International Journal of Hydrogen Energy 32 (2007) 1121–1140 www.elsevier.com/locate/ijhydene

Review Metal hydride materials for solid hydrogen storage: A reviewଁ

Billur Sakintunaa,∗, Farida Lamari-Darkrimb, Michael Hirscherc

aGKSS Research Centre, Institute for Materials Research, Max-Planck-Str. 1, Geesthacht D-21502, Germany bLIMHP-CNRS (UPR 1311), Université Paris 13, Avenue J. B. Clément, 93430 Villetaneuse, France cMax-Planck-Institut für Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany

Received 31 July 2006; received in revised form 21 November 2006; accepted 21 November 2006 Available online 16 January 2007

Abstract Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides. ᭧ 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen storage; Review; Mg-based hydrides; Complex hydrides; Intermetallic compounds; Ball-milling; Kinetics; Storage capacity; Operating temperature and pressure

Contents

1. Introduction ...... 1122 2. Mg-based metal hydrides ...... 1123 2.1. Improvement on surface properties and mechanical ball-milling ...... 1123 2.2. Cyclic stability ...... 1127 2.3. Catalyst effect ...... 1128 2.4. Chemical composition ...... 1129 3. Complex hydrides ...... 1129 3.1. Sodium alanates ...... 1130 3.2. Lithium and potassium alanates ...... 1131 3.3. Lithium nitrides ...... 1131 3.4. Lithium boro- and beryllium hydrides ...... 1132 4. Intermetallic compounds ...... 1133 5. Conclusion ...... 1135 Acknowledgments ...... 1136 References ...... 1136

ଁ The work was carried out in LIMHP-CNRS. ∗ Corresponding author. Tel.: +49 (0) 4152 872673; fax: +49 (0) 4152 872670. E-mail address: [email protected] (B. Sakintuna).

0360-3199/$ - see front matter ᭧ 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.11.022 1122 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

Nomenclature P–C pressure-composition Pabs hydrogen absorption pressure BCC body centered cubic Pdes hydrogen desorption pressure BM ball-milling Tabs hydrogen absorption temperature Cyc. cycle Tdes hydrogen desorption temperature DOE US Department of Energy tabs hydrogen absorption time H/M hydrogen atoms per metal atom tdes hydrogen desorption time Mm misch metals XRD X-ray diffraction

