Introduction

Of late significant investigations are made to develop sustainable and renewable energy storage devices with high energy, power and reliability to meet the challenges of fossil fuel depletion, global warming, atmospheric pollution, needs in medical and electronic equipments, use of intermittent energy sources, etc. (Ref 1,2,3,4). Electrochemical capacitors or supercapacitors have been considered as the most promising energy storage device for sustainable development of human beings than others like lithium-ion batteries, owing to their fast charging-discharging rate, high power density, excellent cyclic stability, safe operation and wide range of working temperature (Ref 5, 6). These may be utilized as primary storer of non-conventional energy like wind, sunlight, etc., and auxiliary (secondary) storer with batteries and fuel cells implanted in cars, mobiles, cordless tools etc., where high power is needed for limited span of time at least occasionally (Ref 7, 8). In general, supercapacitors can be classified into three categories: electrochemical double layer capacitors (EDLCs), pseudocapacitors and hybrid capacitors based on energy storage mechanism (Ref 9, 10). The energy storage mechanism in EDLCs is only non-faradic process. No redox reaction occurs between the electrode and electrolyte. Pseudocapacitors store charges mainly by reversible faradic redox reactions, where the combination of both faradic and non-faradic processes occur in hybrid supercapacitors (Ref 2, 11,12,13). The most significant component of electrochemical supercapacitors is active electrode material (Ref 14, 15). Therefore, to design and fabricate low price and environment friendly novel active electrode materials for high-performance supercapacitors is a crucial task. Nowadays, nickel oxide and copper oxide both are emerging electrode materials among various metal oxide pseudocapacitor electrode materials such as RuO2 and IrO2 etc., for pseudocapacitor application owing to their low cost, enormous thermal stability, natural abundance and eco-friendliness. Despite nickel oxide and copper oxide both have high theoretical value of capacitance (2573 and 1800 F g−1, respectively), their low electrochemical active surface area, poor electrical conductivity, and poor cycling stability limit their wide application in high-performance supercapacitors (Ref 16,17,18,19).

Mixed transition metal oxide (MTMO) is considered promising candidate for wide applications in catalysis, electrocatalysis, sensing, charge storing, etc., (Ref 20,21,22,23,24,25,26). MTMO nanocomposites improve the electrochemical performance by enhancing the electrochemical active surface area and charge transfer efficiency. Moreover, when a semiconductor metal oxide nanomaterial of narrow band gap is mixed with that of broad band gap, the composite may exhibit enhanced physiochemical properties compared with each single material. Among the various transition metal oxides, p-type CuO semiconductor oxide has narrow band gap (Eg = 1.2 eV), while p-type NiO semiconductor oxide has a wider band gap in the range of 3.6-4.0 eV (Ref 27). Homogeneous precipitation, co-precipitation, sol-gel, thermal decomposition and hydrothermal methods have been successfully employed for the synthesis of mixed metal oxides nanoparticles (Ref 12, 28,29,30). The physiochemical properties of these nanocomposites largely depend on their size, shape, surface area and porosities. It is reported that the lattices of NiO in NiO-based nanocomposites could produce more electrochemical active sites for supercapacitive activity (Ref 19, 31). For example, Li et al. (Ref 32) reported that coral-like mesoporous NiO nanobar could achieve 1085 F g−1 at 1 A g−1. Huang et al. (Ref 33) reported that the NiO nanosheets deposited on the nickel foams could exhibit 674.2 F g−1 at 1 A g−1. Yan et al. (Ref 34) reported that mesoporous NiO nanoflake arrays could exhibit 400 F g−1 at 2 A g−1. Du et al. (Ref 35) reported nano-NiO flower-like microspheres with a maximum specific capacitance of 762 F g−1 at 1 A g−1. Min et al. (Ref 36) reported that 2D NiO nanosheet could exhibit high capacitances of 418 F g−1 at the current density of 2 A g−1 with well cycling stability (still maintained 85% after 2000 cycles). The heterostructured semiconductor oxides involve electron-hole separation. Thus, charges are more stable due to slower rate of recombination. So, mixed oxides are expected to contain more active sites at any moment for electrochemical charge storage and charge transfer (Ref 37,38,39,40). Liu et al. (Ref 41) prepared NiO/MnO2 composite contained 20 wt.% of NiO by sol-gel method exhibited much larger specific capacitance of 453 F g−1 at the scanning rate of 10 mV s−1 in 6 M KOH electrolyte solution, than that of each pristine component. Zheng et al. (Ref 42) prepared ZnO-Au-NiO composites electrode material with core-shell morphology for supercapacitors which exhibit high areal capacitance of 4.1 F cm−2 at a current density of 5 mA cm−2, with excellent cycling stability. Ruan et al. (Ref 43) prepared a three-dimensional porous Ni(OH)2/Cu2O/CuO cluster by hydrothermal method exhibited excellent specific capacitance along with excellent cyclic stability. Ray et al. (Ref 44) reported that the three-dimensional mesoporous interlinked tube-like TiO2-V2O5 nanocomposite with molar ratio of 1:2 prepared by wet-chemical method exhibited excellent specific capacitance of 310 F g−1 at a scanning rate of 2 mV s−1 than individual TiO2 and V2O5 material. Yao et al. (Ref 45) prepared core-shell like NiO@NiCo2O4 nanostructures by solvothermal method, which can exhibit excellent specific capacitance along with high cyclic stability.

