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Surface treatments are often applied to carbon materials to impart specific functions to the surface. Surface oxidation is a typical treatment to form oxygen-containing surface functional groups on carbon fiber electrodes of redox flow batteries in order to enhance the performance, which has attracted much attention as a large-scale electric energy storage system. At present, however, little attention has been paid to the effect of the edge plane exposure. In this study, fine etching of the graphitized carbon fiber surface was attained by coating the surface with a metal-containing carbonaceous thin film and thermal oxidation. The etching was caused by the catalysis of the metal species; the mechanism and the effect of the carbonaceous film were demonstrated by in situ X-ray absorption fine structure measurements. The finely etched surface possessed substantially enriched edge planes and an enhanced activity for the positive and negative electrode reactions of the vanadium redox flow battery. The flow cell test with the carbon fiber electrodes after the tuned etching showed a significant decrease in the overpotential and increase in the efficiency as well as stable cycling performance.

The reduction of dioxovanadium(5+) ion (VO 2 + ) was fundamentally investigated using glassy carbon rotating disk electrodes (GC RDEs) as a model of the carbon electrodes of vanadium redox flow batteries (VRFBs). The enrichment of oxygen-containing functional groups on the GC surface was performed by the electrochemical oxidation and checked by X-ray photoelectron spectroscopy (XPS). The XPS results also suggested the coordination of the functional groups to the oxo/dioxovanadium(4+/5+) ions (VO 2+ /VO 2 + ) after immersion into the VO 2+ /VO 2 + -dissolved solution. The enrichment enhanced the VO 2 + reduction and changed the Tafel slope from −0.161 V decade −1 to −0.087 V decade −1 , indicating the reduction mechanism alteration. The Tafel slope was explained by a multi-step reaction mechanism, in which the reduction of the quinone-like functional group generated the hydroxylic functional group that served as the reaction site and coordinated to the oxo/dioxovanadium(4+/5+) species.Electrical energy storage (EES) has been recognized as one of the valuable approaches for leveling intermittent power generation by wind turbines and photovoltaic cells, which would enable the realization of clean and highly efficient energy systems. Redox flow batteries (RFBs) have attracted much interest as a promising EES system due to their suitability for large-scale energy storage and capability to withstand fluctuating power supplies. 1,2Among the various types of RFBs, the VRFBs have various advantages, such as a relatively higher safety and lower risk of crosscontamination by crossover between the electrode compartments upon long-term cycling, and are considered to be in the most advanced stage of research and development. 3-5 Carbon materials are used in the VRFB electrodes and the discharge reactions at the positive and negative electrodes are as follows:The slow kinetics of the VO 2 + reduction is a serious problem in VRFB technology, which has to be overcome to improve the efficiency of the VRFB. 6 Gattrell et al. proposed a mechanism for the positive electrode reactions by considering multi-step chemical and electrochemical reactions of the dissolved vanadium species. 6 In contrast, the potential involvement of the carbon electrode surface in the reaction mechanism, the functional groups on the surface, in particular, has been pointed out and the reaction model shown in Fig. 1 is proposed. 7-9

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