Destabilized Calcium Hydride as a Promising High-Temperature Thermal Battery

Calcium hydride (CaH 2 ) is considered an ideal candidate for thermochemical energy storage (thermal battery) due to its high energy density and low cost. Its very high operating temperature and poor cycling stability are the main factors that hinder its development and implementation as a thermal battery for concentrated solar power (CSP) plants. In this work, CaH 2 was thermodynamically destabilized with aluminum oxide (Al 2 O 3 ) at a 1:1 molar ratio to release hydrogen at a lower temperature than the hydride alone. Temperature-programmed desorption measurements showed that the addition of Al 2 O 3 destabilized the reaction thermodynamics of hydrogen release from CaH 2 by reducing the decomposition temperature to ∼600 °C in comparison to ∼1000 °C for pure CaH 2 at 1 bar of H 2 pressure. The experimental enthalpy and entropy of this system were determined by pressure composition isotherm measurements between 612 and 636 °C. The enthalpy was measured to be Δ H des = 100 ± 2 kJ mol –1 of H 2, and the entropy was measured to be Δ S des = 110 ± 2 J·K –1 mol –1 of H 2 . The XRD after TPD and in situ XRD data confirmed the main product as Ca 12 Al 14 O 33 . The system exhibited a loss of capacity during hydrogen cycling at 636 °C, which was found to be due to sintering of excess Al 2 O 3, as confirmed by X-ray diffraction and scanning electron microscopy. The hydrogen cycling capacity was significantly improved by reducing the initial amount of Al 2 O 3 to a 2:1 molar ratio of CaH 2 to Al 2 O 3, deeming it as a highly promising high-temperature thermal battery for the next generation of CSP plants.