Preparation of binder-free CuO/Cu₂O/Cu composites: a novel electrode material for supercapacitor applications

Abstract

Cuprous(I) oxide (Cu₂O) carries high theoretical specific capacitance (2247.6 F g⁻¹), however, the amount of research about the supercapacitive performance of Cu₂O is relatively small compared with other transition metal oxides. A composite of metal and metal oxide could improve the electrochemical performance efficiently. In this work, the results of XRD and XPS demonstrate that CuO/Cu₂O/Cu is prepared successfully via a facile, eco-friendly, one-step template-free growth process. SEM figures show that cubic CuO/Cu₂O/Cu uniformly and densely covers a skeleton of nickel foam. The binder-free CuO/Cu₂O/Cu electrode exhibits excellent supercapacitive performance with a high specific capacitance of 878 F g⁻¹ at a current density of 5 mA cm⁻² (1.67 A g⁻¹), when the current density is enlarged ten times (50 mA cm⁻² (16.7 A g⁻¹)), the specific capacitance still remains at 545 F g⁻¹. Furthermore, we have first successfully constructed a CuO/Cu₂O/Cu//AC asymmetric supercapacitor, which can achieve an energy density of 42 W h kg⁻¹ at a power density of 0.44 kW kg⁻¹. The good electrochemical performance and simple accessibility prove that the as-prepared CuO/Cu₂O/Cu/NF electrode has a potential application in electrochemical capacitors.

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1. Introduction

Supercapacitors have attracted great attention^1,2 due to the merits of longer cycle life, and higher power density compared with other energy storage devices,^3–5 such as fuel cells, batteries, and conventional capacitors.

According to the mechanism of electric storage, there are two kinds of supercapacitors: (i) electrochemical double layer capacitors (EDLCs) based on carbon materials (activated carbon, graphite, nanotubes (CNTs), etc.) as the electrode, storing energy by adsorption of charges at the surface between the electrode and electrolyte, which usually show lower capacitance and (ii) faradic capacitors based on transition metal oxide or conducting polymer materials as the electrode, storing energy by redox reactions, which always display considerably higher specific capacitance.⁶ Because of the features of lower cost, larger abundance, and lower toxicity, many transition metal oxides, such as MnO_x,^7,8 Ni(OH)₂,^9,10 Co(OH)₂,^11,12 CuO,^13–15 Fe₂O₃,^16–18 V₂O₅,^19,20 and SnO₂,^21,22 have been widely investigated as electrode materials for supercapacitors. However, there is another neglected metal oxide in this field, Cu₂O, which is extensively attractive due to its promising applications in carbon monoxide (CO) oxidation,²³ photocatalysts,²⁴ solar-driven water splitting,²⁵ solar energy conversion,²⁶ negative electrode materials for lithium-ion batteries²⁷ and antibacterial activity.²⁸ To the best of our knowledge, research about the systematic investigation of the supercapacitor performance of Cu₂O is rare and there are not many papers on fabricating an asymmetric supercapacitor with Cu₂O as the positive electrode (as summarized in Table 1). Most importantly, the theoretical capacitance of Cu₂O that can be achieved is as high as 2247.6 F g⁻¹, calculated from the equation: n × F/m × V,²⁹ where n is the number of electrons transferred in the redox reaction, F is the Faraday constant, m is the molecular weight of the metal oxide and V is the charging voltage range, which predicts the potential application of Cu₂O in a supercapacitor. Therefore, further investigations, such as into easier preparation methods, nanoscale morphology, and better supercapacitor performance, are needed for a sensible assessment of its actual potential.

Table 1 Summary of electrochemical data of cuprous oxide based electrodes

Materials	Structure	Current density	Cs (F g^−1)	Ref.
Cu2O	Powder	0.1 A g^−1	144	30
Cu2O@Cu	Nanoneedle array	2.9 A g^−1	510.2	41
Cu2O/Ni	—	500 mV s^−1	427	46
Cu2O/RGO	Cubic particles	1 A g^−1	98.5	34
Cu2O/RGO	Powder	100 mA g^−1	31	31
Cu2O/CuO/RGO	Powder	1 A g^−1	173.4	32
Cu/Cu2O/graphene	Powder	0.1 A g^−1	257	33
Cu2O/Cu	Cube on nickel foam	5 mA cm^−2 (1.67 A g^−1)	878	This work

