Supercapacitors (SCs), one of the most promising energy storage devices, have received extensive attention owning to their high power density, fast charge/discharge speed, long cycling life span, and low-cost (Lu et al., 2014; Xiong et al., 2015; Sami et al., 2017; Dai et al., 2018). According to the reaction mechanisms, SCs can be classified into electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs) (Wei et al., 2012). For EDLCs, the charges are stored electrostatically at the electrode/electrolyte interface while typically taking carbon materials as active materials (Surendran et al., 2018). For PCs, the energy is stored within the electrode through the faradic redox reaction while taking transition metal oxides/hydroxides and conducting polymers as the electrode materials, thus the PCs provide much higher energy density and specific capacitance than EDLCs. Nevertheless, there are many of problems scarcely understood which attract large numbers of investigator devote oneself to resolve, such as inadequate energy density and capacitance, poor electrochemical stability for practical applications.

In recent years, various transition metal oxides/hydroxides, such as RuO₂ (Wang et al., 2014), NiO (Ouyang et al., 2019), Ni(OH)₂ (Kim et al., 2017), MnO₂ (Huang et al., 2015), Co₃O₄ (Liu T. et al., 2018), Co(OH)₂ (Yang et al., 2018), V₂O₅ (Foo et al., 2014), CuO (Bu and Huang, 2017; Li et al., 2017; Liu Y. et al., 2018), Cu₂O (Zhang W. et al., 2016; Ji et al., 2017), have been applied to achieve excellent capacitive performance for PCs. Among these materials, CuO, Cu₂O, or Cu₂O/CuO nanostructures with different configurations including nanoneedle, nanoflowers, nanowires (Dong et al., 2014; Wang et al., 2015; Chen et al., 2016; Xu et al., 2016; Yang et al., 2016), are attracting considerable interest due to their environmental friendliness, numerous reserve, low-cost, chemical stability, and excellent electrochemical properties [theoretical capacitance of CuO up to 1,800 F g⁻¹ (Liu Y. et al., 2018) and Cu₂O is up to 2,247 F g⁻¹ (Wu et al., 2017)]. However, most of metal oxides/hydroxides were poor in electrical conductivity, making it difficult to achieve high specific capacitance (Xu et al., 2016). To resolve this issue, oxides/hydroxides are typically mixed with ancillary carbon black or binder and then bonded to current collector, but leading to a significant decrease of the overall specific capacitance (Yuan et al., 2017). An effective approach is that, nanostructured electrode materials directly grow on current collectors, forming binder-free electrodes, thus achieving higher energy density (Dong et al., 2014).

Carbon material containing carbon nanotube, graphene and carbon fiber is one kind of the preferred current collectors due to their excellent electrical conductivity and electrochemical stability (Prasad et al., 2011; Moosavifard et al., 2014; Bu and Huang, 2017). Among various carbon materials, carbon cloths (CCs) with low-cost, chemical stability and desirable conductivity, are regarded as novel carbonaceous materials, which are consist of numerous uniform carbon fibers with three-dimensional (3D) structure (Guo et al., 2014; Zhang Y. et al., 2016). The 3D network structure is conducive to shorten the diffusion pathway of ions and accelerate the flow of ions during the electrochemical process. Numerous electrode materials of PCs taking CCs as current collectors have been developed, such as NiCo-LDH@NiOOH (622 F g⁻¹ at 1 A g⁻¹) (Liang et al., 2018), MnNiCoO₄@MnO₂ (1931 F g⁻¹ at 0.8 A g⁻¹) (Saray and Hosseini, 2016), MnO₂ nanosheet arrays (2.16 F cm⁻², at 5 mA cm⁻²) (Guo et al., 2014). Currently, copper oxide and its composite materials are mainly grown on copper foam and copper foil (Zhang et al., 2015; Singh and Sarkar, 2017), and the combination of CuO or Cu₂O with CCs is also in the developing situation. For example, Xu et al. (2016) fabricated CuO nanoflower arrays on CCs, the energy density and power density are 10.05 Wh kg⁻¹ and 1,798.5 W kg⁻¹, respectively. Wan et al. (2017) developed forest-like cuprous oxide/copper with the energy density of 24.0 Wh kg⁻¹ at 0.625 kW kg⁻¹. However, it is still challenging to evolve the commercially viable Cu oxides/hydroxides with high energy/power density, specific capacitance, and excellent cycling stability (Dong et al., 2014). Therefore, it will be worthy to make a thorough research on CuO or Cu2O electrodes grown on CCs.

