Understanding the Activation and Concentration Overpotential of Negative and Positive Electrodes in Vanadium Redox Flow Batteries By Using Symmetric Cell Architecture
Authors: Sun Pengfei , Suzuki Takahiro , Tsushima Shohji 
Published: December 2024

Renewable energy, represented by solar and wind energy, has made considerable progress. The vanadium redox flow battery (VRFB) has emerged as the premier option for large-scale energy storage to mitigate the fluctuations inherent in renewable energy sources, thus attracting growing interest [1]. However, the development of VRFB is hindered by their low energy density and current density [2]. Enhancing the electrochemical performance of VRFB is crucial for achieving high energy efficiency [3].
In VRFB, the vanadium ions at the negative electrode (NE) are V2+ and V3+, while the vanadium ions at the positive electrode (PE) are VO2+ and VO2
+. In order to improve the electrochemical performance of VRFB, it is necessary to conduct in-depth analysis of the reaction rates of V2+/V3+ and VO2+/VO2
+. In this study, to clearly illustrate the respective reaction rates of different vanadium ions in NE and PE, symmetrical cells were used. Then, four different electrode combinations were used for symmetrical cell experiments (both electrodes were heated, NE was heated/ PE was not heated, PE was heated/NE was not heated, and both electrodes were not heated) to get more information of separated positive and negative reactions. Lastly, to further analyze the effect of vanadium ion concentration on the reaction rate, different state of charges (SOC) were also used, namely 20% SOC and 80% SOC.
Fig. 1 showed the IV curves of symmetric cells under different electrode combination conditions. It showed that the overpotentials of negative electrolyte were always higher than those of positive electrolyte under the same electrode conditions, indicating that both the activation and concentration overpotential of the V2+/V3+ reaction were higher than those of the VO2+/VO2
+ reaction.
Fig. 1 (a) showed that at 20% SOC, the activation overpotential of the V3+ to V2
+ reaction was higher than that of the V2+ to V3+ reaction at low current density under the NE not heated condition and PE not heated condition. However, the concentration overpotential of the V2+ to V3+ reaction was higher than that of the V3+ to V2+ reaction at high current density. At 80% SOC, both the activation and concentration overpotentials of the V3+ to V2
+ reaction were higher. This was due to the decrease in V3+ concentration as SOC increased from 20% SOC to 80% SOC, resulting in an increase in the concentration overpotential for the V3+ to V2+ reaction.
Fig.1 (b) showed that both the activation and concentration overpotentials of VO2
+ to VO2+ reaction were higher than those of VO2+ to VO2
+ reaction under both not heated conditions. However, the overpotential of the NE not heated condition at 20% SOC was higher than that of the PE not heated condition; but at 80% SOC, the results were opposite. This varaition could be attributed to the change in the concentration of VO2+ and VO2
+ions as SOC increased from 20% to 80%. Specifically, the decease in VO2+ concentration and the increase in VO2
+ concentration significantly affected the activation and concentration overpotentials. That is, when the electrode was not heated, for the positive electrolyte, lower ion concentrations led to higher overpotentials.
In summary, the experiment results showed that the activation overpotential was ordered as V3+>V2+>VO2
+>VO2+ reaction, and the concentration overpotential as V2+>V3+>VO2
+>VO2+ reaction.
Acknowledgement
This work was supported by JSPS KAKENHI Grant Number 21H04540. The first author was supported by the China Scholarship Council.
References
[1] KJ. Kim, MS. Park, YJ. Kim, JH. Kim, SX. Dou, and M. Skyllas-Kazacos, J. Mater. Chem. A, 3 (33), 16913-16933 (2015).
[2] A. Bourke, D. Oboroceanu, N. Quill, C. Lenihan, M.A. Safi, M.A. Miller, R.F. Savinell, J.S. Wainright, V. SasikumarSP, M. Rybalchenko, P. Amini, N. Dalton, R.P. Lynch and D.N. Buckley, J. Electrochem. Soc, 170, 030504 (2023).
[3] C. Choi, H. Noh, S. Kim, R. Kim, J. Lee, J. Heo and H. Kim, J. Energy Storage, 21, 321-327 (2019).

Figure 1