This idea stimulates us to prepare different CuO(x)NiO(100 − x) composites for different mole ratio, x/(100 − x) in order to get improved charge separation and hence synergistic result (Ref 46,47,48,49). Herein, we report a facile approach to prepare CuO and various CuO-NiO nanocomposites in absence of any special capping agent via hydrothermal method followed by calcination and tune the surface defects of these electrode materials by controlling the compositions. After assembling asymmetric supercapacitor with CuO(50)NiO(50) electrode material achieves a high energy and power densities of 3.14 Wh kg−1 and 507 W kg−1, respectively. In addition, the aggregated Janus-type CuO(50)NiO(50) nanocomposite electrode also exhibits excellent specific capacitance of 35.63 F g−1 at current density of 0.3 A g−1 and exhibits excellent cycling stability, as 85.7% of the capacitance still retained at 1000th cycle at a high current density of 10 A g−1 (Ref 50).

Experimental Section

General Information

The UV-vis spectra were taken by UV-1800 SHIMADZU UV-vis spectrophotometer. X-ray powder diffraction patterns (XRD) of as-synthesized nanoparticles were recorded by a Bruker AXS D8-advance instrument, with Ni-filtered CuKα radiation source (λ = 1.5418 Å) at 40 kV and 40 mA, employing a scanning rate of 0.2 degree per second. The morphology of the CuO(50)NiO(50) nanocomposite was obtained using a field emission scanning electron microscope (FESEM, FEI Qunta-250 FEG) operated at 20.00 kV. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected with a Hitachi H-7600 transmission electron microscope.

Materials

Nickel chloride hexahydrate (NiCl2·6H2O), copper sulfate pentahydrate (CuSO4·5H2O) and sodium hydroxide (NaOH) were purchased from Merck, India. All chemicals used were of analytical grade and used as received without any further purification. Milli-Q water was used in all experiments.

