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

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 (1:1,21 and 4:1 in mole ratio). The onset dehydrogenation temperature of MgH2 is observed at around 250 degrees C, which is over 50 degrees C lower from that of as-milled MgH2. 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 MgH2 to form Li(0.92)Mg(4).(08) and Mg(17)Al(12) phases, respectively. The mutual destabilization between MgH2 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). (C) 2008 Elsevier B.V. All rights reserved.