In this study, a simple cobalt oxide modification approach for graphite felt is investigated as the positive electrode for the all-vanadium redox flow battery. A thin coating layer of cobalt oxide on the surface of graphite felt is applied via impregnation in conjunction with ultrasonication and calcination. Cyclic voltammetry and electrochemical impedance spectroscopy reveal that the modified graphite felt exhibits excellent electro-catalytic activity and reversibility toward the VO2+/VO2 + redox reaction and stability after repetitive 50-cycle testing. Full cell performance is assessed via charge-discharge and polarization curve analysis. The coulombic efficiency, voltage efficiency, energy efficiency and discharge capacity of the cell employing the modified electrode (without flow field) are 89.5%, 77.6%, 69.4% and 373.9 mAh at 150 mA cm−2, respectively, 12.7% higher in energy efficiency and 101.7% higher in discharge capacity than a cell employing pristine graphite felt. The polarization curve analysis exhibits a much higher limiting current density and maximum power density at a state of charge of 95% due to the inhibition of ohmic polarization. Overall, the results demonstrate an enhanced and stable catalytic effect of the facile cobalt modification for promoting the electrochemical performance of graphite felt in the vanadium redox flow battery.