Abstract
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.
Supplementary materials
Title
Supporting Information
Description
Chemicals, synthesis, experimental methods, polyRT characterization (SEM and FT-IR), replicate RTFB cycling data, capacity accessibility schematics, DFT computed redox potentials, RTFB efficiency values, derivation of expression generalizing the correlation of RM accessibility to RT accessibility.
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