Abstract
This study discusses the improvement in the hydrogen sorption properties of MgH2 with dual auto-catalysts, MgF2 and CsH. The auto-catalysts are formed due to the reaction between MgH2 and CsF during the dehydrogenation reaction of MgH2. It has been observed that MgF2 and CsH not only improve MgH2's hydrogen sorption properties, also aids its positive thermodynamic tuning, which is favourable for hydrogen storage. The on-set desorption temperature of MgH2 catalysed by MgF2+CsH is 249 °C, which is 106 °C lower than that of ball-milled MgH2 without any additives measured under identical measurement conditions. The catalysts helped in improving both the de/rehydrogenation kinetics of MgH2. The MgH2 catalysed by MgF2+CsH released 4.73 wt % H2 in 15 min at 300 °C. Furthermore, its initial re-hydrogenation rate under isothermal conditon at 300 °C is 4.62 wt % H2 in 5 min. The catalysed sample exhibits negligible hydrogen storage degradation of 0.39 wt % H2 after 25 de/re-hydrogenation cycles. Using the Kissinger method, the activation energy of MgH2 catalysed by MgF2+CsH was estimated to be 98.1 ± 0.5 kJmol-1. From the Van't Hoff plot, the decomposition and formation enthalpies of MgH2 were determined to be 66.6 ± 1.1 kJmol-1 and 63.1 ± 1.2 kJmol-1, respectively. From the experimental observation, a feasible mechanism for the de/re-hydrogenation behaviour of MgH2 with MgF2+CsH is proposed.
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