anion tends to dissolve or when the cations are very similar in size, such as for Mg2+ and into the compound containing the larger anion, and the Mn2+ with identical charge and similar ionic radius, r(Mg2+) = 0.71 structure of the latter tends to be preserved in the obtained and r(Mn2+) = 0.80 Å.65 However, cation substitution can also occur solid solution. This trend can be interpreted as a contraction of for cations that have the same preferred coordination sphere such
1590 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article as Li+ and Mg2+ that both prefer tetrahedral coordination. There- fore the tri-metallic compound Li3MZn5(BH4)15,M=MgandMn may be considered as being derived from the bimetallic compound 83 LiZn2(BH4)5 by Li–Mg cation substitution. Fluorine substitution in hydridocomplexes is clearly possible, however the stability of the produced compounds is limited due to the formation of very stable ionic solids or molecular gases.
The system Na–Al–F–H can provide the complex Na3AlH6 xFx, which has a strong tendency to form fully fluorine substituted 3 238 octahedral complexes [AlF6] . Recall the composition of the rather stable mineral cryolite, Na3AlF6. The metal borohydride– borofluorides, NaBH4–NaBF4 and KBH4–KBF4, provide some new and partly fluorine substituted borohydride complexes, 223,224 NaBH2.1F1.9 built from [B(H,F)4] . However, their stability is rather limited and these systems have a strong tendency to decompose to significantly more stable ionic metal fluorides or possibly the molecular compound, BF3.
4.6 Trends in series of bimetallic borohydride halides % Fig. 11 Room temperature structure (R3m)ofCs3BH4B12H12 viewed In some cases, series of bimetallic alkali metal–d-block metal approximately along the b-axis of the hexagonal lattice. The anti- borohydride halides have been investigated. The monometallic perovskite cubic unit cell of the high temperature polymorph (P23) is borohydrides are described as two different polymorphs for yttrium shown in green. Note the disordered borohydride anion already at room and cadmium. The existence of monometallic scandium and zinc temperature. Cs (blue), B (green) and H (white). borohydrides has been suggested, but they have not been observed experimentally. For the bimetallic systems containing lithium, only is orthorhombic Pna21. Filling the interstitial sites of Na3BH4B12H12 146 + cadmium forms a ternary halide, Li2CdCl4. For the tri-metallic with Li affords (Li0.7Na0.3)3BH4B12H12, where both compounds 2 systems containing lithium, synthesis of Li3MZn5(BH4)15 also yields appear to have ordered B12H12 and BH4 anions, though no 83 some Li2MCl4,M=MgorMn. However, ternary halides are conclusions can be drawn on low frequency reorientations. formed in most cases for sodium and potassium systems. 4.8 Metal borohydride amides The systems MBH4–ZnCl2 (M = Rb, Cs) and MBH4–ScCl3 (M = Rb, Cs) remain incompletely characterised due to their complexity. The first composite of a metal borohydride and a metal amide, 257 Thus, there is a tendency for more complex structural chemistry, LiBH4–LiNH2 (1 : 2), was reported in 2005 with the aim of higher numbers of compounds and solid solutions in the phase improving H2 desorption temperatures, owing to partly negatively diagrams and more diverse compositions for heavier alkali metals. and positively charged H atoms present in the composite. In
addition, it was shown in 2009 that composites of LiBH4–LiNH2 4.7 Metal borohydride closo-boranes are good lithium ion conductors.89 Thesetwofactorshaveinspired There are a few examples of mixed borohydride closo-boranes.1,252 research into various other borohydride–amide composites, e.g. 258 90 Both Cs3BH4B12H12 and Rb3BH4B12H12 were thought to be Mg(BH4)2–LiNH2 and Mg(BH4)2–Mg(NH2)2 (see Table 10). As by-products from the pyrolysis of their respective B3H8 com- described earlier in Section 4, halides may directly substitute for 253 pounds in 1990, which can also be formed as decomposition BH4 on the same crystallographic position. On the other hand, 78 products from bi- and trimetallic borohydride-perovskites. due to difference in size and coordination, the complex NH2 The compound Cs3BH4B12H12 was presented in 2003 (see anion always has unique crystallographic sites compared to BH4 Fig. 11),254 which can be precipitated from water. This com- in borohydride-amide structures. pound crystallizes in a trigonal (R3%m) anti-perovskite structure The first reported crystal structure of a borohydride–amide + type, where Cs is considered as a shared vertex of two [BH4Cs6] was Li4(BH4)(NH2)3, cubic (I213), with three independent octahedra. Interestingly there is dynamic disorder of the BH4 lithium positions that all are coordinated by four ligands, i.e. anion at room temperature, whereas a further structural transi- three BH4 and one NH2 ; one BH4 and three NH2 ; and four 263,264 % tion is believed to occur near 305 1C, which could involve NH2 (Fig. 12). The rhombohedral structure (R3) of 2 rotation of the B12H12 anions. This was also suggested by a Li2(BH4)(NH2) was later determined with two independent more detailed study of the decomposition of borohydride- lithium positions that are coordinated by three BH4 and one 259,260 perovskites, and the symmetry of the high temperature cubic NH2 ; and one BH4 and three NH2 . To our knowledge, 255 anti-perovskite polymorph was identified as P23. the LiBH4–LiNH2 system is unique compared to the other metal
More recently Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12 borohydride amide systems in the way that new compounds are were synthesized by mechanical milling followed by heat treat- readily formed by mechanochemical treatment. In all other ment and investigated for their ion conductivity properties.256 systems, mechanochemical treatment only leads to physical
Na3BH4B12H12 is orthorhombic Cmc21 and (Li0.7Na0.3)3BH4B12H12 mixtures of the starting reactants.
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0 Table 10 Overview of reported composites of M(BH4)m–M (NH2)m
0 0 M in M(BH4)m M in M (NH2)m Ratio Synthesis Product Ref.
Li Li 1 : 1 BM or heat Li2(BH4)(NH2) 259 and 260 Li Li 1 : 2 BM Li2(BH4)(NH2)+Li4(BH4)(NH2)3 257, 261 and 262 Li Li 1 : 3 BM or heat Li4(BH4)(NH2)3 259, 263 and 264 Na Na 1 : 1 Heat Na2(BH4)(NH2)91 Mg Mg 1 : 1 BM + heat Mg(BH4)(NH2)90 Mg Li 1 : 1 BM Mixture of reactants 258 Li Mg 1 : 1 BM Mixture of reactants 265 Ca Li 1 : 1 BM Mixture of reactantsa 266 and 267 Ca Li/Na 1 : 1 BM Mixture of reactantsa 268 Ca Mg/Ca 1 : 2 BM Amorphous mixture 269 a 268 Formation of MCa(BH4)2(NH2) M = Li, Na is proposed.
5. Derivatives of metal borohydrides with neutral molecules
Metal borohydrides have diverse compositions with rich and fascinating structural chemistry, which may be further expanded by the inclusion of other negatively charged anions, discussed in the previous sections of this review. Furthermore, a wide range of metal borohydrides containing neutral molecules have been discovered during the past few years that are the topic of the present section. The introduction of nitrogen in metal boro- hydrides has received significant interest owing to the possible inclusion of dihydrogen bonding via partly positively charged hydrogens, e.g. B–Hd +dH–N. Dihydrogen bonding is considered a new valuable approach for tailoring a material’s physical and
chemical properties. A high number of new ammine, NH3,metal borohydrides have been discovered and trends in composition, structure and properties are first discussed in this section, followed 263,264 Fig. 12 Crystal structure of Li4(BH4)(NH2)3. Remarkably, the crystal by hydrazine, N2H4 and ammonia borane, NH3BH3 derivatives. structure does not contain dihydrogen bonds, although the substance Solvent molecules with nitrogen, oxygen or sulphur as donor releases practically pure hydrogen under mild conditions. Li (red), B atoms may also crystallize with metal borohydrides depending (green), N (blue) and H (white). on synthesis conditions.
5.1 Ammine metal borohydrides
A high temperature polymorph, a-Na2(BH4)(NH2), was prepared Ammine metal borohydrides, M(BH4)m nNH3, have been known by heating NaBH4–NaNH2 to the melting point, 219 1C, for 1 h and sincethe1950’s,butthenumberof new compounds has increased subsequently cooling to RT.91 After longer annealing times (10 days) tremendously during the past B5 years having received interest as at lower temperatures, 98 1C, a-Na2(BH4)(NH2) transforms into both ammonia and hydrogen stores (Table 11). The number of the low temperature polymorph, b-Na2(BH4)(NH2). The cubic BH4 anions (m) in the compounds is fixed and determined by the % a-Na2(BH4)(NH2) crystallizes in the K3SO4F structure type (Pm3m), oxidation state of the metal, whereas the number of NH3 ligands (n) whereas b-Na2(BH4)(NH2) has an orthorhombic structure (Pbcm). may range from 1 to 8. Compared to the borohydride anions, NH3 Both structures can directly be derived from the anti-perovskite coordinates more strongly and less flexibly to the metal, i.e. always type structure ABO3,withthe[BH4] and [NH2] moieties in the via an electron pair donated by N, and acts as a terminal ligand. A and B sites, respectively. The charge balancing Na+ ions occupy 5.1.1 Mono-metallic ammine metal borohydrides. Mono-
2/3 of the O positions. ammine lithium borohydride. LiBH4 NH3, is the only reported
Mg(BH4)(NH2) has been prepared by thermal treatment of ammine alkali metal compound stable at RT. Monoammine 90 Mg(BH4)2–Mg(NH2)2 at 180 1C. The structure is tetragonal lithium borohydride crystallizes in an orthorhombic unit cell with 2+ 271 (I41) with the Mg ions tetrahedrally coordinated by two BH4 symmetry Pnma. Each Li atom is tetrahedrally coordinated by and two NH2 ligands. This material exhibits relatively high three BH4 units and one NH3 molecule, while each BH4 unit is 2+ 6 1 270 Mg ion conductivity of 10 Scm at 150 1C. linked to three Li atoms and four NH3 molecules via dihydrogen LiCa(BH4)2(NH2) and NaCa(BH4)2(NH2) are suggested to interactions. 268 form by ball milling of Ca(BH4)2–MNH2 (M = Li, Na), how- Ammine magnesium borohydrides.Mg(BH4)2 nNH3, are known ever, structure determination is not reported.266,267 for n =1,2,3and6,whilethecrystalstructureforn =1remains
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Table 11 Ammine metal borohydrides, M(BH4)m nNH3, with known crys- NH3 group and five bridging BH4 ligands, creating zig-zag chains tal structures of edge sharing octahedra running along the c-axis forming a 3D 274 Compounds n Ref. network. Ammine strontium borohydrides.Sr(BH) nNH (n =1,2,4), Mono-metallic 4 2 3 have been reported with Sr(BH ) 4NH being isostructural to LiBH4 nNH3 1 271 4 2 3 a 277 Mg(BH4)2 nNH3 1, 2, 3, 6 54, 272 and 273 Ca(BH4)2 4NH3. Sr(BH4)2 2NH3 (orthorhombic, Pnc2) consists b Ca(BH4)2 nNH3 1, 2, 4, 6 54 and 274–277 b of Sr octahedrally coordinated by four bridging BH4 groups Sr(BH4)2 nNH3 1, 2, 4 277 and 278 in-plane and by two axial NH3 groups. Similar to Ca(BH4)2 2NH3, Al(BH4)3 nNH3 1, 6 279–281 V(BH4)3 nNH3 3 282 the four BH4 groups and the metal create a two-dimensional b Cr(BH4)2 nNH3 6 94 a layer in the bc-plane stacked in the sequence AAA along the Mn(BH4)2 nNH3 1, 2, 3, 6 54 b a-axis. The NH3 groups are terminal ligands pointing between the Fe(BH4)2 nNH3 6 94 b Co(BH4)2 nNH3 6 94 layers. Sr(BH4)2 NH3 (orthorhombic, Pbna)hasSrcoordinated a Zn(BH4)2 nNH3 2, 4 119 and 283 octahedrally by five BH4 groups and one NH3 group similar to Y(BH4)3 nNH3 1, 2, 4, 5, 6, 7 53 274 Ca in Ca(BH4)2 NH3. There are two different types of BH4 Zr(BH4)4 nNH3 8 284 La(BH4)3 nNH3 4, 4, 6 285 groups in the structure of Sr(BH4)2 NH3,oneBH4 is bridging Ce(BH4)3 nNH3 4, 4, 5, 6 285 between three Sr atoms by the edge of the tetrahedron (Z2), while Gd(BH4)3 nNH3 4, 5, 6, 7 53 one BH4 groupisbridgingtwoSratomsbyedge(Z2)andface Dy(BH4)3 nNH3 4, 5, 6, 7 53 (Z3) sharing. Sr(BH4)2 NH3 is a double layered structure in the Bimetallic ab-plane, where the identical layers are shifted by an n-glide plane. LiMg(BH ) nNH 2 286 4 3 3 The layers are stacked along the c-axis in the sequence ABAB and Li2Mg(BH4)2 nNH3 6 54 and 287 Li2Al(BH4)5 nNH3 6 288 connected by dihydrogen bonds. LiSc(BH4)4 nNH3 4 282 Ammine aluminum borohydride. Al(BH ) nNH , have been Li Mn(BH ) nNH 654 4 3 3 2 4 2 3 known for n = 1 since 1974,290 whereas the structure of n = 6 was Li2Fe(BH4)2 nNH3 694 281 NaZn(BH4)3 nNH3 2 289 reported more recently. Al(BH4)3 NH3 crystallizes in a monoclinic unit cell (P2 /n) and consists of tetrahedral neutral complexes of Solid solutions 1 [Al(BH4)3(NH3)]. Al(BH4)3 6NH3 crystallizes in an orthorhombic unit Mn1 xMgx(BH4)2 nNH3 654 b 3+ Co(BH4)2 xClx nNH3 6 94 cell (Pbcn)thatconsistsof[Al(NH3)6] complexes, while BH4 act as a Detailed crystal structure remains unknown. b Unstable at RT. counter ions in the crystalline solid. Triammine vanadium borohydride.V(BH4)3 3NH3,crystallizesina cubic system (F23) and consists of neutral [V(BH4)3(NH3)3] com- 272,273 282 unknown. Mg(BH4)2 6NH3 is isostructural to MgCl2 6NH3 plexes, where V is octahedrally coordinated. (if the hydrogen atoms are not considered) and crystallizes in a Hexaammine chromium, iron and cobalt borohydride. % cubic unit cell (Fm3m), where Mg is octahedrally coordinated by M(BH4)2 6NH3 (M = Cr, Fe, Co) with the metals in the oxidation 2+ six NH3 molecules, forming [Mg(NH3)6] complexes, while BH4 state (II), are all isostructural and are also isostructural to 272 2+ act as counter ions in the crystalline solid. Mg(BH4)2 3NH3 M(BH4)2 6NH3 (M = Mg, Ca), i.e. consisting of [M(NH3)6] 54 94 crystallizes in an orthorhombic unit cell with space group Pnma. and [BH4] complexes. Here, Mg coordinates to two terminal BH4 anions and three NH3 Ammine manganese borohydrides.Mn(BH4)2 nNH3, are known for molecules, forming neutral molecular complexes [Mg(BH4)2(NH3)3] n =1,2,3and6,whilethecrystalstructureforn =1remains d d+ connected by dihydrogen bonds between H from BH4 and H unknown, and n = 2, 3 and 6 are all isostructural to the magnesium 54 2+ from NH3. Similarly, Mg(BH4)2 2NH3 forms neutral complexes of analogues. Thus, Mn(BH4)2 6NH3 forms [Mn(NH3)6] complexes [Mg(BH4)2(NH3)2] where Mg is tetrahedrally coordinated by two with BH4 as counter ions, while Mn(BH4)2 3NH3 and Mn(BH4)2 272 BH4 anions and two neutral NH3 ligands. 2NH3 contain neutral molecular complexes.
Ammine calcium borohydrides.Ca(BH4)2 nNH3, are known for n = Tetraammine zinc borohydride.Zn(BH4)2 4NH3 is one of the first
1, 2, 4 and 6, where n = 6 is isostructural to Mg(BH4)2 6NH3 and reported ammine metal borohydrides, but the crystal structure 2+ 119 consists of octahedral complexes [Ca(NH3)6] charge balanced remains unknown. More recently, Zn(BH4)2 2NH3,monoclinic by BH4 . A monoclinic (P21/c) Ca(BH4)2 4NH3 has Ca octa- (P21), was discovered and consists of neutral [Zn(BH4)2(NH3)2] 283 hedrally coordinated by four equatorial NH3 groups and two complexes connected by dihydrogen bonds. axial terminal BH4 groups forming neutral [Ca(NH3)4(BH4)2] Ammine yttrium borohydrides. Y(BH4)3 nNH3 (n =1,2,4,5,6 complexes.277 The structure is composed of hcp octahedra and 7), form the most extensive ammine metal borohydride 53 layers in the bc-plane stacked along the a-axis in the order series. Y(BH4)3 7NH3 (orthorhombic, Pca21) contains cation
AAA, which are connected by dihydrogen bonds. Ca(BH4)2 complexes of Y surrounded by seven NH3 molecules, forming a 3+ 2NH3 has an orthorhombic structure (Pbcn) in which Ca is monocapped trigonal prism. The [Y(NH3)7] prisms form a coordinated octahedrally by four bridging BH4 groups and two hexagonal pattern in the ab-plane, which are stacked in the order % terminal NH3 groups. Ca and the four BH4 groups form a 2D ABAB along the c-axis. The structure of Y(BH4)3 6NH3 (Pa3) is 275 layer stacked in the order ABAB. Ca(BH4)2 NH3 has an orthor- built from Y octahedrally coordinated by six ammonia molecules 3+ 3+ hombic unit cell (Pna21) with Ca octahedrally coordinated by one producing octahedral complexes [Y(NH3)6] .The[Y(NH3)6]
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octahedra are ordered in a hexagonal pattern in (111) layers, which clusters [La(BH4)3(NH3)4]. b-La(BH4)3 4NH3 are isostructural to are stacked in the sequence ABC along the [111] direction. Com- M(BH4)3 4NH3 (M = Y, Gd, Dy). pared to the other ammine yttrium borohydrides, Y(BH4)3 6NH3 Cerium lanthanum borohydrides. Ce(BH4)3 nNH3 (n = a-4, 285 has a low density and the BH4 may be relatively mobile. b-4, 5, 6), have been reported. a-and b-Ce(BH4)3 4NH3 are
Y(BH4)3 5NH3 has a hexagonal unit cell (P63) with Y eight-fold isostructural to a- and b-La(BH4)3 4NH3, while Ce(BH4)3 5NH3 coordinated by five NH3 molecules and three terminal BH4 and Ce(BH4)3 6NH3 are isostructural to M(BH4)3 5NH3 (M = Y, groups in molecular complexes [Y(NH3)5(BH4)3]. The complexes Gd, Dy) and La(BH4)3 6NH3, respectively.
[Y(NH3)5(BH4)3] are packed in the ab-plane in a hexagonal 5.1.2 Bimetallic ammine metal borohydrides. Seven ammine pattern, which are stacked in the order ABAB along the c-axis. bimetallic borohydrides have been reported in the scientific litera-
Y(BH4)3 4NH3 crystallizes in an orthorhombic unit cell (Pna21) ture. Here, NH3 always coordinates to the metal with the highest and Y is seven-fold coordinated by four NH3 and three BH4 , charge density, which also has the higher oxidation number, while creating neutral molecular complex [Y(NH3)4(BH4)3]. The mole- the metal with lower charge density, typically Li or Na, only cular complexes are packed in ab-layers in a hexagonal pattern, coordinate to BH4 . which are stacked in the order ABAB along the c-axis. Y(BH4)3 Diammine lithium magnesium borohydride. LiMg(BH4)3 2NH3, 3+ 2NH3 (orthorhombic, Pbca) with Y coordinated to two NH3 has a hexagonal unit cell (P63), where Mg is five-fold coordi- and to four BH4 groups in an octahedron, where two of the nated and resides in triangular-bipyramidal coordination, with BH4 groups are bridged to other yttrium atoms. Thus, the the N atoms located at the axial positions of this polyhedron and 286 solid Y(BH4)3 2NH3 consists of one-dimensional zig-zag ribbons the BH4 groups at the equatorial positions. Li atoms are running along the a-axis. Y(BH4)3 NH3 is orthorhombic (Cmc21) located in a triangular antiprism formed by six BH4 groups. 3+ with Y coordinated by one NH3 and five BH4 groups, where Hexaammine dilithium magnesium borohydride.Li2Mg(BH4)4 four of the BH4 groups are bridging other yttrium atoms, 6NH3 (tetragonal, P42/mnm) is another known ammine bimetallic forming 2D layers stacked in the order ABAB. borohydride based on lithium and magnesium54,286–291 built 2+ Ammine zirconium borohydride. Zr(BH4)4 nNH3, has the high- from distinct [Mg(NH3)6] complex ions counter-balanced by 2 52,281 est number of coordinated NH3, i.e. n = 8, and crystallizes in an one-dimensional [Li2(BH4)4]n polymers. 284 orthorhombic unit cell (Pnma). The structure is built from Hexaammine dilithium aluminum borohydride.Li2Al(BH4)5 4+ complex cations [Zr(NH3)8] charge balanced by BH4 . 6NH3, was the first ammine bimetallic borohydride reported
Ammine lanthanum borohydrides. La(BH4)3 nNH3 (n = a-4, b- and reveals a promising hydrogen storage capacity and thermal 285 288 4, 6), have recently been reported. La(BH4)3 6NH3 crystallizes in decomposition temperature. The structure of Li2Al(BH4)5 6NH3 3+ % an orthorhombic unit cell (P212121)withLa coordinated by six is hexagonal (P3c), where Al is octahedrally coordinated by NH3 3+ NH3 ligands and two terminal BH4 groups, creating a large forming complex cations [Al(NH3)6] ,asinAl(BH4)3 6NH3.Liis + complex cation [La(BH4)2(NH3)6] that is charged balanced by one tetrahedrally coordinated by BH4 ,andtheLi[BH4]4 tetrahedra are BH4 (Fig. 13) a-La(BH4)3 4NH3 and b-La(BH4)3 4NH3 are, together then vertex-linked through three bridging BH4 units with the with the cerium analogues, the only examples of ammine metal fourth BH4 in the trans position, leading to the formation of a two- borohydrides having two polymorphs. a-La(BH4)3 4NH3 crystallizes dimensional honeycomb-patterned sheet that spreads along the in a monoclinic unit cell (I2), where La3+ is coordinated by ab plane. four NH3 groups and three BH4 groups, creating molecular Tetraammine lithium scandium borohydride. LiSc(BH4)4
4NH3, crystallizes in an orthorhombic unit cell (Pc21n), where Sc is octahedrally coordinated by three NH3 ligands and three 282 BH4 groups.
Hexaammine dilithium manganese borohydride.Li2Mn(BH4)4
6NH3, and hexaammine dilithium iron borohydride, Li2Fe(BH4)4
6NH3, are isostructural to Li2Mg(BH4)2 6NH3 and consist of 2+ 2 54,94 [M(NH3)6] complexes and [Li2(BH4)4]n polymers.
Diammine sodium zinc borohydride. NaZn(BH4)3 2NH3, crys-
tallizes in an orthorhombic unit cell (P21nb) and consists of + large complex bimetallic cations [NaZn(BH4)2(NH3)2] charge 289 balanced by BH4 . 5.1.3 Solid solutions of ammine metal borohydrides. Only two solid solutions of ammine metal borohydrides have been
reported, which are Mn1 xMgx(BH4)2 6NH3 where the metal 54 cation site is statistically disordered, and Co(BH4)2 xClx 6NH3 94 where the anion site is partially occupied by Cl and BH4 .
Both Mn1 xMgx(BH4)2 6NH3 and Co(BH4)2 xClx 6NH3 have the cubic symmetry Fm3%m.
Fig. 13 The structures of [M(BH4)2(NH3)6]BH4 (M = Ce, La) have two 5.1.4 Structural trends for ammine metal borohydrides. different BH4 environments in the same compound. Ammine metal borohydrides are represented by a range of
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Table 12 Structural trends of ammine mono-metallic and bimetallic borohydrides
M Composition Space group Coordination environment of the metal Ref. Structures with one-dimensional chains + Li MBH4 NH3 Pnma Li : coordinated by one NH3 and three BH4 271 3+ Y M(BH4)3 2NH3 Pbca Y : coordinated by two NH3 and four BH4 53 and 291
Structures with two-dimensional layers 2+ Ca M(BH4)2 NH3 Pbcn Ca : coordinated by one NH3 and five BH4 274 2+ Ca M(BH4)2 2NH3 Pna21 Ca : coordinated by two NH3 and four BH4 275 2+ Sr M(BH4)2 NH3 Pbna Sr : coordinated by one NH3 and five BH4 277 2+ Sr M(BH4)2 2NH3 Pnc2Sr: coordinated by two NH3 and four BH4 277 3+ Y M(BH4)3 NH3 Cmc21 Y : coordinated by one NH3 and five BH4 53
Structures with neutral molecular complexes Mg, Mn M(BH4)2 2NH3 Pcab [M(BH4)2(NH3)2] 54 and 272 Mg, Mn M(BH4)2 3NH3 Pnma [M(BH4)2(NH3)3]54 Ca, Sr M(BH4)2 4NH3 P21/c [M(BH4)2(NH3)4] 277 Al M(BH4)3 NH3 P21/n [M(BH4)3(NH3)] 290 Y, Gd, Dy, Ce M(BH4)3 4NH3 Pna21 [M(BH4)3(NH3)4] 53 and 285 Y, Gd, Dy, Ce M(BH4)3 5NH3 P63 [M(BH4)3(NH3)5] 53 and 285 V M(BH4)3 3NH3 F23 [M(BH4)3(NH3)3] 282 Zn M(BH4)2 2NH3 P21 [M(BH4)2(NH3)2] 119 Ce, La M(BH4)3 4NH3 I2 [M(BH4)3(NH3)4] 285
Structures with complex cations and anions 2+ 2 Mg, Mn, Fe Li2M(BH4)4 6NH3 P42/mnm [M(NH3)6] [Li2(BH4)]n 54, 94 and 287 % 2+ Mg, Ca, Mn, Cr, Fe, Co M(BH4)2 6NH3 Fm3m [M(NH3)6] BH4 54, 94, 272 and 277 % 3+ 3 Al Li2M(BH4)5 6NH3 P3c1 [M(NH3)6] [Li2(BH4)]5 288 3+ Al M(BH4)2 6NH3 Pbcn [M(NH3)6] BH4 281 + Zn NaM(BH4)3 2NH3 P21nb [NaM(BH4)2(NH3)2] BH4 289 % 3+ Y, Gd, Dy M(BH4)3 6NH3 Pa3 [M(NH3)6] BH4 53 3+ Y, Gd, Dy M(BH4)3 7NH3 Pca21 [M(NH3)7] BH4 53 4+ Zr M(BH4)3 8NH3 Pnma [M(NH3)8] BH4 284 + Ce, La M(BH4)3 6NH3 P212121 [M(BH4)2(NH3)6] BH4 285
different structure types and an overview is provided in Table 12. The structures of magnesium, calcium, strontium, manganese and yttrium borohydride consist of three-dimensional networks of [M(BH4)6] octahedra or [M(BH4)4] tetrahedra, where each 2 BH4 group bridge between two metal atoms with edge (Z ) 129,137,142,145,197 coordination. Introduction of NH3 molecules interrupts these frameworks. For the series of ammine yttrium borohydrides, the structures vary from two-dimensional layers (n = 1), one-dimensional chains (n =2),molecularcompounds (n = 4 and 5) to those containing large complex cations (n =6 and 7), see Fig. 14. Similar structural variations appear for the other series of compounds. In general, molecular structures appear when the total number of ligands (NH3 +BH4 )matchthe preferred coordination number of the cations, e.g. [Mn(BH4)2(NH3)2] and [Sr(BH4)2(NH3)4]. For compounds with higher numbers of coordinated NH3 molecules, BH4 becomes a counter ion, e.g.
[Ca(NH3)6](BH4)2 and [Y(NH3)n](BH4)3 (n = 6 and 7), whereas one- Fig. 14 The four different structure types observed for Y(BH4)3 nNH3, i.e. or two-dimensional structures are formed for compounds with (a) complex cations (n = 7); (b) molecular compounds (n = 4); (c) one dimensional chains (n = 2); (d) 2D-layered structures (n = 1). lower numbers of coordinated NH3 molecules.
The NH3 coordinated metal cations may be described as complex cations, and the complete structure may be illustrated as a packing of the complex cations (relatively close to being shown in Fig. 15. As expected, the M–N bond distances increase spherical) and borohydride anions. The dihydrogen contacts approximately linearly with increasing ionic radius, and the between them lower the symmetry compared to the simple M–N bond distances do not vary significantly for varying packing of spheres. numbers of NH3 ligands, e.g., the Sr–N bond lengths only varies
Metal–nitrogen bond distances for selected ammine metal from 2.69–2.70 Å for Sr(BH4)2 nNH3 (n = 1, 2 and 4). However, borohydrides plotted as a function of the ionic radius are the Zr–N interactions are outliers for this trend with bond
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1595 Review Article Chem Soc Rev
In ammine bimetallic borohydrides, NH3 coordinates to the metal with the highest charge density, whereas BH4 may coordinate to both metals. The unit cell volumes per formula unit (V/Z) at RT as a function
of the number of NH3 molecules, n,intheformulaunitforM(BH4)m nNH3 (M = Ca, Sr, Mn and Y, Fig. 17) reveals an approximately linear relationship for each series. For the manganese-based series, 3 Mn(BH4)2 nNH3, the slope of the straight line is 34.0 Å per molecule (R2 = 0.9998) corresponding well to the size of one ammonia 3 293 molecule, which in solid NH3 at 80 1CisV(NH3)=33.9Å. In
order to include Li2Mn(BH4)4 intheplot,thevolumeoftwoLiBH4 3 24 (V(LiBH4)=54.2Å) has been subtracted from V(Li2Mn(BH4)4
6NH3), which has a slightly lower V/Z than Mn(BH4)2 6NH3.Simi- larly, the DFT optimized structures of Y(BH4)3 nNH3 increase linearly as a function of n (37.3 Å3, R2 = 0.9916), whereas the experimental values deviate slightly from the linear trend. Fig. 15 Metal–nitrogen bond distances for selected ammine metal boro- All ammine metal borohydride structures contain dihydrogen hydrides plotted as a function of the ionic radius of the metal. The d d+ d+ coordination number (CN) of the metals are provided and represents the H H contacts between partly positively charged hydrogen, H d number of BH4 and NH3 coordinated to the metal. bonded to N and partly negatively charged hydrogen, H bonded 3+ to B. The shortest distance, 1.850 Å, between [Y(NH3)7] and BH4
anions is obtained by DFT optimization of Y(BH4)3 7NH3 lengths ranging 2.15–2.67 Å observed in the [Zr(NH ) ]4+ side- 3 8 determined by X-ray diffraction. However, as discussed in bicapped trigonal prism for [Zr(NH ) ](BH ) .284 3 8 4 4 Section 5.1.5, the strength of the dihydrogen bonds do not play On the other hand, the coordination of BH tetrahedra to a 4 a significant role for the composition of the released gases on metal is more flexible, illustrated by the range of M–B distances thermal decomposition. and varying hapticities from Z0 to Z3 (Fig. 16), i.e. BH can be 4 Structural analogies between ammine metal borohydrides either a terminal or bridging ligand or act as a counter-ion. As and chlorides or bromides exist, considering only heavier atoms, expected, when BH4 coordinates to a metal in a partly covalent i.e. neglecting hydrogen atoms. For example, M(BH4)2 6NH3 (M = manner, i.e. Z1 to Z3 then bond distances increase for increasing Mg and Ca) and MgX2 6NH3 (X = Cl and Br) are isostructural, and ionic radii of the metal, which is also observed for metal 292 Sr(BH4)2 2NH3 and MCl2 2NH3 (M = Ca and Sr) are structurally borohydrides. M–B distances substantially increase for decreasing similar. The borohydride complex, BH4 , is a non-spherical hapticities in the yttrium based series Y(BH ) nNH :whereY–Bisin 4 3 3 anion in contrast to the halide anions; therefore crystal struc- the range of 4–5 Å when BH hasdominantlyionicinteractions 4 tures of ammine metal borohydrides often have lower symmetry with ammine yttrium complexes (Z0)forn = 6, 7 (see Table 13). than their halide analogues, as usually observed when compar- ing metal borohydrides and metal halides. A high degree of structural similarity is observed between compounds with high numbers of coordinated ammonia (n), where both the BH4 anion and the halides, X , act as counter ions with a predominantly ionic bonding, Z0. 5.1.5 Thermal properties of ammine metal borohydrides. The research of ammine metal borohydrides as potential hydrogen storage materials was initiated by Soloveichik et al.
in 2008, studying Mg(BH4)2 2NH3, which mainly releases H2
upon decomposition, whereas Mg(BH4)2 6NH3 mainly releases 272 NH3. Since then, ammine metal borohydrides have attracted attention as promising hydrogen storage materials, e.g., the
compounds Al(BH4)3 6NH3,Li2Al(BH4)5 6NH3 and Zn(BH4)2 2NH3
release 9–12 wt% H2 in the temperature range 115–170 1Cwith 279,283,288 traces of NH3. In comparison, the decomposition of metal
borohydrides, M(BH4)m,below2001Cisalwaysaccompaniedbythe
release of diborane, B2H6. x Fig. 16 Metal–boron distances and hapticities (Z ) for selected ammine Interestingly, ammine metal borohydrides with low electro- metal borohydrides plotted as a function of the ionic radius of the metal. negativities (w o 1.6) often display reduced temperatures for Due to the weak scattering of H, hapticities are tentative for structures that p are not DFT optimized, i.e. Z(Al3+) = 2–3, Z(Mn2+) = 2–3 and Z(Sc3+) = 2–3. gas release when compared to the pure metal borohydride. On The large M–B distances 43.5 occur for BH4 acting as a counter anion, the other hand, ammine metal borohydrides with high electro- 0 Z . negativities (wp 4 1.6) are often more thermally stable than
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53 x Table 13 Overview of the coordination of Y, CN(Y), and selected bond lengths for Y(BH4)3 nNH3. The distances and hapticities (Z ) are obtained from DFT optimized crystal structures with experimental unit cell parameters
n =0 n =1 n =2 n =4 n =5 n =6 n =7 CN(Y)a 6 66786 7 CN(Y)b 12 12 12 10 8 6 7 Y–N (Å) — 2.48 2.51 2.44–2.49 2.40–2.58 2.47–2.48 2.47–2.51 Y–B1 (Å) 2.74 (Z2) 2.50 (Z3) 2.53(Z3) 2.80 (Z2) 3.13 (Z1) 5.04 (Z0) 4.30 (Z0) Y–B2 (Å) 2.75 (Z2) 2.77 (Z2) 2.53(Z3) 2.80 (Z2) 5.15 (Z0) 4.69 (Z0) Y–B3 (Å) 2.80 (Z2) 2.79 (Z2) 2.83 (Z2) 5.41 (Z0) 4.72 (Z0) Y–B4 (Å) 2.80 (Z2) 5.45 (Z0) 4.97 (Z0) Structure type 3D-framework 2D-layers 1D-chains Molecular Molecular Ionic complexes Ionic complexes a b Number of BH4 and NH3 ligands coordinated to Y. Number of H (from BH4 ) and N (from NH3) atoms coordinated to Y.
thermal stability of the ammine metal borohydride is significantly lower than the corresponding metal borohydride (Fig. 18).