In order to improve the kinetics and electrochemical performance of electrodes, two typical methods are usually employed. One straightforward approach is to increase the specific surface area of electrodes to provide more active sites for faradaic redox reaction (Daoping et al., 2014). The other method is to improve the conductivity of electrode material to accelerate electron conduction (Lu et al., 2013). Herein, we firstly synthesized Cu₂O/CuO nanosheets directly grown on CCs which is coated with Cu film by a simple strategy with electroless copper plating, chemical etching and thermal dehydration. The uniform Cu film on carbon microfiber cloth has a strong binding force. In addition, Cu₂O/CuO nanosheets in situ grown on CCs provide sufficient active sites for charge/discharge electronic, which is important for energy storage of supercapacitor. Finally, it is worth mentioning that there are still Cu films between CCs and Cu₂O/CuO nanosheets after chemical etching, which is important for promoting electronic conduction.

The schematic illustration of the growth process of Cu₂O/CuO@Cu-CCs electrodes is shown in Figure 1. Firstly, the Cu film is uniformly coated on the CCs through electroless copper plating, forming Cu-CCs samples. Subsequently, Cu(OH)₂ nanosheet arrays are in situ grown on Cu film by alkaline oxidative enchanting in NaOH and (NH₄)₂S₂O₈ solution, during which the oxidative S₂O82- is attached on the surface of Cu-CCs, and partial CuO are oxidized to Cu²⁺(Chen et al., 2016). With the reaction of Cu²⁺ and OH⁻, Cu(OH)₂ nanosheet arrays are formed and then are decomposed into Cu₂O/CuO nanosheets by thermal dehydration, thus obtaining Cu₂O/CuO@Cu-CCs electrodes

XRD patterns of CCs, Cu-CCs and Cu₂O/CuO@Cu-CCs are shown in Figure 2a. As can be seen by comparing peaks of CCs and Cu-CCs, the Cu films on CCs leads to the decrease of the characteristic peaks of carbon fiber at 2θ = 26.4°. For Cu-CCs samples, there are two strong diffraction peaks at 2θ = 43.5 and 50.6°, corresponding to the (111) and (200) planes of the metallic copper (JCPDS no. 04-0836), respectively (Chen et al., 2016). After the heat treatment at 120°C, the sample exhibits four peaks at 2θ of 35.6, 36.4, 39.1, 42.3°, in which 2θ = 35.6 and 39.1° correspond to (−111) and (200) planes of the CuO substrates (JCPDS no. 48-1548), while the else two peaks (2θ = 36.4 and 42.3°) are attributed to the (111) and (200) reflections of Cu₂O (JCPDS no. 05-0667). It is worthy to note that Cu and CCs peaks are still observed, therefore, the composition is confirmed to be Cu₂O/CuO@Cu-CCs.

Shown in Figure 2b is the SEM image of bare CCs, it can be observed that the surface of the carbon fibers is smooth and the diameter is around 8–10 μm. In Figure 2c, the carbon fibers are uniformly coated with copper films. After being etched in alkaline solution, the morphologies of Cu₂O/CuO nanosheets vary with the change of etching time (Supplementary Figure 1). With an etching time of 25 min, the sample exhibits a highly porous cross-linked structure with abundant thin Cu₂O/CuO nanosheets (Figure 2d). The lamellar nanosheets can effectively increase the number of active sites, which may be beneficial for promoting charge transfer and redox reaction (Liu Y. et al., 2018). As shown in Figure 2e, the porous Cu₂O/CuO nanosheets are ultra-thin, which may enlarge the specific surface area to accelerate the intercalation and de-intercalation of ions (Chen et al., 2016). In addition, the HRTEM image in Figure 2f shows that the measured interplanar spacing of 0.250 and 0.226 nm for the well- defined lattice fringes are consistent well with the (−111) and (200) plane of CuO (JCPDS no. 48-1548), and there is a part of interplanar distances calculated to be 0.214 nm, which can be directed as the (200) plane of Cu₂O (JCPDS no. 05-0667)

