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 limited by the mismatch in redox reaction kinetics during the reverse reactions between the cold and hot cells. In this work, using the I⁻/I₃⁻ and [Fe(CN)₆]^3−/4− redox pairs as an example, a cell-specific electrodes 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 (ΔT) 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 a ΔT 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.