Precise control of protonation of phenazine-TEMPO-coupled hybrid for fully reversible bipolar redox process in aqueous electrolytes

초록

Hydrophilic redox-active organic molecules are promising candidates for aqueous organic redox flow batteries (RFBs), yet only a few effective bipolar systems exist because maintaining water solubility and reversible anodic and cathodic processes within a single electrolyte remains challenging. Although covalent linkage of anolyte- and catholyte-active motifs can impart bipolarity, their optimal pH windows often do not overlap, leading to instability. Here, we report a fully water-soluble phenazine-TEMPO hybrid bearing ammonium chloride functionalities, in which two mechanistically distinct redox units are integrated into a unified bipolar architecture unprecedented in previous systems. This configuration enables both the phenazine-derived anode reaction and the TEMPO-derived cathode reaction to operate reversibly in the same mildly acidic electrolyte (pH approximate to 2.5). Under these conditions, the long-standing stability mismatch between the proton-coupled electron-transfer chemistry of phenazine and the oxoammonium/nitroxide redox cycle of TEMPO is effectively resolved. By pinpointing an acidic pH window in which both redox motifs remain fully reversible, this work establishes a unified bipolar platform for the development of next-generation aqueous organic RFBs.

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