Introduction

With the rapid development of electric vehicles, portable electronic devices and other energy storage devices, the development of efficient, safe, renewable, clean and green energy storage devices has become the focus of attention (Li et al. 2018; Krishnamoorthy et al. 2018). Supercapacitors (SCs) possess high power and energy densities, long cycle life, environmental protection, flexibility and small size and have become an excellent candidate for energy storage (Zhai et al. 2020; Shao et al. 2015; Liu et al. 2017; Chen et al. 2020; Salunkhe 2016). On the basis of the energy storage principle, electrode materials for supercapacitors include electric double-layer capacitors (EDLCs) materials (Ji et al. 2014; Choi et al. 2012) and pseudocapacitors (PCs) materials (Banda et al. 2021; Boota et al. 2019) and battery-type capacitors (BTCs) materials (Zhaon et al. 2022; Wang et al. 2021). The positive and negative electrodes of EDLCs store energy by forming a double layer between the electrode surface and the electrolyte, which is mainly caused by the increase or decrease of high-energy conduction electrons on the electrode surface and does not produce an electrochemical reaction, and the energy storage process is reversible. In general, the electrode materials of EDLCs mainly include activated carbon (AC) (Kostoglou et al. 2018), carbon nanotubes (CNT) (Zhao et al. 2020) and graphene (Xu et al. 2015) and so on. Also, the most common electrode materials for PCs are conductive polymers (Cho et al. 2008) and transition metal compounds (Sathiya et al. 2011). However, PCs (BTCs) are caused by the underpotential deposition method, highly reversible chemical adsorption and desorption or redox reaction of the active electrode materials in the two-dimensional (2D) or quasi-2D space (Salunkhe et al. 2016), and its maximum charge–discharge capacity is controlled by the transfer rate of ions and charges on the surface of the electroactive materials, so charge transfer can be carried out in a short time, that is to say, higher specific power can be obtained. Therefore, PCs (BTCs) have a higher specific capacity (Cp) and an energy density than EDLCs (Jayachandiran et al. 2021; Yang et al. 2020).

Transition metal oxides (TMOs), such as MnO2 (Najafpour et al. 2015), V2O5 (Fleischmann et al. 2017), Co3O4 (Priyadharsini et al. 2020), RuO2 (Ates et al. 2020), NiO (Ates et al. 2020), have been attracting increasing attention on account of their relatively high theoretical capacity, variable oxidation states, environmental protection and low cost. However, TMOs often have some disadvantages such as low mechanical stability and poor electrical conductivity. It is well known that transition metal sulfides (TMSs) such as MoS2 (Savjani et al. 2016), Co3S4 (Shrivastav et al. 2020), NiS2 (Azad et al. 2020) have a smaller band gap, better conductivity and better rate capability than TMOs. Bimetallic sulfides (BMSs) are widely used in energy storage devices such as lithium ion batteries (Lv et al. 2018; Guo et al. 2019), solar cells (Yu et al. 2020; Tang et al. 2020), sensors (Mariyappan et al. 2020) and SCs (Tao et al. 2018; Zhai et al. 2019) due to their diverse crystalline structure, higher conductivity and superior electrochemical activity compared with oxides. Molybdenum-based bimetallic sulfides have widely used in supercapacitors due to their high conductivity, accessibility, additional capacity of Faradic reaction in the center of Mo atom and multiple oxidation states from + 2 to + 6 (Xu et al. 2020). Meanwhile, nickel (Ni) is a common variable metal (Ni3+/Ni2+) and possesses a good magnetism and plasticity. Also, its chemical properties are relatively reactive, but more stable than iron, which is conducive to the redox reaction to improve the electrochemical energy storage performance. Mo-Ni-based bimetallic sulfides show a prospective electrochemical performance, which may be caused by a synergistic effect of Mo and Ni. The spongy NiMoS4 synthesized by Xu et al. shows a Cp of 233 mAh g−1 at 0.75 A g−1 and a retention of 84% after 8000 cycles (Xu et al. 2020). The NiMoS4-A synthesized by Du et al. shows a Cp of 313 C g−1 at 1 A g−1 and a retention rate of 82% after 10,000 cycles (Du et al. 2017). However, there is a certain gap between the reported Cp and theoretical Cp for NiMoS4. Therefore, it is imperative to choose a suitable simple synthesis method and regulate a suitable structure to increase the Cs of NiMoS4 (Shrestha et al. 2021). In this study, a novel NiMoO4@NiMoS4/NF core–shell nanocomposite was obtained by an in situ growth method and a high-temperature calcination method. Due to the unique nanorods@nanosheets core–shell heterostructures, the electrode material possesses a more abundant electrochemical active surface area and sites, showing excellent electrochemical performance in capacitor storage. Additionally, the NiMoO4@NiMoS4 material is directly deposited on NF, which avoids the addition of binders during electrode construction, thus improving the conductivity, rate capability and Cp of the electrode materials.

