The destabilization mechanism and de/re-hydrogenation kinetics of MgH 2–LiAlH 4 hydrogen storage system

Abstract

Unfavourable stability and sluggish de/re-hydrogenation kinetics hamper the application of MgH 2 as a hydrogen storage material for mobile fuel cell systems. In the present work, it can be destabilized effectively by LiAlH 4 in as-synthesized MgH 2–LiAlH 4 composites (1:1, 2:1 and 4:1 in mole ratio). The onset dehydrogenation temperature of MgH 2 is observed at around 250 °C, which is over 50 °C lower from that of as-milled MgH 2. Differential scanning calorimetry (DSC) measurements indicate that the enthalpies of MgH 2-relevant decomposition in MgH 2–LiAlH 4 composites (1:1, 2:1 and 4:1 in mole ratio) are 45, 48.6 and 61 kJ mol −1 H 2, respectively. These values decrease significantly from that of as-milled pristine MgH 2 (76 kJ mol −1 H 2), demonstrating the destabilization of MgH 2 in this system. The destabilization mechanism is investigated by X-ray diffraction (XRD) analyses. It was found that the whole dehydrogenation process can be divided into two stages: the first stage is the two-step decomposition of LiAlH 4 and during the second stage, the yielded LiH and Al phases decompose MgH 2 to form Li 0.92Mg 4.08 and Mg 17Al 12 phases, respectively. The mutual destabilization between MgH 2 and LiH was first observed in this study. Moreover, the reaction is fully reversible. The isothermal kinetics shows that the doped LiAlH 4 is beneficial to the enhancement of the dehydrogenation kinetics of MgH 2.