Ca(BH4)2 nNH3 and Sr(BH4)2 nNH3 clearly become more stable for lower n/m ratios, however, they still decompose at significantly
lower temperatures than M(BH4)2, M = Ca, Sr, and they release NH3
only. At higher temperatures the remaining M(BH4)2,M=Ca,Sr,
decompose and release hydrogen. Similarly, Y(BH4)3 nNH3 (n =7,6
and 5), release NH3 at significantly lower temperatures than the B decomposition temperature of Y(BH4)3, Tdec(Y(BH4)3) 200 1C,
whereas the compounds Y(BH4)3 nNH3 (n = 4, 2 and 1) release decreasing amounts of NH3 in the gas mixture consisting of NH3
and H2 in the temperature range, T B 160–200 1C, approaching
Tdec(Y(BH4)3) B 200 1C. Ammine metal borohydrides based on less stable metal borohydrides, e.g. M=Al,Zn,ZrandV,279,280,283,284
do not release pure NH3 gas, possible because of the lower Fig. 17 Unit cell volumes (V) at RT divided by the number of formula units decomposition temperature of the respective metal borohydrides. (Z) plotted as a function of the number of ammonia molecules, n,inthe Thus, the less stable metal borohydrides react with NH3 upon formula unit, M(BH4)m nNH3. The dashed and solid line correspond to decomposition and release H2 (and in some cases also some NH3). linear fit to the DFT optimized values for Y(BH4)3 nNH3 and the experi- As an example, Al(BH4)3 6NH3 releases H2 and small amounts of mental values for Mn(BH4)2 nNH3, respectively. The unit cell volume of NH3 at B165 1C, where Al(BH4)3 is unstable, Tdec(Al(BH4)3) B 25 1C Li2Mn(BH4)4 6NH3 subtracted the volume of two formula units of LiBH4 is also included. their respective metal borohydride.54 The stabilization could be due to complex formation in ammine metal borohydrides with high electronegativities (wp 4 1.6), causing a shielding effect to the metal cation. For example, Zr(BH4)4 decomposes by sublimation and releases a mixture of B2H6 and H2 at Tsub =
29 1C, whereas [Zr(NH3)8](BH4)4 mainly releases H2 with only 109,284 impurities of NH3 at 130 1C.
In the literature, H2 rich (and NH3 poor) gas streams are often attributed to strong dihydrogen bonds, however, recent 53,54,277 experimental results disagree with this hypothesis. NH3 is released for Y(BH4)3 nNH3 (n = 7 and 6), which reveals the strongest dihydrogen bonds (B1.85 Å) among the Y(BH4)3 nNH3 (n =7,6,5,4,2and1)compounds.TheNH3/BH4 ratios (n/m) also cause higher hydrogen purity for low n/m ratios in some cases, i.e. M(BH4)m nNH3 (M = Mg, Mn and Y) but not others, i.e.
LiBH4 NH3,Ca(BH4)2 NH3 and Sr(BH4)2 NH3.Therefore,the Fig. 18 Decomposition temperatures for selected ammine metal boro- composition of the released gas depends on two factors:277 hydrides and the corresponding metal borohydrides as a function of (i) The stability of the metal borohydride. If the thermal electronegativity (Zn(BH4)2 is not experimentally investigated but consid- ered unstable). Ammonia is released when the metal borohydride is more stability of an ammine metal borohydride is similar to or higher stable than the corresponding ammine metal borohydride (red circle) than the thermal stability of the corresponding metal boro- otherwise hydrogen is released (green square). Adapted with permission hydride, then H2 is released. In contrast, NH3 is released if the from ref. 277. Copyright 2015 John Wiley and Sons.
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279 (Fig. 18). This hydrogen release can be decreased even lower LiBH4 NH2NH2 crystallizes in a monoclinic unit cell (Cc) (B120 1C) by encapsulating Al(BH4)3 6NH3 within a nanoporous with lithium tetrahedrally coordinated by two BH4 and two 294 polymer. NH2NH2 groups. LiBH4 2NH2NH2 crystallizes in an orthorhombic
(ii) The partial pressure of ammonia. In a closed system, the unit cell (Pca21), where lithium coordinates four NH2NH2.NH2NH2 partial pressure of ammonia increases upon ammonia release, bridges between two lithium cations forming chain-like + p(NH3) 4 0, during thermolysis of LiBH4 NH3, Ca(BH4)2 nNH3 cationic complexes [Li(NH2NH2)4] , which are separated by and Sr(BH4)2 nNH3. At higher temperatures, the released BH4 (see Fig. 19). ammonia gas reacts with the metal borohydride, releasing LiBH4 1/2NH2NH2 crystallizes in an orthorhombic unit cell 275,295 + hydrogen exothermically in a solid gas reaction. (P212121) and consists of two-dimensional layers. Li is coordinated 5.1.6 Mechanism for gas releaseofamminemetalborohy- by three BH4 ligands and one NH2NH2 that is bridged to the + drides. Hydrogen release from ammine metal borohydrides appears neighbouring Li .TherearetwotypesofBH4 ligands that both 2 always to be exothermic, which indicates that a chemical coordinate to Li through the edge (Z ). One BH4 is located in the reaction is responsible for the gas release. This may suggest same plane as the Li atoms and coordinates to four Li atoms, while that hydrogen is released in an elimination reaction between one BH4 is positioned slightly above (or below) the plane and Hd+ and Hd . Furthermore, we notice that the thermal stability coordinates to two Li atoms. for ammine metal borohydrides and ammine metal chlorides are LiBH4 1/3NH2NH2 crystallizes in a monoclinic unit cell (P21). similar when comparing the peak temperature of NH3 release, Three independent Li atoms are tetrahedrally coordinated, one by Tdec(NH3). As examples, Y(BH4)3 7NH3 (Tdec(NH3) B 80 1C) is four BH4 and two by three BH4 and one hydrazine molecule. B 291 slightly less stable than YCl3 7NH3 (Tdec(NH3) 100 1C), While borohydrides are shared by three or four Li atoms, the B and Mg(BH4)2 6NH3 (Tdec(NH3) 150 1C) and MgCl2 6NH3 hydrazine molecule only bridges twoLiatoms.Hence,theresulting B 296 (Tdec(NH3) 150 1C) have similar thermal stability, whereas 3D framework is rather complex. Mn(BH4)2 6NH3 (Tdec(NH3) B 130 1C) is slightly more stable NaBH4 NH2NH2 crystallizes in a monoclinic unit cell (P21/c). 297 + than MnCl2 6NH3 (Tdec(NH3) B 105 1C). This observation Each Na coordinates to two bridging NH2NH2 leading to the + may indicate that the release of ammonia is mainly determined formation of [Na(NH2NH2)2]n chains, which are separated by by the strength of the metal–ammonia (M–N) bond in the BH4 with Na–B distances in the range from 2.97 to 3.05 Å (see complex. Thus, the above observations support that there is no Fig. 19). d d % direct relation between dihydrogen bond strength, N–H + H–B, Mg(BH4)2 3NH2NH2 crystallizes in a trigonal unit cell (P31c). 2+ in the solid state and the composition of the released gas. Each Mg is octahedrally coordinated by six NH2NH2 forming 2+ Moreover, hydrogen release from ammine metal borohydrides [Mg(NH2NH2)6] complex cations that are charge balanced by BH4 . may in fact be due to a chemical reaction between the proposed Generally, the hydrazine metal borohydride complexes have intermediate, diborane, B2H6, which may take part in the high hydrogen content and also favourable decomposition tempera- decomposition of metal borohydrides and ammonia released tures, e.g. approx. 13.0 wt% H2 is released from LiBH4 NH2NH2 at simultaneously or at slightly lower temperatures. We note that 140 1C in the presence of Fe–B catalysts.118 However, this leads diborane release is always suppressed for ammine metal boro- to the formation of very stable Li3BN2 and BN according to hydrides as compared to the respective metal borohydrides. reaction scheme (5.1). - 5.2 Hydrazine metal borohydride complexes 3(LiBH4 NH2NH2)(s) Li3BN2(s) + 2BN(s) + N2(g) + 12H2(g) (5.1) Six hydrazine metal borohydride complexes, M(BH4)m nNH2NH2, are known (see Table 14) based on M = Li, Na and Mg, where 118 5.3 Ammonia borane metal borohydride complexes LiBH4 NH2NH2 and LiBH4 2NH2NH2 were first reported in 2012. In 2013 four more hydrazine metal borohydride complexes The first ammonia borane (AB) metal borohydride complexes, were reported, i.e. NaBH4 NH2NH2, Mg(BH4)2 3NH2NH2, LiBH4(NH3BH3)0.5 and Ca(BH4)2(NH3BH3)2 were reported in 117 114 LiBH4 1/2NH2NH2,LiBH4 1/3NH2NH2. In contrast to ammonia, 2010, whereas other ammonia borane metal borohydride
NH3,hydrazine,NH2NH2, is a more flexible ligand that can bridge complexes are now reported (Table 15). They are all prepared by between two metals or be a terminal ligand. The coordination to mechanochemistry, except for Al(BH4)3(NH3BH3)(wherecrystals the metal is always via a lone pair donated by a nitrogen atom. are formed by reacting liquid Al(BH4)3 and solid NH3BH3) and Ca(BH4)2(NH3BH3) (that is formed during decomposition of
NH4Ca(BH4)3). Complex formation is not reported for the more Table 14 Metal borohydride hydrazine complexes with known crystal stable ionic alkali metal borohydrides and ammonia borane, structures but physical mixtures of the reactants are obtained by ball 298 M in M(BH4)m Composition of complex Symmetry Ref. milling with M(BH4)m–NH3BH3 (M = Na, K, Rb and Cs).
Li LiBH4 1/3NH2NH2 Monoclinic, P21 117 NH3BH3 may coordinate to the metal via a lone pair donated Li LiBH4 1/2NH2NH2 Orthorhombic, P212121 117 by nitrogen, i.e. act as a terminal ligand; or it may also coordinate to Li LiBH4 NH2NH2 Monoclinic, Cc 118 the metal by Hd covalently bonded to B, i.e. bridge between two Li LiBH4 2NH2NH2 Orthorhombic, Pca21 118 Na NaBH4 NH2NH2 Monoclinic, P21/c 117 metal atoms; or it may even stay isolated from the metal and only % Mg Mg(BH4)2 3NH2NH2 Trigonal, P31c 117 coordinate via dihydrogen bonds. Ammonia borane complexes
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Fig. 19 (left) 1D-chains of LiBH4 NH2NH2 running along direction [101] in LiBH4 NH2NH2 and (right) a 3D-framework in NaBH4 NH2NH2. Li light blue, Na brown, B and BH4 tetrahedra green, N blue, H light grey.
Table 15 Metal borohydride ammonia borane complexes
M in M(BH4)m Composition of complex Symmetry Ref.
Li LiBH4(NH3BH3)0.5 Orthorhombic, Pnma 114 Li LiBH4(NH3BH3) Monoclinic, P21 301 Mg Mg(BH4)2(NH3BH3)2 Orthorhombic, P212121 115 and 300 Mg Mg(BH4)2(NH3BH3) 2NH3 Tetragonal, P4bm 300 Ca Ca(BH4)2(NH3BH3)2 Orthorhombic, Aba2 114 Ca Ca(BH4)2(NH3BH3) Orthorhombic, P21nb 302 Al a-Al(BH4)3(NH3BH3) Monoclinic, P21/c 116 Al b-Al(BH4)3(NH3BH3) Monoclinic, Cc 116 Li, Al Li2Al(BH4)5(NH3BH3)3 6NH3 Cubic, F23 299
can also be formed from ammine metal borohydrides, result- The crystal structures of Mg(BH4)2(NH3BH3)2 and Al(BH4)3- ing in framework structures, i.e. [Al(NH3)6][Li2(BH4)5] (NH3BH3) both consist of neutral molecular units of [Mg(BH4)2-
3NH3BH3 and [Mg(NH3)2(BH4)2] NH3BH3 where AB is not (NH3BH3)2]or[Al(BH4)3(NH3BH3)] coordinated by intermolecular coordinated to metal atoms but linked with other hydrogen- dihydrogen bonds in the solid state (Fig. 20a).115,116,300 The four containing molecules and ions via dihydrogen bonds.299,300 other known ammonia borane metal borohydride complexes The hydrogen evolution from these composites is significantly show strikingly different connectivity, where BH4 complexes improved compared to the parent compounds containing no and NH3BH3 molecules act as bridging ligands. The borohydride ammonia molecules. complexes bridge three lithium and two NH3BH3 in the structure
Fig. 20 Crystal structure of (a) molecular [Mg(BH4)2(NH3BH3)2] with NH3BH3 being a terminal ligand and (b) LiBH4(NH3BH3)0.5 which is a 3D framework with bridging NH3BH3. Mg (red), B (green), N (blue) and H (white).
This journal is © The Royal Society of Chemistry 2017 Chem. Soc. Rev., 2017, 46, 1565--1634 | 1599 Review Article Chem Soc Rev
of LiBH4(NH3BH3)0.5, creating a three-dimensional polymeric for potentially reversible dehydrogenation of ammonia borane. 114 301 framework. The LiBH4(NH3BH3) structure is more complex, Both complexes Mg(BH4)2(NH3BH3)2 and Al(BH4)3(NH3BH3) containing four independent borohydride anions and four NH3BH3 melt before decomposition, because of the mononuclear com- molecules (Fig. 20b). While the borohydride anions coordinate plexes present in the crystal structures and in the melt.115,116 two to four lithium atoms, the NH3BH3 molecules act as a terminal ligand or even stay isolated from the metal atoms. In 114 5.4 Metal borohydride crystallizing with organic solvents Ca(BH4)2(NH3BH3)2, the borohydride groups bridge to two metal atoms, forming a two-dimensional square planar net, Solvent based synthesis methods have become more common while the two NH3BH3 molecules act as terminal ligands, for the synthesis of metal borohydrides during the past few years as completing the octahedral coordination of Ca. Another ammonia discussed in Section 2 of this review. The solvent may coordinate borane complex, Ca(BH4)2(NH3BH3),wasrecentlyreported,con- strongly to the metal borohydride and form a solvate that needs 302 taining just one NH3BH3 molecule per unit cell. While structu- further treatment in order to obtain a solvent-free product. In most rally related to Ca(BH4)2(NH3BH3)2, the square planar layers in this cases, the solvent contains a donor atom, O, N or S that coordinates net are buckled, and interconnected by a fifth borohydride ligand, to the metal cation of the borohydride. Selected organic solvents which bridges between two Ca atoms. The octahedral coordination and corresponding solvates are presented in Table 16, including polyhedron is again completed by one NH3BH3 molecule, which is bond lengths between the donor atom and the metal. M–S locatedinchannelsformedbythebuckledCa–BH4 substructure. bonds are generally longer than M–O bonds, suggesting that 5.3.1 Thermal properties of metal borohydride ammonia sulphur based solvents are more easily removed than oxygen B borane complexes. NH3BH3 melts at 110 1C and releases one based solvents. equivalent of hydrogen in each of the three decomposition Two different structures of metal borohydrides with dimethyl steps, forming polyaminoborane, [NH2BH2]n (90–120 1C), poly- sulfide, (CH3)2S, are known. The first is the monoclinic structure iminoborane, [NHBH]n (120–200 1C) and finally boron nitride, of M(BH4)2 1/2(CH3)2S (M = Mg, Mn), which is a 3D framework BN (4500 1C).303–306 In addition, the hydrogen release is containing two metal atoms: one atom is tetrahedrally coordi- accompanied by volatile and toxic by-products, such as B2H6 nated to four BH4 groups, and the other to four BH4 groups 3,58 and B3N3H6. Generally, the systems M(BH4)m–NH3BH3 (M = Li, and one S(CH3)2 ligand to form a trigonal bipyramid. Sulfur
Na, K, Mg and Ca) decompose in a similar manner to NH3BH3 coordinates relatively weakly to the metal with Mg–S and Mn–S below 200 1C and do not suppress the release of the toxic gases, bond lengths being 2.669 and 2.705 Å, respectively. Hence, 114,300,301,307 as observed for metal amidoboranes, M(NH2BH3)m. S(CH3)2 is easily removed during drying without decomposing
Thus, it may be concluded that NH3BH3 is insufficiently M(BH4)2. Similarly, halide-free and solvent-free yttrium and modified in MBH4–NH3BH3 (M = Li, Na, K, Mg and Ca), because gadolinium borohydride may be synthesized using dimethyl 114,308 NH3BH3 maintains its molecular form. This is in contrast sulfide with the M(BH4)3 S(CH3)2 (M = Y or Gd) solvates as 66 to metal amidoboranes where one hydrogen atom bonded to intermediates, which have monoclinic structures (P21/c). 307 N is substituted by a metal atom. Al(BH4)3(NH3BH3) on the In U(BH4)4 (C2H5)2O, U is coordinated by four BH4 ligands 2 other hand, endothermically releases two moles of H2 from coordinating by edge sharing (Z ), one BH4 ligand coordinat- 116 3 NH3BH3 per Al at 70 1C. Thereby Al(BH4)3(NH3BH3) differs ing by face sharing (Z ) and one O from ether, giving 14 nearest from pristine ammonia borane, metal amidoboranes and other neighbours. Solvent based methods often provide the most metal borohydride ammonia borane complexes, and may open thermodynamically stable compound, typically a monometallic up possibilities for reversible hydrogen storage. It was reported borohydride or a solvate, as discussed in Section 2. Thus, very 3+ that the ability of the strong Lewis acid Al to coordinate both few bimetallic solvates have been discovered, e.g. LiMn2(BH4)5 61 the initial hydrogenated species as well as their dehydrogena- 2(C2H5)2O and NaMn2(BH4)5 2(C2H5)2O. Here, Mn is coordi- tion products, makes it a good template for chemical transfor- nated by four bridging BH4 groups, while Li/Na is coordinated 309 mations involving light chemical and complex hydrides. tetrahedrally by two BH4 groups and two (C2H5)2O ligands
Therefore the Al(BH4)3(NH3BH3) system can be used as a model Li/Na–O bond lengths ranging from 2.00 to 2.38 Å.
Table 16 Classic organic solvents and selected corresponding solvates
Solvent Selected solvates Symmetry M-donor atom (Å) Ref.
Dimethyl sulfide/DMS, (CH3)2S M(BH4)2 1/2(CH3)2S, M = Mg, Mn Monoclinic, Cc 2.67–2.70 3 and 58 M(BH4)3 (CH3)2S (M = Y or Gd) Monoclinic, P21/c 2.89 66 Diethyl ether/Et2O, (C2H5)2O U(BH4)4 (C2H5)2O Monoclinic, P21 2.485 310 LiMn2(BH4)5 2(C2H5)2O Monoclinic, C2/c 2.00 61 NaMn2(BH4)5 2(C2H5)2O Monoclinic, C2 2.38 61 Tetrahydrofuran/THF, C4H8O M(BH4)2 2THF, M = Sr, Eu, Sm Monoclinic, P21/c 2.55–2.62 311–313 Water, H2O NaBH4 2H2O Orthorhombic, Pbca 2.38–2.47 103 Ethylenediamine/en, C2H4(NH2)2 Al(en)(BH4)3 Monoclinic, P21/c — 160 and 309 Al(en)3(BH4)3 en Monoclinic, P21/a 2.00–2.11 314 Zn(en)4(BH4)2 Triclinic, P1 — 315 Zn(en)3(BH4)2 Orthorhombic, Pbca — 315
1600 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
Similar to the metal borohydrides, THF (C4H8O) solvates of 6.1 Thermodynamics of hydrogen release and uptake 311–313 M(BH4)2 2THF are isostructural for M = Sr, Eu, Sm. They There is a close correlation between the temperature of hydrogen crystallize in P21/c and form one-dimensional chains in which release and uptake, at a given equilibrium pressure, and the each metal atom is surrounded by four borohydrides and two enthalpy and entropy change for metal borohydrides, similar to THF molecules. metal hydrides. Consider a general reaction where a metal, M, Water is a well-known inorganic solvent, but tends to react reacts with hydrogen to form a metal hydride according to with metal borohydrides in particular for pH o 7. NaBH4 2H2O reaction scheme: is the first reported metal borohydride hydrate and crystallizes in an 103,111 2 orthorhombic unit cell (Pbca). Na is octahedrally coordinated M(s) + H2(g) MH2(s) (6.1) by four BH and two H O groups that all bridge between two Na 4 2 The equilibrium constant for this reaction is, atoms. There are three intermolecular O–Hd+ –dH–B dihydrogen 1 Y Y bonds in the compound in the range 1.77–1.95 Å. Interestingly, K = p(H2)/p p 1 bar, standard pressure (6.2) B NaBH4 2H2O decomposes upon heating at 40 1CtoNaBH4 and The Gibbs free energy for the reaction is H2O, which at T 4 40 1C slowly react to release hydrogen. Thus, the hydrate NaBH4 2H2O does not directly release hydrogen but DGr = DHr TDSr = RT ln K (6.3) decomposes into anhydrous NaBH4 and water. This may be because NaBH is stable in basic aqueous solutions. Lithium where DHr is the change in enthalpy, DSr is the change in 4 1 1 112,316 entropy, T is the temperature (K), and R = 8.3145 J K mol borohydride also absorbs water, forming LiBH4 H2O. H2. Combining (6.2) and (6.3) gives the van’t Hoff equation Upon thermal treatment, LiBH4 H2O releases hydrogen at B 1 316 Y 1 1 65 C. Calcium borohydride hydrate, Ca(BH4)2 H2O, has ln(p(H2)/p )=(RT) DHr (R) DSr (6.4) also been reported recently.113 A material’s hydrogen release temperature is usually given
for an equilibrium pressure of peq(H2) = 1.0 bar and is often 6. Thermal properties of metal denoted T(1 bar). For this special case, the van’t Hoff equation borohydrides is reduced to:
T(1 bar) = DHr/DSr (6.5) Metal borohydrides often have a high gravimetric and volumetric hydrogen density and have been of significant interest for many The major contribution to the reaction entropy change, DSr, possible applications including solid-state hydrogen storage. is from the change in state of hydrogen (from solid to gas Y 1 317 However, the reaction mechanisms are often complex and not phase), DS (H2) = 130.7 J (K mol) . Therefore, many metal 1 well understood, showing high kinetic barriers such that decom- hydrogen systems, but not all, have DSr B 130 J (K mol) .As position and hydrogen absorption often only occur at elevated such, in order to reach an equilibrium pressure of p(H2) = 1 bar temperatures, which hampers technical applications of these in the temperature range 50 to 200 1C the decomposition 1 materials. Release and uptake of hydrogen can be denoted enthalpies should be in the range 40–60 kJ mol H2. There a chemical reaction involving two-, three- or multiple-phases is much focus in materials research aimed at identifying such as solid–gas, solid–liquid–gas or solid–solid–gas systems. hydrides fulfilling this enthalpy range, which corresponds to In many cases more than one possible reaction mechanism is the operational temperatures for PEM fuel cells. involved, which may also change as a function of the physical For the metal borohydrides, however, entropy changes that 1 conditions, e.g. partial pressures, temperature and/or stoichio- are significantly different from 130 J (K mol H2) may be metry of involved starting materials. This section focuses on observed due to more complex reaction mechanisms and, in general aspects regarding reaction mechanisms for hydrogen some cases, dynamics in the solid state or liquid–solid transi- release and uptake during thermolysis, i.e. heating, also denoted tions. Dynamics such as ‘motion’ of BH4 complex anions may pyrolysis, along with different approaches for tailoring and contribute to the entropy of the solid state and a lower than optimization of thermodynamic and kinetic properties. This section expected entropy of melting.318 In addition, significantly higher reveals that chemical reactions between different hydrides, i.e. temperatures for hydrogen release and uptake are often reactive hydride composites, and additives can provide new observed in order to obtain reasonable reaction kinetics as perspectives for creating novel hydrogen carrier systems compared to the ideal temperature dictated by thermody- by tailoring physical, chemical, kinetic and thermodynamic namics, e.g. T(1 bar). This is due to kinetic restraints in the properties. Additives have also been investigated along with solid state, and therefore a thermodynamic driving force is their possible catalytic properties for hydrogen release and needed, i.e. Tdec 4 T(1 bar). A drawback is that higher reaction uptake reactions, as well as their function as grain refiners, temperatures often facilitate greater chemical complexity, e.g. and as favourable nucleation sites for the formation and side-reactions, and also dynamic effects, e.g. hydrogen release decomposition of hydride compounds. Nanoconfinement is from high-temperature polymorphs such as h-LiBH4 and, in another approach that has been tested for its potential to some cases, hydrogen exchange from a molten phase. improve both the kinetic and thermodynamic properties of The formation enthalpy, DHf, of metallic hydrides is well metal borohydrides. defined and exothermic for the stable hydrides. The enthalpy
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1601 Review Article Chem Soc Rev characterizes the strength of the metal hydrogen bond and is amounts of additives in the range of a few mol% has been partly determined by the local electron density on the inter- considered for possible catalytic effects and is discussed in stitial site where hydrogen is placed.319–321 The structure of the Section 6.7, whereas larger amounts may react with the metal metallic hydride is often an expanded version of the parent borohydrides and form reactive hydride composites, see Section 6.8. metal structure (DV B 10–20%) and may be considered as Physical conditions such as partial pressures and temperatures also being derived by hydrogen intercalation.322 In contrast, the influence the mechanism of decomposition i.e. the decomposition structure of a metal borohydride is much different from the pathway. The presence of impurities, even in minor quantities, can structure of the residue formed by dehydrogenation of the host also influence bubbling and frothing during melting.45 material. Therefore, the total reaction enthalpy, DHr, for hydrogen Lithium borohydride is the most well investigated metal exchange of metal borohydrides may be considered a sum of borohydride regarding the mechanism for hydrogen release contributions from several partial reactions. Thus, DHr,formetal and uptake. It undergoes a polymorphic transition from o- to 45 borohydrides may not directly correlate with the strength of a h-LiBH4 at 110 1C and melts at 280 1C. The polymorphic single covalent boron hydrogen bond. The decomposition transition and melting events can be associated with minor gas enthalpy, DHdec,isendothermicandischallengingtomeasure release that is often attributed to impurities, e.g. water or oxides, accurately.323,324 The decomposition enthalpy may often depend reacting with lithium borohydride.22,23,45,316,339 Dynamic PCT onthemethodforthemeasurement and the physical conditions, measurements under constant hydrogen flow provide thermo- which may lead to alternative reaction mechanisms or a change of dynamic data via the van’t Hoff equation for the decomposition 1 1 1 state, such as fusion (i.e. then also including DHfus). This is further reaction, DH =74kJmol H2 and DS =115JK mol H2 (see discussed in the following Section 6.2. Table 17).326,331 The mechanism for hydrogen transport in metallic hydrides, Direct decomposition without the presence of intermediate e.g. the early transition metal hydrides, is dominated by diffusion compounds, at elevated temperatures, leads to the production of single hydrogen atoms (or partially positively or negatively of boron, lithium hydride and release of 13.1 wt% hydrogen, charged ions) in the solid state.325 However, more complex scheme (6.6). Thermodynamic results from calorimetry provide 1 1 1 transport phenomena may be involved in the hydrogen release DH = 66.6–68.9 kJ mol H2 and DS = 97.3–100.2 J K mol H2 and uptake reactions in the solid state of hydrides with ionic and (see Table 17) for this suggested reaction. covalent bonds, i.e. complex metal hydrides. There are many LiBH (s) - LiH(s) + B(s) + 3/2H (g) (6.6) indications in the literature that relatively stable molecule-like 4 2 units may participate in hydrogen release and uptake reactions in Partial dehydrogenation can also occur, forming lithium thesolidstateofboron,aluminium and nitrogen based complex closo-borane, Li B H , via less endothermic reactions, scheme hydrides, possibly BH ,AlH or NH , which is further discussed in 2 12 12 3 3 3 (6.7), with a calculated reaction enthalpy of 56 kJ mol 1 H .330 Section 6.6. Thus, the reactions that take place during hydrogen 2 - release and uptake in metal borohydrides may be significantly LiBH4(s) 5/6LiH(s) + 1/12Li2B12H12(s) + 13/12H2(g) (6.7) more complex than suggested by eqn (6.1). The thermodynamics of hydrogen release and uptake may be investigated by the Various quantities of hydrogen in the range from 9–14 wt%
Sievert’s method by measurement of pressure–temperature– are reported to be released from LiBH4 under different experi- composition (PCT) isotherms. Generally, the published literature mental conditions, e.g. 9wt%H2 under evolved hydrogen pressure, suggests increasing difficulty in measuring PCT-isotherms with and 13.8 wt% H is released in flowing He by heating to 600 1C.22,340 well-defined plateau pressures for aluminium o nitrogen o Lithium borohydride has also been decomposed at 600 1Cfor boron based hydrides. This may be due to increasing complexity 5 hours under different partial pressures of hydrogen.341 Only one- of the involved chemistry, an increasing number of alternative third of LiBH4 transforms to Li2B12H12 at p(H2)=50barandno reaction mechanisms, and in some cases, several simultaneous amorphous boron is observed. In the pressure range, 0.1–10 bar, reactions. LiBH4 nearly completely decomposes to Li2B12H12 and amorphous
boron. Under these physical conditions (600 1C, 5 h), LiBH4 appears 6.2 Thermolysis of monometallic borohydrides to decompose more efficiently to amorphous boron at lower 341 Thermolysis denotes the decomposition of a substance upon pressure. Thermolysis of LiBH4 under vacuum at temperatures heat treatment. Monometallic borohydrides may release hydro- up to 600 1C is reported to produce LiH, amorphous B (or lithium 342 gen and, in some cases, also diborane during the formation of boride) via hydrogen-poor Li2B12H12 x. higher metal boranes, metal borides or even pure boron upon The formation of LiBH4 from the elements has also been thermolysis, which has been the topic of much research during attempted, which is equivalent to rehydrogenation or reversing the past decade. The aim of this section is to provide an reaction (6.6). Amorphous boron is known as a rather inert overview of some of the general trends that appear in the scientific material with a complex structure of covalently bonded boron literature for the thermodynamics of hydrogen release and uptake that forms a framework containing B12 clusters. Lithium in selected, well studied, mono-metallic borohydrides. Data hydride is expected to form readily under hydrogen pressure, collected for the same material with different methods are but further hydrogenation can be very slow until melting, compared in Table 17. Additives may significantly change the Tm(LiH) = 689 1C, which facilitates close contact between the reaction mechanism of decomposition. The addition of small reactants.343 The kinetics of hydrogen uptake reactions can be
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Table 17 Thermodynamic data for the decomposition of mono-metallic borohydrides
DHdec DSdec Teq at 1 1 Compound Proposed reaction (kJ mol H2) (J K mol H2) 1 bar H2 (1C) Method Notes Ref. - LiBH4 LiBH4 LiH + B + 3/2H2 74 115 370 van’t Hoff T = 413–517 1C, 326 PCT under constant gas flow - LiBH4 LiH + B + 3/2H2 68.9 100.2 415 Calorimetry — 327 and 328 LiBH4 - LiH + B + 3/2H2 66.6 97.3 411 Calorimetry — 317 LiBH4 - LiH + B + 3/2H2 69.1 — — Unreported — 329 LiBH4 - LiH + B + 3/2H2 75 — — Theoretical — 330 LiBH4 - LiH + B + 3/2H2 56 — — Theoretical — 331 - LiBH4 5/6LiH + 56 — — Theoretical — 330 1/12Li2B12H12 + 13/12H2
NaBH4 NaBH4 - Na+B+2H2 108 133 539 van’t Hoff T = 600–700 1C, 332 PCT under constant gas flow NaBH4 - NaH + B + 3/2H2 89.6 98.6 636 Calorimetry — 327 and 328 - NaBH4 NaH + B + 3/2H2 90.2 93.6 691 Calorimetry — 317 - KBH4 KBH4 KH + B + 3/2H2 113.9 103.6 826 Calorimetry — 327 and 328 KBH4 - KH + B + 3/2H2 112.6 96.9 889 Calorimetry — 317
Mg(BH4)2 Mg(BH4)2 - MgH2 +2B+3H2 39.3 91.3 157 van’t Hoff T = 290–350 1C, 196 PCT under constant gas flow - Mg(BH4)2 MgH2 +2B+3H2 57 128 172 van’t Hoff T = 285–360 1C, Stepwise PCT, 333 p = 1–10 bar Mg(BH4)2 - MgH2 +2B+3H2 41 — — DSC Multiple backpressure 334 Mg(BH4)2 - MgH2 +2B+3H2 40 — — Theoretical — 335 a-Mg(BH4)2 - MgB2 +4H2 38 — — Theoretical — 336
Ca(BH4)2 undefined 87 158 278 van’t Hoff T = 356–413 1C, Stepwise PCT, 337 p = 10–60 bar - Ca(BH4)2 2/3CaH2 + 40.6 109.3 98 Theoretical — 338 1/3CaB6 + 10/3H2 Ca(BH4)2 - CaH2 +2B+3H2 57.3 105.7 269 Theoretical — 338 - Y(BH4)3 b-Y(BH4)3 1/4YH3 + 22.5 114.4 76 Theoretical — 199 3/4YB4 + 45/8H2 - b-Y(BH4)3 1/2YH3 + 28.0 116.4 32 Theoretical — 199 1/2YB6 + 21/4H2 b-Y(BH4)3 - YH3 +3a-B + 9/2H2 36.2 111.0 53 Theoretical — 199
1 1 1 increased by thermal treatment of lithium–boron mixtures at 330 DH = 108 3 kJ mol H2 and DS = 133 3JK mol H2, and 450 1C in argon, which facilitates the formation of more reactive which corresponds to a decomposition temperature of Tdec(1 bar) = 343 332 lithium borides, LiB3 or Li7B6, respectively. The hydrogen uptake 539 1C. These dynamic PCT measurements are conducted at reaction has also been investigated by powder neutron diffraction a constant temperature of 600, 650 or 700 1C and the same 344 starting with lithium boride. The hydrogen/deuterium absorption starting pressure, p(H2) = 180 bar, with a mass flow controller. starts at B200 1C with the formation of LiD, and at 350 1CLiBD4 The decomposition appears to occur in one step with only one also starts to form. However, under the given experimental condi- plateau observed on the pressure composition isotherms.332 tions, Tmax =5001C, p(H2)=180barforB18 hours, a complete The entropic component that is measured with dynamic PCT’s 344 1 1 reaction was not achieved and LiD remains. (DS = 133 J K mol H2) is rather high, and much higher
Sodium borohydride, NaBH4, melts at 505 1C, followed by than the entropy change measured by calorimetry (DS = 45,332,345 1 1 hydrogen release at 534 1C under p(H2) = 1 bar. How- 93.6–98.6 J K mol H2) (see Table 17). It is experimentally ever, hydrogen release can be observed at T B 500 1C when important to note the vapour pressure of Na metal is very high measured in helium or argon, p(He, Ar) = 1 bar.332 Sodium (0.1 bar at 675 1C) and this can result in significant Na loss at borohydride decomposes to the elements (Na and B), see high temperature, especially under gas flow.347 reaction scheme (6.8), due to the lower stability of NaH, Potassium borohydride melts at 625 1C, followed by hydro- compared to LiH that is formed in the decomposition of LiBH4. gen release near 680 1C with a possible reaction mechanism for 33,345 Thermal analysis of NaBH4, however, suggests that intermedi- decomposition similar to that of NaBH4, scheme (6.8). ate compounds may form during the decomposition.346 However, increasing evaporation of potassium from the decom- posed residue is expected due to the high vapour pressure of - NaBH4(s) Na(l) + B(s) + 2H2(g) (6.8) potassium (p(K) = 0.4 bar at 672 1C).348 The high thermal
stability of NaBH4 and KBH4 may be the reason for minimal Dynamic PCT measurements, under constant hydrogen scientific interest and, as such, the decomposition mechanism flow, provide thermodynamic data via the van’t Hoff equation, remains not well investigated. Although, calorimetry indicates
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1 that the decomposition enthalpy of KBH4 is high (DH = 112.6– reaction enthalpy of DH = 38–40 kJ mol H2 for reaction (6.9) 1 335,336 113.9 kJ mol H2) (see Table 17), which is favorable for solar or (6.10). thermal applications given adequate kinetics.349–351 - Mg(BH4)2 MgH2 +2B+3H2 (6.9) Magnesium borohydride, Mg(BH4)2, has received significant interest as a possible hydrogen storage material with a high Mg(BH4)2 - MgB2 +4H2 (6.10) gravimetric hydrogen content of 14.9 wt% H2. Mg(BH4)2 can undergo an array of polymorphic transitions during heating Experimental studies also determine similar enthalpies, and melting depending on gas partial pressure.187,188 At elevated 1 334 with DH = 41 kJ mol H2 from a DSC study, and DH = hydrogen pressures, p(H ) 4 B1 bar, decomposition is suppressed 1 196,333 2 39.3–57 kJ mol H2 fromvan’tHoffplots. The van’t Hoff 45 and the compound melts. The decomposition mechanism of plots were formulated from PCT isotherms in both cases, but the magnesium borohydride is also quite complex and highly difference in these data could be due to one set of isotherms being 183,187,196,333,352–355 dependent on reaction conditions. An in situ measured with constant gas flow and one set being measured step- 1 SR-PXD investigation (constant heating, DT/Dt =41Cmin in wise. This variability also demonstrates the difficulty in obtaining p(H2)=1bar)ofg-Mg(BH4)2 reveals melting of the b-polymorph accurate thermodynamic data from metal borohydrides. B at T 270 1C and observation of a broad diffraction halo at Calcium borohydride’s physical behaviour is similar to d B 5.1 Å.187 Ex situ visual inspection of the melting process Mg(BH4)2, where a molten phase can be observed near the 187 reveals a colour change from white to yellow to brown to grey. decomposition temperature, depending on hydrogen backpressure, Magnesium hydride crystallizes from the melt at 320 1Cand which is also known to control the decomposition pathway.45,361 The disappears at 360 1C due to the formation of Mg observed in the decomposition pathway is temperature dependent and may proceed temperature range 340–580 1C. Boron containing compounds through a CaB2H6 intermediate to either CaB6 (320–350 1C) or 187 362,363 are not observed until the formation of MgB2 at 510 1C. amorphous boron (400–450 1C). Theoretical calculations Hydrogen release occurs in three steps and the first, 270–325 1C, predict Ca(BH4)2 decomposition enthalpies to be quantitatively is assigned to hydrogen release from molten Mg(BH ) . The second, 1 4 2 similar to Mg(BH4)2, where DH = 40.6–57.3 kJ mol H2 for 338 325–360 1C, is assigned to hydrogen release from MgH2 and the decomposition reactions (6.11) or (6.12). third, 360–410 1C, to remaining non-crystalline boranes, which 187 - may also contribute to the second release. Amorphous or nano- Ca(BH4)2 2/3CaH2 + 1/3CaB6 + 10/3H2 (6.11) crystalline closo-borane, MgB12H12, has been reported to be a major Ca(BH4)2 - CaH2 +2B+3H2 (6.12) intermediate product during decomposition of Mg(BH4)2 in vacuum.352 However, when Mg(BH ) is decomposed in dynamic 4 2 However, PCT measurements provide much higher thermo- vacuum in the temperature range 265 to 400 1Cthereaction 1 356 dynamic quantities with DH = 87 kJ mol H2 and DS = pathway is different. Here, Mg(B3H8)2 wasobservedasthemajor 1 1 158 J K mol H2. Here, the entropy is much higher than decomposition product when Mg(BH ) was heated to 265 1C. Trace 4 2 all other entropies for metal borohydrides (Table 17) including amounts of Mg(B3H8)2 were observed after heating to 285 1C, and 1 1 338 theoretical calculations (DS = 105.7–109.3 J K mol H2). none of the boranes Mg(BH4)2,Mg(B3H8)2 or MgB12H12 were 1 356 Strontium borohydride decomposes at 400 C and releases observed in samples heated at T 4 285 1C. Recently, MgB4H10 142,277 H2. Crystalline SrB6 is observed by in situ SR-PXD, which was also proposed as an intermediate in the decomposition 165 suggests that Sr(BH4)2 decomposes into SrH2, SrB6 and H2. reaction.355 Lower dehydrogenation temperatures tend to increase Manganese borohydride decomposes at 160 1C with the evolu- the amount of higher boranes and a large quantity of Mg(B3H8)2 45 357 tion of hydrogen and large quantities of B2H6. For a pure sample was obtained after 5 weeks at 200 1Cinvacuum. Mg(B3H8)2 was of Mn(BH4)2 (9.5 wt% H2), a mass loss of 14.8 wt% is observed in convertedbacktoMg(BH) after 48 h at 250 1C, p(H )=120bar.357 4 2 2 the temperature range 125–175 1C.58 A melting event can also be In a similar fashion, a mixture of Mg(B H ) 2THF–MgH (1 : 2) was 3 8 2 2 observed near this temperature but appears to be dependent on heated to 200 1Cunderp(H2) = 50 bar for 2 h and the product 45 358 hydrogen backpressure and heating rate. Temperature dependent indicated a quantitative conversion to Mg(BH4)2. in situ SR PXD measured for a-Mn(BH4)2 reveals that the diffraction Formation of magnesium borohydride from the elements by peaks disappear and the sample turns X-ray amorphous at 140 1C hydrogenation has also been conducted but is kinetically slow. 58 or 160 1Cforp(Ar) = 1 bar and p(H2) = 10 bar, respectively. The Reactive mechanochemistry in a hydrogen atmosphere has high-pressure polymorph, d-Mn(BH4)2, transforms into a-Mn(BH4)2 been used successfully and B50% of MgB was hydrogenated 2 during heating in the temperature range 67–109 1C, hence the two to amorphous Mg(BH ) at p(H ) = 100 bar (ball-milling time up 4 2 2 polymorphs decomposes similarly.140 to 100 hours).359 Magnesium boride, MgB , has also been 2 Yttrium borohydride forms an X-ray amorphous solid above hydrogenated/deuterated at 400 1C and p(H ) = 800 bar to form 2 200 1C and decomposes near 275 1C.66,98,198,364 When Y(BH ) 11 B 134 4 3 b-Mg( BD4)2 with a 43% yield. A higher borohydride yield is decomposed under H2 back pressure, ranging from p(H2)= was found with pre-milled MgB2 at 400 1C, p(H2) = 950 bar for 1–10 bar, Y(B3H8)3 is observed as the main intermediate 108 h (B75% b-Mg(BH ) ).360 4 2 according to scheme (6.13), along with small amounts of A number of theoretical and experimental studies have been 100 Y2(B12H12)3. conducted to determine the thermodynamics of decomposition for Mg(BH4)2 (see Table 17). Theoretical calculations suggest a Y(BH4)3(s) - 1/3Y(B3H8)3(s) + 2/3YH3(s) + H2(g) (6.13)
