To identify the major degradation mechanism of vanadium redox flow batteries (VRFBs) during cycling under practical operation conditions, an electrochemical impedance analysis is conducted by using dynamic hydrogen electrode (DHE) as a reference electrode. The increased ohmic resistance and charge transfer resistances of the electrodes with cycling are responsible for the performance fade. Spectroscopic and postmortem analysis indicate that the degradation of the carbon felt electrode, which lowers surface conductivity, results in the increases in ohmic and charge transfer resistances. Therefore, the chemical stability of electrode should be improved for achieving more durable VRFBs. (C) 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.