Ester-Substituted Bispyridinylidenes: Double Concerted Two-Electron Bipolar Molecules for Symmetric Organic Redox Flow Batteries

Organic redox-active molecules are promising materialsfor chargestorage in redox-flow batteries (RFBs); however, the development ofall-organic RFBs is hindered by material crossover, limited energydensity, and poor stability of active materials. Here, ester-substitutedbispyridinylidenes are reported as the first examples of intrinsicbipolar molecules that exhibit basically concerted double two-electronredox activity at a potential difference of 1.01 V. All three oxidationstates of the pentylester derivative exhibited excellent temporalstability and good solubility in the electrolyte. Testing this activematerial in symmetric cells, which alleviates crossover issues, revealedgood cyclability (fade of 0.025% and 0.35% per cycle for static andflow cells, respectively), capacities of up to 89% of the theoreticalvalue, and Coulombic efficiencies above 99%. Considering previousevidence for active material solubility limits of & SIM;2 M, andthe benefits of a symmetric design, such double concerted multielectronbipolar active materials will be key to developing energy dense organicRFBs.