Na-modified cast hypo-eutectic Mg–Mg2Si alloys for solid-state hydrogen storage

Mg2Si is a promising catalyst for Mg-based H2 storage materials due to its low cost, light weight, and non-toxic properties. This study investigates the effects of Na in hypo-eutectic Mg-1wt.%Si alloys for H2 storage applications. The addition of trace amounts of Na is vital in improving the H2 sorption kinetics, achieving a H2 storage capacity of 6.72 wt.% H at 350 °C under 2 MPa H2, compared to 0.31 wt.% H in the non-Na added alloy. The hydrogen sorption mechanisms were analysed with Johnson-Mehl-Avrami-Kolmogorov models. It was identified that Na affects the surface of the Mg alloys, forming porous Na2O and NaOH in addition to MgO, facilitating the diffusion of H2. Finally, in-situ synchrotron powder X-ray diffraction showed the Mg2Si catalyst is stable during the H2 sorption reactions. This result demonstrates the potential use of Mg–Mg2Si casting alloys for large scale hydrogen storage and transportation applications.