1. Introduction safety perspective such that some form of conversion or energy input is required to release the hydrogen for use. A great deal Hydrogen is the ideal candidate as an energy carrier for both of effort has been made on new hydrogen-storage systems, in- mobile and stationary applications while averting adverse ef- cluding metal, chemical or complex hydrides and carbon nanos- fects on the environment, and reducing dependence on imported tructures. oil for countries without natural resources. Carbon materials such as activated carbons, carbon nan- Hydrogen storage is clearly one of the key challenges in otubes, and carbon nanofibers have been the subject of intensive developing hydrogen economy. Hydrogen can be stored as (i) research. The research on hydrogen storage in carbon materials pressurized gas, (ii) cryogenic liquid, (iii) solid fuel as chemical was dominated by announcements of high storage capacities or physical combination with materials, such as metal hydrides, in carbon nanostructures. However, the experimental results on complex hydrides and carbon materials, or produced on-board hydrogen storage in carbon nanomaterials scatter over several the vehicle by reforming methanol [1]. Each of these options orders of magnitude. The hydrogen-storage capacity for car- possesses attractive attributes for hydrogen storage [2]. bon materials is reported between 0.2 and 10 wt% [7,8]. The Available technologies permit directly to store hydrogen experiments to date claiming very high values could not inde- by modifying its physical state in gaseous or liquid form in pendently be reproduced in different laboratories. In view of pressurized or in cryogenic tanks. The traditional hydrogen- today’s knowledge although they have good reversibility prop- storage facilities are complicated because of its low boiling erties, carbon nanostructures cannot store the amount of hydro- point (−252.87 ◦C) and low density in the gaseous state gen required for automotive applications [9]. (0.08988 g/L) at 1 atm. Liquid hydrogen requires the addition Hydrogen forms metal hydrides with some metals and alloys of a refrigeration unit to maintain a cryogenic state [3] thus leading to solid-state storage under moderate temperature adding weight and energy costs, and a resultant 40% loss in and pressure that gives them the important safety advantage energy content [4]. High-pressure storage of hydrogen gas is over the gas and liquid storage methods. Metal hydrides have . / 3 limited by the weight of the storage canisters and the potential higher hydrogen-storage density (6 5 H atoms cm for MgH2) for developing leaks. Moreover, storage of hydrogen in liquid than hydrogen gas (0.99 H atoms/cm3) or liquid hydrogen or gaseous form poses important safety problems for on-board (4.2 H atoms/cm3) [3]. Hence, metal hydride storage is a transport applications. Designs involving the use of methane safe, volume-efficient storage method for on-board vehicle as a hydrogen source require the addition of a steam reformer applications. to extract the hydrogen from the carbon which adds weight, There are two possible ways of hydriding a metal, direct dis- additional space requirements, and the need for a device to sociative chemisorption and electrochemical splitting of water. sequester CO2 [1]. These reactions are: The US Department of Energy (DOE) [5] published a long- x M + H ↔ MHx, (1) term vision for hydrogen-storage applications considering eco- 2 2 nomic and environmental parameters. The predicted minimum x x − x − hydrogen-storage capacity should be 6.5 wt% and 65 g/L hy- M + H2O + e ↔ MHx + OH , (2) drogen available, at the decomposition temperature between 60 2 2 2 and 120 ◦C for commercial viability. It was also predicted low where M represents the metal. temperature of hydrogen desorption and low pressure of hydro- Metal hydrides compose of metal atoms that constitute a host gen absorption (a plateau pressure of the order of a few bars lattice and hydrogen atoms. Metal and hydrogen usually form at room temperature) and nonthermal transformation between two different kinds of hydrides, -phase at which only some substrates and products of decomposition as reported by Schulz hydrogen is absorbed and -phase at which hydride is fully [6]. Furthermore, the cost of a storage medium and its toxicity formed. Hydrogen storage in metal hydrides depends on differ- properties need to be carefully considered for the realization of ent parameters and consists of several mechanistic steps. Met- the set goals. als differ in the ability to dissociate hydrogen, this ability being Storage by absorption as chemical compounds or by ad- dependent on surface structure, morphology and purity [10]. sorption on carbon materials have definite advantages from the An optimum hydrogen-storage material is required to have the B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1123 following properties; high hydrogen capacity per unit mass and desorption temperature and to fasten the re/dehydrogenation unit volume which determines the amount of available energy, reactions. These can be accomplished to some extent by chang- low dissociation temperature, moderate dissociation pressure, ing the microstructure of the hydride by ball-milling (mechan- low heat of formation in order to minimize the energy necessary ical alloying) with elements which reduce the stability of the for hydrogen release, low heat dissipation during the exother- hydrides and also by using proper catalysts to improve the mic hydride formation, reversibility, limited energy loss during absorption/desorption kinetics [61]. charge and discharge of hydrogen, fast kinetics, high stability against O2 and moisture for long cycle life, cyclibility, low cost 2.1. Improvement on surface properties and mechanical of recycling and charging infrastructures and high safety. ball-milling Hydrogen-storage as metal hydride has been the focus of in- tensive research. The hydrogen-storage systems have been re- A critical factor for hydrogen absorption by metals is ported in numerous studies [11–17]. The light metals such as Li, the metal surface, which activates dissociation of hydrogen Be, Na, Mg, B and Al, form a large variety of metal–hydrogen molecules and allows easy diffusion of hydrogen into the bulk. compounds. They are especially interesting due to their light Diffusion is not the limiting step initially because no material weight and the number of hydrogen atoms per metal atom, has been reacted and there are sufficient active sites available which is in many cases at the order of H/M = 2. Heavier [62], but chemisorption is the slowest step for pure Mg at this ones may enter the multiple component system only as a low- point [63]. As the reaction progresses, hydrogen diffusion takes abundant additive, most likely for alteration of properties or as place and the hydride layer grows, producing a nearly imper- a catalyst. There is enduring research both on modifying and meable layer. Diffusion through this hydride layer becomes optimizing the known hydrogen-storage materials, and new re- the rate-limiting step in the hydride formation process [63]. sources. The studies are conducted on finding optimum solid In addition to formation of a compact hydride layer, exposure hydrogen-storage system [18]. In this review, we briefly men- to oxygen also lowers absorption rates due to the formation tion about hydrogenation properties, advantages and disadvan- of a highly stable oxide layer [25]. Andreasen et al. [64] have tages of different metal-hydride systems such as Mg-based reviewed the kinetics in terms of apparent activation energies metal hydrides, complex hydrides, alanates and intermetallic and apparent prefactors of Mg-based hydrides. It was suggested compounds. A brief review of state-of-the art is reported on that variations in apparent activation energies correlate with the metal hydrides. This work will serve to evaluate solid fuel presence of MgO surface layer inhibiting diffusion of hydrogen. hydrogen store for industrial on-board hydrogen storage tank Thus, oxidized samples show large apparent activation energies design. and well activated samples show smaller activation energies. The ball-milling creating fresh surfaces during processing is 2. Mg-based metal hydrides an economic process that is widely applied to metal hydrides to achieve good surface properties [15]. The main effects There is considerable research on magnesium and its alloys of ball-milling are increased surface area, formation of mi- for on-board hydrogen storage due to their high hydrogen- cro/nanostructures and creation of defects on the surface and storage capacity by weight and low cost [15]. Besides, the Mg- in the interior of the material. The induced lattice defects may based hydrides possess good-quality functional properties, such aid the diffusion of hydrogen in materials by providing many as heat-resistance, vibration absorbing, reversibility and recy- sites with low activation energy of diffusion. The induced clability. In recent years, therefore, much attention has been microstrain assists diffusion by reducing the hysteresis of hy- paid to investigations on specific material properties of Mg al- drogen absorption and desorption [30]. The increased surface loys for the development of new functional materials. contact with catalyst during ball-milling leads to fast kinetics Magnesium hydride, MgH2, has the highest energy density of hydrogen transformations. It is possible to control proper- (9 MJ/kg Mg) of all reversible hydrides applicable for hydrogen ties of the store material, according to specific applications. storage. MgH2 combines a high H2 capacity of 7.7 wt% with the These include changing the alloy composition, surface prop- benefit of the low cost of the abundantly available magnesium erties, microstructures and grain size by ball-milling without [19–22] with good reversibility [23,24]. the additional cost of catalyst and with minimal loss of stor- The main disadvantages of MgH2 as a hydrogen store is the age capacity [58]. It is used for production of nanocrystalline high temperature of hydrogen discharge, slow desorption kinet- magnesium with superior powder morphology that gives re- ics and a high reactivity toward air and oxygen [25,26]. Ther- markable improvement of kinetics and surface activity for modynamic properties of the magnesium hydride system have hydrogenation [21,25]. The crystalline Mg2Ni alloy obtained been investigated. The results showed high operating tempera- by ball-milling has excellent surface properties compared with ture which is too high for practical on-board applications [27]. those prepared by a conventional metallurgical method [65]. High thermodynamic stability of MgH2 results in a relatively Huot et al. [58] investigated the structural difference between high desorption enthalpy, which corresponds to an unfavorable milled and unmilled MgH2. The specific surface area is de- ◦ desorption temperature of 300 Cat1barH2 [15,20]. creased by milling 10-fold. Faster hydrogen desorption kinetics, Hydrogen absorption/desorption properties of recently stud- reduction in activation energy and enhanced kinetics observed ied Mg-based hydrides are summarized in Table 1. Many efforts for the milled MgH2 compared to the unmilled one are seen in has focused on Mg-based hydrides in recent years to reduce the Fig. 1. The activation energies for desorption were measured 1124 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 [30] [43] [50] [53] [44,45] [22,29] [28] [31,32] [33] [34] [35] [36] [37,38] [39] [40] [41,42] [28] [46] [47] [48] [43] [49] [50] [51] [50] [52] [34] Ref. 2 20 No data 1.37 [28] 3.33 No data40 5.00 No data 5.60 : 30 9 cyc.: stable 4.50 : 83: 1.6: 2.7–19: 0.5–5.8 : No 1 data:5 : 60 No No data:60 data: 10:5 : 15 2 cyc.:: stable 0.45:4 5 cyc.: not stable: 15 2.30 No data:5 3.30 : 2 3.20 3.33 cyc.: 20 stable cyc.:: not 60 stable 3.40 3.43 Not stable: 67 Not stable: 2 3.70 :10 4 3.50 cyc.: stable 3.60 after: 2nd 30 cycle: No 2 data 4.30 :10 4.03 : 90 No data: 90–1440 4.10 No data: 0.83 2000 cyc.:: stable 600 5 cyc.: stable: 600 16.7 cyc.: stable: 1.6 4.49 : 3.33:1.66 4.70 : 60:10 5.00 5.00 No data 5.00 3 5.00 cyc.: stable No data: 60:60 3 cyc.: stable 5 cyc.: not 5.40 5.00 stable 5.50 5.56 5.66 : : : abs abs abs abs abs des abs des abs des abs des des abs des abs des abs des abs abs des des abs des abs abs des abs des abs abs des abs abs abs des abs des t t t t t t t t t t t t t t t t T t t t t t t t t t t t t t t t t t t t t t t : 4–7 : 10–15 :1–3 : 3.6–93.7 : 3–10 des des des des des P P P P P and and and and and : 2–15 :6 : 30:1 :15 :15 :12 :20 :1 No data:11 : 0.5 :10 :1 :1 9 cyc.: stable after fourth cycle: 2.21–11.34 2.70 : 1.62–15.48 :50 :10 : 17–25:15 :30 : 20 bar :1 :2 No data: 77–85 :10 1000 cyc.: stable: 0.15 :10 : 0.15 :30 : 0.15 4.48 :10 :12 :11 : 0.5 abs abs abs des abs abs des abs des abs des abs des des abs abs des abs abs des abs abs abs abs des des abs abs abs abs des abs des abs des abs abs abs abs des P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P C) Pressure (bar) Kinetics (min) Cycling stability Max wt% of H ◦ : 400 : 400 : 300 : 250–303 : 280–400 : 200 : 300 : 350–525 : 500 : 300 des des des des des des des des des des T T T T T T T T T T and and and and and and and and and and : 300 :15 : 250 : 250–350 : 300–350 : 300 : 480 : 270–280 : 250–300 : 450–550 : 300 : 200 : 350 : 330–350 : 300–550 : 473–552 : 200 : 300 : 200 : 300 : 280 : 200 : 300 : 320 abs abs abs abs abs abs des abs abs des des abs abs des abs abs abs abs abs des des abs abs abs abs des abs des abs abs des abs abs abs T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T BM BM BM BM BM BM BM BM Hydriding combustion BM BM Mixing BM BM BM Mixing Mixing BM BM Mixing and BM encapsulation synthesis x ) CoAlMn 6 6 ( 5 . . x 0 0 5 − 3 3 5 5 Cr Cr O O 28 4 4 . O . . 2 2 3 3 5 2 2 1 1 2 2 2 . 6 2 O O 6 1 2 2 2 17 4 FeH . FeH 2 0 2 Mg –2LiNH –40 wt% LaNi 5 5 Fe . 2 6 1 . 17 4 –5 mol% Fe –5 mol% V –2LiNH –5 mol% Al –5 wt% V–Mg –5 at% Ti–5 at% Ni BM –5 at% V BM BM BM 2 2 2 2 2 2 2 2 2 Ni CoH 5 Mg . 2 2 0 1MgH . MaterialMgH Method Temperature ( MgH Mg–50 wt% ZrFe Mg–10 wt% CeO Mg–20 wt% Mm (La, Nd, Ce)Mg–40 wt% ZrFe BM (pellet form) La Mg–50 wt% LaNi Mg–30 wt%Mg–Fe–Mg BMMgH (hexane medium) Mg–30 wt% LaNi Mg–10 wt% Fe Mg–30 wt% CFMmNi MgH Mg MgH 30 wt% Mg–M mNi Mg–10 wt% Al Mg–20 wt% TiO Mg–5 wt% FeTi 90Mg–10Al BM La MgH 1 MmNi MgH MgH MgH Table 1 Hydrogen absorption/desorption properties of Mg-based hydrides B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1125 [59] [54] [43] [34] [50] [55] [56] [57] [58] [26] [20] [60] 60 . 7  4.290 No data6 800 cyc.: stable1.5 5.80 6.00 :33 : 60:60 : 8.33: 13.33 : 6: 10–35 : 150 5 cyc.:: not 2 stable No: data 12.5 (milled): 50 (unmilled) 1000: cyc.: 1 stable 2 No cyc.: data not stable: 420 5.87 No data 6.00 6.40 No data 6.50 No data 7.00 6.70 7.00 7.30 : : : : Fig. 1. Hydrogen desorption curves of unmilled MgH (solid symbols) and abs des abs des abs des abs abs des des abs des des des abs des abs 2 t t t t t t t t t t t t t t t t t ball-milled (hollow symbols) MgH2 under a hydrogen pressure of 0.15 bar [58].

as 120 and 156 kJ/mol for the milled and unmilled powders, : 4.0–1.4 : 1–2 respectively. It has been found that ball-milling of Mg NiH des des 2 4 P P substantially decreases the desorption temperature. Depending 0.15 and and :10 : 0.15 :11 : 0.5 :10 :1 : 8.4 : vacuum : 3–10 : 0.15 : 1: 8.4 : vacuum : 0.6 No data No data 7.00

