Influences of interstitial nitrogen with high electronegativity on structure and hydrogen storage properties of Mg-based metal hydride: A theoretical study

Interstitially doping nonmetal elements has been viewed as an effective way to improve the hydrogen storage properties of Mg-based metal hydride. In this work, the influences of the concentration of N with high electronegativity on the crystal structure, hydrogen absorption/desorption kinetics and thermodynamics of Mg-based system were systematically investigated by first-principle calculations. For Mg₂NiN_x (x = 0, 0.25, 0.5) system and its hydride, the doping positions of interstitial N atom in the crystal structures were firstly determined. The calculation results showed that the addition of N element has the favorable effects on the improvement of both the kinetic and thermodynamic properties. N-doped Mg-based system with x = 0.5 has the best hydrogen absorption/desorption performances. Compared with pure Mg-based system, the formation enthalpy and dehydrogenation energy of Mg₂NiN_0.5 system are reduced respectively by up to 98% and 59% due to the strong hybridization between the orbits of N and H. Besides, Mg₂NiN_0.5 system is able to achieve the reversible hydrogen absorption/desorption reactions at ambient temperature, which makes it suitable for most practical applications.