Organic Redox Targeting Flow Battery Utilizing a Hydrophilic Polymer and Its In‐Operando Characterization via State‐of‐Charge Monitoring of The Redox Mediator

Abstract The hydrophilic poly(2,2,6,6‐tetramethylpiperdinyloxy‐4‐yl‐methacrylamide) (PTMAm) was utilized as redox target material in an aqueous organic redox targeting flow battery (RTFB). This polymer is processed into granules, which contain a conductive agent and an alginate binder. By this, a hydrophilic, yet water‐insoluble redox target can be obtained. The target was combined with the redox mediator molecule N , N , N ‐trimethyl‐2‐oxo‐2‐[(2,2,6,6‐tetramethylpiperidin‐4‐yloxyl)amino]ethan‐1‐ammonium chloride (TEMPOAmide), that has been reported earlier as flow battery active material. This target/mediator combination has been characterized electrochemically and flow battery testing has been done. Furthermore, in‐operando characterization of the redox target via electrolyte state‐of‐charge (SOC) monitoring has been performed for the first time. The approach provides estimates for the redox target's SOC changes during cycling. In addition, a figure of merit – the “redox targetivity” – is proposed, which provides insights into the efficiency of the targeting reaction and supports the future optimization of materials, cell designs, and operational parameters for RTFBs.