Bridging Redox Asymmetry in Hot and Cold Cells for Boosted Power Density in Thermally Regenerative Flow Batteries

Abstract

Thermally regenerative flow batteries (TRFBs) offer a promising route for harvesting low-grade waste heat, however, their power output is constrained by the mismatch in redox reaction kinetics during the reverse reactions between the cold and hot cells. In this work, using the I^-/I^3- and [Fe(CN)₆]^3-/4- redox pairs as an example, a cell-specific electrode design strategy that bridges the redox asymmetry between the hot and cold cells is developed to enable efficient thermoelectric conversion in TRFBs. In the cold cell, the P-type Co₃O₄ electrode promotes the oxidation reaction by facilitating hole-mediated electron extraction and reducing diffusion impedance, while in the hot cell, the N-type CoS₂ electrode with a sulfur-rich surface provides electron-rich active sites, effectively lowering the activation energy of the reduction reaction. This reaction-specific design strategy aligns with the thermodynamic and kinetic requirements of each thermal unit, significantly boosting the power output. The optimised TRFB delivers a normalised power density of 1.41 mW·m^-2·K^-2 at a temperature difference (∆T) of 30 K, nearly double that of state-of-the-art TRFBs, with an efficiency reaching 4.7% of the Carnot efficiency. Moreover, the system maintains an effective power density of 2.3 W·m^-2 over 70 hours at ∆T of 45 K. This work demonstrates the potential of bridging redox asymmetry in cold and hot cells to achieve efficient thermoelectric conversion, providing a new direction for the practical application of TRFBs.