MnCl₂-promoted CaO as an excellent thermochemical material for solar energy storage

The reversible reaction of calcium oxide (CaO) carbonation/calcination is a viable energy storage technology that can be used in concentrated solar power plants. However, the use of pure CaO material is limited by the low Tammann temperature of CaCO₃, which leads to incomplete reaction in the carbonation/calcination cycle process and particle agglomeration and sintering after multiple cycles. These factors affect CO₂ mass transfer and react with deep CaO, resulting in decreased energy storage performance with the increase of number of cycles, which seriously hinders the practical application of the material. In this study, MnCl₂ was employed as a stabilizer to overcome the poor cyclic stability of CaO by using a simple wet impregnation method. Mn²⁺ was introduced into the CaO precursor through impregnation and calcination processes, resulting in the formation of Ca₂MnO₄. This compound acted as a skeleton to support the CaO structure. Additionally, some of the Cl⁻ reacted with Ca²⁺ to produce CaCl₂, which melted during carbonation and facilitated CO₂ mass transfer. It is worth noting that when the concentration of MnCl₂ is 0.05M, the loaded samples exhibit a high energy storage density of 1710 kJ/kg. Moreover, after 50 cycles of carbonation/calcination, the performance only decreases by 5 %, which is 3.7 times that of the CaO materials of the same period. The proposed CA-MnCl₂-0.05 material could be applied in concentrated solar power plants for energy storage.