Mn and Al co-modified CaO-based composites from various calcium precursors for thermochemical energy storage: High energy storage density and excellent solar absorption ability

Thermochemical energy storage (TCES) materials driven by calcium looping (CaL) have great potential to be used in the next generation of concentrated solar power (CSP) plants. However, natural calcium-based materials are easily sintered at high temperatures, and their optical absorption capacities are too low to be directly utilized. To overcome these challenges, Mn and Al co-modified CaO-based composites were developed by the sol-gel method in this work, and three kinds of precursors (calcium acetate, calcium gluconate, and calcium nitrate) were chosen as calcium sources. Considering the energy storage density and the average solar absorption ability, the appropriate doping ratio was determined to be Ca: Mn: Al = 100: 8: 8. In different calcium precursors, the sample fabricated from calcium gluconate forms a stable skeleton and keeps a porous structure during cycles, and its average optical absorption reaches 86.14%. The conversion rate of CaO is maintained at 91.50% over 60 cycles, while that of pure CaO prepared by commercial CaCO3 is only 16.93%. The effect of Mn and Al on the carbonation stage was analyzed by density functional theory (DFT). The CO2 adsorption energies of the perfect CaO surface and the defective CaO surface co-doped with Mn and Al were calculated. It is found that the defective surface is easier to combine with CO2, and the generated C-O bond is strengthened. This work develops a doping strategy for outstanding CaO-based composites and explores the physical mechanism of doping elements for enhancing CaO capability from the microscopic aspect.