: on the ball-milling conditions, the shift of the onset of desorp- ◦ ◦ abs des abs des abs abs des abs des abs des abs des des abs des abs tion temperature can be as large as 100 C for MgH and 40 C P P P P P P P P P P P P P P P P P 2 for Mg2NiH4 [66]. Hydrogenation properties are very sensitive to these surface modifications. The ball-milled powders do not require activa- tion compared to the conventional methods. For all nanocrys- : 300 : 300 : 300 : 300

des des des talline hydrides investigated, the grain boundary does not des T T T T dramatically affect the pressure–composition (P–C) isotherms. and and and and : 200 : 300 : 230–370 : 200 : 300 : 150–250 : 300 (milled) : 350 (milled) : 140–190 : 180 : 335–347 : 50–150 No dataThis describes No the data thermodynamic No data aspects of hydride forma- abs des abs abs abs des abs des abs abs des des abs abs des des T T T T T T T T T T T T T T T T tion which is more visible in the Van’t Hoff plots that illustrate the relationship between equilibrium pressure and changes in enthalpy and entropy. The pressure for hydrogen desorption of the unmilled MgH2 is lower than that of the milled one, as seen in Fig. 2. Aoyagi et al. [67] proposed that ball-milling of the alloys under inert environment leads to reduction in powder size and creation of new surfaces, which are effective for the improve- BM BM BM BM Mixing cyclohexane medium) ment of the hydrogen absorption rate. Hydrogen absorption rate of Mg-based alloys also increases with the milling time. 3

O Liang et al. [30] proposed that ball-milling of Mg–5 wt% 3 2 O 5 2 3 FeTi . produces fine powder with nanometer-sized grains and O 1 2 O 2 2 large microstrain, which results in an increase in the hydrid- –8LiH BM 2 –5 wt% Ni Wet-chemical method ing and dehydriding rates. The high absorption rate is due to ) 2 2 the large quantity of phase boundaries and the porous surface –5 wt% V–5 at% BM Mn–0.2 mol% Cr –2 mol% Ni BM –1 mol% Cr BM –1 at% Al–5 at% Ge BM (benzene and BM NH 2 2 2 2 2 2 2 2 ( MgH

/ structure [40]. Nanocrystalline hydrides enhanced the hydro- MgH MgH Mg–0.5 wt% Nb Mg–10 wt% Cr Mg MgH MgH 3Mg MgH MgH MgH MgH genation kinetics, even at relatively low temperatures. These 1126 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

Fig. 3. Desorption curves of magnesium catalyzed with 0.1 mol% Nb2O5 and milled for 2, 5, 10, 20, 50 and 100 h, at 573 K and vacuum [75].

with 5.56 wt% H2 absorption, by facilitating nucleation by cre- ating defects on the surface of Mg particles and reducing the particle size of Mg and thus by shortening the diffusion dis- Fig. 2. Pressure–concentration–temperature curves of the unmilled and tances of hydrogen atoms. By using the same method, it was ball-milled MgH at 623 K [58]. proposed recently that the presence of nickel lowered the onset 2 ◦ temperature of MgH2 desorption to 225 C by ball-milling un- der hydrogen atmosphere [71]. Tessier et al. [70] synthesized structures can be obtained by means of local change in the Mg2Ni with the same method. Furthermore, Orimo et al. [73] stable atomic positions into the metastable configuration. found that ball-milling of Mg2Ni under hydrogen resulted in / Ball-milling of LiNH2 MgH2 results in low dehydrogenation a significant facilitation of hydrogen desorption. Alloying time ( ◦ ) temperature 200 C but with markedly slow kinetics and has a significant effect on hydride properties of Mg2Ni. Ab- lowered hydrogen uptake capacity [41]. Wagemans et al. [68] dellaoui et al. [74] proposed that absorption capacity changes investigated the quantum chemical perspective of MgH2. Small with alloying duration. Barkhordarian et al. [75] investigated MgH2 clusters have much lower desorption energy than bulk the effect of milling time on the magnesium hydrogen sorp- MgH2, hence enabling hydrogen desorption at lower tempera- tion reaction. Reaction kinetics is enhanced by increasing the tures. The hydrogen desorption energy decreases significantly milling time, as shown in Fig. 3. Ball-milling is generally used when the crystal grain size becomes smaller than 1.3 nm. A to produce powders in the range of 60.100 m [25,76] to cre- MgH2 crystallite size of 0.9 nm corresponds to a desorption ate more active sights for hydrogen penetration. Smaller parti- temperature of only 200 ◦C [68]. Particles with nanocrystalline cle sizes also eliminate the formation of hydride layers greater structure with grain size of 10 nm or less, increase the density than 50 m [25]. of grain boundaries, resulting in easier activation [69]. Although extensive work has been done on hydrogen storage In another approach, a hydrogen-absorbing material can be in bulk materials as thin film hydride is an emerging field of hydrogenated during ball-milling [70]. It was shown that ball- research. Research performed on thin films of pure magnesium milling under hydrogen atmosphere is a convenient method for concluded that the thinner the magnesium sheet, the faster it the formation of metal hydrides which causes simultaneous hy- achieved complete formation of MgH2. It was found that hy- drogen uptake and mechanical deformation resulting from ball- drogen penetrated to an average depth of 30 m and stopped milling. Huot et al. [71] produced MgH2 under H2 atmosphere [77]. Mg portions of the film have converted totally into MgH2 by ball-milling. This method improved the hydride formation at temperatures not higher than 200 ◦C [78]. Hydrogen-storage kinetics. Chen et al. [72] studied also the formation of metal properties of nanocomposite three-layered Pd/Mg/Pd films hydrides under H2 atmosphere. The results indicated that pul- have been previously investigated. After hydrogenation under verization and deformation processes occurring during high- a hydrogen gas pressure of 1 bar at 100 ◦C, Pd layers contain energy ball-milling play a major role in the hydriding reaction. only 0.15–0.30 wt% H2, whereas the Mg film contains 5.0 wt% It is concluded that ball-milling is a simple and inexpensive hydrogen [79]. Pd/Mg films with different degree of crystal- method of producing high hydrogen content metal hydrides. lization in the Mg layer are prepared in different sputtering Mg–10 wt% Fe2O3 is synthesized by mechanical grinding conditions [77]. The dehydriding temperature decreases with under H2 atmosphere [52]. It increases the H2-sorption rates decreasing the degree of crystallization in the Mg layer in B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1127

Table 2 Hydrogen absorption/desorption properties of Mg–Ni-based hydrides

◦ Material Method Temperature ( C) Pressure (bar) Kinetics (min) Cycling stability Max wt% of H2 Ref. T : P : t : Mg2Ni–1 wt% Pd BM abs 200 abs 15 abs 27 4 cyc.: stable 2.50 [86] T P Mg2Ni BM abs: 300 abs: 29 5 cyc.: not stable 3.20 [87] T P t Mg2Ni BM abs: 300 abs: 11.6 abs: 10 4 cyc.: stable 3.50 [86] T T P Mg2Ni BM abs and des: 280 des: 1–2 No data No data 3.53 [74,88] T P t Mg–Mg2Ni BM abs: 300 abs:12 abs: 83 2 cyc.: not stable 3.60 [89] T T P t Mg2Ni BM abs and des: 280–330 abs: 1–15 abs: 1 No data 4.10 [82] Pdes: 1–2 tdes:1 70 wt% Mg–30 wt% LaNi5 BM Tabs and Tdes: 350 Pabs:10 tabs: 30 10 cyc.: stable 4.66 [90] Pdes: 1.5 tdes:10 [91] T P t 65 wt% MgH2–35 wt% Mg2NiH4 BM des: 220–240 des: 0.5 abs: 10 20 cyc.: stable 5.00 [66]