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0 0 The ultimate decomposition results in the formation of YH3, For example: MxM y(BH4)z, (M = Li, Na, K, Rb or Cs, M = Al,
YH2 and YB4, but does not reform the borohydride, even under Y, Sc, Mn, Zn or Cd), may decompose via the formation of 66 1550 bar at 300 1C. Theoretical calculations indicate that Y(BH4)3 mono-metallic alkali metal borohydrides MBH4 but the remaining is thermodynamically unstable at room temperature with a low part may decompose directly to a metal, hydrogen and 1 199 70,146,159 decomposition reaction enthalpy DH = 22.5–36.2 kJ mol H2. diborane. All the Cd-based borohydrides show low Cadmium borohydride’s low-temperature polymorph a-Cd(BH4)2 thermal stability. Their decomposition temperatures exhibit transforms to b-Cd(BH4)2, by a first order phase transition, at the following trend: b-Cd(BH4)2, B75 1C o KCd(BH4)3,
B55 1Candb-Cd(BH4)2 decomposes at B75 1CintometallicCd B80 1C o K2Cd(BH4)4, B85 1C, i.e. the higher the content of 146 and likely a mixture of hydrogen and borane gases according to KBH4 the higher the thermal stability. scheme:146 6.4 Eutectic melting of bi- and trimetallic borohydrides - b-Cd(BH4)2(s) Cd(s) + B2H6(g) + H2(g) (6.14) Two or more metal borohydrides may be combined as discussed previously in Section 6.3, forming compounds with new composi- Molecular metal borohydride. The decomposition mechanism 0 tions and/or structures, e.g. a new bimetallic compound MM (BH4)x. for molecular metal borohydrides, such as Al(BH ) ,Ti(BH) ,or 4 3 4 3 In some cases a eutectic melting mixture may form, which has been Zr(BH ) , remains not fully understood. Apparently these com- 4 4 observed in a few cases involving alkali- and alkaline earth-metal pounds first sublime at low temperatures: around or below 0 1C 45 borohydrides. For example, LiBH4–MBH4 systems reveal eutectic for Al(BH ) 12 and at 29 1C for Zr(BH ) .109,365 In the gas phase all 4 3 4 4 melting, M = K, and several new bimetallic compounds for M = Rb or three compounds tend to release diborane via polymerization 47,156 Cs with increasingly complex phase diagrams. The formation of reactions. The spontaneous diborane release from Al(BH ) and 4 3 an eutectic mixture was observed to cause hydrogen release at lower Ti(BH ) below room temperature may be responsible for the very 4 3 temperatures during thermolysis, which could be a kinetic effect. aggressive decomposition reactions. The stabilization of Ti(BH ) 4 3 However, structural reorganization upon melting may, in principle, was recently reported through its confinement within a metal– result in weakening or strengthening of chemical bonds, which may organic-framework (MOF UiO-66), allowing it to be stored at room influence decomposition, i.e. a thermodynamic effect. These effects temperature for months without decomposition.210 may also influence hydrogen uptake properties. An overview of investigated eutectic mixtures is provided in Table 18. 6.3 Thermolysis of bi- and trimetallic borohydrides Lithium borohydride–sodium borohydride. The LiBH4–NaBH4
The thermal decomposition of the metal borohydrides have system shows almost full solubility, i.e. a solid solution, Li1 xNaxBH4, been shown to occur via a wide range of pathways, often in in the temperature range B110–220 1C with compositions 0.0 o several steps, which may involve simultaneous and, in some x o 0.9. The lowest melting point is observed for Li0.70Na0.30BH4 161,367 cases, coupled chemical reactions. The following focus is on at Tmp B 216 1C. general trends deduced from published literature regarding Lithium borohydride–potassium borohydride. The LiBH4– both the reaction pathways and thermal stability of bi- and KBH4 phase diagram reveals a very deep eutectic melting point 47 trimetallic borohydrides. Their decomposition mechanism is of Tmp = 105 1C (Fig. 21a). Interestingly, a binary stoichio- so far defined by one of two schemes: metric compound, LiK(BH4)2, is formed mechanochemically, (i) Stable metal borohydrides that form the bi- and trime- not thermally, but separates to the monometallic borohydrides tallic borohydride. Here, the bi- and trimetallic borohydride at T 4 95 1C. In contrast, the sodium borohydride–potassium will separate during thermolysis, either splitting into the indi- borohydride system forms a solid solution upon thermal treat- vidual, stable, borohydrides, or possibly melting in an eutectic ment at B200 o T o 450 1C. The reason that LiK(BH4)2 composition. formation is facilitated by compression (mechanochemistry) 0 0 For example: MxM y(BH4)z, M = Li or Na, M = K, Rb, Cs are and NaxK1 x(BH4) formation is facilitated by thermal expansion formed from stable metal borohydrides. During heating, the (heating) is due to the volume (V/Z) of the bimetallic compound bimetallic compounds phase separate and may form an eutectic and solid solution in comparison to their reactants. Compression 0 melt as described in Section 6.4. MxM y(BH4)z, (M = Li, Na, K, Rb can form the bimetallic borohydride LiK(BH4)2 owing to a smaller or Cs, M0 = Mg, Ca or Sr) are formed from stable metal volume than the reactants, whereas thermal treatment can be used borohydrides. They are observed to decompose via the formation to form a bimetallic borohydride with a larger volume than its 47,93 of mono-metallic alkali metal borohydrides MBH4 and alkaline reactants, in this case a solid solution, NaxK1 x(BH4). earth metal borohydrides, which decompose individually at Lithium borohydride–magnesium borohydride. The decomposi- 1,164,165 higher temperatures. tion of the eutectic melting LiBH4–Mg(BH4)2 system initiates above
(ii) A combination of stable and unstable borohydrides that Tmp B 180 1C. At 270 1C, a 0.5LiBH4–0.5Mg(BH4)2 composite form the bi- and trimetallic borohydride. In this case, the releases 7.0 wt% of hydrogen.45,368 The eutectic system therefore unstable borohydride is often stabilized by its incorporation releases hydrogen at significantly lower temperature than the into a bi- and trimetallic borohydride, resulting in decomposition individual lithium and magnesium borohydrides, which occur at at higher than normal temperature. However, the decomposition B400 and B280 1C, respectively. mechanism is unchanged and the remaining stable metal boro- Lithium borohydride–calcium borohydride. The system LiBH4– hydride will stay intact. Ca(BH4)2 has been investigated for several compositions using
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Table 18 Eutectic melting metal borohydride compositions, melting temperatures for the two individual compounds Tmp1 and Tmp2, melting 45 temperature for the eutectic composition Tmp, temperature for major hydrogen release, Tdec and morphology of the melt by visual inspection
Eutectic mixture Tmp1 (1C) Tmp2 (1C) Tmp (1C) Tdec (1C) Eutectic morphology Ref.
0.62LiBH4–0.38NaBH4 280 510 215 300 Transparent liquid 366 and 367 0.725LiBH4–0.275KBH4 280 605 105 420 Transparent liquid 47 0.55LiBH4–0.45Mg(BH4)2 280 280 180 250 Froth 368 0.7LiBH4–0.3Ca(BH4)2 280 370 200 350 Froth 369 0.68NaBH4–0.32KBH4 510 605 460 465 Dark liquid 93 and 366 0.45NaBH4 + 0.55Mg(BH4)2 510 280 205 360 Froth 370
47 Fig. 21 (a) Binary phase diagram for the LiBH4–KBH4 system. (b) Binary phase diagram for the NaBH4–KBH4 system. Adapted with permission from ref. 93. Copyright 2015 American Chemical Society.
DSC, in situ SR-PXD, etc.369,371 Interestingly, this composite metal borohydrides, or form higher metal boranes, metal undergoes eutectic melting with a minimum temperature of hydrides, or metal borides. 11B NMR is particularly useful in
Tmp B 200 1C in a relatively wide composition range 0.6 o x o identifying decomposition products if they are non-crystalline, 0.8.369,371 where diffraction is less useful. The 11B spectra can be used to
Sodium borohydride–potassium borohydride. The NaBH4– differentiate between many different metal borohydrides with
KBH4 phase diagram (Fig. 21b) reveals full solubility in the different cations, due to their varied interactions with the boron entire compositional range 0 o x o 1 for Na1 xKxBH4 above nucleus (Fig. 23). Trends in the NMR chemical shifts of boro- 200 1C. The melting point of the solid solution is reduced hydrides have been investigated recently.372 Other spectroscopic slightly compared to the individual reactants with a minimum methods such as IR and Raman are also useful in characterising 93 at Tmp B 460 1C for Na0.682K0.318BH4. crystalline and non-crystalline metal borohydrides, where different Sodium borohydride–magnesium borohydride. The system, bonding environments display characteristic vibrations (Fig. 24). xNaBH4–(1 x)Mg(BH4)2, x = 0.4–0.5, shows melting/frothing Progress in the spectroscopic understanding of alkali and alkaline 370 between 205–220 1C. However, the sample does not become earth metal borohydrides, M(BH4)n (M = Li, Na, K, Rb, Cs, Be, a transparent melt. In contrast, no eutectic is observed for Mg, Ca) and reactive hydride composites has been recently 370 373–375 NaBH4–Ca(BH4)2. reviewed.
An empirical correlation between the thermal stability, Tdec, 6.5 Trends in thermal decomposition for metal borohydrides of metal borohydrides and Pauling electronegativity was first Upon heating, stable mono-metallic borohydrides tend to melt suggested to exist in 1955391 and has been further investigated and then start to decompose at slightly higher temperatures, theoretically and experimentally by several research groups.6,392,393 see Fig. 22. On the other hand, the less stable molecular metal The more stable metal borohydrides tend to follow a linear borohydrides tend to sublime, rather than melt, and decom- correlation between the electronegativities and decomposition pose in the gas phase. The decomposition products from the temperature. This approximation, however, is less pronounced group of stable mono-metallic borohydrides appear to change for compounds with higher electronegativities. A comparison of color, morphology, volume etc. largely related to the degree of a linear and exponential correlation is shown in the inset in 45 decomposition achieved in each system. Metallic decomposi- Fig. 22b. The decomposition temperatures, Tdec, reported in tion products are not observed by visual inspection, X-ray Fig. 22b are measured by in situ SR-PXD or thermal analysis diffraction or nuclear magnetic resonance, suggesting that after (TGA/DSC) in inert atmosphere, often p(Ar) = 1 bar. In the case heating to modest temperatures the products likely remain as of the bimetallic borohydrides, the values for the Pauling
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Fig. 22 (a) Experimentally determined melting points for metal borohydrides as a function of the electronegativity of the metal. (b) Experimentally 0 0 observed decomposition temperatures, Tdec for metal borohydrides M (BH4)n and MxM y(BH4)z plotted as a function of the electronegativity of the metal, 0 M . The Tdec values refer to the temperature for the maximum gas release and are measured by TGA/DSC or in situ SR-PXD at p(He) or p(Ar) = 1 bar. The inset in Fig. 22b reveal a comparison of linear and exponential correlation between observed decomposition temperatures, Tdec, as a function of the electronegativity of the metal.
Fig. 23 Experimentally determined 11B NMR chemical shifts of relevant metal borohydride and borane compounds. Bimetallic compounds are presented at the electronegativity of the most electronegative cation.158,166,197,275,342,376–389
electronegativities, wp, are taken from the more electronegative MSc(BH4)4, MZn2(BH4)5 and MZn(BH4)3 reveal only minor elements, i.e. Sc, Y, Zn, Cd and Al. This approach is justified by variations in the decomposition temperature with changes in the stronger coordination between the more electronegative alkali metal.70,166,380 This indicates that the decomposition metal and BH4, causing the formation of discrete metal boro- temperature is specific for these complex anions rather than hydride anions (see the discussion in Section 3.8). determined by the overall composition of the compound. This The less electronegative metal is often an alkali metal with a underlines the strong correlation between the decomposition dominantly ionic interaction. The validity of this assumption is temperature and the electronegativity of the complex-forming illustrated by the decomposition of KCd(BH4)3 to K2Cd(BH4)4 metal and the key role of the complex M–BH4 anions in the and Cd(BH4)2. The latter immediately decomposes to Cd, H2 structural stability of bimetallic borohydrides. and B2H6. Upon further heating, K2Cd(BH4)4 decomposes to 6.6 The released gas from metal borohydrides, stability of KBH4 and Cd(BH4)2, where the latter again immediately decom- 146 diborane poses to Cd, H2 and B2H6. This example illustrates that the more electronegative cation is controlling the stability. Further- Materials containing complex ions were overlooked as hydro- more, metal borohydrides containing complex anions, such as gen carriers for possible hydrogen storage applications until [Sc(BH4)4] , [Zn2(BH4)5] and [Zn(BH4)3] in the compounds the late nineties due to their relatively high stability and the fact
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390 Fig. 24 IR and Raman active fundamental vibrations commonly observed for MBH4. that hydrogen uptake only occurs under extreme conditions. A and ionic hydrides (mentioned in Section 2.5).95,403 On the paradigm shift was initiated by the discovery that titanium other hand, an ionic hydride, e.g. LiH or NaH may also be catalyzed NaAlH4 releases and uptakes hydrogen at moderate observed in the dehydrogenated state: conditions,394 which initiated the research activities in hydro- AH + LiH 2 LiAH A = B, N, or Al (6.15) gen storage systems containing complex ions. 3 4 The boron, aluminium and nitrogen based complex hydrides, Lithium borohydride, LiBH releases hydrogen at T 4 400 1C, 4 e.g. LiBH4,LiAlH4 or LiNH2, contain the complexes, BH4 ,AlH4 but was also found to emit diborane at the impurity level.339 This or NH2 . These complexes form stable crystalline solids with contrasts with, for example, Mn(BH4)2, which releases significant many alkali or alkaline earth metals, which act as counter ions amounts of diborane at T B 160 1C.58 Thus, metal borohydrides in the solid state. Reversible nitrogen-based complex hydride have a tendency to release increasing amounts of diborane when systems were discovered by Ping Chen et al. in 2002 and metal they have lower decomposition temperatures.97,404 The extreme borohydrides were also found to be reversible, albeit under case is for LiZn (BH ) that decomposes at T B 100 1Cand 329,395 2 4 5 harsh conditions, e.g. LiBH4 in 1980. presumably releases a 1 : 1 mixture of hydrogen and diborane A drawback for these light element complex hydrides, e.g. LiBH4, due to a reduction of Zn2+ to Zn.70 A similar type of metal ion LiAlH4 or LiNH2, appears to be that their neutral counterparts (AH3) 146 reduction is also observed for the decomposition of Cd(BH4)2. are neutral molecules, BH3 or NH3,ormetastablesolids,AlH3.In Varying mixtures of hydrogen and diborane in the released gas fact, BH3 is in equilibrium with its dimer, B2H6, where the partial suggests that more than one reaction mechanism is involved. 8 pressure of BH3 is roughly 2 10 of the pressure of B2H6 at room Therefore, the composition of the released gas may depend on 396 temperature. However, at higher temperature the BH3 partial physical conditions, such as temperature and hydrogen partial 4 1 pressure is higher, i.e. 10 times higher at 200 C than RT, possibly pressure, which may favour a particular competing reaction. promoting hydrogen release via polymerization reactions. Ammonia, Diborane can react with an ionic hydride and form a metal NH3, is a reactive gas at room temperature that liquefies below borohydride via a solid–gas addition reaction as shown for 1 33.3 Catp(NH3) = 1 bar. Ammonia is an industrially useful and lithium borohydride at T 4 B120 1C, reaction scheme (6.16). At energy dense compound (18.6 MJ kg 1), but may be converted to slightly higher temperatures, 150 o T o 200 1C, diborane can N2 and H2 at high temperature, with catalysts, with a Na/NaNH2 further react with lithium borohydride and form metal closo- cracking system or by the omnipresent NaOH in NaNH2 as boranes (M B H ), according to reaction schemes (6.17) and 397,398 x n n shown recently. Alane, AlH3, is a polymeric solid built (6.18).95,252,405 399,400 from octahedra of AlH6. Alane is thermodynamically - unstable at room temperature, but is stabilized in the bulk form 2LiH(s) + B2H6(g) 2LiBH4(s) (6.16) by surface aluminium oxide layers.401,402 2LiBH4 +4B2H6 - Li2B10H10 + 11H2 (6.17) There are many indications that these ‘molecules’ (AH3) play a crucial role in the mechanism for hydrogen release and 2LiBH4 +5B2H6 - Li2B12H12 + 13H2 (6.18) uptake in complex hydrides, e.g. LiBH4, LiAlH4 or LiNH2. This is exemplified by the fact that the light element complex Boron chemistry is extremely diverse and all neutral boranes, hydrides can be prepared by a reaction between the ‘molecule’ BmHn, can be prepared, directly or indirectly, from diborane with
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the product mainly depending on the temperature and partial of Nb2O5 and other additives these materials have a beneficial effect 406 pressure of the reactants. Thus, the borane molecule, BH3, on the ball-milling process leading to the formation of smaller Mg may play an active role for hydrogen release and uptake in metal particles and crystallites, which, in turn, improves kinetics due borohydrides. These arguments may suggest that hydrogen to surface area or grain boundary area enhancements.426–429 release from metal borohydrides occurs via the partial reduction A significant variety of other additives have been investigated in of borane, BH3, via the formation of higher boranes, or complete the MgH2 system, including metals, intermetallics, oxides, reduction forming metal borides or amorphous boron. Low total hydrides and halides.430–432 gas pressure may lead to a relatively high partial pressures of Additives for metal borohydrides. The above mentioned diborane, which appears to facilitate the formation of the rather examples have been used as inspiration in the exploration of stable metal closo-boranes.361 High partial pressures of hydrogen a wide range of additives for hydrogen release and uptake in appear to facilitate the full reduction of BH3 and the formation metal borohydrides. Trends in additive effects may appear of metal borides. when they are grouped into elements, and compounds such as hydrides, borides, oxides and halides. Mainly lithium and 6.7 Additives to improve reaction kinetics magnesium borohydrides have been examined systematically. Thehydrogenreleasetemperaturesofthestablemetalborohydrides Elemental metallic additives. A range of transition metals are usually considered too high for practical applications and have been considered as additives in MBH4 systems, e.g. Sc, Ti, catalysis has been widely investigated in an attempt to overcome V, Cr. Nanoscale transition metal particles are very important in kinetic barriers and reduce the activation energy for hydrogen release organic chemistry with a long history, involving the chemical and uptake. The more stable metal borohydrides are of particular transformation of boron compounds, e.g. palladium nanoparticles interest because they appear to have the potential to release pure for catalyzed Suzuki–Miyaura cross-coupling reactions, forming hydrogen with no trace of diborane. Small amounts of additives have new C–C bonds. Homo-coupling reactions can also occur, which been sought to act as catalysts and promote both hydrogen release form new C–B bonds.17 For mono-metallic nanoparticles, the and absorption in metal borohydrides. Additives may also promote activity follows a sequence of Pd 4 Cu 4 Ru 4 Pt.17 the formation of metal borides rather than metal closo-boranes, The adsorption of BH4 on the more noble metal surfaces i.e. selective formation of a dehydrogenated state that facilitates has also been investigated and reveals a weaker interaction with absorption of hydrogen at more moderate conditions. The present Ag and Au when compared to other noble metals, Pd, Rh, Ru, Section 6.7 focuses on smaller quantities of additives investigated for Pt, Ir and Os. This important result is related to the hydrolysis possible catalytic properties and Section 6.8 treats larger amounts of of borohydrides,433 which is further discussed in Section 6.9. additives that may purposefully react with the metal borohydride to Metal borohydrides may melt prior to the release of hydrogen, form a reactive hydride composite. which also increases their reactivity. Gold powder was used as
Additive effects for aluminum and magnesium based hydrides. an X-ray standard in a sample of LiBH4, but a reaction took
A significant proportion of the research conducted on metal boro- place at T 4 Tmp(LiBH4) B 290 1C and a lithium gold alloy hydride additives also tends to explore analogies with other hydrides, formed.9 A range of metals (Mg, Al, Ti, V, Cr, Sc) have been such as magnesium hydride, MgH2, and sodium tetrahydrido mixed with LiBH4 in stoichiometric proportions with reported 407–410 B 434 aluminate, denoted sodium alanate, NaAlH4. Catalytic nano- decreases of 50 1C in its decomposition temperature. particles have enabled breakthroughs in low-temperature hydrogen However, with high doping levels the entire reaction pathway 394,411,412 release and uptake in sodium alanate, NaAlH4. Several can be shifted, as in the case of reactive hydride composites, metals, such as titanium, scandium or cerium, have been shown forming alternate reaction products with different reaction to promote both hydrogen release and uptake, undertaking a thermodynamics. bifunctional catalytic role.413 Typically, a transition metal chloride Elemental non-metallic additives. The addition of excess is added to NaAlH4, which then irreversibly reacts, forming inert quantities of amorphous boron to LiBH4 was found to enhance NaCl and catalytically active Al or Ti–Al nanoparticles, possibly with the decomposition process by leading to an alternative reaction 412,413 the catalytically active metal on the surface of NaAlH4. mechanism, which reduces the decomposition temperature by B B 1 435,436 Hydrogen release and uptake in magnesium hydride, MgH2, DT 150 1C and the activation energy by DEA 55 kJ mol . has been shown to be promoted by other principles. Alloying Pendolino et al. propose a reaction of the intermediate Li2B12H12 effects with other metals such as Al, Cu, or Pd and the and B to cause this change in desorption behaviour, by surplus B formation of Mg17Al12,Mg2Cu/MgCu2,orMg6Pd have been facilitating faster formation and growth of the solid intermediate by 414–417 2 436 shown to influence hydrogen release and uptake. Other forming a polymer chain or layers with the boron of B12H12 . hydrides may also form and possibly act as an intermediate for Carbon. Carbon has been investigated as an additive hydrogen exchange, e.g. formation of Mg2Ni and Mg2NiH4 in to promote hydrogen exchange in metal borohydrides, both 418–420 421–425 MgH2–Ni samples or Mg2FeH6 in MgH2–Fe samples. theoretically and experimentally. Theoretical modelling suggests that
Another mechanism is suggested for the Mg–Nb2O5 system increased curvature of the carbon surface decreases the ‘hydrogen where MgH2 tends to reduce niobium oxide forming a removal energy’, i.e. C60 is expected to have a stronger effect in 437 Mg1 xNbxO solid solution, which possibly facilitates both hydro- comparison to graphene and graphite. Density functional theory gen splitting at the surface as well as hydrogen diffusion through (DFT) was used to evaluate a wide range of chemical reactions the very inert and dense MgO layer. Because of the high hardness suggesting the formation of bimetallic carbides, e.g. LiBC in a
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reaction between LiBH4 and carbon, which releases 11.95 wt% H2 Similarly, the reaction between LiBH4 and TiF3 results in an 1 438 448 with a moderate reaction enthalpy (DU0 =45.1kJmol H2). In a exothermic reaction to form LiF and TiB2. Borofluorides, similar manner, the formation of Li2C2 was observed in mixtures of NaBF4 and KBF4, are described in Section 4 and enable fluoride
LiBH4 with carbon nanotubes, which could be rehydrogenated substitution in metal borohydrides to possibly promote the 439 back to LiH. Another example is the reaction between LiBH4, formation of closo-boranes during thermolysis. In general, fluorides ScH2 and C, which is suggested to form ScB2C and LiH in the tend to ‘destabilize’ the metal borohydride often by promoting decomposed state with a relatively low enthalpy change of DU0 B irreversible chemical reactions that tend to form stable metal 1 438 52 kJ mol H2. The ‘carbon-effect’ on metal borohydrides is fluorides, closo-boranes and possibly diborane gas. Halides of Al, further discussed in Section 6.10 in regard to nanoconfinement. Ti, Zr and Hf may form volatile molecular borohydrides, which Sulphur. The reactions between metal borohydrides and then sublime and decompose in the gas phase. elemental sulfur have been investigated in situ during thermal Heavier halides: chlorides, bromides and iodides, as additives. treatment and were found to be highly exothermic (up to 700 J g 1). Heavier halides of metals with relatively low electronegativity These reactions are exceptionally rapid and result in the sudden (i.e. alkali- and alkali earth metals) may lead to anion substitution release of substantial quantities of hydrogen gas and should be in the metal borohydride, as discussed in Section 4. Anion handled with caution. The metal borohydrides react with molten substitution tends to stabilize the hydrogenated state, and
Sat140,190and1801C, respectively for LiBH4–S, NaBH4–S and provide hydrogen release at slightly higher temperatures, tend-
KBH4–S. Remarkably, NaBH4–S release pure hydrogen, whereas ing to facilitate the hydrogenation of the sample. Titanium
LiBH4–S and KBH4–S systems also release some H2SorB2H6 trichloride has a well-documented catalytic effect on the hydro- 440 394,413 gases. The drawback may be the very complex metal sulphur gen release and uptake from NaAlH4 and appears to 449 chemistry that results in the formation of multiple complex decrease the decomposition temperature of LiBH4. A color compounds and intermediates, as observed for the NaAlH4–S change from light red/purple to gray/black of a LiBH4–TiCl3 system.441 The reaction between metal borohydrides and sulphur (2 mol%) sample when kept a few hours at RT in an argon-filled could have implications for their use as solid state electrolytes in glove box indicates that a chemical reaction takes place.9 This Li–S battery applications. chemical reaction can be suppressed when the sample is kept Metal hydrides as additives. Different types of hydrides have at low temperatures, e.g. T B 35 1C and in situ SR-PXD reveals also been considered in order to promote hydrogen exchange in the formation of LiCl initiated at RT.9 Thus, the reaction 434,438,442 metal borohydrides. Many of these systems alter the between LiBH4 and TiCl3 results in the formation of Ti(BH4)3, decomposition pathway, resulting in altered thermodynamics which decomposes at room temperature by an unknown and are discussed in the following section on reactive hydride mechanism.210 One of the most effective additives was suggested composites. to be LiBH4–0.2MgCl2–0.1TiCl3, which released about 5 wt% H2 at
Metal oxides as additives. A variety of metal oxides have also T 4 60 1C and absorbed B4.5 wt% H2 at T = 600 1C, p(H2)= 450 been considered to enhance the hydrogen exchange properties 70 bar. The addition of TiCl3 also changes the hydrogen 451 of metal borohydrides. This was initiated by the study of glass exchange properties for other metal borohydrides, e.g. Mg(BH4)2. powders having an effect on lithium borohydride, i.e. LiBH4– Metal borides as additives. Metal borides are known as inert
SiO2, which enabled hydrogen release at lower temperatures materials with high melting points that have also been under 22 than observed for pure LiBH4. However, in situ diffraction consideration as additives. Titanium boride, TiB2, appears to shows an irreversible reaction between SiO2 and molten LiBH4 have a limited effect on the decomposition pathway of the at T 4 370 1C and the formation of orthosilicate, Li4SiO4, for LiBH4–1.5Al system, but seems to facilitate the formation of small amounts of additive (o10 mol% SiO2, i.e. excess of intermediate compounds (i.e. Li–Al–B solid solutions) at lower 452 lithium) and lithium metasilicate (Li2SiO3) for larger amounts temperatures compared to samples without an additive. Similarly, 9 of additive. The fate of boron remain unclear but the for- zirconium boride, ZrB2, appears to stabilize the hydrogen storage mation of lithium borates may occur. capacity for reactive hydride composites, e.g. LiBH4–MgH2, discussed
A minor change in hydrogen release profile for g-Mg(BH4)2 in the following section, possibly by facilitating nucleation and 453 with 0.02 mol% TiO2 and MoO3 is observed, i.e. the DSC events growth of layered magnesium diboride. appeared broader and were shifted to slightly lower tempera- Assessment of additive effects by comparative studies. A series 443 tures (ca. 20 1C), as compared to the pure g-Mg(BH4)2. A large of nickel-based materials, Ni powder (30 nm), Ni3B, NiF2 and NiCl2 decrease in the dehydrogenation temperature was found for (2 mol%), have been added to g-Mg(BH4)2 and the hydrogen 447 stoichiometric ratios of LiBH4 with TiO2 (down to 150 1C), but storage properties were evaluated. EXAFS spectra indicate that 444 was due to a redox reaction forming LiTiO2. Several transi- NiCl2 and NiF2 react with g-Mg(BH4)2 and transform to amorphous tion metal oxides have also been explored, which have some nickel boride. The g-Mg(BH4)2–NiF2 sample releases 6.5 wt% gas tendency for release of diborane due to reduction of the metal and the other three samples release 2.7 wt% at moderate by BH4 , accompanied by the irreversible formation of borates conditions, 220 o T o 264 1C, 10 o t o 60 h, under static or ternary metal oxides.445,446 vacuum. Different side-reactions in the fluoride-containing sample
Metal fluorides as additives. Nickel fluoride, NiF2, tends to may be responsible for the larger gas release. Spectroscopy react with g-Mg(BH4)2 and form Ni3B and possibly Mg(B3H8)2,which suggests the formation of Mg(B3H8)2 in the decomposed sample 447 absorbs hydrogen at moderate conditions to form b-Mg(BH4)2. due to the relatively low thermolysis temperature. The four
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samples absorb 1–2 wt% H2 at 210 o T o 262 1C, 10 o t o 48 h, identified by a mass loss, which scales with the amount of 447 and 100 o p(H2) o 155 bar. additive. Irreversible chemical reactions will lead to significant
A series of cobalt-based materials Co2B, CoF3, CoCl2 and losses of the hydrogen storage capacity in the first cycle,
Co3O4 (2 mol%), have been added to g-Mg(BH4)2 in a similar illustrating the importance of conducting experimental evalua- manner to the nickel additives.454 Three full cycles of hydrogen tions over several cycles of hydrogen release and uptake. release and uptake were conducted and the first cycle, at The available knowledge on the exact mechanism for hydro-
T B 286 1C, 2.6 o p(H2) o 4bar,10o t o 40 h, releases gen release/uptake remains limited partly owing to difficulties
3.1–4.4 wt% H2, which is very similar to that of pure g-Mg(BH4)2, in determining the decomposition products from metal boro-
4.1–4.2 wt% H2. Hydrogen absorption was conducted at hydrides, which are often non-crystalline. In several cases,
280–290 1Cat110o p(H2) o 125 bar. The third cycle releases more than one reaction mechanism for hydrogen exchange is
1.4–2.0 wt% H2 and the kinetics for hydrogen release and uptake proposed, which may change as a function of the physical 454 is similar to pure g-Mg(BH4)2. conditions, e.g. p, T, as discussed in Section 6.2. Thus, the rate General discussion of the effect of additives on metal determining step in the mechanism remain unknown, which borohydrides. Overall, no real catalyst for B–H bond breaking also hampers the design of a catalyst. or formation has been identified. The investigated additives Another largely overlooked problem is the fact that many may be divided into either reactive or inert additives. Reactive metal borohydrides melt before or during hydrogen release. additives may react irreversibly with metal borohydrides and Fusion eliminates the grain structure and reduces the surface typically enhance the mass loss from the system in the first area of multicomponent samples. The partly molten state of the hydrogen release cycle. Reactive additives may contain a metal metal borohydride – additive system usually has a relatively with a relatively high oxidation state and/or electronegativity, high viscosity, thus a gas release leads to bubbling, foaming which may be reduced during release of diborane and hydro- and possible phase segregation, which further limits reversi- gen. Reactive anions may react irreversibly with the metal bility and the use of catalysts. borohydride and form metal borates or more stable boranes, e.g. closo-borane. Inert additives may be metals or metal borides. 6.8 Reactive hydride composites However, the latter may have a beneficial effect and facilitate the Larger amounts of additives may lead to stoichiometric reac- crystallization of metal borides in the decomposed state. Small tions among the components in a system, denoted Reactive amounts of transition metal compounds like VCl3,ZrCl4 Hydride Composites (RHC). P. Chen (China) was first to apply or Zr-isopropoxide added prior to the ball-milling process of this approach to nitrogen containing systems like Mg(NH2)2– reactive hydride composites (discussed in Section 6.8), e.g. 2LiH.456 Later the approach was extended to borohydride
2LiBH4–MgH2 have beneficial kinetic effects. The additives containing systems by J. J. Vajo (USA), Y. W. Cho (South Korea) may transform to transition metal boride nanoparticles during and R. Bormann (Germany).37,39,42 Several systems combining thermolysis or aggressive ball-milling, which could facilitate the metal borohydrides and magnesium hydride have been studied formation of metal borides in the dehydrogenated state and intensively during the past decade, e.g. LiBH4–MgH2, NaBH4– 455 37,38,457 thereby hydrogen absorption. MgH2 and Ca(BH4)2–MgH2. A chemical reaction takes In most cases, only the first dehydrogenation is investigated, place between two or more components during decomposition, which hampers the distinction between irreversible chemical which lowers the overall reaction enthalpy (Fig. 25). This reactions and possible catalytic effects. Chemical reactions concept is analogous to Medima principles applied to d-block often show a significant enhancement of the first hydrogen metals and the well-known Hess law from chemistry. The release reaction, which may occur with faster kinetics and gravimetric hydrogen storage capacity is then the weighted therefore also at apparently lower temperatures. A disadvantage average of the individual hydrogen storage capacity of the for the use of volumetric or Sievert’s methods for measuring the components of the RHC system.8 In the following, selected gas release is that measured changes in pressure is converted to RHC systems are discussed in more detail and Table 19 pro- an equivalent amount of hydrogen, which is incorrect if gases vides an overview of the thermodynamic data. other than hydrogen are released. A chemical reaction between Lithium borohydride–magnesium hydride, 2LiBH4–MgH2. The a reactive additive and a metal borohydride may often be reactive hydride composite lithium borohydride–magnesium
Fig. 25 Schematic illustration of the concept of reactive hydride composites illustrated for 2LiBH4–MgH2.
