1 Introduction

The huge demand of the energy consumption made fossil fuels to perish earlier which enabled to rely on the most feasible energy sources such as renewable energy sources. Numerous experiments were conducted to store this energy and eventually found the more efficient energy storage devices such as supercapacitors, batteries, etc. [1,2,3]. These supercapacitors have some appreciable characteristics such as fast-charging capabilities, greater power densities and longer life period. Hence, they are also termed as electrochemical capacitors that have more industrial and technical applications [4, 5]. Depending on their charge storage phenomenon, the supercapacitors are categorized into two types such as (1) electrical double-layer supercapacitors, where the capacitance occurs because of the ion adsorption/desorption in between the carbon electrode and redox electrolyte, and (2) Faraday (redox) supercapacitors, where the higher capacitances are observed at reversible faster redox reactions.

EDLC supercapacitor electrodes are widely fabricated using the porous carbon materials. At the same time, the pseudocapacitor electrodes are made from transition metal oxides, hydroxides, sulfides and conducting polymers, etc. For various applications in solar cells, supercapacitors and photocatalysis [6,7,8,9,10]. In practical applications such as electric vehicles, there are some limitations when using the high-cost transition metal oxides (RuO2 and V2O5) [11, 12]. Hence, there is a greater importance to fabricate pseudocapacitors with low-cost materials. There are some reports in recent time that suggest mixing two metal oxides increases the specific capacitance and results in greater electrical conductivity [13].

Because of their good electrochemical abilities, transition metal sulfides (TMS) were found to be the good option for electrode materials in supercapacitor applications [14]. Different TMS such as CoSx, Ni3S2 and CuS can be used as electroactive materials for supercapacitor applications [15,16,17]. Tungsten disulfide (WS2) is one of the typical layered transition metal dichalcogenide (TMD) has found to be the efficient SC electrode material observed from its good layered structure and excellent conductivity [18,19,20]. Higher capacitive values can be achieved when used as nanostructured materials than bulk materials. Further, this exhibits an enhanced surface area and minimal transfer channels for ions/electrons resulting in good capacitance [21]. Though CuS has a greater usage in supercapacitor applications, there is a huge research going on the WS2 to identify the electrochemical capacitance properties of nanostructured WS2 [22, 23]. Tu et al. synthesized WS2 nanostructures and showed a specific capacitance of 398.5 F g−1 for sheet-like WS2 structures [24]. Pan et al. [25] synthesized a CuS-decorated Ti3C2 MXene for asymmetric supercapacitors and obtained a specific capacitance of 169.5 C g−1 at 1 A g−1. Kumar et al. [26] fabricated the CuS/Cu(OH)2 nanocomposite material for supercapacitor applications and achieved the 845.5 F/g at 1 mA/cm2. Raghavendra et al. [27] synthesized the CuS electrode and obtained the specific capacity of 164.053 m Ah g−1 at 1 A g−1.

Inspired from the above works, the present work reports the method of chemical bath deposition for the synthesis of nanorice-like structured CuS@WS2 that is used for various supercapacitor applications. Further, this CBD method is very simple to operate, low cost and can be implemented for deposition of large area. This paper includes the synthesis of nanorice-like structured CuS@WS2 electrode was prepared by CBD on the surface of Ni foam substrate and that can be employed as supercapacitor electrode material. The electrochemical studies were analyzed through cyclic voltammetry (CV) and galvanostatic charge–discharge measurements in a 3 M KOH solution using three-electrode setup. The formation of a nanorice-like structured CuS@WS2 gives rise to better electrical conductivity, faster electrolyte diffusion and rapid electron pathways compared to those of bare CuS and WS2 electrode materials. As a result, the nanorice-like structured CuS@WS2 electrode exhibits a higher specific capacitance than the bare CuS and WS2 electrodes.

2 Experimental

2.1 Materials

All the chemicals used in the synthesis were of analytical grade purchased from Sigma-Aldrich. Copper sulfate (CuSO4), urea (CH4N2O), sodium thiosulfate (Na2S2O3), sodium tungstate dihydrate (Na2WO4) were used as obtained.