The aims of this investigation are: (1) to characterize the NiMoO4/NF and NiMoO4@NiMoS4/NF nanocomposite via X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) techniques; (2) to study the electrochemical performance of NiMoO4/NF and NiMoO4@NiMoS4/NF with the use of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge (GCD); and (3) to investigate the electrochemical performance of hybrid supercapacitor based on NiMoO4@NiMoS4/NF and activated carbon (AC).

Experimental

Chemicals

NaMoO4 2H2O (100 g, 99.95%), potassium hydroxide (KOH), (NH4)2S, Ni(NO3)2 6H2O, other chemicals (analytical reagent) were purchased from Xilong Chemical Co., Ltd. (Guangdong, China).

Pretreatment of NF

The NF was cut to the size of 1 × 1 cm2 and then, respectively, placed in hydrochloric acid (3 M HCl), ethanol and ultra-pure water with ultrasonic for 10 min to remove surface oxides and impurities.

Preparation of NiMoO4/NF

Ni(NO3)2.6H2O (0.87 g) and NaMoO4.2H2O (0.73 g) were placed into a 200 mL of beaker, and 60 mL ultrapure water was added for stirring for 15 min continuously. Then the pretreated NF and the mixed solution were together transferred into a 100-mL reactor (Teflon-lined autoclave) and heated at 150 °C for 5 h. The NF loaded with materials was washed with ultra-pure water (UPW) and absolute alcohol several times in sequence and dried in vacuum at 80 °C for 12 h. Finally, the above sample (NiMoO4/NF) was calcined in argon (Ar) at 300 °C in a tube furnace for 2 h.

Preparation of NiMoO4@NiMoS4/NF

The synthesized NiMoO4/NF was vulcanized at 70 °C for 8 h in 60 mL (NH4)2S solution with different concentrations (5, 10 and 15 mM) to obtain NiMoO4@NiMoS4/NF (marked as NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3, respectively). The obtained samples (NiMoO4@NiMoS4/NF) were cleaned with UPW and absolute alcohol several times in turn and dried in vacuum at 80 °C for 12 h. The active material attached to NF was approximately 1.0 mg. Scheme 1 is the schematic diagram of the synthesis method for NiMoO4@NiMoS4/NF.

Scheme 1
scheme 1

The schematic diagram of the synthesis method for NiMoO4@NiMoS4/NF

Full size image

Material characterizations

Crystal information, sample composition and morphology were obtained by XRD with the 2θ range from 5° to 80° (Bruker D8 Advance), XPS (Thermo Fisher, Nexsa), SEM (ZEISS MERLIN Compact) and TEM (Tecnai G2 F20).

Electrochemical measurement

Electrochemical methods including CV, EIS and GCD were performed on an electrochemical workstation (CHI660A, Shanghai Chenhua Co., LTD) in 1 M KOH electrolyte. Ag/AgCl, Pt wire and NiMoS4@NiMoO4/NF were used as the reference, auxiliary and working electrodes, respectively. In a two-electrode system, a NiMoO4@NiMoS4/NF-2//AC hybrid device was constructed through NiMoO4@NiMoS4/NF-2 as the positive electrode and AC as the negative electrode. The specific process is as follows: Acetylene black, polyvinylidene fluoride and AC were accurately weighed according to the mass ratio of 1:1:8 and transferred to an agate mortar. An appropriate amount of N-methyl-2-pyrrolidinone (NMP) was added and ground into a homogeneous paste. A certain amount of paste was coated on the NF and dried in vacuum for 12 h to obtain AC electrode. Both NiMoO4@NiMoS4/NF and AC electrodes should be immersed into 1 M NaOH electrolyte to activate for 12 h before use.

