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

Organic redox-active molecules are promising materials for charge storage in redox-flow batteries (RFBs); however, the development of all-organic RFBs is hindered by material crossover, limited energy density, and poor stability of active materials. Here, ester-substituted bispyridinylidenes are reported as the first examples of intrinsic bipolar molecules that exhibit basically concerted double two-electron redox activity at a potential difference of 1.01 V. All three oxidation states of the pentylester derivative exhibited excellent temporal stability and good solubility in the electrolyte. Testing this active material in symmetric cells, which alleviates crossover issues, revealed good cyclability (fade of 0.025% and 0.35% per cycle for static and flow cells, respectively), capacities of up to 89% of the theoretical value, and Coulombic efficiencies above 99%. Considering previous evidence for active material solubility limits of ∼2 M, and the benefits of a symmetric design, such double concerted multielectron bipolar active materials will be key to developing energy dense organic RFBs.