Redox flow battery is a promising large-scale grid energy storage technology. Its advantage of separating energy and power allows independent designs of tank volume and stack size and thus offers tremendous flexibility for different energy/power ratio applications. Traditional inorganic redox materials have suffered limited solubility, low electrochemical reactivity and/or high cost. Instead, organic compounds are being considerably pursued because of their molecular diversity, structural tailorability, environmental benignity, and potential low cost. A number of organic flow battery systems have been investigated.^1-3

Here we report several organic redox flow chemistries in aqueous and nonaqueous electrolytes.^4,5Promising redox material candidates have been identified with desirable redox potentials, high solubilities, and good electrochemical properties. Flow cells of these systems demonstrated remarkable cell efficiencies and high rate performance. Functional groups present in these molecules play an important role to tune solubility and chemical stability of redox species. Rational molecular engineering will be discussed as a general strategy to improve the battery performance and durability.

References

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(2) K. Gong, Q. Fang, S. Gu, S. Li, Y. Yan, Energ. Environ. Sci. 2015, DOI: 10.1039/C5EE02341F.

(3) G. L. Soloveichik, Chem. Rev. 2015, DOI: 10.1021/cr500720t.

(4) X. Wei, W. Xu, J. Huang, L. Zhang, E. Walter, C. Lawrence, M. Vijayakumar, W. A. Henderson, T. Liu, L. Cosimbescu, B. Li, V. Sprenkle, W. Wang, Angew. Chem. Int. Ed. 2015, 54, 8684.

(5) T. Liu, X. Wei, Z. Nie, V. Sprenkle, W. Wang, Adv. Energy Mater. 2016, 6, DOI: 10.1002/aenm.201501449.

Figure 1. Flow cell cycling performance of: (a) nonaqueous N-methylphthalimide (anolyte)/di-t-butyl-dialkoxybenzene (catholyte) system; and (b) aqueous methyl viologen (anolyte)/HO-TEMPO (catholyte) system.⁵