Ultrathin free-standing electrospun carbon nanofiber web (ECNFW) used for the electrodes of the vanadium flow battery (VFB) has been fabricated by the electrospinning technique followed by the carbonization process in this study to reduce the ohmic polarization of the VFB. The microstructure, surface chemistry and electrochemical performance of ECNFW carbonized at various temperatures from 800 to 1400 degrees C have been investigated. The results show that ECNFW carbonized at 1100 degrees C exhibits the highest electrocatalytic activity toward the V2+/V3+ redox reaction, and its electrocatalytic activity decreases along with the increase of carbonization temperature due to the drooping of the surface functional groups. While for the VO2+/VO2+ redox couple, the electrocatalytic activity of ECNFW carbonized above 1100 degrees C barely changes as the carbonization temperature rises. It indicates that the surface functional groups could function as the reaction sites for the V2+/V3+ redox couple, but have not any catalytic effect for the VO2+/VO2+ redox couple. And the single cell test result suggests that ECNFW carbonized at 1100 degrees C is a promising material as the VFB electrode and the VFB with ECNFW electrodes obtains a super low internal resistance of 250 m Omega cm(2). (C) 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.