Synthesis of CuO, NiO and Various CuO-NiO Composites

In a typical optimized synthesis of CuO(100), 40 mL of 0.5 (M) CuSO4-5H2O was added into 40 mL of 1 (M) NaOH solution dropwise under constant stirring at 25 °C for 1 h in a beaker. The obtained blue solution was then transferred into a 100 mL Teflon-lined stainless steel autoclave and then heated at 180 °C for 24 h, and then it was allowed to cool at room temperature. The obtained black precipitate was separated from the solution by centrifugation at 6500 rpm for 5 min (REMI PR-24), washed with Millipore water for three times and finally with ethanol. Then the collected sample was dried in an oven at 80 °C for 24 h. Finally, the as-prepared sample was annealed at 400 °C for 6 h in a hot-air furnace. For synthesis of binary metal oxide composites containing CuO and NiO in the molar ratio of x/(100 − x), 0.4x mL 0.5 M CuSO4·5H2O solution was mixed with same volume of 1 M NaOH solution and 0.4(100 − x) mL of 0.5 M NiCl2·6H2O solution was also mixed with same volume of 1 M NaOH solution separately. The latter solution was added into the former solution with constant stirring for 1 h, at room temperature. Then, the whole mixture was taken in an autoclave for hydrothermal reaction at 180 °C for 24 h, and the rest procedure is maintained as that it was done for the synthesis of CuO(100) (Fig. 1). The general designation of synthesized material was given CuO(x)NiO(100 − x). By this way three different mixed oxides namely CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80) were prepared. In addition, similarly, NiO(100) was also synthesized by using 0.5 M NiCl2·6H2O aqueous solution under same method to investigate the synergistic property of CuO-NiO nanocomposites.

Fig. 1
figure 1

Schematic representation of the synthesis of CuO(x)NiO(100 − x) active electrode materials

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Electrochemical Studies

The electrochemical performance of the electrode material was greatly influenced by the structure and morphology of the nanocomposites. Electrochemical measurements such as cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed with a multichannel electrochemical analyzer (CS313, CorrTest, China) using a three-electrode system immersed in 1 M Na2SO4 aqueous solution as an electrolyte at room temperature, taking Ag/AgCl (sat. KCl), a platinum foil (1 cm × 1 cm) and the fabricated nanomaterials deposited on carbon support as reference, counter and working electrodes, respectively. To prepare the working electrode, a Teflon-coated graphite rod (4 mm diameter) was used as the current collector. The working electrode was prepared by mixing the as-prepared active electrode material (80 wt.%), activated carbon (15 wt.%) and polyvinylidene difluoride (PVDF, 5 wt.%) in 200 μL of N-methyl pyrrolidine (NMP) solution and sonicated it for 30 min to form a gel. Next, the gel was drop-casted on the one flat end of the Teflon-covered graphite rod, and finally the electrode was dried at 60 °C for 6 h, and the deposited mass of the as-synthesized active electrode material was approximately 1.0 mg. Electrochemical impedance spectroscopy study was performed in the frequency range of 0.01 Hz to 100 kHz with an AC perturbation amplitude of 10 mV. EC-Lab software was used to fit the EIS data. The accurate specific capacitance (Csp) in F g−1 for all CuO and CuO-NiO nanocomposite active electrode materials in three-electrode system has been calculated from the galvanostatic discharge curves based on Eq 1.

$$C_{\text{sp}} = \frac{I \times \Delta t}{m \times \Delta V}$$
(1)

where m is the mass of the active electrode material in working electrode (g), I/m is the current density (A g−1), Δt is the discharge time (s), and ΔV is the applied potential (V) window.

The energy density (Ecell) (Wh kg−1) and power density (Pcell) (W kg−1) of the fabricated active electrode materials were calculated using Eq 2 and 3, respectively,

$$E_{\text{cell}} = \frac{1}{2 \times 3.6} \times C_{\text{sp}} \times \Delta V^{2}$$
(2)
$$P_{\text{cell}} = \frac{{3600 \times E_{\text{cell}} }}{\Delta t}$$
(3)

where Csp is the specific capacitance (F g−1), Δt is the discharge time (s), and ΔV is the applied potential (V) window.

Results and Discussion

The UV-vis absorption spectra of the synthesized CuO nanoparticles and CuO-NiO nanocomposites are presented in Fig. 2. It is evident that the CuO(50)NiO(50) nanocomposite shows sharp absorption peak at 323 nm, while others synthesized nanomaterials exhibit weak absorption peaks. This clearly indicates that the decrease in the value of band gap of CuO(50)NiO(50) nanocomposite may be attributed to easy charge separation, improvement in the crystalline quality of the 3D nano network as well as to the decrease in grain size (Ref 51). The peak positions are shifted toward lower wavelength as mol.% of NiO is increased in CuO(x)NiO(100 − x) mixed materials, as evidenced in Fig. 2(a) and (b). Figure 2(b) is the expanded picture of a part of Fig. 2(a). The absorption peaks of CuO(100), CuO(80)NiO(20) and CuO(20)NiO(80) are 389, 348 and 264 nm, respectively.

