The electrochemical performance of a Cu₂O supercapacitor electrode (Cu₂O/Cu/GCP) was investigated in a three-electrode system using 6 mol/L KOH as the electrolyte. Figure 1(a) shows the galvanostatic charge-discharge (GCD) curves of the Cu₂O/Cu/GCP electrode obtained after electrooxidation of Cu/GCP in KOH solution for different times. The results show that the discharge time significantly increases with increasing electrooxidation time. Since discharge time is proportional to specific capacitance, this indicates that increasing electrooxidation time increases the specific capacitance of Cu₂O/Cu/GCP. Figure 1(b) shows that as the electrooxidation time increases from 100 s to 25,600 s, the specific capacitance increases from 160 F/g to 583 F/g (a 2.6-fold increase) due to the increased conversion of Cu to Cu₂O. When the polarization time is extended from 25600 s to 51200 s and further to 102400 s, the capacitance increase is not significant (approximately 1.7% increase from 25600 to 51200 s and approximately 2.1% increase from 25600 to 102400 s). This is likely due to the near-complete conversion of the surface Cu layer to Cu₂O. XRD analysis of the Cu₂O/Cu/GCP composites confirms this hypothesis. As shown in Figure 2, the XRD signal of the Cu₂O component gradually intensifies with increasing polarization time. For Cu₂O/Cu/GCP-51 200 and Cu₂O/Cu/GCP-102 400 (Cu₂O/Cu/GCP-x represents Cu₂O/Cu/GCP obtained by electrooxidation of Cu/GCP for x seconds), the XRD signal intensities of the Cu₂O component are almost identical.

Figure 1. (a) GCD curves and (b) gravimetric capacitance of Cu₂O/Cu/GCP electrodes obtained by electrooxidation of Cu/GCP for different times at a current density of 5 A/g; (c) GCD curves and (d) gravimetric capacitance of Cu₂O/Cu/GCP-102400 at different current densities; (e) Relationship between energy density and power density of Cu₂O/Cu/GCP-102400; (f) CV curves of Cu₂O/Cu/GCP102400 at different scan rates; (g) Nyquist plot of Cu₂O/Cu/GCP-102400, with the inset showing a magnified image of the high-frequency region; (h) Cycling performance and Coulombic efficiency of Cu₂O/Cu/GCP-102400 at a current density of 50 Ag.

Figure 2. XRD patterns of Cu/GCP and Cu₂O/CUGCP obtained with different electrooxidation times. The line below is the standard JCPDS for Cu. No.04-0836 and JCPDS No.05-0667 for Cu₂O

Figure 1(c) shows the GCD curves of the Cu₂O supercapacitor electrode Cu₂O/Cu/GCP-102 400 at different current densities (2–100 A/g) in the potential window of 0–0.4 V. As shown, the charge-discharge curves maintain good symmetry, demonstrating outstanding capacitive characteristics and electrochemical reversibility. The nonlinear characteristics of the charge-discharge behavior reflect the capacitive properties of the electrode. Figure 1(d) shows the mass-specific capacitance values at different current densities, calculated based on the discharge curves. The maximum specific capacitance (601 F/g) for Cu₂O/Cu/GCP-102400 was achieved at a current density of 2 A/g. This value is higher than that of many reported copper-based electrodes (see Table 1), such as 3D highly ordered nanoporous CuO (431 F/g), Cu₂O-Cu(OH)₂ nanosheets/graphene/stainless steel (425 F/g), rose rock-like nano-Cu₂O anchored graphene (416 F/g), Cu-supported CuO nanoribbons (392 F/g), TiO₂/Cu₂O-Mn₃O₄ (312 F/g), and Cu₂O/CuO/reduced graphene oxide (rGO) (173 F/g). The specific capacitance gradually decreases with increasing current density, which may be due to the increased internal resistance of the redox reaction and insufficient active species participating in the reaction at higher current densities. The specific capacitance of Cu₂O/Cu/GCP-102 400 at current densities of 50 A/g and 100 A/g is 88% and 83% of that at a current density of 2 A/g, respectively. The high capacitance retention clearly demonstrates the excellent rate capability of this Cu₂O supercapacitor electrode at high current densities, which is of great significance for the practical application of supercapacitors. Furthermore, the electrochemical properties of the GCP electrode were tested (see Figure 3). The mass specific capacitance of GCP at a current density of 2 A/g was only 1.5 F/g (calculated from its GCD curve), indicating that the capacitance of Cu₂O/Cu/GCP is primarily due to the pseudocapacitance of the Cu₂O component.

