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

Abstract The reactions x LiNH 2  + (1 −  x )LiBH 4 and x NaNH 2  + (1 −  x )NaBH 4 have been investigated and new phases identified. The lithium amide–borohydride system is dominated by a body centred cubic compound of formula Li 4 BH 4 (NH 2 ) 3 . In the sodium system, a new hydride of approximate composition Na 2 BH 4 NH 2 has been identified with a primitive cubic structure and lattice parameter a  ≈ 4.7 A. The desorption of gases from the two amide–borohydrides on heating followed a similar pattern with the relative proportions of H 2 and NH 3 released depending critically on the experimental set-up: in the IGA, ammonia release occurred in two steps – beginning at 60 and 260 °C for Li 4 BH 4 (NH 2 ) 3 – the second of which was accompanied by hydrogen release; in the TPD system the main desorption product was hydrogen—again at 260 °C for Li 4 BH 4 (NH 2 ) 3 accompanied by around 5% ammonia. We hypothesize that the BH 4 − anion can play a similar role to LiH in the LiNH 2  + LiH system, where ammonia release is suppressed in favour of hydrogen. The reaction x LiNH 2  + (1 −  x )LiAlH 4 did not result in the production of any new phases but TPD experiments show that hydrogen is released from the mixture 2LiNH 2  + LiAlH 4 , 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.