DOI: 10.1002/ente.201500020 A Highly Soluble Organic Catholyte for Non-Aqueous Redox Flow Batteries Aman Preet Kaur, [a] Nicolas E. Holubowitch, [b] Selin Ergun, [a] Corrine F. Elliott, [a] and Susan A. Odom* [a] A phenothiazine derivative with high solubility in carbonate solvents containing lithium salts showed extensive over- charge protection and, as a result, has been evaluated as a catholyte for non-aqueous redox flow batteries. We report the testing of 3,7-bis(trifluoromethyl)-N-ethylphenothiazine as a catholyte and 2,3,6-trimethylquinoxaline as the anolyte in redox flow batteries containing 0.05, 0.15, and 0.35 m active material and found the longest capacity retention over about 60 cycles at 0.15 m. To our knowledge, this is the most soluble catholyte candidate with a robust radical cation. Introduction As greenhouse gases continue to warm the Earth, increasing the number of renewable energy sources connected to the electrical grid has become a topic of worldwide interest. [1] Our current electrical grid is predicted to become unstable if solar and wind power rise to supply more than 20 % of its energy—the amount that we are predicted to reach by 2030—because the grid lacks the ability to accommodate fluctuating energy sources. [2] To remedy this limitation, large- scale electrical energy storage (EES) systems have been in- vestigated for the purpose of storing energy during peak pro- duction and releasing it to relieve strain on the grid during periods of peak end-user demand, resulting in a load-leveling effect. [3] For grid storage using EES, aqueous redox flow batteries (RFBs) have shown great promise due to their low costs and long lifetimes [4] and have been demonstrated on scales of up to 10 MW. [4b, 5] Of the many flow systems that have been in- vestigated, the aqueous all-vanadium system is the most ad- vanced. [6] However, the voltage window of aqueous systems is limited to approximately 1.5 V by the electrolysis of water, and they employ high concentrations of extremely corrosive supporting electrolytes such as sulfuric acid, hydrobromic acid, hydrochloric acid, or nitric acid. [4] These two factors have led to increased interest in non-aqueous redox flow sys- tems, which can potentially be charged to 4 V, depending on the solvent used. [7] Half- and full-cell designs have been dem- onstrated on small scales, employing a variety of solvents and a handful of electro-active materials as the electron donors and acceptors, [8] including phthalimide, anthracene, quinone, or quinoxaline derivatives as the anolyte; [9] 2,2,6,6- tetramethylpiperidin-1-yl)oxyl or dimethoxybenzene deriva- tives as the catholyte; [9a, c, 10] and 9,10-butyl-2,3,6,7-tetracyano- 1,4,5,8,9,10-hexaazaanthracene as both the anolyte and cath- olyte. [11] Recently, suspensions of solid electrode materials used in Li-ion batteries have also been reported for use as the charge-carrying electro-active species. [12] The main factors preventing the commercialization of non- aqueous RFBs are the poor voltage and energy efficiencies and the rapid decay in capacity with cycling. These faults are attributed to one or more of the following problems with the electro-active species: the limited stability of oxidized and/or reduced forms, irreversible reaction with electrode surfaces, and membrane crossover. Despite the development of func- tionalized derivatives [9c, 10b] and tailored electrolytes [1c, 8d, i, 10a] for increased solubility, the instances of battery cycling re- ported in the majority of non-aqueous RFB publications have been limited to systems with low concentrations of elec- tro-active species ( 0.05 m), perhaps due to more rapid ca- pacity fade upon testing the electro-active materials at higher concentrations. The ability to tailor the structure of organic compounds to lead to more-soluble, more-stable spe- cies offers an opportunity to improve upon these limiting fac- tors. Our group has studied electro-active organic compounds for energy storage applications, including the stability of these compounds in oxidized and/or reduced states. Through this work, we identified an electrochemically reversible elec- tron donor with high solubility and stability in carbonate sol- vents containing lithium salts: 3,7-bis(trifluoromethyl)-N-eth- ylphenothiazine (BCF3EPT, Figure 1). [13] These characteris- tics support its use as a potential electro-active material in RFBs. Herein we demonstrate BCF3EPT as a promising new Figure 1. Chemical structures of the catholytes 3,7-bis(trifluoromethyl)-N-eth- ylphenothiazine (BCF3EPT) and 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)ben- zene (DBBB), and the anolyte 2,3,6-trimethyl quinoxaline (TMeQ). [a] Dr. A. P. Kaur, Dr. S. Ergun, C. F. Elliott, Prof. S. A. Odom Department of Chemistry University of Kentucky Lexington, KY 40506-0055 (USA) E-mail: susan.odom@uky.edu [b] Dr. N. E. Holubowitch Center for Applied Energy Research University of Kentucky Lexington, KY 40511 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ente.201500020. Energy Technol. 2015, 3, 476 – 480 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 476