2.2 Preparation of CuS, WS2 and CuS@WS2 electrodes

In the synthesis of CuS, 0.1 M of CuSO4, 0.4 M of urea and 0.4 M of Na2S2O3 were dissolved in 70 mL of deionized (DI) water and constantly stirred for 30 min. Nickel (Ni) foams (of size 2 × 5 cm2) cleaned by ultrasonication process were dipped in the as-prepared CuS solution and kept in the oven at 90 °C for 6 h. After cooling to room temperature, the Ni foams were further washed with ethanol, DI water, dried in over at 60 °C for overnight and stored properly.

For WS2 electrode preparation, 0.1 M of Na2WO4, 0.4 M of urea and 0.4 M of Na2S2O3 were dissolved in 70 mL of DI water. The ultrasonically cleaned Ni foams were immersed in the as-prepared WS2 solution and kept in hot-air oven for 90 °C for 6 h. Further, the obtained electrodes were washed properly and dried in the oven at 60 °C for 12 h and used for electrochemical measurement.

In the synthesis of CuS@WS2, 0.1 M of CuSO4, 0.4 M of urea and 0.4 M of Na2S2O3, 0.1 M of Na2WO4 were dissolved in 70 mL of DI water and further stirred for 30 min to make a constant homogenous solution. Then, Ni foams were immersed in this solution and kept in oven at 90 °C for 6 h. After this, the Ni foams were washed with ethanol and DI water and dried in the oven at 90 °C for 12 h and stored properly for the characterizations.

2.3 Characterization

The electrode morphology can be characterized by field emission scanning electron microscopy (FE-SEM S2400, Hitachi). Transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS, KBSI, Busan) was performed to examine the chemical states of the elements. The phase structure of the copper sulfide-based electrode was analyzed by X-ray diffraction (XRD, D/Max-2400, Rigaku) using Cu kα source.

2.4 Electrochemical measurements

The electrochemical measurements were carried out using BioLogic-SP150 workstation, and the experimental tests were performed using a three-electrode setup with CuS@WS2 as the working electrode, Ag/AgCl as the reference electrode and Pt wire as the counter electrode. The aqueous 3 M KOH is used as the electrolyte. The specific capacitance (Cs, F g−1), energy density (E, W h kg−1) and power density (P, W kg−1) of the as-prepared electrodes were calculated using the following equations:

$$C_{\text{s}} = \frac{I \times \Delta t}{m \times \Delta V}$$
(1)
$$E = \frac{{0.5 \times C_{\text{s}} \times \Delta V^{2} }}{3.6}$$
(2)
$$P = \frac{E \times 3600}{\Delta t}$$
(3)

where I, \(\Delta t,\, \Delta V\) and m represent the current (A), discharge time (s), potential window (V) and mass (g) of the electroactive material, respectively.

3 Results and discussion

The CuS, WS2 and CuS@WS2 electrode materials were grown on Ni foam surface using facile chemical bath deposition method. SEM and TEM analysis were performed to study the morphologies of the electrodes under a variety of magnifications, as shown in Fig. 1. From Fig. 1a, the CuS nanoparticles were grown steadily on the surface of the Ni foam substrate. In Fig. 1a1, a2, every nanoparticle was composed of the very small tiny bundle of widespread nanorice-like particles. Further, it is observed that from Fig. 1b, b1, b2 that the smooth surface of agglomerated nanosheets was covered over the Ni foam surface homogenously exhibiting an intermediate resistance of WS2 electrode. Concurrently, Fig. 1c, c1, c2 shows that the nanorice-like particles were distributed on the Ni foam.

Fig. 1
figure 1

Low- and high-magnification SEM images of (a, a1, a2) CuS, (b, b1, b2) WS2 and (c, c1, c2) composite CuS@WS2 electrodes on Ni foam surface

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The structure of as-prepared material on Ni foam was further confirmed by TEM characterization. Figure 2 depicts the TEM images of the as-prepared CuS, WS2 and CuS@WS2 electrodes, respectively. The TEM image of CuS material exhibits the agglomerated nanoparticle structures (Fig. 2a, a1). Figure 2b, b1 shows the TEM image of nanosheet-like structures. The TEM image in Fig. 2c, c1 represents the homogenous distribution of nanorice-like structures which were interconnected with each other. The TEM results are consistent with the SEM studies. The observed unique surface area and unique structure are advantageous to electrolyte diffusion and electron transfer, which promotes the electrochemical activity of CuS@WS2.

