Redox-active oxide-molten salt composites as a new family of high-capacity thermal energy storage materials†

This study introduces a new family of redox-active oxide-molten salt (ROMS) composites for high-capacity thermal energy storage. Porous perovskite oxides serve as active support materials, facilitating thermochemical energy storage through redox reactions, while latent heat from the phase change of the salt mixture enables high energy density within a narrow temperature swing. We demonstrated the compatibility between perovskites and salt mixtures, with 12 out of 25 tested combinations proving successful. The diverse properties of perovskites and salt mixtures resulted in ROMS compositions with different functionality and performance, three of which are highlighted in this work. La_0.8Sr_0.2FeO_3−δ:NaF–CaF₂–LiF exhibited excellent latent heat-based energy storage as well as long-term stability with a total capacity of ∼530 kJ kg⁻¹ (510–660 °C). Sr_0.125Ca_0.875Fe_0.25Mn_0.75O_3−δ:NaF–CaF₂ achieved the overall energy density of ∼523 kJ kg⁻¹ (670–820 °C) through both phase-transition and redox-based mechanisms, though gradual deactivation was observed over long-term operation. Lastly, La_0.8Sr_0.2FeO_3−δ:Li₂MoO₄, a highly redox-active ROMS composition, delivered up to 875 kJ kg⁻¹ when applied for waste heat recovery from fuel-containing exhaust gas streams.