All-Vanadium Flow Batteries (VFBs) are an attractive technology for energy storage, especially in conjunction with a renewable energy source such as wind or solar¹. The cells typically have carbon felt electrodes for both half-cells and are separated by an ion-exchange membrane. The catholyte and the anolyte circulate through the electrodes from reservoirs. Electrode performance can be enhanced by thermal, chemical, or electrochemical treatment. Oxygen surface species are often introduced on the surface after such treatment. The effects of those oxygen surface species on electrode performance are known but not yet well understood, with a considerable variation in the reported activity of carbon electrodes toward both redox couples, V^IV-V^V and V^II-V^III. Many researchers^2-4 have concluded that redox reaction has slower kinetics, but some^4-6 have concluded that redox reaction has slower kinetics.

We have previously reported^7-9 that cathodic treatment enhances the kinetics of the positive electrode (V^IV-V^V) but inhibits the kinetics of the negative electrode (V^II-V^III), while anodic treatment inhibits the kinetics of the positive electrode but enhances the kinetics of the negative electrode. We observed this for all carbon materials investigated including carbon fibres, glassy carbon, reticulated vitreous carbon (RVC) and carbon paper.

The lack of agreement in the literature on which electrode has faster kinetics is not surprising because of the sensitivity of carbon materials to changes in their environment.¹⁰ Such discrepancies can result from using different treatment methods leading to different surface histories. Therefore, it is not straightforward to compare reported results because of the variety of different surface treatments used, and it is only reasonable to compare the kinetics for electrodes that have been treated in the same manner.

In this paper, we further investigate the influence of the treatment potentials on the optimised electrode kinetics and show how electrode history can change which redox couple has the fastest kinetics.

References:

[1] D.N. Buckley, C. O’Dwyer, N. Quill, R.P. Lynch, Issues in Environmental Science and Technology, 2019-January (46), 115–149 (2019).

[2] E. Sum, M. Rychcik, and M. Skyllas-Kazacos, Journal of Power Sources, 16, 85–95 (1985).

[3] E. Sum and M. Skyllas-Kazacos, Journal of Power Sources, 15, 179–190 (1985).

[4] T. Yamamura, N. Watanabe, T. Yano, and Y. Shiokawa, Journal of The Electrochemical Society, 152, A830 (2005).

[5] X. W. Wu, T. Yamamura, S. Ohta, Q. X. Zhang, F. C. Lv, C. M. Liu, K. Shirasaki, I. Satoh, T. Shikama, D. Lu, and S. Q. Liu, Journal of Applied Electrochemistry, 41, 1183– 1190 (2011).

[6] M. Gattrell, J. Park, B. MacDougall, J. Apte, S. McCarthy, and C. W. Wu, Journal of The Electrochemical Society, 151, A123 (2004).

[7] A. Bourke, M. A. Miller, R. P. Lynch, X. Gao, J. Landon, J. S. Wainright, R. F. Savinell, and D. N. Buckley, J. Electrochem. Soc., 163, A5097 (2016)

[8] A. Bourke, M. A. Miller, R. P. Lynch, J. S. Wainright, R. F. Savinell, and D. N. Buckley, J. Electrochem. Soc. 162, A1547 (2015)

[9] M. A. Miller, A. Bourke, N. Quill, J. S. Wainright, R. P. Lynch, D. N. Buckley, and R. F. Savinell, J. Electrochem. Soc. 163 A2095 (2016)

[10] P. Chen, M. A. Fryling, and R. L. McCreery, Analytical Chemistry, 67, 3115–3122 (1995).

Fig.1: The activity of V^IV-V^V (graph on the right) and V^II-V^III (graph on the left) electrodes plotted as a function of the anodic (circular markers) and cathodic (square markers) treatment. The oxidation potential is 1.2 V, while the reduction potential limit in V^II/V^III is – 1.2 V (the orange marker) and –1.5 V (the grey marker) and in V^IV /V^V is – 0.7 V (the pink marker), – 0.9 V (the green marker), – 1.2 V (the orange marker) and –1.5 V (the grey marker). It is clearly shown that the optimised kinetics of V^II-V^III and V^IV-V^V are less favourable with more cathodic treatment potentials.