Organic redox targeting flow battery utilizing a hydrophilic polymer and its in-operando characterization via state-of-charge monitoring of the redox mediator.

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.