Monitoring the State of Charge Imbalance of Vanadium Redox Flow Batteries Via Dual Online UV/Visible Spectroscopy

Abstract

Electrolyte imbalance in Redox Flow Batteries (RFBs) is known to result in significant capacity decay over cycling [1]. This drawback is inherent to the flow battery technology and is caused by unwanted physico-chemical phenomena, such as species and water crossover, hydrogen evolution, air oxidation and precipitation of vanadium ions. The time evolution of electrolyte imbalance is often disregarded or underestimated when measuring the open-circuit voltage (OCV) for keeping track of the battery State of Charge (SOC). In order to correct such asymmetry, in operando real-time monitoring of the SOC of the positive and negative electrolytes would be highly desirable.In this work, we present an experimental test bench for the continuous online monitoring of the anolyte and catholyte SOCs via UV/visible spectroscopy during cycling of an all-vanadium redox flow battery (VRFB). The system notably includes 3D printed microfluidic optical flow cells, an optical switch, and an in-house microfluidic electrochemical cell. Key steps for the calibration of UV/visible spectroscopy with vanadium are presented and applied independently to both electrolytes to accurately estimate the SOC while taking into account the total vanadium concentration [2]. An experimental campaign is then carried out under varying conditions (initial average oxidation state (AOS), air oxidation, crossover, etc.) to illustrate the real-time evolution of electrolyte imbalance during VRFB battery cycling. The results suggest that the imbalance may arise from asymmetric initial Average Oxidation State (AOS ≠ 3.5) [3], vanadium crossover and also hydrogen evolution, which notably reduces the charging speed of the negative electrolyte [4]. The results highlight the benefits of UV/Visible spectroscopy to obtain SOC data for any of the three stable vanadium electrolytes: i) negative, VII/VIII ii) positive VIV/VV and even the iii) VIII/VIV pair. Moreover, the measurements are online, non-invasive and could provide real-time data to a battery management system either in conventional or microfluidic VRFBs to correct the SOC imbalance and maintain the battery capacity and efficiency [1]. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Project PID2019-106740RB-I00, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid. A. A. Maurice Energy acknowledges the support of an MCSF-Cofund “Energy for Future” (E4F) postdoctoral research fellowship by the Spanish Iberdrola Foundation (GA-101034297).