Pd/Mg films. The lowest crystallization of films absorb 5.6 wt% dehydriding temperature is 250.300 ◦C at desorption pressure of hydrogen and all hydrogen desorbed at a temperature lower of 2.1–3.0 bar [22,66,74,84,88]. High hydrogen capacity of four ◦ than 190 C in a vacuum [77]. hydrogen atoms per Mg2Ni, combined with the small specific Mechanical alloying can be preformed in the presence of or- weight of the alloy is the most important advantage of Mg2Ni ganic solvents such as benzene, cyclohexane or carbon materi- over other magnesium hydrides [93]. als [80,81]. Nanosized MgH2 ball-milled with benzene showed There are numerous publications regarding the upgrade of reversible hydriding/dehydriding with high capacity under 1 bar hydrogenation properties of Mg2NiH4. Their hydriding prop- hydrogen pressure. It was proposed that the organic additives erties are thought to be strongly affected by their nanometer- of benzene or cyclohexane are essential for decisive character- scale structures by means of thermodynamic and kinetic istics of the resulting magnesium to sustain nanosized magne- aspects. MgNi2 prepared by ball-milling is found to react with sium with high-degree of dispersion [20]. Wet milling in the hydrogen even at room temperature, whereas polycrystalline presence of toluene leads to hydrogen and carbon pick up in the material needs hydrogenation temperature of 250.350 ◦C and powder, and also require much longer milling times [82]. Hy- pressure of 15–50 bar [84]. Haussermann et al. [92] inves- driding properties of Mg-based composites which are prepared tigated the structural stability and bonding properties of the by mechanical milling of magnesium powder and graphite or Mg2NiH4 using ab initio density functional calculations. Par- graphite supporting 5 wt% Pd in the presence of various ad- ticular changes of the hydriding properties of the nanostruc- ditives (tetrahydrofuran, benzene or cyclohexane) have been tured Mg2NiH4 have been also reported [19,22,25,73,94]. studied [80]. In particular, the presence of tetrahydrofuran in The nanocrystalline Mg2Ni intermetallic compound formed the milling process strongly affected the hydriding and dehy- by mechanical alloying of Mg and Mg2Ni can absorb hydro- + driding kinetics of the resulting composites [80]. There is a gen rapidly without activation. The Mg Mg2Ni composites synergetic interaction between Mg and aromatic carbon atoms need activation, but once activated, they absorb hydrogen more ◦ of graphite containing charge transfer to some extent. rapidly than Mg2Ni at low temperature of 150 C under 12 bar It has been found that the hydrogenation properties of Mg, with high capacity of 4.2 wt% [89]. Nanoparticles of Mg2Ni such as hydrogenation/dehydrogenation temperature and hy- leads to superior hydrogenation behavior including easy acti- drogenation rate, can be more or less improved by forming vation and hydrogen uptake in the first cycle itself compared to composite structures. Zhu et al. [22,29] reported that the hy- the conventional crystal phase [86,87]. Nanocrystalline Mg2Ni drogen sorption properties of Mg and Mg–Ni-based alloys can with Pd exhibits much faster absorption kinetics at 200 ◦C than significantly be improved by forming composites having proper the as-ball-milled samples (reaction halftime of 2 and 33 min, microstructural feature. respectively) [86]. Mg2NiH4 attracts wide interest for being a promising hydro- gen store material due to its relatively high capacity, low cost, 2.2. Cyclic stability light weight and low-toxicity [83] and for its unusual struc- tural and bonding properties [84,85]. The hydrogen absorption Cyclic stability is one of the major criteria for applicability and desorption properties of Mg–Ni-based alloys are listed in of metal/metal hydride systems for reversible hydrogen storage. Table 2.Mg2NiH4 forms eagerly by hydrogenating the alloy Depending on the nature of the additives, cycling temperatures Mg2Ni [92]: and starting microstructures, various structures and intermedi- + → ate phases can be obtained. Song et al. [34] synthesized magne- Mg2Ni 2H2 Mg2NiH4. (3) sium hydrides with additives of Cr2O3,Al2O3 and CeO2. All Upon hydrogenation, Mg2Ni reacts with 3.6 wt% of hydro- the samples absorb and desorb less hydrogen at the fifth cycle gen and transforms into the hydride phase, Mg2NiH4, and the than at the first cycle due to the agglomeration of the particles 1128 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

2.3. Catalyst effect

Catalysis is one of the critical factors in the improvement of hydrogen sorption kinetics in metal hydride systems that enables fast and effective dissociation of hydrogen molecules [21]. Effective catalysts, even added in small amounts enhance the formation of a hydride in reasonable extent. There is in- tensive research about finding a proper catalyst to enhance the hydriding properties. It was reported that the rate of absorption is controlled by the following factors: the rate of hydrogen dis- sociation at the surface, the capability of hydrogen to penetrate from the surface which is typically covered by an oxide layer into metal, the rate of hydrogen diffusion into the bulk metal Fig. 4. Cyclic stability of chemical Ni-doped Mg at 503–643 K and 4.0 bar, and through the hydride already formed. time for one full cycle 3 h [43]. Palladium is a good catalyst for hydrogen dissociation re- action. The hydriding properties are enhanced by catalysis during hydrogenation/dehydrogenation cycling. Misch metals through nanoparticles of Pd located on magnesium surface (Mm) such as Ce, La, Nd and Pr, are used to increase the cyclic [25]. The reactivity of palladium after exposure to oxygen is stability [95,35]. However, the increased concentration of Mm recovered during exposure to hydrogen because of the easy in the samples has an adverse effect on hydrogen-storage ca- decomposition of palladium oxide [93]. However, high cost pacity and their reaction rate [35]. Gross et al. [37] have re- of palladium is the main disadvantage for the industrial ap- ported formation phase changes, segregation and disintegration plications [15]. Zaluski et al. [86] have also reported on the + of La2Mg17 40 wt% LaNi5 during cycling at temperatures presence of Pd as a catalyst in nanocrystalline Mg2Ni, LaNi5 up to 350 ◦C. It was also reported that hydrogen capacity of and FeTi systems, enhance the absorption rates even at lower La0.5Ni1.5Mg17 decreases with cycling [39]. temperatures and maintain less sensitivity to air exposures. There are few publications regarding high number of cyclic Hydrogen molecules have a strong affinity for nickel and tests. Reiser et al. [43] investigated the behavior of Ni-doped readily dissociate and adsorb onto surface-layer nickel clusters Mg and Mg2CoH5. They indicated that they are almost stable [101,102]. Through the addition of 1 at% of nickel to magne- even after 800 cycles with small fluctuations in the hydrogen ca- sium, Holtz and Imam [76] achieved a 50% increase in hy- pacity, as shown in Fig. 4. The cyclic stability of MgH2–5 wt% drogen capacity, a decrease in the temperature for the onset of V is studied by Dehouche et al. [48] up to 2000 cycles. They hydrogenation from 275 to 175 ◦C, and a lowering of the de- concluded that there is no change in isotherms and no disinte- hydrogenation onset temperature from 350 to 275 ◦C. gration of the materials even when hydrogen content reached In addition to Pd and Ni, Ge can be used for the catalysis + . 5 wt%. Also the cyclic stability of MgH2 0 2 mol% Cr2O3 is of hydrogenation kinetics [60]. The presence of Ge decreases previously examined at 1000 cycles. Although desorption time the hydride decomposition temperature in a range from 50 to ◦ is increased, an increase of H2 storage capacity is reported by 150 C, depending on the catalyst amount. But the catalytic ef- about 8% between the first and the 500 or 1000 cycle due to fect of Ge disappears after few hydrogen absorption/desorption structural relaxations and crystallite growth [55]. Friedlmeier cycles [60]. Vanadium also acts as a catalyst for the dissoci- et al. [96] observed a decrease in the kinetics of hydrogen ab- ation of hydrogen molecules. It was also reported using V as sorption after 4300 cycles, but no loss in the hydrogen capacity a catalyst, hydrogen capacity can be increased up to 5.8 wt% of Mg–2 at% Ni alloy. However, to achieve the same storage while the thermodynamic parameters of MgH2 were not altered capacity, the system temperature had to be increased. Neverthe- [54], as shown in Fig. 5. less, Dehouche et al. [97] showed 15% decrease in hydrogen Titanium and vanadium block the oxidation of the alloy sur- capacity after 2100 cycles with a starting material of nanocrys- face, and therefore, increase the discharge capacity over mul- talline Mg2Ni. This was attributed to the formation of the non- tiple cycles [103]. A nanocrystalline Mg1.9Ti0.1Ni alloy shows hydride forming MgNi2 phase, during the cycling process. good absorption kinetics at room temperature [104]. The Ti de- The resistance of metal hydrides to impurities is one of the creases the kinetic barriers of absorption while the Ni protects critical issues for on-board applications in order to maintain the alloy from deactivation due to oxide layer formation. performance over the lifetime of the material. The effects of The poor kinetics of MgH2 are greatly improved by addition N2,O2,CO2 and CO on a pure magnesium powder have been of different oxide catalysts that enhance hydriding properties studied [98]. Both O2 and N2 slowed the rate of hydrogen ab- at relatively low temperature, such as V2O5 [28] and Cr2O3 sorption, while CO and CO2 entirely prevented the uptake of [34,55]. The oxide particles may operate as a milling ball dur- . . hydrogen [98,99]. The performance of MgH2 V Ti was eval- ing high-energy ball-milling that creates many defects in the uated after 1000 cycles under a H2 atmosphere containing Mg powder. Defects provide hydrogen an easy path to Mg [28]. 101 ppm moisture. Hydrogen-storage capacity increased 5% but It was also proposed that Cr2O3 yields fast hydrogen absorp- desorption properties deteriorated due to surface modification tion, whereas V2O5 and Fe3O4 cause the most rapid desorp- of the particles [100]. tion of hydrogen [57]. The addition of TiO2 also resulted in B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1129

