Interplay of Redox Mediators and Polymeric Targets for High-Capacity Accessibility in Redox Targeting Flow Batteries

Redox targeting flow batteries (RTFBs) allow for enhanced energy density over traditional flow batteries by including immobilized redox-active solids in the tank, charged and discharged via soluble redox mediators (RMs). This study presents a nonaqueous polymeric RTFB using crosslinked poly[(4-(N,N-dimethylaminomethyl)ferrocenylmethyl)styrene] hexafluorophosphate (xPs–[(FcNMe2)+][PF6–]) as the polymeric redox target (polyRT) and evaluates three RM systems: a single-mediator redox targeting (SMRT) system with [FcNMe₃⁺][PF₆⁻] or MEEPT, and a dual-mediator redox targeting (DMRT) system combining MEEPT and Fc. Discharge capacity utilizations of 90% and 63% were achieved for the [FcNMe₃⁺][PF₆⁻]- and MEEPT-based SMRT systems, respectively, while the DMRT system reached 106%, indicating near-complete or enhanced accessibility through cooperative RM action. In particular, MEEPT and Fc enabled staged discharge across different redox potentials, reducing voltage losses and improving voltage efficiency. A Nernstian thermodynamic model was applied to rationalize the performance trends by coupling RM and polyRT state-of-charge (SOC) profiles. Two metrics were introduced: η_RM, the accessible fraction of mediator discharge capacity, and η_RT^*, the model predicted accessibility of the polyRT. The model showed reasonable agreement with experimental data, highlighting that small redox potential offset and high η_RM are critical for optimal SMRT performance. Together, these results establish design criteria for selecting and pairing RMs with polyRT, offering a practical framework for advancing high-capacity, high-efficiency RTFBs.