Thermodynamic destabilization of magnesium hydride using Mg-based solid solution alloys

Thermodynamic destabilization of magnesium hydride is a difficult task that has challenged researchers of metal hydrides for decades. In this work, solid solution alloys of magnesium were exploited as a way to destabilize magnesium hydride thermodynamically. Various elements were alloyed with magnesium to form solid solutions, including: indium (In), aluminum (Al), gallium (Ga), and zinc (Zn). Thermodynamic properties of the reactions between the magnesium solid solution alloys and hydrogen were investigated. Equilibrium pressures were determined by pressure-composition-isothermal (PCI) measurements, showing that all the solid solution alloys that were investigated in this work have higher equilibrium hydrogen pressures than that of pure magnesium. Compared to magnesium hydride, the enthalpy (δH) of decomposition to form hydrogen and the magnesium alloy can be reduced from 78.60 kJ/(mol H₂) to 69.04 kJ/(mol H₂), and the temperature of 1 bar hydrogen pressure can be reduced to 262.33 °C, from 282.78 °C, for the decomposition of pure magnesium hydride. Further, in situ XRD analysis confirmed that magnesium solid solutions were indeed formed after the dehydrogenation of high-energy ball-milled MgH₂ with the addition of the solute element(s). XRD results also indicated that intermetallic phases of Mg with the solute elements were present along with MgH₂ in the rehydrogenated magnesium solid solution alloys, providing a reversible hydrogen absorption/desorption reaction pathway. However, the alloys were shown to have lower hydrogen storage capacity than that of pure MgH₂.