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Table 19 Thermodynamic and hydrogen storage properties of selected reactive hydride systems (RHC) and their components. Selected experimental thermodynamic data for individual components have been included from Table 17
1 1 1 System rm (wt% H) DH (kJ mol H2) DS (J K mol H2) T1bar (1C) Onset Tdec (1C) Peak Tdec (1C) Ref.
LiBH4 18.5 74 115 370 280 420–460 45, 326 and 342 NaBH4 10.7 108 133 539 475 510 332 and 458 Ca(BH4)2 11.6 87 158 278 350 375 45 and 337 MgH2 7.7 74.1 133.4 282 240 315 323 and 426 2LiBH4–MgH2 14.4 40.5 81.3 225 300/350 350/420 39 2NaBH4–MgH2 9.9 64 102 282 280/420 325/470 459–462 Ca(BH4)2–MgH2 10.5 — — — 270 330 463 2LiBH4–Al 11.4 — — — 310 375–410 452 and 464 2NaBH4–Al 7.9 — — — 340 500 464 CaBH4–Al 4.9 — — — 345 365 464 4LiBH4–MgH2–Al 12.9 — — — 270/370 290/420 465
hydride releases and absorbs hydrogen under less harsh conditions e.g. small amounts of transition metal compounds like VCl3,
(p, T, t) compared to the individual components. This is due to an ZrCl4 or Zr-isopropoxide. Initially, the additives have a beneficial exothermic formation of MgB2 during the endothermic dehydro- effect on the ball-milling process through the reduction of grain and 455 genation reaction of the 2LiBH4–MgH2 system, i.e. achemical particle sizes and thereby reducing hydrogen diffusion pathways. reaction between LiBH4 and Mg, which leads to hydrogen release, Further investigations using extended X-ray absorption fine structure the formation of LiH and a reduction of the total reaction enthalpy, (EXAFS), X-ray absorption near edge structure (XANES) and anom- see reaction scheme (6.19). In addition, hydrogen uptake from alous small angle scattering (ASAXS) measurements indicate the
MgB2–2LiH occurs at more moderate conditions than for the formation of the respective transition metal boride nanoparticles 453,455,473 decomposition products from pure LiBH4, i.e. amorphous during aggressive ball-milling or subsequent thermolysis. 37 boron, lithium boride and/or higher boranes like Li2B12H12. The transition metal compounds with the best enhancement of
This is caused by the open layered structure of MgB2 as well as reaction kinetics for RHC systems form transition metal borides 466 the non-covalent bonding between Mg and B in MgB2. The full with small directional and interplanar misfits to MgB2 by 0.1 and reversibility of the 2LiBH4–MgH2 system is reported to only be 2.8%. Thus, these transition metal borides may act as favorable obtained when the decomposition occurs in a hydrogen back heterogeneous nucleation sites for the formation of the MgB2 453 pressure of p(H2) B 1–5 bar, which facilitates the formation of during decomposition. Therefore, hydrogen absorption kinetics
MgB2 by suppressing the individual decomposition of LiBH4 and are significantly faster when compared to RHC without additives. 39,467,468 MgH2. Nanoscale MgH2 synthesized via an organometallic method and The phase diagram has recently been investigated for the used as a starting component appears to suppress the formation of 2 474 2LiBH4–MgH2 system from a comprehensive pressure–composition- closo-boranes, e.g. [B12H12] . 469 isotherm study. It was shown that there is a single equilibrium Sodium borohydride–magnesium hydride, 2NaBH4–MgH2. pressure plateau below 413 1C represented by reaction (6.19a), but Sodium borohydride readily forms by the hydrogenation of a 37 there are two plateaus above 413 1C represented by reaction (6.19b). 2NaH–MgB2 composite, see reaction (6.20). The composite 240 A similar phenomena exists for NaMgH2F. 2NaH–MgB2 has a calculated gravimetric hydrogen capacity of 1 rm = 7.8 wt% H2, and reaction enthalpy of DH = 62 kJ mol H2, 2LiBH + MgH 2 2LiH + MgB +4H (6.19a) 4 2 2 2 corresponding to a decomposition temperature of T(1 bar) = 1 459 2 2 350 C. Hydrogenation of the 2NaH–MgB2 composite also 2LiBH4 + MgH2 2LiBH4 +Mg+H2 2LiH + MgB2 +4H2 occurs at low pressure i.e. p(H ) = 25 bar (t = 63 h) at 400 1C (6.19b) 2 475 B where 6.2 wt% H2 is absorbed. Hydrogenation starts at T 1 A calculation of the decomposition enthalpy change yields 250 C but appears to become more complex at lower hydrogen DH B 40–46 kJ mol 1 H , corresponding to T(1 bar) B pressures, proceeding via an unknown compound and dec 2 475 169 1C.39,41,469 A similar reaction occurs for a magnesium-rich NaMgH3. system, e.g. 0.3LiBH –MgH that decomposes to MgB in p(H ) 4 1 4 2 2 2 2NaBH4 + MgH2 2 2NaH + MgB2 +4H2 (6.20) 470,471 bar. In contrast, the system LiBH4–MgH2 (0.3 : 1) and
(2 : 1) decompose through a different reaction in dynamic The decomposition pathway of 2NaBH4–MgH2 RHC system vacuum forming a- and b-alloys of MgxLi1 x, x B 0.82 and 0.70, appears to be a two-step reaction similar to 2LiBH4–MgH2, see respectively.470,472 Hydrogen absorption also occurs at rather reaction scheme (6.19b). However, the reactivity appears to be low pressures and temperatures p(H2) o 50 bar and T o 300 1C. lower and some NaBH4 often remains in the sample after the 476 These conditions are substantially more favourable than those for reaction with Mg to form MgB2.
LiBH4 and are considered a breakthrough in utilizing borohydrides Calcium borohydride–magnesium hydride, Ca(BH4)2–MgH2. for reversible hydrogen storage. The calculated gravimetric hydrogen capacity for the Ca(BH4)2–
The kinetics for hydrogen release and uptake reactions can MgH2 system is rm =8.3wt%H2 and it shows a complex reaction be significantly enhanced by the addition of suitable additives, pathway with a number of intermediates, e.g. formation of CaB6,
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338,477–479 Ca4Mg3H14 and other Ca–B–H compounds. The idealized hydrogenation reaction takes place as illustrated in reaction scheme (6.21), which can take place at p(H2) = 140 bar and T = 250 1C,
CaH2 + MgB2 +4H2 - Ca(BH4)2 + MgH2 (6.21)
Until now, only partial reversibility has been obtained, however, transition metal-based additives facilitate the formation of MgB2 in the Ca(BH4)2–MgH2 system in a similar way to the LiBH4–MgH2 system.250,463
Lithium borohydride–aluminium, 2LiBH4–Al. Access to the hydrogen content of LiBH4 may be facilitated by the addition of a reactive metal, e.g. the 2LiBH4–Al system. The 2LiBH4–Al composite is more prone to rehydrogenation when compared to LiBH4, e.g. at Fig. 26 Sieverts desorption measurements of LiBH –Al–TiB (39.3, 58.9 T = 400 1Candp(H )=100bar.471,480–483 In fact, the 2LiBH –Al, 4 2 2 4 and 1.8 mol%). The dashed line is the temperature profile and the ten 2NaBH4–Al and CaBH4–Al systems can all be hydrogenated under desorption measurements are numbered (1) to (10). Dehydrogenation was 100 bar at 350–400 1C, but better kinetics are obtained under conducted by heating to 300 1C(DT/Dt =101Cmin 1) and subsequently to 464 1 600 bar. However, cycling of hydrogen release and uptake in 500 1C(DT/Dt =11Cmin ), and then kept at 500 1C for 150 min at p(H2) B 10 2 bar. Adapted with permission from ref. 452. Copyright 2013 2LiBH4–Al has shown that the system suffers from a significant American Chemical Society. degradation in hydrogen storage capacity.452,484,485 The composite
2LiBH4–Al has been suggested to release hydrogen in two steps at T o 500 1C represented by reaction schemes:471,480–483 p(H2) = 110 bar. During hydrogenation, the diffracted intensity from LiAl decreases at T B 200 1C and the formation of LiH and 2LiBH4(l) + Al(s) - AlB2(s) + 2LiH(s) + 3H2(g) (6.22) Al is observed, while the consumption of AlB2 occurs at T B LiH(s) + Al(s) - LiAl(s) + 0.5H (g) (6.23) 2 280 1C. Crystalline h-LiBH4 forms upon cooling at T B 260 1C. Apparently, the loss of sodium vapour prevents the formation of The enthalpy change for reaction (6.22) is calculated as 484 1 486 sodium alanate. DH = 59.3 kJ mol H2, which corresponds to T(1 bar) = 483 The lithium borohydride–magnesium hydride–aluminium 166 1C. Reactions (6.22) and (6.23) illustrate that the full H2 system LiBH4–MgH2–Al. Several reactive hydride composites capacity of LiBH4 (rm = 18.3 wt% H2) can be utilized for the (RHC) may also be combined to form more complex systems, e.g. composite of LiBH4–1.5Al. However, the addition of Al lowers the the LiBH4–MgH2 and LiBH4–Al RHC systems are combined to form gravimetric H2 capacity of this composite to only rm =6.5wt%H2. 465 LiBH4–MgH2–Al, which has an even lower decomposition tem- The decomposition of LiBH4–Al results in the formation of perature via another mechanism and results in the formation of LiAl, AlB2 and Li2B12H12 via several reactions and intermediate Mg Al B as shown in the idealized reaction scheme:465 compounds. Apparently, there is significant formation of x 1 x 2
Li2B12H12, rather than a diborane release or amorphous boron 2LiBH4(s) + xMgH2(s) + (1 x)Al(s) - 2LiH(s) production.452 The hydrogen storage capacity reduces significantly over ten cycles of hydrogen release and uptake for the reactive +MgxAl1 xB2(s) + (4 + x)H2(g) (6.25) 452 composite LiBH4–1.5Al, see Fig. 26.
Lithium borohydride–sodium alanate composite, LiBH4–NaAlH4. The hydrogen storage capacity of the LiBH4–MgH2–Al system Other metal hydrides have also been explored as possible Al sources, decreasesuponcontinuoushydrogencycling(seeTable20). 484 e.g. the system, 2LiBH4–NaAlH4, rm =11.9wt%H2. However, the Boroninthehydrogenatedsample4LiBH4–MgH2–Al exists as samples prepared by ball milling reveal diffraction from both LiAlH4 Li2B12H12 (19.1 mol%) and LiBH4 (80.9 mol%) after three cycles of 487 and NaBH4, due to a metathesis reaction (6.24). hydrogen release and uptake, when dehydrogenated in low hydro- gen pressure, p(H2) = 0.15 bar. However, the ratio Li2B12H12–LiBH4 LiBH (s) + NaAlH (s) - LiAlH (s) + NaBH (s) (6.24) 4 4 4 4 is only 8.7 : 91.3 mol% when the sample is dehydrogenated at Several (coupled) chemical reactions are involved in hydrogen p(H2) = 5.0 bar. For the sample 4LiBH4–MgH2–5Al only 2.4 mol% release and uptake. NaBH4 decomposes at a much lower tempera- ture than normal, at B340 1C, after which some Na vapour is also Table 20 Molar composition of LiBH4–MgH2–Al (4 : 1 : 1) and LiBH4– released due to dynamic vacuum, leaving a LiAl alloy and amor- MgH2–Al (4 : 1 : 5) after three hydrogen release and uptake cycles applying 465 phous boron in the solid state. Sodium borohydride, NaBH4,only different hydrogen backpressure during desorption B partly decomposes to B and NaH at T = 400 1Cunderp(H2) 1bar. Sample p(H2) (bar) Li2B12H12 (mol%) LiBH4 (mol%) The formation of AlB2 is observed at T =4501C along with LiBH4–MgH2–Al (4 : 1 : 1) 0.15 19.1 80.9 AlBx or Al1 xLixB2, which stabilizes boron in the solid state. LiBH4–MgH2–Al (4 : 1 : 1) 5.0 8.7 91.3 The LiBH4–NaAlH4 system is partially reversible and LiBH4 was LiBH4–MgH2–Al (4 : 1 : 5) 0.15 8.0 92.0 rehydrogenated under moderate conditions, T = 400 1Cand LiBH4–MgH2–Al (4 : 1 : 5) 5.0 2.4 97.6
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4 4 4 490–492 Li2B12H12 and 97.6 mol% LiBH4 is formed using the higher anions including [FeH6] ,[CoH5] , and [NiH4] . The dehydrogenation partial pressure of hydrogen, p(H2)=5.0bar. magnesium iron hydride–lithium borohydride system also stores These results show that both sample composition and hydrogen hydrogen reversibly but forms a monometallic boride, FeB, in partial pressure, e.g. p(H2) 4 1bar,duringthermolysisiscrucial the decomposed state. to suppress the formation of more stable closo-boranes, Li B H , 2 12 12 - and thereby preserve the hydrogen storage capacity. 2Mg2FeH6 + 2LiBH4 2LiH + 4MgH2 + 2FeB + 5H2 (6.27)
Manganese borohydride–metal hydrides. Mn(BH4)2 is an interesting candidate for hydrogen storage, owing to its high The enthalpy change for this system is slightly larger, DH = 1 1 1 stability at room temperature, low decomposition temperature 72 4 kJ mol H2 and DS = 147 7JK mol H2, and occurs in the temperature range 310–400 1C.493 of 130–180 1C, high theoretical hydrogen capacity of 9.5 wt% H2 and the high abundance of manganese in the lithosphere. A Yttrium hydride. The reactive hydride composite, 4LiBH4– YH releases B7 wt% H at T B 350 1C following reaction drawback for the utilization of Mn(BH4)2 is that the gas released 3 2 during decomposition contains high levels of diborane. The mecha- scheme (6.28), which corresponds to a calculated hydrogen content 494 nism for decomposition remains not fully understood, in particular of rm =8.5wt%H2. thepossiblesidereactionsthatproducediborane.488 Thus, the 4LiBH +YH - 4LiH + YB + 7.5H (6.28) hydrogen storage capacity is expected to decrease due to the loss 4 3 4 2 of boron from the system during continued hydrogen release and This reaction is reversible at relatively mild rehydrogenation uptake in manganese borohydride. In the reactive hydride compo- 468 conditions, e.g. T =3501Candp(H2) = 90 bar. Also, a hydrogen sites, Mn(BH4)2–MHx (M = Li, Na, Mg, Ca), the formation of the pressure of e.g. p(H2) = 90 bar facilitates the formation of metal more stable M(BH4)x for M = Li, Na and Ca suppresses the release of borides during decomposition.494 diborane and provides a higher hydrogen content in the released gas Rare earth metal hydrides are also used as additives, in during the decomposition (Fig. 27).489 6LiBH –MH composites (M = La, Ce), which provide hydrogen Bimetallic transition metal hydrides. Reactive hydride composites 4 2 release after LiBH4 melting, but below its normal dehydrogenation including Mg2NiH4,Mg2FeH6 etc. have also been explored to temperature.495 PCT measurements performed on 6LiBH –CeH promote hydrogen release and uptake in metal borohydrides. 4 2 provide thermodynamic data for the decomposition reaction The first example was discovered by J. J. Vajo (US, 2010), LiBH4– 1 1 1 DHdec =58 3kJmol H2 and DSdec =113 4JK mol H2 Mg2NiH4, which reversibly stores hydrogen at moderate condi- 496 1 1 1 corresponding to T(1 bar) = 240 32 1C. The change in thermo- tions, DH = 15.4 2 kJ mol H2 and DS = 62.2 3JK mol 490 dynamics suggests the formation of a reactive hydride composite. H2. This is due to the formation of a bimetallic boride in the Summary. The concept to lower both the total reaction decomposed state, see reaction (6.26), which also absorbs enthalpy and thus the hydrogen release temperatures of hydride- hydrogen at moderate condition. borohydride mixtures (RHC) is currently working. In some cases,
4LiBH4 + 5Mg2NiH4 - 2MgNi2.5B2 + 4LiH + 8MgH2 +8H2 quite astonishing properties are found, such as enhanced absorption (6.26) kinetics, in spite of a lowered driving force for absorption, however, the progress reached so far is limited. The sorption temperatures are This result suggests that there could be other similar systems still too high for many technical applications. Less stable borohy- because there are numerous complex transition metal hydride drides, decomposing at lower temperatures, show not only the release of hydrogen but also diborane, as mentioned above. To reach a significant breakthrough hydrides and/or reactive additives have to be found that react (reversibly) with the borohydrides at much lower temperatures and thereby capture the boron and prevent the release of diborane or the production of higher boranes.
6.9 Hydrolysis – decomposition by reaction with water In an alternative process to thermal decomposition, hydrogen can also be released from hydrides in a reaction with water, denoted hydrolysis.497 An advantage is that half of the released hydrogen originates from the water and increases the hydrogen capacity of the system.30,498,499 Sodium borohydride can be stabilized in basic solution with a typical composition of 20%
NaBH4 and 1% NaOH (solubility of NaBH4 is 55 g per 100 mL at 20 1C). The hydrolysis reaction is controlled by a catalyst, e.g. a late transition metal, such as Ru. The catalytic effect of Pt for Fig. 27 Mass spectroscopy signal showing hydrogen (m/z = 2) and the generation of H2 from BH4 hydrolysis was discovered in diborane (m/z = 26) for thermolysis of Mn(BH4)2 (upper plot) and
Mn(BH4)2–MHx (M = Li or Na) (lower plot). Adapted with permission from the early 1960s and this reaction has been extensively investi- ref. 489. Copyright 2014 American Chemical Society. gated on a variety of high-surface area catalysts such as Pt–C.500
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DFT calculations indicate that the hydrolysis reaction takes decomposition products and keep them in intimate close 433 place for BH4 co-adsorbed with H2O on a surface. contact. Thereby, nanoconfinement may improve hydrogen
The low concentration of NaBH4 reduces the storage capacity absorption properties of metal hydrides. Phase segregation of of the system to 4 wt% H2. The product obtained from hydrolysis decomposition products upon bulk dehydrogenation can limit is sodium metaborate NaBO2 and hydrogen, see reaction (6.29). hydrogen uptake reactions, which can be circumvented with nano- The concentration of the reactants should be kept sufficiently confinement. A large amount of heat may also be exchanged during low in order to prevent precipitation of NaBO2, which may hydrogen release and uptake in metal hydrides and heat transport block the catalytic active sites (the solubility of NaBO2 in water may be a kinetic limiting factor. Metal hydride powders have low is 26 g per 100 mL at 20 1C).501 thermal conductivities of B0.1 W m 1 K 1 in contrast to carbon based support materials, which can have high thermal conduc- - NaBH4(aq) + 2H2O(l) 4H2(g) + NaBO2(s) (6.29) tivities, e.g. 12–75 W m 1 K 1 for graphite.510 Nanoconfined chemistry is receiving increasing general interest and is becom- A significant drawback of the hydrolysis approach is that ing an important tool within several research areas.502,511–515 The the dehydrogenated product is typically a stable oxide, e.g. aim of the following is to highlight selected results obtained by sodium metaborate NaBO2, with a heat of formation of DHf = the nanoconfinement of metal borohydrides. 1 1058 kJ mol . The heat of formation for NaBH4, DHf = Nanoconfinement of lithium borohydride, LiBH . Lithium 1 4 191kJmol is significantly less negative, illustrating that a large borohydride can be melt infiltrated in ordered mesoporous amount of energy is required for the preparation of NaBH4 from SiO2 scaffolds (Dmax B 5–10 nm), e.g. at T B 295 1C, and high NaBO2, e.g. from a reaction using molten magnesium. Thus, hydrogen pressure, e.g. p(H2) = 100 bar, can suppress a reaction hydrolysis often provides hydrogen at RT, but is considered difficult with the scaffold, which would otherwise produce lithium 30 to reverse because off-board regeneration is needed. 516 silicates, Li2SiO3 or Li4SiO4. Lithium borohydride has a higher stability when nanocon- 6.10 Nanoconfinement fined in carbon materials and absorbs hydrogen at significantly
Nanoparticles may have significantly different properties com- more mild conditions when compared to bulk LiBH4. Lithium pared to bulk materials due to (i) increased surface area, (ii) borohydride, solvent infiltrated in mesoporous carbon scaf- 510 nanoscale diffusion distances, (iii) increased numbers of atoms folds (Dmax B 4 nm), releases hydrogen at T B 300 1C. About in the grain boundaries and (iv) intimate contact between 70% of the initial hydrogen content is released during the third 502,503 different reacting solids, or a melt. These nanoscale hydrogen release cycle for LiBH4, confined in microporous properties facilitate the release and uptake of hydrogen and activated carbon (Davg o 2 nm), when hydrogen absorption is enhance reaction kinetics. Thermodynamic properties may also conducted at p(H2) = 100 bar, T = 400 1C for two hours. A lower be improved and theoretical studies predict that the increased stability of the hydrogen storage capacity is observed for scaf- surface area to bulk volume destabilizes MgH2 nanoparticles folds with larger pores, e.g. carbon aerogel, Davg = 25 nm, where 323,503–506 and thereby reduces the reaction enthalpy. However, LiBH4 only releases 40% of the initial hydrogen content in the thermodynamic improvements are suggested to occur only for third release cycle. However, the observed increase in hydrogen B MgH2 nanoparticles with sizes less than 5 nm, while kinetic release and uptake rates with decreasing pore size may be a enhancement also occurs for larger nanoparticles below kinetic effect, which may also contribute to the apparently B50 nm.323,506–508 The most widely used technique for the higher cyclic stability. The hydrogen storage capacity appears preparation of hydride nanoparticles is probably mechano- to be stabilized and preserved to a higher degree for nanoconfined chemical treatment, i.e. ball milling. In this top-down approach systems in comparison to bulk systems. This effect may be the size of the bulk material is reduced mechanically, in some associated with a different hydrogen exchange mechanism due to cases to o100 nm. However, the samples are often contami- the nanoconfinement of reactants and decomposition products, i.e. 52 516–519 nated with trace material from the vial and balls, and are Li2B12H12,Li2B10B10,orLiBx. However, systematic investiga- challenging to characterize.509 Furthermore, nanosized parti- tion into the effect of varying pore sizes suggests that diborane cles may grow into larger particles upon continuous hydrogen releaseandtheformationofcloso-boranes may be reduced using release and uptake reactions. Nano materials may be directly carbon scaffolds with smaller pores.520 synthesized and stabilized in a nanoporous scaffold, denoted Nanoconfinement of sodium borohydride, NaBH4. Sodium nanoconfinement, which is discussed below. borohydride has a high decomposition temperature, Tdec 4 Nanoconfinement. The topic for this section is the utiliza- 500 1C, with an onset at B470 1C, which may be reduced to tion of nanoporous materials as scaffolds for the preparation 250 1C upon nanoconfinement in porous carbon. Here, 43% of and confinement of nanosized metal borohydrides. This the hydrogen content is reversible under mild conditions 521 bottom-up approach limits the particle size of the borohydride p(H2) = 60 bar, T = 325 1C for five hours. Encapsulation of to the pore size of the scaffold material, which allows direct NaBH4 nanoparticles by a Ni layer, forming a core shell nano- production of smaller particles than mechanically obtainable. structure, provides effective nanoconfinement of the molten
Furthermore, particle growth and agglomeration may be NaBH4 core and the dehydrogenation products. These nano- hindered by the compartmentalization of nanoparticles within structures have high stability and were cycled 5 times with the scaffold material and may also limit the mobility of the hydrogen release (p(H2) = 1 mbar) and uptake (p(H2) = 40 bar) at
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T = 350 1C, demonstrating a hydrogen capacity of 5 wt% where 390 1C, p(H2) = 2 bar after 20 h, in accordance with the calculated 522 ca. 80% of the H2 capacity was exchanged within 1 h. hydrogen content for the sample of 3.9 wt% H2.Thehydrogen
Nanoconfinement of magnesium borohydride, Mg(BH4)2. The absorption was performed under relatively mild conditions and the synthesis of Mg(BH4)2/carbon nanocomposites from MgH2 nano- fourth desorption released 3.0 wt% H2, i.e. 92 and 74% of the 529 particles supported on carbon aerogel in a B2H6/H2 atmosphere has capacity is preserved over three and four cycles, respectively. been conducted.523 The nanocomposite exhibits a lower kinetic Similar results were obtained with other methods for infiltration barrier and a lower peak hydrogen release temperature, T =1601C, and other scaffold materials.492,530,531 However, improved capacity when compared to bulk Mg(BH4)2. Furthermore, hydrogen uptake as been achieved for 2LiBH4–MgH2 encapsulated within graphene, 523 occurs at mild conditions, T =2001Candp(H2) = 80–150 bar. where an impressive reversible capacity of 8.9 wt% H2 is reported Magnesium borohydride has also been infiltrated into ordered after 25 cycles at 350 1C.532 mesoporous carbon (CMK-3) functionalized by Ni nanoparticles Functionalization of nanoporous scaffolds may further (5 wt%).524 The hydrogen release temperature was significantly improve kinetics during hydrogen release and uptake. The stability 533 534 535 decreased with onset at T =751C and peak release at T =1551C, during cycling was explored for TiCl3, TiCl4 and ZrCl4. compared to 270 1Cand4350 1C, respectively, for pure Mg(BH4)2. Several other composites based on lithium borohydride have also 536 Hydrogen release also occurs with faster kinetics in the nano- been successfully nanoconfined, e.g. 2LiBH4–NaAlH4, LiBH4– 524 537 538 confined system. Mg2NiH4 and LiBH4–NaBH4.
