In this work, the electrochemical activity of carbon felt (CF) for the V2+/V3+ redox reaction is improved by combining KOH-etched pretreatment with uniform Bi nanoparticles deposition. CF is functionalized by the KOH activated pretreatment, resulting in micropores structure on the surface and an increase of oxygen-containing functional groups up to 16.49%. The micropores structure and high content of oxygen-containing functional groups of KOH-etched carbon felt (CFE) promote the uniform distribution of Bi nanoparticles on the surface of CFE with an average particle size of 45 nm. The resulting carbon felt (CFE-Bi) significantly enhances the electrochemical activity for V2+/V3+ redox couple because of the synergistic catalysis between the CFE and the Bi nanoparticles. The peak potential separation (Delta Ep) and the charge transfer resistance of CFE-Bi is 182 mV and 0.160 Omega cm(2), which are much lower than that of CF (427 mV and 3.238 Omega cm(2)) under the same conditions. Moreover, the energy efficiency of VRFB based on the CFE-Bi as negative electrode achieves 79.3% at 160 mA cm (2), which is 36.2% higher than that of the VRFB with raw carbon felt. Overall, CFE-Bi is a promising electrode for the V2+/V3+ redox couple reaction in VRFB. (C) 2017 Elsevier Ltd. All rights reserved.