Enhanced hydriding kinetics of Mg-10 at% Al composite by forming Al₁₂Mg₁₇ during hydriding combustion synthesis

Mg-based alloys are widely considered as promising materials for hydrogen storage. However, owing to the high chemical activity of Mg, the surface of Mg powder is always covered with a layer of close packed MgO, which can retard the hydrogenation of Mg. In this paper, 10 at% Al was added to accelerate the hydriding kinetics of Mg through tabletting and grinding treatments. After heating to 420 °C and subsequent holding at 340 °C for 120 min under 1.5 MPa hydrogen pressure, the Mg-10 at% Al composite could absorb 6.34 wt% H₂ in the process of hydriding combustion synthesis (HCS) while the same treated pure Mg powder was barely hydrided. Upon heating from room temperature to 420 °C, Al alloyed with Mg to form an Al₁₂Mg₁₇ phase on the surface of Mg particles and the newly formed alloy was oxide-free, which would react with H₂ when the temperature decreased to 340 °C. Then hydrogen was able to get into the inner part of the Mg particle from the former Al₁₂Mg₁₇ sites where the MgO shell had been broken in the alloying period to further react with unhydrided Mg.