Gaseous complex hydrides NaMH4 and Na2MH5 (M = B, Al) as hydrogen storage materials: a quantum chemical study.

Metal hydrides are feasible for energy storage applications as they are able to decompose with hydrogen gas release. In this work, gaseous complex sodium hydrides, NaMH4 and Na2MH5 (M = B or Al), have been investigated using DFT/B3P86 and MP2 methods with 6-311++G(d,p) basis set; the optimized geometry, vibrational spectra and thermodynamic (TD) properties have been determined. Based on TD approach, a stability of the hydrides to different dissociation channels is analysed; the enthalpies of formation ∆fH°(0) of gaseous species have been obtained: - 1 ± 17 kJ mol-1 (NaBH4), 91 ± 14 kJ mol-1 (NaAlH4), - 13 ± 16 kJ mol-1 (Na2BH5), and 71 ± 16 kJ mol-1 (Na2AlH5). The complex hydrides are confirmed to produce gaseous products with hydrogen gas release at elevated temperature, whereas heterophase reactions, with NaH and B/Al products in condensed state, are predicted to occur spontaneously at lower temperature. Graphical abstract.