According to the charge balance theory of the asymmetric device (ASC) (Q+  = Q−), the charge stored in a capacitor depends on the Cp and active mass of the electrode. In this study, the active materials of NiMoO4@NiMoS4/NF and AC were 1 and 15 mg, respectively. That is, the mass ratio of positive electrode (NiMoS4@NiMoO4) and negative electrode (AC) is 0.067.

The specific capacitance (Cs, F g−1), Cp (C g−1), energy density (E) and power density (P) are calculated by the following formula (1–4) (D.W. Du et al. 2017).

$$C_{s} = I\Delta t/\Delta V \times m$$
(1)
$$C_{p} = I\Delta t/ \times m$$
(2)
$$E = \, \left( {{1}000/{2 } \times { 36}00} \right)C_{p} \left( {\Delta V^{{2}} } \right)$$
(3)
$$P = { 36}00E/ \, \Delta t$$
(4)

Results and discussion

Characterization of NiMoO4 and NiMoO4@NiMoS4

XRD was performed to study the crystal structure of the prepared NiMoO4 and NiMoO4@NiMoS4. The XRD pattern of NiMoO4 shows responses (Fig. 1A) at 2θ (degree) = 14.3, 18.9, 28.9, 32.8, 38.8, 43.9, 47.5 and 56.7° are attributed to the NiMoO4 (110), (− 201), (220), (022), (112), (330), (− 204) and (024), respectively (JCPDS No. 86–0361) (M.J. Gao et al. 2019). Additionally, the patterns of NiMoO4 are in good accordance with NiMoO4·xH2O (JCPDS No.13–0128), indicating the successful synthesis for the NiMoO4 electrode material. However, it is difficult to determine the full crystal structure because the detail information of NiMoO4·xH2O is still not clear. The characteristic diffraction peaks of NiMoO4@NiMoS4 are consistent with the characteristic diffraction peak of NiMoO4, showing that NiMoS4 is a typical amorphous characteristic, so no other obvious characteristic peaks appear. Additionally, the some characteristic peaks of NiMoO4@NiMoS4 are weaker than that of NiMoO4, which further proves that NiMoS4 on the surface is a typical amorphous structure (Gao et al. 2019).

Fig. 1
figure 1

A XRD patterns of NiMoO4@NiMoS4 and NiMoO4; B XPS patterns of NiMoO4@NiMoS4; C Ni 2p, D Mo 3d and E S 2p of NiMoO4@NiMoS4

Full size image

XPS is an important means of characterization of materials, and it can provide not only general chemical information, but also surface, microregion and depth distribution information of materials. Figure 1B indicates full scan spectrum of NiMoS4@NiMoO4 nanoparticles; Ni, Mo, S and O elements can be clearly observed in full scan spectrum. According to Fig. 1C, the Ni 2p spectrum is formed by fitting four distinct peaks. Peaks at 856.3 and 873.8 eV correspond to Ni 2p3/2 and Ni 2p1/2, respectively, while the peaks at 862.2 and 880.1 eV correspond to their satellite peaks, respectively, which are in line with Ni2+. From Fig. 1D, two obvious peaks at 232.5 and 235.6 eV correspond to Mo 3d5/2 and Mo 3d3/2, and their dissociative binding energy is 3.1 eV, which conforms to Mo6+. In Fig. 1E, the binding energies at 161.8 and 169.2 eV correspond to S 2p3/2 and S 2p1/2, respectively, indicating that the valence of S in the NiMoS4 sample is − 2 (Xu et al. 2020).

Figure 2 shows SEM images of the NiMoO4 and NiMoO4@NiMoS4 samples. From Figs. 2A-2F, rod and columnar nanostructured materials are observed with approximately 35–50 nm in diameter and 0.1–0.3 μm in length, some of the powder agglomerating. The biggest difference between NiMoO4 (Figs. 2A-2C) and NiMoO4@NiMoS4 (Figs. 2A-2E) is the surface structure of two kinds of materials. It is completely obvious that the surface morphology of NiMoO4 is relatively smooth, and that of NiMoO4@NiMoS4 is rough and uneven, and a large number of nanosheets attach to the surfaces of cylinders to form a core–shell structure.

