A Water-Miscible Quinone Flow Battery with High Volumetric Capacity and Energy Density

A water-miscible anthraquinone with polyethylene glycol (PEG)-based solubilizing groups is introduced as the redox-active molecule in a negative electrolyte (negolyte) for aqueous redox flow batteries, exhibiting the highest volumetric capacity among aqueous organic negolytes. We synthesized and screened a series of PEG-substituted anthraquinones (PEGAQs) and carefully studied one of its isomers, namely, 1,8-bis(2-(2-(2-hydroxyethoxy)ethoxy)-ethoxy)anthracene-9,10-dione (AQ-1,8-3E-OH), which has high electrochemical reversibility and is completely miscible in water of any pH. A negolyte containing 1.5 M AQ-1,8-3E-OH, when paired with a ferrocyanide-based positive electrolyte across an inexpensive, nonfluorinated permselective polymer membrane at pH 7, exhibits an open-circuit potential of 1.0 V, a volumetric capacity of 80.4 Ah/L, and an energy density of 25.2 Wh/L.