Mn and Mg synergistically stabilized CaO as an effective thermochemical material for solar energy storage

Calcium oxide (CaO) is a very promising candidate energy storage material for concentrated solar power plants because of high energy storage density, high temperatures of energy release, and readily available precursors. However, CaO itself not only sinters severely at typical operating temperatures but also has low light absorption capacity. To overcome these challenges, modifying CaO with nanoparticle of magnesium (Mg) and manganese (Mn) oxides was proposed in this work. The Mg and Mn co-modified sample (Ca:Mn:Mg = 9:1:1) demonstrates exceptional thermal stability at 800 °C, the density of energy storage deceases by about 1.1 and 0.7% after 10 and 25 cycles, respectively. Microstructure analysis reveals that MgO and Ca₂MnO₄ nanoparticles assemble into a net or chain that distributed in the CaO or CaCO₃ particles, which effectively suppresses sintering and provides stable channels for CO₂ to diffuse in CaO or CaCO₃ layer. After 60 cycles, the density of energy storage declines by 7.46%. Additionally, introducing manganese both makes O atoms have more electrons and creates more oxygen vacancies that favor CO₂ activation. The synergistic effect between Mn and Mg achieves high stability, fast calcination reaction and high conversion rate of the energy storage material. The presence of Mn also greatly improves the solar absorption capacity of CaO material, and the co-modified sample exhibits much higher solar absorptance after long-term testing. The proposed CaMnMg911 material could be applied in a concentrated solar power plant for solar energy storage.