Understanding the role of vanadium in enhancing the low-temperature hydrogenation kinetics of an Mg thin film.

Mg100-xVx (x = 0 to 15) thin films capped with Pd were prepared by electron beam codeposition and studied for their hydrogenation/dehydrogenation kinetics and cycling properties at 140 °C under hydrogenation pressures of 0.1 MPa. It has been found that the Mg100-xVx thin films show significantly higher reversible hydrogen-storage capacity and faster kinetics in comparison with a pure Mg thin film; for instance, the maximum hydrogen absorption (3.7% mass fraction hydrogen) can be obtained in the fifth cycle for Mg90V10 in less than 5 min. The addition of V clearly plays a favorable role in improving the reversible hydrogen-storage capacity of an Mg film; however, with increasing hydrogenation/dehydrogenation cycles the hydrogen-storage capacity gradually deteriorates. To explore the origin of the effect of V on the improved hydrogenation of an Mg thin film, in this work we focused on studying the structural variations of the Mg90V10 thin film before and after hydrogenation at different stages of cycling; the films were investigated by X-ray diffraction as well as scanning and transmission electron microscopy. We concluded that (1) early in the absorption/desorption cycling the as-deposited structure of percolating layers of nanocrystalline V throughout a Mg matrix is preserved; (2) the percolating V layers envelope fine Mg grains and act as (a) dispersers that isolate small Mg grains, (b) fast diffusers of hydrogen, and (c) hydrogen catalysts at the Mg/V interface to form MgH2; and (3) with progressive cycling, the continuous layers of V aggregate to spherical nanoparticles, which interrupts the continuity of fast hydrogen diffusion through V.