Carbon felt co-doped with nitrogen and phosphorus is suggested as an electrode for catalytically enhancing the redox reaction of the VO2+/VO(2)(+)redox couple. N and P can be simultaneously incorporated with the carbon felt via a one-step facile synthesis method using aniline and triphenylphosphine as the N and P sources. Successful co-doping is confirmed using energy-dispersive spectroscopy and X-ray photoelectron spectroscopy analyses. The electrochemical activity of the proposed carbon felt electrode toward the VO2+/VO(2)(+)redox reaction is significantly enhanced when compared to those of carbon felt electrodes doped with only nitrogen or phosphorus, owing to the synergetic effect of the N, P co-doping. The all-vanadium redox flow battery (VRFB) assembled with the N, P co-doped carbon felt (NPCF) electrode exhibits a highly stable cyclic performance accompanied by considerably improved energy efficiencies of 84.94% and 84.33% in the 1st and 50th cycles, respectively. In addition, the cell employing the NPCF electrode exhibits an energy efficiency of 79.1%, approximately 11% higher than that of bare carbon felt (BCF: 68.1%), at a high current density of 100 mA cm(-2). This improvement is mainly attributed to abundant electrochemical active sites for the oxidation and reduction of vanadium ions provided by the N and P functional groups formed on the carbon felt via our proposed treatment. These mitigate electrochemical polarization and accelerate the redox reaction.