Lithium–potassium borohydride, LiBH4–KBH4. Eutectic melting, reactive hydride composites and nanoconfinement all have the potential to improve the reversible hydrogen storage capacity in 7. New properties and future metal borohydrides. The combined effect of all three methods has perspectives of metal borohydrides been explored in an attempt to improve the lithium–potassium Metal borohydrides have diverse compositions with overwhelmingly borohydride system, 0.725LiBH4–0.275KBH4, which has a low 48 rich structural chemistry, which has led to new perspectives for the melting temperature Tm =1051C. The bulk eutectic mixture future utilization of this fascinating class of new materials. Further- and the RHC with magnesium, LiBH4–KBH4–Mg and LiBH4– more, a wide range of approaches for tailoring the composition and KBH4–MgH2, have been melt-infiltrated into a nanoporous carbon structure of metal borohydrides was discussed in previous sections, scaffold. The decomposition of 0.725LiBH4–0.275KBH4 (and the RHC) initiates simultaneously with melting at 105 1C, but remains along with the effect of additives, reactive hydride composites and slow until higher temperatures are reached T 4 300 1C. Eutectic nanoconfinement etc., in Section 6. This section provides the melting appears to improve the kinetics of the hydrogen de- and state-of-the-art for new perspectives on the utilization of metal absorption, while nanoconfinement lowers the main hydrogen borohydrides for practical applications, in addition to those release temperature in the first cycle by up to 200 1C.48 involving their extreme energy densities. This chapter reveals multiple novel properties for metal borohydrides, e.g. optical, Lithium–magnesium borohydride, LiBH4–Mg(BH4)2. The decomposition of nanoconfined lithium–magnesium borohydride, magnetic, ion or semi conductivity. 11 11 Li BD4–Mg( BD4)2, was investigated by thermal analysis and small-angle neutron scattering.525 The dehydrogenation tempera- 7.1 Porous metal borohydrides for gas adsorption ture was decreased by up to 60 1C in the nanoconfined system and The extreme structural flexibility found for metal borohydrides occurred without the formation of diborane, B2D6,orcloso-boranes, is highlighted by magnesium borohydride, as described in which evolve from the bulk mixture.525 detail in Section 3.1. The g polymorph has 33% ‘empty space’
Lithium–calcium borohydride, LiBH4–Ca(BH4)2. Eutectic and a 3D network of interpenetrated channels with pore sizes melting LiBH4–Ca(BH4)2 has been nanoconfined in different in the range 5.8–8.8 Å, being the first complex metal hydride types of carbon scaffolds with surface areas 500 to 2600 m2 g 1 with large permanent porosity.3 Furthermore, it is the first and pore sizes 5–50 nm. In general, the nanoconfined systems complex metal hydride capable of reversibly adsorbing hydro- provide faster kinetics and higher stability over several cycles of gen, nitrogen and other small molecules such as CH2Cl2. The hydrogen release and uptake, which also occurs at more mild adsorption of dichloromethane, CH2Cl2 is reversible and fast 369,526–528 conditions. Nanoconfined LiBH4–Ca(BH4)2 in CO2- (within minutes), occurring near room temperature with negligible activated scaffolds release up to 80 or 73% of the calculated unit cell expansion, forming g-Mg(BH4)2 0.18CH2Cl2. Here, the only hydrogen content in the second and third hydrogen release cycle, host–guest contacts are the dihydrogen bonds B–Hd d+H–C of respectively. They can potentially reach hydrogen storage densities 1.9 and 2.0 Å. There are neither substantial guest–guest interactions in the range B10–12 wt% H2 for scaffolds with large pore volumes, nor coordination of the guest to the metal atom (Mg H–C 3.5 Å, 3.1 mL g 1, and large surface areas, 2660 208 m2 g 1.527 Mg Cl B 4 Å). The host–guest interactions illustrate the
Reactive hydride composites, 2LiBH4–MgH2. The first selectivity of the anionic borohydride groups being capable of nanoconfined reactive hydride composite (RHC) 2LiBH4–MgH2 binding positively charged moieties. Neutral molecules such as B was infiltrated in a nanoporous RF-CA scaffold (Davg 21 nm) in nitrogen (N2) and hydrogen (H2) also adsorb in the g-Mg(BH4)2 an attempt to combine the beneficial effect from nanoconfinement framework at elevated pressure and low temperature. In situ 529 and RHC. The onset temperature for hydrogen release is reduced SR-PXD data revealed storage capacities of g-Mg(BH4)2 0.63N2 by 90 1Candthenanoconfinedsampledesorbed3.9wt%H2 at at p(N2) = 30.6 bar and g-Mg(BH4)2 0.80H2 at p(H2) = 105 bar at
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193 1C. The latter corresponds to a total of 17.4 wt% H2, including the hydrogen in the framework.3 Nitrogen or hydrogen molecules are grouped into diffuse rods within the structure, centered around (1/8, 1/8, 1/8) and extended along the cube diagonal (x, x, x), which indicates van der Waals interactions occur between gas molecules and BH4 groups. The desorption isobars (p(H2)=105bar,p(N2) = 30.6 bar) indicate that the gas molecules are leaving the framework at relatively high tempera- tures; desorption of hydrogen starts at 143 1C, with B50% H2 remaining inside the pores at 73 1C. This suggests the potential for efficient hydrogen storage in related materials at moderate temperatures. The isosteric heats of adsorption, Qst,ofnitrogen B and hydrogen in g-Mg(BH4)2 were determined to be: Qst(N2) 1 1 15 kJ mol and Qst(H2) B 6kJmol (average value), with Qst 4 1 1 3 7kJmol at a loading of 15 mg H2 g . The latter value is among the highest values measured for MOFs and other porous solids.539 The structural similarity between metal borohydride frame- works and covalently bonded metal organic frameworks (MOFs) is illustrated by the significant degree of directionality in the interaction between a metal and the complex borohydride Fig. 28 Li+ ion conductivity of a range of borohydride-based materials anion. In particular, the same framework topology is observed and higher boranes.88,89,168,546–550 for both g-Mg(BH4)2 and the zinc imidazolate framework ZIF-72,3,540 and several metal borohydrides have interpenetrated framework structures similar to MOFs, e.g. MZn2(BH4)5 (M = mixed-anion electrolytes have been developed that exhibit fast 49 Li and Na) and Li3MZn5(BH4)15 (M = Mg, Mn). Although, there Li- and Na-ion conduction. are other borohydrides that mimic the tetrahedral framework Lithium borohydride halides as Li-ion conductors. Above of cristobalite, such as d-Mg(BH4)2,ora-andb-Cd(BH4)2. the polymorphic transition temperature of 110 1C, the Li ion g-Mg(BH4)2 shares gas adsorption properties with MOFs and conductivity of the HT-polymorph h-LiBH4 is three orders of also shares an ‘amorphous’ to ‘crystalline’ phase transition at magnitude higher than that of the RT-orthorhombic polymorph, elevated pressures. However, metal borohydride frameworks give o-LiBH4. It was shown that anion substitution BH4 2 X (X = rise to specific guest–host interactions with hydridic atoms of Halide) stabilizes the h-LiBH4 polymorph down to room tem- 551 the BH4 anion, which do not occur in MOFs. perature, e.g. h-Li(BH4)1 xIx. The RT Li-ion conductivity in the 5 1 550 halide stabilized h-LiBH4 reaches 2 10 Scm . 7.2 Metal borohydrides as ion conductors Alkali rare-earth spinel-like borohydride halides as Li-ion
Hydrides of intermetallic compounds such as LaNi5H6 have conductors. Recently, a new series of mixed-cation mixed-anion 0 widely been used as electrodes in rechargeable batteries, repla- borohydride chlorides based on rare-earths elements, LiM (BH4)3Cl, cing toxic metal Cd in the Ni–Cd pair. Following the report on M0 = La, Gd, and Ce, was discovered using combined mechano- 541 0 the conversion reaction of the redox couple MgH2/Li many chemical synthesis and heat treatment from M Cl3–LiBH4 (1 : 3) other complex metal hydrides have been studied as battery mixtures.4,88 This growing class of materials has a new structure type 7,542,543 4 electrodes. The use of borohydrides in batteries is very and contains isolated tetranuclear anionic clusters [M4Cl4(BH4)12] + recent, and is actually limited to solid-state electrolytes (Fig. 28). with a distorted cubane M4Cl4 core, charge-balanced by Li cations. This is despite several reactions having been proposed theoretically This compound can also be understood as containing fragments of + by ab initio calculations for conversion electrodes in Li-batteries, the spinel structure, i.e. LiCl4 tetrahedra and M4Cl4 cubane. The Li including a promising reaction, based on Mg(BH4)2, that results in ions are disordered and occupy 2/3 of the 12d Wyckoff sites and DFT 1 544 an enormous theoretical charge capacity of 3969 mA h g . The calculations indicate that LiCe(BH4)3Cl is stabilized by larger entropy numerous advantages of metal borohydrides as battery materials rather than smaller enthalpy. Thus, the structural model and DFT include their light weight (high charge density), apparent material calculations agree well with the very high lithium ion conductivity 4 1 compatibility with Li and Na electrodes, thermal and electro- measured for LiCe(BH4)3Cl, 1.03 10 Scm at T =201C. The chemical stability, and easy manufacturing of the electrolyte/ other members of this series of isostructural compounds, electrode interface and thus of the whole battery. LiLa(BH4)3Cl and LiGd(BH4)3Cl, also have high Li-ion conductivities Fast Li-ion conduction in a borohydride was first discovered at RT and low electronic conductivities, see Table 21. This class of 545 in the high temperature polymorph h-LiBH4, where conductivities materials, LiM(BH4)3Cl, holds significant potential in the design of exceeding 2 10 3 Scm 1 above 110 1C (the polymorphic other new ion conductors, which is not yet fully explored. Cation and transition temperature) were reported. Since then, several anion substitution may allow for the preparation of new sodium or other solid-state metal-borohydride and borohydride-based magnesium ion conductors.
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4,88 Table 21 Ion and electronic conductivity at RT and hydrogen storage properties of LiM(BH4)3Cl, M = La, Ce and Gd
3 3 4 1 8 1 Compound Unit cell a (Å) r (g cm ) rm (wt%) rV (kg H2 m ) Tdec (1C) Li-ion 10 (S cm ) Electronic 10 (S cm )
LiLa(BH4)3Cl 11.7955 1.86 5.36 96.4 260 2.3 1.4 LiCe(BH4)3Cl 11.7204 2.61 5.33 99.8 260 1.03 — LiGd(BH4)3Cl 11.5627 2.16 4.95 102.6 260 3.6 9
Other borohydride-based mixed-anion ion conductors. large-scale facilities, which operate at over 300 1C in the case of Some metal amides and imides are good ion conductors, such Na–S power grids, for instance. as lithium imide Li2NH with an excellent solid-state Li-ion The paddle-wheel mechanism for ion conduction. Many conductivity at RT (2.5 10 4 Scm 1), but with very low potential solid-state ionic conductors can be rationally designed electrochemical stability, 0.7 V.552 Addition reactions between from salts containing dynamically disordered complex anions. In nitrogen-based compounds and metal borohydrides have therefore such materials, the conduction is not purely vacancy-dependent. The n been studied, leading to several novel solid state electrolytes such structure typically contains polyanions [ABy] with covalent A–B 89 3 2 as Li2(BH4)(NH2)andLi4(BH4)(NH2)3 with Li-ion conductivities of bonds (e.g. PO4 ,SO4 and NO3 ), where the rotational diffusion 4 1 n up to 2 10 Scm at RT. The disordered anti-perovskite of [ABy] promotes cationic conductivity, decreasing the asso-
Na2(BH4)(NH2) was also recently shown to have a Na-ion conduc- ciated activation energy via a so-called ‘‘paddle-wheel’’ tivity of 2 10 6 Scm 1 at RT,553 and Mg-ion conductivity in mechanism.556 After the first reports on a possible ‘‘paddle-wheel’’ 6 1 270 tetragonal Mg(BH4)(NH2) reaches 1 10 Scm at 150 1C. mechanism of the complex anion [BH4] in superionic metal Alkali rare-earth garnet borohydrides as Li-ion conductors. borohydrides,557–559 it was shown by solid state NMR and quasi- The oxide-borohydride analogy,193 is a new tool for the design elastic neutron scattering,560,561 that the high rotational mobility of novel materials with useful properties, clearly illustrated promotes superionicity in the respective materials (Fig. 29). by the recently discovery of metal borohydride-garnets. Metal There is also structural evidence for the ‘‘paddle-wheel’’ Ag+ 168 + borohydride-garnets were synthesized, based on the fact migration mechanism in Ag2B12H12,whereAg cations can only that doped lanthanide-oxide garnets Li3+xLa3M2O12 (where M occupy particular atomic sites in the crystal structure for certain 554 2 562 denotes various metals) are excellent Li-ion conductors. The [B12H12] anion orientations. new metal borohydride garnet analogues, Li3K3Ce2(BH4)12 and
Li3K3La2(BH4)12 show high room temperature Li-ion conductivity (3 10 7 and 6 10 7 Scm 1). Most importantly, these values are 5 to 6 orders of magnitude above those of undoped metal oxide garnet conductors. The effect of heterovalent cation substitution was investigated as a means of tailoring the ion conductivity. Substitution with divalent Sr2+ and Eu2+ showed that the Li-ion conductivity can be increased by one order of magnitude in the whole temperature range measured.168 Higher-boranes and mixed anion higher borane-borohydrides 2 as Li- and Na-ion conductors. Higher boranes such as B10H10 2 or B12H12 form inorganic salts which, in the case of Na- and Li- based compounds, are good ionic conductors when the cations become disordered in HT-phases,546,547,555 and are the focus of a 252 recent review. The Na-ion conductivity of Na2B12H12 increases by 3 orders of magnitude between the phase transition from the ordered monoclinic phase to the disordered cubic phase at 256 1C, and reaches a conductivity value of close to 0.1 S cm 1.546 The concept of anion-mixing was applied to this novel class of solid electrolytes generating Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12, capable of conducting either Na+ or both Na+ and Li+ ions.256 Unlike order–disorder governed dodeca- and decaboranes the cationic mobility is not entropically activated in Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12, thus a HT phase transition is not required to enable fast ion conduction. Na3BH4B12H12 reaches 3 1 RT conductivity values close to 10 Scm . (Li0.7Na0.3)3- BH4B12H12, on the other hand, forms only by a reversible chemical reaction at 230 1C, which is accompanied by a 1 1 dramatic increase in ionic conductivity 410 Scm . Such Fig. 29 Polymorphic transition temperatures for fast anion rotational high temperatures are nevertheless within reasonable limits for motion in selected boron-based solids.24,101,132,555,560,563–572
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7.3 Metal borohydrides with optical properties imposed on potential candidates are, in addition to weight and Recent reports on perovskite-type borohydrides motivated volume, the absence of any long-range ordering down to very 576 investigations into lanthanide-luminescence in perovskite and low temperatures, in order to replace costly He-3. The mass 1 of the borohydride ligand is close to that of the oxide anion, double perovskite host lattices such as CsCa(BH4)3 or LiCs2Y(BH4)6. These systems provided the structural simplicity required in order and hence much lighter than most of the organic ligands 577 to study light emission in metal borohydrides, but also enabled employed in MOF materials currently under investigation. tailoring of the emitted spectrum and the efficiency of the light- The magnetic entropy change per unit mass is thus highly emitting process in these potential solid-state borohydride phos- competitive and the recently reported K2Gd(BH4)5 reaches values that lie amongst the 5 highest reported to date,50 and phors. Very little is known about the Eu(II) luminescence in metal hydrides. Kunkel et al. have reported relatively low quantum yields of further increases in the magnetic to non-magnetic mass ratio + 2 + yellow-red emission in simple binary alkaline earth hydrides can be expected due to cation substitution NH4 K . 573 Furthermore, many metal borohydrides, both pseudo-binary EuxM1 xH2 (M = Ca, Sr, Ba) as well as in perovskite hosts based on alkali and alkaline earth metals such as KMgH ,NaMgH and and -ternary, crystallize in packed structure types derived from 3 3 193 574 ccp or hcp, which allows sensible values concerning the LiBaxSr1 xH3, where the emission ranges from yellow to green. magnetic entropy change per unit volume. Both pseudo-binary While such red-shifts of the Eu(II) emission were explained on the basis of the nephelauxetic effect, in metal borohydrides the bright Mn(BH4)2 as well as Mn-bearing double-salts are stable compounds that are paramagnetic down to at least 2 K and their magnetic Eu(II) emission is centred in the blue regime, which is far more refrigeration properties will be investigated in the future. The common for Eu(II). To our knowledge, the first reports on Eu(II) luminescence in a complex hydride date back only very recently,313 thermal conductivities of some simple metal borohydrides lie in 1 1 578,579 and is similar to the unsolvated homoleptic borohydrides.1 These the range of 2–5 W m K , which is appropriate for the heat compounds show little concentration quenching and a high quan- transfer in magnetic cooling devices. tum yield, rare observations for bulk Eu(II)-based phosphors. 7.5 Perspectives for design of novel metal borohydrides CsPb(BH4)3 crystallizes in the perovskite type, and is the first borohydride to show a bandgap in the visible light regime, Borohydrides are beginning to play a role as interesting materi- while metal borohydrides commonly are wide gap insulators als in a variety of technical fields. For instance, a semiconduct- 1 with band gaps above 5 eV.1 This material was synthesized in ing compound CsPb(BH4)3 was recently reported, which view of the current revolution in photovoltaics, triggered by the relates well to the plumbohalides also under investigation for photovoltaics, however, it is not chemically stable for a long hybrid perovskites AMX3, where A is a bulky organic cation time period. Neutron shielding applications were also proposed such as CH3NH3 or HC(NH2)2, M is a group IV metal (Pb, Sn, Ge) and X is a halogen. The substitution X 2 BH may harness due to the high absorption cross section of boron (767 barn for 4 580 alterations to the band gap of the material and hence to the natural isotope abundancy). In this context, the energy transfer light spectrum that may be harvested by the material. from the neutron to the material could also be exploited to develop neutron detectors (or scintillators in general) where a luminescent RE-ion emits light upon irradiation of the host, similar to what has 7.4 Metal borohydrides with magnetic properties been done with borates in the past. Most physical properties related to magnetism arise from long-range The hydrolysis of metal borohydrides has been investigated magnetic order, which is dependent on the exchange interaction since 1953 as a mechanism to quickly generate hydrogen gas between magnetic centers, and requires unpaired electrons in open- from a solid-state storage medium.581 This research field is still shell configurations. The interesting transition metals used to evolving and current research strives for a cost reduction for generate such ordering are prone to reduction by the borohydride using hydrolysis as a reversible energy generation and storage group and hence not stable. Thus, magnetic/electronic correlation system. In a similar manner, metal borohydrides are also has hitherto not been observed in metal borohydrides. Indeed, the investigated as reducing agents in direct borohydride fuel only transition metal with an open shell reported amongst borohy- cells.582 A redox reaction occurs between the borohydride anion drides is Mn, with oxidation state Mn2+.Ontheotherhand,andas itself and an oxidant to generate electricity. These fuel cells shown throughout this review, all di- and many trivalent oxidation offer advantages over traditional fuel cells due to their operating states of rare-earth elements form stable borohydride compounds. temperatures, voltages, and high power densities. Further A series of bimetallic Gd-borohydrides was investigated with the aim reduction reactions have also recently been investigated with of identifying paramagnetic salts and investigating their properties the sulfurization of metal borohydrides.440 related to the magnetic entropy change DSM, which is one of the Perovskite-type compounds were briefly discussed in Section key properties of magnetic refrigerants.50 Owed to its isotropic f7 3.4 and 3.5. It was also mentioned that the homopolar dihydro- electronic configuration, and hence its large ground state spin gen contacts between hydridic species pertaining to adjacent 3+ multiplicity, most competitive coolers have been based on Gd , BH4 ligands generate lattice instabilities. It can be assumed such as the benchmark material GGG (gadolinium gallium garnet that such close contacts constitute a novel kind of ‘‘weak’’ 575 Gd3Ga5O12). Additionally, a paradigm shift is currently taking interaction in perovskites that may be further exploited regard- place, attempting to develop Mn2+-based materials for sub-Kelvin ing the stabilization of low symmetries and the possibility cooling. Similar to most energy-related materials, restrictions of switching between polar bi-stable configurations of the
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1619 Review Article Chem Soc Rev structure, thus potentially culminating in a novel type of properties need to be optimized simultaneously, which span wide geometric ferroelectricity. From an applied viewpoint, such a areas including low cost, fast energy recharging, large gravimetric scheme would be closely related to the supramolecular ferro- and volumetric energy storage densities, low operation temperature electrics where ferroelectricity arises from various kinds of weak etc. This review has illustrated numerous examples of the rational interactions.583 From a fundamental point of view such hydridic design of novel metal borohydrides with interesting properties, e.g. interactions are both highly interesting and timely, forming part using borohydride–oxide analogies, and also the possibilities of of the ever growing family of intriguing weak interactions.584 post-modification of the materials, e.g. by anion substitution etc. Homoleptic metal borohydrides, but also heteroleptic ones, offer Thus, new classes are likely to be discovered in the near future. a unique opportunity of studying this hydridic interaction, which Two decades ago, alloying metals that reacted with hydrogen to currently challenges all levels of ab initio theory. different degrees tended to provide an alloy with intermediate Indeed, homo- and heteropolar interactions between protons properties. During the past decade, reactive hydride composites and hydrides could potentially be meticulously implemented in were explored, which have an average hydrogen storage capa- order to tailor lattice distortions in hybrid organic–inorganic city compared to the individual components. Recently, the first borohydride-based materials. Further means of taking influence stabilization of the metastable compound, NH4BH4 was dis- on symmetry-determining parameters such as order, polarizabil- covered by a reaction with the more stable metal borohydride, ity and dipole moments include anion-substitution of the BH4 Ca(BH4)2. The product, NH4Ca(BH4)3, has a different decom- group by amides or halides, but also within the BH4 group by the position profile and may be considered as the first member of a + scheme H 2 F . As such, this inorganic family of materials larger series of compounds. Notice that, the series BH4 ,NH4 , could be placed into the landscape of molecular and supra- CH4 are isoelectronic and the hydrogen density of NH4BH4, rV = 1 molecular chemistry. In this context, we could extend the focus 157 g H2 mL and rm = 24.5 wt% H2, is even higher than liquid 1 from homoleptic metal borohydrides to the heteroleptic com- natural gas, CH4(l), rV = 137 g H2 mL and rm = 24.8 wt% H2. pounds, where the tetrahydridoboranate anion may be employed Recently, other borane based materials have been discovered, 588 as a hydridic building block in the crystal engineering of e.g. hydrazine bisborane, BH3N2H4BH3. molecular and metalorganic compounds. All in all, this young but vast family of materials offers exotic and enticing new chemistry to materials scientists working in 8. Conclusions energy-related fields. Metal borohydrides have received increasing interest over the past 7.6 CO2-recycling with metal borohydrides decade with a wide range of new materials discovered. An extremely Our current energy system is, to a large extent, based on fossil fuels, rich chemistry has been outlined including fascinating structural i.e. carbon-based materials, and our energy system is not considered flexibility and a wide range of elemental compositions. Furthermore, a ‘closed materials cycle’, because we consume these fuels by derivatives of metal borohydrides have also been discovered, which combustion on a much faster rate than they are produced by strongly contribute to the diverse chemistry. These materials have a photosynthesis and geological processes. Hence, in order to close wide range of unexpected properties, which may lead to new thecarboncycle,wemustfindaroutetoconverttheproducedCO2 applications in the future. The systematic work on crystalline metal into useful fuel. Some metal borohydrides have recently been borohydrides has provided enough results to understand the basic investigated in this context, of CO2 capture and recycling. It has principles which control their crystal structures, and consequently been suggested to generate hydrogen by a CO2-enhanced hydrolysis their properties. One of the most importantunderstandingsisthe of KBH4 where the waste products of fossil fuel combustion, i.e. CO2 description of borohydrides as ionocovalent solids comparable and H2O,canbeusedinafurthercyclefor hydrogen production for with many other inorganic compounds such as oxides or halides. fuel cells.585 The proposed reaction results in the formation of a The crystal chemistry of borohydrides may then be explained by carbonate salt, water and gaseous hydrogen. The porous polymorph known principles such as Pauling rules, involving the polarisation g-Mg(BH4)2 was also found to have potential in CO2 recycling where of the rather soft borohydride anion, various factors based on upon CO2-reduction chemical products such as formate or methoxy- ionic radii etc. A consequences is that the huge crystal chemistry species are produced.586 Owing to the large specific surface area of knowledge accumulated for oxides or halides can be now applied to g-Mg(BH4)2,CO2 conversion occurs with unprecedented fast kinetics borohydrides in designing new compounds with desired chemical at 30 1Candp(CO2) = 1 bar. Metal borohydrides with low surface composition, atomic coordination and, in the best case, with area, such as KBH4, readily react with CO2 under mechanochemical desired properties. conditions at room temperature,587 yielding mainly formylhydrobo- This review has outlined trends across the synthetic approaches rates. Whilst, solid–gas reactions in the KBH4–CO2 system at high for metal borohydrides and their halide and neutral molecule temperature reveal the formation of methanol and methane under complexes, classing reactions into addition, metathesis, or complex water-free conditions. metathesis-addition mechanisms. Trends in structural chemistry have also been listed and over 100 compounds are reported for 7.7 A possible paradigm shift for solid state hydrogen storage mono-, bi- and tri-metallic borohydrides. Many similarities can be The design of a possible successor for liquid fuels like found within certain classes of borohydride, for example those rich diesel and gasoline faces numerous challenges. About twenty in alkali metals, or rare earth metals. The metal borohydride halides
1620 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article can also be classed into those compounds that exhibit completely 2 A. V. Skripov, A. V. Soloninin, O. A. Babanova, H. Hagemann ordered, partly ordered, or completely disordered structures. Many and Y. Filinchuk, J. Phys. Chem. C,2010,114, 12370–12374. new metal borohydrides have been discovered that are complexed 3 Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, with neutral molecules that can exhibit many interesting properties D. Chernyshov and H. Hagemann, Angew. Chem., Int. Ed., suitable for technical applications, but have also been shown to 2011, 50, 11162–11166. influence the decomposition pathway of the parent borohydride. A 4 M. B. Ley, D. B. Ravnsbæk, Y. Filinchuk, Y.-S. Lee, R. Janot, number of metal borohydrides have been shown to melt individually Y. W. Cho, J. Skibsted and T. R. Jensen, Chem. Mater., 2012, or in eutectic compositions, but often decompose shortly after in 24, 1654–1663. often multi-step pathways that are temperature and pressure depen- 5 M. B. Ley, L. H. Jepsen, Y.-S. Lee, Y. W. Cho, J. M. Bellosta dent. The kinetics of hydrogen release and uptake have typically von Colbe, M. Dornheim, M. Rokni, J. O. Jensen, M. Sloth, been poor for metal borohydrides and traditional additives have not Y. Filinchuk, J. E. Jørgensen, F. Besenbacher and been successful in enhancing kinetics, as they have been in many T. R. Jensen, Mater. Today, 2014, 17, 122–128. other metal–hydrogen systems. However, techniques including reac- 6 L. H. Jepsen, M. B. Ley, Y.-S. Lee, Y. W. Cho, M. Dornheim, tive hydride composites and nanoconfinement have offered new J. O. Jensen, Y. Filinchuk, J. E. Jørgensen, F. Besenbacher possibilities in improving kinetics and thermodynamics of hydrogen and T. R. Jensen, Mater. Today, 2014, 17, 129–135. release and uptake. The rehydrogenation of metal borohydrides can 7 A. Unemoto, M. Matsuo and S.-I. Orimo, Adv. Funct. Mater., be performed under more mild conditions by controlling the 2014, 24, 2267–2279. desorption by-products to restrict the quantity of higher boranes, 8 L. H. Rude, T. K. Nielsen, D. B. Ravnsbæk, U. Bo¨senberg, i.e. with lower desorption temperatures or with hydrogen back- M. B. Ley, B. Richter, L. M. Arnbjerg, M. Dornheim, pressure. Recently, metal borohydrides have been studied for new Y. Filinchuk, F. Besenbacher and T. R. Jensen, Phys. Status applications, taking advantage of cation conduction in the solid- Solidi A, 2011, 208, 1754–1773. state, optical properties, porous structures for gas adsorption, and 9 L. Mosegaard, B. Moller, J.-E. Jorgensen, Y. Filinchuk, also magnetic properties. Y. Cerenius, J. C. Hanson, E. Dimasi, F. Besenbacher and The knowledge gained mainly during the past decade of T. R. Jensen, J. Phys. Chem. C, 2008, 112, 1299–1303. research within metal borohydrides is presented in this review, 10 M. D. Riktor, Y. Filinchuk, P. Vajeeston, E. G. Bardaji, which provides analysis of the trends in synthesis, structure and M. Fichtner, H. Fjellvag and M. H. Sørby, J. Mater. Chem., properties. An overview of the current frontiers of research and 2011, 21, 7188–7193. new perspectives for the future utilization of this fascinating 11 Y.-S. Lee, Y. Filinchuk, H.-S. Lee, J.-Y. Suh, J. W. Kim, J.-S. Yu class of new materials is also provided. A wide range of and Y. W. Cho, J. Phys. Chem. C,2011,115, 10298–10304. approaches for tailoring the composition and structure of metal 12 H. I. Schlesinger, R. T. Sanderson and A. B. Burg, J. Am. borohydrides is also discussed, which provides new methods for Chem. Soc., 1940, 62, 3421–3425. tailoring properties. This is an important step towards rational 13 A. B. Burg and H. I. Schlesinger, J. Am. Chem. Soc., 1940, 62, design of new functional materials. This review also demon- 3425–3429. strates that there is still plenty of room for discovering new metal 14 H. I. Schlesinger and H. C. Brown, J. Am. Chem. Soc., 1940, borohydrides since these types of materials have extreme flex- 62, 3429–3435. ibility in composition, structure and properties. 15 H. I. Schlesinger and H. C. Brown, J. Am. Chem. Soc., 1953, 75, 219–221. 16 H. I. Schlesinger, H. C. Brown, B. Abraham, A. C. Bond, Acknowledgements N. Davidson, A. E. Finholt, J. R. Gilbreath, H. Hoekstra, L. Horvitz, E. K. Hyde, J. J. Katz, J. Knight, R. A. Lad, D. L. The work was supported by the Danish National Research Mayfield, L. Rapp, D. M. Ritter, A. M. Schwartz, I. Sheft, Foundation, Center for Materials Crystallography (DNRF93), L. D. Tuck and A. O. Walker, J. Am. Chem. Soc., 1953, 75, The Innovation Fund Denmark (project HyFill-Fast), and by the 186–190. Danish Research Council for Nature and Universe (Danscatt) and 17 Y. Zhu and N. S. Hosmane, Coord. Chem. Rev., 2015, 293– the Danish Council for Independent Research (HyNanoBorN, 294, 357–367. DFF – 4181-00462). MP acknowledges financial support from 18 A. M. Soldate, J. Am. Chem. Soc., 1947, 69, 987–988. The Danish Council for Independent Research for DFF Mobility 19 S. C. Abrahams and J. Kalnajs, J. Chem. Phys., 1954, 22, 1325-00072. We are grateful to the Carlsberg Foundation. This 434–436. work was partly supported by the Swiss National Science Foun- 20 C. J. Dain, A. J. Downs, M. J. Goode, D. G. Evans, K. T. dation and Fonds de la Recherche Scientifique – FNRS. Nicholls, D. W. H. Rankin and H. E. Robertson, J. Chem. Soc., Dalton Trans., 1991, 967–977. References 21 R. Cˇerny´, N. Penin, H. Hagemann and Y. Filinchuk, J. Phys. Chem. C, 2009, 113, 9003–9007. 1 P. Schouwink, M. B. Ley, A. Tissot, H. Hagemann, T. R. 22 A. Zu¨ttel, S. Rentsch, P. Fischer, P. Wenger, P. Sudan, Jensen, L´. Smrcˇok and R. Cˇerny´, Nat. Commun., 2014, P. Mauron and C. Emmenegger, J. Alloys Compd., 2003, 5, 5706. 356–357, 515–520.
