Introduction

Energy is the material basis of human activities, the core driving force for economic development, and the essential condition for a country’s core competitiveness and sustainable economic and social development [1,2,3,4,5,6,7]. Supercapacitors (SCs), also known as electrochemical capacitors, electrochemical double-layer capacitors, pseudocapacitors, ultracapacitors, power capacitors, and gold capacitors, etc. [8, 9], are devices that store energy through a double electrical layer at the electrode/electrolyte interface, or through the electrochemical Faraday redox reactions [10, 11]. As a new type of energy storage device with high efficiency and cleanness, SC has higher energy density and power density than traditional dielectric capacitor. Compared with traditional battery, supercapacitor has the advantages of high power density, fast charging and discharging speed, long cycle life, and wide working temperature range [12]. It can be widely used in backup batteries, energy storage, and auxiliary peak power and so on. The situation has great market value and commercial potential in industrial control, military, electric power, new energy vehicles, etc. [13, 14]. However, the low energy density of SCs is an important reason to limit their development. At present, the main solution is to develop electrode materials with high electrochemical performance. Previous studies on electrode materials for SCs are mainly focused on transition metal oxides or hydroxides with pseudocapacitive properties, for example, NiO [15, 16], RuO2 [17], MnO2 [18], Co(OH)2 [19], V2O5 [20], and Ni(OH)2 [21]. The RuO2 exhibits the best electrochemical performance. However, its high price and toxicity limit its wide commercial application [22].

In recent years, the multi-hybrid nanomaterials of SCs (such as NiMoO4 [23,24,25], CoMoO4 [26], ZnCo2O4 [27], NiCo2O4 [28], MnMoO4 [29], NiO@CeO2 [30], NiO@MnO2 [31], NiCo2O4@NiWO4 [32], NiMoO4/CoMoO4 [33], NiCo2S4@NiMoO4 [34], and CoMoO4-NiMoO4·xH2O [35]) have become a strong exploration trend due to their intrinsic properties, such as low cost, natural abundance, reliable redox transformation, and synergistic effects [36]. It is worth mentioning that the good electrochemical activity of nickel ions and the better conductivity of molybdenum elements can be attributed to the excellent electrochemical capacitance of the obtained bimolybdate [37, 38]. For examples, Huang et al. [39] successfully prepared wall-like hierarchical metal oxide MMoO4 (M = Ni, Co) nanosheet arrays electrode material by a facile hydrothermal method, which exhibited a high specific capacitance of 1483 and 452 F g−1 for NiMoO4 and CoMoO4 at a current density of 2 A g−1. Cai et al. [40] synthesized NiMoO4 nanospheres and nanorods by a facile hydrothermal method, which exhibited a high specific capacitance of 974.4 F g−1 at a current density of 1 A g−1. Furthermore, Tian et al. [41] rationally designed porous worm-like NiMoO4 by electrostatic spinning, which offered good rate capability (860.3 F g−1 at a current density of 20 A g−1), high specific capacitance (1088.5 F g−1 at a current density of 1 A g−1), and long cycle life with a capacity retention of 73.9% after 5000 cycles. Usually, NiMoO4 electrode material can offer high capacitance and low cycle stability, while CoMoO4 exhibits a lower capacitance and good rate capability [33]. Zhao et al. [42] prepared CoMoO4 nanorod electrode, which exhibited a specific capacitance of 89.5 F g−1 at a current density of 1 mA cm−2. Veerasubramani et al. [43] synthesized plate-like CoMoO4 nanostructures via a facile sonochemical approach, which showed a specific capacitance of 133 F g−1 at a current density of 1 mA cm−2, and the capacitance retention is about 84% after 1000 cycles. Zhang et al. [44] prepared NiMoO4@CoMoO4 nanospheres on Ni foam, which delivered a greatly enhanced specific capacitance of 1601.6 F g−1 at a current density of 2 A g−1, as well as better cycling stability and rate capability than pure NiMoO4 or CoMoO4 material. Therefore, a large number of scientists trend to investigate composite electrode materials for SCs, and use the synergistic effect between different components to increase the rapid diffusion and transport of electrons and ions, which illustrate that the electrochemical performance of the composite material is superior to that of the single material.

