In Situ UV-Vis Spectroscopic Monitoring of Real-Time Negolyte State-of-Charge in Iron-Chromium Flow Batteries

The growing integration of renewable energy necessitates advanced large-scale energy storage solutions. Iron-chromium flow batteries (ICFBs) are a promising candidate due to their safety and cost-effectiveness, yet the lack of reliable methods for monitoring the electrolyte state of charge (SOC) has impeded performance optimization. To address this, we developed a novel in situ UV–vis spectroscopic method for monitoring the SOC of the negative electrolyte. The molar absorptivity of Cr 2+ at 800 nm is determined and verified to exhibit a strong linear relationship with its concentration (R 2 = 0.9999). To ensure adaptability to varying electrolyte compositions, a correction model for chloride ion concentration is established, enabling accurate quantification of Cr 2+ concentration and SOC determination. Furthermore, a dual-wavelength detection method (407 and 800 nm) is designed to simultaneously monitor [Cr(H 2 O) 6 ] 3+ and Cr 2+ , allowing for precise measurement of total chromium concentration with a deviation of less than 2.5% compared to ICP-OES results. This work provides a robust solution for ICFB management, enabling real-time SOC tracking, adaptability to varying electrolytes, and accurate determination of total Cr concentration at high SOC.