Firstly, crystalline structure and phase purity of the products are analyzed by XRD. Figure 1a shows the XRD spectra of the samples as-prepared samples. The three samples have sharp diffraction peaks at 2 theta value of 44.5°, 51.8° and 76.4°, corresponding to the surface index (111), (200) and (220) of Ni foam the standard comparison card (JCPDS No. 04-0850), respectively. It is found that there are several distinct diffraction peaks at 21.8°, 31.1°, 37.8°, 50.1° and 55.2° corresponding to (101), (110), (003), (211) and (122) crystal planes of NiCo2S4 (JCPDS No.20–0782), respectively. Other peaks at 21.8°, 31.1°, 37.8°, 50.1° and 55.2° corresponding to (101), (110), (003), (211) and (122) crystal planes of can be indexed to NiMoO4 (JCPDS No.12–0348). There is no diffraction peak of other impurities, which indicates that the sample is NiCo2S4@NiMoO4 phase with high purity.
The composition and surface valence of the elements were analyzed by XPS NiCo2S4@NiMoO4 samples. As shown in Fig. 2(a-d). Figure 2a shows Co 2p spectra, two distinct characteristic peaks at the binding energy of 777.8 eV and 795.1 eV, which are consistent with Co 2p3/2 and Co 2p1/2, respectively [20]. The existence of Co2+ and Co3+ can be proved by spin orbit coupling. In addition, the satellite peaks at the binding energies of 784.8 eV and 879.2 eV are named Sat., which are caused by the electronic transition in the valence band [21, 22]. Mo 3d spectra are shown in Fig. 2b. NiCo2O4@NiMoO4 samples exhibit two peaks at 229.9 and 232.5 eV, which correspond to Mo 3d5/2 and Mo 3d3/2. Binding energy at 235.23, 230.4 and 226.5 eV corresponds to Mo-S bond [23, 24]. Ni 2p emission spectra are fitted with two kinds of nickel species containing Ni2+ and Ni3+ (Fig. 2c). Binding energies at 853.4 eV and 856.5 eV correspond to Ni 2p3/2 and those at 874.5 and 871.6 eV for Ni 2p1/2. Those at 787 and 873 eV could be indexed to shakeup satellites (noted as Sat.,), revealing that most of Ni exists in the form of Ni2+ ion [25, 26]. The S 2p spectrum in Fig. 2d shows two characteristic peaks at 162.59 eV and 163.2 eV, which can be ascribed to S 2p1/2 and S 2p3/2, respectively, indicating that the S2+ valence exists in NiCo2S4@NiMoO4. Furthermore, a satellite peak of S was checked at 168.2 eV [27], which may be owing to the high oxidation of S on the surface of NiCo2S4@NiMoO4 sample during the test procedure. XPS characterization further proved that the prepared NiCo2S4@NiMoO4 sample had high purity and good crystal quality [28].
SEM and TEM were used to analyze the surface morphology and structure of as-prepared products. Figure 3(a, b) shows the SEM images of the prepared products at different magnification. Ni foam surface is covered with a three-dimensional nanosheets structures (Fig. 3a). From high magnification SEM images (Fig. 2b), it is found that adjacent nanometers are linked to each other. Figure 3(c, d) show SEM images of hybrid NiCo2S4@NiMoO4 samples, it is seen that the thickness and surface roughness of the nanosheets increased significantly, which was more conducive to the exposure of active sites. From the TEM images of Fig. 3e, a layer of nanosheets uniformly coat on the surface of NiCo2S4 nanosheest, which exhibited fill consistency with the observed SEM images. The HRTEM image of Fig. 3f shows that the lattice distances of 0.28 and 0.237 nm correspond to the (110) and (003) faces of NiCo2S4 and NiMoO4, respectively.
