First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg2Ni, of aluminum and silver substituted alloys Mg2−xMxNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg2NiH4, Mg2−xMxNiH4 (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH4 units in Mg2NiH4. The NiH4 unit near the Al/Ag atom became tripod-like NiH3 in Mg2−xMxNiH4 (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg2Ni, Mg2−xAlxNi and Mg2−xAgxNi are −65.14, −51.56 and −53.63 kJ/mol H2, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.