Thermodynamics and performance of the Mg-H-F system for thermochemical energy storage applications

Magnesium hydride (MgH₂) is a hydrogen storage material that operates at temperatures above 300 °C. Unfortunately, magnesium sintering occurs above 420 °C, inhibiting its application as a thermal energy storage material. In this study, the substitution of fluorine for hydrogen in MgH₂ to form a range of Mg(H_xF_1-x)₂ (x = 1, 0.95, 0.85, 0.70, 0.50, 0) composites has been utilised to thermodynamically stabilise the material, so it can be used as a thermochemical energy storage material that can replace molten salts in concentrating solar thermal plants. These materials have been studied by in situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, temperature-programmed-desorption mass spectrometry and Pressure-Composition-Isothermal (PCI) analysis. Thermal analysis has determined that the thermal stability of Mg-H-F solid solutions increases proportionally with fluorine content, with Mg(H_0.85F_0.15)₂ having a maximum rate of H₂ desorption at 434 °C, with a practical hydrogen capacity of 4.6 ± 0.2 wt% H₂ (theoretical 5.4 wt% H₂). An extremely stable Mg(H_0.43F_0.57)₂ phase is formed upon the decomposition of each Mg-H-F composition of which the remaining H₂ is not released until above 505 °C. PCI measurements of Mg(H_0.85F_0.15)₂ have determined the enthalpy (ΔH_des) to be 73.6 ± 0.2 kJ mol^-1 H₂ and entropy (ΔS_des) to be 131.2 ± 0.2 J K^-1 mol^-1 H₂, which is slightly lower than MgH₂ with ΔH_des of 74.06 kJ mol^-1 H₂ and ΔS_des = 133.4 J K^-1 mol^-1 H₂. Cycling studies of Mg(H_0.85F_0.15)₂ over six absorption/desorption cycles between 425 and 480 °C show an increased usable cycling temperature of ∼80 °C compared to bulk MgH₂, increasing the thermal operating temperatures for technological applications.