DFT Study of the Cooperative Promotion of MgO and ZnO on CaCO₃/CaO Thermochemical Heat Storage Performance of CaO

CaCO3/CaO thermochemical heat storage is one of the most prospective schemes for large-scale heat storage in the next-generation concentrated solar power plants. MgO and ZnO can cooperatively enhance the heat storage performance of CaO. However, the underlying mechanism for the cooperative promotion of MgO and ZnO on the heat storage performance of CaO remains unidentified. In this work, density functional theory (DFT) calculations were applied to analyze the role of MgO and ZnO in CaCO3/CaO heat storage processes. The thermodynamic properties, structural evolution, and exothermic reactivity of CaO-based materials were studied by DFT. Because of the influence of ZnO on Ca-O bonds, ZnCa3O4 has a lower binding energy than Ca4O4. ZnO enlarges the lattice heat capacity and the phonons average velocity of CaO, thus improving the thermal conductivity. The sintering resistance of M-C is mainly due to the strong interaction of Mg-s-O-c, but the interaction of Ca-c-O-s is weak. With the doping of ZnO, the strength of Ca-c-O-s is improved, resulting in a stronger sintering resistance of M-ZC. The exothermic reactivity of CaO is simultaneously affected by the interactions between C and O in CaO and between Ca and O in CO2. MgO-ZnO codoping facilitates the interactions between Ca and O but harms the interactions between C and O. With their combined influence, the Hirshfeld charge transfer of M-ZC for CO2 is -0.5 e, which is 1.39 times that of CaO. Besides, MgO-ZnO-codoped CaO was prepared by the wet-mixing method and had heat storage tests. The experimental results support the result obtained by the DFT calculation. Therefore, the probable mechanism of the cooperative promotion on the CaCO3/CaO heat storage process of CaO resulting from MgO-ZnO codoping was determined.