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

Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH(4), as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of II) and the second dissociation step of Ti-doped NaAlH(4), (3.0 wt% H) of 37 and 47 kT/mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH(4) particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253-254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe-doped NaAlH(4) within cycle tests, reaching a constant storage capacity of similar to 4 wt% H(2), has been demonstrated. By means of (57)Fe Mossbauer spectroscopy of the Ti/Fe-doped NaAlH(4), 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 after the first dehydrogenation, the nanosize Fe particles with NaAlH(4), present are probably transformed into an Fe-Al-alloy which throughout the cycle test remains practically unchanged. (C) 2000 Elsevier Science S.A. All rights reserved.