The Effect of the Upper and Lower Electrochemical Treatment Potentials on the Electrode Activity Towards Vanadium Redox Couples

Abstract

Vanadium flow batteries (VFBs) are a promising energy storage technology. They have long cycle lives and are essentially immune to cross contamination through their ion-exchange membranes1. Battery energy efficiency depends on voltage and coulombic efficiency. Sluggish electrode kinetics reduces voltage efficiency by increasing overpotential at the electrodes and thereby can result in an increase in the fraction of current associated with unwanted side reactions at the electrodes, reducing columbic efficiency. It follows that study of the VFB electrode kinetics is very important.The kinetics of the redox couples at the positive and negative electrodes have been reported in flow batteries and three-electrode cell experiments for a range of carbon materials. The reports show that the kinetics of these reactions are not fully understood and discrepancies exist regarding which half-cell redox couple has slower kinetics. Many researchers2-4 have concluded that the redox couple has slower kinetics but some4-6 have concluded that the redox couple has slower kinetics.Carbon electrodes are very sensitive to changes in their environment which can subsequently alter the kinetics of redox couples.10 Thus, comparing the reported kinetics in the literature is complicated. We have previously reported7-9for a range of carbon materials – including carbon fibres, glassy carbon, reticulated vitreous carbon (RVC) and carbon paper – that the kinetics of are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. In contrast, the electrode kinetics of are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Therefore, it is more reasonable when comparing the electrode kinetics towards different redox couples to compare electrodes that have been treated in the same manner.We showed previously7-9 that the optimised kinetics at the positive electrode is faster than at the negative electrode. In this presentation we further investigate the effect of the electrochemical treatment on the electrode kinetics by controlling the upper and lower treatment potentials experienced by the electrode. We will present the resulting dependency of electrode kinetics on the range of the electrochemical treatment window experienced by the electrode and will show, as can be seen in Fig. 1, that determining which redox couple has the fastest kinetics at an electrode is highly dependent on the range of potentials experienced by the electrode. Fig.1: The activity of VIV-VV (circular markers) and VII-VIII (triangular markers) electrodes plotted as a function of the anodic (red markers) and cathodic (blue markers) treatment potentials for two different potential windows/ranges of maximum upper and minimum lower potential. It is clearly shown that the optimised kinetics of VII-VIII exceed those of VIV-VV for a window of upper and lower potentials of –1.2 V and +1.2 V, respectively (graph on left), while the optimised kinetics of VIV-VV exceed those of VII-VIII for a window of upper and lower potentials of –1.2 V and +1.5 V, respectively (graph on right).