Effects of surface dynamic behavior on hydrogen storage properties of sputter-deposited MgNi films

We have studied experimentally the distribution profiles of accommodated oxygen and hydrogen atoms in Mg–Ni films after different stages of hydrogenation at 8 bar pressure in the range of temperatures 210–250 °C by Elastic Recoil Detection Analysis (ERDA) and Nuclear Reaction Analysis (NRA) techniques and performed X-ray diffraction (XRD) structural and surface topography analysis. In agreement with previous publications, it is shown that even small impurity levels of uncontrollable oxygen during hydrogenation result in the formation of the oxide barrier on the surface changing hydrogen permeation properties. It is registered that the quantity of stored hydrogen during the first 3 h of hydrogenation increases up to 40 at.%, decreases up to 20 at.% after 6 h and increases again up to 40 at.% after 72 h. The quantity of the released hydrogen after 3 h hydrogenation increases with the increase in hydrogenation temperature. The observed experimental results are explained assuming that properties of the surface barrier layer changes during hydrogenation and modify hydrogen transport mechanism. The growing hydride phase in the bulk generates stresses that induce cracks and holes in the oxide barrier formed during the initial stages of hydrogenation, and hydrogen release becomes possible. At steady state, the hydrogenation properties adjust to dynamic processes on the surface and in the bulk.