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

The reactions xLiNH2 + (1 − x)LiBH4 and xNaNH2 + (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 Na2BH4NH2 has been identified with a primitive cubic structure and lattice parameter a ≈ 4.7 Å. The desorption of gases from the two amide–borohydrides on heating followed a similar pattern with the relative proportions of H2 and NH3 released depending critically on the experimental set-up: in the IGA, ammonia release occurred in two steps – beginning at 60 and 260 °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 °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 LiNH2 + LiH system, where ammonia release is suppressed in favour of hydrogen. The reaction xLiNH2 + (1 − x)LiAlH4 did not result in the production of any new phases but TPD experiments show that hydrogen is released from the mixture 2LiNH2 + 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.