Figure 4(a-c) shows the CV curve of NiCo2S4@NiMoO4, NiCo2S4 and NiMoO4 electrodes at different scan rates. At different scanning rates, there are obvious oxidation and reduction peaks, which are caused by the reversible redox reaction. With the increase of scanning rate, the positions of oxidation peak and reduction peak move to positive voltage and negative voltage respectively, and the CV curve still keeps the similar shape and envelope area becomes larger, which proves that NiCo2S4@NiMoO4 electrode has the characteristics of fast charge discharge and high-rate capacity. Figure 4(d-f) shows the GCD curves of NiCo2S4@NiMoO4, NiCo2S4 and NiMoO4 electrodes between 0 and 0.5 V at different current densities. It can be observed that these curves are symmetrical, and each curve shows a relatively flat area, which reveals the Faraday characteristics of the electrode material and high reversibility of its Faraday reaction. In addition, the capacitance of NiCo2S4@NiMoO4-8 electrode is 1102.5, 843.6, 704.5, 385.4 F g− 1 at the current densities of 1, 2, 4 and 6 A g− 1, respectively.
In order to explore the electrochemical performance of the synthesized products, cycle voltammetry (CV) and galvanostatic charge-discharge (GCD) curves were measured in a three-electrode-system in 3.0 M KOH solution. Figure 5a depicts the CV curves of NiCo2S4@NiMoO4, NiCo2S4 and NiMoO4 electrodes at 20 mV s− 1. It is discovered that the envelope area of the CV curve of the NiCo2S4@NiMoO4 electrode is larger than that of NiCo2S4 and NiMoO4 samples, indicating that the NiCo2S4@NiMoO4 electrode has a large capacitance. The GCD curves of three electrode materials at 1 A g− 1 are shown in Fig. 5b, in which NiCo2S4@NiMoO4 electrode material has longer discharge time than NiCo2S4 and NiMoO4 samples, indicating its high specific capacitance. The dynamic characteristics of different electrodes in the frequency range of 100 kHz to 0.01 Hz with the amplitude of 0.01 V are analyzed by electrochemical impedance spectroscopy (EIS), as shown in Fig. 3c. In the high frequency region, the intercept of the real axis corresponds to the equivalent series resistance (Rs), and the radius of the semicircle corresponds to represents the transfer resistance (Rct). In the low frequency region, the slope of the line is attributed to the Warburg resistance [29]. The lower Rs value (0.90 Ω· cm2) of NiCo2S4@NiMoO4 electrode indicates that it has higher conductivity. The NiCo2S4@NiMoO4 electrode showed a more vertical line along the imaginary axis, indicating that the ion diffusion process was relatively fast. NiCo2S4@NiMoO4 electrode has excellent electrical conductivity. In order to study the cycle stability of three electrode materials, 10000 cycles of charge discharge experiments were carried out at 3 A g− 1 current density. As shown in Fig. 5d, the curve tends to be stable after 2000 cycles, and the specific capacitance remains at 72.7 % after 10000 cycles, indicating that NiCo2S4@NiMoO4 has good cycle stability.
In order to further explore the practical application of the as-prepared samples, the asymmetric supercapacitor (ASC) was prepared with NiCo2S4@NiMoO4 as positive electrode and AC as negative electrode. The CV curves of NiCo2S4@NiMoO4 electrode (0-0.6 V) and activated carbon (AC) electrode (-1.0-0 V) at three electrode mode scan rate of 50 mV s− 1 are shown in Fig. 6a. Figure 6b shows the CV curves of ASC devices under different operating voltage windows. Therefore, the stable voltage windows of the ASC device should be 0-1.6V. Figure 6c shows the CV curves of the device at different scan rates. With the increase of scan rate, its shape of the curve remains unchanged, but its area increases gradually, which indicates that the device has excellent capacitance performance. GCD curves of the assembled capacitor under different current densities are shown in Fig. 6d. The device delivers a long discharge time of 234.2 s at 1 A g− 1. Figure 6e shows the Ragone diagram of NiCo2S4@NiMoO4//AC ASC. The as-assembled devices possess an energy density of 37.6 W h kg− 1 at power density of 2880 W kg− 1, reveals that the achieved energy density of our device is distinctly than previously reported capacitive devices[30–34]. Figure 6e shows the cycle stability of the device at 10 A g− 1. After 10000 charge discharge cycles, the capacitance retention of the device reaches 75.0%.