1 Crystal structure, chemical composition and pore structure of forest-like Cu₂O/Cu array supported by cellulose fibers

The crystal structure was studied by X-ray diffraction (XRD). Comparing Figures 1(a) and (b), it can be seen that the coverage of the Cu array leads to the disappearance of the characteristic peaks of the cellulose type Ⅰcrystal structure, while three peaks appear at 43.8°, 50.9° and 74.6°, corresponding to the (111), (200) and (220) crystal planes of Cu (JCPDS No.04-0836), respectively. In addition, a weak peak corresponding to the Cu₂O (111) crystal plane appears in Figure 1(c), which verifies that part of Cu is converted into Cu₂O due to electro-oxidation, further indicating the feasibility of Cu₂O/Cu array for supercapacitors. The chemical valence of the Cu element in the Cu₂O/Cu/cellulose composite paper was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 1(d) shows the Cu 2p core level XPS spectrum, where the two peaks at 932.5 eV and 952.3 eV correspond to the Cu 2p_3/2 and Cu 2p_1/2 peaks of Cu⁺ and Cu⁰, respectively . It is noteworthy that the Cu 2p_3/2 binding energies of Cu₂O and Cu0 are close to each other; the reported values are 932.5 eV and 932.6 eV, respectively. Therefore, Cu⁺ and Cu⁰ cannot be distinguished by Cu 2p_3/2 in the XPS spectrum. On the other hand, the two fitted peaks with binding energies of 933.5 eV and 953.2 eV and the peaks at 942.7 eV and 962.2 eV are assigned to the characteristic peaks of Cu²⁺, which indicates that a small amount of Cu⁺ is oxidized to Cu²⁺ in air. The O 1s core level XPS spectrum can be deconvoluted into two peaks [Figure 1(e)]. The peak at 530.6 eV is oxygen in the Cu₂O lattice, further confirming the presence of Cu⁺. The peak at 531.6 eV corresponds to the adsorbed OH group. Therefore, both XPS and XRD results indicate that the composite paper mainly contains Cu₂O, Cu and cellulose components. As shown in Figure 1(f), the Bnunauer-Emmett-Teller (BET) surface area can be calculated based on the N₂ adsorption-desorption isotherm. Compared with cellulose paper (3.4 m² g) and Cu (13.2 m²/g), the coexistence of multi-scale Cu rods and Cu₂O branches makes the BET surface area of Cu₂O/Cu cellulose composite paper reach 16.6 m²/g. This higher specific surface area further enhances the energy storage capacity of Cu₂O/Cu array for supercapacitors. According to the above data and the loading amount of Cu₂O in the composite paper, the surface area of the Cu₂O component can be roughly calculated to be as high as 261.5m²/g. The calculation process is as follows:

The BET surface areas of Cu cellulose paper and Cu₂O/Cu/cellulose composite paper are 16.6m²/g and 13.2m²/g respectively. In addition, the mass percentage of Cu₂O in Cu₂O/Cu/cellulose composite paper is about 1.3%. Therefore, we can roughly calculate the surface area of ​​the Cu₂O component: [(16.6-13.2)/1.3%]m²/g≈261.5 m²/g.

Figure 1 (a) XRD patterns of cellulose paper, (b) Cu/cellulose paper and (c) Cu₂O/Cu/cellulose composite paper, where the bottom lines in (b) and (c) are the standard JCPDS cards of Cu and Cu₂O, respectively (the number of Cu is 04-0836 and the number of Cu₂O is 05-0667); (d) Cu 2p and (e) O 1s core level XPS spectra of Cu₂O/Cu/cellulose composite paper; (f) N₂ adsorption-desorption isotherms of cellulose paper, Cw/cellulose paper and Cu₂O/Cu cellulose composite paper

Larger surface area helps to provide more electrochemical active sites for electrolyte ion reactions, thereby improving electrochemical performance. The pore size distribution of Cu₂O/Cu cellulose composite paper was calculated using the Barrett-Joyner-Halenda (BJH) method based on the N₂ adsorption-desorption isotherm, as shown in Figure 2. As can be seen from the figure, the pore size range is 3.1~193.8 nm, indicating that the composite paper is composed of mesopores (2~50 nm) and macropores (>50 nm). Such a pore size distribution is conducive to the penetration of electrolytes, allowing reactants to quickly enter the electrode. It can be seen that the structural characteristics of the Cu₂O/Cu array for supercapacitors can effectively promote electrochemical reactions.

Figure 2 Barrett-Joyner-Halenda (BJH) pore size distribution of Cu₂O/Cu/cellulose composite paper

2 Electrochemical performance and energy storage mechanism of cellulose fiber-supported forest-like Cu₂O/Cu arrays

In order to evaluate the potential of cellulose fiber-supported forest-like Cu₂OCu0/Cu array structure as a supercapacitor electrode, a series of electrochemical tests were carried out under a three-electrode system. Figure 3(a) shows the CV curves of the Cu₂O/Cu/cellulose composite paper electrode at various scan rates. These CV curves are completely different from the rectangular shape of the double-layer capacitor. This is mainly attributed to the Faradaic redox reaction during the Cu(Ⅰ)/Cu(Ⅱ) conversion process:

                                                (1)

                                                (2)

                                                    (3)

                                                    (4)

Figure 3 CV curves of Cu₂O/Cu/cellulose composite paper electrode (a) at different scan rates and (b) GCD curves at different current densities; (c) area specific capacitance and mass specific capacitance of Cu₂O/Cu/cellulose composite paper electrode at different current densities; (d) energy density and power density of Cu₂O/Cu/cellulose composite paper electrode compared with data of other Cu₂O-based electrodes in the literature (Ragone diagram); (e) at a current density of 16 mA/cm² Cycling performance of Cu₂O/Cu/cellulose composite paper electrode; (f) Nyquist plot of Cu₂O/Cu/cellulose composite paper electrode

After electrochemical testing, XPS analysis of Cu₂O/Cu/cellulose composite paper confirmed the large formation of Cu²⁺ (see Figure 4). In addition, with the increase of scan rate, the potential of the cathode peak shifted to the negative direction, which is related to the internal resistance of the electrode and further proved the capacitive properties of Cu₂O/Cu array for supercapacitors.

