To improve the conductivity of multi-layer CuO and its structural stability during electrochemical reactions, as shown in Figure 1(a), a green porous wood-derived anisotropic charcoal—a suitable porous carbon for supercapacitor—was introduced and combined with multi-layer CuO through a simple ball milling method. Mechanical ball milling is an effective composite method that can mill various materials into fine powders and combine them together. During the ball milling process, the materials are continuously broken down, and their size is reduced to the micrometer or even nanometer range. This process provides mechanical force to exfoliate and mix the materials to obtain nanocomposites. Therefore, through ball milling, the CuO component can be attached to the surface of the charcoal. The porous structure of the anisotropic charcoal can effectively inhibit the aggregation of CuO and promote the utilization of CuO.

To understand the microstructure of the anisotropic charcoal/multi-layer CuO composite, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) tests were performed. As shown in Figure 1(b), the SEM image of the anisotropic charcoal shows a highly open and layered porous 3D structure with medium/large channels. As a typical porous carbon for supercapacitor, these low-curvature channels can not only store liquid electrolytes but also promote ion transport during electrochemical reactions. The SEM image in Figure 1(c) shows a magnified view of the CuO component, where the layered structure is clearly visible. The SEM image of the anisotropic charcoal/multi-layer CuO is shown in Figure 1(d), where the CuO component is highly dispersed after ball milling, and good mixing with the charcoal is achieved. In addition, the porous structure of the charcoal is well-maintained after ball milling. TEM and HRTEM images are shown in Figures 1(e) and (f), where the ordered lattice fringes with a spacing of 0.226 nm correspond to the (111) plane of single-crystal CuO. Furthermore, the SAED pattern in Figure 1(g) shows distinct diffraction rings, indicating the polycrystalline nature of the composite material. These rings correspond to the (110), (202), (311), and (004) planes of CuO, indicating the successful synthesis of CuO. This composite structure provides a reliable structural basis for the subsequent development of high-performance electrode materials by XUANSN.

Figure 1. Synthesis strategy and microstructure of anisotropic charcoal/multilayer CuO composite material

(a) Schematic diagram of the ball milling process; (b) SEM image of anisotropic charcoal, (c) multilayer CuO, and (d) interface between anisotropic charcoal and CuO; (e) TEM image, (f) HRTEM image, and (g) SAED image of the anisotropic charcoal/multilayer CuO composite material

The crystal structure of the materials was analyzed by XRD. As shown in Figure 2(a), the wood-derived anisotropic charcoal exhibits two peaks at 24.9° and 44.9°, corresponding to the (002) and (101) crystal planes of amorphous carbon, which further indicates that its porous carbon for supercapacitor material, has a typical amorphous carbon structure. For pure multilayer CuO, these diffraction peaks correspond to the (110), (111), (111), (202), (020), (202), (I13), (311), (220), (311), and (004) planes of CuO (JCPDS No. 05-0661, a=4.684Å, b=3.425Å, c=5.129Å, β=99.47°).  Furthermore, no characteristic peaks of other possible impurities [such as Cu₂O or Cu(OH)₂] were observed, indicating the high purity of the synthesized CuO. In addition, the peak intensity in the XRD pattern of the composite material is weaker compared to that of pure multilayer CuO. This phenomenon can be attributed to the reduction in CuO crystal size after the ball milling process. The chemical valence states of the elements in the anisotropic charcoal/multilayer CuO composite material were studied by XPS analysis. As shown in Figure 2(b), the presence of O, C, and Cu elements can be determined from the XPS full scan spectrum, where the photoelectron binding energy values of 532 eV, 285 eV, 934 eV, and 79 eV are attributed to O1s, C1s, Cu2p, and Cu3p, respectively. The Cu 2p core level XPS spectrum in the range of 930–970 eV is shown in Figure 2(c), where the peaks at 933.95 eV and 953.75 eV, with a spin energy splitting of 19.8 eV, correspond to Cu2p_3/2 and Cu2p_1/2, respectively, indicating the presence of Cu²⁺ in the composite material. In addition, there are three satellite peaks of Cu 2p_3/2 and Cu 2p_1/2, located at 944.25 eV, 941.45 eV, and 962.85 eV, respectively. The energy difference between these satellite peaks and their main peaks is approximately 9 eV, which further confirms the presence of CuO. In the O1s spectrum [Figure 2(d)], the peaks at binding energies of 529.7 eV and 532.2 eV are associated with oxygen atoms in CuO and adsorbed water, respectively. Furthermore, the intensity of the peak at 532.2 eV indicates a large amount of adsorbed oxygen on the CuO surface, suggesting abundant surface sites and high electrochemical activity. These results are consistent with the findings of XUANSN in their research on porous carbon for supercapacitor/metal oxide composite systems.

The pore structure was analyzed by nitrogen adsorption-desorption method. As shown in Figure 2(e), according to the International Union of Pure and Applied Chemistry (IUPAC) classification, the anisotropic charcoal exhibits a type IV adsorption isotherm, and the hysteresis loop is attributed to capillary condensation occurring in mesopores, further confirming the hierarchical porous characteristics of the porous carbon for supercapacitor material. In addition, the significant increase in initial adsorption volume indicates the presence of micropores. Therefore, the results reflect the hierarchical porous structure of the wood-derived carbon. As shown in Figure 3, TEM images also show a hierarchical pore structure composed of micropores (<2 nm), mesopores (2～50 nm), and macropores (>50 nm). Through calculation, its Brunauer-Emmet-Teller (BET) surface area reached as high as 365.1 m²/g. By combining CuO with porous carbon, the BET surface area significantly increased from 2.2 m²/g to 163.6 m²/g due to the layered pore structure of the anisotropic charcoal. Based on the adsorption isotherm, the pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method. As shown in Figure 2(f), the pore sizes of these materials are all in the range of 1–100 nm, belonging to the range of micropores, mesopores, and macropores. According to previous studies, a porous structure containing micropores and mesopores or macropores is considered ideal because the actual energy storage mainly occurs in the smaller micropores, while the larger mesopores and macropores act as connecting media, providing transport channels for the rapid entry and exit of electrolytes into and out of the micropores.

Figure 2 Crystal structure, chemical valence, and pore structure of anisotropic charcoal/multilayer CuO composite materials

(a) XRD pattern; (b) XPS full scan spectrum; (c) Cu2p and (d) O1s core level XPS spectra; (e) Nitrogen adsorption-desorption isotherm; (f) Pore size distribution

Figure 3 TEM images of anisotropic charcoal at different magnifications