However, most current NiMoO4 or CoMoO4 literatures [33, 45,46,47] adopt cladding paste electrode, making its actual discharge capacity much smaller than its theoretical value. Based on the above considerations, an integrated NiMoO4/CoMoO4 electrode (with binder-free) based on Ni foamed is prepared by a simple hydrothermal method, which is helpful to reduce the contact resistance between electrode materials and collector, and to improve the capacitive performance. As far as we know, the novel NiMoO4/CoMoO4 micron structure has rarely been reported. And the excellent electrochemical properties of the composites are attributed to the synergistic effect of NiMoO4 and CoMoO4. What’s more, the unique structure can provide channels for rapid diffusion process and enrich active reaction sites; shorten electron/ion transport pathways, improving the redox reaction of active material not only on the surface of electrode but also in the electrolyte; and then improve the utilization of active materials. Using these advantages, the as-prepared NiMoO4/CoMoO4 electrode materials have great potential application value in the development of electrochemical energy storage devices.

Experimental

Material preparation

Nickel foam was cleaned with acetone, deionized water, hydrochloric acid, and deionized water for 10 min through ultrasonic cleaning, and then completely dried in air. In the typical hydrothermal synthesis process, all reagents are used as raw materials without further purification. The molar ratios of Ni and Co were controlled to be 1:2, 1:1, 2:1, 3:1, 4:1, and 5:1, respectively. The mass of Ni(NO3)2·6H2O and Co(NO3)2·6H2O at different nickel-cobalt ratios is calculated, and then, 0.4839 g Na2MoO4·2H2O and 0.1089 g Na acetate are weighed. The volume ratio of water to ethanol is 2:1, that is, 40 mL water and 20 mL ethanol are mixed evenly and divided into 3 equal parts to dissolve the above substances. Then, the Ni(NO3)2 solution, Co(NO3)2 solution, and Na acetate solution were added to Na2MoO4 solution drop by drop and stirred fully under a magnetic stirrer. After that, the homogeneous solution and the prepared nickel foam are transferred to the Teflon-lined stainless steel autoclave. The hydrothermal reaction was maintained at 150 °C for 6 h. After a reactor was naturally cooled to room temperature, a precursor was washed several times with distilled water and anhydrous ethanol, and then dried completely in air at 60 °C throughout the night. Finally, the NiMoO4/CoMoO4 composite material was obtained after annealing at 300 °C for 5 h.

Structure characterizations

The phase and crystal structure of the prepared samples were examined by X-ray powder diffraction (XRD, D8, Bruker) equipped with Cu Kα radiation in the 2θ range of 10–80° at a scanning rate of 4° min−1. The chemical composition of the samples were determined by Fourier transform infrared spectroscopy (FTIR). The morphology and microstructures of the as-products were characterized by scanning electron microscopy (SEM; JSM 6490) and energy-dispersive spectrometry (EDS). The surface chemical compositions of the obtained sample were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with an Al Kα source.

Electrochemical measurements

The electrochemical performance of the NiMoO4/CoMoO4 composites was examined by CHI 660E electrochemical workstation using three-electrode cell system in 3 mol L−1 KOH aqueous solution. The prepared NiMoO4/CoMoO4, platinum foil, and Hg/HgO were used as a working electrode, counter electrode, and reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) measurement is also operated in the frequency range of 0.01~100 kHz with AC amplitude of 5 mV. The specific capacitance (Cm, F g−1), energy density (E, Wh kg−1), and power density (P, W kg−1) were calculated using the following equations [48]:

$$ {C}_{\mathrm{m}}=C/m=\frac{i\times \varDelta t}{m\times \varDelta u} $$
(1)

where i (A), m (g), Δt (s), and Δu (V) represent the discharge current, the mass of active electrode material, the total discharge time, and the potential window, respectively.

