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 is the major barrier preventing AORFBs from being commercialized. Structural modifications can improve molecular stability at the expense of increased synthetic cost and molecular weight. Utilizing 2,6-dihydroxy-anthraquinone (DHAQ), without further structural modification, we demonstrate that electrochemical regeneration could be a viable route to achieve low-cost, long-lifetime AORFBs. In situ (online) NMR and EPR and complementary electrochemical analyses reveal that decomposition compounds i.e., 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL), can be converted back to DHAQ in two steps: first DHA(L)2− are oxidized to the dimer (DHA)24− at − 0.32 V vs. SHE by one-electron transfer; subsequently, the (DHA)24− is oxidized to DHAQ2− at +0.57 V vs. SHE by three-electron transfer. Electrochemical regeneration rejuvenates not only DHAQ2−, but also the positive electrolyte – rebalancing the states of charge of both electrolytes without introducing extra ions. We demonstrate the repeated capacity recovery with DHAQ | potassium ferro-/ferricyanide flow battery in basic conditions, and show the approach is also effective for anthraquinone-2,7-disulfonate in acid. Electrochemical regeneration strategies may extend the useful lifetime of many water-soluble organic molecules with anthraquinone core structures in electrochemical cells.
Supplementary figures for Electrochemical Regeneration of Anthraquinones for Lifetime Extension in Flow Batteries
The AQDS preliminary studies in the SI further show both the feasibility of electrochemical regeneration at pH 0 and that this is not a peculiar feature of DHAQ. The electrochemical regeneration method should extend to other anthraquinones susceptible to anthrone formation; similar strategies may also be applicable for other redox-active organic molecules. Electrochemical regeneration could permit redox-active organic molecules to reach the combination of performance, cost, and lifetime necessary for AORFBs to become an attractive solution to the intermittent renewable electricity storage problem. Besides, the SI provides the experimental details about DHAQ and AQDS chemical and electrochemical regenerations, including in situ, ex situ NMR data, cyclic voltammetry, linear sweep voltammetry, cell cycling, LC-MS, and photo of our 3-electrode setup etc.