Fig. 2
figure 2

SEM images of NiMoO4 (A, B and C) and NiMoO4@NiMoS4 (D, E and F)

Full size image

Figure 3 is the color mapping of oxygen (O), nickel (Ni), molybdenum (Mo) and sulfur (S) elements (B, C, D and E) of NiMoO4@NiMoS4. It is very apparent that the O, Ni, Mo and S elements were detected in the NiMoO4@NiMoS4 nanocomposite, indicating that partial O element in the NiMoO4@NiMoS4 composite was indeed replaced by the S element, which further demonstrates the successful synthesis for the NiMoO4@NiMoS4 nanocomposite. According to Fig. 3B, a large number of O elements exist in NiMoO4@NiMoS4, due to the fact that the NiMoO4 mainly exists in the core part of NiMoO4@NiMoS4. From Fig. 3C, only some sparse dots are observed, but this does not mean the lack of Ni element in NiMoO4@NiMoS4 composite, which can be interpreted that the Ni element mainly exists in the inner “core,” and it is less distributed in the outer “shell” of materials (Shrestha et al. 2021).

Fig. 3
figure 3

SEM image of the NiMoO4@NiMoS4 (A); the corresponding elemental mapping of NiMoO4@NiMoS4 of O, Ni, Mo and S (B, C, D and E)

Full size image

The detailed morphology and microstructure of prepared materials were further investigated by TEM, and the NiMoO4 and NiMoO4@NiMoS4 samples were obtained through an ultrasonic stripping method, respectively. TEM images of the NiMoO4 and NiMoO4@NiMoS4 samples are shown in Figs. 4(A and B) and Figs. 4 (D and E). A rod-like nanostructure was observed in NiMoO4 and NiMoO4@NiMoS4 samples. However, compared with NiMoO4 (Fig. 4(A and B)), the NiMoO4@NiMoS4 (Fig. 4(D and E)) has an core–shell structure, and such core–shell structure makes NiMoO4@NiMoS4 nanocomposite have a lot of active surface, reduce the dead volume, promote the rapid redox process, generate rich induced current and improve the stability of electrochemical cycle. Additionally, the high-magnification TEM (HRTEM) images for the NiMoO4 and NiMoO4@NiMoS4 samples are shown in Figs. 4 (C and F). From Fig. 4C, the lattice fringes of NiMoO4 sample were observed distinctly, and the lattice spacings were measured to be 0.621 and 0.228 nm, which correspond to (110) and (112) lattice planes of NiMoO4. Figure 4F shows the HRTEM image of NiMoO4@NiMoS4, and the lattice spacings of 0.621 and 0.228 nm were also observed, corresponding to (110) and (112) lattice planes of NiMoO4, which further confirms that the “core” part of the material is made up of NiMoO4.

Fig. 4
figure 4

TEM and HRTEM images of NiMoO4 (A, B and C) and NiMoO4@NiMoS4 (D, E and F)

Full size image

In order to clearly distinguish the differences in material properties before and after vulcanization, the specific surface area and pore size distribution of the NiMoO4/NF and NiMoO4@NiMoS4/NF-2 electrode materials were examined. The adsorption isotherms of the two electrode materials are shown in Figs. 5A and 5B. Typical isotherm type IV was presented according to the IUPAC classification. Based on BET-specific surface area measurements and the BJH method, the NiMoO4@NiMoS4/NF-2 (20.488 m2/g) had a larger BET surface area than the NiMoO4 (18.128 m2/g). The pore size distributions (PSD) of the NiMoO4/NF and NiMoO4@NiMoS4/NF-2 electrodes are shown in Figs. 5C and 5D. The average pore sizes of NiMoO4/NF and NiMoO4@NiMoS4/NF-2 were calculated to be ~ 17.377 and ~ 14.414 nm through BJH method, respectively, which supports the observation as the majority of the pores lie in the mesoporous region in all of the specimens.

