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Journal of the American Chemical Society Communication These observations demonstrate that operating conditions that access most, but not all, of the theoretical capacity greatly extend battery lifetime. Although the strategy of avoiding high states of charge reduces the amount of DHA formation, it does not eliminate it completely unless impractically low (less than ∼60%) states of charge are utilized (Figure S21). To divert the remaining DHA from irreversible dimerization and prevent the associated loss of capacity, we used lower oxidation overpotentials to slow the rate of DHA dimerization. We then added molecular oxygen to favor the oxidation of DHA back to DHAQ over DHA dimerization (Scheme 1). We tested this strategy by symmetric cell cycling of a DHAQ electrolyte under anaerobic conditions within its full SOC range (0−99.9%) but with an oxidative overpotential of 200 mV. The capacity decreased by 13% (Figures 4B and S22B) over 2 days of cycling. At this point, the electrolyte was removed from the glovebox in the discharged (oxidized) state, aerated, and returned to service. This operation resulted in the recovery of approximately 70% of the lost capacity when cycling was resumed. In a large-scale flow battery, the same process might be made more effective by purging the DHAQ negolyte reservoir with air after each full discharge to exploit the high DHAQ recovery rate at low DHA concentrations. Aeration could be optimized to match the DHA concentration to avoid unnecessary current efficiency loss (Figure S24). Our findings establish that the progressive loss of capacity in a DHAQ-based flow battery is primarily due to anthrahy- droquinone disproportionation followed by irreversible anthrone dimerization. Computational analysis indicates that the propensity for anthrahydroquinones to disproportionate is highly correlated to the redox potential of the anthraquinone. Therefore, the use of anthraquinones with lower reduction potentials, enabling apparently higher cell voltages, may be limited by a decrease in stability. This correlation suggests that ongoing efforts to improve quinone-based negolytes might profitably be redirected toward the manipulation of properties such as their solubility and synthetic costs, rather than redox potential alone. We have also shown that the loss of battery capacity can largely be ameliorated by avoiding high states of charge and by negolyte aeration. We estimate that the combination of these two strategies would reduce the capacity fade rate of anthraquinone-based flow batteries by approximately 2 orders of magnitude, from 5.6%/day to (0.3)(0.14%/day) = 0.042%/ day. These modified operating procedures should not substantially increase the estimated large-scale capital cost (∼$20/kWh for the DHAQ negolyte).66 By comparison, the United States Department of Energy has set a $150/kWh goal for grid-based energy storage systems. With further optimiza- tion of the battery operating conditions, this goal may be a■ttained in the near future. ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b13295. Experimental details (PDF) ■ AUTHOR INFORMATION Corresponding Authors *gordon@chemistry.harvard.edu *maziz@harvard.edu *ekwan@fas.harvard.edu ORCID Liuchuan Tong: 0000-0001-6211-6322 Daniel P. Tabor: 0000-0002-8680-6667 Susan A. Odom: 0000-0001-6708-5852 Alań Aspuru-Guzik: 0000-0002-8277-4434 Roy G. Gordon: 0000-0001-5980-268X Michael J. Aziz: 0000-0001-9657-9456 Present Addresses ⊥Form Energy Inc., Somerville, Massachusetts 02143, United States ¶Department of Chemistry and Department of Computer Science; Vector Institute for Artificial Intelligence, University of Toronto, Toronto, Ontario M5S 1A1, Canada Author Contributions ∥These authors contributed equally. Notes The authors declare the following competing financial interest(s): Harvard University has filed a patent application b■ased on the methods described here. ACKNOWLEDGMENTS This research was supported by U.S. DOE award DE-AC05- 76RL01830 through PNNL subcontract 428977, by U.S. DOE ARPA-E award DE-AR-0000767, by Innovation Fund Den- mark via the Grand Solutions project “ORBATS” file no. 7046- 00018B, by the Massachusetts Clean Energy Technology Center, and by the Harvard School of Engineering and Applied Sciences. D.A.P. acknowledges funding support from the NSF Graduate Research Fellowship Program, no. DGE1144152 and DGE1745303. A.A.-G. acknowledges support from the Canada 150 Research Chair Program. We thank Prof. Cyrille Costentin, Prof. Luke M. Davis, Dr. Eugene S. Beh, Prof. Sergio Granados-Focil, and Prof. Alison E. Wendlandt for ■helpful discussions and Eric Fell for experimental support. REFERENCES (1) Stehly, T.; Heimiller, D.; Scott, G. 2016 Cost of Wind Energy Review; National Renewable Energy Laboratory, 2017. (2) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical energy storage for the grid: a battery of choices. Science 2011, 334 (6058), 928−35. (3) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical energy storage for green grid. Chem. Rev. 2011, 111 (5), 3577−613. (4) Nguyen, T.; Savinell, R. F. Flow Batteries. Electrochem. Soc. Interface 2010, 19, 54−56. (5) Rugolo, J.; Aziz, M. J. Electricity storage for intermittent renewable sources. Energy Environ. Sci. 2012, 5, 7151. (6) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Gostick, J. T.; Liu, Q. Redox flow batteries: a review. J. Appl. Electrochem. 2011, 41 (10), 1137−1164. (7)Leung,P.;Li,X.;PoncedeLeoń,C.;Berlouis,L.;Low,C.T.J.; Walsh, F. C. Progress in redox flow batteries, remaining challenges and their applications in energy storage. RSC Adv. 2012, 2 (27), 10125. (8) Biello, D. Solar Wars. Sci. Am. 2014, 311 (5), 66−71. (9) Skyllas-Kazacos, M.; Chakrabarti, M. H.; Hajimolana, S. A.; Mjalli, F. S.; Saleem, M. Progress in Flow Battery Research and Development. J. Electrochem. Soc. 2011, 158 (8), R55. (10) Soloveichik, G. L. Flow batteries: current status and trends. Chem. Rev. 2015, 115 (20), 11533−58. (11) Darling, R. M.; Gallagher, K. G.; Kowalski, J. A.; Ha, S.; Brushett, F. R. Pathways to low-cost electrochemical energy storage: a DOI: 10.1021/jacs.8b13295 J. Am. Chem. Soc. 2019, 141, 8014−8019 8017PDF Image | Extending organic flow batteries via redox state management
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