Cellulose paper can not only act as a 3D porous framework to support the forest-like Cu₂O/Cu array and as an electrolyte storage unit to promote ion transport, but can also be directly used as an electrical insulating separator to assemble supercapacitor device. In this chapter, a symmetrical supercapacitor can be easily obtained by growing a forest-like Cu₂O/Cu array structure on both surfaces of cellulose paper. The Cu₂O/Cu composites on the two surfaces can be used as positive and negative electrodes, respectively. The structural schematic diagram of the Cu₂O/Cu/cellulose composite paper-based SSC device is shown in Figure 1(a). The device was immersed in 1 mol/L KOH solution and all electrochemical tests were performed in a two-electrode system.

Figure 1 (a) Schematic diagram of the structure of the Cu₂O/Cu/cellulose composite paper-based SSC device; (b) CV curves of the device at different scan rates; (c) GCD curves of the device at different current densities; (d) Comparison of the energy density and power density of the Cu₂O/Cu/cellulose composite paper-based SSC device with the data of other Cu₂O-based SSC or ASC devices in the literature (Ragone plot);(e) Cyclic performance of the device measured at a current density of 16 mA/cm₂

Figure 1(b) shows the CV curves of the device at different scan rates. A pair of significant anodic and cathodic peaks can be clearly observed on each CV curve, indicating the reversible and continuous Faradaic reaction involving copper oxide during the charging and discharging process. In order to study the contribution of the current collector (platinum sheet) to the device capacitance, the CV curve of the platinum sheet was tested (Figure 2). Compared with the area enclosed by the CV curve of the device, the area enclosed by the CV curve of the platinum sheet is significantly smaller, indicating that the capacitance contribution of the platinum sheet is negligible. The GCI curve of the supercapacitor device at different current densities is shown in Figure 1(c). During the device charging, the outer layer of the positive electrode is oxidized to form CuO or Cu(OH)₂; while on the negative electrode, K⁺ ions are physically adsorbed on the electrode surface without Faraday reaction. The discharge curve contains two regions, namely a rapid IR drop and a subsequent slow potential decay. The IR drop originates from the internal resistance of the device (including resistance and solution resistance). Figure 3 shows the changes in the area specific capacitance and mass specific capacitance of the device with the change of discharge current density. The supercapacitor device can achieve a maximum specific capacitance of 409F/g (213 mF/cm²) at a current density of 1.9Ag, which is higher than some previously reported Cu₂O-based symmetric/asymmetric supercapacitors (SSC and ASC) (see Table 1), such as Cu₂O@Cu nanoneedle array//activated carbon ASC (77 F/g), rose rock-like nano-Cu₂O anchored graphene SSC (92 F/g), Cu₂O-Cu(OH)₂ nanosheets/graphene/stainless steel SSC (104 F/g) and graphene/polypyrrole/Cu₂O-Cu(OH)₂/nickel foam SSC (225 F/g). The Ragone plot of Figure 1(d) is drawn based on GCD data. The device has a high energy density of 24.0Wh/kg at a power density of 0.625kW/kg, and maintains a high energy density of 2.5 Wh/kg at a high power density of 10kW/kg. These energy and power data are similar to or even better than the above-mentioned Cu₂O-based ASC and SSC devices (12~35.6 Wh/kg and 0.162~8 kW/kg). Figure 1(e) shows the test results of the device’s continuous charge and discharge test at 16 mA/cm² (30.8 Ag). As shown in the figure, even after 10,000 cycles, the capacitance can maintain 90.2% of its initial value.

Figure 2 (a) CV curves of platinum sheet at different scan rates measured under a three-electrode system; (b) CV curves of platinum sheet and Cu₂O/Cu/cellulose composite paper-based SSC device at a scan rate of 5mV/s

Figure 3 Area specific capacitance and mass specific capacitance of Cu₂O/Cu/cellulose composite paper-based SSC device at different current densities

Table 1 Some Cu₂O-based ASC and SSC Comparison of mass specific capacitance, energy density and cycling stability of devices

It is worth noting that, in addition to the Cu₂O/Cu/cellulose composite paper-based supercapacitor device SSC reported in this chapter, other types of ASC and SSC devices with a wide potential range can also be easily prepared by depositing various electrochemically active materials (such as metal oxides, conductive polymers and carbon materials) on the two surfaces of cellulose paper through various physicochemical techniques (such as vapor deposition and vacuum filtration).

The forest-like Cu₂O/Cu array structure was grown on the 3D fiber framework of flexible cellulose paper through two fast and simple steps including magnetron sputtering and electro-oxidation. This self-supporting flexible composite paper electrode achieved a high specific capacitance of 915 F/g (238 mF/cm²) and presented a high energy density of 53.7 WM/kg at a power density of 1.25 kW/kg as well as excellent cycling stability and rate performance. These excellent electrochemical properties are mainly attributed to the synergistic effect of the three components (strongly hydrophilic cellulose fibers, highly conductive Cu, and highly electrochemically active Cu₂O) and the advantages of the multi-scale forest-like array structure (which can provide a large reaction area). More importantly, the SSC device can be easily constructed by growing the forest-like Cu₂O/Cu array on both surfaces of cellulose paper. The device has a high specific capacitance of 409F/g (213 mF/cm²) and exhibits a high energy density of 24.0Wh/kg at a power density of 0.625kW/Kg. In addition, the device also has good cycling stability (capacitance retention rate after 10,000 cycles is 90.2%). These satisfactory results demonstrate the feasibility of designing flexible composite paper with forest-like array structure into a new type of flexible supercapacitor device electrode.