Electrode Kinetics in All-Vanadium Flow Batteries: Effects of Electrochemical Treatment

Abstract

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 carbon fibers, glassy carbon, reticulated vitreous carbon (RVC), carbon paper, and graphite using cyclic voltammetry and electrochemical impedance spectroscopy. In all cases 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 VII/VIII are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. 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. The surface of carbon felt samples after treatment was analyzed using x-ray photoelectron spectroscopy (XPS), and contact angle measurements. The results highlight the role of oxygen containing functional groups on the activation and deactivation process of the electrode.