Wang et al. [33,36] combined both the catalytic and milling effect in a system of fine ZrFe1.4Cr0.6 particles covering Mg particles that resulted in an enhancement in absorp- tion/desorption rates. ZrFe1.4Cr0.6 dispersing homogeneously on the particle surfaces and in the interior of the material played an important role in promoting the decomposition of MgH2. That probably could account for the decrease in activation en- ergy. In another study [51], mechanically alloyed Mg–30 wt% LaNi2.28 reported to have good hydriding properties with 5.40 wt% hydrogen storage under 30 bar hydrogen pressure. With energetic ball-milling, the solubility limit of magne- sium in lithium slightly increased [106]. Luo [44] developed / LiNH2 MgH2 system by partial substitution of Li by Mg which has a storage capacity of 4.5 wt% at 200 ◦C and 30 bar. Me- chanical alloying of Mg with some elements such as Zn, Al, Ag, Ga, In or Cd resulted in reducing the stability of magne- sium hydride. Indium and cadmium gave the best results [107]. Bogdanovic et al. [49] and Reiser et al. [43] considered the Mg2FeH6–MgH2 systems. The maximum amount of stored hy- drogen is 5.0 wt% which is one third less than magnesium dihy- . dride with 7.7 wt%, but the Mg2FeH6 MgH2 system possesses numerous advantages including the low price of starting mate- rials, the free choice and constancy of the heat delivery temper- ature by controlling the applied hydrogen pressure and absence Fig. 5. Hydrogen desorption curves of mechanically milled MgH2–5 at% V composite (hollow symbols) and MgH2 (solid symbols) under 0.15 bar [54]. of heat losses with time. The intermetallic hydride Mg2FeH6 / 3 shows the high volumetric hydrogen density of 150 kgH2 m , which is more than double of liquid hydrogen [108]. a markedly improved hydrogenation performance of Mg, rapid Nanocomposites of MgH2 and 3d transition metals, Ti, V, kinetics, low working temperature and excellent oxidation re- Mn, Fe and Ni, have been investigated intensively [48,50,109]. sistance [46]. Liang et al. [40] suggested that La showed a cat- The composites containing Mg–Ti and Mg–V exhibited the alytic effect on Mg–50 wt% LaNi5 nanocomposite. rapid absorption kinetics such as absorption time of 2–5 min. In addition to catalyst type, the amount of catalysts used Formation enthalpy and entropy of magnesium hydride is dis- has a significant effect on hydrogen absorption behavior. torted by milling with transition metals. Furthermore, the acti- Oelerich et al. [57] investigated different amounts of oxides vation energy of desorption for magnesium hydride is reduced for catalysis but it is shown that only 0.2 mol% of the cata- at 200 ◦C. lyst is sufficient to provide fast sorption kinetics. The effect of Nb O concentration on the kinetics of magnesium hydro- 2 5 3. Complex hydrides gen sorption reaction at 300 ◦C is studied. Fastest kinetics are obtained using 0.5 mol% Nb2O5 with a 7.0 wt% of hydrogen Another class of light-weight storage materials is com- capacity [26]. plex hydrides. Complex hydrides are known as “one-pass” In the search for efficient and inexpensive catalysts for hy- hydrogen-storage systems which mean that H2 evolves upon drogen sorption reactions, a new type of catalytic compounds contact with water. Sodium, lithium and beryllium are the only is developed [105]. These catalytic complexes demonstrated re- elements lighter than magnesium that can also form solid-state markable enhancement in sodium alanates and magnesium, as compounds with hydrogen. The hydrogen content reaches the well as in hydrogen generation through hydrolysis [105]. value of 18 wt% for LiBH4. Use of complex hydrides for hydrogen storage is challenging because of both kinetic and 2.4. Chemical composition thermodynamic limitations. Intense interest has developed in low weight complex hy- − − The type and the chemical composition of the metal alloy drides such as alanates [AlH4] , amides [NH2] , imides and − is one of the most important factors in the metal–hydrogen borohydrides [BH4] . In such systems, the hydrogen is often system [21]. Bouaricha et al. [31] prepared Mg/Al alloys by located at the corners of a tetrahedron. The alanates and borates high-energy ball-milling. The measured hydrogen capacity of are especially interesting because of their light weight and the the material decreases with Al content, from H/M = 1.74 for capacity for large number of hydrogen atoms per metal atom. pure unmilled Mg, to 1.38 for Mg:Al (90:10), and then to 1.05 Borates are known to be stable and decompose only at ele- for Mg:Al (75:25). But it improves the kinetics. Ball-milling has vated temperatures. Alanates are remarkable due to their high a possibility to combine two or more phases having different storage capacities; however, they decompose in two steps upon hydrogenation characteristics [21]. dehydriding. 1130 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

3.1. Sodium alanates Moreover the surface of sodium alanate is damaged by the catalyst which causes decreasing the cyclic capacity. Sodium alanates are complex hydrides of aluminum and The dry-doping TiCl3 catalyst is investigated by the same sodium. Sodium tetrahydroaluminate–NaAlH4 and trisodium group [123]. It largely eliminates the high catalyst weight, hexahydroaluminate–Na3AlH6 have been known for decades. low capacity and contamination problems noted above for the Sodium aluminum hydride, NaAlH4, would seem to be a alkoxide catalysts [116]. Both the hydriding and dehydriding possible candidate for application as a practical on-board rates increases by doping [123]. Under the same rehydrogena- hydrogen-storage material due to the theoretically reversible tion conditions, the amount of evolved hydrogen from the hydrogen-storage capacity of 5.6 wt%, low cost and its avail- Ti-doped NaAlH4 is about 50 times as much as that of the ability in bulk. undoped NaAlH4. This clearly indicates that the addition of ti- Although they have good hydrogen-storage capacity, com- tanium species enhances kinetically not only the dehydrogena- plex aluminum hydrides are not considered as rechargeable tion, but also the rehydrogenation reaction of NaAlH4 [118]. hydrogen carriers due to irreversibility and poor kinetics. With 2 mol% TiN as a doping agent, cyclic storage capacity However, by using appropriate transition or rare-earth metals of 5 wt% H2 is achieved after 17 cycles. However, decrease in as catalysts, the complex hydrides can be made reversible. hydrogenation rate with number of cycles is observed [122]. Bogdanovic and Schwickardi [110,111] demonstrated upon Zirconium doping is also used for the same purpose. The dehy- doping with proper titanium compounds, the dehydriding of driding kinetics of NaAlH4 is significantly improved. Although aluminum hydrides could be kinetically enhanced and main- effects of zirconium are poorer than titanium as a catalyst for tain reversibility under moderate conditions in the solid state. the dehydriding of NaAlH4 to Na3AlH6 and Al, it is a superior Unlike intermetallic hydrides, these complex-based hydrides catalyst for the dehydriding of Na3AlH6 to NaH and Al [119]. release hydrogen through a series of decomposition reactions Sun et al. [124] examined the details of the doping role as described in two equations below, for the dehydrogenation of titanium and zirconium on NaAlH4 with XRD. Significant reactions of NaAlH4: changes occur in the lattice parameters of sodium alanates upon doping. Hence the enhancement of dehydriding kinetics is as- ↔ 1 + 2 + NaAlH4 3 Na3AlH6 3 Al H2, (4a) sociated with dopant induced lattice distortions rather than the catalytic effect [124]. Phase transitions and crystal structure ↔ + + 3 Na3AlH6 3NaH Al 2 H2. (4b) modifications of doped-NaAlH4 were also observed by Gross et al. [125]. Contrary to Sun et al. [124], they proposed the cat- Theoretically, NaAlH4 and Na3AlH6 contain large amounts alytic effects of doping materials in the enhancement of kinet- of hydrogen, 7.4 and 5.9 wt%, respectively. However, the re- ics of NaAlH4. It was earlier reported that the presence of Ti is lease of hydrogen does not occur in a single-step reaction. not enough rather a particular local arrangement is required for NaAlH4 first decomposes evolving molecular hydrogen and the dehydrogenation reaction [126]. Ti-cluster-doped NaAlH4 form an intermediate compound, Na3AlH6 and metallic Al. is investigated [122,127]. The hydrogenation and dehydrogena- This intermediate phase then decomposes to NaH with ad- tion measurements indicate that the state of the precursor is of ditional metallic Al formation and hydrogen evolution. The even greater importance than the simple amount of Ti in the reversibility of these two reactions is a critical factor for the material. The small size of the Ti clusters seem to be responsi- practical applications. ble for the increase of the reaction rate [127]. Stoichiometrically, the first step consists of 3.7 wt% H2 re- Although titanium has positive effects as a catalyst, kinet- lease and the second step 1.9 wt%, for a theoretical net reaction ics is not fully enhanced. The hydrogen desorption can last of 5.6 wt% reversible gravimetric hydrogen storage [111,112]. up to 4200 min in some cases [123]. To improve the kinet- The process is characterized by slow kinetics and reversibility ics Thomas et al. [128] investigated the effects of mechanical only under severe conditions. The operating temperatures are ball-milling on the microstructural character of NaAlH4 in the between 185 and 230 ◦C, and 260 ◦C for the first and the sec- presence of catalyst. Fracture and fragmentation of particles ond reaction, respectively. Finally, the decomposition of NaH are observed in particle morphology. The direct synthesis of occurs at a much higher temperature, with the total hydrogen Na3AlH6 and Na2LiAlH6 by energetic mechanical alloying was release of 7.40 wt%. For hydrogen storage, only the first two also investigated earlier [129]. The milled NaAlH4 or Na3AlH6 reactions need to be considered, because the decomposition of exhibited great improvement of the kinetics of absorption and NaH occurs at too high temperature of 425 ◦C for practical desorption. The addition of carbon in the milling process im- storage systems. proved their performance as well [120]. Mechanically alloyed Previously Jensen et al. [113] reviewed the catalytically en- Na3AlH6 exhibits faster kinetics than Na3AlH6 obtained from hanced sodium aluminum hydrides. Recently, numerous stud- the decomposition of NaAlH4 [130]. Moreover, the hydrogen- ies are being carried out to enhance the hydriding properties of storage performance of NaAlH4 was found to be highly de- NaAlH4 as listed in Table 3. pendent on the milling time which increases with time [117]. n i The effects of liquid alkoxides Ti(Obu )4 + Zr(OPr )4 on Consequently mechano-chemical synthesis method which in- hydriding properties of sodium alanates were investigated volves simply ball-milling of the appropriate reagents at high by Sandrock et al. [116]. The liquid alkoxides contribute energy, is applied to prepare Na3AlH6 [114]. It can desorb the to hydrocarbon contamination of the released hydrogen. same amount of hydrogen at 200 ◦C within only 150 min and B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1131