Fig. 2
figure 2

TEM images of (a, a1) CuS, (b, b1) WS2 and (c, c1) CuS@WS2 electrodes

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XRD analysis was performed to obtain the crystal structures and phase purity of CuS, WS2 and CuS@WS2 precursors as shown in Fig. 3. The XRD peaks for CuS observed at 21.78°, 27.71°, 31.82°, 35.05°, 43.14° and 53.27° 2θ were given to the (0 0 4), (1 0 1), (1 0 3), (1 0 4), (1 0 6), and (1 1 4) planes of the CuS peaks (JCPDS No: 01-079-2321), respectively [28]. For WS2, the peaks observed at 14.3°, 32.7°, 39.5°, 57.3°, 60.0°, 70.1° and 78.3° were given to the (0 0 2), (1 0 0), (1 0 3), (1 1 0), (0 0 8), (1 0 8) and (2 0 0) planes of the hexagonal-phase WS2 structure (JCPDS 84-1398), respectively [29].

Fig. 3
figure 3

XRD pattern of CuS, WS2 and composite CuS@WS2 electrode on Ni foam

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XPS was performed to find the chemical composition and the oxidation states of the various compounds like CuS, WS2, CuS@WS2. Figure 4a shows the XPS survey spectrum of the CuS, WS2, CuS@WS2 electroactive materials on Ni foam. The bare CuS and WS2 survey scans show the existence of Cu and S and W and S signals. More interestingly, the composite of CuS@WS2 survey spectrum shows the presence of the Cu, W and S elements. The Ni peak in the XPS survey spectrum is due to the background of Ni foam. Figure 4b shows the high-resolution Cu2p spectrum. The Cu2p spectrum exhibits the two strong peaks at the binding energies of 934.0 and 953.87 eV, which are assigned to Cu2p3/2 and Cu2p1/2, respectively [30]. The difference in binding energy of these two peaks was approximately 19.87 eV. These peaks can be ascribed to the Cu2+ state in CuS structures. Figure 4c depicts the high-resolution XPS spectrum of W signal. The W elements are typical W4+ profile at 33.3, 35.7 and 39.7 eV and are assigned to the W4f7/2, W4f5/2 and W5p3/2 states [31]. The high-resolution spectrum of the S 2p, characteristic of the S2− species, is observed at 161.78 and 163.03 eV and is ascribed to the S2p3/2 and S2p1/2, respectively [32]. The XRD and XPS studies confirm that the CuS@WS2 nanostructures were successfully grown on the Ni foam surface.

Fig. 4
figure 4

a XPS survey spectra of as-prepared CuS, WS2 and CuS@WS2 electrodes on Ni foam. High-resolution XPS spectra of b Cu2p, c W4f and d S2p signals in CuS@WS2 electrodes