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As seen in Table 1, the common preparation technique for Cu₂O electrodes is to firstly synthesise pure Cu₂O³⁰ or Cu₂O and carbonous material composites^31–34 and then smear the electrochemically active materials with a conductive agent and binder onto the current collector. Compared to the above synthesis method, active materials directly grown on conductive substrates are more beneficial to improving supercapacitor performance, because the design of active materials directly grown on a three-dimensional current collector could efficiently facilitate the transport of electrons and ions, improving the utilization of active materials and substantially enhancing the energy density and power density. The most commonly used substrate in supercapacitors and batteries is nickel foam due to its porous architecture, large surface area, good electric conductivity and excellent stability in various kinds of liquid electrolytes.

Herein, we report a successful preparation of self-supported cubic cuprous oxide and copper composites on nickel foam (CuO/Cu₂O/Cu/NF) via a simple template-free hydrothermal method. We characterized the microstructure of the electrode and investigated systematically the electrochemical performance. The CuO/Cu₂O/Cu/NF electrode displays excellent supercapacitor properties. Besides, we firstly fabricate the asymmetric supercapacitor using the CuO/Cu₂O/Cu/NF as the positive electrode and activated carbon (AC) as the negative electrode, which achieves a high energy density of 42 W h kg⁻¹. This indicates that the future of CuO/Cu₂O/Cu/NF in practical applications for energy storage is promising.

2. Experimental

2.1. Preparation and characterization of the positive electrode

All of the chemicals were of analytical grade and were used without further purification. 4.832 g of Cu(NO₃)₂·3H₂O was first dissolved in 60 mL of deionized water under continuous magnetic stirring to form a homogeneous solution. Then, 1.2 g of CO(NH₂)₂ (urea) was added into the above solution and the mixture was stirred for another 15 min to obtain a homogeneous solution, which was then transferred into a Teflon-lined autoclave with a volume of 80 mL. A piece of nickel foam (30 mm × 50 mm × 1.5 mm, 110 PPI, 320 g m⁻², Changsha Lyrun Material Co., Ltd. China) was degreased with acetone, etched with 6.0 mol dm⁻³ HCl for 5 min, rinsed with deionized water and then immersed into the above autoclave. The autoclave was first purged with N₂ to remove oxygen and was heated at 140 °C for 14 h to induce the direct growth of CuO/Cu₂O/Cu composites on the nickel foam. After cooling down naturally to room temperature, the electrode material was picked out, washed with deionized water and dried in a vacuum oven at 60 °C. The scheme is briefly shown in Scheme 1.

Scheme 1 Scheme for the preparation of the Cu₂O/Cu/NF electrode.

The morphology was examined using a scanning electron microscope (SEM, JEOL JSM-6480) and transmission electron microscope (TEM, FEI Teccai G2 S-Twin, Philips). The crystallographic phases of all of the samples were investigated using an X-ray diffractometer (XRD, Rigaku TTR III) with Kα radiation (λ = 0.1506 nm) at a scan rate of 10° min⁻¹ with a step width of 0.02°.

2.2. Electrochemical measurements

Electrochemical tests were carried out in a conventional three-electrode electrochemical cell using a computerized potentiostat (VMP3/Z Bio-Logic) controlled by the EC-lab software. The as-prepared electrode (1 × 1 cm² nominal planar area) acted as the working electrode, platinum foil (1 × 2 cm²) served as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The asymmetric supercapacitor, using the CuO/Cu₂O/Cu/NF nanosheet as the positive and AC as the negative electrode, was tested in a two-electrode electrochemical cell. The cycle life tests were conducted on a Land battery program-control test system. All of the electrochemical measurements were performed in 6.0 mol dm⁻³ KOH electrolyte. The solutions were made with analytical grade chemical reagents and Milli-Q water (18 MΩ cm, Millipore). EIS measurements were performed by applying an alternating voltage with a 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz at the open circuit potential.

3. Results and discussion

3.1. Characterization of electrodes

The crystal structure of the electrode was firstly studied via XRD analysis. As can be seen in Fig. 1a, the peaks located at 43.5, 50.4 and 74.2 can be ascribed to the (111), (200) and (220) planes of metallic Cu (JCPDS file no. 03-1015), respectively. The diffraction peaks at 29.5, 36.4, 42.3, 61.3, 73.5 and 77.3 can be attributed to the (110), (111), (200), (220), (311) and (222) crystal planes of cubic Cu₂O (JCPDS file no. 78-2076), respectively. The position of the strongest peak corresponds to the (111) crystal plane of Cu₂O, indicating the [111] preferential orientation.^35–37 Fig. 1b shows the structure of the (111) surface of Cu₂O, the ordered pore framework between Cu and O supplies electrolyte accessible channels, ensuring more effective reduction oxidation reactions to supply capacitance, resulting in considerably increased specific capacitance and a decrease in concentration polarization at a high current, leading to improved power density.

