Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen

Chem. Rev. 2004, 104, 1283−1315 1283 Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen Wojciech Grochala*,†,‡ and Peter P. Edwards*,§ The School of Chemistry, The University of Birmingham, Edgbaston, Birmingham, B15 2TT U.K., The Department of Chemistry, The University of Warsaw, Pasteur 1, 02093 Warsaw, Poland, and Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR U.K. Received June 27, 2003 Contents 12. Acknowledgment 1305 13. Note Added in Proof 1306 1. Introduction 1283 14. References and Notes 1306 1.1. Nomenclature and the Coverage of the 1284 Review 1.2. Contents 1285 “Hydrogen is the most important consistent of the universe.” 2. The Storage of Hydrogen 1286 s 1 -I Gerhard Herzberg 3. Which Equilibrium Is Easier To Play On: H /H2 1290 +I or H /H2? 1. Introduction 4. How Does the Thermal Decomposition of a 1291 Hydride Proceed? Hydrogen is nature’s simplest, lightest atom, con- sisting of a single proton and a single electron.2 5. How May Tdec Be Tuned, Therefore, in Binary 1293 and Multinary Hydrides of the Chemical Paradoxically, this “simplicity” in electronic structure Elements? leads to unique consequences for the chemical and 6. Standard Enthalpy of Decomposition as an 1297 physical properties of the element and its atomic, Important Predictor of Tdec cationic, and anionic siblings. In its desire to possess 7. Novel Binary and Ternary Hydrides of MIII (M ) 1299 either a totally empty or a totally filled 1s electron Ga, In, Tl, Sc, Ti, V, Yb, Eu), MIV (M ) Sn, Pb, shell, hydrogen exhibits three common oxidation S‚‚‚Po, Ti‚‚‚Hf, Ce, Th‚‚‚Am), MV (M ) P‚‚‚Bi, states: +1, 0, and -1 (reflecting a 1s0,1s1,or1s2 Cl‚‚‚At, V‚‚‚Ta, Pa‚‚‚Am), MVI (M ) S‚‚‚Po, electronic configuration, respectively). The relative Cr‚‚‚W, U‚‚‚Am), and MVII (M ) Cl‚‚‚I, change in the number of electrons surrounding the Mn‚‚‚Re) nucleus associated with the transformations H0 f III - + 7.1. Notes on the Existence of M H3 (M ) Ga, 1299 H I and H0 f H I ((1e, (100%) is thus the largest In, Tl, Sc, Ti, V, Yb, Eu) among the chemical elements of the Periodic Table; 7.2. Notes on the Existence of Binary Hydrides of 1300 so are the relative changes of many key chemical and MIV (M ) S‚‚‚Po, Ti‚‚‚Hf, Ce, Th‚‚‚Am), MV physical properties of these three unitary species (see ) ‚‚‚ ‚‚‚ ‚‚‚ ‚‚‚ VI (M P Bi, Cl At, V Ta, Pa Am), M section 3). ) ‚‚‚ ‚‚‚ ‚‚‚ VII (M S Po, Cr W, U Am), and M (M Hydrogen dominates the 15-billion-year tale of our ) Cl‚‚‚I, Mn‚‚‚Re, U‚‚‚Am) universe (see, e.g., ref 1). It is by far-and-away the 8. Prediction of Thermal Stability of Several Novel 1301 most abundant element in the cosmos, of which it Ternary Hydrides makes up 88.6% of the composition by weight. The 9. Prediction of the Standard Enthalpies of 1302 chemical evolution of the stars depends crucially Formation for Binary and Multinary Hydrides upon the advance of hydrogen fusion. Bewildering 10. New Materials and the Future of Hydrogen 1302 amounts of life-giving solar rays provide energy for Storage our own hospitable planet in the form of heat and 10.1. Catalytic Enhancement of Hydrogen Storage 1303 light to our plant world via the remarkable photo- 10.2. Mechanochemical Synthesis 1304 synthetic cycle. Hydrogen as water is pivotal to life 10.3. Thermal Decomposition of Metal Hydrides 1304 + - and is present also in the overwhelming majority of Based on the Recombination of H and H organic compounds. But hydrogen is also one of the 10.4. Hydrogen Storage in Carbon-Based Materials 1305 most important elements in the chemistry of the 11. Summary and Outlook 1305 energy-giving materials, such as hydrocarbons (min- eral oil and methane gas). * Address correspondence to Prof. Peter P. Edwards, Inorganic The inevitable socio-economic impact of recurring Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK, E-mail [email protected], telephone fuel crises, the threat of the early end of the fossil +44-(0)-1865-272-652, fax +44-(0)-1865-272-690, or to Dr. Wojciech fuels era in the coming 50 years,3 the increasing Grochala, The Department of Chemistry, The University of Warsaw, Pasteur 1, 02093 Warsaw Poland, E-mail wg22@ pollution of our environment, and the problem of cornell.edu, telephone +48-22-822-0211 ext. 276, fax +44-22-822- anthropogenic-induced climate changeswidely, if not 2309. 4s † universally accepted have been the catalysts for the The University of Birmingham. 