1 Cu2O composite paper supercapacitor:micromorphology and elemental composition of Cu₂O/Cu/graphite/cellulose composite paper

Figure 1 compares the optical images, surface and cross-sectional morphologies, and elemental compositions of cellulose paper, GCP, Cw/GCP, and Cu₂O/Cu/GCP. Ordinary cellulose paper has a porous structure consisting of a cross-linked 3D fiber network [Figure 1(a), (e), and (i)]. Cellulose paper changes from white to black by pencil painting [Figure 1(b)]. According to scanning electron microscopy (SEM) observation, a thin layer of graphite flakes uniformly covers the surface of the paper [Figure 1(f) and (j)]. Compared with the energy dispersive X-ray spectrum (EDX) of cellulose paper [Figure 1(m)], the EDX spectrum of GCP shows that the presence of graphite coating leads to a significant increase in the C/O ratio, and some other new elements (such as Mg, Al, Si, Ca, and Fe) come from the clay component in the pencil [Figure 1(n)]. As a electrode of Cu₂O composite paper supercapacitor, the composite material exhibits excellent porous structure and conductive properties, providing a good foundation for the design of high-performance supercapacitors.

After electrodeposition of Cu on GCP, the color of the composite paper changes to purple-red [Figure 1(c)]. The SEM image of Cu/GCP shows that the thickness of the Cu layer is about 70wm [Figure 1(g) and (k)]. In addition, its EDX spectrum confirms the presence of abundant Cu components (about 93.62%, mass fraction) on the surface of Cu/GCP [Figure 1(o)]. The electrodeposition process is mainly divided into two steps. First, Cu²⁺ achieves a high surface coverage on the cathode (i.e., GCP) surface through physical adsorption. Second, under the action of an external electric field, Cu²⁺ is strongly adsorbed on the cathode surface by Coulomb force and quickly obtains electrons, forming a pure Cu core on the cathode surface. The cations in the electrolyte continue to migrate to the cathode surface and continue to be reduced to metallic copper. According to the research of Gruiicic and Pesic, the use of relatively high pH values ​​and Cu²⁺ concentrations tends to produce cores with coarser texture and lower distribution density, which is consistent with the SEM observations in this section [Figure 1(g)]. This preparation method provides an important material basis for the development of Cu₂O composite paper supercapacitor.

Figure 1 (a) Optical images of cellulose paper, (b) GCP, (c) Cu/GCP, and (d) Cu₂O/Cu/GCP; (e) SEM images of cellulose paper, (f) GCP, (g) Cu/GCP, and (h) Cu₂O/Cu/GCP, and the insets in (e) to (h) show the corresponding magnified images; (i) Cross-sectional SEM images of cellulose paper, (j) GCP, (k) Cu/GCP, and (l) Cu₂O/Cu/GCP; (m) EDX spectra of cellulose paper, (n) GCP, (o) Cu/GCP, and (p) Cu₂O/Cu/GCP

Electro-oxidation of the copper layer in alkaline solution leads to the formation of Cu₂O, which turns the color of the composite paper black [Figure 1 (d)]. The expected core reactions are Cu→Cu⁺+e^– and 2Cu⁺+20H^–→Cu₂O+H₂O. SEM observations show that the surface of Cu₂O/Cu/GCP is more uniform after electro-oxidation treatment [Figure 1(h) and (I)]. High-magnification SEM images show that the composite paper surface has abundant micro/nano pyramid-like structures. Since the oxygen atom density of the (111) crystal plane of cuprous oxide is the highest and the growth rate is low at low potential, the morphology of Cu₂O depends on the (111) crystal plane, resulting in a four-sided pyramid-like crystal surface morphology. This micro/nanostructure can effectively improve the surface/interface area of the composite material and is expected to promote the diffusion of electrolytes in the gaps of the Cu/Cu₂O multi-scale structure. The EDX spectrum of Cu₂O/Cu/GCP shows that the oxygen content (mass fraction) increases significantly from 3.89% to 7.09% [Figure 1(p)], confirming the oxidation of the surface Cu layer. The above morphological and elemental analysis show that Cu₂O/Cu/GCP forms a multilayer core-shell structure. The purpose of designing this multilayer paper electrode is: ① Cellulose paper as a substrate has the characteristics of light weight, and the fibrous network can absorb electrolytes, which is convenient for ion transport as an electrolyte reservoir; ② The pencil painting method can easily deposit thin layers of stone on cellulose paper to improve the conductivity of the paper; ③ The copper layer can provide an electronic “highway” for charge storage and transmission due to its excellent conductivity, and can be used as the initial raw material for synthesizing Cu₂O; ④ Cu₂O has an extremely high theoretical specific capacitance, and its micro/nanoscale pyramid-like morphology is conducive to greatly improving the electroactive specific surface area. These characteristics have significantly improved the performance of Cu₂O composite paper supercapacitor.

