Size and stress dependent hydrogen desorption in metastable Mg hydride films

Mg is a promising light-weight material that has superior hydrogen storage capacity. However H2 storage in Mg typically requires high temperature, ∼500-600 K. Furthermore it has been shown that there is a peculiar film thickness effect on H2 sorption in Mg films, that is thinner Mg films desorb H2 at higher temperature [1]. In this study we show that the morphology of DC magnetron sputtered Mg thin films on rigid SiO2 substrate varied from a continuous dense morphology to porous columnar structure when they grew thicker. Sputtered Mg films absorbed H2 at 373 K and evolved into a metastable orthorhombic Mg hydride phase. Thermal desorption spectroscopy studies show that thinner dense MgH2 films desorb H2 at lower temperature than thicker porous MgH2 films. Meanwhile MgH2 pillars with greater porosity have degraded hydrogen sorption performance contradictory to general wisdom. The influences of stress on formation of metastable MgH2 phase and consequent reduction of H2 sorption temperature are discussed.