Surface oxide barrier effects on hydrogen storage in Mg-Ni films

Abstract

Mg2NiH4 is an interesting example of a complex metal hydride switchable mirror and hydrogen storage material. However, the strong tendency of surfaces for oxidation hinders the hydrogen absorption and desorption. Even minute impurity levels of oxygen and water can result in the formation of surface oxides and must be considered in all practical uses. So the study ion irradiation effects on the reactivity to oxygen and air and the passivation of Mg-Ni materials is a major area of interest in this field, both form a practical and fundamental point of view. The hydrogen and oxygen distribution profiles in 2 pm thick Mg-Ni films formed by magnetron-sputter deposition technique on the quartz substrate and hydrogenated at 8 bar pressure in the temperature range 210-260°C have been investigated using NRA and ERDA techniques. We have demonstrated a strong influence of ion irradiation effects on the hydrogen uptake rate. The emphasis is made on the studies of changes of the surface barrier properties during hydrogenation. It is shown that in the case of the presence of a small amount of oxygen surface must be considered as a dynamic when addressing hydrogen uptake of materials. It adjusts to the changes of chemical environment and phase transformations in the near surface region. It is shown that hydrogen uptake rate is a not-monotonous function of time. As synthesis of hydride phases proceeds in the bulk, this leads to the expansion of material. The surface oxide barrier film becomes highly defected with many defects and cracks available for hydrogen transport into and out of the film. We have registered that the increase of temperature in the range 210-250°C decreases the hydrogen storage capacity as a result of interplay between the mean surface diffusion length of adsorbed hydrogen adatoms and the mean distance sink defects in the oxide layer. The hydrogenation properties were compared with hydrogen desorption kinetics measured by TDS technique and correlated t