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1621 Review Article Chem Soc Rev
23 A. Zuttel, P. Wenger, S. Rentsch, P. Sudan, P. Mauron and 48 E. Roedern, B. R. S. Hansen, M. B. Ley and T. R. Jensen, C. Emmenegger, J. Power Sources, 2003, 118, 1–7. J. Phys. Chem. C, 2015, 119, 25818–25825. 24 J. P. Soulie´, G. Renaudin, R. Cˇerny´ and K. Yvon, J. Alloys 49 M. Matsuo and S.-i. Orimo, Adv. Energy Mater., 2011, 1, Compd., 2002, 346, 200–205. 161–172. 25 L. Schlapbach and A. Zu¨ttel, Nature, 2001, 414, 353–358. 50 P. Schouwink, E. Didelot, Y.-S. Lee, T. Mazet and R. Cˇerny´, 26 A. Zu¨ttel, Naturwissenschaften, 2004, 91, 157–172. J. Alloys Compd., 2016, 664, 378–384. 27 J. A. Ritter, A. D. Ebner, J. Wang and R. Zidan, Mater. 51 A. Almenningen, G. Gundersen and A. Haaland, Chem. Today, 2003, 6, 18–23. Commun., 1967, 557–559. 28 B. K. Sovacool, Energy Policy, 2009, 37, 1529–1541. 52 J. Huot, D. B. Ravnsbæk, J. Zhang, F. Cuevas, M. Latroche 29 D. J. C. MacKay, Sustainable Energy - Without the Hot Air, and T. R. Jensen, Prog. Mater. Sci., 2013, 58, 30–75. UIT, Cambridge, 2009. 53 L. H. Jepsen, M. B. Ley, R. Cˇerny´, Y.-S. Lee, Y. W. Cho, 30 M. Felderhoff, C. Weidenthaler, R. von Helmolt and D. Ravnsbæk, F. Besenbacher, J. Skibsted and T. R. Jensen, U. Eberle, Phys. Chem. Chem. Phys., 2007, 9, 2643–2653. Inorg. Chem., 2015, 54, 7402–7414. 31 M. Fichtner, J. Alloys Compd., 2011, 509, S529–S534. 54 L. H. Jepsen, M. B. Ley, Y. Filinchuk, F. Besenbacher and 32 J. M. Pasini, C. Corgnale, B. A. van Hassel, T. Motyka, T. R. Jensen, ChemSusChem, 2015, 8, 1452–1463. S. Kumar and K. L. Simmons, Int. J. Hydrogen Energy, 2013, 55 E. Wiberg and R. Bauer, Z. Naturforsch., B: J. Chem. Sci., 38, 9755–9765. 1950, 5, 397–398. 33 S. i. Orimo, Y. Nakamori, J. R. Eliseo, A. Zuttel and 56 K. Chłopek, C. Frommen, A. Le´on,O.ZabaraandM.Fichtner, C. M. Jensen, Chem. Rev., 2007, 107, 4111–4132. J. Mater. Chem., 2007, 17, 3496–3503. 34 Q. Lai, M. Paskevicius, D. A. Sheppard, C. E. Buckley, 57 P. Zanella, L. Crociani, N. Masciocchi and G. Giunchi, A. W. Thornton, M. R. Hill, Q. Gu, J. Mao, Z. Huang, Inorg. Chem., 2007, 46, 9039–9041. H. K. Liu, Z. Guo, A. Banerjee, S. Chakraborty, R. Ahuja 58 B. Richter, D. B. Ravnsbaek, N. Tumanov, Y. Filinchuk and and K.-F. Aguey-Zinsou, ChemSusChem, 2015, 8, 2789–2825. T. R. Jensen, Dalton Trans., 2015, 44, 3988–3996. 35 U. Eberle, M. Felderhoff and F. Schuth, Angew. Chem., Int. 59 V. I. Mikheeva and L. V. Titov, Zh. Neorg. Khim., 1964, Ed., 2009, 48, 6608–6630. 9, 789. 36 E. Callini, Z. O¨. K. Atakli, B. C. Hauback, S.-I. Orimo, C. Jensen, 60 H. I. Schlesinger, R. T. Sanderson and A. B. Burg, J. Am. M.Dornheim,D.Grant,Y.W.Cho,P.Chen,B.Hjo¨rvarsson, Chem. Soc., 1939, 61, 536. P. Jongh, C. Weidenthaler, M. Baricco, M. Paskevicius, 61 N. A. Tumanov, D. A. Safin, B. Richter, Z. Lodziana, T. R. T. R. Jensen, M. E. Bowden, T. S. Autrey and A. Zu¨ttel, Appl. Jensen, Y. Garcia and Y. Filinchuk, Dalton Trans., 2015, 44, Phys. A: Mater. Sci. Process., 2016, 122,353. 6571–6580. 37 G. Barkhordarian, T. Klassen, M. Dornheim and 62 T. Jaron´, P. A. Orłowski, W. Wegner, K. J. Fijałkowski, R. Bormann, J. Alloys Compd., 2007, 440, L18–L21. P. J. Leszczyn´ski and W. Grochala, Angew. Chem., Int. Ed., 38 J. J. Vajo, F. Mertens, C. C. Ahn, R. C. Bowman and B. Fultz, 2015, 54, 1236–1239. J. Phys. Chem. B, 2004, 108, 13977–13983. 63 T. Jaron´, W. Wegner, K. J. Fijałkowski, P. J. Leszczyn´ski and 39 J. J. Vajo, S. L. Skeith and F. Mertens, J. Phys. Chem. B, 2005, W. Grochala, Chem. – Eur. J., 2015, 21, 5689–5692. 109, 3719–3722. 64 R. A. Varin, L. Zbroniec, M. Polanski, Y. Filinchuk and 40 U. Bo¨senberg, S. Doppiu, L. Mosegaard, G. Barkhordarian, R. Cˇerny´, Int. J. Hydrogen Energy, 2012, 37, 16056–16069. N. Eigen, A. Borgschulte, T. R. Jensen, Y. Cerenius, 65 R. Cˇerny´, N. Penin, V. D’Anna, H. Hagemann, E. Durand O. Gutfleisch, T. Klassen, M. Dornheim and R. Bormann, and J. Ru˚ˇicz ˇka, Acta Mater., 2011, 59, 5171–5180. Acta Mater., 2007, 55, 3951–3958. 66 M. B. Ley, M. Paskevicius, P. Schouwink, B. Richter, D. A. 41 S. V. Alapati, J. K. Johnson and D. S. Sholl, J. Phys. Chem. B, Sheppard, C. E. Buckley and T. R. Jensen, Dalton Trans., 2006, 110, 8769–8776. 2014, 43, 13333–13342. 42 Y. W. Cho, J. H. Shim and B. J. Lee, CALPHAD: Comput. 67 M. B. Ley, M. Jørgensen, R. Cˇerny´, Y. Filinchuk and T. R. Coupling Phase Diagrams Thermochem., 2006, 30, 65–69. Jensen, Inorg. Chem., 2016, 55, 9748–9756. 43 J. Huot, Handbook of Hydrogen Storage, Wiley-VCH Verlag 68 T. D. Humphries, M. B. Ley, C. Frommen, K. T. Munroe, GmbH & Co. KGaA, 2010, ch. 4, pp. 81–116, DOI: 10.1002/ T. R. Jensen and B. C. Hauback, J. Mater. Chem. A, 2015, 3, 9783527629800. 691–698. 44 H. H. Van Mal, K. H. J. Buschow and A. R. Miedema, J. Less- 69 A. R. West, Solid State Chemistry and its Applications, John Common Met., 1974, 35, 65–76. Wiley and Sons Ltd, 1984. 45 M. Paskevicius, M. B. Ley, D. A. Sheppard, T. R. Jensen and 70 D. B. Ravnsbæk, Y. Filinchuk, Y. Cerenius, H. J. Jakobsen, C. E. Buckley, Phys. Chem. Chem. Phys., 2013, 15, 19774–19789. F. Besenbacher, J. Skibsted and T. R. Jensen, Angew. Chem., 46 Z. Zhao-Karger, R. Witter, E. G. Bardaji, D. Wang, Int. Ed., 2009, 48, 6659–6663. D. Cossement and M. Fichtner, J. Mater. Chem. A, 2013, 71 I. Llamas-Jansa, N. Aliouane, S. Deledda, J. E. Fonneløp, 1, 3379–3386. C. Frommen, T. Humphries, K. Lieutenant, S. Sartori, 47 M. B. Ley, E. Roedern and T. R. Jensen, Phys. Chem. Chem. M. H. Sørby and B. C. Hauback, J. Alloys Compd., 2012, Phys., 2014, 16, 24194–24199. 530, 186–192.
1622 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
72 J. E. Olsen, M. H. Sørby and B. C. Hauback, J. Alloys 95O.Friedrichs,A.Borgschulte,S.Kato,F.Buchter,R.Gremaud, Compd., 2011, 509, L228–L231. A. Remhof and A. Zu¨ttel, Chem. – Eur. J., 2009, 15, 5531–5534. 73 J. W. Kim, J.-H. Shim, J.-P. Ahn, Y. W. Cho, J.-H. Kim and 96 O. Friedrichs, J. W. Kim, A. Remhof, D. Wallacher, K. H. Oh, Mater. Lett., 2008, 62, 2461–2464. A. Hoser, Y. W. Cho, K. H. Oh and A. Zuttel, Phys. Chem. 74 S. Doppiu, L. Schultz and O. Gutfleisch, J. Alloys Compd., Chem. Phys., 2010, 12, 4600–4603. 2007, 427, 204–208. 97 O. Friedrichs, A. Remhof, A. Borgschulte, F. Buchter, 75 M. Fichtner, C. Frommen and O. Fuhr, Inorg. Chem., 2005, S. I. Orimo and A. Zuttel, Phys. Chem. Chem. Phys., 2010, 44, 3479–3484. 12, 10919–10922. 76 H. W. Brinks, A. Istad-Lem and B. C. Hauback, J. Phys. 98 A. Remhof, A. Borgschulte, O. Friedrichs, P. Mauron, Chem. B, 2006, 110, 25833–25837. Y. Yan and A. Zu¨ttel, Scr. Mater., 2012, 66, 280–283. 77 P. Kuziora, M. Wyszyn´ska, M. Polanski and J. Bystrzycki, 99 K. Park, H.-S. Lee, A. Remhof, Y.-S. Lee, Y. Yan, M.-Y. Kim, Int. J. Hydrogen Energy, 2014, 39, 9883–9887. S. J. Kim, A. Zu¨ttel and Y. W. Cho, Int. J. Hydrogen Energy, 78 Y. Sadikin, K. Stare, P. Schouwink, M. Brix Ley, T. R. 2013, 38, 9263–9270. Jensen, A. Meden and R. Cˇerny´, J. Solid State Chem., 100 Y. Yan, A. Remhof, D. Rentsch, Y.-S. Lee, Y. Whan Cho and 2015, 225, 231–239. A. Zuttel, Chem. Commun., 2013, 49, 5234–5236. 79 V. N. Konoplev, Russ. J. Inorg. Chem., 1980, 25, 964–966. 101 Y. Filinchuk, D. Chernyshov and R. Cˇerny´, J. Phys. Chem. C, 80 R. Cˇerny´, G. Severa, D. B. Ravnsbæk, Y. Filinchuk, V. d’Anna, 2008, 112, 10579–10584. H. Hagemann, D. Haase, C. M. Jensen and T. R. Jensen, 102 D. Chernyshov, A. Bosak, V. Dmitriev, Y. Filinchuk and J. Phys. Chem. C, 2010, 114, 1357–1364. H. Hagemann, Phys. Rev. B: Condens. Matter Mater. Phys., 81 H. Hagemann, M. Longhini, J. W. Kaminski, T. A. Wesolowski, 2008, 78, 172104. R. Cˇerny´, N. Penin, M. H. Sørby, B. C. Hauback, G. Severa and 103 Y. Filinchuk and H. Hagemann, Eur. J. Inorg. Chem., 2008, C. M. Jensen, J. Phys. Chem. A, 2008, 112, 7551–7555. 3127–3133. 82 I. Solinas and H. D. Lutz, J. Solid State Chem., 1995, 117, 104 R. L. Luck and E. J. Schelter, Acta Crystallogr., Sect. C: Cryst. 34–38. Struct. Commun., 1999, 55, IUC9900151. 83 R. Cˇerny´, P. Schouwink, Y. Sadikin, K. Stare, L. Smrcˇok, 105 D. S. Marynick and W. N. Lipscomb, Inorg. Chem., 1972, 11, B. Richter and T. R. Jensen, Inorg. Chem., 2013, 52, 9941–9947. 820–823. 84 D. B. Ravnsbæk, L. H. Sørensen, Y. Filinchuk, D. Reed, 106 Y. Filinchuk, R. Cˇerny´ and H. Hagemann, Chem. Mater., D. Book, H. J. Jakobsen, F. Besenbacher, J. Skibsted and 2009, 21, 925–933. T. R. Jensen, Eur. J. Inorg. Chem., 2010, 1608–1612. 107 S.Aldridge,A.J.Blake,A.J.Downs,R.O.Gould,S.Parsonsand 85 R. Cˇerny´, D. B. Ravnsbæk, P. Schouwink, Y. Filinchuk, C. R. Pulham, J. Chem. Soc., Dalton Trans., 1997, 1007–1012. Y. Penin, J. Teyssier, L. Smrcˇok and T. R. Jensen, J. Phys. 108 P. Schouwink, A. Ramel, E. Giannini and R. Cˇerny´, Cryst- Chem. C, 2012, 116, 1563–1571. EngComm, 2015, 17, 2682–2689. 86 L. H. Rude, O. Zavorotynska, L. M. Arnbjerg, D. B. 109 L. H. Rude, M. Corno, P. Ugliengo, M. Baricco, Y.-S. Lee, Ravnsbæk, R. A. Malmkjær, H. Grove, B. C. Hauback, Y. W. Cho, F. Besenbacher, J. Overgaard and T. R. Jensen, M. Baricco, Y. Filinchuk, F. Besenbacher and T. R. Jensen, J. Phys. Chem. C, 2012, 116, 20239–20245. Int. J. Hydrogen Energy,2011,36, 15664–15672. 110 B. Hilczer, M. Połomska, A. Pawłowski and J. Wolak, Ferro- 87 D. B. Ravnsbæk, M. B. Ley, Y.-S. Lee, H. Hagemann, electrics, 1994, 155, 177–182. V. D’Anna, Y. W. Cho, Y. Filinchuk and T. R. Jensen, Int. 111 R. Custelcean and J. E. Jackson, Chem. Rev., 2001, 101, J. Hydrogen Energy, 2012, 37, 8428–8438. 1963–1980. 88 M. B. Ley, S. Boulineau, R. Janot, Y. Filinchuk and 112 H. Yamawaki, H. Fujihisa, Y. Gotoh and S. Nakano, J. Alloys T. R. Jensen, J. Phys. Chem. C, 2012, 116, 21267–21276. Compd., 2012, 541, 111–114. 89 M. Matsuo, A. Remhof, P. Martelli, R. Caputo, M. Ernst, 113 T. Noritake, M. Aoki, M. Matsumoto, K. Miwa, S. Towata, Y. Miura, T. Sato, H. Oguchi, H. Maekawa, H. Takamura, H. W. Li and S. Orimo, J. Alloys Compd., 2010, 491, 57–62. A. Borgschulte, A. Zu¨ttel and S. Orimo, J. Am. Chem. Soc., 114 H. Wu, W. Zhou, F. E. Pinkerton, M. S. Meyer, G. Srinivas, 2009, 131, 16389–16391. T. Yildirim, T. J. Udovic and J. J. Rush, J. Mater. Chem., 90 T. Noritake, K. Miwa, M. Aoki, M. Matsumoto, S.-I. Towata, 2010, 20, 6550–6556. H.-W. Li and S.-I. Orimo, Int. J. Hydrogen Energy, 2013, 38, 115 L. H. Jepsen, V. Ban, K. T. Møller, Y.-S. Lee, Y. W. Cho, 6730–6735. F. Besenbacher, Y. Filinchuk, J. Skibsted and T. R. Jensen, 91M.Somer,S.Acar,C.Koz,I.Kokal,P.Ho¨hn, R. Cardoso-Gil, J. Phys. Chem. C, 2014, 118, 12141–12153. U. Aydemir and L. Akselrud, J. Alloys Compd., 2010, 491, 98–105. 116 I. Dovgaliuk, C. S. Le Duff, K. Robeyns, M. Devillers and 92 L. Tuan, C. K. Nguyen and T. D. Huan, Phys. Status Solidi B, Y. Filinchuk, Chem. Mater., 2015, 27, 768–777. 2014, 251, 1539–1544. 117 T.He,H.Wu,J.Chen,W.Zhou,G.Wu,Z.Xiong,T.Zhangand 93 S. R. H. Jensen, L. H. Jepsen, J. Skibsted and T. R. Jensen, P. Chen, Phys. Chem. Chem. Phys., 2013, 15, 10487–10493. J. Phys. Chem. C, 2015, 119, 27919–27929. 118 T. He, H. Wu, G. Wu, J. Wang, W. Zhou, Z. Xiong, J. Chen, 94 E. Roedern and T. R. Jensen, Inorg. Chem., 2015, 54, T. Zhang and P. Chen, Energy Environ. Sci., 2012, 5, 10477–10482. 5686–5689.
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1623 Review Article Chem Soc Rev
119 A. B. E. Zirngieb, W. Ger. Pat., 070148, 1959. 141 Y. Filinchuk, A. V. Talyzin, H. Hagemann, V. Dmitriev, 120 M. Mostajeran, D. J. Wolstenholme, C. Frazee, G. S. McGrady D. Chernyshov and B. Sundqvist, Inorg. Chem., 2010, 49, andR.T.Baker,Chem. Commun., 2016, 52, 2581–2584. 5285–5292. 121 R. Cˇerny´ and Y. Filinchuk, Z. Kristallogr., 2011, 226, 882–891. 142 D. B. Ravnsbæk, E. A. Nickels, R. Cˇerny´, C. H. Olesen, 122 D. B. Ravnsbæk, Y. Filinchuk, R. Cˇerny´ and T. R. Jensen, W. I. F. David, P. P. Edwards, Y. Filinchuk and T. R. Jensen, Z. Kristallogr., 2010, 225, 557–569. Inorg. Chem., 2013, 52, 10877–10885. 123 Y. Filinchuk, D. Chernyshov and V. Dmitriev, in Boron hydrides, 143 M. Sharma, E. Didelot, A. Spyratou, L. M. L. Daku, R. Cˇerny´ high potential hydrogen storage materials, ed. U. B. Demirci and and H. Hagemann, Inorg. Chem., 2016, 55, 7090–7097. P. Miele, Nova publishers, 2010, pp. 138–164. 144 T. Sato, K. Miwa, Y. Nakamori, K. Ohoyama, H.-W. Li, 124B.R.S.Hansen,K.T.Moller,M.Paskevicius,A.-C.Dippel, T. Noritake, M. Aoki, S.-I. Towata and S.-I. Orimo, Phys. P.Walter,C.J.Webb,C.Pistidda,N.Bergemann,M.Dornheim, Rev. B: Condens. Matter Mater. Phys., 2008, 77, 104114. T. Klassen, J.-E. Jorgensen and T. R. Jensen, J. Appl. Crystallogr., 145 C. Frommen, N. Aliouane, S. Deledda,J.E.Fonneløp,H.Grove, 2015, 48, 1234–1241. K. Lieutenant, I. Llamas-Jansa, S. Sartori, M. H. Sørby and 125 T. R. Jensen, T. K. Nielsen, Y. Filinchuk, J.-E. Jorgensen, B. C. Hauback, J. Alloys Compd., 2010, 496, 710–716. Y. Cerenius, E. M. Gray and C. J. Webb, J. Appl. Crystallogr., 146D.B.Ravnsbæk,L.H.Sørensen,Y.Filinchuk,F.Besenbacher 2010, 43, 1456–1463. andT.R.Jensen,Angew. Chem., Int. Ed., 2012, 51, 3582–3586. 126 R. W. Broach, I. S. Chuang, T. J. Marks and J. M. Williams, 147 J. E. Olsen, C. Frommen, T. R. Jensen, M. D. Riktor, M. H. Inorg. Chem., 1983, 22, 1081–1084. Sorby and B. C. Hauback, RSC Adv., 2014, 4, 1570–1582. 127 P. H. Bird and M. R. Churchill, Chem. Commun., 1967, 403. 148 W. Wegner, T. Jaron´ and W. Grochala, Int. J. Hydrogen 128 J. H. Her, P. W. Stephens, Y. Gao, G. L. Soloveichik, Energy, 2014, 39, 20024–20030. J. Rijssenbeek, M. Andrus and J. C. Zhao, Acta Crystallogr., 149 B. Richter, Aarhus University, Denmark, Personal Commu- Sect. B: Struct. Sci., 2007, 63, 561. nication, 2016. 129 R. Cˇerny´, Y. Filinchuk, H. Hagemann and K. Yvon, Angew. 150 J. E. Olsen, C. Frommen, M. H. Sorby and B. C. Hauback, Chem., Int. Ed., 2007, 46, 5765–5767. RSC Adv., 2013, 3, 10764–10774. 130 E. A. Nickels, M. O. Jones, W. I. F. David, S. R. Johnson, 151 H. R. Hoekstra and J. J. Katz, J. Am. Chem. Soc., 1949, 71, R. L. Lowton, M. Sommariva and P. P. Edwards, Angew. 2488–2492. Chem., Int. Ed., 2008, 47, 2817–2819. 152 A. C. Dunbar, PhD thesis, University of Illinois, Urbana- 131 Y. Filinchuk, D. Chernyshov, A. Nevidomskyy and Champaign, USA, 2011. V. Dmitriev, Angew. Chem., Int. Ed., 2008, 47, 529–532. 153 E. R. Bernstein, W. C. Hamilton, T. A. Keiderling, S. J. La 132 O. A. Babanova, A. V. Soloninin, A. P. Stepanov, A. V. Skripov Placa, S. J. Lippard and J. J. Mayerle, Inorg. Chem., 1972, 11, and Y. Filinchuk, J. Phys. Chem. C, 2010, 114, 3712–3718. 3009–3016. 133 Y. Filinchuk, A. V. Talyzin, D. Chernyshov and V. Dmitriev, 154 P. Charpin, M. Nierlich, D. Vigner, M. Lance and D. Baudry, Phys. Rev. B: Condens. Matter Mater. Phys.,2007,76, 092104. Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,1987,43, 134 M. P. Pitt, C. J. Webb, M. Paskevicius, D. Sheptyakov, C. E. 1465–1467. Buckley and E. M. Gray, J. Phys. Chem. C, 2011, 115, 155 R. H. Banks, N. M. Edelstein, B. Spencer, D. H. Templeton 22669–22679. and A. Zalkin, J. Am. Chem. Soc., 1980, 102, 620–623. 135 G. Renaudin, S. Gomes, H. Hagemann, L. Keller and 156 P. Schouwink, L. Smrcok and R. Cˇerny´, Acta Crystallogr., K. Yvon, J. Alloys Compd., 2004, 375, 98–106. Sect. B: Struct. Sci., Cryst. Eng. Mater., 2014, 70, 871–878. 136 R. S. Kumar, E. Kim and A. L. Cornelius, J. Phys. Chem. C, 157 R. Cˇerny´, K. C. Kim, N. Penin, V. D’Anna, H. Hagemann 2008, 112, 8452–8457. and D. S. Sholl, J. Phys. Chem. C, 2010, 114, 19127–19133. 137 Y. Filinchuk, E. Ro¨nnebro and D. Chandra, Acta Mater., 158D.B.Ravnsbæk,C.Frommen,D.Reed,Y.Filinchuk,M.Sørby, 2009, 57, 732–738. B. C. Hauback, H. J. Jakobsen, D. Book, F. Besenbacher, 138 F. Buchter, Z. Łodziana, A. Remhof, O. Friedrichs, J.SkibstedandT.R.Jensen,J. Alloys Compd., 2011, 509, A. Borgschulte, P. Mauron, A. Zu¨ttel, D. Sheptyakov, S698–S704. G. Barkhordarian, R. Bormann, K. Chlopek, M. Fichtner, 159 I. Lindemann, R. Dome`nech Ferrer, L. Dunsch, Y. Filinchuk, M. H. Sorby, M. D. Riktor, B. C. Hauback and S. Orimo, R. Cˇerny´,H.Hagemann,V.D’Anna,L.M.LawsonDaku, J. Phys. Chem. B, 2008, 112, 8042–8048. L. Schultz and O. Gutfleisch, Chem. – Eur. J., 2010, 16, 139 F. Buchter, Z. Łodziana, A. Remhof, O. Friedrichs, 8707–8712. A. Borgschulte, P. Mauron, A. Zu¨ttel, D. Sheptyakov, 160 I. Dovgaliuk, Synthesis, structure and properties of Al-based L. Palatinus, K. Chłopek, M. Fichtner, G. Barkhordarian, borohydrides for hydrogen storage, PhD thesis, 2016, http:// R. Bormann and B. C. Hauback, J. Phys. Chem. C, 2009, dial.uclouvain.be/handle/boreal:169194?site_name=UCL. 113, 17223–17230. 161 E.Roedern,Y.-S.Lee,M.B.Ley,K.Park,Y.W.Cho,J.Skibsted 140 N. A. Tumanov, E. Roedern, Z. Łodziana, D. B. Nielsen, andT.R.Jensen,J. Mater. Chem. A, 2016, 4, 8793–8802. T. R. Jensen, A. V. Talyzin, R. Cˇerny´, D. Chernyshov, 162 S. P. GharibDoust, M. Heere, M. H. Sørby, M. B. Ley, V. Dmitriev, T. Palasyuk and Y. Filinchuk, Chem. Mater., D. B. Ravnsbæk, C. H. Bjørn, R. Cˇerny´ and T. R. Jensen, 2016, 28, 274–283. Dalton Trans., 2016, 45, 19002–19011.
1624 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
163 W. Wegner, T. Jaron´ and W. Grochala, Acta Crystallogr., 186 M. Fichtner, K. Chłopek, M. Longhini and H. Hagemann, Sect. C: Cryst. Struct. Commun., 2013, 69, 1289–1291. J. Phys. Chem. C, 2008, 112, 11575–11581. 164 P. Schouwink, V. D’Anna, M. B. Ley, L. M. Lawson Daku, 187 M. Paskevicius, M. P. Pitt, C. J. Webb, D. A. Sheppard, B. Richter, T. R. Jensen, H. Hagemann and R. Cˇerny´, U. Filsø, E. M. Gray and C. E. Buckley, J. Phys. Chem. C, J. Phys. Chem. C, 2012, 116, 10829–10840. 2012, 116, 15231–15240. 165 K. T. Møller, M. B. Ley, P. Schouwink, R. Cˇerny´ and 188 W. I. F. David, S. K. Callear, M. O. Jones, P. C. Aeberhard, T. R. Jensen, Dalton Trans., 2016, 45, 831–840. S. D. Culligan, A. H. Pohl, S. R. Johnson, K. R. Ryan, 166 R. Cˇerny´, D. B. Ravnsbæk, G. Severa, Y. Filinchuk, V. D’Anna, J. E. Parker, P. P. Edwards, C. J. Nuttall and A. Amieiro- H. Hagemann, D. Haase, J. Skibsted, C. M. Jensen and T. R. Fonseca, Phys. Chem. Chem. Phys., 2012, 14, 11800–11807. Jensen, J. Phys. Chem. C, 2010, 114, 19540–19549. 189 A. Amieiro-Fonseca, S. R. Ellis, C. J. Nuttall, B. E. Hayden, 167 T. Jaron and W. Grochala, Dalton Trans., 2011, 40, S. Guerin, G. Purdy, J. P. Soulie, S. K. Callear, S. D. Culligan, 12808–12817. W. I. F. David, P. P. Edwards, M. O. Jones, S. R. Johnson and 168 M. Brighi, P. Schouwink, Y. Sadikin and R. Cˇerny´, J. Alloys A. H. Pohl, Faraday Discuss., 2011, 151, 369–384. Compd., 2016, 662, 388–395. 190 J.-H. Her, P. W. Stephens, Y. Gao, G. L. Soloveichik, 169 I. Dovgaliuk, V. Ban, Y. Sadikin, R. Cˇerny´, L. Aranda, J. Rijssenbeek, M. Andrus and J.-C. Zhao, Acta Crystallogr., N. Casati, M. Devillers and Y. Filinchuk, J. Phys. Chem. C, Sect. B: Struct. Sci., 2007, 63, 561–568. 2014, 118, 145–153. 191 R. Cˇerny´, Y. Filinchuk, H. Hagemann and K. Yvon, Angew. 170 W. Wegner, T. Jaron, M. A. Dobrowolski, L. Dobrzycki, Chem., 2007, 119, 5867–5869. M. K. Cyranski and W. Grochala, Dalton Trans., 2016, 45, 192 B. Dai, D. S. Sholl and J. K. Johnson, J. Phys. Chem. C, 2008, 14370–14377. 112, 4391–4395. 171 T. Jaron, W. Wegner and W. Grochala, Dalton Trans., 2013, 193 R. Cˇerny´ and P. Schouwink, Acta Crystallogr., Sect. B: Struct. 42, 6886–6893. Sci., Cryst. Eng. Mater., 2015, 71, 619–640. 172 P. Schouwink, M. B. Ley, T. R. Jensen, L. Smrcok and 194 V. Ban, A. V. Soloninin, A. V. Skripov, J. Hadermann, R. Cˇerny´, Dalton Trans., 2014, 43, 7726–7733. A. Abakumov and Y. Filinchuk, J. Phys. Chem. C, 2014, 173 N. Bindzus, F. Cargnoni, B. B. Iversen and C. Gatti, J. Phys. 118, 23402–23408. Chem. C, 2013, 117, 2308–2316. 195 L. George, V. Drozd, S. K. Saxena, E. G. Bardaji and 174 N. Bindzus, F. Cargnoni, C. Gatti, B. Richter, T. R. Jensen, M. Fichtner, J. Phys. Chem. C, 2009, 113, 486–492. M. Takata and B. B. Iversen, Comput. Theor. Chem., 2015, 196 T. Matsunaga, F. Buchter, P. Mauron, M. Bielman, 1053, 245–253. Y. Nakamori, S. Orimo, N. Ohba, K. Miwa, S. Towata and 175 F. Buchter, Z. Lodziana, A. Remhof, P. Mauron, O. Friedrichs, A. Zu¨ttel, J. Alloys Compd., 2008, 459, 583–588. A. Borgschulte, A. Zu¨ttel,Y.FilinchukandL.Palatinus,Phys. 197 D. B. Ravnsbæk, Y. Filinchuk, R. Cˇerny´, M. B. Ley, D. Haase, Rev. B: Condens. Matter Mater. Phys., 2011, 83, 064107. H. J. Jakobsen, J. Skibsted and T. R. Jensen, Inorg. Chem., 176 N. Bindzus, F. Cargnoni, B. B. Iversen and C. Gatti, J. Phys. 2010, 49, 3801–3809. Chem. C, 2015, 119, 12109–12118. 198 T. Jaron and W. Grochala, Dalton Trans., 2010, 39, 160–166. 177 M. R. Hartman, J. J. Rush, T. J. Udovic, R. C. Bowman and 199 Y.-S. Lee, J.-H. Shim and Y. W. Cho, J. Phys. Chem. C, 2010, S. J. Hwang, J. Solid State Chem., 2007, 180, 1298–1305. 114, 12833–12837. 178 Y. Filinchuk and D. Chernyshov, Acta Crystallogr., Sect. A: 200 H. Hagemann and R. Cˇerny´, Dalton Trans., 2010, 39, Found. Crystallogr., 2007, 63, s240–s241. 6006–6012. 179 Y. Yao and D. D. Klug, Phys. Rev. B: Condens. Matter Mater. 201 F. C. Gennari and M. R. Esquivel, J. Alloys Compd., 2009, Phys., 2012, 86, 064107. 485, L47–L51. 180 S. Nakano, H. Fujihisa, H. Yamawaki and T. Kikegawa, 202 B. J. Zhang, B. H. Liu and Z. P. Li, J. Alloys Compd., 2011, J. Phys. Chem. C, 2015, 119, 3911–3917. 509, 751–757. 181 V. Dmitriev, Y. Filinchuk, D. Chernyshov, A. V. Talyzin, 203 J. Andrade-Gamboa, J. A. Puszkiel, L. Ferna´ndez-Albanesi A. Ilewski, O. Andersson, B. Sundqvist and A. Kurnosov, and F. C. Gennari, Int. J. Hydrogen Energy, 2010, 35, Phys. Rev. B: Condens. Matter Mater. Phys.,2008,77, 174112. 10324–10328. 182 P. Fischer and A. Zu¨ttel, Mater. Sci. Forum, 2004, 443–444, 204 G. Wulfsberg, Principles of Descriptive Inorganic Chemistry, 287–290. University Science Books, 1991. 183 M. D. Riktor, M. H. Sorby, K. Chlopek, M. Fichtner, 205 D. A. Knight, R. Zidan, R. Lascola, R. Mohtadi, C. Ling, F. Buchter, A. Zuttel and B. C. Hauback, J. Mater. Chem., P. Sivasubramanian, J. A. Kaduk, S.-J. Hwang, D. Samanta 2007, 17, 4939–4942. and P. Jena, J. Phys. Chem. C, 2013, 117, 19905–19915. 184 K. Miwa, M. Aoki, T. Noritake, N. Ohba, Y. Nakamori, 206 Z. Z. Fang, L. P. Ma, X. D. Kang, P. J. Wang, P. Wang and S. Towata, A. Zu¨ttel and S. Orimo, Phys. Rev. B: Condens. H. M. Cheng, Appl. Phys. Lett., 2009, 94, 044104. Matter Mater. Phys., 2006, 74, 155122. 207 A. Jarid, A. Lledos, Y. Jean and F. Volatron, Inorg. Chem., 185 P. C. Aeberhard, K. Refson, P. P. Edwards and W. I. F. 1993, 32, 4695–4699. David, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 208 R. G. Egdell and J. C. Green, Inorg. Chim. Acta, 2008, 361, 83, 174102. 462–466.