The XPS spectrums of the surface atomic composition and chemical state of the Cu₂O/CuO@Cu-CCs samples are obtained by Gaussian curve-fitting. As illustrated in Figure 3A, the complete spectrum indicates the existence of C, Cu, and O elements in the sample. As shown in Figure 3B, there are two sharp peaks located at 932.43 and 952.5 eV, which are correspond to Cu 2p3/2 and Cu 2p1/2, respectively, illustrating the coexistence of Cu⁺ and Cu⁰ species (Wan et al., 2017). At the same time, there are three satellite peaks with binding energies of 934.2, 943.1, and 953.9 eV indicated the existence of CuO in the samples. Therefore, it can be concluded that the copper is mainly Cu²⁺, Cu⁺, and Cu⁰ (Liu Y. et al., 2018). The existence of Cu⁰ can be beneficial for improving the electronic conduction of electrodes. As shown in Figure 3C, the O 1s XPS spectrum can be deconvoluted into two peaks, one is the peak at 530.3 eV, which represents the oxygen in Cu₂O lattice. Another is the high intensity peak at 531.1 eV, which is attributed to the CuO. This result further confirms the coexistence of CuO and Cu₂O (Singh and Sarkar, 2017). In the Figure 3D, C 1s spectrum shows a high intensity peak at 284.5 eV, demonstrating that the intensity of C-C functional group peak is notably strong, further illustrating carbon fiber is stable in Cu₂O/CuO@Cu-CCs.

The CV curves of CCs, Cu-CCs and Cu₂O/CuO@Cu-CCs at a scan rate of 30 mV s⁻¹ are shown in Figure 4A. It is obvious that strong pair of anodic and cathodic peaks is clearly visible for Cu₂O/CuO@Cu-CCs and Cu-CCs samples, mainly due to the Faradaic redox reaction (Dong et al., 2014). For Cu-CCs, copper ions mainly derive from the reaction of Cu and KOH electrolyte solution during electrochemical measurement. The pronounced pseudocapacitive characteristic of Cu₂O/CuO@Cu-CCs is mainly attributed to the porous cross-linked Cu₂O/CuO nanosheets while the contribution of capacitance for CCs can be negligible. Figure 4B shows the GCD curves of different electrodes at a constant current density of 10 mA cm⁻². The non-linear behavior of GCD curves further verifies that the main sources for charge storage originate from Faradaic reactions. The Cu₂O/CuO@Cu-CCs electrode discussed above is the sample etched for 25 min (CV and GCD curves of other samples are shown in Supplementary Figures 2A,B), and this sample shows the best pseudocapacitive characteristic with a specific capacitance of 1.71 F cm⁻² (835.2 F g⁻¹) at 10 mA cm⁻² (3.57 A g⁻¹) (Figure 4C), which is outperform the previously published values of Cu₂O/CuO-based electrodes (1.674 F cm⁻², equivalent to 594.27 F g⁻¹, at 2 mA cm⁻²; 839.9 Fg⁻¹, at 1 mVs⁻¹; 357 F g⁻¹, at 10 A g⁻¹) and more exhaustive data were displayed in Supplementary Table 4. The EIS analysis was studied to further clarify the electrochemical behaviors of different electrodes. The Nyquist diagrams are shown in Figure 4D, which consist of an approximate semicircle in the high-frequency region and a line in the low-frequency region. All real-axis intercepts are as low as approximately 0.5 Ω, illustrating all the samples have excellent electronic conduction due to the CCs and Cu-CCs current collectors. The depressed semicircle at the high frequency region corresponds to charge transfer resistance (Rct) caused by Faradaic reactions (Ensafi et al., 2018). The Cu₂O/CuO@Cu-CCs electrode has the smallest semicircle, illustrating an enhanced charge transfer. Also, the straight line in low-frequency region can be ascribed to Warburg impedance related to the fast charge diffusion in the electrolyte (Ensafi et al., 2018).

