Efficient Edge Plane Exposure on Graphitic Carbon Fiber for Enhanced Flow-Battery Reactions

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.