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1625 Review Article Chem Soc Rev
209 D. M. Goedde and G. S. Girolami, Inorg. Chem., 2006, 45, 234 L. M. Arnbjerg, D. B. Ravnsbæk, Y. Filinchuk, R. T. Vang, 1380–1388. Y. Cerenius, F. Besenbacher, J.-E. Jørgensen, H. J. Jakobsen 210 E. Callini, P. A. Szilagyi, M. Paskevicius, N. P. Stadie, and T. R. Jensen, Chem. Mater., 2009, 21, 5772–5782. J. Rehault, C. E. Buckley, A. Borgschulte and A. Zuttel, 235 M. Quilichini, P. Bernede, J. Lefebvre and P. Schweiss, Chem. Sci., 2016, 7, 666–672. J. Phys.: Condens. Matter, 1990, 2, 4543–4558. 211 L. Seballos, J. Z. Zhang, E. Ro¨nnebro, J. L. Herberg and 236 H. K. Roobottom, H. D. B. Jenkins, J. Passmore and E. H. Majzoub, J. Alloys Compd., 2009, 476, 446–450. L. Glasser, J. Chem. Educ., 1999, 76, 1570–1573. 212 J. Mullay, J. Am. Chem. Soc., 1985, 107, 7271–7275. 237 O. Zavorotynska, M. Corno, E. Pinatel, L. H. Rude, 213 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., P. Ugliengo, T. R. Jensen and M. Baricco, Crystals, 2012, Theor. Gen. Crystallogr., 1976, 32, 751–767. 2, 144–158. 214 I. Lindemann, R. D. Ferrer, L. Dunsch, R. Cˇerny´, H. Hagemann, 238 H. W. Brinks, A. Fossdal and B. C. Hauback, J. Phys. Chem. V.D’Anna,Y.Filinchuk,L.SchultzandO.Gutfleisch,Faraday C, 2008, 112, 5658–5661. Discuss., 2011, 151, 231–242. 239 N. Eigen, U. Bo¨senberg, J. Bellosta von Colbe, T. R. Jensen, 215 L. Pauling, J. Am. Chem. Soc., 1929, 51, 1010–1026. Y. Cerenius, M. Dornheim, T. Klassen and R. Bormann, 216 Z. Łodziana and M. J. van Setten, Phys. Rev. B: Condens. J. Alloys Compd., 2009, 477, 76–80. Matter Mater. Phys., 2010, 81, 024117. 240 D. A. Sheppard, C. Corgnale, B. Hardy, T. Motyka, R. Zidan, 217 C. W. F. T. Pistorius, Z. Phys. Chem., 1974, 88, 253–263. M. Paskevicius and C. E. Buckley, RSC Adv., 2014, 4, 218 P. A. Madden and M. Wilson, Chem. Soc. Rev., 1996, 25, 26552–26562. 339–350. 241 E. R. Pinatel, M. Corno, P. Ugliengo and M. Baricco, 219 C. Frommen, M. H. Sørby, P. Ravindran, P. Vajeeston, J. Alloys Compd., 2014, 615, S706–S710. H. Fjellvag and B. C. Hauback, J. Phys. Chem. C, 2011, 242 M. Corno, E. Pinatel, P. Ugliengo and M. Baricco, J. Alloys 115, 23591–23602. Compd., 2011, 509, S679–S683. 220 E. Grube, C. H. Olesen, D. B. Ravnsbaek and T. R. Jensen, 243 C. Pistidda, F. Karimi, S. Garroni, A. Rzeszutek, C. Bonatto Dalton Trans., 2016, 45, 8291–8299. Minella, C. Milanese, T. T. Le, L. H. Rude, J. Skibsted, 221 L. H. Rude, E. Groppo, L. M. Arnbjerg, D. B. Ravnsbæk, T. R. Jensen, C. Horstmann, C. Gundlach, M. Tolkiehn, R. A. Malmkjær, Y. Filinchuk, M. Baricco, F. Besenbacher P. K. Pranzas, A. Schreyer, T. Klassen and M. Dornheim, and T. R. Jensen, J. Alloys Compd., 2011, 509, 8299–8305. J. Phys. Chem. C, 2014, 118, 28409–28417. 222 D. B. Ravnsbæk, L. H. Rude and T. R. Jensen, J. Solid State 244 K. S. Alcantara, J. M. Ramallo-Lopez,U.Boesenberg,I.Saldan, Chem., 2011, 184, 1858–1866. C.Pistidda,F.G.Requejo,T.Jensen,Y.Cerenius,M.Sørby, 223 L. H. Rude, U. Filso, V. D’Anna, A. Spyratou, B. Richter, J.Avila,J.B.vonColbe,K.Taube,T.KlassenandM.Dornheim, S. Hino, O. Zavorotynska, M. Baricco, M. H. Sorby, B. C. J. Phys. Chem. C, 2012, 116, 7207–7212. Hauback,H.Hagemann,F.Besenbacher,J.Skibstedand 245 J. Puszkiel, F. C. Gennari, P. Arneodo Larochette, T. R. Jensen, Phys. Chem. Chem. Phys., 2013, 15, 18185–18194. H. E. Troiani, F. Karimi, C. Pistidda, R. Gosalawit–Utke, 224 R. H. Heyn, I. Saldan, M. H. Sorby, C. Frommen, B. Arstad, J. Jepsen, T. R. Jensen, C. Gundlach, M. Tolkiehn, J. Bellosta A. M. Bougza, H. Fjellvag and B. C. Hauback, Phys. Chem. von Colbe, T. Klassen and M. Dornheim, J. Power Sources, Chem. Phys., 2013, 15, 11226–11230. 2014, 267,799–811. 225 S. Hino, J. E. Fonneløp, M. Corno, O. Zavorotynska, A. Damin, 246I.Saldan,M.Schulze,C.Pistidda,R.Gosalawit-Utke, B. Richter, M. Baricco, T. R. Jensen, M. H. Sørby and O.Zavorotynska,L.H.Rude,J.Skibsted,D.Haase, B. C. Hauback, J. Phys. Chem. C, 2012, 116, 12482–12488. Y. Cerenius, T. R. Jensen, G. Spoto, M. Baricco, K. Taube and 226 H. Grove, L. H. Rude, T. R. Jensen, M. Corno, P. Ugliengo, M. Dornheim, J. Phys. Chem. C, 2013, 117, 17360–17366. M. Baricco, M. H. Sorby and B. C. Hauback, RSC Adv., 2014, 247 I. Saldan, R. Gosalawit-Utke, C. Pistidda, U. Bo¨senberg, 4, 4736–4742. M. Schulze, T. R. Jensen, K. Taube, M. Dornheim and 227 J. Y. Lee, Y.-S. Lee, J.-Y. Suh, J.-H. Shim and Y. W. Cho, T. Klassen, J. Phys. Chem. C, 2012, 116, 7010–7015. J. Alloys Compd., 2010, 506, 721–727. 248 R. Gosalawit-Utke, K. Suarez, J. M. Bellosta von Colbe, 228 L. H. Rude, Y. Filinchuk, M. H. Sørby, B. C. Hauback, U. Bo¨senberg, T. R. Jensen, Y. Cerenius, C. Bonatto Mine- F. Besenbacher and T. R. Jensen, J. Phys. Chem. C, 2011, lla, C. Pistidda, G. Barkhordarian, M. Schulze, T. Klassen, 115, 7768–7777. R. Bormann and M. Dornheim, J. Phys. Chem. C, 2011, 115, 229 B. Brehler, Z. Kristallogr., 1961, 115, 373–402. 3762–3768. 230 Y. Filinchuk, D. Chernyshov and V. Dmitriev, Z. Kristallogr., 249 K. Suarez-Alcantara, M. H. Sørby, C. Pistidda, F. Karimi, 2008, 223,649–659. I. Saldan, B. C. Hauback, T. Klassen and M. Dornheim, 231 T. W. Mak and F.-C. Mok, J. Cryst. Mol. Struct., 1978, 8, J. Phys. Chem. C, 2015, 119, 11430–11437. 183–191. 250 C. Bonatto Minella, S. Garroni, C. Pistidda, R. Gosalawit- 232 E. B. Brackett, T. E. Brackett and R. L. Sass, J. Phys. Chem., Utke, G. Barkhordarian, C. Rongeat, I. Lindemann, 1963, 67, 2132–2135. O. Gutfleisch, T. R. Jensen, Y. Cerenius, J. Christensen, 233 A. Haase and G. Brauer, Z. Anorg. Allg. Chem., 1978, 441, M. D. Baro´, R. Bormann, T. Klassen and M. Dornheim, 181–195. J. Phys. Chem. C, 2011, 115, 2497–2504.
1626 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
251 D. Blanchard, J. B. Marosson, M. D. Riktor, J. Kheres, 274 Z. Tang, Y. Tan, Q. Gu and X. Yu, J. Mater. Chem., 2012, 22, D. Sveinbjo¨rnsson, E. G. Bardaji, A. N. Leon, F. Juranyi, 5312–5318. J. Wuttke and K. Lefmann, J. Phys. Chem. C, 2012, 116, 275 H. Chu, G. Wu, Z. Xiong, J. Guo, T. He and P. Chen, Chem. 2013–2023. Mater., 2010, 22, 6021–6028. 252 B. R. S. Hansen, M. Paskevicius, H.-W. Li, E. Akiba and 276 O. V. Kravchenko and S. E. S. Kravchencko, Zh. Obshch. T. R. Jensen, Coord. Chem. Rev., 2016, 323, 60–70. Khim., 1989, 59, 1730–1734. 253 O. A. Kanaeva, N. T. Kuznetsov and K. A. Solntsev, Russ. 277 L. H. Jepsen, Y.-S. Lee, R. Cˇerny´, R. S. Sarusie, Y. W. Cho, J. Inorg. Chem., 1990, 35, 1421. F. Besenbacher and T. R. Jensen, ChemSusChem, 2015, 8, 254 I. Tiritiris, Untersuchungen zu Reaktivita¨t, Aufbau und 3472–3482. struktureller Dynamik von salzartigen closo–Dodekaboraten, 278 A. Gradisˇek, L. H. Jepsen, T. R. Jensen and M. S. Conradi, Institut fu¨r Anorganische Chemie der Universita¨t Stuttgart, J. Phys. Chem. C, 2016, 120, 24646–24654. 2003. 279 Y. Guo, X. Yu, W. Sun, D. Sun and W. Yang, Angew. Chem., 255 P. Schouwink, Y. Sadikin, W. van Beek and R. Cˇerny´, Int. Int. Ed., 2011, 50, 1087–1091. J. Hydrogen Energy, 2015, 40, 10902–10907. 280 Y. Guo, Y. Jiang, G. Xia and X. Yu, Chem. Commun., 2012, 256 Y. Sadikin, M. Brighi, P. Schouwink and R. Cˇerny´, Adv. 48, 4408–4410. Energy Mater., 2015, 5, 1501016. 281 Z. Tang, Y. Tan, H. Wu, Q. Gu, W. Zhou, C. M. Jensen and 257 F. E. Pinkerton, G. P. Meisner, M. S. Meyer, M. P. Balogh X. Yu, Acta Mater., 2013, 61, 4787–4796. and M. D. Kundrat, J. Phys. Chem. B, 2005, 109, 6–8. 282 Z. Tang, F. Yuan, Q. Gu, Y. Tan, X. Chen, C. M. Jensen and 258 X. B. Yu, Y. H. Guo, D. L. Sun, Z. X. Yang, A. Ranjbar, Z. P. X. Yu, Acta Mater., 2013, 61, 3110–3119. Guo, H. K. Liu and S. X. Dou, J. Phys. Chem. C, 2010, 114, 283 Q. Gu, L. Gao, Y. Guo, Y. Tan, Z. Tang, K. S. Wallwork, 4733–4737. F. Zhang and X. Yu, Energy Environ. Sci., 2012, 5, 259 H. Wu, W. Zhou, T. J. Udovic, J. J. Rush and T. Yildirim, 7590–7600. Chem. Mater., 2008, 20, 1245–1247. 284 J. Huang, Y. Tan, J. Su, Q. Gu, R. Cˇerny´, L. Ouyang, D. Sun, 260 P. A. Chater, W. I. F. David and P. A. Anderson, Chem. X. Yu and M. Zhu, Chem. Commun., 2015, 51, 2794–2797. Commun., 2007, 4770–4772. 285 M. B. Ley, L. H. Jepsen, Y. S. Lee, M. Jørgensen, Y. W. Cho 261 M. Aoki, K. Miwa, T. Noritake, G. Kitahara, Y. Nakamori, and T. R. Jensen, Personal communication, 2017. S. Orimo and S. Towata, Appl. Phys. A: Mater. Sci. Process., 286 W. Sun, X. P. Chen, Q. Gu, K. S. Wallwork, Y. Tan, Z. Tang 2005, 80, 1409–1412. and X. Yu, Chem. – Eur. J., 2012, 18, 6825–6834. 262 G. P. Meisner, M. L. Scullin, M. P. Balogh, F. E. Pinkerton 287 Y. Yang, Y. Liu, H. Wu, W. Zhou, M. Gao and H. Pan, Phys. and M. S. Meyer, J. Phys. Chem. B, 2006, 110, 4186–4192. Chem. Chem. Phys., 2014, 16, 135–143. 263 Y. E. Filinchuk, K. Yvon, G. P. Meisner, F. E. Pinkerton and 288 Y. Guo, H. Wu, W. Zhou and X. Yu, J. Am. Chem. Soc., 2011, M. P. Balogh, Inorg. Chem., 2006, 45, 1433–1435. 133, 4690–4693. 264 P. A. Chater, W. I. F. David, S. R. Johnson, P. P. Edwards 289 G. Xia, Q. Gu, Y. Guo and X. Yu, J. Mater. Chem., 2012, 22, and P. A. Anderson, Chem. Commun., 2006, 2439–2441. 7300–7307. 265 X. Y. Chen, Y. H. Guo and X. B. Yu, J. Phys. Chem. C, 2010, 290 E. B. Lobkovskii, A. L. Dorosinskii and K. N. Semenenko, 114, 17947–17953. Zh. Strukt. Khim., 1974, 15, 70–73. 266 H. Chu, Z. Xiong, G. Wu, J. Guo, T. He and P. Chen, Dalton 291 F. Yuan, Q. Gu, Y. Guo, W. Sun, X. Chen and X. Yu, J. Mater. Trans., 2010, 39, 10585–10587. Chem., 2012, 22, 1061–1068. 267 X. B. Yu, Z. X. Yang, Y. H. Guo and S. G. Li, J. Alloys Compd., 292 D. B. Ravnsbaek, PhD dissertation, Aarhus University, 2011, 509, S724–S727. 2011. 268 N. Poonyayant, V. Stavila, E. H. Majzoub, L. E. Klebanoff, 293 I. Olovsson and D. H. Templeton, Acta Crystallogr., 1959, R. Behrens, N. Angboonpong, M. Ulutagay-Kartin, 12, 832–836. P. Pakawatpanurut, E. S. Hecht and J. S. Breit, J. Phys. 294 Z. Tang, Y. Tan, X. Chen, L. Ouyang, M. Zhu, D. Sun and Chem. C, 2014, 118, 14759–14769. X. Yu, Angew. Chem., Int. Ed., 2013, 52, 12659–12663. 269 H. Chu, G. Wu, Y. Zhang, Z. Xiong, J. Guo, T. He and 295 X. Zheng, G. Wu, W. Li, Z. Xiong, T. He, J. Guo, H. Chen P. Chen, J. Phys. Chem. C, 2011, 115, 18035–18041. and P. Chen, Energy Environ. Sci., 2011, 4, 3593–3600. 270 S. Higashi, K. Miwa, M. Aoki and K. Takechi, Chem. 296 C. H. Christensen, R. Z. Sørensen, T. Johannessen, Commun., 2014, 50, 1320–1322. U. J. Quaade, K. Honkala, T. D. Elmøe, R. Køhler and 271 S. R. Johnson, W. I. F. David, D. M. Royse, M. Sommariva, J. K. Nørskov, J. Mater. Chem., 2005, 15, 4106–4108. C. Y. Tang, F. P. A. Fabbiani, M. O. Jones and P. P. Edwards, 297 H. Reardon, J. M. Hanlon, M. Grant, I. Fullbrook and Chem. – Asian J.,2009,4, 849–854. D. H. Gregory, Crystals, 2012, 2, 193–212. 272 G. Soloveichik, J.-H. Her, P. W. Stephens, Y. Gao, 298 L. H. Jepsen, J. Skibsted and T. R. Jensen, J. Alloys Compd., J. Rijssenbeek, M. Andrus and J. C. Zhao, Inorg. Chem., 2013, 580, S287–S290. 2008, 47, 4290–4298. 299 Y. Tan, Q. Gu, J. A. Kimpton, Q. Li, X. Chen, L. Ouyang, 273 Y. Yang, Y. Liu, Y. Li, M. Gao and H. Pan, Chem. – Asian J., M. Zhu, D. Sun and X. Yu, J. Mater. Chem. A, 2013, 1, 2013, 8, 476–481. 10155–10165.
This journal is © The Royal Society of Chemistry 2017 Chem. Soc. Rev., 2017, 46, 1565--1634 | 1627 Review Article Chem Soc Rev
300 X. Chen, F. Yuan, Q. Gu and X. Yu, Dalton Trans., 2013, 42, 325 S. J. van der Molen, M. S. Welling and R. Griessen, Phys. 14365–14368. Rev. Lett., 2000, 85, 3882–3885. 301 J. Luo, H. Wu, W. Zhou, X. Kang, Z. Fang and P. Wang, Int. 326 P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, J. Hydrogen Energy, 2012, 37, 10750–10757. M. Bielmann, C. N. Zwicky and A. Zuttel, J. Phys. Chem. B, 302 P. Schouwink, F. Morelle, Y. Sadikin, Y. Filinchuk and 2008, 112,906–910. R. Cˇerny´, Energies, 2015, 8, 8286–8299. 327 W. D. Davis, L. S. Mason and G. Stegeman, J. Am. Chem. 303 G. Wolf, J. Baumann, F. Baitalow and F. P. Hoffmann, Soc., 1949, 71, 2775–2781. Thermochim. Acta, 2000, 343, 19–25. 328 M. B. Smith and G. E. Bass, J. Chem. Eng. Data, 1963, 8, 304 F. Baitalow, J. Baumann, G. Wolf, K. Jaenicke-Ro¨ßler and 342–346. G. Leitner, Thermochim. Acta, 2002, 391, 159–168. 329 A. Muller, F. Mathey and J. Bensoam, US4193978 A, 1980. 305 A. Staubitz, A. P. M. Robertson and I. Manners, Chem. Rev., 330 N. Ohba, K. Miwa, M. Aoki, T. Noritake, S. Towata, 2010, 110, 4079–4124. Y. Nakamori, S. Orimo and A. Zu¨ttel, Phys. Rev. B: Condens. 306 A. C. Stowe, W. J. Shaw, J. C. Linehan, B. Schmid and Matter Mater. Phys., 2006, 74, 075110. T. Autrey, Phys. Chem. Chem. Phys., 2007, 9, 1831–1836. 331 K. Miwa, N. Ohba, S.-I. Towata, Y. Nakamori and S.-I. 307 Y. S. Chua, P. Chen, G. Wu and Z. Xiong, Chem. Commun., Orimo, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 2011, 47, 5116–5129. 69, 245120. 308 H. Wu, W. Zhou and T. Yildirim, J. Am. Chem. Soc., 2008, 332 P. Martelli, R. Caputo, A. Remhof, P. Mauron, A. Borgschulte 130, 14834–14839. and A. Zu¨ttel, J. Phys. Chem. C, 2010, 114, 7173–7177. 309 I. Dovgaliuk and Y. Filinchuk, Int. J. Hydrogen Energy, 2016, 333 H. W. Li, K. Kikuchi, Y. Nakamori, N. Ohba, K. Miwa, 41, 15489–15504. S. Towata and S. Orimo, Acta Mater., 2008, 56, 1342–1347. 310 R. R. Rietz, A. Zalkin, D. H. Templeton, N. M. Edelstein and 334 Y. Yan, H.-W. Li, Y. Nakamori, N. Ohba, K. Miwa, S.-I. Towata L. K. Templeton, Inorg. Chem., 1978, 17, 653–658. and S.-I. Orimo, Mater. Trans., 2008, 49, 2751–2752. 311 M. Bremer, H. No¨th, M. Thomann and M. Schmidt, Chem. 335 E. R. Pinatel, E. Albanese, B. Civalleri and M. Baricco, Ber., 1995, 128, 455–460. J. Alloys Compd., 2015, 645, S64–S68. 312 F. Jaroschik, F. Bonnet, X.-F. Le Goff, L. Ricard, F. Nief and 336 M. J. van Setten, G. A. devWijs, M. Fichtner and G. Brocks, M. Visseaux, Dalton Trans., 2010, 39, 6761–6766. Chem. Mater., 2008, 20, 4952–4956. 313 S. Marks, J. G. Heck, M. H. Habicht, P. On˜a-Burgos, 337 J. Mao, Z. Guo, C. K. Poh, A. Ranjbar, Y. Guo, X. Yu and C. Feldmann and P. W. Roesky, J. Am. Chem. Soc., 2012, H. Liu, J. Alloys Compd., 2010, 500, 200–205. 134, 16983–16986. 338 Y. Kim, D. Reed, Y.-S. Lee, J. Y. Lee, J.-H. Shim, D. Book and 314 Q. Gu, Z. Wang, Y. Filinchuk, J. A. Kimpton, H. E. A. Brand, Y. W. Cho, J. Phys. Chem. C, 2009, 113, 5865–5871. Q. Li and X. Yu, J. Phys. Chem. C, 2016, 120, 10192–10198. 339 J. Kostka, W. Lohstroh, M. Fichtner and H. Hahn, J. Phys. 315 E. Roedern and T. R. Jensen, ChemistrySelect, 2016, 1, Chem. C, 2007, 111, 14026–14029. 752–755. 340 S. Orimo, N. Nakamori, N. Ohba, K. Miwa, M. Aoki, 316 L. Mosegaard, B. Møller, J.-E. Jørgensen, U. Bo¨senberg, S. Towata and A. Zu¨ttel, Appl. Phys. Lett., 2006, 89, 021920. M. Dornheim, J. C. Hanson, Y. Cerenius, G. Walker, 341 Y. Yan, A. Remhof, S.-J. Hwang, H.-W. Li, P. Mauron, S.-I. H. J. Jakobsen, F. Besenbacher and T. R. Jensen, J. Alloys Orimo and A. Zuttel, Phys. Chem. Chem. Phys., 2012, 14, Compd., 2007, 446–447, 301–305. 6514–6519. 317 HSC Chemistry, Chemistry Software, Outukumpu, Houston, 342 M. P. Pitt, M. Paskevicius, D. H. Brown, D. A. Sheppard and 6.1 edn, 2006. C. E. Buckley, J. Am. Chem. Soc., 2013, 135, 6930–6941. 318 W. Adamczyk, P. Połczyn´ski, A. Mika, T. Jaron´, Z. Mazej, 343 O. Friedrichs, F. Buchter, A. Borgschulte, A. Remhof, K. J. Fijalkowski, R. Jurczakowski and W. Grochala, C. N. Zwicky, P. Mauron, M. Bielmann and A. Zu¨ttel, Acta J. Fluorine Chem., 2015, 174, 22–29. Mater., 2008, 56, 949–954. 319 P. Nordlander, J. K. Norskov and F. Besenbacher, J. Phys. F: 344 A. Remhof, O. Friedrichs, F. Buchter, P. Mauron, A. Zuttel Met. Phys., 1986, 16, 1161–1171. and D. Wallacher, Phys. Chem. Chem. Phys., 2008, 10, 320 J. K. Nørskov and F. Besenbacher, J. Less-Common Met., 5859–5862. 1987, 130, 475–490. 345 D. S. Stasinevich and G. A. Egorenko, Russ. J. Inorg. Chem., 321 T. B. Flanagan and W. A. Oates, in Hydrogen in Intermetallic 1968, 13, 341–343. Compounds I: Electronic, Thermodynamic, and Crystallo- 346 J. Urgnani, F. J. Torres, M. Palumbo and M. Baricco, Int. graphic Properties, Preparation, ed. L. Schlapbach, Springer J. Hydrogen Energy, 2008, 33, 3111–3115. Berlin Heidelberg, Berlin, Heidelberg, 1988, pp. 49–85, 347 M. M. Makansi, C. H. Muendel and W. A. Selke, J. Phys. DOI: 10.1007/3540183337_10. Chem., 1955, 59, 40–42. 322 E. H. Kisi, E. Mac, A. Gray and S. J. Kennedy, J. Alloys 348 K. J. Bowles, Vapor Pressure of Potassium to 2170 Degrees K, Compd., 1994, 216, 123–129. National Aeronautics and Space Administration, TN 323 M. Paskevicius, D. A. Sheppard and C. E. Buckley, J. Am. D-4535, 1968. Chem. Soc., 2010, 132, 5077–5083. 349 D. N. Harries, M. Paskevicius, D. A. Sheppard, T. Price and 324 D. P. Broom, Int. J. Hydrogen Energy, 2007, 32, 4871–4888. C. E. Buckley, Proc. IEEE, 2012, 100, 539–549.
1628 | Chem. Soc. Rev., 2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
350 M. Fellet, C. E. Buckley, M. Paskevicius and D. A. Sheppard, 374 D. Reed and D. Book, Curr. Opin. Solid State Mater. Sci., MRS Bull.,2013,38, 1012–1013. 2011, 15, 62–72. 351 D.A.Sheppard,M.Paskevicius,T.D.Humphries,M.Felderhoff, 375 S. F. Parker, Coord. Chem. Rev., 2010, 254, 215–234. G. Capurso, J. Bellosta von Colbe, M. Dornheim, T. Klassen, 376 M. Mancini, P. Bougeard, R. C. Burns, M. Mlekuz, P.A.Ward,J.A.Teprovich,C.Corgnale,R.Zidan,D.M.Grant B. G. Sayer, J. I. A. Thompson and M. J. McGlinchey, Inorg. andC.E.Buckley,Appl. Phys. A: Mater. Sci. Process., 2016, Chem., 1984, 23, 1072–1078. 122,395. 377 V. Stavila, J.-H. Her, W. Zhou, S.-J. Hwang, C. Kim, L. A. M. 352 G. L. Soloveichik, Y. Gao, J. Rijssenbeek, M. Andrus, Ottley and T. J. Udovic, J. Solid State Chem., 2010, 183, S. Kniajanski, R. C. Bowman Jr, S.-J. Hwang and J.-C. 1133–1140. Zhao, Int. J. Hydrogen Energy, 2009, 34, 916–928. 378 I. Tiritiris, T. Schleid and K. Mu¨ller, Appl. Magn. Reson., 353 R. J. Newhouse, V. Stavila, S.-J. Hwang, L. E. Klebanoff and 2007, 32, 459–481. J. Z. Zhang, J. Phys. Chem. C, 2010, 114, 5224–5232. 379 A. Remhof, Y. Yan, D. Rentsch, A. Borgschulte, C. M. Jensen 354 N. Hanada, K. Chlopek, C. Frommen, W. Lohstroh and and A. Zuttel, J. Mater. Chem. A, 2014, 2, 7244–7249. M. Fichtner, J. Mater. Chem., 2008, 18, 2611–2614. 380 R. Cˇerny´, G. Severa, D. B. Ravnsbæk, Y. Filinchuk, 355 J. G. Vitillo, S. Bordiga and M. Baricco, J. Phys. Chem. C, V. D’Anna, H. Hagemann, D. R. Haase, C. M. Jensen and 2015, 119, 25340–25351. T. R. Jensen, J. Phys. Chem. C, 2009, 114, 1357–1364. 356 Y. Yan, A. Remhof, D. Rentsch and A. Zuttel, Chem. 381 G. N. Boiko, I. M. Yu and N. S. Kirill, Russ. Chem. Rev., Commun., 2015, 51, 700–702. 1975, 44, 1–8. 357 M. Chong, A. Karkamkar, T. Autrey, S.-I. Orimo, S. Jalisatgi 382 S. Narasimhan and R. Balakumar, Aldrichimica Acta, 1998, and C. M. Jensen, Chem. Commun., 2011, 47, 1330–1332. 31, 19–26. 358 M. Chong, M. Matsuo, S.-I. Orimo, T. Autrey and 383 J. Andrieux, U. B. Demirci, J. Hannauer, C. Gervais, C. M. Jensen, Inorg. Chem., 2015, 54, 4120–4125. C. Goutaudier and P. Miele, Int. J. Hydrogen Energy, 2011, 359C.Pistidda,S.Garroni,F.Dolci,E.G.Bardajı´,A.Khandelwal, 36, 224–233. P.Nolis,M.Dornheim,R.Gosalawit,T.Jensen,Y.Cerenius, 384 R. Caputo, S. Garroni, D. Olid, F. Teixidor, S. Surinach and S. Surin˜ach, M. D. Baro´,W.LohstrohandM.Fichtner,J. Alloys M. D. Baro, Phys. Chem. Chem. Phys., 2010, 12, 15093–15100. Compd., 2010, 508, 212–215. 385 Z. P. Li, B. H. Liu, N. Morigasaki and S. Suda, J. Alloys 360 G. Severa, E. Ronnebro and C. M. Jensen, Chem. Commun., Compd., 2003, 354, 243–247. 2010, 46, 421–423. 386 A. C. Dunbar, J. A. Macor and G. S. Girolami, Inorg. Chem., 361 Y. Kim, S.-J. Hwang, Y.-S. Lee, J.-Y. Suh, H. N. Han and 2013, 53, 822–826. Y. W. Cho, J. Phys. Chem. C, 2012, 116, 25715–25720. 387 D. F. Gaines, J. L. Walsh, J. H. Morris and D. F. Hillenbrand, 362 Y. Yan, A. Remhof, D. Rentsch, A. Zuttel, S. Giri and Inorg. Chem., 1978, 17, 1516–1522. P. Jena, Chem. Commun., 2015, 51, 11008–11011. 388 T. A. Salupo, Preparation of ytterbium and europium borides 363 M. D. Riktor, M. H. Sorby, K. Chlopek, M. Fichtner and from Yb(II) and Eu(II) boron hydride precursors, PhD thesis, B. C. Hauback, J. Mater. Chem., 2009, 19, 2754–2759. Ohio State University, 1993. 364 Y. Yan, H.-W. Li, T. Sato, N. Umeda, K. Miwa, S.-I. Towata 389 M. Ehemann and H. No¨th, Z. Anorg. Allg. Chem., 1971, 386, and S.-I. Orimo, Int. J. Hydrogen Energy,2009,34, 5732–5736. 87–101. 365 F. C. Gennari, L. Ferna´ndez Albanesi and I. J. Rios, Inorg. 390 T. J. Marks and J. R. Kolb, Chem. Rev., 1977, 77, 263–293. Chim. Acta, 2009, 362, 3731–3737. 391 G. N. Schrauzer, Naturwissenschaften, 1955, 42, 438. 366 K. N. Semenenko, A. P. Chavgun and V. N. Surov, Russ. 392 Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, J. Inorg. Chem., 1971, 16, 271–273. S. Towata, A. Zu¨ttel and S. I. Orimo, Phys. Rev. B: Condens. 367 E. M. Dematteis, E. Roedern, E. R. Pinatel, M. Corno, Matter Mater. Phys., 2006, 74, 045126. T. R. Jensen and M. Baricco, RSC Adv., 2016, 6, 60101–60108. 393 P. Błon´ski and Z. Łodziana, Phys. Rev. B: Condens. Matter 368 E. G. Bardaji, Z. Zhao-Karger, N. Boucharat, A. Nale, Mater. Phys., 2014, 90, 054114. M. J. van Setten, W. Lohstroh, E. Ro¨hm, M. Catti and 394 B. Bogdanovic´ and M. Schwickardi, J. Alloys Compd., 1997, M. Fichtner, J. Phys. Chem. C, 2011, 115, 6095–6101. 253–254, 1–9. 369 H.-S. Lee, Y.-S. Lee, J.-Y. Suh, M. Kim, J.-S. Yu and 395 P. Chen, Z. Xiong, J. Luo, J. Lin and K. L. Tan, Nature, 2002, Y. W. Cho, J. Phys. Chem. C, 2011, 115, 20027–20035. 420, 302–304. 370 M. B. Ley, E. Roedern, P. V. Thygesen and T. R. Jensen, 396 S. H. Bauer, A. Shepp and R. E. McCoy, J. Am. Chem. Soc., Energies, 2015, 8, 2701–2713. 1953, 75, 1003–1004. 371 J. Y. Lee, D. Ravnsbæk, Y.-S. Lee, Y. Kim, Y. Cerenius, J.-H. 397 W. I. F. David, J. W. Makepeace, S. K. Callear, H. M. A. Shim, T. R. Jensen, N. H. Hur and Y. W. Cho, J. Phys. Chem. Hunter, J. D. Taylor, T. J. Wood and M. O. Jones, J. Am. C, 2009, 113, 15080–15086. Chem. Soc., 2014, 136, 13082–13085. 372 Z. Łodziana, P. Błon´ski, Y. Yan, D. Rentsch and A. Remhof, 398 L. H. Jepsen, P. Wang, G. Wu, Z. Xiong, F. Besenbacher, J. Phys. Chem. C, 2014, 118, 6594–6603. P. Chen and T. R. Jensen, Phys. Chem. Chem. Phys., 2016, 373 V. D’Anna, A. Spyratou, M. Sharma and H. Hagemann, 18, 25257–25264. Spectrochim. Acta, Part A, 2014, 128, 902–906. 399 J. W. Turley and H. W. Rinn, Inorg. Chem., 1969, 8, 18–22.
