In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries

Abstract

Aqueous organic redox flow batteries (AORFBs) 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 while 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 AORFBs. 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.