(Invited) Effect of Electrochemical Treatment and pH on the Kinetics of VII-VIII Reactions on Glassy Carbon Electrodes
Authors: Sasikumar S P Varsha , Lynch Robert P. , Al Hajji Safi Maria , Rybalchenko Maria , Buckley D. Noel , Bourke Andrea 
Published: February 2024

Vanadium flow batteries (VFBs) are known for their long cycle life, high efficiency, and scalability, which make them suitable for various energy storage applications, including grid-scale energy storage, renewable energy integration, and remote power systems. There are several areas of active research on VFBs. These include electrolyte chemistry, electrode materials, cell design, system integration, modeling and simulation, performance optimization, scale-up and commercialization1.
Carbon felts are often used as electrodes in VFBs due to their high electrical conductivity, chemical stability, and availability2. However, the surface properties of carbon materials can be influenced by the surrounding environment, leading to changes in their surface chemistry and reactivity. These changes in the environment can alter the kinetics of the vanadium reactions at the carbon surfaces, which in turn can impact the performance and efficiency of the battery system. Various electrochemical, chemical, and thermal treatments have been reported to have a significant impact on the carbon electrode kinetics3. It is thus important to investigate electrode kinetics to improve the performance of VFBs.
Previously our group has reported that electrochemical treatments of carbon electrodes at different potentials significantly affect the electrode kinetics of VIV-VV and VII-VIII redox reactions4,5. The cathodic treatment of carbon electrode enhances the kinetics of VIV-VV redox reaction but inhibits the kinetics of VII-VIII redox reaction, while anodic treatment inhibits the kinetics of the VIV-VV redox reaction but enhances the kinetics of the VII-VIII redox reaction6-9. We have further investigated the effect of electrochemical treatment on the VIV-VV kinetics at glassy carbon electrodes following treatment in electrolytes of various pH. For all pHs investigated (pH 0 – pH 6), VIV-VV kinetics are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. The activating effect of cathodization increased markedly with increasing pH, whereas the deactivating effect of anodization appears approximately the same for all pHs. The observed effects are attributed to changes to electrode surface states by electrochemical polarization reactions, the rates of which appear to be pH dependent10.
We have extended our investigation into the VII-VIII reaction kinetics. We will present results on the effect of electrochemical treatment and pH on the kinetics of VII-VIII reactions on glassy carbon electrodes. The kinetic rates for the VII-VIII couple on these electrodes are measured and compared using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Acknowledgements

Varsha Sasikumar S P would like to thank the Irish Research Council (IRC) for a postgraduate scholarship to perform this research and acknowledge facilitation of this research by University of Limerick (UL), Ireland and South East Technological University (SETU), Ireland.

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