Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of V II -V III and V IV -V V in H 2 SO 4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers. It was shown that, for all carbon materials investigated, the kinetics of V II -V III is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of V IV -V V is inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for V IV -V V than for V II -V III except when anodized electrodes were compared, which may explain discrepancies in the literature. The observed effects are attributed to oxygen-containing functional-groups on the electrode surface. The considerable differences between the potentials at which enhancement of V II -V III and inhibition of V IV -V V occur indicates that they do not correspond to a common oxidized state of the electrode. Likewise inhibition of V II -V III and enhancement of V IV -V V do not correspond to a common reduced state of the electrode. It is possible that enhancement of both V II -V III and V IV -V V is due to the same (active) state of the electrode. There is considerable interest in vanadium flow batteries (VFBs), also known as vanadium redox flow batteries (VRFBs or VRBs), for storage of electrical energy particularly in conjunction with renewable energy sources such as wind and solar. [1][2][3][4][5][6] Active areas of research include cell design and modelling, [7][8][9] performance and state-of-charge monitoring, 10-16 coulombic and energy efficiencies, 5,17,18 electrolytes, [11][12][13][14][15][16]19,20 membranes, 4,21 and electrodes. Cells typically have porous carbon electrodes and electrode performance can depend strongly on electrode treatment. Various electrochemical, 22-27,36-41 chemical, 36,40,43,44 and thermal [45][46][47][48][49] treatments have been reported. These treatments often have the effect of oxidizing or reducing the surface, and the influence of surface oxygen species on electrochemical kinetics at carbon electrodes is recognized, 22,57-60 although often not well understood. Thermal 45-49 and chemical 36,40,43,44 treatments of electrodes for VFBs have been tested on a range of carbon-based electrodes and, in general, these treatments result in higher activities of the electrode toward the vanadium redox reactions. There are also a number of reports of the effect of electrochemical treatment of electrodes. Anodic treatment of carbon felt was reported 22,36 to cause a decrease in the kinetic rates of the V IV -V V redox couple. In contrast, there are also reports of enhancement of V IV -V V kinetics after electrochemical oxidation [38][39][40][41] (of graphite and carbon felt electrodes) and of V II -V III kinetics after potential cycling 61 (of highly-oriented-pyrolytic-graphite and glassy carbon electrodes). However, in considering the effects of anodization on a c...

Novel carbon fiber microelectrode (CFME) and flow cell experiments were used to investigate electrode treatments for vanadium flow batteries (VFBs). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) on CFMEs showed that electrode treatments at positive potentials enhance the kinetics of V 2+ /V 3+ and inhibit the kinetics of VO 2+ /VO 2 + , while electrode treatments at negative potentials inhibit the kinetics of V 2+ /V 3+ and enhance the kinetics of VO 2+ /VO 2 + . XPS analysis showed changes in oxygen-containing species on electrode surfaces after treatment, supporting the suggestion that such species are responsible for the observed effects. The kinetics of VO 2+ /VO 2 + are significantly faster than that of V 2+ /V 3+ . Based on the CFME results, the range of potential experienced by a negative electrode in a flow cell during operation corresponds to a region where it is being deactivated by reduction, and the redox potential of the positive half-cell falls in a region where the electrode is activated for the V 2+ /V 3+ reaction. This is supported by flow cell experiments which showed that the overpotential at the negative electrode increases with charge-discharge cycling but decreases significantly when the positive and negative electrolytes are interchanged. The all-vanadium flow battery (VFB) has received attention as a load leveling technology for large-scale energy storage.1-4 This technology is capable of interfacing with renewable energy sources and provides an alternative solution to balancing power consumption and generation. Despite the advantages, VFBs have not yet been widely commercialized. Significant improvements are needed to enhance flow battery systems. Limitations include ion transport through the membrane, mass transport resistances within the electrodes, and electrode reaction kinetics. 5 Recently, attention has been directed toward improvement of electrochemical properties of carbon based electrode materials. Modifications via thermal treatments, chemical oxidation, or electrochemical oxidation are thought to enhance electrochemical activity.6-12 The presence of oxygen containing functional groups has been shown to directly affect the kinetics; surface oxides resulting from the aforementioned treatments are thought to act as active sites, catalyzing the vanadium reactions.13 Some researchers have reported an increase [6][7][8][9][10][11][12] while others reported a decrease 14 in activity upon functionalization of carbon electrodes. Many of these studies have been conducted using glassy carbon, 15,16 graphite, 17 carbon paper, 18 multi-walled carbon nanotubes, 19 or carbon composites. 20 One group concluded that the kinetics for the VO 2+ /VO 2 + reaction are faster than the V 2+ /V 3+ reaction at a plastic formed carbon electrode, but found the opposite when using pyrolytic graphite. 21 Previously, we reported that electrochemical oxidation treatments enhanced the V 2+ /V 3+ reaction kinetics, whereas electrochemical reduction treatments enhanced the VO 2...