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.