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First-principles investigation of ammonia borane for hydrogen storage

Solid state ammonia borane (AB, H3BNH3) has attracted considerable attention as a promising hydrogen storage material. In this paper, electronic structures, bonding characters and hydrogen release mechanism of AB are investigated by first-principles calculations. From electronic structure calculations, we show that B (or N) atom first constructs sp hybrids and then forms covalent bonds with surrounding H atoms. The B–N bonds are formed via B–p and N–p hybridization and are dative in nature. Electron deficiency of B atoms induces a charge transfer, which results in hydrogen bonds and dipole–dipole interactions among H3BNH3 complexes. The hydrogen-bonding energy in AB is estimated to be 15.1 kJ mol−1. The formation enthalpy calculations indicate that, in the initial stage of the hydrogen release process, the H atoms forming an H2 molecule are decomposed from two adjacent H3BNH3 complexes rather than from the same H3BNH3 complex and the process is almost thermoneutral.