200-fold lifetime extension of 2,6-dihydroxyanthraquinone during flow battery operation

We study the capacity fade rate of a flow battery utilizing 2,6-dihydroxyanthraquinone (DHAQ) and its dependence on hydroxide concentration, state of charge, cutoff voltages for the discharge step and for the electrochemical regeneration (oxidation of decomposition compounds back to active species) step, and period of performing the electrochemical regeneration events. Our observations confirm that the first decomposition product, 2,6-dihydroxyanthrone (DHA), is stable but after electro-oxidative dimerization, the anthrone dimer decomposes. We identify conditions for which there is little time after dimerization until the dimer is rapidly re-oxidized electrochemically to form DHAQ. Combining these approaches, we decrease the fade rate to 0.02%/day, which is 18 times lower than lowest rate reported previously of 0.38%/day, and over 200 times lower than the value under standard cycling conditions of 4.3%/day. The findings and their mechanistic interpretation are expected to extend the lifetime and enhance the effectiveness of in-situ electrochemical regeneration for other electroactive species with finite lifetimes.