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.

1 Nguyen, T., Savinell, R. F. Flow batteries. Soc. Interface, 54-56 (2010).

2 Liu, W., Lu, W., Zhang, H., Li, X. Aqueous flow batteries: research and development. Eur. J. 24, 17, doi:10.1002/chem.201802798 (2018).

3 Soloveichik, G. L. Flow batteries: current status and trends. Rev. 115, 11533-11558, doi:10.1021/cr500720t (2015).

4 Li, L., Kim, S., Wang, W., Vijayakumar, M., Nie, Z., Chen, B., Zhang, J., Xia, G., Hu, J., Graff, G., Liu, J., Yang, Z. A stable vanadium redox-flow battery with high energy density for large-scale energy storage. Energy Mater. 1, 7, doi:10.1002/aenm.201100008 (2011).

5 Kwabi, D. G., Lin, K., Ji, Y., Kerr, E. F., Goulet, M.-A., Porcellinis, D. D., Tabor, D. P., Pollack, D. A., Aspuru-Guzik, A., Gordon, R. G., Aziz, M. J. Alkaline quinone flow battery with long lifetime at pH 12. Joule 2, 13, doi:0.1016/j.joule.2018.07.005 (2018).

6 Beh, E. S., De Porcellinis, D., Gracia, R. L., Xia, K. T., Gordon, R. G., Aziz, M. J. A neutral pH aqueous organic–organometallic redox flow battery with extremely high capacity retention. ACS Energy Lett. 2, 639-644, doi:10.1021/acsenergylett.7b00019 (2017).

7 Ji, Y., Goulet, M-A., Pollack, D. A., Kwabi, D. G., Jin, S., De Porcellinis, D., Kerr, E. F., Gordon, R. G., Aziz, M. J. A phosphonate-functionalized quinone redox flow battery at near-neutral pH with record capacity retention rate. Energy Mater., doi:No. aenm. 201900039 (2019).

8 Hu, B., DeBruler, C., Rhodes, Z., Liu, T. Leo. Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage. Journal of the American Chemical Society 139, 1207-1214, doi:10.1021/jacs.6b10984 (2017).