Measuring Electrochemical Potentials for Health Monitoring of Redox Flow Batteries

Abstract

Long-duration energy-storage (LDES) systems have the potential to transform the electric grid. Redox Flow Batteries (RFBs) have many unique attributes that make this battery architecture ideally suited for grid-scale LDES, namely: independent power and energy; long cycle life, even with deep cycles; inherently superior safety [1]; and easy recyclability. However, despite a recent renaissance in RFB technologies, assessing the durability of RFB cells is a topic that has not received much attention in the open literature, and further research is needed to better understand the performance and degradation mechanisms [2]. In-situ health monitoring of RFBs is critical for maximizing their lifetime and efficiency. Measurements of the electrochemical potentials of the electrodes and the electrolyte can provide insights into whether operating parameters are being adhered to and can potentially detect failure mechanisms occurring real-time within the battery. Since the electrochemical potential of both the positive and negative electrolytes exhibits Nernstian behavior, they are convertible to state-of-charge (SOC) measurements. Ensuring that the electrolyte stays within a desired SOC window during operation is essential for the mitigation of undesirable side reactions, such as the hydrogen-evolution reaction (HER) [3]. In addition, measurements of the potential difference between the electrode and the electrolyte, commonly known as overpotential, during charging and discharging processes allow for independent quantification of the reaction efficiency at each electrode. Factors affecting this reaction efficiency may include reaction kinetics, the mass transport of active species to the electrodes, the plating of impurities onto the electrode surface, and the mobility of ions across the ion-exchange membrane. Analysis of overpotential can also be useful in detecting failure mechanisms relating to these various factors [4]. Largo Clean Energy has developed multiple methods to measure electrochemical potentials in all-vanadium RFBs (VRFBs). The focus of this talk will be on how one can use these different measurements to assess the performance of VRFB cells. References R. Wittman, M. L. Perry, T. N. Lambert, B. R. Chalamala, and Y. Preger, “Perspective – On the Need for Reliability and Safety of Grid-Scale Aqueous Batteries,” J. Electrochemical Society, 167 (2020).Y. Yao, J. Lei; Y. Shi; F. Ai; T. C. Lu, “Assessment methods and performance metrics for redox flow batteries,” Nature Energy, 6 (2021).C. Tai-Chieh Wan, K. E. Rodby, M. L. Perry, Y.-M. Chiang, F. R. Brushett, “Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte,” J. of Power Sources, 554 (2023).M. L. Perry and R. M. Darling, “Development of a Simple and Rapid Diagnostic for Redox Flow-Battery Cells,” Symposium I02: Frontiers of Chemical/Molecular Engineering in Electrochemical Energy Technologies, 242nd ECS Meeting, (2022).