Table 3 Hydrogen absorption/desorption properties of sodium alanates

Material Method Temperature (C◦) Pressure (bar) Kinetics (min) Cycling stability Max Ref. wt% of H2

Na3AlH6 Mechano-chemical Tdes: 200 Pdes:1 tdes: 150 No data 2.50 [114] synthesis Na2LiAlH6 BM Tabs: 211 Pabs:45 tabs: 100 No data 2.50 [115] NaAlH4–2 mol% BM Tabs and Tdes: 125–165 Pabs and Pdes: 101–202 tabs: 60 5 cyc.: not 3.00 [116] n Ti(Obu )4–2 mol% tdes: 180 stable i Zr(OPr )4 NaAlH4–4 mol% Ti BM Tabs: 120 Pabs: 120 tabs: 60 8 cyc.: not 3.30 [117] Tdes: 150 Pdes:1 tdes: 600 stable NaAlH4–2 mol% Ti BM Tdes: 25–160 Pabs: 20–120 tabs: 300–720 No data 3.80 [118] Tabs: 25–193 Pdes:1 tdes:40 NaAlH4–2 mol% Mixing Tabs: 120 Pabs and Pdes: 60–150 tabs: 1020 25 cyc.: not 4.00 [111] n (Ti(Obu )4 Tdes: 180–260 tdes: 120–300 stable [112] NaAlH4–2 mol% Mixing Tabs: 135–120 Pabs and Pdes: 150–130 tabs: 330 33 cyc.: not 4.00 [110] n Ti(Obu )4 Tdes: 180–160 tdes: 90 stable NaAlH4–2 mol% TiCl3 BM Tdes: 125–100 Pdes: 83–91 tdes: 20 5 cyc.: not stable 4.00 [116] NaAlH4–2 mol% Mixing Tdes: 200 Pdes: 1 No data 3 cyc.: stable 4.00 [119] Zr(OPr)4 3 after second cyc. NaAlH4–2 mol% Mixing Tabs: 104 Pabs:88 tabs: 1020 3 cyc.: stable 4.00 [113] n (Ti(Obu )4) after second cyc. NaAlH4 Mechano-chemical Tabs and Tdes: 80–180 Pabs and Pdes: 76–91 tabs: 120–300 2 cyc.: not 5.00 [120] synthesis tdes: 300 stable NaAlH4–2 mol% Mixing Tdes: 200 Pdes: 1 No data not stable 5.00 [121] n Ti(Obu )4-C NaAlH4–2 mol% TiN BM Tabs: 104–170 Pabs: 115–140 tdes: 30–1200 25 cyc.: stable 5.00 [122] after 17th cycle

without a catalyst, giving reaction rate about 10 times faster very high temperatures of above 680 ◦C [114]. Therefore, the than conventionally produced, non-catalyzed hydrides [114]. commercialization of lithium-based compounds is hindered by their slow kinetics and high temperature absorption and 3.2. Lithium and potassium alanates desorption. The reversible hydrogen decomposition of KAlH4 has been A survey of important studies on lithium-based hydrogen- studied previously [138]. The hydrogen capacity was above storage compounds are summarized in Table 4. It shows clearly 3.5 wt% under 10 bar of hydrogen in a temperature range of ◦ high capacities of stored hydrogen as weight percent material. 250–330 C, the reversible reaction smoothly proceeds without In theory, lithium alanates are very attractive for hydro- any catalyst, which is different from the reactions of NaAlH4 gen storage, because of their high hydrogen content. The to- and LiAlH4 [138]. tal hydrogen content is 10.5 and 11.2 wt% for LiAlH and 4 3.3. Lithium nitrides Li3AlH6, respectively. Unfortunately, LiAlH4 has an extremely high equilibrium pressure of hydrogen, even at room temper- Lithium nitride is usually employed as an electrode, or as a ature. LiAlH is in fact an example of an unstable hydride, 4 starting material for the synthesis of binary or ternary nitrides. which decomposes easily, but which cannot be re-hydrogenated Although the temperature required to release the hydrogen [114]. The desorption of LiAlH occurs in two steps: 4 at usable pressures is too high for hydrogen-storage appli-

3LiAlH4 → Li3AlH6 + 2Al + 3H2, (5a) cations it was suggested that the metal–N–H system could prove to be a promising route for reversible solid hydrogen → + + 3 Li3AlH6 3LiH Al 2 H2. (5b) storage [45]. The idea of hydrogen storage in lithium compounds goes The hydrogen released in the two reactions shown, corre- back to 1910, when Dafert and Miklauz [139] reported the sponds to 5.3 wt% from the decomposition of LiAlH4 and reaction between Li3N and H2 to Li3NH4. In fact Li3NH4 has 2.65 wt% from the decomposition of Li3AlH6 at temperatures ◦ been proved to be the product of the following reaction, which between 160 and 200 C. After completion of the reactions, was proposed by Ruff and Georges [140]: 2.65 wt% of the total hydrogen content in LiAlH4 remains unreleased in form of LiH which can be desorbed only at Li3N + 2H2 → LiNH2 + 2LiH. (6) 1132 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

Table 4 Hydrogen absorption/desorption properties of Li-based hydrides

Material Method Temperature (◦C) Pressure (bar) Kinetics (min) Cycling stability Max wt% Ref. of H2

LaNi3BH3 Arc melting Tabs:25 Pabs:4 tabs: 30 2 cyc.: not stable 0.84 [131] Li2NH Mixing Tabs and Tdes: 230–200 Pabs:7 tabs: 10 15 cyc.: not 3.10 [132] Pdes:1 tdes: 300 stable LiNH2–LiH–1 mol% BM Tdes: 150–250 Pabs:30 tabs: 30 3 cyc.: stable 5.00 [133] TiCl3 Tabs: 180 Li2O–Li3N Partial oxidation Tabs: 180 Pabs:7 tabs: 3 6 cyc.: stable 5.20 [134] Li2MgN2H2 Mixing Tabs and Tdes: 180 Pabs:90 tabs: 60 No data 5.50 [42] Pdes:1 tdes:60 Li3N Mixing Tabs:50 Pabs: 0.5 tabs: 20 No data 6.00 [135] Tdes: 240–270 Pdes:1 Li2NH Mixing Tabs and Tdes: 255–285 Pabs: 10 No data No data 6.50 [45] Pdes: 1.5 Li3BN2H8 BM Tdes: 250–364 Pdes:1 tdes: 228 No data 10.00 [136] 1 T T P t LiBH4- 2 MgH2–2 mol% BM abs and des: 315–450 abs: 4.5–19 abs: 240 3 cyc.: not 10.00 [137] TiCl3 Pdes: 2–3 tabs: 240 stable