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The energy storage performance of the CuS, WS2 and CuS@WS2 was analyzed by CV, GCD and EIS measurements. The electrochemical properties of the CuS@WS2 were tested using three-electrode setup in 3 M KOH electrolyte. The KOH electrolyte was found to have very high ionic conductivity; hence, it is the most widely used electrolyte for supercapacitor applications [33]. In the three-electrode setup, platinum wire was selected as the counter electrode, Ag/AgCl as the reference electrode and the Ni foam coated with CuS@WS2, CuS and WS2 as the working electrodes at various conditions. Figure 5a shows the comparative CV curves of CuS, WS2 and CuS@WS2 electrodes at a constant scan rate of 10 mV s −1. Interestingly, the CuS@WS2 electrode exhibits the higher potential window range of -0.4 to +0.7 V, which is larger than the CuS (-0.4 to +0.6 V) and WS2 electrode (− 0.4 to +0.6 V). As shown in Fig. 5a, all the electrodes exhibit a pair of redox peaks, which might be attributed to reversible Faradaic redox reactions. Obviously, CuS@WS2 electrode achieves the larger integration than that of CuS and WS2 electrodes, which is attributed to the synergistic effect of redox reactions associated with CuS and WS2 materials in composite electrode, thereby achieving superior energy storage performance. Moreover, CV tests were conducted for CuS, WS2 and CuS@WS2 electrodes at various scan rates of 10–50 mV s−1 and the corresponding CV profiles are shown in Fig. 5b–d. With the increase in scan rate, the peak current increases and also exhibits the similar CV shape with redox peaks, which indicate that the capacitance characteristics are mainly governed by Faradaic redox reactions. Furthermore, the anodic peak in positive direction and cathodic peak in negative direction were shifted with increasing the scan rate because the charge transfer kinetics limits the reaction [34]. From the following CV curves, composite CuS@WS2 electrode was found to be the excellent energy storage material when compared to the CuS and WS2 electrodes.

Fig. 5
figure 5

a Comparison CV curves of the CuS, WS2 and CuS@WS2 electrodes at the scan rate of 10 mV s −1. CV curves of the b CuS, c WS2, d CuS@WS2 electrodes at different scan rates of 10–50 mV s−1 in 3 M KOH solution

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In addition, galvanostatic charge/discharge (GCD) measurements were conducted to examine the charge storage performance of the as-prepared electrodes. Figure 6a shows the GCD plots of as-developed CuS, WS2 and CuS@WS2 electrodes at various current densities within the potential window of 0 and +0.4 V. Based on GCD plots and Eq. (1), the specific capacitance values of the CuS@WS2 composite, CuS and WS2 are illustrated in the following table.

Fig. 6
figure 6

GCD curves of the a CuS, b WS2 and c CuS@WS2 composite electrodes at different current densities of 6.25, 7.5 and 8.75 A g−1 in 3 M KOH solution. d Specific capacitance as a function of current density

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The specific capacitance decreased gradually with increasing current density from 3.75 to 15.0 A g−1 due to the resistance of the electrode and insufficient Faradaic redox reaction at high current densities. The higher specific capacitance of CuS@WS2 was attributed to its higher surface area with the improved surface morphology of nanostructures [35] (Tables 1, 2).

Table 1 Specific capacitance values of the CuS, WS2 and CuS@WS2
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Table 2 Comparison table of recently reported similar works to CuS@WS2
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The energy density and power density are the two important parameters for supercapacitors in real-time applications. Based on the as-obtained specific capacitance values, the energy and power densities of the CuS, WS2 and CuS@WS2 electrodes were calculated and the resulting Ragone plot is shown in Fig. 7. The energy density of the composite CuS@WS2 material was found to be 19.16 W h kg−1 at a power density of 750 W h kg−1, which is much higher than the CuS and WS2 electrodes, respectively. Consequently, we concluded that the CuS@WS2 exhibited a good energy density and power density in supercapacitor applications.

Fig. 7
figure 7

Ragone plots of the CuS-, WS2- and CuS@WS2-based electrodes

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4 Conclusion

In summary, the synthesis of nanorice-like structured CuS@WS2 nanoparticles was successfully prepared in 3 M KOH electrolyte in the three-electrode system by using a facile chemical bath deposition method. In comparison with other electrodes, such as CuS and WS2, the CuS@WS2 electrode showed excellent electrochemical properties including specific capacitance of 887.15 F g−1 at 1 A g−1. The energy density of the composite CuS@WS2 material was found to be 19.16 W h kg−1 at a power density of 750 W h kg−1, which is much higher than the CuS and WS2 electrodes, respectively. Since there are a huge number of Faradaic reaction active locations in the contact area with the electrolyte, the CuS@WS2 electrode supports faster redox reactions and hence it delivers an excellent electrochemical behavior. Therefore, from the detailed analysis, the CuS@WS2 electrode referred to as a battery-type electrode material can be a potential electrode for high-performance supercapacitor applications.