by Samuel Kopfinger, Hongyi Zhang, Garrett Grocke, Hassan Harb, Rajeev Assary, Stuart Rowan, Shrayesh Patel
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.