Fig. 5
figure 5

Nitrogen adsorption–desorption isotherms of A NiMoO4/NF and B NiMoO4@NiMoS4/NF-2; PSD of C NiMoO4/NF and D NiMoO4@NiMoS4/NF-2

Full size image

Electrochemical performance of NiMoO4/NF and NiMoO4@NiMoS4/NF-2

In a three-electrode system, the electrochemical tests of NiMoO4/NF and NiMoO4@NiMoS4/NF-2 were carried out by CV, EIS and GCD measurements in 1 M KOH solution. Figure 6A is the CV curves of NiMoO4@NiMoS4/NF-2, NiMoO4/NF and NF within a voltage window of 0–0.6 V at 5 mV s−1. A pair of redox peaks are observed, showing that the redox reaction is a quasi-reversible process, which demonstrates the battery-type characteristics for the NiMoO4/NF and NiMoO4@NiMoS4 /NF-2 electrode materials. In general, the integral area enclosed by CV curve corresponds to the Cp value of the electrode material. From Fig. 6A, the electrochemical signal for the NF is negligible compared with the NiMoO4/NF and NiMoO4@NiMoS4/NF-2. So the capacity contribution of the NiMoO4/NF and NiMoO4@NiMoS4/NF-2 mainly originates from the active material themselves (Xu et al. 2020). The area of NiMoO4@NiMoS4/NF-2 is much larger than that of NiMoO4/NF, indicating that NiMoO4@NiMoS4/NF-2 electrode material has relatively excellent electrochemical performance and can generate the maximum specific capacity. The capacity of NiMoO4/NF and NiMoO4@NiMoS4/NF-2 is mainly contributed by the Faraday redox reaction of Ni2+/Ni3+ (Eqs. (6 and 7) (Gao et al. 2019).

$${\text{Ni}}^{{{2} + }} + {\text{ 2OH}}^{ - } = {\text{ Ni }}\left( {{\text{OH}}} \right)_{{2}}$$
(5)
$${\text{Ni }}\left( {{\text{OH}}} \right)_{{2}} + {\text{ OH}}^{ - } = {\text{ NiOOH }} + {\text{ H}}_{{2}} {\text{O }} + {\text{ e}}^{ - }$$
(6)
Fig. 6
figure 6

A CVs of NF, NiMoO4@NiMoS4 and NiMoO4 at 5 mV s−1; B GCDs of NF, NiMoO4@NiMoS4 and NiMoO4 at a current density of 1 A g−1; C the corresponding specific capacity values at different current densities for NiMoO4@NiMoS4 and NiMoO4; D Nyquist plots of NiMoO4@NiMoS4 and NiMoO4 at open-circuit potential

Full size image

Figure 6B is the GCDs of NiMoO4@NiMoS4/NF-2, NiMoO4/NF and NF within a voltage window of 0–0.45 V at a current density of 1 A g−1. It is clear that the NF cannot be charged and discharged, which fits well with the CV results. The GCD curves of NiMoO4@NiMoS4/NF-2 and NiMoO4/NF deviate from the regular triangle, showing a deviation from linearity, which indicates that the NiMoO4@NiMoS4/NF-2 and NiMoO4/NF are typical battery-type electrodes. According to the calculation, when the current density is 1 A g−1, the Cp of NiMoO4@NiMoS4/NF-2 and NiMoO4/NF is 1996 and 279 C g−1, respectively. Compared with NiMoO4/NF, the Cp of NiMoO4@NiMoS4/NF-2 increased by 7 times. These could be explained that O element is gradually replaced by S element with low electronegativity, which generates a more flexible core–shell structure to effectively prevent the disintegration of the structure and facilitate the transfer of electrons inside the materials. So, NiMoS4@NiMoO4/NF nanocomposite with core–shell structure possesses a larger surface area and more electrochemical active sites to facilitate the ion/electron transfer between the electrolyte and electrode materials. Figure 6C shows the Cp values of the NiMoO4@NiMoS4/NF-2 and NiMoO4/NF at different current densities. When the current density is 1, 2, 4, 6, 8, 10, 15, 20, 25 and 30 A g−1, the Cp of NiMoO4/NF is 279, 249.8, 220.4, 204.6, 188.0, 171.0, 141.0, 106.0, 75.0 and 42.0 C g−1, respectively, and the Cp of NiMoO4@NiMoS4/NF-2 is 1996.0, 1748.0, 1603.6, 1482.6, 1394.4, 1338.0, 1207.5, 1124.0, 1067.5 and 945.0 C g−1, respectively.