Results and discussion

Determination of the optimum ratio sample

Figure 1a displays the typical cyclic voltammetry (CV) curves of NiMoO4/CoMoO4 composite electrodes with different Ni/Co molar ratios at a scan rate of 5 mV s−1 recorded in a potential window of 0–0.5 V. Apparently, the CV curves of the resulting NiMoO4/CoMoO4 composite electrode supply typical Faradaic capacitive behavior with a pair of well-defined redox peaks are based on Ni and Co diffusion controlled reversibly changing their oxidation states (Ni2+/Ni3+ and Co2+/Co3+) [49], which is distinct from that of EDLCs characterized by nearly a rectangular shape. And the oxidation peak potential and reduction peak potential of NiMoO4/CoMoO4 composites are about 0.47 V and 0.37 V, respectively. Furthermore, although Mo is a transition metal, it does not participate in the redox reaction directly, but rather enhances the electrical conductivity, thus improving the electrochemical performance of the electrode [50].

Fig. 1
figure 1

a CV curves of NiMoO4/CoMoO4 composite electrodes with different Ni/Co molar ratios (1:2, 1:1, 2:1, 3:1, 4:1, and 5:1) at 5 mV s−1. b GCD curves of NiMoO4/CoMoO4 composite electrodes with different Ni/Co molar ratios (1:2, 1:1, 2:1, 3:1, 4:1, and 5:1) at 1 A g−1

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In order to further compare and analyze the electrochemical properties of NiMoO4/CoMoO4 composite electrodes, a series of experiments on Ni/Co molar ratio were carried out, and the corresponding galvanostatic charge-discharge (GCD) curves recorded in a potential window of 0–0.5 V at 1 A g−1 are shown in Fig. 1b. The GCD curves of NiMoO4/CoMoO4 composite electrodes displayed lines with charge-discharge platform rather than smooth lines, which indicates the pseudocapacitive nature and consistent with the CV results. The near symmetry of the GCD curves indicates that the Faraday redox reaction is highly reversible. More importantly, according to Eq. (1), the specific capacitance of the electrode can be calculated using the charge-discharge curve. A maximum specific capacitance was observed for the composite with a Ni/Co mass ratio of 1:1, which may be promising candidates for the practical application of SCs.

The electrochemical impedance spectroscopy (EIS) is used to investigate the internal resistance of the electrode material, and the resistance between the electrode material and the electrolyte. Figure 2 shows the EIS plots of NiMoO4/CoMoO4 composites with different Ni/Co molar ratios recorded from 0.01~100 kHz with a perturbation amplitude of 5 mV. Figure 2a shows a semicircle section in the high-frequency region and a slant line in the low-frequency region. All the plots are almost the same, including semicircles and oblique lines. High-frequency semicircles represent the induced resistance caused by electron transfer of interfacial active substances. Diameter determines the resistance of electron transfer on the surface of electrodes, and the sloped lines at the low frequency region represent the Warburg impedance caused by diffusion [51]. In addition, the intercepts of the semicircles and the real axis at high frequency can also be obtained. Figure 2b shows the intercepts of NiMoO4/CoMoO4 composites with different Ni/Co molar ratios are approximately the same. In other words, the equivalent series resistance does not change much. The slope value of the composites with a Ni/Co molar ratio of 1:1 is higher than that of other samples, which indicates that the Warburg impedance is smaller and the diffusion impedance of the active substance in the electrolyte is smaller, which thus accelerates the diffusion of ions in active substances and electrolytes, as well as the transfer of electrons and ions, and improves the degree of the Faraday reaction [52]. As mentioned above, when the molar ratio of Ni/Co is 1:1, the obtained NiMoO4/CoMoO4 composite has the best electrochemical performance.

Fig. 2
figure 2

a EIS plots of NiMoO4/CoMoO4 composite electrodes with different Ni/Co molar ratios (1:2, 1:1, 2:1, 3:1, 4:1, and 5:1). b The enlarged EIS at the high-frequency region

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Characterization of the optimum ratio sample

Figure 3a shows the CV curves of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1 at various scan rates of 2, 5, 10, 15, and 20 mV s−1 in the potential range of 0–0.5 V. With the increase of scanning rate from 2 to 20 mV s−1, the redox peak potential almost remains unchanged, but the area of CV curve and peak current gradually increased. Therefore, accelerating electron transport and optimizing electrode structure can realize the rapid redox reaction process for energy storage. The shape of CV curve changed slightly, which indicated that the sample had good rate characteristics and excellent electrochemical properties [53,54,55,56].