The electrochemical performances of Cu₂O/CuO@Cu-CCs at various scan rates and current densities (Figures 4E,F) demonstrate a perfect reversibility during the charge-discharge process. Clearly, the slope of GCD curves decline suddenly at 0.18–0.25 V in charge part and the same as discharge part, corresponding the pseudocapacitance behavior in the CV scans, which is associated with the Faradaic redox reactions of Cu²⁺/Cu⁺ redox pairs related to OH⁻ as bellows (Guan et al., 2017; Sami et al., 2017).

Remarkable, with the current density increases from 5 to 30 mA cm⁻², the GCD curves present a gradually decreased discharge time but tends to preserve similar shape (Figure 4F) and the electrode retains 68.5% of its capacitance (Figure 4G), suggesting an excellent rate capability. Furthermore, the Cu₂O/CuO@Cu-CCs electrode delivers excellent cycling stability with only 14.4% loss in specific capacitance after 5,000 cycles at 5 mA cm⁻² (Figure 4H), which can be explained by the stable structure of electrodes after cycling (the inset in Figure 4H).

For further exploring of the application, the electrochemical performances of the ASC device are investigated. As shown in Figure 5A, the device is sandwiched with the Cu₂O/CuO@Cu-CCs positive electrode, active carbon negative electrode, and diaphragm separator soaked with 6 M KOH aqueous solution. Figure 5B shows the exactly complementary potential windows range of simple AC and Cu₂O/CuO@Cu-CCs electrode, which suggest the high potential window of the ACS device. Furthermore, the calculated mass ratio of the electroactive materials of negative and positive electrodes according to Equation (3) is about 1:20. Figures 5C,D show the CV curves at a scan rate of 30 mV s⁻¹ and GCD curves at a current density of 1 A g⁻¹ with different potential windows, respectively. It is obvious that the shapes of the CV curves stay nearly same at different potential windows and the maximum potential window is extended to 1.6 V. The perfect symmetry and nearly unchanged shapes at different potential windows of GCD curves also contribute to the outstanding capacitive performance of this ASC device.

Figure 5E shows the CV curves of the ASC device at a scan rate ranging from 10 to 100 mV s⁻¹. Apparently, the excellent synergy effect of the two electrodes leads to the high operation voltage of 1.6 V, which is three times as wide as the potential window of Cu₂O/CuO@Cu-CCs electrode in the three-electrode system. Meanwhile, the curve shape retains the same at different scan rates. The GCD curves at current densities from 0.5 to 10 A g⁻¹ are shown in Figure 5F. It is obvious that very low voltage drops are visible compared with three-electrode test even at high current densities. And the symmetrical shape indicates high reversibility of the device. Thus, the device shows excellent rate capability (Supplementary Figure 3). In addition, owing to the broad potential window and huge specific capacitance, the ASC device shows a high energy density of 60.26 Wh kg⁻¹ at a power density of 299.73 W kg⁻¹, higher than some other literatures (Figure 5G). In order to investigate the long-term cycling stability and durability of the device, we performed 5,000 continuous GCD cycles at a current density of 2 A g⁻¹. The ASC device exhibits an excellent cycling stability with keeping 90.35% in its specific capacitance after 5,000 GCD cycles (Figure 5H). This kind of electrode material will be a promising electrode for further engineering all-solid-state high-performance supercapacitor due to its excellent capacitor performance and flexibility characteristic.

In short, we constructed Cu₂O/CuO@Cu-CCs electrodes by a simple process with electroless copper plating, chemical etching and thermal dehydration. The ASC device with Cu₂O/CuO@Cu-CCs positive electrode and AC negative electrode showed high energy density of 60.26 Wh kg⁻¹ at a power density of 299.73 W kg⁻¹ using 6 M KOH aqueous solution as the electrolyte. Also, the ASC device express an excellent cycling stability with keeping 90.35% in its specific capacitance after 5,000 GCD cycles. Also, this kind of electrode material will be a promising electrode for further engineering all-solid-state high-performance supercapacitor.

The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.

LX synthesized Cu₂O/CuO@Cu-CCs samples and analyzed part of characterizations. JL was the supervisor of this research work. HS and XG helped with the data analysis. JX analyzed XPS measurements. HZ organized a part of the data. XZ supplemented a part of the experiment.