Catalyzed Complex Metal Hydrides
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Catalyzed Complex Metal Hydrides
Borislav Bogdanovic´ and Gary Sandrock Abstract Complex hydrides are mixed ionic–covalent compounds that can serve as reversible H2 storage media only when they are catalyzed by a transition metal such as Ti. As the prime example, the phenomenology of Ti-catalyzed sodium alanate (NaAlH4) is reviewed from a historical perspective. Dehydriding yields a theoretical 5.6 wt% H2 during two-step decomposition, NaAlH4 l Na3AlH6 l NaH Al, although 100% recovery of that H2 is not currently possible. H2 can be discharged and recharged at practical rates at 125C. More work is needed on the alanates, in particular, as well as the identification and optimization of the catalytic mechanism and a broad extension of the concept to other than Na-based alanates. The possibility of an even further extension of the concept to other complex hydrides (e.g., the borohydrides and transition-metal complexes) is discussed. Keywords: hydrogen storage, kinetic properties, metal hydrides.
Introduction There is a practical need for improved H2 storage methods with high inherent gravimetric capacity, for example, to carry hydrogen for fuel-cell vehicles. The history and potential of rechargeable metal hydrides for this purpose are presented elsewhere in this issue, clearly showing that we in the hydride field have been hampered by fundamental thermodynamic and kinetic limitations. On the one hand, interstitial hydrides that are easily reversible around room temperature (e.g., those based on V or the AB, AB2, and AB5 intermetallic compounds) represent essentially metallic H bonding and are limited to only about 1.5–2.5 wt% reversible gravimetric H capacity. At the other extreme, we have reversible hydrides that exhibit strong covalent or ionic H bonding (e.g., MgH2 and LiH, respectively) that can provide good gravimetric H capacity (7–13 wt%), but unfortunately require temperatures greater than 250C to release the bound H. This basic dilemma has existed for many years. Bogdanovic´ et al. suggested a new and different approach, starting in 1996, namely, the use of catalyzed alkali metal alanates, for example, Ti-doped NaAlH4 and Na3AlH6.1,2 Because this approach offers the clear potential of achieving 5 wt% H storage capability at temperatures that are only modestly greater than ambient (e.g., 50–100C), past thermodynamic barriers were considered to be breached. Early
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work at the Max-Planck-Institut für Kohlenforschung in Mülheim, Germany (MPI—Mülheim), was quickly confirmed by other investigators and has resulted in the generation of a great deal of worldwide interest (both fundamental and practical) over the past five years. It is our purpose here to briefly review the fundamentals of the catalyzed alanates and the recent practical progress made and then to comment on the need for future research and development in this area. In particular, we will suggest that the catalyzed alanate complex hydrides may have more general applicability to other complex hydrides. We do not cover here the aqueous comple
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