Abstract
Due to increasing energy demands, interest in hydrogen storage is substantial. Magnesium is one of the most attractive systems, yet, development for practical application remains challenging. By combination of X-ray diffraction, electron microscopy and in-situ measurements of resistivity we determine the diffusion coefficient of hydrogen in MgH2 at technically relevant pressure (20 bar). Pd coated thin films of well-defined thickness enable a quantitative evaluation of the hydrogenation rate. From this, we detect linear to parabolic kinetic transition and obtain the diffusion coefficient of hydrogen in MgH2. Measurements at different temperatures (RT-300 °C) demonstrate an Arrhenius behaviour with an activation energy Ea = 28.1 kJ mol−1. This low value and the transformation into a nanocrystalline microstructure upon hydrogenation indicate grain boundary diffusion as the essential mechanism. In completion, the interface Pd/Mg is studied. Mg5Pd2 and Mg6Pd form at elevated temperatures required for dehydrogenation. These phases affect, but do not prevent, further hydrogen loading.
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