Hydrogen Desorption Properties of the MgH2–AlH3 Composites

In this work, the hydrogen desorption properties of the novel Mg–Al–H hydrogen storage composites prepared by ball-milling the as-received magnesium hydride (MgH 2 ) and the as-prepared aluminum hydride (AlH 3 ) with different molar ratios (1:1, 1:0.5, and 1:0.25) are investigated systematically. Nonisothermal desorption measurements show that more than 7.51 wt % of hydrogen is released from the Mg–Al–H composites when they are heated from room temperature to 500 °C. Thermal analysis by differential scanning calorimeter (DSC) and hydrogen desorption spectrum by mass spectrometer (MS-H 2 ) are used synchronously to study the desorption process of the Mg–Al–H. The onset hydrogen desorption temperature of MgH 2 in the MgH 2 + AlH 3 composite is dramatically reduced from 320 °C for the pure MgH 2 to below 252 °C. The DSC/MS-H 2 and the X-ray diffraction (XRD) analyses show that the desorption of MgH 2 in the Mg–Al–H composite is composed of two steps with the reaction between MgH 2 and Al forming Mg 17 Al 12 as the first step and the self-decomposition of the residual MgH 2 as the second step. Isothermal desorptions show that the desorption kinetics of the MgH 2 in the Mg–Al–H composite is significantly improved. The apparent activation energy for the desorption of MgH 2 in the MgH 2 + 0.25AlH 3 is reduced to 144.6 kJ/mol, which contributes to the improvement in the desorption kinetics of MgH 2 . The Johnson–Mehl–Avrami (JMA) kinetic studies indicate that the desorption kinetic model of MgH 2 in the Mg–Al–H composite is different from the pure MgH 2 . Concerning the reversibility, under the conditions of 300 °C and 5 MPa H 2, MgH 2 in the Mg–Al–H composite is almost fully recovered after absorption for 5 h while it is partially recovered for the pure MgH 2 . Among the samples studied, the MgH 2 + 0.25AlH 3 composite exhibits the highest reversible hydrogen storage capacity of 6.10 wt % (H/M).