Redox flow batteries (RFBs) have garnered increasing attention for energy storage applications with non-aqueous RFBs offering several advantages such as larger voltage windows and the prospect of high energy densities. To increase the efficacy of non-aqueous RFBs, much effort has gone into the design of redox moieties that are highly soluble, cheaper, and robust, with desired electrochemical properties. To evaluate the capability of these redox moieties for RFB applications, the kinetics and thermodynamics of electron transfer events need to be understood. These insights could help towards rational design of redox moieties and to predict RFB performance. Techniques such as cyclic voltammetry and rotating disk electrode are commonly used to study the fundamental parameters of electron transfer process and thermodynamic properties of the redox moieties in both the neutral and charged states.

In this work, we investigate the effects of supporting electrolytes on the thermodynamics and kinetics of the redox moieties. Vanadium (III) acetylacetonate and iron complexes are the bipolar model redox moieties used as both negative and positive electrolyte in non-aqueous RFB applications.¹ While many redox moieties are stable in the neutral state, it is during cycling that degradation of the complexes typically occurs. As shown previously, the supporting electrolyte composition plays a key role in the performance and stability of the redox species in RFBs.² We found that ion pairing between the charged redox moieties and the supporting electrolyte resulted in lower diffusion coefficients, it also has a negative impact on the kinetics of charge transfer process. We propose that the observed ion pairing can explain the stabilizing effects of supporting electrolytes on redox moieties. The magnitude of this effect can be explained by hard soft acid base theory (HSAB) which can be further used to predict the extent of ionic interactions of the charged species in the RFB electrolytes.

Acknowledgements:

Los Alamos National Laboratory is operated by Triad, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (Contract No. 89233218NCA000001). Authors would like to thank Dr. Imre Gyuk and financial support from the U.S. Department of Energy’s Office of Electricity for the publication of this work is gratefully acknowledged.

1. Sharma, G.A. Andrade, S. Maurya, I.A. Popov, E.R. Batista, B.L. Davis, R. Mukundan, N.C. Smythe, A.M. Tondreau, P. Yang, J.C. Gordon, Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications, Energy Storage Mater., 37 (2021), pp. 576-586
2. Wei X, Xu W, Huang J, Zhang L, Walter E, Lawrence C, Vijayakumar M, Henderson WA, Liu T, Cosimbescu L, Li B, Sprenkle V, Wang W. “Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All-Organic Redox Flow Battery” Angew Chem Int Ed Engl. 2015, 54 (30): 8684-7.