The Activating Effect of Carnallite and Hydrated MgCl2 on Hydrogen Absorption in Magnesium

Abstract We present activation strategies to enhance hydrogen absorption in magnesium. Magnesium chips, produced by mechanical chipping with a laboratory-scale milling device, were hydrogenated after adding activators and compared to pure Mg chips. It was found that following activation by turbomolecular evacuation at 300 °C for 3 h, MgCl2·6H2O-added Mg chips absorbed hydrogen at temperatures as low as 200 °C. Also, the addition of the activator eliminated the incubation period and improved the hydrogen absorption rate by a factor of three compared with pure Mg chips. These enhancements are attributed to the decomposition of MgCl2·6H2O during the activation process, which releases HCl gas. The HCl gas reacts with surface constituents on the Mg chips to form MgCl2. This reaction is associated with breaking the oxide and carbonate layers, subsequently forming cracks and exposing an active surface. The modified surface chemistry promotes hydrogen dissociation and diffusion, thereby accelerating the first hydrogen absorption. Kinetic analysis of the first hydrogenation of activated Mg chips reveals that the process is governed by a low-dimensional nucleation and growth mechanism, like the mechanism taking place during the first hydrogen absorption in pure Mg chips. Application of the Avrami model further shows that the apparent activation energies for hydrogen absorption in activated Mg chips are lower than the values previously reported in the literature for pure Mg. This study highlights the benefits of adding small amounts, 4 wt % or less, of activator, which eliminates the incubation period, lowers the hydrogen absorption temperature, and increases the hydrogenation rate. These results demonstrate the potential of adding activators to produce highly reactive hydrogen storage materials.