2 Crystal structure of Cu₂O/Cu/graphite/cellulose composite paper

Raman spectroscopy is an ideal means to characterize crystalline, nanocrystalline and amorphous carbon. The Raman spectrum of GCP is shown in Figure 2(a). The D peak at 1345 cm^-1 is generally considered to be the disordered vibration peak of graphene. The specific location of this peak is related to the laser wavelength. It is caused by the lattice vibration leaving the center of the Brillouin zone and is used to characterize structural defects or edges in graphite samples. Another significant Raman characteristic peak of graphite is the G peak at 1580 cm^-1. The G peak is the main characteristic peak of graphite, which is caused by the in-plane vibration of sp² carbon atoms. This peak can effectively reflect the number of graphite layers, but is easily affected by stress. For sp² carbon materials, in addition to the typical Raman G peak and D peak, there are some other second-order Raman scattering peaks. A large number of studies have shown that in the material of Cu₂O composite paper supercapacitor, graphene contains some second-order sum frequency and harmonic frequency Raman peaks, which are often ignored due to their weak intensity.

The crystal structure was studied by X-ray diffraction (XRD) analysis. As shown in Figure 2(b), cellulose paper has three peaks at 14.9°, 16.6° and 22.6°, corresponding to the (110), (110) and (200) planes of the cellulose I crystal structure, respectively. For the XRD spectrum of GCP, the characteristic peaks of cellulose remain unchanged, while two new peaks appear at 26.6° and 54.7°, which are derived from the (002) and (004) crystal plane reflections of graphite in pencil lead (JCPDS No.41-1487). Cu/GCP has several new peaks at 43.4°, 50.5° and 74.2°, which are attributed to the (111), (200) and (220) crystal plane reflections of metal Cu (JCPDS No.04-0836). In addition, due to the presence of the Cu coating layer, the XRD characteristic signals of cellulose are completely shielded. After electro-oxidation, Cu₂O/Cu/GCP produces peaks related to Cu₂O at 36.4°, 42.3° and 61.4° (JCPDS No.05-0667), indicating that part of Cu is successfully converted into Cu₂O, further proving the structural characteristics of the material of Cu₂O composite paper supercapacitor. In order to further study the chemical valence of Cu, X-ray photoelectron spectroscopy (XPS) analysis was performed on Cu₂O/Cu/GCP. From the full XPS scan spectrum, it can be seen [Figure 2(c)] that C, O and Cu elements exist on the surface of Cu₂O/Cu/GCP, and there are no obvious impurities, which is consistent with the results of EDX analysis. Figure 2(d) shows the XPS spectrum of Cu 2p core level, in which the two strong peaks near 932.3 eV and 952.4 e\ are related to Cu 2p_3/2 and Cu 2p_1/2, respectively, indicating the existence of Cu⁺ and Cu⁰. It is worth mentioning that the Cu 2p_3/2 binding energy of Cu₂O and Cu is very close, 932.5eV and 932.6 eV, respectively. Therefore, the Cu 2p_3/2 core level XPS spectrum cannot distinguish between Cu⁺ and Cu⁰. The other two small peaks with binding energies of 935.1eV and 955.1eV correspond to Cu 2p_3/2 (Cu 2p_1/2) in CuO or Cu(OH)₂, respectively, which indicates that a small amount of Cu⁺ in Cu₂O is oxidized to Cu²⁺ in air. The O 1s core level XPS spectrum can be fitted into three peaks [Figure 2(e)]. The peak at 530.4eV represents oxygen in the Cu₂O lattice, further confirming the existence of Cu⁺ in the Cu₂O lattice, and the peaks at 531.6eV and 532.6eV represent the adsorbed OH groups [or OH^– in Cu(OH)₂] and molecular water on the surface of Cu₂O/Cu/GCP, respectively.

Figure 2 (a) Raman spectrum of GCP; (b) XRD spectra of cellulose paper, GCP, Cu/GCP and Cu₂O/Cu/GCP. The lines below are the standard JCPDS No.41-1487 for graphite, the standard JCPDS No.04-0836 for Cu and the standard JCPDS No.05-0667 for Cu₂O; (c) XPS full scan spectrum, (d) Cu 2p core level XPS spectrum and (e) O 1s core level XPS spectrum of Cu₂O/Cu/GCP