To further investigate the practical applications of this interesting electrode design, XUANSN assembled an asymmetric supercapacitor ASC device with a “sandwich” structure，device consisting primarily of a Cu₂O/Cu/graphite composite on a Cu₂O/Cu/GCP-102 400 surface as the positive electrode, a cellulose paper substrate as the separator, and a carbon paper as the negative electrode, labeled Cu₂O/Cu/GCP//CP. Other components include two current collectors (nickel foam) and two PET films, as shown in Figure 1(a). The presence of the carbon paper widens the potential window to 0.8 V in 6 mol/L KOH aqueous solution (Figure 2). The shape of the CV curve of this Hybrid Supercapacitor differs significantly from the ideal rectangular double-layer capacitor (EDLC) (Figure 1(b)), indicating that the capacitance of the Cu₂O/Cu/GCP//CP ASC device stems primarily from the Faradaic redox reaction. The anodic peaks are primarily attributed to the oxidation of Cu₂O and/or CuOH to CuO and/or Cu(OH)₂, while the broad cathodic peaks are primarily attributed to the reduction of CuO and/or Cu(OH)₂ to Cu₂O and/or CuOH. A small amount of oxidation of nickel foam may also contribute to these redox signals. Furthermore, the broad peaks may be due to the overlap of redox signals.

Figure 1 (a) Schematic diagram (left) and digital photograph (right) of the Cu₂O/Cu/GCP//CP ASC device; (b) CV curves of the Cu₂O/Cu/GCP//CP ASC device at different scan rates; (c) GCL curves and (d) area-specific capacitance of the Cu₂O/Cu/GCP//CP ASC device at different current densities; (e) Relationship between the power density and energy density of the Cu₂O/CWGCP//CP ASC device and comparison with some recently reported SSC and ASC device data; (f) Nyquist plot of the Cu₂O/Cu/GCP//CP ASC device and inset showing a magnified image of the high-frequency region; (g) Cycling performance and Coulombic efficiency of the Cu₂O/Cu/GCP//CP ASC device at a current density of 8 mA cm².

Figure 2 CV curves of the Cu₂O/Cu/GCP//CP ASC device at different voltage windows at a scan rate of 100 mV/s.

Figure 1(c) shows GCD curves of the Cu₂O/Cu/GCP//CP ASC device at different current densities in the potential range of 0–0.8 V. Their nonlinear shapes further indicate that the effective charge storage originates from Faradaic reactions, which is consistent with the results of CV analysis. At a current density of 1 mA/cm², the Cu₂O/Cu/GCR//CP ASC Hybrid Supercapacitor achieves its highest area-specific capacitance (122 mF/cm²), while maintaining an area-specific capacitance of 36 mF/cm² at a high current density of 16 mA/cm² (Figure 1(d)). The supercapacitor achieves its maximum energy density (10.8 μWh/cm²) at a power density of 402.5 μW/cm² and maintains an energy density of 3.2 μWh/cm² at a power density of 6400 μW/cm² (Figure 1(e)). These energy and power density values are comparable to or higher than those of recently reported symmetric/Hybrid Supercapacitor (SSCs and ASCs) (Table 1), such as graphene/cellulose paper SSC (20 μW/cm², 5.8 μWh/cm²), rGO/MnO₂//rGO ASC (37.5 μW/cm², 35.1 μWh/cm²), graphene gel SSC (369.8 mW/cm², 2.7 μWh/cm²), RGO+carbon nanotube (CNT)@carboxymethyl cellulose SSC (20 μW/cm², 3.8 μWh/cm²), RGO/PPy SSC (10 mW/cm², 61.4 μWh/cm²), GaN/graphite SSC (250 μW/cm², 7.4 μWh/cm²), and TiN@CNT SSC (809 μW/cm², 2.3 μWh/cm²). These favorable electrochemical properties meet the requirements of applications such as harvesting and storing electricity generated from thermoelectric and ambient airflow. The Nyquist plot of the Cu₂O/Cu/GCP//CP Hybrid Supercapacitor is shown in Figure 1(f). The low intercept on the real axis (approximately 1.1 Ω) in the high-frequency region is associated with the low solution resistance. Similar to the three-electrode system, the assembled device exhibits no distinct semicircular pattern in the high-frequency region, reflecting the device’s extremely low internal resistance. In the low-frequency region, the slope of the curve approaches 90°, indicating good capacitive behavior. Furthermore, the Cu₂O/Cu/GCP//CP device was subjected to 5000 continuous charge-discharge cycles at a high current density of 8 mA cm², as shown in Figure 1(g). The device retained 91.5% of its initial capacitance at the end of the cycle, demonstrating excellent cycling stability. Furthermore, the device exhibits superior stability to some recently reported copper-based supercapacitors, such as the Cu₂O-Cu(OH)₂ nanosheet/graphene/stainless steel SSC (87% after 2000 cycles) and the Cu₂O/CuMoO₄/activated carbon ASC (86.6% after 3000 cycles). Furthermore, the device achieves a high Coulombic efficiency of 95.3%–99.8% throughout the cycling process, demonstrating high energy utilization.

Table 1 Comparison of the areal capacitance and energy density of the Cu₂O/Cu/GCP//CP ASC device with some recently reported SSC and ASC devices.

XUANSN’s extensive experience in supercapacitor electrode materials and device processing provided strong support for the performance optimization and experimental implementation of this asymmetric supercapacitor. This research not only demonstrates the promising application of the Cu2O/Cu/GCP electrode in high-performance energy storage but also provides valuable insights for the development of new, green supercapacitors.