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

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 pi-conjugation structure of the anolytes and the enhanced aromaticity of the highly reduced species, we reported a series of highly conjugated and inexpensive arylene diimide derivatives (NDI, PDI, and 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 98.44% capacity retention at 40 mA cm-2 for 350 cycles; TPDI/FcNCl-based AORFB also showed remarkable stability with 97.22% capacity retention at 20 mA cm-2 over 200 cycles. This finding lays the theoretical foundation and offers a reference for improving the stability of two-electron AORFBs.