Compositionally Unbalanced Symmetric Cell Cycling As a Method for Quantifying Crossover Rates in Redox Flow Cells

Abstract

Redox flow batteries (RFBs) are a promising grid-scale energy storage platform, but broad deployment is stymied by technical and economic constraints.1 Within this context, the membrane serves a central role in determining battery performance, selectively facilitating the conduction of supporting salts to mitigate resistive losses while preventing the undesired crossover of active species.2 Tradeoffs between these two facets—namely, conductivity and permeability—requires a thorough understanding of how transport proceeds under the dynamic conditions observed in practical embodiments (e.g., varying compositions, alternating polarity). To this end, a wide range of experimental tools exists for characterizing membrane properties under controlled environments, but few methods directly assess crossover and resistive losses in conventional redox flow cells.3 In this presentation, we introduce a facile method for quantifying crossover rates in flow cells—compositionally unbalanced symmetric cell cycling (CUSCC). Using a previously developed zero-dimensional modeling framework,4 we first examine fundamental processes that give rise to a characteristic “capacity gain” profile. We then validate the technique experimentally using FeCl2 / FeCl3 and Nafion as a representative system, confirming theoretical predictions and establishing the efficacy of CUSCC for characterizing crossover. Finally, we apply the model to estimate transport parameters from experimental capacity gain profiles, yielding results that are consistent with conventional ex situ measurements. Overall, CUSCC is a robust method for quantifying crossover rates with standard flow cell hardware, potentially expanding the experimental toolkit for RFB development. Acknowledgments This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, contract number DE-AC02-06CH11357. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 2141064. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. References M. L. Perry and A. Z. Weber, J. Electrochem. Soc., 163, A5064–A5067 (2016).J. Yuan, Z.-Z. Pan, Y. Jin, Q. Qiu, C. Zhang, Y. Zhao, and Y. Li, Journal of Power Sources, 500, 229983 (2021).Y. A. Gandomi, D. S. Aaron, J. R. Houser, M. C. Daugherty, J. T. Clement, A. M. Pezeshki, T. Y. Ertugrul, D. P. Moseley, and M. M. Mench, J. Electrochem. Soc., 165, A970 (2018).B. J. Neyhouse, J. Lee, and F. R. Brushett, J. Electrochem. Soc., 169, 090503 (2022).