(Invited) Effect of Electrochemical Treatment and pH on V^IV/V^V Electrode Kinetics

Flow batteries are a promising technology for flexible large-scale energy storage systems to tackle the problem of intermittency with renewable energy sources. Several flow battery systems have been successfully developed and deployed to date, including all-vanadium, zinc-iron, zinc-bromine, and all-iron flow batteries.¹ Among these, vanadium flow batteries (VFBs) have received a lot of attention in the large-scale energy storage sector due to their excellent characteristics such as long cycle life with almost zero electrode and electrolyte degradation, safe and non-flammable aqueous electrolytes, short response times, and the ability to operate at reasonable temperatures.² The operation of flow batteries involves simultaneous oxidation and reduction reactions at the anodic and cathodic compartments. In the VFB, the redox couple at the anode is V^II-V^III and at the cathode is V^IV-V^V.

Voltage inefficiencies at the electrode-electrolyte interface are a source of concern for the operation of these systems.³ Electrode polarization, slow kinetics, and iR losses associated with contact and ionic resistances all contribute to battery voltage inefficiencies. It is widely acknowledged that the electrode material and pretreatments (chemical, thermal, and electrochemical) have significant impacts on the electrode kinetics and hence the performance of the VFB system.⁴ Investigation of the electrode kinetics is thus important in order to better understand and improve the performance of VFBs.

Previous research from our group has shown that electrochemical treatments of carbon electrodes at different potentials significantly affect the electrode kinetics of V^IV-V^V and V^II-V^III oxidation-reduction reactions. The electrode kinetics of V^IV-V^V are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. In contrast, the electrode kinetics of V^II-V^III are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Furthermore, the carbon electrode can be toggled repeatedly and reproducibly between the enhanced (activated) and the inhibited (deactivated) state.^5,6,7 Other factors affecting the electrode activity include ageing of the electrode, the final treatment potential, the redox rest potential, and the treatment potential window.⁸ This previous research involves treatments in strong acid, low pH electrolytes.

In this study, we investigate the electrochemical treatment of glassy carbon electrodes in electrolytes at a range of pH. The resulting electrode kinetics of the vanadium redox couples at each treatment potential are studied using electrochemical characterization techniques including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Acknowledgements

Varsha Sasikumar S P would like to thank the Irish Research Council (IRC) for providing funding for this research work.

References

1. Xinyou Ke et al., Chemical Society Reviews, 47, 8721-8743 (2018).

2. R. Chalamala et al., Proceedings of the IEEE, 102(6), 976-999, (2014).

3. A. H. Whitehead and M. Harrer, Journal of Power Sources, 230, 271–276 (2013).

4. Ki Jae Kim, et al., Journal of Materials Chemistry A, 33(3), 16913-16933 (2015).

5. A. Bourke et al., Journal of Electrochemical Society, 163, A5097 (2016).

6. A. Bourke et al., Journal of Electrochemical Society, 162, A1547 (2015).

7. M. A. Miller, et al., Journal of Electrochemical Society, 163, A2095-A2102, (2016).

8. D. N. Buckley, et al., ECS Transactions, 98(9), 223-239 (2020).