Fig. 1 (a) XRD patterns of the CuO/Cu₂O/Cu/NF electrode, (b) structure of the Cu₂O (111) crystal plane, (c) XPS spectrum of Cu 2p, and (d) EDS images of the CuO/Cu₂O/Cu/NF electrode.

XPS was performed in the Cu 2p region to further identify the valence state of the Cu in the composite and the spectra were fitted in Fig. 1c. The main peak at the binding energy of 933.0 eV and the satellite peak at 943.6 eV were used for decomposition of the Cu peak. The broad Cu 2p_3/2 peak at 933.0 eV can be divided into three peaks (932.46, 933.24 and 935.1 eV), corresponding to Cu₂O, Cu and CuO,^38,39 respectively. The reason for the existence of CuO may be due to the quite small fraction of superficial oxidation during sample drying, because no corresponding peaks for CuO are observed in the XRD pattern.^39,40 It is acceptable that the result from the XPS is in accordance with the XRD, further indicating that the composite of Cu₂O and Cu has been prepared successfully.

In addition, the EDS (Fig. 1d) reveals that the molar ratio of Cu : O is 69.5 : 30.5 with a chemical formula of Cu_2.28O, demonstrating that the primary phase of the CuO/Cu₂O/Cu composites is Cu₂O, which matches well with the results of the XRD and XPS. It should be pointed out that metallic Cu, as a perfect electric conductor, favors the transportation of electrons, improving the utilization of active materials, achieving substantially high specific capacitance.

Fig. 2a is a SEM image of the nickel foam substrate, showing a three dimensional network structure supplying a large surface area for supporting active materials. Fig. 2b is a SEM image at low magnification, showing that CuO/Cu₂O/Cu forms uniform cubes and densely covers the skeleton surface of the nickel foam. At high magnification (Fig. 2c), the CuO/Cu₂O/Cu cubes cross over each other forming a coarse surface, which could increase the contact area of the electrode and electrolyte, improving the specific capacitance. Fig. 2d shows a TEM image, in which we can clearly see that the edge length of the cube is about 1.75 μm. In the magnified figures, the lattice spacings measured are about 0.208 nm and 0.246 nm, which fit close to the (111) crystal plane of Cu and (111) crystal plane of Cu₂O (Fig. 2e), respectively. Definitely, the direct contact of the CuO/Cu₂O/Cu composite to the current collector (nickel foam) avoids the conductive additive agent and polymer binder, which facilitate the transportation of electrons and ions, making the most use of the active materials, guaranteeing effective and rapid oxidation and reduction reactions to provide capacitance.

Fig. 2 SEM images of (a) nickel foam, and (b) and (c) the CuO/Cu₂O/Cu/NF electrode. (d) TEM image of Cu₂O/Cu, and (e) HRTEM of CuO/Cu₂O/Cu.

3.2. Electrochemical properties of the electrodes

The CuO/Cu₂O/Cu/NF as a positive electrode was investigated first via three-electrode measurements in 6 M KOH.

The CV curves of the CuO/Cu₂O/Cu/NF electrode at different scan rates are shown in Fig. 3a. An obvious pair of redox peaks are clearly seen, indicating that the faradic reactions of Cu₂O when the electrode is charged and discharged is reversible and continuous. According to literature reporting on the meaning of redox peaks of copper oxide or copper hydroxide in KOH electrolytes,^{29,32,41–44} the charge storage mechanism of the CuO/Cu₂O/Cu/NF electrode in the potential range of 0–0.5 V is suggested to be as follows:

½Cu₂O + OH⁻ ⇌ CuO + ½H₂O + e⁻ (1)

Fig. 3 (a) CV curves of the CuO/Cu₂O/Cu/NF electrode at different scan rates and (b) peak current as a function of square root of scan rate.

