Improved hydrogen storage properties of MgH2 by ball milling with AlH3: preparations, de/rehydriding properties, and reaction mechanisms

Organometallically prepared AlH3 and as-received Al powders were mixed with MgH2 to improve the dehydriding and rehydriding properties of MgH2. Thermal analysis shows that the onset dehydriding temperature of MgH2 is reduced by 55 °C (or 25 °C) when mixed with AlH3 (or Al). The destabilization of MgH2 is attributed to the formation of Mg–Al alloys through the reaction between MgH2 and Al. Isothermal dehydriding measurements demonstrate that AlH3 and Al both improve the dehydriding kinetic of MgH2 to some extent, and it only takes 44 min for MgH2 + AlH3 (5.4 h for MgH2 + Al) to release 60% of the hydrogen of MgH2 at 300 °C, but 8.6 h are required for as-milled MgH2. The apparent activation energy for the dehydriding of MgH2 is reduced from 174.6 kJ mol−1 for as-milled MgH2 to 154.8 and 138.1 kJ mol−1 for MgH2 mixed with Al and AlH3 respectively; this is responsible for the improvement in the dehydriding kinetics of MgH2. Despite this, AlH3 is better in destabilizing MgH2 than the as-received Al for the fact that Al* formed in situ from the decomposition of AlH3 is oxide-free on the particle surfaces, which effectively increases the chemical activity of Al*. Furthermore, the brittleness of AlH3 makes it easier to mix MgH2 with AlH3, which would result in uniform distributions of Mg and Al and shortening of the diffusion length. Concerning the reversibility, at 300 °C and 5 MPa H2 and MgH2 are fully recovered in the dehydrided MgH2 + Al, and MgH2 + AlH3 samples after rehydriding for 10 h. The rehydriding kinetic of MgH2 is significantly enhanced for the dehydrided MgH2 + AlH3, but not in the case of the MgH2 + Al.