Mg-Fe thin films: A phase-separated structure with fast kinetics of hydrogenation

In this paper we suggest a new approach to improve the kinetics of hydrogenation based on a material with a high density of interfaces between hydride-forming solid (HFS) and hydrogen-diffusing solid media (HDM). Such materials can be realized for phase-separated systems when synthesized in the conditions limiting kinetics of phase separation, e.g., in thin film deposition. Mg-Fe was selected as a model system: (1) It has phase separation; (2) MgH ₂ is a high-capacity hydride; (3) Fe has high diffusivity of hydrogen. Mg _1-xFe _x thin films (x = 0-0.30) capped with Pd were prepared by electron beam codeposition and their hydrogenation/ dehydrogenation kinetics and cycling properties were studied at 413 K. The structures of the thin films before and after hydrogenation during different cycles were investigated by X-ray diffraction and transmission electron microscopy. It has been found that there is a substantial improvement in hydrogen absorption and desorption kinetics for the Mg _1-xFe _x films in comparison to pure Mg film. The improvement is attributed to the presence of Fe layers percolating throughout the Mg matrix. For the Mg _1-xFe _x films with x = 0.05-0.15 more than 3.5% mass fraction hydrogen can be absorbed under hydrogen pressures of 0.1 MPa in less than 2 min, and above 3.0% mass fraction hydrogen can be desorbed in 15 min. For x > 0.15 films, the reversible hydrogen storage properties degrade significantly; structural study of the higher concentration films shows the presence of a stable ternary hydride Mg ₂FeH ₆, formation of which is responsible for interruption of fast hydrogen delivery.