5 ‡ The University of Warsaw. utilization of clean and renewable energy resources. § University of Oxford. Hydrogen is undoubtedly one of the key alternatives 10.1021/cr030691s CCC: $48.50 © 2004 American Chemical Society Published on Web 02/18/2004 1284 Chemical Reviews, 2004, Vol. 104, No. 3 Grochala and Edwards plications.6 Interest in hydrogen experienced a re- naissance in the late 1960s and in the 1970s,6g and has grown even more dramatically since 1990, partly catalyzed by the deuterium cold fusion controversy;7 many advances in hydrogen production8 and utilization technologies have been made during the past decade. However, there remain a number of funda- mental scientific, technological, and socio-economic problems to be overcome before any large-scale utilization of hydrogen in our day-to-day human activities could occur. These problems, out which one can identify hydrogen storage at the very forefront, are serious inhibitors for acclaiming hydrogen as a potential savior element of the 21st century.9 In principle, hydrogen can be stored either in its el- Wojciech Grochala was born in Warsaw, Poland, in 1972. He received emental form, as a gas or liquid, or in a chemical his Ph.D. degree from the University of Warsaw in 1998, and came to form. Briefly, an ideal chemical hydrogen storage Cornell University as a postdoctoral fellow in 1999. He studied chemical aspects of vibronic coupling phenomena with Prof. Roald Hoffmann, with material will have a low molar weight (to decrease support from The Kosciuszko Foundation and The Cornell Center for the reservoir/storage mass), be inexpensive, have Materials Research. His research focused on prediction of new super- rapid kinetics for absorbing and desorbing H2 in the conducting materials in the solid state. He then moved to The University 25-120 °C temperature range, and store large quan- of Birmingham (UK) to work with Prof. Peter P. Edwards as a Royal tities of hydrogen reversibly (a much more detailed Society Fellow. He searched for experimental evidence of superconductivity “wish-list” is expanded-upon shortly). It is important in doped fluorides of AgII, and simultaneouslysin cooperation with the FUCHSIA European projectshe advanced theoretical approaches to to stress at the outset that, at present, no single hydrogen storage in solid hydrides. He has obtained various awards, material fulfills all of these requirements although, including the prestigious Crescendum Est-Polonia and The Foundation as we will illustrate, the physicochemical properties for Polish Science fellowships. His present research, back at his parent of hydrides of the chemical elements allow us at least University of Warsaw, now concentrates on advancing prospects for to discern a viable path toward these challenging, superconductivity in chemical hydrides and on the search for catalysts closely linked goals. for low-temperature hydrogen evolution from hydrides. His hobbies include mountaineering, photography, and martial arts. Romaniasits people, In the present work, therefore, we review key history, and culturesis his true passion. aspects of the thermal decomposition of multinary or mixed hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of such hydrides, Tdec. We have also at- tempted to predict the thermal stability of as-yet unknown, elusive or even unknown hydrides, and we discuss the future of a particularly promising class of materials for hydrogen storage, namely the cata- lytically enhanced complex metal hydrides. We support our predictions by thermodynamics consider- ations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition. 1.1. Nomenclature and the Coverage of the Peter Edwards was born in Liverpool, UK, in 1949. He received both his Review undergraduate and graduate degrees in chemistry at the Department of At this point, we mention something of definition Pure and Applied Chemistry at Salford University (UK), obtaining his Ph.D. within, and coverage across, our Chemical Reviews in 1974 with Dr. Ron Catterall and Sir Nevill Mott (Cavendish Laboratory, Cambridge) for his work on the synthesis and electronic properties of article. alkali metal−ammonia solutions. In 1975, he was awarded a Fulbright Hydrogen combines with many elements to form Scholarship and National Science Foundation Fellowship to work with binary hydrides, MHn. It is customary to group the Prof. Mike Sienko at the Baker Laboratory of Chemistry at Cornell binary hydrides of the elements into various classes University on a variety of systems traversing the metal−insulator transition. according to their presumed mode of chemical bond- He moved to Oxford in 1977 to work with Professor John B. Goodenough ing. A traditional division into three distinct groups on oxide superconductors, specifically the superconductor−insulator transition in oxides. In 1979, he moved to a faculty position at Cambridge is used in chemistry textbooks: (a) ionic (such as HF), University, where he remained until 1991, when he took the Chair of also including a subgroup of so-called saline hydrides Inorganic Chemistry at Birmingham University. He has recently (October (such as LiH); (b) covalent (or molecular, such as 2003) taken up the position of Professor and Head of Inorganic Chemistry SiH4); and (iii) interstitial (or “metallic”, such as at Oxford University.

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