(Invited) Understanding Benzothiadiazole Based Anolyte Materials for Nonaqueous Redox Flow Cells

Abstract

Redox-active organic materials (ROMs) have shown great promise for redox flow battery applications but generally encounter limited cycling efficiency and stability at relevant redox material concentrations in nonaqueous systems. In this talk, a new family of heterocyclic organic anolyte molecules, 2,1,3-benzothiadiazole (BZNSN) and derivatives,1-2 will be discussed, which demonstrated high solubility, low redox potential, and fast electrochemical kinetics. By coupling BZNSN with a benchmark catholyte ROM (DBMMB), the nonaqueous organic flow battery delivered significant improvement in cyclable redox material concentrations and cell efficiencies compared to the state-of-the-art nonaqueous systems. In order to obtain an insightful understanding of this family of molecules, a series of derivatives have been developed by varying the 5-position substitutions with tunable electronic accepting/withdrawing features. The substituent effects on their properties of interest have been examined, including redox potentials, calendar lives of charged ROMs in an electrolyte, and the flow cell cycling performance. While the calendar life of energized fluids can be tuned in a predictable fashion over a wide range, the improvements in the calendar life do not directly translate into the enhanced cycling performance. This finding strongly indicates the significant role of structural engineering in achieving improvement of radical ion stability, suggesting a generic guidance for developing long-cycling ROM-based flow cells. Figure 1 Structures and cyclic voltammogram of benzothiadiazole derivatives and the correlation of the half-decay lifetime of the radical anions with the redox potentials. Mind the logarithmic vertical scale. Duan, W.; Huang, J.; Kowalski, J. A.; Shkrob, I. A.; Vijayakumar, M.; Walter, E.; Pan, B.; Yang, Z.; Milshtein, J. D.; Li, B.; Liao, C.; Zhang, Z.; Wang, W.; Liu, J.; Moore, J. S.; Brushett, F. R.; Zhang, L.; Wei, X.,. ACS Energy Letters 2017, 2(5), 1156-1161. 2. Huang, J.; Duan, W.; Zhang, J.; Shkrob, I. A.; Assary, R. S.; Pan, B.; Liao, C.; Zhang, Z.; Wei, X.; Zhang, L..Journal of Materials Chemistry A 2018, 6 (15), 6251-6254. Figure 1