Multi-doping strategy modified calcium-based materials for improving the performance of direct solar-driven calcium looping thermochemical energy storage

Calcium-looping thermochemical energy storage is a promising candidate in concentrated solar power plants. Solar-driven calcium-looping process via directly capturing sunlight can solve the challenges faced by traditional calcium-looping system, such as excessively high wall temperature, large thermal resistance, and low utilization efficiency of solar energy. In this work, we aim to improve optical absorption and cyclic stability of the direct solar-driven calcium-looping process with modified calcium-based materials for thermochemical energy storage. A simple sol-gel method was used to synthesize calcium-based composites via multi-doping strategy and the doping elements mainly included cerium (Ce), cobalt (Co), and manganese (Mn). Firstly, the role of calcium based materials using single-doping element on the performance of thermochemical energy storage was revealed. The doping of Ce alone could generate oxygen vacancies and increase the adsorbed ability of CO2, accelerating the carbonization kinetics of CaO. The doping of Co alone played an important role in enhancing optical absorption while the doping of Mn alone was beneficial to cycling stability via providing a strong framework to inhibit sintering effect. Calcium-based materials using multi-doping strategy retained the function of single-doping effect on the performance of calcium-looping thermochemical energy storage, synergistically improving optical absorption, carbonization kinetics, and cyclic stability. Compared to pure CaCO3, Ce/Co/Mn co-doped Ca-based materials (CaCO3/5Ce-3Co-6Mn) exhibited a higher light trapping ability with optical absorption reached to 83.26% and more excellent cycling stability with effective conversion rate remained above 64% after 20 cycles. Furthermore, an interesting phenomenon that phase transformation of doped metal oxides formed by multi-doping strategy occurred during the calcination/carbonation cycling process was discovered and the underlying mechanisms were clarified. Finally, the feasibility of experiment by directly capturing concentrated sunlight to drive CaCO3 decomposition for storing solar energy was verified.