Fig. 2
figure 2

(a) The UV-vis. spectra of diluted aqueous solutions of same mass of CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80) nanomaterials (b) parts of the same spectra in expanded form

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The crystal structure and phase purity of the fabricated nanomaterials were determined by x-ray diffraction (XRD). The profiles were presented in Fig. 3. Figure 3(a) shows the x-ray diffraction patterns of CuO(100) nanoparticles, CuO(80)NiO(20) and CuO(20)NiO(80) nanocomposites. The x-ray diffraction patterns of CuO(50)NiO(50) nanocomposite and NiO(100) are shown in Fig. 3(b) and (c), respectively. The diffraction peaks of CuO(100) at 32.55°, 35.56°, 38.80°, 48.79°, 53.6°, 58.37°, 66.29°, 68.17°, 72.5° and 75.3° correspond to the (110), (\(\bar{1}\)11), (111), (\(\bar{2}\)02), (020), (202), (\(\bar{3}\)11), (220), (311) and (\(\bar{2}\) 22) planes of end-centered-monoclinic CuO (JCPDS card No. 80-1268), while the diffraction peak at 2θ value of 61.4° correspond to the (220) plane of cubic Cu2O (JCPDS card No. 78-2076), respectively. The diffraction peaks of CuO(80)NiO(20), CuO(20)NiO(80), and CuO(50)NiO(50) nanocomposites at 32.55°, 35.56°, 38.80°, 48.79°, 53.6°, 58.37°, 66.29°, 68.17°, 72.5°, 75.3° correspond to the (110), (\(\bar{1}\)11), (111), (\(\bar{2}\)02), (020), (202), (\(\bar{3}\)11), (220), (311), (\(\bar{2}\)22) planes of end-centered-monoclinic CuO (JCPDS card No. 80-1268), while the peaks at 37.2°, 43.29°, 62.8°, 75.4°, and 79.4° are associated with (111), (200), (220), (311), and (222) planes of face-centered-cubic NiO (JCPDS card No. 78-0429), respectively. For the CuO(80)NiO(20) sample, while the rest of the diffraction peaks at 2θ values of 29.6° and 61.4° are due to the (110) and (220) planes of cubic Cu2O (JCPDS card No. 78-2076), respectively (Ref 52). It is to be noted that no characteristic Cu2O peaks were found in CuO(20)NiO(80), and CuO(50)NiO(50) nanocomposites. The average grain sizes of CuO and NiO nanoparticles of CuO(50)NiO(50) nanocomposite have been determined from the most intense peak of XRD pattern using Debye-Scherrer formula, are 18, and 6 nm for NiO and CuO nanoparticles, respectively (Ref 6). Similarly the average grain size of CuO(80)NiO(20) and CuO(20)NiO(80) nanocomposites can be calculated from the most intense peaks of the corresponding profile as 36 and 17 nm for CuO and 34 and 9 nm for NiO, respectively. Moreover, the average grain diameter of CuO(100) nanomaterial is 17 nm.

Fig. 3
figure 3

X-ray diffraction patterns of the (a) CuO(100), CuO(80)NiO(20), CuO(20)NiO(80), (b) CuO(50)NiO(50), and (c) NiO(100) nanomaterials, respectively

