Effect of Electro-Oxidation Current Density on Performance of Graphite Felt Electrode for Vanadium Redox Flow Battery

Electro-oxidation method is used to improve the electrolyte accessibility and reaction activity of the graphite felt (GF) electrode for vanadium redox flow battery (VRB). The effect of electro-oxidation current density on activity of oxidized graphite felt (OGF) is investigated and the optimized parameter is suggested. Scanning electron microscope result indicates that the surface of GF is etched and becomes rougher by electro-oxidation treatment. Cyclic voltammetry and electrochemical impedance spectroscopy results show that the OGF obtained at 50 mA/cm2 (OGF-50) has the highest oxidation peak current density and lowest charge transfer resistance towards VO2+/VO2+ redox reaction, due to the introduction of surface oxygen-containing active functional groups, which is further verified by X-ray photoelectron spectroscopy. VRB single cell using OGF-50 electrode also exhibits the best cell performance, with a voltage and energy efficiency of 87% and 77% at 60 mA/cm2, respectively. Electro-oxidation method is environmentally friendly and controllable comparing with the tradition activation methods, such as concentrated acid treatment and high-temperature thermal treatment.