First-principles study on energetics and electronic structures of Ni and M (M = Zr, Nb) co-doped MgH2

By utilizing density functional theory (DFT), we have conducted first-principles calculations to investigate the stability, structure, and electronic properties of MgH2 co-doped with Ni and M (where M can be Zr or Nb). The calculations of formation enthalpy and substitution enthalpy serve as a basis for evaluating the energy changes and effects associated with the formation and substitution of compounds, respectively. By minimizing the total electronic energy, the most favorable positions for dopants can be determined. It is observed that the stability of the hydrides follows an increasing trend in the sequence of Mg6Ni8Nb4H36, Mg10Ni8H36, and Mg6Ni8Nb4H36. Additionally, the analysis of hydrogen desorption enthalpies reveals that the combination of Ni and M (M=Zr, Nb) compared to pure Ni doping, MgH2 demonstrates favorable dehydrogenation properties. The strong bonding interaction between the dopant M and Mg and Ni atoms is due to their relatively low hydrogen desorption enthalpy. Furthermore, the investigation of electronic structures elucidates the micro-mechanisms responsible for the improved dehydrogenation properties observed in Ni and M co-doped MgH2. These mechanisms involve the weakening of Ni-H and Mg-H bonds, The movement of the conduction band below the Fermi level (E-F), reduction in the bonding electronic number below E-F.