Alkali Metal Hydride Modification on Hydrazine Borane for Improved Dehydrogenation

Hydrazidotrihydridoborates of various alkali metals, i.e., NaN2H3BH3 and KN2H3BH3, were synthesized successfully via a liquid approach. The crystal structures of NaN2H3BH3 and KN2H3BH3 were determined, and their dehydrogenation properties were compared with LiN2H3BH3 and N2H4BH3. A clear correlation between sizes of metal cations in hydrazidotrihydridoborates and their corresponding melting and dehydrogenation temperatures was observed. The dehydrogenation temperature was found dependent on the melting temperature. Upon approaching the melting points, alkali metal hydrazidotrihydridoborates dehydrogenate rapidly in the first step, giving rise to the formation of intermediates that possess N2BH2, N2BH, and NBH3 species. Further increasing temperature leads to the release of additional H-2 and the formation of N2BH species. Compared to pristine N2H4BH3, the alkali-metal-substituted hydrazidotrihydridoborates demonstrate significantly improved dehydrogenation behavior with no N2H4 emission and greatly suppressed NH3 release.