Role of vacancy defects on the dehydrogenation properties of the ternary hydride ZrNiH3: Ab-initio insights

ZrNi is considered a promising candidate for hydrogen storage and nickel-metal hydride rechargeable batteries (Ni-MH). The effect of creating zirconium and nickel vacancy defects on the dehydrogenation properties of ZrNiH3 is investigated by means of first-principles calculations. The results indicate that nickel vacancy is energetically more favorable to form in ZrNiH3 than zirconium vacancy, because of the lesser formation energy of Ni-vacancy. For both Zr and Ni vacancy defects, the formation enthalpy decreases with increasing the concentration of vacancy and, vice versa. In particular, it is found that with ~2.4% of zirconium vacancy defects or with ~4.5% of nickel vacancy defects in ZrNiH3, the formation enthalpy is around - 40 kJ/mol.H2, which is recommended by the U.S. Department of Energy (DOE). It is worth noting also that with slightly higher vacancy defects ~2.8 of Zr-vacancy or ~5.3% of Ni-vacancy in ZrNiH3, it becomes harder to store hydrogen in these systems without cooling. Moreover, the density of states (DOS) analysis indicates that the stability of ZrNiH3 decreases with increasing Zr-vacancy and Ni-vacancy concentrations, through the shrinkage in the size of the total DOS and shifting in the valence bands near to Fermi level.