Table 1 Comparison of the mass-specific capacitance of Cu₂O/Cu/GCP-102 400 with recently reported copper-based electrodes

 

Figure 3 (a) CV curves of GCP at different scan rates; (b) GCD curve of GCP at a current density of 2Ag

The energy density and power density calculated from the GCD curves are key factors for the practical application of supercapacitors. As shown in Figure 1(e), the Cu₂O/Cu/GCP-102 400 electrode has a high energy density, ranging from 13.4 Whkg (0.40 kW/kg) to 11.1 Wh/kg (20 kW/kg), which is superior to some reported copper-based electrodes, such as carbon nanofiber/CuO nanorod array (0.20 kW/kg, 8.8 Whkg), CuO nanoflower/carbon fiber cloth (0.31 kW/kg, 10.1 Wh/kg) and NiCo2O4/Cu-based nanowire (0.34 kW/kg, 12.6 Wb/kg). Figure 1(f) shows the CV curves of the Cu₂O supercapacitor electrode Cu₂O/Cu/GCP-102 400 at different scan rates in the range of 5~100 mV/s. As can be seen from the figure, increasing the scan rate has no obvious effect on the shape of the CV curve, further indicating that the electrode has good rate performance and reversibility. Furthermore, none of these CV curves are quasi-rectangular, indicating that the charge storage capacity of the electrode originates from the Faradaic redox reaction. In alkaline electrolyte, the redox behavior of the Cu₂O/Cu/GCF electrode is mainly attributed to the oxidation of Cu⁺ to Cu²⁺ and the reduction of Cu²⁺ to Cu⁺. The reaction of Cu₂O in alkaline electrolyte is as follows:

   (1)

   (2)

     (3)

    (4)

The excellent electrochemical behavior of the Cu₂O/Cu/GCP electrode may be due to the following reasons: ① The highly porous cellulose paper can absorb electrolytes as an electrolyte reservoir and promote ion transport; ② The highly conductive copper layer can serve as a “highway” for electron transport; ③ The Cu₂O nanostructures grown directly on the surface of the copper layer increase the electrochemical active centers for reversible redox reactions; ④ Cu₂O has an extremely high theoretical capacitance. These factors work together to enable the Cu₂O supercapacitor electrode to exhibit excellent performance in practical applications. To further understand the electrochemical behavior of Cu₂O/Cu/GCP, AC impedance spectroscopy (ECI) of the Cu₂O/Cu/GCP was measured in the 0.01–10 Hz range. The Nyquist plot of the electrode is shown in Figure 1(g). The solution resistance (the intercept of the curve with the real axis) is as low as 0.63Ω, and no significant semicircle, reflecting the charge transfer resistance within the electrode, is observed in the high-frequency region, indicating rapid charge transfer rates and low electrode resistance. Long cycle life is also a key performance indicator for an ideal supercapacitor. Cu₂O/Cu/GCP-102400 was subjected to 8000 charge-discharge cycles at a current density of 50 A/g. As shown in Figure 1(h), the electrode exhibits excellent long-term electrochemical stability, with a capacitance retention of 95.3% after 8000 cycles. Furthermore, the Coulombic efficiency, calculated from the ratio of discharge time to charge time, remained high at 97.3%–99.6% throughout the cycling process.