As the scan rate is increased, the redox peaks shift to high and low potentials, respectively, implying the quasi-reversible feature of the redox reactions. In general, the peak current ratio i_pa/i_pc in the CV curves is a useful judgement of reversibility. This ratio would be unity under ideal conditions. For our studied systems, the value is 1.07, approximately unity, at a scan rate of 5 mV s⁻¹, demonstrating an excellent reversibility of electrochemical processes occurring in Cu₂O, as given by eqn (1).

The peak currents of i_pa (anodic peak current) and i_pc (cathodic peak) against ν^1/2 are shown in Fig. 3b. It is widely known that we can deduce the electrochemical reaction mechanism from the following relationship: i = aν^b (b = 1.0 indicates a solid-phase diffusion controlled process; b = 0.5 indicates a surface confined charge transfer process). As can be seen, the two better linear relationships between scan rate and peak current, of which the regression equations are i_pa (mA) = 30.36ν^1/2 (mV s⁻¹) − 19.41 (R² = 0.999) and i_pc (mA) = −24.83ν^1/2 (mV s⁻¹) + 9.19 (R² = 0.999), indicate that the redox reactions of the CuO/Cu₂O/Cu/NF electrode are diffusion controlled processes. The remaining double-layer capacitance and background current lead to the non-zero intercepts on the fitting line.⁴⁵

Additionally, Fig. 4a shows GCD curves at current densities from 5 mA cm⁻² to 50 mA cm⁻². According to the CV curves (Fig. 3a), to avoid the oxygen evolution destroying the electrode structure in the process of charging, we chose 0–0.4 V as the GCD potential range. Obviously, the charge time is equal to the corresponding discharge time for all of the curves, indicating that the electricity charged can be released completely, which demonstrates the perfect reversibility of the CuO/Cu₂O/Cu/NF electrode. The area specific capacitance (C_s (F cm⁻²)) and mass specific capacitance (C_m (F g⁻¹)) are calculated based on the following equations:

where I_d (A) is the charging/discharging current, Δt (s) is the discharging time, ΔV (V) is the potential window, S (cm²) is the area of the CuO/Cu₂O/Cu/NF electrode (1 × 1 cm²) and m (mg) is the mass loading of active materials. The relationship of the calculated specific and area capacitance to the discharge current density is displayed in Fig. 4b. The area and mass specific capacitance values of the CuO/Cu₂O/Cu/NF electrode are 2.64 F cm⁻² (878 F g⁻¹), 2.33 F cm⁻² (779 F g⁻¹), 1.99 F cm⁻² (661 F g⁻¹), 1.77 F cm⁻² (591 F g⁻¹) and 1.64 F cm⁻² (545 F g⁻¹) at the current densities of 5 (1.67), 10 (3.33), 20 (6.67), 30 (10) and 50 (16.7) mA cm⁻² (A g⁻¹), respectively. The capacitance values of the CuO/Cu₂O/Cu/NF electrode obtained in this work were higher than present Cu₂O materials. For example, Cu₂O powder prepared by Chen et al. via the polyol reduction method exhibits a specific capacitance of 144 F g⁻¹ at a current density of 0.1 A g⁻¹;³⁰ Deng et al. synthesized Cu₂O on three-dimensionally ordered macroporous Ni with an expensive polystyrene bead template displaying 502 F g⁻¹ at 10 mV s⁻¹;⁴⁶ the Zhang et al. group developed Cu₂O via a complex two-step electrochemical process, exhibiting 510.2 F g⁻¹ at 2 A g⁻¹.⁴¹ Furthermore, the binder-free direct growth of CuO/Cu₂O/Cu on nickel foam results in a much superior supercapacitor performance compared to the composites of Cu₂O and carbonous materials (Cu₂O/RGO, 98.5 F g⁻¹ at 1 A g⁻¹;³⁴ Cu₂O/CuO/RGO, 173.4 F g⁻¹ at 1 A g⁻¹;³² Cu/Cu₂O/graphene, 257 F g⁻¹ at 0.1 A g⁻¹;³³ Cu₂O/RGO, 31 F g⁻¹ at 100 mA g^−1 31). In addition, a high specific capacitance of 545 F g⁻¹ at the not too high current density of 50 mA cm⁻² (16.7 A g⁻¹) is scarce for transition metal oxide materials, such as MnO₂ (381 F g⁻¹ at 10 A g⁻¹),⁴⁷ Co(OH)₂ (385 F g⁻¹ at 16 A g⁻¹),⁴⁸ and NiO (368 F g⁻¹ at 20 A g⁻¹).⁴⁹ The superior capacitance can be rationalized as follows. First, the 3D continuous skeleton of nickel foam supplies a large surface area for supporting more active material to produce capacitance and the 3D porous structure provides efficient electrolyte diffusion channels for oxidation and reduction reactions. Second, the cubic CuO/Cu₂O/Cu directly grown on nickel foam serving as an integrated electrode without a conductive agent and binder could form an expedited transportation path for electronics, reducing electrochemical polarization. Third, the interlaced cubic CuO/Cu₂O/Cu expands the contact area of the electrolyte and active materials, improving the utilization of active materials. Furthermore, the existence of metallic Cu could definitely associate the transportation of electrons in the composite. Therefore, considering the easy one-step template-free hydrothermal synthesis method and the perfect electrochemical performance, our work is meaningful to the research into Cu₂O as a supercapacitor electrode.

