In-Situ Electrode Potential Measurements in Vanadium Redox Flow Batteries
Authors: Yang Zhiwei , Perry Mike L. 
Published: February 2020

Vanadium redox flow battery (VRFB) cells typically consist of a proton-conducting membrane sandwiched by two porous carbon-fiber based electrodes. The V2+/V3+ and V4+/V5+ redox couples, dissolved in acidic electrolytes, flow through the negative and positive electrodes, respectively. And, the primary reactions during charge and discharge are the redox reactions of these two vanadium couples. The round-trip efficiency (RTE) of a VRFB cell is a complex combination of multiple factors, including the electrodes, membrane, electrolytes, and cell-design configuration, as well as the operating conditions, i.e. charge/discharge current, range of the electrolyte state-of-charge (SOC) and electrolyte flow rates.1,2 The ohmic losses in the membrane and overpotentials on the two electrodes are two major factors that contribute to the cell’s efficiency losses. Undesired side reactions, including hydrogen evolution on the negative electrode and carbon corrosion on the positive electrode, may also occur, especially when the cell is overcharged, which may lead to a change in the electrolyte average vanadium oxidation state and degradation of the carbon electrodes and bipolar plates.3-5 Since both the vanadium redox reactions and these side reactions are strongly potential driven, it is useful to have a reliable reference electrode integrated into the VRFB system so that the electrode potentials vs. reversible hydrogen electrode (RHE) can be measured under in-situ conditions. The use of reference electrodes, such as the dynamic hydrogen electrode, have been reported in VRFB cells to measure in-situ electrode potentials.6,7 However, a significant challenge is the vanadium ion contamination that shifts the RHE potential higher, up to ~ 0.3V by 1.6M vanadium electrolyte.
Here we describe a specially designed reference-electrode apparatus (REA), which is depicted in Fig.1 and consists of a small VRFB cell, a RHE, and a continuously acid-washed region between the VRFB cell and the RHE to prevent vanadium contamination of the RHE. This unique REA enables a reliable RHE for a VRFB cell operating under various operating conditions. For example, the REA can be used to measure both the overpotentials and electrochemical potentials (vs. RHE) of the working electrodes in a VRFB cell during a complete charge and discharge cycle, as shown in Fig. 2. The REA also provides real-time measurement of the electrolyte OCV, which can be used to determine the SOC of the posolyte and negolyte individually. The REA can also be used to provide insights into VRFB-electrolyte rebalancing options. Therefore, the REA that will be introduced here can be a useful tool to guide the selection of both electrode materials and the optimization of VRFB operating protocols, and examples of both will be presented.

Acknowledgements

This work was made possible by financial supported from Vionx Energy.

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Figure 1