Catalyzed complex metal hydrides

Complex hydrides are mixed ionic-covalent compounds that can serve as reversible H-2 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% H-2 during two-step decomposition, NaAlH4 --> Na(3)AIH(6) --> NaH + Al, although 100% recovery of that H-2 is not currently possible. H-2 can be discharged and recharged at practical rates at 125degreesC. 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.