Comparison of Capacity Retention Rates During Cycling of Quinone-Bromide Flow Batteries
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Comparison of Capacity Retention Rates During Cycling of Quinone-Bromide Flow Batteries Michael R. Gerhardt1, Eugene S. Beh1,2, Liuchuan Tong2, Roy G. Gordon1,2, and Michael J. Aziz1 1
Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA 02138, U.S.A. 2 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, U.S.A. ABSTRACT We use cyclic charge-discharge experiments to evaluate the capacity retention rates of two quinone-bromide flow batteries (QBFBs). These aqueous QBFBs use a negative electrolyte containing either anthraquinone-2,7-disulfonic acid (AQDS) or anthraquinone-2-sulfonic acid (AQS) dissolved in sulfuric acid, and a positive electrolyte containing bromine and hydrobromic acid. We find that the AQS cell exhibits a significantly lower capacity retention rate than the AQDS cell. The observed AQS capacity fade is corroborated by NMR evidence that suggests the formation of hydroxylated products in the electrolyte in place of AQS. We further cycle the AQDS cell and observe a capacity fade rate extrapolating to 30% loss of active species after 5000 cycles. After about 180 cycles, bromine crossover leads to sufficient electrolyte imbalance to accelerate the capacity fade rate, indicating that the actual realization of long cycle life will require bromine rebalancing or a membrane less permeable than Nafion® to molecular bromine. INTRODUCTION Cost effective grid scale energy storage would enable large-scale decarbonization of our electric grid. Redox flow batteries (RFBs) are a potentially low-cost solution to the intermittency problem faced by renewable energy sources such as wind and solar, but their viability requires long cycle life as well as long calendar life [1]. Recently, quinones have been demonstrated as inexpensive electroactive species for aqueous flow batteries [2-5]. One hundred charge-discharge cycles have been demonstrated in an all-quinone flow battery using a tiron posolyte (positive electrolyte) and an anthraquinone-2,6disulfonic acid negolyte (negative electrolyte), at 35% depth of discharge, with no mention of capacity retention [5]. Our group demonstrated 750 deep cycles of a quinone-bromide flow battery (QBFB) using anthraquinone-2,7-disulfonic acid (AQDS, figure 1a), with 99.84% capacity retention per cycle [6].
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Figure 1. The structures of a. AQDS and b. AQS.
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The AQDS-bromine battery voltage could be increased by using anthraquinone-2sulfonic acid (AQS, figure 1b) as the negative electrolyte in place of AQDS, because AQS has a lower reduction potential than AQDS. In this work, we examine the long term stability of AQS and AQDS electrolytes in QBFBs through full-cell cycling and chemical analysis. Additionally, we surpass our previous c
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