## 3 Results and Discussion  
The precursor ZIF-67 was grown in situ on CF, which was different from the traditional postbonding method. In order to study the crystallization course of the ZIF-67, the SEM images of the ZIF-67 at different reaction times (0, 5, 12, and 20 h) were collected, as shown in Figure 1a-d. Firstly, Co\({}^{2+}\) ions replaced the H\({}^{+}\), which combined with the CF and captured the 2-methylimidazole surrounding it in the solution, and many buds grew in situ on the surface of the CF (Figure 1b). This was the growing point for the ZIF-67, and determined the in situ growth state and how the ZIF-67 bonded with the CF. With the reaction proceeding, an increasing amount of 2-methylimidazole was absorbed on the surface, and bridged as shown in Figure 1c. Finally, the Co\({}^{2+}\) ions bonded with the 2-methylimidazole to form 3D structures, such as diamond 12-face bodies and cubes. Those structures were called ZIF-67, and the as-treated CF was called ZIF-67@CF. In order to determine the changes to the CF surface, the SEM images of pristine CF, ZIF-67@CF, and Co-CN@CF were measured; these are shown in Figure 2. In the SEM images of ZIF-67, it can be seen that the original Co-MOF precursor had a regular shape, and its particle size was about 200-300 nm. In addition, the EDS of the ZIF-67 confirmed that the Co-MOF was evenly distributed on the surface. After the ZIF-67@CF was heated to 600 \({}^{\circ}\)C for 2 h at a rate of 2 \({}^{\circ}\)C-min\({}^{-1}\) under an Ar atmosphere, the SEM image showed that some of the product particles nearly maintained the morphology of the original MOF, and some product particles became smaller, in the manner of a glass splinter adhering to the CF. This proved that carbonization of ZIF-67 at a high temperature led to the decrease in the particles' size. The XRD was used to analyze the phase composition of the ZIF-67, ZIF-67@CF, and Co-CN@CF. The XRD of the ZIF-67 powder centrifugal separated from the solution showed the ZIF-67@CF reaction (Figure 3a). The results also indicated that the as-prepared Co-MOF agreed with the simulated results, demonstrating that the as-prepared Co-MOF had a crystalline feature. The XRD of the ZIF-67@CF and Co-CN@CF (Figure 3b,c) showed a strong peak at around 25\({}^{\circ}\) for both the ZIF-67@CF and Co-CN@CF that was assigned to the (002) reflection of carbon. The other peaks were in agreement with ZIF-67 for the Co-CN@CF, while the other peaks were in agreement with the Co standard pattern (PDFCard No. 15-0806). This meant that during the heating process, organic ligands and some absorbed compounds in the ZIF-67 were decomposed to form a C mixed N structure, while the residual Co\({}^{2+}\) ions were changed to Co metallics. No impurity peaks appeared, implying that the product was pure Co metallics. The XPS curves are shown in Figure 4. We determined that the functional group of the ZIF-67@CF was changed, and the Co metal element was combined with the CF. As the surveys (Figure 4a,f) of all chemical elements showed, the peaks of the Co-CN@CF were different from those of the ZIF-67@CF; there were Co 3s and Co 3p peaks in the curves of the Co-CN@CF, and the peak intensities for Co and O in the curves of Co-CN@CF were higher than those in the curves of ZIF-67@CF. However, the peak intensities for N and C in  
Figure 3: XRD results: (**a**) ZIF-67 powder; (**b**) ZIF-67@CF; (**c**) Co-CN@CF. Figure 2: SEM images of ZIF-67 at different times: (**a**) before treatment; (**b**) after 5 h; (**c**) after 12 h; (**d**) after 20 h. (**e-h**) EDS images of the ZIF-67@CF. SEM images of: (**i,m**) CF; (**j,n**) ZIF-67@CF (after 24 h); (**k,q**) Co-CN@CF. the curves of Co-CN@CF were lower than those in the curves of ZIF-67@CF. This meant that the structure of the ZIF-67 changed after annealing, as Co and O were brought onto the surface of CF.