Accelerated hydrogen desorption from MgH2 by high-energy ball-milling with Al2O3

The effect of the addition of Al2O3 (50 wt%) on the dehydrogenation of MgH2 was investigated. Composites of the oxide and the hydride were prepared in two ways: by milling the components separately or by co-milling them together in a gear-driven planetary ball mill for 10 min. The co-milled composite (MgH2–Al2O3) released approximately 90% of the maximum hydrogen storage capacity within 30 min under a pressure of 0.003 MPa at 250 °C. In contrast, the composite of the separately milled components did not release hydrogen even after 2 h under the same conditions. BET measurement with nitrogen gas showed a negligible difference in the specific surface areas between the co-milled and separately milled composites. However, the saturation amount of hydrogen gas for the co-milled composite was 30% larger than that of the mixture of separately milled hydride and oxide. The activation energy for hydrogen desorption from the co-milled composite, calculated on the basis of the surface-controlled model was 80 kJ mol−1, a value that is 50 kJ mol−1 lower than that of mixture of the separately milled MgH2 and Al2O3.