Fig. 4 (a) GCD curves of the CuO/Cu₂O/Cu/NF electrode at different current densities and (b) variation of specific capacitance as a function of current density.

Fig. 5a presents the cycling stability of the CuO/Cu₂O/Cu/NF electrode which was performed at a current density of 10 mA cm⁻². The coulombic efficiency could be calculated via the equation as follows:

where t_c is the charge time and t_d is the discharge time. The specific capacitance gradually decreases for the first 1000 cycles, after which it was almost constant at ∼529 F g⁻¹ after 8000 cycles, indicating that the CuO/Cu₂O/Cu/NF electrode has been in a stable state after 1000 continuous and repetitive charge–discharge cycles. The morphology after 8000 cycles is shown in the inset of Fig. 5a, indicating that the electrode still remained cubic in structure. A reason for the capacitance decrease may be the falling off of active materials. Moreover, the coulombic efficiency remains about 100% within 8000 cycles.

Fig. 5 (a) Specific capacitance retention and the coulombic efficiency on the charge/discharge cycle numbers at a current density of 10 mA cm⁻² for the Cu₂O/Cu/NF electrode; and (b) representative CV curves of the CuO/Cu₂O/Cu/NF electrode after the 1^st, 1000^th and 8000^th charge–discharge cycle.

The representative CV curves for the electrode are shown in Fig. 5b. The area enclosed by the CV curve after the 8000^th cycle is much smaller than that for the first cycle of the electrode, but is overlapping with that of the 1000^th, which is in accordance with the result of Fig. 5a.

EIS is tested to evaluate the reaction kinetics. The Nyquist plots of the fresh CuO/Cu₂O/Cu/NF electrode and the electrode after 8000 charge/discharge cycles are shown in Fig. 6 and were measured at open circuit potential. Both plots show two distinct sections: in the high frequency region, there is a semicircle, of which the diameter represents the charge transfer resistance R_ct (Ω) and the calculated R_ct of the CuO/Cu₂O/Cu/NF electrode before and after cycling is 0.05 Ω cm⁻² and 0.15 Ω cm⁻², respectively; and a straight line with a slope around 75°, exhibiting the features of the supercapacitor. The above results indicate that the CuO/Cu₂O/Cu/NF electrode is a suitable supercapacitor electrode material.

Fig. 6 (a) CV curves of the CuO/Cu₂O/Cu/NF//AC asymmetric supercapacitor at different scan rates, (b) GCD curves of the Cu₂O/Cu/NF//AC asymmetric supercapacitor at different current densities, (c) a Ragone plot related to energy and power densities of the Cu₂O/Cu/NF//AC asymmetric supercapacitor, and (d) the cycle performance of the Cu₂O/Cu/NF//AC supercapacitor.

3.3. Electrochemical properties of the asymmetric supercapacitor

We first fabricated the asymmetric supercapacitor by utilizing CuO/Cu₂O/Cu/NF and AC as positive and negative electrodes, respectively. The electrochemical performance of the asymmetric supercapacitor CuO/Cu₂O/Cu/NF//AC was investigated via two-electrode measurements in 6 M KOH.

The CV curves at various scan rates of the asymmetric supercapacitor are shown in Fig. 6a. Because of the mutual effect of the positive and negative electrodes, the operation potential window could achieve as high as 1.6 V. Unlike the CV feature of the CuO/Cu₂O/Cu/NF electrode, owing to the influence of the electrochemical double layer of the negative electrode on the whole device, the device exhibits a quasi-rectangular CV shape. With the increasing scan rate, the shape of the CV curves remain unaltered, which demonstrates the high rate capability of the device. The constant current charge–discharge curves are shown in Fig. 6b. Similarly, the DCD curves display a triangle-like shape, rather than an obvious plateau, which is well consistent with the CV curves. In addition, the good symmetry of all of the charge and discharge curves indicates the excellent electrochemical reversibility of the device.

