Structural prediction, analysis and decomposition mechanism of solid M(NH2BH3)n (M = Mg, Ca and Al)

Abstract

Multivalent Metal amidoboranes (MABs) are considered to be the important candidates for hydrogen storage materials. Whereas CaAB crystal structure has been determined experimentally, the crystal structures of MgAB and AlAB have not been obtained, thus hindering the further development of MABs. In this work, we determine the crystal and electronic structures of MABs (M = Mg, Ca, Al), and obtain their phononic density of states and thermodynamic properties. By means of M–N population analysis, we conclude that Al–N and Ca–N bonds have a covalent character whereas the Mg–N bond is ionic. We furthermore observe that HOMO–LUMO gaps and thus stability follow the trend MgAB ≈ AlAB > CaAB. Thermodynamic properties and their dependence on temperature seem to be very similar for AlAB and MgAB compounds compared to CaAB. CaAB has the lowest enthalpy and thus the lowest internal energy in the series of MABs (M = Mg, Ca, Al). We also propose the most probable dehydrogenation pathways during which 4n H₂ molecules are released (in 4 steps) from MgAB and CaAB and 6n H₂ (in 6 steps) from AlAB indicating that dehydrogenation from the same AB group twice in a row is less likely to occur. Furthermore, we reveal that the energy barriers for the 1st dehydrogenation step follow the trend CaAB > MgAB > AlAB. This is in agreement with the experiments showing that AlAB will start to dehydrogenate at the lowest temperature followed by MgAB. Finally, we propose that complete dehydrogenation rates at 400 °C follow the trend MgAB (1.73 × 10⁻¹⁴ min⁻¹) ≈ CaAB (1.72 × 10⁻¹⁴ min⁻¹) > AlAB (2.18 × 10⁻¹⁶ min⁻¹).