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 H ₂ at 373 K and evolved into a metastable orthorhombic Mg hydride phase. Thermal desorption spectroscopy studies show that thinner dense MgH ₂ 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.