The specific capacitance (C_m (F g⁻¹)), specific energy (E_m (W h kg⁻¹)), and specific power (P_m (kW kg⁻¹)) were calculated according to the following equations:

where I_d (mA) is the current density, Δt (s) is the discharge time, V (V) is the discharge potential range, and m is the total mass of the two electrodes. The calculated electrochemical parameters for CuO/Cu₂O/Cu/NF//AC are summarized in Table 2. The assembled asymmetric capacitor achieved a high specific capacitance of 117 F g⁻¹ at a current density of 5 mA cm⁻² (0.56 A g⁻¹), which was comparable with recent reports for other asymmetric capacitors, such as CuO//AC (72.4 F g⁻¹ at 7.5 mA cm⁻²),⁵⁰ MnO₂//graphite (72.7 F g⁻¹ at 0.5 A g⁻¹),⁴⁷ V₂O₅//C (104 F g⁻¹ at 1 A g⁻¹)⁵¹ and Co₃O₄//AC (57.4 F g⁻¹ at 1 A g⁻¹).⁵² Furthermore, the as-assembled capacitor can deliver a high energy of 42 W h kg⁻¹ at a power density of 0.44 kW kg⁻¹. When the power density achieves 4.4 kW kg⁻¹, the energy density still remains at 18 W h kg⁻¹. The relationship of the energy density to the power density, namely the Ragone plot, is displayed in Fig. 6c. Such a performance is much higher than that of other copper based composite asymmetric supercapacitors such as CuO//AC (19.7 W h kg⁻¹ at 700 W kg⁻¹),⁵⁰ CuO//AC (29.4 W h kg⁻¹ at 12 W kg⁻¹),⁵³ Cu(OH)₂//AC (18.3 W h kg⁻¹ at 0.3 kW kg⁻¹),⁴² Cu₂O//AC (35.6 W h kg⁻¹ at 0.9 kW kg⁻¹)⁴¹ and CuO@rGO//rGO (31.2 W h kg⁻¹ at 1.69 kW kg⁻¹).⁵⁴ The high energy and power density reveal the practicability of our supercapacitor.

Table 2 Electrochemical parameters of Cu₂O/Cu/NF//AC

Current density (mA cm^−2)	Discharging time (s)	Specific capacitance (F g^−1)	Specific energy (W h kg^−1)	Specific power (kW kg^−1)
5	340	117	42	0.44
10	150	104	37	0.89
20	62	86	30	1.78
30	33	69	25	2.67
50	14	50	18	4.4

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The cycling-life test was performed for the CuO/Cu₂O/Cu/NF//AC asymmetric supercapacitor via constant current charge–discharge at 20 mA cm⁻² (Fig. 6d). As expected, the specific capacitance kept a similar trend compared with that of the single CuO/Cu₂O/Cu/NF electrode, decreasing first and remaining stable later after 5000 repeated charge–discharge cycles. The final achieved specific capacitance was 77% of the initial value and the coulombic efficiency remained at about 100%. These results indicate that CuO/Cu₂O/Cu/NF is promising for electrochemical applications as an efficient electrode material.

4. Conclusions

In conclusion, we successfully prepared cubic cuprous oxide and copper composites (CuO/Cu₂O/Cu) free-standing on nickel foam via a simple hydrothermal method. The CuO/Cu₂O/Cu/NF electrode in 6 mol dm⁻³ KOH electrolyte displays a superior electrochemical performance with a specific capacitance of 2.635 F cm⁻² (878 F g⁻¹) at a current density of 5 mA cm⁻² (1.67 A g⁻¹). Furthermore, the fabricated asymmetric supercapacitor can achieve a wide potential window of 0–1.6 V with a high energy density of 42 kW h kg⁻¹ at a power density of 0.44 kW kg⁻¹ and still remains at 18 kW h kg⁻¹ at a high power density of 4.4 kW kg⁻¹. These encouraging studies demonstrate the practical value of the low cost CuO/Cu₂O/Cu material in supercapacitors.