In order to further understand the electrochemical properties of the electrode materials, the Faraday resistance for the NiMoO4@NiMoS4/NF-2 and NiMoO4/NF was compared using EIS (Fig. 6D). The arc in a high frequency is usually related to the charge-transfer resistance (Rct). In general, the smaller the radius of the arc, the faster the rate of charge transfer. From Fig. 6D, the Rct values of NiMoO4@NiMoS4/NF-2 and NiMoO4/NF are calculated to be 0.51 and 4.80 Ω, and the Rct of NiMoO4@NiMoS4/NF-2 is far less than that of NiMoO4/NF, indicating that NiMoO4@NiMoS4/NF-2 has a fast charge transfer rate and a fast redox reaction, which attributes to the transport and transfer between the electrolyte and electrode materials caused by highly conductive NiMoO4@NiMoS4 core–shell structure (M.J. Gao et al. 2019).

The effects of different concentrations of S2− on the Cp of NiMoO4@NiMoS4/NF were investigated. When the concentrations of S2− are 5 mM, 10 mM and 15 mM, the NiMoO4@NiMoS4/NF electrode materials were marked as NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3, respectively. Figure 7A shows the CVs of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3. It is very obvious that the enclosed area of NiMoO4@NiMoS4/NF-2 is larger than those of NiMoO4@NiMoS4/NF-1 and NiMoO4@NiMoS4/NF-3, indicating that the NiMoO4@NiMoS4/NF-2 has the largest Cp. Figure 7B shows the GCDs of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3. By calculation, when the current density is 1 A g−1, the Cp of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3 is 465.1, 1996.0 and 1311.0 C g−1, respectively. Moreover, the Cp of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3 at different current densities is shown in Fig. 7C. When the current density is 1, 2, 4, 6, 8, 10, 15, 20, 25 and 30 A g−1, the specific capacity of NiMoO4@NiMoS4/NF-1 is 465.1, 436.6, 398.0, 361.2, 340.8, 319.0, 262.5, 212.0, 152.5 and 96.0 C g−1, respectively, and the specific capacity of NiMoS4/NF-3 is 1311.0, 1169.6, 1047.2, 978.6, 915.2, 852.0, 775.4, 704.0, 650.0 and 633.0 C g−1, respectively. Figure 8 is SEM images of NiMoO4@NiMoS4/NF-1 (A), NiMoO4@NiMoS4/NF-2 (B) and NiMoO4@ NiMoS4/NF-3 (C). According to Fig. 8A, a large number of loose laminated structures (NiMoS4) are observed after the partial O element in the NiMoO4@NiMoS4 composite was indeed replaced by the S element. With the increasing concentration of (NH4)2S, NiMoS4 nanosheets get denser and denser (Fig. 8B). However, when the S2− concentration further increases, too many sheet structures give rise to serious agglomeration on the surface of the samples (Fig. 8C), which results in a decline in effective active area and active sites of the electrode materials (Lv et al. 2022). Therefore, when the S2− concentration is suitable, the NiMoO4@NiMoS4/NF electrode material will show a better electrochemical performance. Table 1 compares the value of Cp and cycling stability for the NiMoO4@NiMoS4/NF with those of some other Ni and Mo bimetallic oxide and sulfide electrode materials for SCs reported in the previous literatures. The results showed that the Cp of the NiMoO4@NiMoS4/NF-2 is higher than those of similar materials, which may be related to the good electrical conductivity of the materials.