This journal is © The Royal Society of Chemistry 2017 Chem. Soc. Rev., 2017, 46, 1565--1634 | 1629 Review Article Chem Soc Rev
400 L. Andrews and X. Wang, Science, 2003, 299, 2049–2052. 421J.Puszkiel,F.Gennari,P.A.Larochette,F.Karimi,C.Pistidda, 401 M. Paskevicius, D. A. Sheppard and C. E. Buckley, J. Alloys R. Gosalawit-Utke, J. Jepsen, T. R. Jensen, C. Gundlach, J. B. von Compd., 2009, 487, 370–376. Colbe, T. Klassen and M. Dornheim, Int. J. Hydrogen Energy, 402L.V.Dinh,D.A.Knight,M.Paskevicius,C.E.Buckleyand 2013, 38, 14618–14630. R. Zidan, Appl. Phys. A: Mater. Sci. Process., 2012, 107, 173–181. 422 C. Gosselin, S. Deledda, B. C. Hauback and J. Huot, J. Alloys 403 A. Remhof, Y. Yan, O. Friedrichs, J. W. Kim, M. Ph, Compd., 2015, 645, S304–S307. A. Borgschulte, D. Wallacher, A. Buchsteiner, A. Hoser, 423 M. Polanski, T. K. Nielsen, Y. Cerenius, J. Bystrzycki and K. H. Oh, Y. W. Cho and A. Zu¨ttel, J. Phys.: Conf. Ser., 2012, T. R. Jensen, Int. J. Hydrogen Energy, 2010, 35, 3578–3582. 340, 012111. 424 M. Polanski, K. Witek, T. K. Nielsen, L. Jaroszewicz and 404 A. Borgschulte, E. Callini, B. Probst, A. Jain, S. Kato, J. Bystrzycki, Int. J. Hydrogen Energy, 2013, 38, 2785–2789. O. Friedrichs, A. Remhof, M. Bielmann, A. J. Ramirez-Cuesta 425 M. Polanski, J. Bystrzycki, R. A. Varin and T. Plocinski, Int. and A. Zu¨ttel, J. Phys. Chem. C, 2011, 115, 17220–17226. J. Hydrogen Energy, 2011, 36, 1059–1065. 405 O. Friedrichs, A. Remhof, S. J. Hwang and A. Zu¨ttel, Chem. 426 M. P. Pitt, M. Paskevicius, C. J. Webb, D. A. Sheppard, Mater., 2010, 22, 3265–3268. C. E. Buckley and E. M. Gray, Int. J. Hydrogen Energy, 2012, 406 S. H. Bauer, J. Am. Chem. Soc., 1958, 80, 294–298. 37, 4227–4237. 407 R. A. Varin, L. Zbroniec, M. Polanski and J. Bystrzycki, 427 M. Dornheim, N. Eigen, G. Barkhordarian, T. Klassen and Energies, 2011, 4, 1–25. R. Bormann, Adv. Eng. Mater., 2006, 8, 377–385. 408 P. Jena, J. Phys. Chem. Lett., 2011, 2, 206–211. 428 M. Dornheim and T. Klassen, in Encyclopedia of Electro- 409 H.-W. Li, Y. Yan, S.-I. Orimo, A. Zu¨ttel and C. M. Jensen, chemical Power Sources, ed. C. K. Dyer, P. T. Moseley, Energies, 2011, 4, 185–214. Z. Ogumi, D. A. J. Rand, B. Scrosati and J. Garche, Elsevier 410 E. Callini, K.-F. Aguey-Zinsou, R. Ahuja, J. R. Ares, S. Bals, Science, 2013. N. Bilisˇkov, S. Chakraborty, G. Charalambopoulou, A.-L. 429 M. Dornheim, S. Doppiu, G. Barkhordarian, U. Boesenberg, Chaudhary,F.Cuevas,B.Dam,P.deJongh,M.Dornheim, T. Klassen, O. Gutfleisch and R. Bormann, Scr. Mater.,2007, Y. Filinchuk, J. Grbovic´ Novakovic´,M.Hirscher,T.R.Jensen, 56, 841–846. P. B. Jensen, N. Novakovic´,Q.Lai,F.Leardini,D.M.Gattia, 430 C. J. Webb, J. Phys. Chem. Solids, 2015, 84, 96–106. L.Pasquini,T.Steriotis,S.Turner,T.Vegge,A.Zu¨ttel and 431 J.-C. Crivello, B. Dam, R. V. Denys, M. Dornheim, A. Montone, Int. J. Hydrogen Energy, 2016, 41, 14404–14428. D. M. Grant, J. Huot, T. R. Jensen, P. Jongh, M. Latroche, 411 M. P. Pitt, M. Paskevicius, C. J. Webb, M. H. Sørby, C. Milanese, D. Milcˇius, G. S. Walker, C. J. Webb, C. Zlotea S. Delleda, T. R. Jensen, B. C. Hauback, C. E. Buckley and and V. A. Yartys, Appl. Phys. A: Mater. Sci. Process., 2016, E. M. Gray, Int. J. Hydrogen Energy, 2011, 36, 8403–8411. 122, 97. 412 M. P. Pitt, P. E. Vullum, M. H. Sørby, H. Emerich, 432 J.-C. Crivello, R. V. Denys, M. Dornheim, M. Felderhoff, M. Paskevicius, C. E. Buckley, E. M. Gray, J. C. Walmsley, D. M. Grant, J. Huot, T. R. Jensen, P. Jongh, M. Latroche, R. Holmestad and B. C. Hauback, J. Alloys Compd., 2012, G. S. Walker, C. J. Webb and V. A. Yartys, Appl. Phys. A: 527, 16–24. Mater. Sci. Process., 2016, 122, 85. 413M.P.Pitt,P.E.Vullum,M.H.Sørby,H.Emerich, 433 M. C. S. Escan˜o, E. Gyenge, R. L. Arevalo and H. Kasai, M. Paskevicius, C. E. Buckley, J. C. Walmsley, R. Holmestad J. Phys. Chem. C, 2011, 115, 19883–19889. and B. C. Hauback, J. Phys. Chem. C, 2012, 116, 14205–14217. 434 J. Yang, A. Sudik and C. Wolverton, J. Phys. Chem. C, 2007, 414 A. Andreasen, M. B. Sørensen, R. Burkarl, B. Møller, A. M. 111, 19134–19140. Molenbroek, A. S. Pedersen, T. Vegge and T. R. Jensen, 435 F. Pendolino, P. Mauron, A. Borgschulte and A. Zu¨ttel, Appl. Phys. A: Mater. Sci. Process., 2005, 82, 515–521. J. Phys. Chem. C, 2009, 113, 17231–17234. 415 A. Andreasen, M. B. Sørensen, R. Burkarl, B. Møller, 436 F. Pendolino, J. Phys. Chem. C, 2012, 116, 1390–1394. A. M. Molenbroek, A. S. Pedersen, J. W. Andreasen, 437 P. A. Berseth, A. G. Harter, R. Zidan, A. Blomqvist, M. M. Nielsen and T. R. Jensen, J. Alloys Compd., 2005, C. M. Arau´jo, R. H. Scheicher, R. Ahuja and P. Jena, Nano 404–406, 323–326. Lett., 2009, 9, 1501–1505. 416 L. Pasquini, E. Callini, M. Brighi, F. Boscherini, 438 S. V. Alapati, J. K. Johnson and D. S. Sholl, Phys. Chem. A. Montone, T. R. Jensen, C. Maurizio, M. Vittori Antisari Chem. Phys., 2007, 9, 1438–1452. and E. Bonetti, J. Nanopart. Res., 2011, 13, 5727–5737. 439 X. B. Yu, Z. Wu, Q. R. Chen, Z. L. Li, B. C. Weng and 417 E. Callini, L. Pasquini, L. H. Rude, T. K. Nielsen, T. R. Jensen T. S. Huang, Appl. Phys. Lett., 2007, 90, 034106. and E. Bonetti, J. Appl. Phys., 2010, 108, 073513. 440 M. Paskevicius, B. Richter, M. Polanski, S. P. Thompson 418 E. Callini, L. Pasquini, T. R. Jensen and E. Bonetti, Int. and T. R. Jensen, Dalton Trans., 2016, 45, 639–645. J. Hydrogen Energy, 2013, 38, 12207–12212. 441 D. A. Sheppard, L. H. Jepsen, T. R. Jensen, M. Paskevicius 419 M. Polanski, T. K. Nielsen, I. Kunce, M. Norek, T. Płocin´ski, and C. E. Buckley, J. Mater. Chem. A, 2013, 1, 12775–12781. L. R. Jaroszewicz, C. Gundlach, T. R. Jensen and 442 D. J. Siegel, C. Wolverton and V. Ozolin- ˇs, Phys. Rev. B: J. Bystrzycki, Int. J. Hydrogen Energy, 2013, 38, 4003–4010. Condens. Matter Mater. Phys., 2007, 76, 134102. 420 R. A. Varin, Z. Zaranski, T. Czujko, M. Polanski and 443I.Saldan,C.Frommen,I.Llamas-Jansa,G.N.Kalantzopoulos, Z. S. Wronski, Int. J. Hydrogen Energy, 2011, 36, 1177–1183. S.Hino,B.Arstad,R.H.Heyn,O.Zavorotynska,S.Deledda,
1630 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
M. H. Sørby, H. Fjellvåg and B. C. Hauback, Int. J. Hydrogen 464 K. T. Møller, A. S. Fogh, M. Paskevicius, J. Skibsted and T. R. Energy, 2015, 40, 12286–12293. Jensen, Phys. Chem. Chem. Phys., 2016, 18, 27545–27553. 444 X. B. Yu, D. M. Grant and G. S. Walker, J. Phys. Chem. C, 465 B. R. S. Hansen, D. B. Ravnsbaek, J. Skibsted and 2008, 112, 11059–11062. T. R. Jensen, Phys. Chem. Chem. Phys., 2014, 16, 8970–8980. 445 M. Au, W. Spencer, A. Jurgensen and C. Zeigler, J. Alloys 466 P. de la Mora, M. Castro and G. Tavizon, J. Solid State Compd., 2008, 462, 303–309. Chem., 2002, 169, 168–175. 446 X. B. Yu, D. M. Grant and G. S. Walker, J. Phys. Chem. C, 467 U. Bo¨senberg, D. B. Ravnsbæk, H. Hagemann, V. D’Anna, 2009, 113, 17945–17949. C. B. Minella, C. Pistidda, W. van Beek, T. R. Jensen, 447 I. Saldan, S. Hino, T. D. Humphries, O. Zavorotynska, R. Bormann and M. Dornheim, J. Phys. Chem. C, 2010, M. Chong, C. M. Jensen, S. Deledda and B. C. Hauback, 114, 15212–15217. J. Phys. Chem. C, 2014, 118, 23376–23384. 468 J.-H. Shim, J.-H. Lim, S.-U. Rather, Y.-S. Lee, D. Reed, 448 Y. H. Guo, X. B. Yu, L. Gao, G. L. Xia, Z. P. Guo and Y. Kim, D. Book and Y. W. Cho, J. Phys. Chem. Lett., H. K. Liu, Energy Environ. Sci., 2010, 3, 464–469. 2010, 1, 59–63. 449 M. Au and A. Jurgensen, J. Phys. Chem. B, 2006, 110, 469 F. Cova, E. C. E. Ro¨nnebro, Y. J. Choi, F. C. Gennari and 7062–7067. P. A. Larochette, J. Phys. Chem. C, 2015, 119, 15816–15822. 450 M. Au, A. Jurgensen and K. Zeigler, J. Phys. Chem. B, 2006, 470 T. E. C. Price, D. M. Grant, D. Weston, T. Hansen, 110, 26482–26487. L. M. Arnbjerg, D. B. Ravnsbæk, T. R. Jensen and 451 H.-W. Li, D. Matsumura, Y. Nishihata, E. Akiba and S.-I. G. S. Walker, J. Am. Chem. Soc., 2011, 133, 13534–13538. Orimo, J. Jpn. Inst. Met. Mater., 2013, 77, 627–630. 471 O. Friedrichs, J. W. Kim, A. Remhof, F. Buchter, 452 B. R. S. Hansen, D. B. Ravnsbæk, D. Reed, D. Book, A. Borgschulte, D. Wallacher, Y. W. Cho, M. Fichtner, C. Gundlach, J. Skibsted and T. R. Jensen, J. Phys. Chem. K. H. Oh and A. Zuttel, Phys. Chem. Chem. Phys., 2009, C, 2013, 117, 7423–7432. 11, 1515–1520. 453 U. Bo¨senberg, J. W. Kim, D. Gosslar, N. Eigen, T. R. Jensen, 472 X. B. Yu, D. M. Grant and G. S. Walker, Chem. Commun., J. M. Bellosta von Colbe, Y. Zhou, M. Dahms, D. H. Kim, 2006, 3906–3908. R. Gu¨nther, Y. W. Cho, K. H. Oh, T. Klassen, R. Bormann 473 E. Deprez, M. A. Mun˜oz-Ma´rquez, M. A. Rolda´n, and M. Dornheim, Acta Mater., 2010, 58, 3381–3389. C. Prestipino, F. J. Palomares, C. B. Minella, U. Bo¨senberg, 454 O. Zavorotynska, I. Saldan, S. Hino, T. D. Humphries, M. Dornheim, R. Bormann and A. Ferna´ndez, J. Phys. Chem. S. Deledda and B. C. Hauback, J. Mater. Chem. A, 2015, 3, C, 2010, 114, 3309–3317. 6592–6602. 474 J. Hu, R. Witter, H. Shao, M. Felderhoff and M. Fichtner, 455 U. Bo¨senberg, U. Vainio, P. K. Pranzas, J. M. Bellosta von J. Mater. Chem. A, 2014, 2, 66–72. Colbe, G. Goerigk, E. Welter, M. Dornheim, A. Schreyer 475 C. Pistidda, S. Garroni, C. B. Minella, F. Dolci, T. R. Jensen, and R. Bormann, Nanotechnology, 2009, 20, 204003. P. Nolis, U. Bo¨senberg, Y. Cerenius, W. Lohstroh, 456 Z. Xiong, G. Wu, J. Hu and P. Chen, Adv. Mater., 2004, 16, M. Fichtner, M. D. Baro´, R. Bormann and M. Dornheim, 1522–1525. J. Phys. Chem. C, 2010, 114, 21816–21823. 457 G. Barkhordarian, T. Klassen and R. Bormann, Int. Pat. 476 D. Pottmaier, C. Pistidda, E. Groppo, S. Bordiga, G. Spoto, WO2006063627, 2006. M. Dornheim and M. Baricco, Int. J. Hydrogen Energy, 2011, 458 T. D. Humphries, G. N. Kalantzopoulos, I. Llamas-Jansa, 36, 7891–7896. J. E. Olsen and B. C. Hauback, J. Phys. Chem. C, 2013, 117, 477 G. Barkhordarian, T. R. Jensen, S. Doppiu, U. Bo¨senberg, 6060–6065. A. Borgschulte, R. Gremaud, Y. Cerenius, M. Dornheim, 459 S. Garroni, C. Pistidda, M. Brunelli, G. B. M. Vaughan, T. Klassen and R. Bormann, J. Phys. Chem. C, 2008, 112, S. Surinach and M. D. Baro, Scr. Mater., 2009, 60, 2743–2749. 1129–1132. 478 Y. Kim, D. Reed, Y.-S. Lee, J.-H. Shim, H. N. Han, D. Book 460 C. Pistidda, G. Barkhordarian, A. Rzeszutek, S. Garroni, and Y. W. Cho, J. Alloys Compd., 2010, 492, 597–600. C. B. Minella, M. D. Baro´, P. Nolis, R. Bormann, T. Klassen 479 M. Gonzalez-Silveira, R. Gremaud, H. Schreuders, M. J. V. and M. Dornheim, Scr. Mater., 2011, 64, 1035–1038. Setten, E. Batyrev, A. Rougier, L. Dupont, E. G. Bardajı´, 461 C. Milanese, S. Garroni, A. Girella, G. Mulas, V. Berbenni, W. Lohstroh and B. Dam, J. Phys. Chem. C, 2010, 114, G. Bruni, S. Surin˜ach, M. D. Baro´ and A. Marini, J. Phys. 13895–13901. Chem. C, 2011, 115, 3151–3162. 480 X.-D. Kang, P. Wang, L.-P. Ma and H.-M. Cheng, Appl. Phys. 462 S. Kato, A. Borgschulte, M. Bielmann and A. Zuttel, Phys. A: Mater. Sci. Process., 2007, 89, 963–966. Chem. Chem. Phys., 2012, 14, 8360–8368. 481 S. A. Jin, J. H. Shim, Y. W. Cho, K. W. Yi, O. Zabara and 463 F. Karimi, P. Klaus Pranzas, C. Pistidda, J. A. Puszkiel, M. Fichtner, Scr. Mater., 2008, 58, 963–965. C. Milanese, U. Vainio, M. Paskevicius, T. Emmler, 482 Y. Zhang, Q. Tian, J. Zhang, S.-S. Liu and L.-X. Sun, J. Phys. A. Santoru, R. Utke, M. Tolkiehn, C. B. Minella, A.-L. Chem. C, 2009, 113, 18424–18430. Chaudhary, S. Boerries, C. E. Buckley, S. Enzo, 483 A. Remhof, O. Friedrichs, F. Buchter, P. Mauron, J. W. Kim, A. Schreyer, T. Klassen and M. Dornheim, Phys. Chem. K. H. Oh, A. Buchsteiner, D. Wallacher and A. Zu¨ttel, Chem. Phys., 2015, 17, 27328–27342. J. Alloys Compd., 2009, 484, 654–659.
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1631 Review Article Chem Soc Rev
484 D. B. Ravnsbæk and T. R. Jensen, J. Phys. Chem. Solids, 513 J. J. Vajo, Curr. Opin. Solid State Mater. Sci., 2011, 15, 52–61. 2010, 71, 1144–1149. 514 M. Fichtner, Nanotechnology, 2009, 20, 204009. 485 D. B. Ravnsbæk and T. R. Jensen, J. Appl. Phys., 2012, 515 P. E. de Jongh and P. Adelhelm, ChemSusChem, 2010, 3, 111, 112621. 1332–1348. 486 M. J. van Setten and M. Fichtner, J. Alloys Compd., 2009, 516 P. Ngene, P. Adelhelm, A. M. Beale, K. P. de Jong and 477, L11–L12. P. E. de Jongh, J. Phys. Chem. C, 2010, 114, 6163–6168. 487 Q. Shi, X. Yu, R. Feidenhans’l and T. Vegge, J. Phys. Chem. 517 S. Zhang, A. F. Gross, S. L. Van Atta, M. Lopez, P. Liu, C, 2008, 112, 18244–18248. C. C. Ahn, J. J. Vajo and C. M. Jensen, Nanotechnology, 488 R. Liu, D. Reed and D. Book, J. Alloys Compd., 2012, 515, 2009, 20, 204027. 32–38. 518 Z. Zhao-Karger, J. Hu, A. Roth, D. Wang, C. Ku¨bel, 489 E. Roedern and T. R. Jensen, J. Phys. Chem. C, 2014, 118, W. Lohstroh and M. Fichtner, Chem. Commun., 2010, 46, 23567–23574. 8353–8355. 490 J. J. Vajo, W. Li and P. Liu, Chem. Commun., 2010, 46, 519 R. Bogerd, P. Adelhelm, J. H. Meeldijk, K. P. de Jong and 6687–6689. P. E. de Jongh, Nanotechnology, 2009, 20, 204019. 491 K. Yvon and G. Renaudin, in Encyclopedia of Inorganic- 520 X. Liu, D. Peaslee, C. Z. Jost, T. F. Baumann and Chemistry, ed. R. B. King, John Wiley & Sons, Ltd, 2006, E. H. Majzoub, Chem. Mater., 2011, 23, 1331–1336. DOI: 10.1002/0470862106.ia087. 521 P. Ngene, R. van den Berg, M. H. W. Verkuijlen, K. P. de 492 M. Norek, T. K. Nielsen, M. Polanski, I. Kunce, T. Płocin´ski, Jong and P. E. de Jongh, Energy Environ. Sci., 2011, 4, L. R. Jaroszewicz, Y. Cerenius, T. R. Jensen and J. Bystrzycki, 4108–4115. Int. J. Hydrogen Energy, 2011, 36, 10760–10770. 522 M. L. Christian and K.-F. Aguey-Zinsou, ACS Nano, 2012, 6, 493 M. R. Ghaani, M. Catti and A. Nale, J. Phys. Chem. C, 2012, 7739–7751. 116, 26694–26699. 523 Y. Yan, Y. S. Au, D. Rentsch, A. Remhof, P. E. de Jongh and 494 K.-B. Kim, J.-H. Shim, Y. W. Cho and K. H. Oh, Chem. A. Zuttel, J. Mater. Chem. A, 2013, 1, 11177–11183. Commun., 2011, 47, 9831–9833. 524 M. A. Wahab, Y. Jia, D. Yang, H. Zhao and X. Yao, J. Mater. 495 F. C. Gennari, Int. J. Hydrogen Energy, 2011, 36, 15231–15238. Chem. A, 2013, 1, 3471–3478. 496 P. Mauron, M. Bielmann, A. Remhof, A. Zu¨ttel, J.-H. Shim 525 S. Sartori, K. D. Knudsen, F. S. Hage, R. H. Heyn, E. G. and Y. W. Cho, J. Phys. Chem. C, 2010, 114, 16801–16805. Bardaji, Z. Zhao-Karger, M. Fichtner and B. C. Hauback, 497 M. B. Ley, M. Meggouh, R. Moury, K. Peinecke and Nanotechnology, 2012, 23, 255704. M. Felderhoff, Materials, 2015, 8, 5891–5921. 526 A. Ampoumogli, G. Charalambopoulou, P. Javadian, 498 S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, B. Richter, T. R. Jensen and T. Steriotis, J. Alloys Compd., N. C. Spencer, M. T. Kelly, P. J. Petillo and M. Binder, 2015, 645, S480–S484. Int. J. Hydrogen Energy, 2000, 25, 969–975. 527 P. Javadian, D. A. Sheppard, C. E. Buckley and T. R. Jensen, 499 J.-H. Wee, J. Power Sources, 2006, 155, 329–339. Nano Energy, 2015, 11, 96–103. 500 H. C. Brown and C. A. Brown, J. Am. Chem. Soc., 1962, 84, 528 Y. Yan, A. Remhof, P. Mauron, D. Rentsch, Z. Łodziana, Y.- 1493–1494. S. Lee, H.-S. Lee, Y. W. Cho and A. Zu¨ttel, J. Phys. Chem. C, 501 Y. Shang and R. Chen, Energy Fuels, 2006, 20, 2142–2148. 2013, 117, 8878–8886. 502 C.-j. Liu, U. Burghaus, F. Besenbacher and Z. L. Wang, ACS 529 T. K. Nielsen, U. Bo¨senberg, R. Gosalawit, M. Dornheim, Nano, 2010, 4, 5517–5526. Y. Cerenius, F. Besenbacher and T. R. Jensen, ACS Nano, 503 V. Berube, G. Chen and M. S. Dresselhaus, Int. J. Hydrogen 2010, 4, 3903–3908. Energy, 2008, 33, 4122–4131. 530 R. Gosalawit-Utke, C. Milanese, T. K. Nielsen, F. Karimi, 504 S. Cheung, W. Q. Deng, A. C. T. van Duin and W. A. I. Saldan, K. Pranzas, T. R. Jensen, A. Marini, T. Klassen Goddard, J. Phys. Chem. A, 2005, 109, 851–859. and M. Dornheim, Int. J. Hydrogen Energy, 2013, 38, 505 J. J. Liang, Appl. Phys. A: Mater. Sci. Process., 2005, 80, 1932–1942. 173–178. 531 R. Gosalawit Utke, T. K. Nielsen, K. Pranzas, I. Saldan, 506 R. W. P. Wagemans, J. H. van Lenthe, P. E. de Jongh, C. Pistidda, F. Karimi, D. Laipple, J. Skibsted, T. R. Jensen, A. J. van Dillen and K. P. de Jong, J. Am. Chem. Soc., 2005, T. Klassen and M. Dornheim, J. Phys. Chem. C, 2011, 116, 127, 16675–16680. 1526–1534. 507 X. Yao and G. Lu, Chin. Sci. Bull., 2008, 53, 2421–2431. 532 G. Xia, Y. Tan, F. Wu, F. Fang, D. Sun, Z. Guo, Z. Huang 508 V. Be´rube´, G. Radtke, M. Dresselhaus and G. Chen, Int. and X. Yu, Nano Energy, 2016, 26, 488–495. J. Energy Res., 2007, 31, 637–663. 533 R. Gosalawit-Utke, C. Milanese, P. Javadian, J. Jepsen, 509 C. Weidenthaler, Nanoscale, 2011, 3, 792–810. D. Laipple, F. Karmi, J. Puszkiel, T. R. Jensen, A. Marini, 510 S. Cahen, J. B. Eymery, R. Janot and J. M. Tarascon, J. Power T. Klassen and M. Dornheim, Int. J. Hydrogen Energy, 2013, Sources, 2009, 189, 902–908. 38, 3275–3282. 511 T. K. Nielsen, F. Besenbacher and T. R. Jensen, Nanoscale, 534 R. Gosalawit-Utke, C. Milanese, P. Javadian, A. Girella, 2011, 3, 2086–2098. D. Laipple, J. Puszkiel, A. S. Cattaneo, C. Ferrara, 512 F. E. Pinkerton, J. Alloys Compd., 2011, 509, 8958–8964. J. Wittayakhun, J. Skibsted, T. R. Jensen, A. Marini,
1632 | Chem.Soc.Rev.,2017, 46, 1565--1634 This journal is © The Royal Society of Chemistry 2017 Chem Soc Rev Review Article
T. Klassen and M. Dornheim, J. Alloys Compd., 2014, 599, 557 P. Martelli, A. Remhof, A. Borgschulte, R. Ackermann, 78–86. T. Stra¨ssle, J. P. Embs, M. Ernst, M. Matsuo, S.-I. Orimo 535 R. Utke, S. Thiangviriya, P. Javadian, T. R. Jensen, and A. Zu¨ttel, J. Phys. Chem. A, 2011, 115, 5329–5334. C. Milanese, T. Klassen and M. Dornheim, Mater. Chem. 558 N. Verdal, T. J. Udovic, J. J. Rush, H. Wu and A. V. Skripov, Phys., 2016, 169, 136–141. J. Phys. Chem. C, 2013, 117, 12010–12018. 536 S. Thiangviriya, P. Plerdsranoy, N. Wiset, P. Javadian, T. R. 559 A. V. Skripov, A. V. Soloninin, M. B. Ley, T. R. Jensen and Jensen and R. Utke, J. Alloys Compd., 2015, 633, 484–493. Y. Filinchuk, J. Phys. Chem. C, 2013, 117, 14965–14972. 537 P. Javadian, C. Zlotea, C. M. Ghimbeu, M. Latroche and 560 A. V. Skripov, O. A. Babanova, A. V. Soloninin, V. Stavila, T. R. Jensen, J. Phys. Chem. C, 2015, 119, 5819–5826. N. Verdal, T. J. Udovic and J. J. Rush, J. Phys. Chem. C, 2013, 538 P. Javadian, D. A. Sheppard, C. E. Buckley and T. R. Jensen, 117, 25961–25968. Int. J. Hydrogen Energy, 2015, 40, 14916–14924. 561 N. Verdal, T. J. Udovic, V. Stavila, W. S. Tang, J. J. Rush and 539 B. Schmitz, U. Mu¨ller, N. Trukhan, M. Schubert, G. Fe´rey A. V. Skripov, J. Phys. Chem. C, 2014, 118, 17483–17489. and M. Hirscher, ChemPhysChem, 2008, 9, 2181–2184. 562 M. Paskevicius, B. R. S. Hansen, M. Jørgensen, B. Richter 540 A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, and T. R. Jensen, Nat. Commun., 2017, accepted. M. O’Keeffe and O. M. Yaghi, Acc. Chem. Res., 2010, 43, 563 M. Paskevicius, M. P. Pitt, D. H. Brown, D. A. Sheppard, 58–67. S. Chumphongphan and C. E. Buckley, Phys. Chem. Chem. 541 Y. Oumellal, A. Rougier, G. A. Nazri, J. M. Tarascon and Phys., 2013, 15, 15825–15828. L. Aymard, Nat. Mater., 2008, 7, 916–921. 564 N. Verdal, J.-H. Her, V. Stavila, A. V. Soloninin, 542 V. Yartys, D. Noreus and M. Latroche, Appl. Phys. A: Mater. O. A. Babanova, A. V. Skripov, T. J. Udovic and J. J. Rush, Sci. Process., 2016, 122, 43. J. Solid State Chem., 2014, 212, 81–91. 543 S. Sartori, F. Cuevas and M. Latroche, Appl. Phys. A: Mater. 565 N. Verdal, H. Wu, T. J. Udovic, V. Stavila, W. Zhou and Sci. Process., 2016, 122, 135. J. J. Rush, J. Solid State Chem., 2011, 184, 3110–3116. 544 T. H. Mason, X. Liu, J. Hong, J. Graetz and E. H. Majzoub, 566 N. Verdal, M. R. Hartman, T. Jenkins, D. J. DeVries, J. Phys. Chem. C, 2011, 115, 16681–16687. J. J. Rush and T. J. Udovic, J. Phys. Chem. C, 2010, 114, 545 M. Matsuo, Y. Nakamori, S.-I. Orimo, H. Maekawa and 10027–10033. H. Takamura, Appl. Phys. Lett., 2007, 91, 224103. 567 V. V. Volkov, M. Khikmatov, U. Mirsaidov, S. P. Gabuda 546 T. J. Udovic, M. Matsuo, A. Unemoto, N. Verdal, V. Stavila, and S. G. Kozlova, J. Struct. Chem., 1988, 29, 58–61. A. V. Skripov, J. J. Rush, H. Takamura and S.-I. Orimo, 568 D. J. Huettner, J. L. Ragle, L. Sherk, T. R. Stengle and Chem. Commun., 2014, 50, 3750–3752. H. J. C. Yeh, J. Chem. Phys., 1968, 48, 1739–1744. 547 L. He, H.-W. Li, H. Nakajima, N. Tumanov, Y. Filinchuk, S.- 569 E. C. Reynhardt and J. A. J. Lourens, J. Phys. C: Solid State J. Hwang, M. Sharma, H. Hagemann and E. Akiba, Chem. Phys., 1980, 13, 2765–2779. Mater., 2015, 27, 5483–5486. 570 E. C. Reynhardt and J. A. J. Lourens, J. Chem. Phys., 1984, 548 N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, 80, 6240–6244. R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, 571 N. Verdal, T. J. Udovic, W. Zhou, J. J. Rush, D. J. De Vries K. Kawamoto and A. Mitsui, Nat. Mater., 2011, 10, and M. R. Hartman, J. Phys. Chem. C, 2013, 117, 876–883. 682–686. 572 H. Wu, W. S. Tang, V. Stavila, W. Zhou, J. J. Rush and 549 W. S. Tang, A. Unemoto, W. Zhou, V. Stavila, M. Matsuo, T. J. Udovic, J. Phys. Chem. C, 2015, 119, 6481–6487. H. Wu, S.-I. Orimo and T. J. Udovic, Energy Environ. Sci., 573 N. Kunkel, H. Kohlmann, A. Sayede and M. Springborg, 2015, 8, 3637–3645. Inorg. Chem., 2011, 50, 5873–5875. 550 H. Maekawa, M. Matsuo, H. Takamura, M. Ando, Y. Noda, 574 N. Kunkel, A. Meijerink, M. Springborg and H. Kohlmann, T. Karahashi and S.-I. Orimo, J. Am. Chem. Soc., 2009, 131, J. Mater. Chem. C, 2014, 2, 4799–4804. 894–895. 575 B. Daudin, R. Lagnier and B. Salce, J. Magn. Magn. Mater., 551 H. Oguchi, M. Matsuo, J. S. Hummelshøj, T. Vegge, J. K. 1982, 27, 315–322. Nørskov, T. Sato, Y. Miura, H. Takamura, H. Maekawa and 576 A. M. Tishin and Y. I. Spichkin, The Magnetocaloric Effect S. Orimo, Appl. Phys. Lett., 2009, 94, 141912. and its Applications, Institute of Physics, Bristol, 2003. 552 B. A. Boukamp and R. A. Huggins, Phys. Lett. A, 1979, 72, 577 J.-L. Liu, Y.-C. Chen, F.-S. Guo and M.-L. Tong, Coord. 464–466. Chem. Rev., 2014, 281, 26–49. 553 M. Matsuo, S. Kuromoto, T. Sato, H. Oguchi, H. Takamura 578 B. Sundqvist and O. Andersson, Phys. Rev. B: Condens. and S.-I. Orimo, Appl. Phys. Lett., 2012, 100, 203904. Matter Mater. Phys., 2006, 73, 092102. 554 V. Thangadurai, D. Pinzaru, S. Narayanan and A. K. Baral, 579 B. Sundqvist and O. Andersson, Int. J. Thermophys., 2009, J. Phys. Chem. Lett., 2015, 6, 292–299. 30, 1118–1129. 555 T. J. Udovic, M. Matsuo, W. S. Tang, H. Wu, V. Stavila, 580 T. Hayashi, K. Tobita, Y. Nakamori and S. Orimo, J. Nucl. A. V. Soloninin, R. V. Skoryunov, O. A. Babanova, Mater., 2009, 386–388, 119–121. A. V. Skripov, J. J. Rush, A. Unemoto, H. Takamura and 581 H. I. Schlesinger, H. C. Brown, A. E. Finholt, J. R. Gilbreath, S.-I. Orimo, Adv. Mater., 2014, 26, 7622–7626. H. R. Hoekstra and E. K. Hyde, J. Am. Chem. Soc., 1953, 75, 556 M. Jansen, Angew. Chem., Int. Ed., 1991, 30, 1547–1558. 215–219.
This journal is © The Royal Society of Chemistry 2017 Chem.Soc.Rev.,2017, 46, 1565--1634 | 1633 Review Article Chem Soc Rev
582 J. Ma, N. A. Choudhury and Y. Sahai, Renewable Sustainable 586 J. G. Vitillo, E. Groppo, E. G. Bardaji, M. Baricco and Energy Rev., 2010, 14, 183–199. S. Bordiga, Phys. Chem. Chem. Phys.,2014,16, 22482–22486. 583 A. S. Tayi, A. Kaeser, M. Matsumoto, T. Aida and S. I. Stupp, 587 C. V. Picasso, D. A. Safin, I. Dovgaliuk, F. Devred, Nat. Chem., 2015, 7, 281–294. D. Debecker, H.-W. Li, J. Proost and Y. Filinchuk, Int. 584 J. Echeverrı´a, G. Aullo´n, D. Danovich, S. Shaik and J. Hydrogen Energy, 2016, 41, 14377–14386. S. Alvarez, Nat. Chem., 2011, 3, 323–330. 588 S. Pylypko, E. Petit, P. G. Yot, F. Salles, M. Cretin, 585 I. Dovgaliuk, H. Hagemann, T. Leyssens, M. Devillers and P. Miele and U. B. Demirci, Inorg. Chem., 2015, 54, Y. Filinchuk, Int. J. Hydrogen Energy,2014,39, 19603–19608. 4574–4583.
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