## 3 Results and Discussion
As shown in the C 1s (Figure 4b,g) and N 1s (Figure 4a,c,h) peaks, there were only Co-coordinated-N and NSP2-C bonds in the ZIF-67@CF, because the 2-MI reacted with Co\({}^{2+}\) and formed an MOF structure. After annealing, the Co-coordinated-N and NSP2-C bonds broke down, and C combined with CF, O, and N to form C=C, C\(-\)O, and C-N bonds, while only a few Co-coordinated-N bonds were kept. For the Co 2p (Figure 4e,i) peaks and O 1s (Figure 4f,j) peaks, there were Co metals on the surface of the CF that originated from the ZIF-67 MOFs, as the -OH group in the Co-CN@CF could provide more reaction points for electrode reactions. Figure 5a provides CV curves of the CF electrode using different treatment methods. There were two main peaks in all the curves, representing the oxidation and reduction peaks corresponding to the VO\({}^{2+}\)/VO\({}_{2}\)\({}^{+}\) couple. The current difference in the peak potential separation between the two oxidation and reduction peaks could be used to evaluate the redox reaction kinetics and reversibility for the VO\({}^{2+}\)/VO\({}_{2}\)\({}^{+}\) couple. For the pristine CF electrode, the oxidation and reduction peak potentials appeared at 0.596 and 0.201 V (vs. SCE); the peak potentials of the CF/600 \({}^{\circ}\)C electrode appeared at 0.585 and 0.201 V (vs. SCE); the peak potentials of the ZIF-67@CF electrode appeared at 0.58 and 0.194 V (vs. SCE); while the peak potentials of the Co-CN@CF/600 \({}^{\circ}\)C electrode appeared at 0.551 and 0.215 V (vs. SCE). It was clear that the peak potential separation of the Co-CN@CF/600 \({}^{\circ}\)C electrode (0.33 V) was the smallest among all the electrodes, which indicated a better reversibility of the redox reaction on the Co-CN@CF/600 \({}^{\circ}\)C electrode. Moreover, the oxidation and reduction peak current densities of the Co-CN@CF/600 \({}^{\circ}\)C electrode were 83.84 and 85.36 mA cm\({}^{-2}\), which were the highest among those GF electrodes. Moreover, for the \(\rm{1I_{p}}^{a}/\rm{I_{p}}^{c}\,\rm{\mid}\) of those electrodes (as shown in Figure 5b), the \(\rm{1I_{p}}^{a}/\rm{I_{p}}^{c}\,\rm{\mid}\) of the Co-CN@CF/600 \({}^{\circ}\)C electrode and the ZIF-67@CF electrode were higher than 90% and better than the others; the highest one was shown for the Co-CN@CF/600\({}^{\circ}\)C electrode (98%), which was close to 100%. The remarkable increases in the peak current density and the \(\rm{1I_{p}}^{a}/\rm{I_{p}}^{c}\,\rm{\mid}\) of the Co-CN@CF/600 \({}^{\circ}\)C meant that the electrocatalytic activity and reversibility of the CF were enhanced by introducing the ZIF-67 group and Co-CN structure. However, it seriously degraded with a heat treatment temperature higher than 600 \({}^{\circ}\)C, such as 900 \({}^{\circ}\)C. All the results showed that the ZIF-67 and Co-CN grown in situ on the CF were excellent catalysts for the VO\({}^{2+}\)/VO\({}_{2}\)\({}^{+}\) couple. Figure 4: XPS results: **(a)** the surveys of all chemical elements in ZIF–67@CF; **(b–e)** the peaks of C 1s, N 1s, Co 2p, and O 1s in ZIF–67@CF, respectively; **(f)** the surveys of all chemical elements in Co\(-\)CN@CF; **(g–j)** the peaks of C 1s, N 1s, Co 2p, and O 1s in Co\(-\)CN@CF, respectively. EIS measurements were also used to study the electric charge transfer properties of the electrodes. Nyquist plots for the pristine CF and Co-CN@CF electrodes measured at open circuit potential (OCP), as well as in the corresponding equivalent circuit based on measured data, are shown in Figure 5c. The Nyquist plots for the two electrodes maintained the same shape, including a high-frequency semicircle and a low-frequency sloping line. For the equivalent circuit, there were two resistances, R1 and R2. R1 was the sum of the electrolyte solution and electrode resistance, while R2 represented the charge transfer resistance in the interface between the electrolyte solution and electrode.