Aqueous anthraquinone redox flow batteries (AARFBs) offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables and are especially well-suited for large-scale stationary deployment.^1,2 However, production cost of anthraquinone-based electrolytes and molecular decomposition are the two major challenges preventing them from being commercialized.^3,4 We report electrochemical approaches to both these problems: anthraquinone electrosynthesis from lower-cost anthracene feedstock, and the electrochemically-induced reversal of decomposition.

We demonstrate the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a continuous flow cell without the use of hazardous oxidants or noble metal catalysts. The anthraquinone, generated in situ, was used as the active species in a flow battery electrolyte without further modification or purification.^5,6

Utilizing 2,6-dihydroxy-anthraquinone (DHAQ) without further structural modification, we demonstrate that the regeneration of the original molecule after decomposition represents a viable route to achieve low-cost, long-lifetime AARFBs. We used in situ (online) NMR and EPR and complementary electrochemical analyses to show that the decomposition compounds 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL), can be recomposed to DHAQ electrochemically through two steps: oxidation of DHA(L)²⁻ to the dimer (DHA)₂⁴⁻ by one-electron transfer followed by oxidation of (DHA)₂⁴⁻ to DHAQ²⁻ by three-electron transfer per DHAQ molecule. This electrochemical regeneration process also rejuvenates the positive electrolyte – rebalancing the states of charge of both electrolytes without introducing extra ions.⁷

References:

1. Huskinson, B. T.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A metal-free organic-inorganic aqueous flow battery. Nature 2014, 505 (7482), 195.
2. Lin, K.; Chen, Q.; Gerhardt, M. R.; Tong, L.; Kim, S. B.; Eisenach, L.; Valle, A. W.; Hardee, D.; Gordon, R. G.; Aziz, M. J.et al. Alkaline quinone flow battery. Science 2015, 349 (6255), 1529.
3. Kwabi, D. G.; Ji, Y.; Aziz, M. J. Electrolyte lifetime in aqueous organic redox flow batteries: A critical review. Chem. Rev. 2020, 120 (14), 6467.
4. Brushett, F. R.; Aziz, M. J.; Rodby, K. E. On lifetime and cost of redox-active organics for aqueous flow batteries. ACS Energy Letters 2020, 5, 879.
5. Wu, M.; Jing, Y.; Wong, A. A.; Fell, E. M.; Jin, S.; Tang, Z.; Gordon, R. G.; Aziz, M. J. Extremely stable anthraquinone negolytes synthesized from common precursors. Chem 2020, 6, 11.
6. Jing, Y.; Wu, M.; Wong, A. A.; Fell, E. M.; Jin, S.; Pollack, D. A.; Kerr, E. F.; Gordon, R. G.; Aziz, M. J. In situ electrosynthesis of anthraquinone electrolytes in aqueous flow batteries. Green Chemistry 2020, 22 (18), 6084.
7. Jing, Y.; Zhao, E. W.; Goulet, M.-A.; Bahari, M.; Fell, E. M.; Jin, S.; Davoodi, A.; Jónsson, E.; Wu, M.; Grey, C. P.; Gordon, R. G.; Aziz, M. J. Closing the Molecular Decomposition-Recomposition Loop in Aqueous Organic Flow Batteries. Nature Chemistry 14, in press (2022); preprint: ChemRxiv, 2021, 10.33774/chemrxiv-2021- x05x1.