## 3 Results and Discussion  
Figure 1 shows the SEM images of all composites. The Co-MOF-derived CNC has the sheet structure in general (Figure 1a). As shown in Figure 1b-d, under the action of the binder PVP-K30, the 3D-graphene particles can be well dotted on the surface of the CNC after the high-temperature pyrolysis. In addition, it can be observed that the average dotted density of the 3D-graphene on the CNC was positively correlated with the additive amount of the 3D-graphene. Compared with the CNCG-1 and CNCG-3, CNCG-2 has the best morphology and spatial structure. Figure 2 shows the HR-TEM image of the CNC. The lattice fringe with the spacing of 0.21 nm corresponds with the (111) plane of the Co crystal, and the lattice fringe with the spacing of 0.35 nm corresponds with the graphite carbon. Figure 1: The SEM images of all composites. **(a)** The SEM image of CNC; **(b)** The SEM image of CNCG-1; **(c)** The SEM image of CNCG-2; **(d)** The SEM image of CNCG-3. The crystal structures of all composites were studied by XRD patterns (Figure 3a). There is an obvious characteristic peak around 26\({}^{\circ}\), which is derived from the response of the (002) plane of carbon (JCPDS no. 41-1487). In addition, the three diffraction peaks at 44.16\({}^{\circ}\), 51.40\({}^{\circ}\), and 75.84\({}^{\circ}\) correspond to the (111), (200), and (220) crystal planes of Co (JCPDS: 15-0806), indicating that Co\({}^{2+}\) was successfully reduced to Co under the action of carbon. The states of carbon in all composites were studied by Raman spectroscopy (Figure 3b). The two characteristic bands at 1350 and 1580 cm\({}^{-1}\) correspond to the D band and G band, which are attributed to the lattice defects of the carbon atom and the in-plane vibration of the sp2 carbon atom, respectively. The intensity ratios of the D band to G band (\(I_{\rm D}/I_{\rm G}\)) can be used to judge the graphitization degree of the carbon-containing composites. The \(I_{\rm D}/I_{\rm G}\) of all composites are 0.850, 0.905, 1.075, and 1.107, respectively. The magnetic properties of all composites were studied by VSM (Figure 4). The saturation magnetization (Ms) of all composites was 33.63, 18.99, 15.97, and 13.81 emu/g, respectively, all of which show the ferromagnetic behavior. In addition, it is obvious that the CNC has the highest Ms, mainly because the carbon content of the CNC is relatively low. Figure 3: The XRD patterns and Raman spectra of all composites. (**a**) The XRD patterns of all composites; (**b**) The Raman spectra of all composites. Figure 2: The HR-TEM image of CNC. The surface chemical composition and states of the CNCG-2 were studied by XPS. The XPS spectrum of CNCG-2 demonstrates the existence of Co, C, O, and N elements (Figure 5a). The fitting peak of Co 2p at 778.47 eV indicates that the existence of Co atom (Figure 5b). The two fitting peaks of C 1s at the binding energies of 284.74 and 285.42 eV are related to C-C/C=C and C-O groups (Figure 5c), respectively. The two fitting peaks of N 1s at binding energies of 398.99 and 400.77 eV are directly related to pyridine nitrogen and pyrrole nitrogen (Figure 5d). Oxygen-containing functional groups, pyridine nitrogen, and pyrrole nitrogen can act as polarization centers to enhance the intensity of dipole polarization [10]. The microwave absorption capacity of the material is determined by its electromagnetic parameters, including complex permittivity (\(\epsilon_{\rm r}=\epsilon^{\prime}-j\epsilon^{\prime\prime}\)) and complex permeability (\(\mu_{\rm r}=\mu^{\prime}-j\mu^{\prime\prime}\)). Generally speaking, the \(\epsilon^{\prime}\) and \(\mu^{\prime}\) represent the storage capacity of the electrical energy and the magnetic energy, while the \(\epsilon^{\prime\prime}\) and \(\mu^{\prime\prime}\) represent the loss capacity of the electrical energy and the magnetic energy.