Figure 4 Cu 2p core level XPS spectrum of Cu₂O/Cu/cellulose composite paper after electrochemical testing proves the large formation of CuO or Cu(OH)₂

Figure 3(b) shows the constant current charge-discharge (GCD) curves of Cu₂O/Cu/cellulose hybrid paper electrode at various current densities. The nonlinear characteristics also verify the capacitive properties of the composite electrode. In addition, the charging curve is almost symmetrical with its corresponding discharge curve, which indicates good reversibility. Figure 3(c) plots the functional relationship between the area specific capacitance and mass specific capacitance and current density. At a current density of 1 mA/cm² (3.8 Ag), the maximum capacitance is 238 mF/cm² (915 F/g). When the current density increases to 32 mA/cm² (123.1 A/g), the electrode still maintains a capacitance of 128 mF/cm² (492 F/g), which is equivalent to 54% of the capacitance at 1 mA/cm², showing its excellent rate performance. For comparison, this chapter summarizes the recently reported Cu₂O-based supercapacitor electrodes, which are prepared by different methods such as polyol reduction, hydrothermal, chemical deposition-calcination, and electrodeposition. As shown in Table 1, it can be seen that the maximum specific capacitance of the Cu₂O/Cu cellulose composite paper electrode is significantly higher than or similar to that of the mono-Cu₂O electrodes [such as Cu₂O microspheres (144 F/g) and microcubes [(660 F/g)], binary Cu₂O-based electrodes [such as rose rock-like nano-Cu₂O anchored graphene (416 F/g), 3D ordered macroporous Cu₂O/Ni inverse opal (502 F/g) and Cu₂O@Cu nanoneedle arrays (510.2 F/g)] and multi-Cu₂O-based electrodes [such as nanosheet-structured Ni/Cu/Cu₂O/CuO/ZnO (241 F/g), Cu₂O/CuO/Co₃O₄ core-shell nanowires (318 F/g), Cu₂O-Cu(OH)₂ nanosheets/graphene stainless steel (425 F/g) and graphene/polypyrrole/Cu₂O-Cu(OH)₂/nickel foam (997F/g)]. In addition to capacitance, energy density and power density are also key factors in the application of supercapacitors. Although the energy density and power density parameters under the three-electrode system cannot represent the performance of actual devices, these data still have certain guiding significance for the development of actual devices. The Cu₂O/Cu fiber stack composite electrode shows excellent energy density and power density. When the power gradually increases from 1.25kW/kg to 40kW/kg, the energy density only decreases from 53.7 Wh/kg to 28.9 Wh/kg. These data are better than the above-mentioned Cu₂O-based electrodes [see Table 1 and Figure 3(d)], namely Cu₂O microcubes (0.2kW/Kg, 14.6Wh/kg), 3D ordered macroporous Cu₂O/Ni inverse opal (1.2 kW/kg, 26.5 Whkg), and nanosheet-structured Cu₂O/Ni inverse opal (1.2 kW/kg, 26.5 Whkg). Ni/Cu/Cu₂O/CuO/ZnO (0.55 kW/kg, 32 Wh/kg) and rose rock-like nano-Cu₂O anchored graphene (1.5 kW/kg, 42 Wh/kg). In addition, the Cu₂O/Cu array for supercapacitors electrodes have excellent cycling stability, with the initial capacitance decreasing by only 8.3% after 10,000 cycles [Figure 3(e)], which is also better than most efficient Cu₂O-based electrodes, as shown in Table 1. Therefore, these comparisons not only show the advantages of Cu₂O/Cu cellulose composite paper electrodes in capacitance, energy, and cycling performance, but also demonstrate the advantages of the synthesis strategy reported in this chapter (i.e., controlled emission and electrochemical).

Table 1 Cu₂O obtained by different preparation strategies Comparison of mass-specific capacitance, energy disk density, cycling stability, specific surface area and conductivity of supercapacitor electrodes

The reasons why Cu₂O/Cu cellulose composite paper has these good electrochemical properties can be summarized as follows: ① The fiber rope paper with high porosity and good wettability can absorb electrolyte and serve as an electrolyte storage unit to promote ion transport; ② The hierarchical multi-scale structure of the forest-like array with a large surface area significantly increases the electrochemical active sites for reversible redox reactions; ③ The highly conductive copper rod array provides an electron highway for the transfer of charge; ④ The generated Cu₂O branches have an ultra-high theoretical capacitance of 207S F/g.

Figure 3(f) is the Nyquist plot of the Cu₂O/Cu cellulose composite paper electrode. Its equivalent series resistance (real axis intercept) is as low as about 45 Ω. In addition, the semicircle width in the high-frequency region indicates the charge transfer resistance. It is well known that the Faraday interface charge transfer resistance is the core limiting factor of the power density of pseudocapacitors. The electrode has an extremely low charge transfer resistance, which helps to improve its power density, indicating that the Cu₂O/Cu array for supercapacitors has excellent power density characteristics.