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

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 NH4 I 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  /NH4 I 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.