(313a) Redox-Active Oxide–Molten Salt Composites: A New Class of Materials for Thermal Energy Storage

Thermal energy storage (TES) materials can harness industrial waste heat to enable load shifting and power generation. Among high-temperature TES materials, phase change composites are attractive as they stabilize molten salts for latent heat storage by encapsulating them within inert support materials. However, inert supports reduce the overall energy storage capacity of the composites, requiring large temperature swings for effective operation. To address these challenges, we present a novel class of materials called redox-active oxide molten salt (ROMS) composites, which integrate (i) latent heat from the phase transition of salts; (ii) thermochemical heat from redox activity of perovskite oxides; and (iii) sensible heat to achieve high energy storage capacities within narrow temperature swings. In these composites, porous perovskite oxides act as an active support material to stabilize the molten salts while also contributing to overall energy density. We investigated the compatibility between perovskites and salt mixtures via XRD; identifying 12 compatible pairs out of 25 tested. TGA-DSC results showed that the tunability of perovskites and molten salt mixtures allows the design of composite materials with distinct properties. Three compositions were focused on: Sr_xCa_1-x Fe_yMn_1-yO_3−δ:NaF-CaF₂ achieved an overall capacity of ~523 kJ/kg between 670-820 ^oC through both thermochemical and latent heat storage, despite showing deactivation over extended cycling. Meanwhile, La_xSr_1-xFeO_3−δ:NaF-CaF₂-LiF demonstrated outstanding stability, delivering an overall capacity of ~530 kJ/kg between 510-660 °C via a phase transition-based storage mechanism. Finally, La_xSr_1-xFeO_3−δ :Li₂MoO₄ was identified as a highly redox active composite, reaching energy densities up to 875 kJ/kg when utilized waste fuels from flue gas streams.