Scandium and vanadium borohydride ammoniates: Enhanced dehydrogenation behavior upon coordinative expansion and establishment of Hδ+ ... -δH interactions

LiSc(BH4)(4)center dot 4NH(3) and V(BH4)(3)center dot 3NH(3), two novel metal borohydride ammoniates (MBAs), have been successfully synthesized via ball-milling the mixtures of MCl3 center dot xNH(3) (M = Sc, V and x = 3, 4) with LiBH4. Structure analysis reveals that LiSc(BH4)(4)center dot 4NH(3) crystallizes in an orthorhombic structure with lattice parameters of a = 7.4376(3) angstrom, b = 11.1538(5) angstrom and c = 14.5132(7) angstrom and space group of Pc2(1)n, in which the base octahedral units are composed of central metal and an equivalent number of BH4 and NH3 units, distinct from other reported MBAs. Base units with the above constitution are also observed in the crystal structure of V(BH4)(3)center dot 3NH(3), which is identified as a cubic structure with lattice parameters of a = 10.78060(25) angstrom and space group of F23. These two compounds exhibit a favorable dehydrogenation capability, releasing 15.1 and 14.3 wt.% high-purity hydrogen, respectively, below 300 degrees C. Isothermal measurements reveal that, at a constant temperature of 110 degrees C, which meets the operation requirement of fuel cells, >8 and >10 wt.% pure hydrogen is released from the two compounds with favorable kinetics, respectively. Moreover, by reacting with N2H4 in liquid ammonia, the decomposed LiSc(BH4)(4)center dot 4NH(3) can be partly hydrogenated and can possibly establish a system that will undergo reversible dehydrogenation. These favorable properties point to potential on-board application. The dehydrogenation capacity, purity and temperature of the two systems can be adjusted, by tuning the ratios of the starting reagents LiBH4 and MCl3 center dot xNH(3), to achieve expected stoichiometric proportions of BH4 and NH3 units, which provides a facile and viable strategy for the synthesis of modified, mono-, di- or polymetal borohydride ammoniate systems and thus tunable hydrogen storage performances. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.