Fig. 3
figure 3

a CV curves of NiMoO4/CoMoO4 composite electrode with Ni/Co molar ratio 1:1 at various scan rates of 2, 5, 10, 15, and 20 mV s−1. b GCD curves of NiMoO4/CoMoO4 composite electrode with Ni/Co molar ratio 1:1 at various current densities of 1, 4, 7, and 10 A g−1. c Specific capacitances of the electrode as a function of current densities

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Figure 3b shows the GCD curves of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1 at various current densities of 1, 4, 7, and 10 A g−1, respectively. A well-defined platform can be observed in the GCD curves, and the discharge specific capacitance calculated according to Eq. (1) is shown in Fig. 3c. When the discharge current densities are 1, 4, 7, and 10 A g−1, the excellent discharge specific capacitances of the composite are 2221, 1868, 1678, and 1568 F g−1, respectively. When the current density increases from 1 to 10 A g−1, the discharge time and discharge specific capacitance decrease gradually. It is impressive that the highest discharge specific capacitance is 2221 F g−1 at 1 A g−1. When the current density increases to 10 A g−1, the discharge specific capacitance (1568 F g−1) is 70.6% retention for 1 A g−1. However, according to literature report [41], the specific capacitance of NiMoO4 is 1088.5 F g−1 at current density of 1 A g−1, which indicates that the synergistic effect of NiMoO4/CoMoO4 composite makes it a high capacity and excellent rate material [48].

To demonstrate the advantages of the material, in Table 1, we compare the related properties (such as specific capacitance at low and high current densities) of NiMoO4/CoMoO4 electrode with other recently reported transition metal oxide–based electrodes in the literature. Compared with previous studies, the NiMoO4/CoMoO4 electrode reported in this paper has higher specific capacitance and bigger rate capability. This is due to the fact that NiMoO4 and CoMoO4 components exhibit good synergies, making the composite exhibit better capacitive properties, indicating that NiMoO4/CoMoO4 composite is an ideal material for building SCs.

Table 1 Comparison with previously reported supercapacitor electrode materials
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Figure 4 shows the XRD results of the as-synthesized NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1. The diffraction patterns of single NiMoO4 and pure CoMoO4 are consistent with the standard spectra of NiMoO4 (JCPDS card no. 33-0948) and CoMoO4 (JCPDS card no. 25-1434), respectively. In addition, the present both the characteristic diffraction peaks of NiMoO4 and CoMoO4 phases are appeared in the spectra of NiMoO4/CoMoO4 composite, indicating the coexistence of NiMoO4 and CoMoO4. For the hybrid composite, the diffraction peaks at 14.3°, 25.3°, 28.8°, 32.7°, 43.8°, and 47.5° are attributed to the reflections of (110), (002), (220), (022), (330), and (204) planes, which is in good agreement with the standard spectrum of NiMoO4 [51]. In addition, the other diffraction peaks occurring at 14.1°, 25.1°, 28.5°, 32.3°, and 43.2° can be readily indexed to CoMoO4 [78,79,80]. Therefore, XRD analysis shows that we have successfully synthesized NiMoO4/CoMoO4 composite on Ni foams. Besides, there are still some weak peaks, showing a lower crystallinity.

Fig. 4
figure 4

XRD pattern of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1

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Figure 5 shows the FTIR spectra of the prepared samples. Obviously, the typical peaks 958 cm−1, 873 cm−1, and 738 cm−1 were observed on the NiMoO4/CoMoO4 curve, corresponding to the absorption vibrations of Mo–O, Co–O, and Ni–O bonds, respectively. The extra peak at 3423 cm−1 is due to the stretching vibration of -OH. In addition, the bending vibration of the FTIR peak at 1620 cm−1 is due to the physical adsorption of water molecules in the sample, this can indicate the presence of crystal water in the sample [81].

