The continued evolution and greening of the electric grid has spurred interest in redox flow batteries (RFBs) as scalable, flexible and potentially low cost energy storage devices¹. Nonaqueous RFBs that utilize organic molecules for charge storage are an emerging concept which has garnered significant interest in the scientific community². Of particular interest is the ability to tailor the molecular structure of the organic compounds to achieve desired properties such increased solubility, higher/lower redox potential and longer lifetimes³. However, because this is an infant field, while many candidate organic molecules have been reported, validations of performance and durability are inconsistent. Materials are evaluated under different conditions, including different charging rates, state-of-charge swings, reactant concentrations, and cell configurations^3–5. This presents changes in terms of both comparative analysis (e.g., is material A better than material B?) and absolute metrics (e.g., what is the true stability of material A?). Here, we compare bulk electrolysis techniques, varying charging rates, concentrations, and cycling methods, to gain insight into how different testing protocols influence observed behavior. The overarching goal is to determine the most appropriate methods for quantifying cycle stability for new redox active materials and to provide guidance on best practices.

References

(1) Su, L.; Kowalski, J. A.; Carroll, K. J.; Brushett, F. R. Recent Developments and Trends in Redox Flow Batteries. In Rechargeable Batteries; Zhang, Z., Zhang, S. S., Eds.; Green Energy and Technology; Springer International Publishing, 2015; pp 673–712.

(2) Kowalski, J. A.; Su, L.; Milshtein, J. D.; Brushett, F. R. Recent Advances in Molecular Engineering of Redox Active Organic Molecules for Nonaqueous Flow Batteries. Curr. Opin. Chem. Eng. 2016, 13, 45–52.

(3) Sevov, C. S.; Brooner, R. E. M.; Chénard, E.; Assary, R. S.; Moore, J. S.; Rodríguez-López, J.; Sanford, M. S. Evolutionary Design of Low Molecular Weight Organic Anolyte Materials for Applications in Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2015, 137(45), 14465–14472.

(4) Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries. Energy Environ. Sci. 2016, 9(11), 3531–3543.

(5) Huang, J.; Su, L.; Kowalski, J. A.; Barton, J. L.; Ferrandon, M.; Burrell, A. K.; Brushett, F. R.; Zhang, L. A Subtractive Approach to Molecular Engineering of Dimethoxybenzene-Based Redox Materials for Non-Aqueous Flow Batteries. J. Mater. Chem. A 2015, 3 (29), 14971–14976.