1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA.
2 Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge, UK.
3 Magnetic Resonance Research Center, Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen, the Netherlands.
4 John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
5 Department of Chemical and Materials Engineering, Concordia University, Montreal, Quebec, Canada.
6 Materials and Metallurgical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad (FUM), Mashhad, Iran.
7 Sichuan University-Pittsburgh Institute, Sichuan University, Chengdu, China.
8 Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge, UK. cpg27@cam.ac.uk.
9 Department of Chemistry and Chemical

Aqueous organic redox flow batteries offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition represents a major barrier to commercialization-and although structural modifications can improve stability, it comes at the expense of synthetic cost and molecular weight. Now, 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 aqueous organic redox flow batteries. We used in situ (online) NMR and electron paramagnetic resonance, and complementary electrochemical analyses to show that the decomposition compound 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)₂^4- by one-electron transfer followed by oxidation of (DHA)₂^4- to DHAQ^2- 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.