Fig. 5
figure 5

FTIR spectra of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1

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In order to further investigate the valence states of elements in the as-synthesized NiMoO4/CoMoO4 composite, XPS experiments were performed, and the corresponding results are shown in Fig. 6. In detail, the survey spectra (Fig. 6a) of the composite sample exhibits the distinct peaks of Co 2p, Ni 2p, Mo 3d, and O 1s peaks located at 781.2, 854.7, 231.2, and 530.1 eV, revealing the presence of Co, Ni, Mo, and O elements, respectively. Figure 6b shows the Co 2p core level spectrum; the two main fitted peaks at 796.7 and 780.6 eV, accompanied two diminutive satellite peaks at 803.1 and 784.2 eV, are assigned to Co 2p1/2 and Co 2p3/2 energy level, respectively. The primary peaks and shakeup satellite peaks of Co 2p1/2 and Co 2p3/2 indicate the Co2+ valence state [33]. As depicted in Fig. 6c, the Ni 2p spectrum was fitted by four peaks. The major peak at 873.4 eV and its satellite peak at 879.9 eV are owning to Ni 2p1/2 level, whereas those at 855.6 and 861.7 eV are ascribed to Ni 2p3/2 level. Further, the gap in binding energy between the main peaks of Ni 2p1/2 and Ni 2p3/2 is 17.8 eV, proving the existence of the Ni2+ oxidation state [44]. The deconvoluted Mo 3d spectrum (Fig. 6d) exhibits two major peaks at 235.3 and 232.2 eV which can be assigned to Mo 3d3/2 and 3d5/2 energy level, respectively. The two peaks are separated by a binding energy of 3.1 eV, confirming the existence of an oxidation state of Mo6+ [82], which is consistent with the previous reports [83]. In addition, the O 1s spectrum (Fig. 6e) can deconvoluted into three oxygen peaks, located at 529.9, 530.5, and 532.2 eV of O1, O2, and O3 components, respectively. The O1 component is related to metal–oxygen bond, while O2 component is attributed to functional groups and defect sites, and O3 component is ascribe to the surface physical adsorption of H2O [84].

Fig. 6
figure 6

a Full XPS spectra and the deconvoluted b Co 2p, c Ni 2p, d Mo 3d, and e O 1s spectra of NiMoO4/CoMoO4 composite

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Figure 7a is the EDS spectrum of NiMoO4/CoMoO4 composite, which shows the surface composition of the component. The results show that the material is composed of Co, Ni, Mo, and O elements, and the ratio of Co to Ni is close to 1:1, suggesting that the sample is mainly composed of Co, Ni, Mo, and O, which is consistent with the XRD results [44, 85]. As shown in Fig. 7b–e, the element mapping images indicate the uniform distribution of Co, Ni, Mo, and O in NiMoO4/CoMoO4 composite sample, suggesting the coexistence of NiMoO4 and CoMoO4.

Fig. 7
figure 7

a EDS pattern of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1. EDS distribution mapping of b Co, c Ni, d Mo, and e O

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Figure 8a and b show the SEM images of synthesized NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1. SEM images of prepared composite show grain-like morphology. The low-magnification SEM images in Fig. 8 depict the as-prepared composites are uniformly distributed on the surface of nickel foam after mild hydrothermal procedures. The aggregation of these small particles forms a porous structure, and their sizes are between 2 and 3 μm. This porous structure composite material formed on nickel foam provides abundant space and electroactive sites for electrochemical reaction, and shortens the length of diffusion path, so that its electrochemical properties can be significantly improved.

Fig. 8
figure 8

a, b Typical SEM images of NiMoO4/CoMoO4 composite with Ni/Co molar ratio 1:1

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Conclusions

In summary, NiMoO4/CoMoO4 composite was successfully synthesized by an affinity hydrothermal method, which have the advantages of simplicity and cost-effectiveness. The results indicated that the as-prepared composite with the Ni/Co molar ratio of 1:1 has the best electrochemical properties, which are uniformly distributed on the surface of nickel foam with diameters between 2 and 3 μm. The specific capacitance of NiMoO4/CoMoO4 composite was 2221, 1868, 1678, and 1568 F g−1 at the current density of 1, 4, 7, and 10 A g−1 in 3 mol L−1 KOH electrolytes. In terms of specific capacitance, rate capability, cost, and simple synthesis process, its excellent electrochemical performance is satisfactory, even better than that reported in the literature, indicating its broad application prospect in high-performance SCs. The enhancement of electrochemical performance can be mainly due to the introduction of CoMoO4 and the synergistic effect of cobalt molybdates and nickel molybdates, which can provide channel for quick diffusion and transport of electrons and ions and a large number of active sites.