Effects of nonmetal element (B, C and Si) additives in Mg2Ni hydrogen storage alloy: A first-principles study

Nonmetal atoms (B, C and Si) are designed to add into Mg2Ni hydrogen storage alloy and its hydride. First-principles density-functional theory calculations have been performed to investigate their crystal structures, electronic and thermodynamic properties. The calculation results present that nonmetal additions in Mg2NiH4 show more effective destabilization than metal additions. Especially for B and C, the decreases of formation enthalpies of Mg2NiH4 reach 0.19 and 0.21 eV/atom. The NiH4 structure near B or C in Mg2NiH4 hydride becomes the tripod-like NiH3 structure. The results show that the thermodynamic stabilities of Mg2Ni and Mg2NiH4 exhibit a nearly linear decrease with the increasing content of nonmetal atoms. The calculated dehydrogenation energies are 59.39, 58.12, 55.84 and 55.30 kJ/mol H2 for Mg2NiH4, Mg2NiB0.5H4, Mg2NiC0.5H4 and Mg2NiSi0.5H4, respectively. It is found that the addition of nonmetal atoms favors the dehydrogenation reaction for Mg2Ni hydrogen storage material. In addition, the effects of nonmetals to the heat capacities and vibrational entropies of Mg2Ni and Mg2NiH4 are also analyzed.