Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials

Thermodynamics and kinetics of the reversible dissociation of metal-doped N aAlH4, as a hydrogen (or heat) storage system have been investigated in som e detail. The experimentally determined enthalpies for the first (3.7 wt% o f II) and the second dissociation step of Ti-doped NaAlH4, (3.0 wt% H) of 3 7 and 47 kT/mol are in accordance with low and medium temperature reversibl e metal hydride systems, respectively. Through variation of NaAlH4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved w ith respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloy s Comp. 253-254 (1997) 1]. In particular, using combinations of Ti and Fe c ompounds as dopants, a cooperative (synergistic) catalytic effect of the me tals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe- doped NaAlH4 within cycle tests, reaching a constant storage capacity of si milar to 4 wt% H-2, has been demonstrated. By means of Fe-57 Mossbauer spec troscopy of the Ti/Fe-doped NaAlH4, before and throughout a cycle test, it has been ascertained that(1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt)(2) and (2) already a fter the first dehydrogenation, the nanosize Fe particles with NaAlH4, pres ent are probably transformed into an Fe-Al-alloy which throughout the cycle test remains practically unchanged. (C) 2000 Elsevier Science S.A. All rig hts reserved.