A Water-Miscible Quinone Flow Battery with High Volumetric and Energy Density
Authors: Jin Shijian , Jing Yan , Aziz Michael J. , Gordon Roy G. 
Published: February 2020

Aqueous redox flow batteries are potentially better candidates than lithium-ion batteries for grid scale energy storage for their safety, cost-effectiveness, longevity, and decoupled power and energy enabling long discharge durations at rated power.1,2 The penetration of the state-of-the-art all-vanadium flow battery is limited by the limited supply and high and fluctuating price of vanadium.3,4 Although aqueous flow batteries with organic redox active materials have demonstrated promising results, including high stability, > 1 V open-circuit potential and safer operation environment (less extreme pH), few have showed volumetric capacity and energy density comparable with vanadium flow batteries.4,5,6,7 In this work, a water-miscible anthraquinone 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 substituted anthraquinones and carefully studied one that has high electrochemical reversibility and is completely miscible with water of any pH. A negolyte containing 3 M electron concentration, when paired with a ferrocyanide-based positive electrolyte across an inexpensive, non-fluorinated 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, which is comparable with that of most vanadium flow batteries.
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