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The structure and morphology of the as-synthesized CuO(50)NiO(50) nanocomposite was investigated by field emission scanning electron microscopy and high-resolution transmission electron microscopy. Figure 4 shows the FESEM image of as-prepared CuO(50)NiO(50) nanocomposite. FESEM image clearly reveals that the aggregated hierarchical three-dimensional (3D) nanostructures are consisted of parallel self-assembled nanosheets, which may provide high electrochemical active specific surface area as well as higher degree of diffusion of the ions of electrolyte to the CuO(50)NiO(50) electrode and thus result in higher capacitance. The TEM, HRTEM images, SAED pattern, and the particles distribution curves of the as-synthesized CuO(50)NiO(50) nanocomposite are shown in Fig. 5. TEM image (Fig. 5a) shows aggregated but two definitely separated black and gray parts which might be called Janus-type nanostructure of CuO(50)NiO(50) nanocomposite. Figure 5(b) shows the HRTEM image of as-synthesized CuO(50)NiO(50) nanocomposite, clearly reveal the stacked aggregated nanostructure, which is consistent with the FESEM result. In addition, HRTEM exhibits the lattice-fringes with spacing between the adjacent lattice planes of 0.212 and 0.241 nm, corresponding to the (200) plane of fcc CuO [JCPDS card No. 78-0428] and (111) plane of fcc NiO [JCPDS card No. 78-0429] of as-synthesized CuO(50)NiO(50) nanocomposite as shown in Fig. 5(b). The SAED pattern of CuO(50)NiO(50) nanocomposite is shown in Fig. 5(c), which confirms the presence of (111), (220), and (311) planes corresponding to the lattice fringes with the spacing of 0.241, 0.147, and 0.126 nm of cubic crystalline NiO and (200) plane corresponding to the lattice fringes with the spacing of 0.212 nm of cubic CuO, respectively. Additionally, the typical TEM image (the inset of Fig. 5a) and the HRTEM image (Fig. 5d) show the Janus-type asymmetric nanostructures of CuO(50)NiO(50) nanocomposite. Figure 5(e) and (f) depict the particle size distribution profiles of CuO and NiO of CuO(50)NiO(50) nanocomposite. The average diameter of CuO and NiO nanoparticles are found 4.0 nm and 23.1 nm, respectively.

Fig. 4
figure 4

FESEM image of the CuO(50)NiO(50) nanocomposite

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Fig. 5
figure 5

(a) TEM image showing Janus-type structure at the inset (b) HRTEM image, (c) SAED pattern and (d) HRTEM image showing, Janus-type structure of CuO(50)NiO(50) nanocomposite, with particles size distribution curve of (e) CuO and (f) NiO nanoparticles from TEM images

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Figure 6 shows the cyclic voltammograms of CuO(100) and CuO(x)NiO(100 − x)-based electrodes immersed in 1 M Na2SO4, obtained by application of various scan rates ranging between 2 to 100 mV s−1 within the potential window of 0 to 1.0 V, at room temperature. The shape of the CV curves of all the four electrodes clearly reveals the typical pseudocapacitive behaviors of the electrodes. Figure 6 clearly reveals that the area of the CV curves is increased with increasing the scan rate from 2 to 100 mV s−1. Moreover, the cyclic voltammograms of CuO(80)NiO(20) and CuO(100) exhibit a prominent anodic peak at potential around 0.23 V at the scan rates of 2 and 5 mV s−1 as shown in Fig. 6(b) and (e), respectively, due to the following faradic reaction (Ref 18, 52):

Fig. 6
figure 6

CV curves of (a) CuO(100), (b) CuO(80)NiO(20), (c) CuO(50)NiO(50) and (d) CuO(20)NiO(80) nanomaterials at different scan rates ranging from 2 to 100 mV s−1 and (e) cyclic voltammogram of the CuO(100) at the scan rates of 2 and 5 mV s−1

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$${\text{Cu}}_{2} {\text{O}} + 2{\text{OH}}^{ - } \leftrightarrow 2{\text{CuO}} + {\text{H}}_{2} {\text{O}} + 2{\text{e}}^{ - }$$

