Real-time monitoring of the state of charge (SOC) in vanadium redox-flow batteries using UV–Vis spectroscopy in operando mode

Abstract

Abstract The state of charge (SOC) monitoring method of the vanadium redox flow battery (VRFB) is presented by UV–Vis spectroscopy of the charging-discharging of positive [V(IV)/V(V)] and negative [V(III)/V(II)] electrolytes in operando mode. While a low concentration of the vanadium ion is required for measuring the UV–Vis spectroscopy, the practical VRFB system has a high concentration of vanadium ions in both positive and negative electrolytes. Furthermore, a reference blank solution for the UV–Vis spectrum can complicate the UV–Vis equipment connected with the VRFB system. To address this complication, quartz cuvette windows with notably thin path lengths below 0.1 mm and a concept of difference absorbance (Adiff) are introduced for the SOC monitoring system to be simplified. Through these methods, we experimentally demonstrate that the SOC monitoring based on Adiff can be easily achieved in the anolyte (linear fitting), rather than the catholyte (parabolic fitting), in which the calibrations of Adiff vs. SOC were performed at the UV–Vis peaks of 410 nm in the anode side and 575 nm in the cathode side, respectively. The UV–Vis in-operando mode has the potential ability to monitor the real-time SOC in VRFB, thereby contributing to the simple VRFB-SOC monitoring system.