The Hydrogen Storage Properties of Destabilized MgH2–AlH3 (2:1) System

Systematic investigations on the hydrogen storage properties of AlH3 destabilized MgH2 is investigated. It is found that the MgH2/AlH3 (2:1) composite system showed improved dehydrogenation performance compared with that of as-milled MgH2 alone. The dehydrogenation process in the MgH2/AlH3 composite can be divided into two stages. During the first dehydrogenation process, AlH3 decomposed first to produce Al with hydrogen release. In the second dehydrogenation stage, the as-formed Al phase reacts with MgH2 to form Mg17Al12 phase at a temperature of about 250 °C, which is about 80 °C lower than the decomposition temperature of as-milled MgH2. The second step decomposition enthalpy of the system was determined by differential scanning calorimetry measurements and the enthalpies change to be 45.0 kJ mol-1 H2, which is smaller than that of MgH2 alone (75.7 kJ mol-1 H2). Kissinger analysis indicated that the apparent activation energy, EA, for the MgH2-relevent decomposition in MgH2-AlH3 composite was 94.0 kJ mol-1, which is 68.0 kJ mol-1 less than for as-milled MgH2 (162.0 kJ mol-1). Rehydrogenation processes show that Mg17Al12 is fully reversible. It is believed that the formation of the Al12Mg17 phase during the dehydrogenation process alters the reaction pathway of the MgH2-AlH3 (2:1) composite system and improves its thermodynamic properties accordingly.