## 3 Results and Discussion
Q1 and Q2 were the constant-phase elements, representing the diffusion capacitance caused by VO\({}^{2+}\) and VO\({}_{2}\) diffusion and electric double-layer capacitance in the electrode-solution interface, respectively. According to the fitting results, the R1 for the pristine CF and the Co-CN@CF were almost the same (0.29 \(\Omega\)), which meant that the two electrodes had a similar resistance. However, the R2 value of the Co-CN@CF (0.56 \(\Omega\)) was lower than that of the pristine CF (0.68 \(\Omega\)), which meant that the charge transfer resistance of the Co-CN@CF was lower, and the structure could accelerate the electron transfer. The wettability of the Co-CN@CF and ZIF-67@CF was better than that of the pristine CF. As shown in Figure 5d, the contact angle of the pristine CF was \(>\)90\({}^{\circ}\), and the Co-CF@CF and ZIF-67@CF were superhydrophilic. With the increase in the -OH functional group on the surface of CF, the wettability of the CF increased, and the wettability of the Co-CN@CF was better than that of the ZIF-67@CF. This caused the electrolyte to react on the CF faster and more efficiently. A charge-discharge test and rate performance of the single cells were implemented in a redox flow battery (Figure 6d) that was designed by our laboratory in order to further  
Figure 5: (**a**) CV curves of CF electrode using different treatment methods; (**b**) performance analysis of CF electrode using different treatment methods; (**c**) EIS measurement of electrode; (**d**) wettability of different CFs. understand the effect of the Co-CN catalyst on the electrochemical performance of the CF. As shown in Figure 5a, lifetime tests were further carried out to study the long-term stability of the cells using different membranes. The tests were terminated when the VE decay reached 65%. The cell with a CO-CN@CF electrode exhibited an initial VE of 86%, and maintained stability (about 75% after 1000 cycles) during the entire cyclic process, whereas the cell with the pristine CF electrode declined from 75% to 65% at only 600 cycles, as shown in Figure 6a. Similarly, the EE slightly dropped from 84% to 73% in the cell with the CO-CN@CF electrode after 1000 cycles (over 400 h), whereas the cell with a pristine CF electrode decreased from 74% to 64% in just 600 cycles (about 150 h). Figure 5b presents the coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the pristine CF and CO-CN@CF electrodes at a current density ranging from 30 to 250 mA cm\({}^{-2}\). The CE of the cell with the CO-CN@CF electrode showed high stability over the entire testing range, while that of the pristine CF electrode exhibited some fluctuation, especially at low current densities (e.g., 50 mA cm\({}^{-2}\)). When returning from 250 to 30 mA cm\({}^{-2}\), the CE for the former recovered to the initial value, whereas the CE for the latter declined to a lower value. This implied that the Co-CN catalyst on the CF could keep the reaction stable in a large-scale current density, and that the wettability was also a key point for the VRFB reaction. Moreover, the efficiency of the cell with a CO-CN@CF electrode was higher than that of the pristine CF electrode throughout the rate performance test. The EE of the cell with a CO-CN@CF electrode still retained \(\approx\)52.6% at 250 mA cm\({}^{-2}\), as  
Figure 6: (a) Long-term stability, coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) of the VRFB single cells with pristine CF and CO-CN@CF electrodes; (b) the rate performance of the VRFB single cells with pristine CF and CO-CN@CF electrodes at a current density of 30 to 250 mA cm\({}^{-2}\); (c) charge–discharge curves at 80 mA cm\({}^{-2}\) for the 10 th cycle, 100 th cycle, and 200 th cycle; (d) the VRFB single cells. compared to 48.7% for the CF electrode counterpart. This indicated that the Co-CN catalyst could improve the electrochemical performance. Figure 6c compares the charge-discharge curves of the cells with pristine CF and COvCN@CF electrodes at 80 mA cm\({}^{-2}\).