Figure 7(a) reveals that the specific capacitance is decreased with increasing the potential speed. On increasing the potential speed, the electrolyte ions become unable to move at par and thus unable to contact with the inner surface of the electrode fully, causing a decrease in specific capacitance. To investigate the charge storage mechanism of CuO(100) and CuO-NiO nanocomposites having different compositions the relationship between scan rates (ν) and anodic peak currents (Ip) are analyzed with help of the following equation (Eq 4):

$$I_{\text{p }} = a \times v^{b}$$
(4)

where ‘a’ and ‘b’ are adjustable parameters. For b = 1, the charge storage mechanism is mainly surface-controlled, while b = 0.5 represents that the said mechanism is diffusion-controlled process. The obtained b values of CuO(100) and CuO-NiO nanostructures (Fig. 7b) are 0.61, 0.60, 0.82 and 0.61, respectively, for CuO(100), CuO(80)NiO(20), CuO(50)NiO(50), and CuO(20)NiO(80) nanomaterials, respectively, suggesting that the total specific capacitance is due to both surface-controlled and diffusion-controlled processes. Surface-controlled capacitance (Cin) and diffusion-controlled capacitance (Cout) are expressed by the Trasatti relation (Eq 5).

Fig. 7
figure 7

(a) Specific capacitance variation with different scan rates, (b) log(ip) vs. log(ν) plots, (c) CTotal vs. ν1/2 and (d) CTotal vs. ν−1/2 of CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80)-based electrodes

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$$C_{\text{Total}} \left( {{\text{F}}\,{\text{g}}^{ - 1} } \right) = C_{\text{in}} + C_{\text{out}}$$
(5)

The maximum possible capacity, CTotal and the capacity due to outer surface, Cout can be determined by extrapolating the specific capacitance (Csp) versus (ν)1/2 (Fig. 7c) and specific capacitance (Csp) versus (ν)−1/2 (Fig. 7d), respectively. CTotal (F g−1) for CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80) are found 2.74, 2.8, 6.03 and 26.16, respectively. Moreover, Cout (F g−1) for these electrodes is 0.33, 0.30, 1.23 and 1.20, respectively, indicating that the contribution of capacity by outer surface is almost same for the last two electrodes. Overall the electrodes which contain sufficient amount say 50-80 mol.% of NiO seem to be better than others.

The galvanostatic charge/discharge (GCD) cycles at different current densities for CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80) are shown in Fig. 8. The specific capacitance for all the electrode materials computed from the GCD plots (Fig. 9a) shows that the maximum Csp values obtained are 3.95, 0.6, 22.6 and 6.37 F g−1 at current density of 1 A g−1 for CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80), respectively. GCD profiles (Fig. 8) reveal that the capacity of the electrodes follows the order CuO(50)NiO(50) > CuO(20)NiO(80) > CuO(100) > CuO(80)NiO(20). Figure 9(b) displays that the specific capacitance decreased with increasing charge–discharge current density. The specific capacitance values are 35.6, 29.6, 25.5, 22.6, 22.56 and 14.4 F g−1 at current densities of 0.3, 0.5, 0.7, 0.9, 1.0 and 3.0 A g−1 for CuO(50)NiO(50) nanocomposite electrode. In order to examine synergistic effect of CuO(50)NiO(50) nanocomposite, we have synthesized NiO(100) active electrode materials by the same procedure as stated above like CuO(100), and GCD experiments was performed for these three electrodes at current density of 0.3 A g−1 within the potential window of 0 to 1.0 V. Figure 10 shows the galvanostatic charge/discharge curves of CuO(100), NiO(100), and CuO(50)NiO(50) electrodes at a mass normalized current of 0.3 A g−1. The calculated specific capacitances using Eq 1, are 5, 25 and 36 F g−1, respectively, indicating CuO(50)NiO(50) nanocomposite exhibits excellent supercapacitve performance than CuO(100) and NiO(100) due to synergistic effects between nickel oxide and copper oxide (Ref 6, 53).