Acknowledgements

We gratefully acknowledge the financial support of this research from the National Natural Science Foundation of China (51572052, 21403044 and 51472077), the Heilongjiang Postdoctoral Fund (LBH-Z14054), and the Fundamental Research Funds for the Central Universities (HEUCF20151004), Program for New Century Excellent Talents in University (NCET-13-0779).

Notes and references

1. P. J. Hall, M. Mirzaeian, S. I. Fletcher, F. B. Sillars, A. J. R. Rennie, G. O. Shitta-Bey, G. Wilson, A. Cruden and R. Carter, Energy Environ. Sci., 2010, 3, 1238–1251.
2. X. Zhao, B. M. Sanchez, P. J. Dobson and P. S. Grant, Nanoscale, 2011, 3, 839–855.
3. X. Lai, J. E. Halpert and D. Wang, Energy Environ. Sci., 2012, 5, 5604–5618.
4. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
5. H. Wang and H. Dai, Chem. Soc. Rev., 2013, 42, 3088–3113.
6. T. Xue, C.-L. Xu, D.-D. Zhao, X.-H. Li and H.-L. Li, J. Power Sources, 2007, 164, 953–958.
7. G. Zhu, H. Li, L. Deng and Z.-H. Liu, Mater. Lett., 2010, 64, 1763–1765.
8. O. Ghodbane, F. Ataherian, N.-L. Wu and F. Favier, J. Power Sources, 2012, 206, 454–462.
9. D.-D. Zhao, S.-J. Bao, W.-J. Zhou and H.-L. Li, Electrochem. Commun., 2007, 9, 869–874.
10. H. Jiang, T. Zhao, C. Li and J. Ma, J. Mater. Chem., 2011, 21, 3818–3823.
11. L. Cao, F. Xu, Y. Y. Liang and H. L. Li, Adv. Mater., 2004, 16, 1853–1857.
12. V. Gupta, T. Kusahara, H. Toyama, S. Gupta and N. Miura, Electrochem. Commun., 2007, 9, 2315–2319.
13. G. Wang, J. Huang, S. Chen, Y. Gao and D. Cao, J. Power Sources, 2011, 196, 5756–5760.
14. Y.-K. Hsu, Y.-C. Chen and Y.-G. Lin, J. Electroanal. Chem., 2012, 673, 43–47.
15. J. Huang, H. Wu, D. Cao and G. Wang, Electrochim. Acta, 2012, 75, 208–212.
16. P. M. Kulal, D. P. Dubal, C. D. Lokhande and V. J. Fulari, J. Alloys Compd., 2011, 509, 2567–2571.
17. X. Xia, Q. Hao, W. Lei, W. Wang, D. Sun and X. Wang, J. Mater. Chem., 2012, 22, 16844–16850.
18. M.-S. Wu, R.-H. Lee, J.-J. Jow, W.-D. Yang, C.-Y. Hsieh and B.-J. Weng, Electrochem. Solid-State Lett., 2009, 12, A1–A4.
19. A. Ghosh, E. J. Ra, M. Jin, H.-K. Jeong, T. H. Kim, C. Biswas and Y. H. Lee, Adv. Funct. Mater., 2011, 21, 2541–2547.
20. G. Wee, H. Z. Soh, Y. L. Cheah, S. G. Mhaisalkar and M. Srinivasan, J. Mater. Chem., 2010, 20, 6720–6725.
21. R. K. Selvan, I. Perelshtein, N. Perkas and A. Gedanken, J. Phys. Chem. C, 2008, 112, 1825–1830.
22. W. Chen, R. B. Rakhi, L. Hu, X. Xie, Y. Cui and H. N. Alshareef, Nano Lett., 2011, 11, 5165–5172.
23. Q. Hua, T. Cao, H. Bao, Z. Jiang and W. Huang, ChemSusChem, 2013, 6, 1966–1972.
24. D. Barreca, G. Carraro, V. Gombac, A. Gasparotto, C. Maccato, P. Fornasiero and E. Tondello, Adv. Funct. Mater., 2011, 21, 2611–2623.
25. A. Paracchino, V. Laporte, K. Sivula, M. Grätzel and E. Thimsen, Nat. Mater., 2011, 10, 456–461.
26. R. N. Briskman, Sol. Energy Mater. Sol. Cells, 1992, 27, 361–368.
27. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496–499.
28. H. Pang, F. Gao and Q. Lu, Chem. Commun., 2009, 1076–1078,  10.1039/b816670f.
29. B. Vidhyadharan, I. I. Misnon, R. A. Aziz, K. P. Padmasree, M. M. Yusoff and R. Jose, J. Mater. Chem. A, 2014, 2, 6578–6588.
30. L. Chen, Y. Zhang, P. Zhu, F. Zhou, W. Zeng, D. D. Lu, R. Sun and C. Wong, Sci. Rep., 2015, 5, 9672.
31. B. Li, H. Cao, G. Yin, Y. Lu and J. Yin, J. Mater. Chem., 2011, 21, 10645–10648.
32. K. Wang, X. Dong, C. Zhao, X. Qian and Y. Xu, Electrochim. Acta, 2015, 152, 433–442.
33. Y. Li, Q. Wang, P. Liu, X. Yang, G. Du and Y. Liu, Ceram. Int., 2015, 41, 4248–4253.
34. X. Dong, K. Wang, C. Zhao, X. Qian, S. Chen, Z. Li, H. Liu and S. Dou, J. Alloys Compd., 2014, 586, 745–753.
35. L. Kim, J. Kim, D. Jung, C.-Y. Park, C.-W. Yang and Y. Roh, Thin Solid Films, 2000, 360, 154–158.
36. W. Gong, J.-F. Li, X. Chu, Z. Gui and L. Li, Acta Mater., 2004, 52, 2787–2793.
37. J. Q. Hu, Y. Bando, J. H. Zhan, Y. B. Li and T. Sekiguchi, Appl. Phys. Lett., 2003, 83, 4414–4416.
38. S. Shi, D. Gao, Q. Xu, Z. Yang and D. Xue, CrystEngComm, 2015, 17, 2118–2122.
39. B. Zhou, Z. Liu, H. Wang, Y. Yang and W. Su, Catal. Lett., 2009, 132, 75–80.
40. J. J. Teo, Y. Chang and H. C. Zeng, Langmuir, 2006, 22, 7369–7377.
41. C. Dong, Y. Wang, J. Xu, G. Cheng, W. Yang, T. Kou, Z. Zhang and Y. Ding, J. Mater. Chem. A, 2014, 2, 18229–18235.
42. P. Xu, K. Ye, M. Du, J. Liu, K. Cheng, J. Yin, G. Wang and D. Cao, RSC Adv., 2015, 5, 36656–36664.
43. S. M. Abd el Haleem and B. G. Ateya, J. Electroanal. Chem. Interfacial Electrochem., 1981, 117, 309–319.
44. J. Huang, P. Xu, D. Cao, X. Zhou, S. Yang, Y. Li and G. Wang, J. Power Sources, 2014, 246, 371–376.
45. W. Liu, C. Ju, D. Jiang, L. Xu, H. Mao and K. Wang, Electrochim. Acta, 2014, 143, 135–142.
46. M.-J. Deng, C.-Z. Song, P.-J. Ho, C.-C. Wang, J.-M. Chen and K.-T. Lu, Phys. Chem. Chem. Phys., 2013, 15, 7479–7483.
47. M. Huang, X. L. Zhao, F. Li, L. L. Zhang and Y. X. Zhang, J. Power Sources, 2015, 277, 36–43.
48. F. Cao, G. X. Pan, P. S. Tang and H. F. Chen, J. Power Sources, 2012, 216, 395–399.
49. X. Yan, X. Tong, J. Wang, C. Gong, M. Zhang and L. Liang, J. Alloys Compd., 2014, 593, 184–189.
50. S. E. Moosavifard, M. F. El-Kady, M. S. Rahmanifar, R. B. Kaner and M. F. Mousavi, ACS Appl. Mater. Interfaces, 2015, 7, 4851–4860.
51. Z. Lin, X. Yan, J. Lang, R. Wang and L.-B. Kong, J. Power Sources, 2015, 279, 358–364.
52. W. Liu, X. Li, M. Zhu and X. He, J. Power Sources, 2015, 282, 179–186.
53. V. Senthilkumar, Y. S. Kim, S. Chandrasekaran, B. Rajagopalan, E. J. Kim and J. S. Chung, RSC Adv., 2015, 5, 20545–20553.
54. D. P. Dubal, N. R. Chodankar, G. S. Gund, R. Holze, C. D. Lokhande and P. Gomez-Romero, Energy Technol., 2015, 3, 168–176.

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