Synthesis and characterization of amide-borohydrides: New complex light hydrides for potential hydrogen storage

The reactions xLiNH(2) + (1- x)LiBH4 and xNaNH(2) + (1 - x)NaBH4 have been investigated and new phases identified. The lithium amide-borohydride system is dominated by a body centred cubic compound of formula Li4BH4(NH2)(3). In the sodium system, a new hydride of approximate composition NaBH4NH2 has been identified with a primitive cubic structure and lattice parameter alpha approximate to 4.72 angstrom. The desorption of gases from the two amide-borohydrides on heating followed a similar pattern with the relative proportions of H, and NH3 released depending critically on the experimental set-up: in the IGA, ammonia release occurred in two steps - beginning at 60 and 260 degrees C for Li4BH4(NH2)(3)- the second of which was accompanied by hydrogen release; in the TPD system the main desorption product was hydrogen-again at 260 degrees C for Li4BH4(NH2)(3) accompanied by around 5% ammonia. We hypothesize that the BH4- anion can play a similar role to LiH in the UNH2 + LiH system. where ammonia release is suppressed in favour of hydrogen. The reaction xLiNH(2) + (1 - x)LiAlH4 did not result in the production of any new phases but TPD experiments show that hydrogen is released from the mixture 2LiNH(2) + LiAlH4, over a wide temperature range. We conclude that mixed complex hydrides may provide a means of tuning the dehydrogenation and rehydrogenation reactions to make viable storage systems. (C) 2007 Elsevier B.V. All richts reserved.