Effect of Polarization Pretreatment of Glassy Carbon on the Kinetics of the Redox Reactions in Vanadium Redox Flow Batteries
In a VRFB the electroactive species are dissolved in the electrolyte. The electrode reactions may be written as:
V3+ + e- ↔ V2+
VO2+ + H2O ↔ VO2+ + 2H+ + e-
The negative half-cell reaction appears to be a simple one-electron transfer reaction while the positive half-cell reaction appears to involve bond breaking. Thus the kinetics would be expected to be faster at the negative electrode than at the positive. However, in actual flow cells, the overpotential is much larger at the negative electrode than at the positive.3
The detailed kinetics of the redox processes occurring at the positive and negative electrode in a VRFB are not well established in the literature and there is some disagreement between results obtained on various substrates at fixed and rotating disk electrodes.4-11
As part of an investigation of the kinetics of electrode processes in VRFBs, we have examined the kinetics of the VO2+/VO2+ and V2+/V3+ couples on glassy carbon electrodes. We have found that polarization pretreatment of glassy carbon has an effect on the activity of the surface for oxidation and reduction of the VO2+/VO2+ and V2+/V3+ couples. When glassy carbon electrodes are pretreated by polarization at potentials more positive than 0.5 V (sat. Hg/Hg2SO4) they are subsequently less active for oxidation of VO2+ and reduction of VO2+. The activity of the electrode is recovered by polarization at negative potentials.11 However, when glassy carbon electrodes are pretreated by polarization at potentials more positive than 0.5 V (sat. Hg/Hg2SO4) they are subsequently more active for oxidation of V2+ and reduction of V3+. The electrode can be deactivated by polarization pretreatment at negative potentials.
The results of this investigation will be presented and discussed.
A. Bourke wishes to thank the Irish Research Council (IRC) for a postgraduate scholarship to perform this research.
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