A new modification of graphitic carbon felt (GCF) as an electrode for all-vanadium redox flow batteries (VRBs) was developed with cross-linked polymer film which helps to improve long-term cycling ability of the cells. The polymer modification which causes the vanadium coordination effect with tertiary amine groups of the main chains avoids the severe oxidation and preserves the concentration of vanadium species during long-term redox cycling. Besides the ability of polymer films keeps the membrane surface clean without precipitation of vanadium compounds [e.g. vanadium (V) sulfate, V2(SO4)5 or vanadium (IV) oxide sulfate, VOSO4] arising at high SOC which causes increasing temperature due to the electrostatic attraction between the ionized amine groups and SO42- ions. This is a good way to keep the internal resistance stable. The schematic illustrations of typical VRB system and with polyethyleneimine cross-linked with glutaraldehyde (PG) modified GCF electrodes are shown in Fig. 1. The PG modified GCF electrode was fabricated via stamping method with PEI (5 Wt.%, branched) and GA (1Wt.%) solutions. One-fifth of pristine GCF electrode (5*6 cm2) is immersed in each solution one by one two times. Then it is dried at 70ºC in vacuum for 5 hours and washed with de-ionized (DI) water several times. The PEIGA modified GCF (PGGCF) electrode dried overnight are used as positive and negative electrodes. This modification method has been proposed or demonstrated as proof of concepts for the new interphase between membrane and carbon electrode. Fig. 2 shows SEM images of (a, b and c) pristine GCF and (d, e and f) PGGCF electrodes with different magnifications. After PG coating, the string surface of PGGCF electrode became smooth with very thin polymer films like a spider web formed between carbon strings. Thus, the coating method is efficient way to get thin PG films on each string and between them. Based on SEM-EDX analysis, the polymer films are consisting of carbon, oxygen and nitrogen, as shown in Fig. 3. FT-IR spectrum of PGGCF electrode shows typical N-H and C-N stretching vibrations corresponding to PEI while the pristine GCF does not show them meaning that PEI was successfully coated on GCF electrode, as shown in Fig.4. The chemical reaction to form the PEIGA is shown in Fig. 5. The electrochemical performance of PGGCF and pristine GCF electrodes was evaluated in vanadium 3+ and 4+ solutions as negative and positive electrolytes (1.5M VOSO4 with 3.5M H2SO4), respectively with N115 membrane. The flow rate of electrolytes was 30ml/min and current was 50mA/cm2 operated in cut-off voltage range of 0.8-1.6V. As shown in Fig. 6, cycling stability of PGGCF cell with high cycling Coulombic efficiency (CE) is much better than GCF baseline cell for 100 cycles although the energy efficiency (EE) of PGGCF cell is lower than baseline cell due to their lower electrical conductivity and smaller surface area caused by polymer coating than pristine electrode. Fig. 7 shows the charge-discharge curves of the GCF baseline and PGGCF cells at 1st, 50th and 100th. The capacity decrease of baseline cell is observed in Fig. 6a with increasing overpotential and decreasing charge-discharge time as cycling proceeded, as shown in Fig. 7a while cycling ability of PGGCF cell is much more stable without severe increasing cell polarization. The poor cycling stability of baseline cell could be attributed to the concentration polarization of active species as cycling proceeded. Basically the concentration polarization results from vanadium crossover over prolonged cycling, but we found out different reason in this study. Based on ICP analysis for the electrolytes after 100 cycles, the oxidation of vanadium 4+ especially in positive electrolyte is considered as the main reason why the baseline has the concentration polarization. As a result, the amount of redox-active species (V4+) in positive side was significantly reduced and it becomes irreversible, as shown in Fig. 1a. Another reason we found is the precipitation of vanadium compounds on the membrane which may cause the decreasing proton conductivity and further increasing cell polarization. On the other hand, the concentration of V4+ in positive electrolyte of PGGCF cell after 100 cycles is almost same as the beginning of cell test meaning that PG modification helped to stabilize the oxidation state of V4+ for 100 cycles due to the vanadium coordination effect of PEI, as shown in Fig. 1b. To verify it, the cycled electrodes were analyzed by SEM-EDX showing that vanadium and sulfur atoms were detected in the polymer phase coated on the strings. A lot of tertiary amine groups on PEI backbone and their branches and ionized amine groups in the acidic electrolytes can coordinate vanadium and sulfur ions during the prolonged cycling. The effect of PG modification on membrane is shown in Fig. 9 showing digital photographs and SEM images of Nafion 115 membranes in the GCF baseline and PGGCF cells after 100 cycles. Theoretically the vanadium compounds are formed in vanadium electrolytes with different oxidation states [(a) 3+, (b) 4+ and (c) 5+] at high temperature (50ºC, stored for 2 weeks), as shown in Fig. 10. Therefore it is likely that vanadium compounds form at high SOC which causes increasing temperature in the cell especially during long-term cycling. In case of GCF baseline cell, the cycled membrane has lots of precipitates on the membrane surface while that of PGGCF cell has a clean surface without precipitates meaning that PG modification helps to avoid the precipitation which causes loosing redox-active species and increasing cell polarization as cycling proceeded.