Exploring halide destabilised calcium hydride as a high-temperature thermal battery.

CaH 2 is a metal hydride with a high energy density that decomposes around 1100 °C at 1 bar of H 2 pressure. Due to this high decomposition temperature, it is difficult to utilise this material as a thermal battery for the next generation of concentrated solar power plants, where the currently targeted operational temperature is between 600 and 800 °C. In this study, CaH 2 has been mixed with calcium halide salts (CaCl 2 , CaBr 2 and CaI 2) and annealed at 450 °C under 100 bar of H 2 pressure to form CaHCl, CaHBr and CaHI. These hydride-halide salts incur a thermodynamic destabilisation of their hydrogen release, compared to CaH 2 , so that they can operate between 600 and 800 °C within practical operating pressures (1–10 bar H 2) for thermochemical energy storage. The as-synthesised metal hydrides were studied by in-situ synchrotron X-ray diffraction, temperature programmed desorption and pseudo pressure composition isothermal analysis. Each of the calcium hydride-halide salts decomposed to form calcium metal and a calcium halide salt after hydrogen release. In comparison to pure CaH 2, their decomposition reactions were faster when heated up to 850 °C, and the experimental values of the desorbed hydrogen gas were very close to the theoretical ones. All samples after their decomposition showed signs of sintering, which hindered their rehydrogenation reaction. • CaH 2 was thermodynamically destabilised when mixed with calcium halide salts. • The new compounds exhibited faster reaction kinetics compared to CaH 2. • All three compounds exhibited equilibrium pressures between 1 and 10 bar at 650 °C. [ABSTRACT FROM AUTHOR]