In our contribution we study the long-term durability of two different graphite felt materials serving as negative electrode in vanadium redox flow battery. Both electrodes differ in the precursor material and the way of activation which causes significant differences in relevant properties such as electric conductivity, specific surface area and electrochemical double layer capacitance. The performance stability of the felts within 2000 charge-discharge cycles conducted in lab-scale single-cell is evaluated from 4-point characterization before the cycling and after each 1000 cycles. Significant deterioration of electrocatalytic activity is observed for commercially activated polyacrylonitrile-based negative electrode, whereas the in-house activated rayon-based negative electrode provides stable performance over the whole tested period. A comprehensive post-mortem characterization is performed to identify the causes of different durability of both negative electrodes. X-ray photoelectron spectroscopy analysis reveals very similar changes in surface functionalization by oxygen-containing groups for both felts; however, the decrease of sp2-hybridized carbon and simultaneous increase of sp3-hybridized one is significantly higher for the unstable polyacrylonitrile-based felt. Although the performance deterioration of the polyacrylonitrile-based felt cannot be related to the decrease of the surface area or double-layer capacitance, the high surface area of activated rayon-based felt most probably contributes to its excellent performance stability.