Kinetic investigation of vanadium (V)/(IV) redox couple on electrochemically oxidized graphite electrodes

• The VO2+/VO2+ redox reaction of the electrode could be facilitated to some extent with the increasing anodic corrosion.
• A real reaction kinetic equation for the oxidation of VO2+ on the electrochemically oxidized electrode has been firstly obtained.
• The establishment of the kinetic equation is conducive to predict polarization behaviors of the electrodes in engineering application.

The morphology, surface composition, wettability and the kinetic parameters of the electrochemically oxidized graphite electrodes obtained under different anodic polarization conditions have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, steady-state polarization and cyclic voltammetry (CV) tests, with an attempt to investigate the inherent correlation between the physicochemical properties and the kinetic characteristics for carbon electrodes used in an all-vanadium redox flow battery (VRFB). When the anodic polarization potential raises up to 1.8 V vs. SCE, the anodic corrosion of the graphite might happen and a large number of oxygen-containing functional groups generate. The VO2+/VO2+ redox reaction can be facilitated and the reaction reversibility tends to become better with the increasing anodic potential, possibly owing to the increased surface oxides and the resulting improved wettability of the electrode. Based on this, a real reaction kinetic equation for the oxidation of VO2+ has been obtained on the electrode polarized at 1.8 V vs. SCE and it can be also well used to predict the polarization behavior of the oxidized electrode in vanadium (IV) acidic solutions.

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