Fig. 8
figure 8

GCD curves of (a) CuO(100), (b) CuO(80)NiO(20), (c) CuO(50)NiO(50) and (d) CuO(20)NiO(80) electrodes at different current densities

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Fig. 9
figure 9

(a) GCD curves at a constant current density of 1 A g−1 (b) specific capacitance at different current densities of CuO(100), CuO(80)NiO(20), CuO(50)NiO(50) and CuO(20)NiO(80) electrodes, (c) Nyquist plots with equivalent circuit diagram at the inset (d) Nyquist plots at high frequency region of CuO(50)NiO(50), CuO(20)NiO(80), CuO(80)NiO(20), CuO(100)-based electrodes and (e) coulombic efficiency vs. cycle number for CuO(50)NiO(50)-based electrode at a current density of 10 A g−1

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Fig. 10
figure 10

Galvanostatic charge/discharge profiles of CuO(100), NiO(100) and CuO(50)NiO(50) nanocomposite-based electrodes at a current density of 0.3 A g−1

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Figure 9(c) depicts the EIS plot of as-prepared electrodes in the frequency range of 0.01 Hz to 100 kHz in 1 M Na2SO4 at a fixed dc potential of 0.5 V. The inset of Fig. 9(c) presents the fitting circuit diagram. The EIS plots for as-prepared electrode show small semicircles at high frequency region and the sloped lines in the low-frequency region, indicating the charge-transfer resistance (Rct) and Warburg impedance (W0), respectively. Moreover, in the fitting circuit, Rs, Cdl and Cp are the solution resistance, double layer capacitance and the pseudocapacitance, respectively. The different electrochemical parameters obtained from the EIS study are presented in Table 1. The smaller the charge-transfer resistance (Rct), solution resistance (Rs) and the larger pseudocapacitance (Cp) value of CuO(50)NiO(50) nanocomposite compared to the other materials due to synergistic effect suggesting faster electrochemical reaction on the electrode. The lower value of Warburg impedance (W0) of CuO(50)NiO(50) nanocomposite also signifies the better diffusion of electrolyte ions with the stacked 2D nanosheets.

Table 1 Electrochemical parameters of CuO(100), and CuO-NiO nanocomposites estimated through EIS study
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Figure 9(e) shows that the coulombic efficiency of CuO(50)NiO(50) electrode, at a current density of 10 A g−1. The reduction of coulombic efficiency of CuO(50)NiO(50) electrode in the beginning has been observed due to the activation effect of the active electrode materials. As the number of repeated charge–discharge (CD) cycles increases, the adsorption-desorption of electrolyte ions is increased into the stacked nanosheets of active electrode materials and more electrochemically active redox sites of electrodes get in direct contact with the electrolyte, leading to increase of coulombic efficiency. Up to a certain number of charge–discharge cycles, the maximum activation process of the stacked nanosheets of CuO(50)NiO(50) nanocomposite occur, and after that it gets saturated. During the CD cycles the volume expansion of the electrode materials occur, which improves the infiltration of the electrolyte ions within the stacked nanosheets of the electrodes (Ref 54). The specific capacitances of CuO(50)NiO(50) nanocomposite are found to be 14, 12 and 12 F g−1 for first, 500, and 1000 cycles, respectively, the specific capacitance could keep 85.7% capacity retention at 1000 cycle, demonstrating its good cycling stability (Fig. 11). This CuO(50)NiO(50) nanocomposite electrode offers the highest energy density (Ecell) and power density (Pcell) compared to other electrodes as shown in Table 2.

Fig. 11
figure 11

Galvanostatic charge/discharge profiles of CuO(50)NiO(50) at the 1st, 500th and 1000th cycles at a current density of 10 A g−1

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Table 2 Values of energy densities (Ecell) and power densities (Pcell) of different electrodes at current density of 1 A g −1
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Conclusions

In summary, different CuO-NiO mixed oxides containing NiO in the range of 20–80 mol.% are synthesized for efficient supercapacitor application. The CuO(50)NiO(50) nanocomposite is found to be the best one for improved and synergistic specific capacity, energy density, power density, etc., because of its optimum composition and unique Janus-type nanostructure with stacked 2D nanosheet morphology. The particle dimension can be reduced and porosity may be increased by suitable modification of the synthesis protocol and better fabrication for more efficient devices.