Vanadium redox flow batteries (VRFBs) have been highlighted for use in energy storage systems. In spite of the many studies on the redox reaction of vanadium ions, the mechanisms for positive and negative electrode reaction are under debate. In this work, we conduct an impedance analysis for positive and negative symmetric cells with untreated and heat-treated carbon felt (CF) electrodes to identify the reaction mechanisms. The negative electrode reaction (V2+/V3+) is highly dependent on the heat treatment and reaction temperature, which is a feature of an inner-sphere mechanism, whereas the positive electrode reaction (VO2+/VO2+) reaction is rather insensitive to the heat treatment and reaction temperature, suggesting an outer-sphere mechanism. An atomistic molecular dynamics simulation suggests that the different mechanisms are quite feasible considering the difference in the structure of the hydration shell for the vanadium ions. The deeper understanding of the reaction mechanism and its influence on cell performance will be helpful to advance VRFBs.