DFT Insights into the Interactive Effect of Ni + N Cosubstitution on Enhanced Dehydrogenation Properties of Mg(BH₄)(NH₂)-like Complex Hydride for Hydrogen Energy Storage

This work proposes a new method of transition metal and nonmetal partial cosubstitution for improving the dehydrogenation properties of magnesium borohydride that has a high hydrogen capacity of 14.9 wt % but poor dehydrogenation properties. Compared with the frequently used method of single substitution, it is found that the cosubstitution has better positive effects on the properties including thermal stability, hydrogen dissociation energy, and dehydrogenation temperature. The Ni + N partial cosubstitution for Mg + B causes the formation enthalpy of Mg(BH₄)₂ to increase from -0.2438 to atmost -0.2039 eV·atom^-1, and the thermal stability is reduced as a result. Moreover, the hydrogen dissociation energy and the onset dehydrogenation temperature after the Ni + N partial cosubstitution are also reduced by about 75% and more than 160 °C, respectively. The density functional theory calculations show that the interactive effect between Ni-B and N-Mg bonds appears in the cosubstituted Mg(BH₄)₂, which significantly weakens the strong interaction between Mg and B/BH₄ atoms. Besides, the dual bonding effects of Ni-H and N-H forming the groups NiH₆ and NH₂ break the stable BH₄ groups into BH₃ and BH₂ groups, consequently facilitating the dehydrogenation process of Mg(BH₄)₂.