Highly Soluble Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Abstract Two-electron neutral aqueous organic redox flow batteries (AORFBs) hold more promising applications in the power grid than one-electron batteries because of their higher capacity. However, their development is strongly limited by the structural instability of the highly reduced species. By combining the extended π-conjugation structure of the anolytes and the enhanced aromaticity of the highly reduced species, we reported a series of highly water-soluble and inexpensive arylene diimide derivatives (NDI, PDI, TPDI) as novel two-electron storage anolyte materials for ultrastable AORFBs. Matched with (ferrocenylmethyl)trimethylammonium chloride (FcNCl) as catholyte, arylene diimide derivative-based AORFBs showed the highest stability in two-electron AORFBs to date. The NDI/FcNCl-based AORFB delivered 98.4% capacity retention at 40 mA cm− 2 for 350 cycles, and TPDI/FcNCl-based AORFB showed remarkable stability (1600 cycles) with 95.5% capacity retention at 20 mA cm− 2. This finding lays the theoretical foundation and offers a reference for improving the stability of two-electron AORFBs.