Compositional Tuning of Ca1–xSrxCrO4 for Redox-Stable High-Temperature Thermochemical Heat Storage

Abstract

Thermochemical energy storage by utilizing concentrated solar power through high-temperature processes requires redox materials with high energy density and good structure stability throughout multiple cycles. In this research, chemical tuning of the calcium chromate series of materials, Ca_1–xSr_xCrO₄, has been conducted to improve the thermodynamics and structure stability of the redox reaction. Structural analysis shows that the structure evolves from a zircon to a monazite type of structure for x ≥ 0.4. It significantly alters the redox reaction mechanism. While CaCrO₄ has a multistep redox reaction with low reversibility, and severe sintering is observed for SrCrO₄, a single-step redox reaction with improved oxygen exchange is observed for intermediate compositions. The optimal composition has a high energy density of ∼ 1.06 MJ/kg, an excellent cycling stability with 99.8% reversibility over 20 cycles, and small thermomechanical losses. The XPS measurements reveal reversible Cr valence states associated with the formation of oxygen vacancies. DFT + U and ab initio molecular dynamics calculations identify the composition at x = 0.6 as the most promising candidate, combining a favorable electronic structure with enhanced oxygen mobility. MSD and diffusion analysis show that oxygen migration is fast and activation energy is low (0.29 eV), which emphasizes the significance of chromate lattice engineering in developing TCES materials for future use.