CaCr2O4 spinel doped calcium-based material with hollow sphere microstructure for directly solar-driven thermochemical energy storage: An experimental and density functional theory study

The development of efficient and stable calcium-based materials is of great importance for achieving directly solar-driven thermochemical energy storage (TCES). To address key challenges such as poor cyclic energy storage performance and low photothermal conversion of calcium-based materials, this study proposes a synergistic strategy combining microstructure regulation with the doping of multifunctional additive. Cr-doped calcium-based material with a hollow microsphere structure was synthesized by the hydrothermal method, and its cyclic TCES capacity was tested. The results indicate that Cr exists primarily in form of CaCr 2 O 4 spinel, functioning simultaneously as the inert support, reaction promoter, and photothermal conversion agent. The hollow microsphere structure not only shortens the diffusion paths of CO 2 and provides larger reaction interfaces, but also effectively buffers against pore structure degradation caused by the sintering during TCES cycles. Experimental results demonstrate that the CaCr 2 O 4 -doped hollow microsphere structured calcium-based material maintains an energy storage density of 2090 kJ/kg in the 20th cycle, which is 2.28 times that of commercial CaO. Furthermore, owing to the intrinsic light-absorbing properties of CaCr 2 O 4 and the multi-level scattering effect of the hollow structure, CaCr 2 O 4 -doped hollow microsphere structured calcium-based material exhibits enhanced heating rates and steady-state temperatures under simulated sunlight. Mechanistic studies based on Density functional theory calculations reveal that the spinel structure of CaCr 2 O 4 significantly reduces the oxygen vacancy formation energy, thereby enhancing reaction activity. And CaCr 2 O 4 effectively inhibits the sintering of calcium-based materials by anchoring CaO clusters. This study provides a novel design strategy and theoretical foundation for developing high-performance calcium-based materials for direct solar-driven TCES.