High-temperature thermochemical energy storage using metal hydrides: Destabilisation of calcium hydride with silicon

Highlights: • Calcium hydride has been thermodynamically destabilised with silicon. • A multistep decomposition pathway was determined by theoretical calculations. • The experimental thermal decomposition pathway and thermodynamics were established. • The Ca–Si system is a viable thermal energy storage material. -- Abstract: The thermochemical energy storage properties of calcium hydride (CaH₂) destabilised with either silicon (Si) or Ca_xSi_y compounds at various molar ratios, were thoroughly studied by a combination of experimental and computer assisted thermodynamic calculations. Particularly, the destabilisation effect of Si on CaH₂ at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH₂ to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH₂ and Si forming Ca_xSi_y at varying temperatures, which was confirmed by in-situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and mass-spectroscopic measurements. The most suitable destabilisation reactions between CaH₂ and Si or Ca_xSi_y that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH₂ and CaSi system (in a 2:3 molar ratio of CaH₂ to CaSi) showed desirable operating conditions with a decomposition temperature of 747 ± 33 °C at a hydrogen pressure of 1 bar. Pressure composition isothermal measurements were conducted on this system to determine its practical enthalpy of decomposition to form Ca₅Si₃. The calculated value (107.3 kJ mol⁻¹ H₂) was lower compared to the experimentally determined value (154 ± 4 kJ mol⁻¹ H₂). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca₅Si₃ phase.