Consequently, Li3N can theoretically store 10.4 wt% hydro- storage capacity of above 10 wt%, however, reversibility is lack- gen. Li3N has two plateaus in the P–C isotherm. Desorption ing. Although desorption conditions seem to be advantageous, isotherms cannot return to the origin. Under P–C isotherm con- irreversibility stands as an important problem for use of these ditions, about 55% of hydrogen can be desorbed at tempera- materials to be utilized as storage medium. Authors reported tures above 230 ◦C. More important for hydrogen storage, is their attempts to rehydride the decomposition product by heat- that this mixture could decompose to release hydrogen gas upon ing at 8 MPa but without success. heating, according to the following reaction: A proposed way for lowering the hydrogen desorption temperatures of Li-based complex hydrides is partial cation LiNH + 2LiH → Li NH + LiH + H . (7) 2 2 2 substitutions using different valence cations with larger elec- Hydrogen storage leads to the formation of LiNH2 and LiH. tronegativities. It was predicted that the dehydriding reactions Theoretically 7 wt% of hydrogen can be reversibly stored in of LiNH2 with partial Mg substitutions are useful as hydrogen- Li2NH [45]. storage materials for fuel–cell applications [142]. The starting A critical potential issue regarding this N-based storage ma- and ending temperatures for the hydrogen desorption reac- terial is the generation of NH3, which consumes some H2 and tion from LiNH2 are lowered about 50 K by the partial cation also poisons the downstream processes. Hu et al. [141] demon- substitution of Li by Mg [143,144]. strated that NH3 produced via the decomposition of LiNH2 is completely captured by LiH. This ultrafast reaction between 3.4. Lithium boro- and beryllium hydrides NH3 and LiH inhibits NH3 formation during the hydrogenation of Li3N [141]. The importance of boron for hydrogen-storage technologies Hu et al. [135] studied the temperature-programmed hydro- have been reported [145]. LiBH4 has a gravimetric hydrogen genation and dehydrogenation of Li3N and reported a reversible density of 18 wt%. The compound was first synthesized by hydrogen capacity of 6 wt%, within 240–270 ◦C temperature Schlesinger and Brown [146] in an organic solvent. Accord- range. Another study of the same group was to investigate the ing to the work of Stasinevich and Egorenko [147] hydrogen ◦ fast kinetics of LiO2/Li3N mixture for hydrogen storage [134]. desorbs from LiBH4 at temperatures greater than 470 C. They improved the activation characteristics of Li3N with the Despite its great storage capacity, all attempts to synthesize ◦ hydrogenation–dehydrogenation pretreatment, partially oxidiz- LiBH4 from the elements at elevated temperatures up to 650 C ing the surface layer. They reported a maximum of 5.20 wt% and pressure of 150 bar H2 failed to date [108,148]. Moreover, hydrogen capacity with very fast kinetics such as 3 min for LiBH4 is an expensive compound [15]. desorption. It was found that the compound releases hydrogen in A catalytic study of Ichikawa et al. [133] reported a 5.50 wt% different reaction steps and temperature regimes. The low storage with high reaction rate after three cycles, with the uti- temperature desorption releases only a small amount (0.3 wt%) lization of TiCl3 catalyst during ball-milling of the LiNH2 and of hydrogen. The high temperature phase releases up to LiH mixture. Recently Hu et al. [132] reported a reversible hy- 13.5 wt% of hydrogen. A total of 4.5 wt% of the hydrogen drogen capacity of 3.10 wt%, that increases with the number remains as LiH in the decomposition product. of cycles, due to improved porous structure of the material. Vajo et al. [137] show that LiBH4 can be reversibly store Another study on a different class of solid hydride has been 8–10 wt% hydrogen at temperatures of 315–400 ◦C by addi- done by Pinkerton et al. [136], reporting an extraordinarily high tion of MgH2 including 2–3 mol% TiCl3. Formation of MgB2 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1133 stabilizes the dehydrogenated state and destabilizes the LiBH4 The early studies on LaNi5 as hydrogen-storage material has [137]. However, the kinetics is too slow with the absorption been reported by Aoyagi et al. [67]. The effect of ball-milling time up to 6000 min hinder practical applications. on hydrogen absorption properties has been investigated, and Besides alanates, nitrides and borohydrides, lithium–beryllium maximum hydrogen content has been reported as 0.25 wt%. It hydrides are a new group of metal hydrides for hydrogen stor- is concluded that activation treatment is necessary for LaNi5 age. They show high reversible hydrogen capacity with more to be useful for hydrogen-storage applications. Kaplan et al. ◦ than 8 wt% at 150 C. The reaction of hydride formation is [181] have investigated the hydrogen absorption in pure LaNi5 fully reversible. Since lithium and beryllium are the lightest both theoretically and experimentally. The group reported that hydride-forming metals, reversible hydrogen capacity in these LaNi5 has 1.28 wt% hydrogen-storage capacity for 8.3 min hy- complex compounds is higher than in any other known hy- driding process. They characterized the hydriding process by drides [149]. On the other hand, the Li3Be2H7 is a highly toxic exothermic reaction between LaNi5 and H2 that leads to rapid material [15]. temperature increase. These supported the results from theoret- ical calculations. 4. Intermetallic compounds With different stoichiometric addition of other metals, LaNi5 compounds were shown to be enhanced for hydrogen-storage Research on intermetallic compounds for hydrogen storage by the works of Chen et al. [182], Corre et al. [175], and Lu et al. was already attempted more than 20 years ago. The discovery [174]. They achieved hydrogen-storage capacities with differ- of hydrogen absorption by LaNi5 [150] and FeTi [151], opened ent stoichiometric LaNi5 samples such as 1, 1.44 and 1.32 wt%, new possibilities for industrial developments. However, for on- respectively. Chen et al. [182] applied chemical coating by cop- board storage, they remained at the stage of prototypes due per and achieved kinetics and cycling behavior during hydro- their weight penalty and low hydrogen-storage capacity [152]. genation /dehydrogenation process. By CO surface treatment, The different families of intermetallic compounds classified 1.44 wt% hydrogen capacity is achieved in LaNi5.Luetal. on the basis of their crystal structures, such as AB2 type (Laves [174] processed the LaNi5 using a twin roll process in order to phase), AB5 type phases and Ti-based body centered cubic, achieve uniform nanocrystalline structure with 1.32 wt% stor- BCC, alloys are well known as hydrogen-storage materials. age capacity. Intermetallic compounds are often obtained by combining an Apart from the above reported works, a few important stud- element forming a stable hydride with an element forming a ies in the literature are worth mentioning. Liu et al. [183], Wang nonstable hydride. As for the metallic hydrides, the dissociative et al. [184], and Suda et al. [185] reported on the fluorination of chemisorption of hydrogen is followed by hydrogen diffusion hydriding alloys and its effects on properties. The aim of these into the interstitial sites. experiments was to form a Ni-rich layer in order to improve The hydriding properties of these compounds are summa- the initial activation and impurity tolerance. There are also rized in Tables 5 and 6. For practical applications at ambient theoretical studies found in the literature, on LaNi5 utilization temperature and pressure, their low energy density per unit for hydrogen storage, tank design and optimization. Kikkinides weight is an important critical disadvantage. For instance, et al. [186] showed the direct relation between hydrogen- the hydrogen capacity of the most popular LaNi5-based al- storage performance and optimization in LaNi5 systems. loys operating at moderate temperature does not exceed Storage time could be improved by 60% through optimization. 1.4 wt%. The mechanism of hydrogen diffusion in LaNi3BHx is inves- Among the AB5 type alloys, due to their low working tem- tigated. The results suggested that hydrogen occupies sites co- perature and pressure, metal alloys containing high amounts of ordinated by lanthanum and nickel only, while the basal La–B LaNi5 have been studied as hydrogen-storage materials by var- planes act as barriers to hydrogen diffusion along the hexag- ious research groups around the world [176–180]. Some of the onal axis. Boron has an adverse effect on the hydrogen sorp- studies were carried with pure compounds, while others stud- tion properties of intermetallic compounds because it acts as ied blending of the material with various metals through melt- a barrier for hydrogen diffusion and favors localization of the ing or mechanical alloying techniques. Important results from hydrogen atoms in the metal matrix away from the boron atom these studies are represented below. sites [131]. The parent compound LaNi5 absorbs about 1.0 H/LaNi5 The AB2 type compounds are derived from the Laves phases (1.5 wt%), however, its cost is relatively high and the plateau crystal structures. The potential AB2 types are obtained with pressure is low. The P–C diagram shows a flat plateau, low Ti and Zr on the A site. The B elements are represented mainly hysteresis, but unfortunately the hydrogen capacity is de- by different combinations of 3d atoms, V, Cr, Mn and Fe. The graded after a few cycles. Therefore, these materials are still hydrogen-storage capacity can reach up to 2 wt% in Laves phase far from meeting the US DOE goal of 6.5 wt% reversible V–7.4%Zr–7.4%Ti–7.4%Ni [166]. The Laves phase series of hydrogen capacity. Due to their potential for commercial appli- compounds has attracted large attention in the last decade due cations, researchers have been studying the effects of milling, to their good hydrogen-storage capacity. Most of Laves phases mechanical alloying or melting with other metals and surface show relatively high capacities faster kinetics, longer life and a treatment techniques such as carbon monoxide treatment. relatively low cost in comparison to the LaNi5-related systems Table 6 presents the major outputs from a number of studies [155]. However, their hydrides are too stable at room temper- on AB5 type compounds for hydrogen storage. ature [187]. In general the AB2 type compounds seem to be 1134 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 [153] [155] [157] [158] [160] [161] [160] [162] [163] [165] [166] [167] [169] [161] [172] [156] [156] [159] [159] [164] [168] [170] [170] [171] [173] [154] Ref. 2 of H : 6.6 No data 0.95 des t and : 60: 60: 5: 100 15 cyc.: stable 100 cyc.: stable 11 cyc.: stable 11 after cyc.: ninth stable cycle after ninth cycle 1.10 1.30 1.44 : 1 1.30 :5 1000 cyc.: stable: 1200: 8.3 No data: 8.3 1.80 No data: 20 No data: 15:10 50 2.23 cyc.:stable 3.01 6 cyc.: not stable 3.36 3.90 5.00 abs des des des abs abs des abs abs abs abs abs des t t t t t t t t t t t t t : 50: 5–10 : 0.24 : 30:10 No data: 3–4: 3.5:10 No data:25 No data:35 No data No: data 100: 6.8 : No 17 data: 0.5 1000 No cyc.: data: stable 100 5:81 cyc.: stable: No 5 data: 0.33 No data: 47 No data:33 0.50 :1 No data 1.16 : No 100 data: 0.99 No 10 data No data:1 1.30 : 50 No data: 0.2 No data:80 No data No data: No 100 data 1.45 :30 No data:1 :30 10 1.50 cyc.: not No stable data:1 1.55 : No 17 data: 0.5 : 100 1.56 :10 No data: 1.75 No 30 data: 2.00 0.03 :40 1.92 :1 No data No data 2.20 No data 2.80 3.50 3.98 abs abs des abs des des des des des abs abs des abs des abs des abs des abs abs des abs abs des abs des abs abs abs des abs des abs des abs des abs des abs des P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P C) Pressure (bar) Kinetics (min) Cycling stability Max wt% ◦ : 100, 25 :80 :25 :25 :30 :30 :23 :30 :40 :30 : 27–327 des abs des des des des des des des des des T T T T T T T T T T T 196 20 20 − − − and and and and and and and and and and and : : 120 :25 : :25 :20 :20 :40 :25 :40 : 300 : 427 :60 :60 : : 300 : 247–472 abs abs abs des des des des abs abs abs abs abs abs abs abs abs abs abs abs des abs abs abs abs abs des des abs T T T T T T T T T T T T T T T T T T T T T T T T T T T T BM BM + + Melting Induction melting RF melting Arc melting Induction melting RF melting Melting Melting RF levitation melting RF levitation melting Melting Arc melting Arc melting BM Melting Arc melting BM 0.32 Sn 4.63 0.90 Ni 0.2 Co V 0.075 0.01 0.1 0.4 –20 wt% Melting 0.2 0.5 Pr Mn 13 0.9 Mn Cr Mn Ni N V 0.04 5 5 2 0.30 ) 1.6 1.6 1.5 14 5 2.65 1.1 Ni Ni Nd 7.5 Cr 0.4 0.4 0.2 Cr Cr Ni Ni 17 Mn Mg 0.2 Fe 0:15 Al Fe 0.25 0.9 0.05 0.3 Ni 0.25 Mo 0.03 0.25 0.2 0.45 Sn 49 V 0.15 4.6 4.6 38 Ca Ca Ti Ce Y Ti 0.8 Zr V Mg Ca 4.8 5 Zr CrMn Arc melting 1.1 Zr 0:85 0.75 0.90 0.55 0.7 1.8 0.75 0.9 0.97 1.1 45 43.5 0.375 MaterialMgS Method BM Temperature ( Ti–V–Cr Arc melting FeTi BM La Zr(Cr Ml La Ti 80 wt% TiCr LaNi Ti La Zr Ti V–7.4%Zr–7.4%Ti–7.4%NiV Ti Arc melting Ti–10Cr–18Mn–27V–5FeTi–10Cr–18Mn–32VTiCr Magnetic levitation melting Ti Ti–V–Cr–Mn Magnetic levitation melting La Magnetic levitation melting LaNi MmNi MmNi Ml Quasicrystal powders Table 5 Hydrogen absorption/desorption properties of intermetallic compounds B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140 1135

