A pH‐Neutral, Metal‐Free Aqueous Organic Redox Flow Battery Employing an Ammonium Anthraquinone Anolyte

Abstract Redox‐active anthraquinone molecules represent promising anolyte materials in aqueous organic redox flow batteries (AORFBs). However, the chemical stability issue and corrosion nature of anthraquinone‐based anolytes in reported acidic and alkaline AORFBs constitute a roadblock for their practical applications in energy storage. A feasible strategy to overcome these issues is migrating to pH‐neutral conditions and employing soluble AQDS salts. Herein, we report the 9,10‐anthraquinone‐2,7‐disulfonic diammonium salt AQDS(NH4)2, as an anolyte material for pH‐neutral AORFBs with solubility of 1.9 m in water, which is more than 3 times that of the corresponding sodium salt. Paired with an NH4I catholyte, the resulting pH‐neutral AORFB with an energy density of 12.5 Wh L−1 displayed outstanding cycling stability over 300 cycles. Even at the pH‐neutral condition, the AQDS(NH4)2 /NH4I AORFB delivered an impressive energy efficiency of 70.6 % at 60 mA cm−2 and a high power density of 91.5 mW cm−2 at 100 % SOC. The present AQDS(NH4)2 flow battery chemistry opens a new avenue to apply anthraquinone molecules in developing low‐cost and benign pH‐neutral flow batteries for scalable energy storage.