Electrospun carbon fibers are featured with abundant electroactive sites but large mass transport resistances as the electrodes for vanadium redox flow battery. To lower mass transport resistances while maintaining large specific areas, electrospun carbon fibers with different structural properties, including pore size and pore distribution, are prepared by varying precursor concentrations. Increasing the polyacrylonitrile concentration from 9 wt % to 18 wt% results in carbonized fibers with an average fiber diameter ranging from 0.28 mu m to 1.82 mu m. The median pore diameter, in the meantime, almost linearly increases from 1.32 mu m to 9.05 um while maintaining the porosity of higher than 82%. The subsequent electroactivity evaluation and full battery testing demonstrate that the mass transport of vanadium ions through the electrode with larger fiber diameters are significantly improved but not scarifying the electrochemical activity. It is shown that the flow battery with these electrodes obtains an energy efficiency of 79% and electrolyte utilization of 74% at 300 mA cm(-2). Hence, all these results eliminate the concern of mass transport when applying electrospun carbon fibers as the electrodes for redox flow batteries and guide the future development of electrospun carbon fibers. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.