Table 6 Hydrogen absorption/desorption properties of LaNi5 compounds

Material Method Temperature (◦C) Pressure (bar) Kinetics (min) Cycling stability Max wt% Ref. of H2

LaNi5 BM Tabs:20 Pabs :20 tabs: 1.6 8 cyc.: not stable 0.25 [67] La0.59Ce0.29Pr0.03Ni4Co0.45Mn0.45Al0.3 Twin-rolling Tabs:60 Pabs: 10 No data No data 1.27 [174] Pdes: 0.6 La0.9Ce0.1Ni5 CO surface Tabs: 0–100 Pabs:50 tdes: 1.8 20 cyc.: stable 1.40 [175] treatment after fifth cycle Tdes:25 LaNi5 CO surface Tabs: 0–100 Pabs:50 tdes: 13.6 20 cyc.: stable 1.44 [175] treatment after fifth cycle Tdes:25

more sensitive to gaseous impurities than the AB5 type com- mance, hydrogen absorption–desorption plateau pressure, hy- pounds. Thus, a small amount of oxygen can be a poison for drogen desorption capacity and reduce the hysteresis of hydro- the AB2s, while, for the AB5s, it acts as a reactant, reducing the gen absorption–desorption plateau, and the cost of alloy [170]. storage capacity slightly. CO is a poison for both types of com- The quasicrystals have a new type of translational long-range pounds, although capacity recovery is possible by recycling in order, display non-crystallographic rotational symmetry. They pure hydrogen [188]. contain high amounts of interstitial sites. The quasicrystalline Hydrogen absorption of FeTi powders has previously been Ti45Zr38Ni17 has 2.23 wt% storage capacity [168]. The sorp- extensively studied. FeTi is a well-known hydrogen-storage tion of hydrogen between the layers of the multilayered wall compound with a total hydrogen capacity of around 1.90 wt% of nanotubular TiO2 was studied in the temperature range of with inexpensive elements. Hydrogen capacity of FeTi can be 195–200 ◦C and at pressures of up to 6 bars. Hydrogen can in- accomplished to 1.90 wt% by the catalytic effect of 1 wt% Pd tercalate between layers in the walls of TiO2 nanotube [189]. addition [165]. However, the activation process of FeTi is trou- blesome due to the formation of titanium oxide layer. Both 5. Conclusion high-pressure and high temperature are required to achieve a reproducible absorption/desorption of the maximum amount of Hydrogen storage is a key issue in the success and realiza- hydrogen in the compound [151,187]. tion of hydrogen technology and economy. According to US New BCC solid solution alloys have been reported to ab- DOE, the hydrogen-storage capacity target for commercializa- sorb more hydrogen than the conventional intermetallic com- tion is 6.5 wt% at the decomposition temperature between 60 pounds. In recent years, study of Ti-based BCC phase alloys and 120 ◦C with high cycle life. Although pure water contains has been studied in several laboratories because of their remark- 11.1 wt% of hydrogen, its decomposition requires much ther- able hydrogen capacities. However, the high cost is one of the mal, electric, or chemical energy [15]. critical drawbacks limiting their successful practical applica- Since the conventional hydrogen fuel storage methods of tions. Ti–10Cr–18Mn–27V–5Fe and Ti–10Cr–18Mn–32V has pressurized H2 gas and cryogenic liquid H2 pose safety and per- hydrogen-storage capacities of 3.01 and 3.36 wt%, respectively meation problems along with high cost, they do not meet future [170]. Increasing the V content is effective in accelerating hy- on-board applications goals set for hydrogen economy. Solid drogen absorption, enhancing the hydrogen absorption capacity state hydrogen fuel storage either absorption in the interstices and flattening the hydrogen desorption plateau, while decreas- of metals and metallic alloys or adsorption on high surface area ing hydrogen desorption plateau pressure. The maximum and materials such as activated carbons gain the attention for pos- effective hydrogen-storage capacities of Ti–V–Cr–Mn alloys sible future hydrogen applications. The present article reviews are 3.98 and 2.51 wt%, respectively [172]. the hydrogen-storage materials for transport applications which It is well known that vanadium is expensive; therefore, de- require research into further aspects of tank technology, heat creasing of the vanadium content is another goal of the investi- management and solid fuel recycling. The work is carried out gations. In Ti–V–Cr–Mn compounds, increasing the Cr content in an attempt to facilitate prospectus material choice for further and the addition of Mn is necessary to increase the effective hy- tank design aiming at on-board vehicle applications. drogen capacity by increasing the plateau pressure, but also to Hydrogen can be stored in metal hydrides under moderate decrease costs by decreasing the vanadium content. The BCC temperature and pressure. Metal hydrides are the promising phase solid solution of V0.375Ti0.25Cr0.30Mn0.075 exhibited an candidates due to their safety advantage with high volume- effective hydrogen capacity of 2.20 wt% [167]. efficient storage capacity for on-board applications. Intensive In order to improve the hydrogen-storage properties and re- research has been recently conducted on the metal hydrides for duce the cost of Ti–V-based BCC alloys, the effect of Fe substi- improving adsorption/desorption properties based on hydrogen- tution in Ti–10Cr–18Mn–32V alloy is also investigated [170]. storage capacity, kinetics, thermal properties, toxicity, cycling It was reported that Fe addition increases the activation perfor- behavior and cost. 1136 B. Sakintuna et al. / International Journal of Hydrogen Energy 32 (2007) 1121–1140

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