Fig. 7
figure 7

A CVs of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3 at a scan rate of 5 mV s−1 B GCDs of NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@ NiMoS4/NF-3 at a current density of 0.5 A g−1, C the corresponding specific capacity values at different current densities for NiMoO4@NiMoS4/NF-1, NiMoO4@NiMoS4/NF-2 and NiMoO4@NiMoS4/NF-3

Full size image
Fig. 8
figure 8

SEM images of NiMoO4@NiMoS4/NF-1 A, NiMoO4@NiMoS4/NF-2 B and NiMoO4@ NiMoS4/NF-3 C

Full size image
Table 1 Comparison of Cp and cycling stability for the NiMoO4@NiMoS4/NF with reported Ni and Mo bimetallic oxide and sulfide electrode materials for SCs
Full size table

In Fig. 9A, the CV curves of NiMoO4@NiMoS4/NF-2 at different scanning speeds are investigated. With the increase of scanning rates, the oxidation and reduction peaks shift to the high and low potential (Tang et al. 2020), respectively. At different scanning rates, the CVs in shape are similar, showing excellent redox reversibility. In Figs. 9B and 9C, the GCDs of NiMoO4@NiMoS4/NF-2 at different current densities are investigated. With the increase of current density, the charging and discharging times decrease, which is attributed to the fact that the ion diffusion is difficult in the electrolyte and the charge transfer at electrode materials is slow at high current density. The cyclic stability of NiMoO4@NiMoS4/NF-2 at a current density of 20 A g−1 is shown in Fig. 9D, and the results show that the capacity retention rate of NiMoO4@NiMoS4/NF-2 reaches 83.1% after 5000 cycles. At the beginning, the Cp decays rapidly, and at the later stage, the Cp is relatively stable. This may be because it is difficult for ions/electrons to enter the electrode material inside at a higher current density, thus causing the Cp to decline.

Fig. 9
figure 9

(A) CVs of NiMoO4@NiMoS4/NF-2 at different scan rates; (B and C) GCDs of NiMoO4@NiMoS4/NF-2 at different current densities; (D) cyclic stability of NiMoO4@NiMoS4/NF-2 at a current density of 20 A g−1

Full size image

In a two-electrode system, the hybrid SC—NiMoO4@NiMoS4/NF-2//AC was constructed with NiMoO4@NiMoS4/NF as the positive electrode and AC as the negative electrode in 1 M KOH electrolyte. Combined with the voltage window of 0–0.45 V for NiMoO4@NiMoS4/NF-2, the voltage window of − 1.0–0 V for AC and the maximum polarization effect of the application window, the voltage window of the NiMoO4@NiMoS4/NF-2//AC asymmetric SC is extended to 0 to 1.45 V (X.Y. Xu et al. 2020). The electrochemical tests of NiMoO4@NiMoS4/NF-2//AC at different scanning rates and current densities are shown in Fig. 10. It can be clearly seen from Fig. 10 that the curve of electrode material presents non-rectangular CV curve (Fig. 10A) and nonlinear GCD curve (Fig. 10B), which shows that the Cp value is mainly attributed to Faraday redox reaction. When the current density is 1, 2, 4, 6, 8, 12 and 16 A g−1, the Cp value is 162.6, 147.8, 122.4, 144.6, 128.8, 112.8 and 100.8 C g−1, respectively. At high current density, the Cp of the device is gradually reduced due to the inherent resistance in the electrode and the lack and loss of Faraday reaction for electrode materials. Moreover, the energy density and power density are calculated to be 32.75 Wh kg−1 and 725.09 W kg−1, respectively.

Fig. 10
figure 10

A CVs of NiMoO4@NiMoS4/NF-2//AC at different scan rates (5, 10, 20, 30, 50 and 100 mV s−1); B GCDs of NiMoO4@NiMoS4/NF-2/AC at different current densities (1, 2, 4, 6, 8, 12 and 16 A g−1)

Full size image

Conclusions

In conclusion, a novel NiMoO4@NiMoS4 core–shell nanocomposite is facilely grown on NF via hydrothermal reaction and high-temperature calcination methods. The NiMoO4@NiMoS4/NF-2 shows ultra-high Cp and good cycle stability, and these could be mainly explained that the typical core–shell structure possesses a larger active surface area and abundant electrochemical active sites, which is conducive to maximize the utilization of the active substance of NiMoO4@NiMoS4/NF-2 in energy storage. The nanosheet structure as the “shell” can promote the charge transfer from electrode material to the NF so as to realize the Faraday process quickly. At a current density of 1 A g−1, the Cp value of the NiMoO4@NiMoS4/NF-2 reaches 1996.0 C g−1, which is much higher than the Cp values of similar materials reported in the literature. So, the NiMoO4@NiMoS4/NF is an outstanding electrode material for hybrid SCs.