Insights into a Thermodynamically Optimal Synthesis of the Ternary Complex Hydride Mg₂FeH₆for High-Density Thermal Energy Storage

Mg2FeH6 hydride has been viewed as a high-density and efficient material for thermal energy storage systems. However, the synthesis of the pure hydride is still a big problem for its application. This work combines experiment and density functional theory calculations to explore the conditions under which the synthesis is thermodynamically optimal. The experimental results show that Mg2FeH6 can be successfully synthesized by reactive ball milling within 5 h. The synthesis mechanism is revealed by numerical simulation to be indirect hydrogenation in two steps: one accelerating mode, in which MgH2 is probably formed with the reaction enthalpy of 75.57 kJ/mol H2, and then, a sluggish mode, in which the evolution of Mg2FeH6 happens via the reaction between MgH2 and Fe with the reaction enthalpy of 65.27 kJ/mol H2. In addition, the rehydrogenation of the mixture under low pressure and high temperature >400 °C results in a high degree of crystallinity of Mg2FeH6 and a maximum hydrogen capacity of 5.01 wt %.