Mg/Al/Mn Mono- to Ternary-Doped CaO-Based Microspheres via Dual-Fluid Spray Pyrolysis and Combustion Synthesis for Enhanced Calcium Looping

Calcium looping (CaL) is a promising technology for thermochemical energy storage (TCES), benefiting from the high energy storage density and operating temperature of CaO-based heat carriers. However, CaO suffers from severe sintering and low optical absorptance. In this work, Mg/Al/Mn mono-, binary-, and ternary-doped CaO-based microspheres were synthesized via dual-fluid spray pyrolysis (SP) and spray combustion (SC), respectively. The effects of doping schemes on cyclic energy storage, optical absorption, and microstructure were systematically investigated. Mg mono doping refined grains and optimized the pore structure, with 30 wt % MgO as the optimum. Al mono doping formed Ca 3 Al 2 O 6, reducing reactivity but improving cyclic stability. Mg−Al binary doping displayed a synergistic effect, balancing energy storage density and stability. With a total doping content of 20 wt %, introducing Mn boosted energy storage performance and optical absorption: The Mg-Mn system exhibited superior reactivity with Ca 2 MnO 4 formed, and the SP-derived sample with 15 wt % MgO/5 wt % MnO 2 showed a 50-cycle accumulated energy storage density of 104 kJ/g. The Al-Mn system produced CaMnO 3 with enhanced stability, and the maximum optical absorptance reached 87.41%. Mg−Al−Mn ternary doping yielded microspheres with balanced overall performance. The SC-derived sample with 5 wt % MgO/5 wt % Al 2 O 3 /10 wt % MnO 2 showed a 50-cycle decay rate of 9.94% and average solar absorptance of 79.19%. All microsphere surfaces possessed uniform elemental distribution. Parametric comparison revealed that SP favored porous structures for higher energy storage density, whereas SC improved crystallinity, cyclic stability, and optical absorption, though requiring more anti-sintering dopants to offset the effects of high-temperature synthesis.