Hydrogen sorption kinetics in MgH₂ thin films

Storage in metal hydrides is suggested as a natural solution to solve the hydrogen storage problem. Magnesium hydride (MgH₂) is one of the attractive materials, studied intensively for the hydrogen fuel based economy, mostly for stationary applications. This reversible system has however some drawbacks. Hydrogen (H) and Mg are strongly bonded by an ionic bond, and this entails 300°C for the stable compound to be used. During this work we study the hydrogenation kinetics in nano-metric dimensions in thin films. Mg thin films (50-800nm) enable us to monitor the growth process of the hydride. Palladium (Pd) is used as a catalyst coating (5-40nm) for decreasing the pressure and temperature of hydrogenation to 5bars and 200℃. The hydride formation is followed by XRD crystallographic structure characterization. Despite the fact that electron microscopy damages the hydride phase, microscopic imaging of the co-existence of MgH₂ and Mg is presented by SEM-FIB and TEM. The microstructure change is clearly visible in the micrographs (from columnar to equi-axed grainy structure). These combined techniques allow us to follow the kinetics of hydride formation within the layer and determine the diffusion coefficient in β phase. The mechanism of hydrogenation at 200°C and 300°C and at different H-pressures (1-100bars) is studied. Surprisingly, a transition between linear, at 200°C, and parabolic growth regime, at 300°C, is observed. Diffusion coefficients and surface transport coefficients are determined and discussed.