Theoretical investigation of the surface orientation impact on the hydrogen vacancy formation of MgH2

Abstract MgH2 is one of the most promising materials for solid-state hydrogen storage, but its slow kinetics and relatively high dehydrogenation temperature have yet to be overcome. We have theoretically investigated surface energy and surface H vacancy formation on various MgH2 surfaces, including all possible H vacancy configurations in the studied size of MgH2(110), (100), (101), and (001), up to (3 × 2), (2 × 3), (2 × 2) and (2 × 2) slabs respectively, based on density functional theory. Consequently, the hydrogen diffusion paths on the surfaces are also studied. The most unstable surface, MgH2(001), has the most drastic change in vacancy formation energy. The relative high barriers for vacancy H formation, especially on the most stable surface, MgH2(110), reflect the slow kinetics of H release. The introduction of vacancy into MgH2 surfaces often induces defect levels in the energy-gap region. In general, the release of surface H atoms exhibits a pronounced dependence on the surface stability and the concentration of surface H vacancy, in line with the remarkable effect of ball milling in experiments.