Electrode kinetics in all-vanadium flow batteries: effects of electrochemical treatments

Electrochemical reactions in all-vanadium flow batteries (VFBs) were investigated by examining the effects of electrochemical treatments of carbon on the electrode kinetics of the oxidation-reduction reactions of both VIV/VV and VII/VIII. A range of carbon materials were investigated, including glassy carbon, graphite, reticulated vitreous carbon (RVC), carbon paper, carbon xerogel and carbon fibres.

The electrode kinetics of VIV/VV are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. In contrast, the electrode kinetics of V II/VIII are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment.

The effects of treatment potential and treatment time on both VIV/VV and VII/VIII kinetics were investigated in detail for both anodic and cathodic treatment. Pronounced activation for VIV/VV occurs in a range from about +0.1 V to –0.3 V (all potentials referenced to a Hg/Hg2SO4 electrode) and the effect begins to saturate at about –0.6 V. Pronounced deactivation occurs at potentials more positive than about +0.7 V. At typical potentials, effects are significant in time-scales of ~60 s. Pronounced activation for VII/VIII occurs in the potential range from about –0.4 V to +0.5 V and the effect begins to saturate at about +1.0 V. Pronounced deactivation occurs at potentials more negative than about –0.9 V. Again, at typical potentials, effects are significant in time-scales of ~60 s.

The observed activation and deactivation effects occur regardless of whether vanadium is present in the electrolyte and are attributed to oxygen-containing functional-groups on the electrode surface. A model was developed based on the rates of oxidation and reduction of active sites on the carbon surface as a function of potential. A good fit of the model to the experimental data was obtained.

X-ray photoelectron spectroscopy (XPS) of treated electrodes provides supporting evidence for the role of oxygen functional groups. Contact angle measurements show an increase in wettability of carbon felt after treatments at potentials where either anodic deactivation of VIV/VV or cathodic deactivation of VII /VIII is observed. This suggests that the functional groups involved in deactivation of electrode surfaces also cause increased wettability.

Based on our results, we propose three types of surface site on carbon electrodes: oxidised, intermediate, and reduced. Electrodes that predominantly have intermediate-type surface sites are active for both VIV/VV and VII /VIII while electrodes that predominantly have either oxidised or reduced surface sites are deactivated. Thus anodically treated electrodes are deactivated for VIV/VV while cathodically treated electrodes are deactivated for VII/VIII. Neither cathodic deactivation of VIV/VV or anodic deactivation of VII/VIII is observed because the effect is masked in each case by oxidation or reduction of the surface in the respective electrolyte.

For all five carbon materials the kinetic rates of VIV/VV are greater than those of V II/VIII . However, for any particular material, the rate can vary significantly depending on electrode history and pretreatment, which may help to explain apparent anomalies in the literature.

These results are important to the operation of a VFB because, for both the positive and the negative electrode, the potentials during charging are close to where deactivation can occur. Overcharging of the battery could therefore lead to deactivation of both electrodes. The potentials during discharging are close to where activation can occur, however, for both electrodes. Overdischarging of the battery could therefore lead to activation of both electrodes. Based on this, a novel and simple procedure for improvement of the performance of VFBs is presented.