1 Introduction

The shortage of fossil energy resources leads to urgent requirement for the exploration of sustainable energy conversion and storage equipment [1,2,3]. Among them, the supercapacitor (SC) is an excellent energy storage device due to high power density and long cycle life [4, 5]. According to energy storage mechanism, SCs can be classified into electrical double-layer capacitors and pseudo-capacitors. The latter type possesses a greater potential than the former in terms of specific capacitance, due to highly reversible redox reactions of electrode materials [6,7,8]. However, low energy density restricts their practical application. Therefore, it is extremely important to develop high-performance electrode materials for this type of SCs.

At present, transition metal oxides are considered to be promising candidates for SC electrode materials [9,10,11,12,13]. However, these traditional cathode materials still show relatively poor conductivity and low specific capacitance. The ternary transition metal oxides show better conductivity than some binary counterparts due to the multiple oxidation valence states [14,15,16]. Co-based materials have been used as cathodes for SCs [17,18,19,20]. It is still crucial to tailor their shapes and structures to improve the electrochemical performance by constructing a Co3O4-based hybrid structure [21,22,23].

Herein, we report Co3O4@NiMoO4 nanowire structures grown on porous nickel (Ni) foam via a two-step hydrothermal method. With conductive Ni foam as the skeleton, electrode materials with high capacitance can be obtained. The as-obtained material delivers a capacity of 600 C/g at 1 A/g. Asymmetric SCs are assembled, with Co3O4@NiMoO4 as cathode and activated carbon as anode (Co3O4@NiMoO4//AC). The device shows an energy density of 36.1 Wh/kg and long cycle stability.

2 Methods

2.1 Synthesis of Co3O4 nanowires

First, the Ni foam (2 cm × 1 cm, as substrate) was washed three times with absolute ethanol and deionized (DI) water- by ultrasonic cleaner (SK7200H, Shanghai Kedao). Then, 5 mmol CoCl2·6H2O, 10 mmol NH4F and 3 mmol urea were added into 60 mL DI water. The solution and one piece of cleaned Ni foam were transferred into a 100 mL autoclave and kept for 8 h at 120 °C. The as-obtained Co3O4 nanowire sample was washed with DI water and absolute ethanol, and dried for 8 h at 60 °C. Finally, this sample was calcined in a muffle furnace (KSL-1100X, Hefei Kejing) at 400 °C for 2 h at a heating rate of 2 °C/min.

2.2 Synthesis of Co3O4@NiMoO4 composite

Briefly, as-prepared Co3O4 nanowires were utilized as the core structure for the growth of Co3O4@NiMoO4 composite. 3 mmol Ni(NO3)2·6H2O and 2 mmol NaMoO4·2H2O were dissolved in 60 mL deionized water. The solution and the previously obtained Co3O4 nanowires sample was kept in a reactor for 2 h at 160 °C. The final Co3O4@NiMoO4 composite sample was calcined in air at 400 °C for 2 h. Single NiMoO4 samples were obtained according to the above route without the addition of Co3O4 precursors.

2.3 Structure characterization

X-ray diffraction (XRD) patterns of the samples were measured on a X-ray diffractometer (BRUKER D8) using Cu Kα radiation (λ = 1.5406 Å). Morphological and structural features were characterized through scanning electron microscope (SEM, Sigma500 field emission) and X-ray photoelectron Spectroscope (XPS, ESCALAB250).

2.4 Fabrication of the asymmetric supercapacitor (ASC) device

Activated carbon, acetylene black and polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 7:2:1. The mixture was coated on a cleaned Ni foam (2 cm × 1 cm) to be used as anode of the ASC. The synthesized Co3O4@NiMoO4 composite sample was used as cathode. The electrolyte was prepared as follows: 30 mL of water was poured into a beaker and heated to 95 °C with a magnetic stirrer. Then adding 3 mg PVA and 3 g KOH and stirring it until a clear solution was obtained. A separator was used to isolate the anode from the cathode. The ASC device was sealed with an aluminum-plastic film.

2.5 Electrochemical measurements


In a three-electrode system, the electrochemical performance of the electrodes was measured in 3 mol KOH electrolyte, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) curves. Three samples (Co3O4, NiMoO4, and Co3O4@NiMoO4) were employed as the working electrode respectively, Hg/HgO as a reference electrode, and Pt plate as a counter one. The specific capacitance (Cs) of the samples can be obtained by applying discharge time (Δt):

$$C_{{\text{s}}} = I\Delta t/m,$$
(1)

where I stands for current density, m represents the mass of the electrode.

An ASC was assembled by using the Co3O4@NiMoO4 electrode as the cathode and AC electrode as the anode. Energy density (E) and power density (P) can be obtained through the equations as follows:

$$E = {1}/{2}C_{{\text{s}}} \left( {\Delta V} \right)^{{2}} ,$$
(2)
$$P = {36}00E/t.$$
(3)

3 Results and discussion

Figure 1 shows the schematic of the structure of productsCo3O4@NiMoO4 composite samples. Ni foam is directly used to grow Co3O4 nanowires due to its 3D conductive porous characteristics. Co3O4 samples and Co3O4@NiMoO4 composite samples are prepared as described in Sects. 2.1 and 2.2. Then we observed the morphologies of the samples with different magnifications using SEM. As shown in Fig. 2a, the Co3O4 sample presented a wire-like shape and was evenly covered on the Ni foam. From Fig. 2b, it can be seen that the average diameter of nanowires was 20 nm. Figure 2c and d present the morphologies of Co3O4@NiMoO4 composite samples, which show the NiMoO4 nanosheets were coated on the surface of Co3O4 nanowires, forming a core–shell structure as illustrated in Fig. 1. The surface of nanosheet becomes obviously rough. Figure 2e shows that the four elements of Co, Ni, Mo and O were evenly distributed on the surface of the sample.

Fig. 1 
figure 1

Schematic of the structure of Co3O4@NiMoO4 composite samples

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Fig. 2 
figure 2

Morphology characterization. a and b Co3O4 samples. c and d Co3O4@NiMoO4 samples. e Elemental mappings

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The XRD patterns of Co3O4@NiMoO4 samples are shown in Fig. 3a. The peaks at 18.95°, 55.27° and 59.28° correspond to (111), (422) and (511) of Co3O4 phase (JCPS: 42-1467). The peaks at 31.14°, 36.69° and 64.98° can be assigned to (220), (311) and (440) of NiMoO4 (JCPS: 12-0348). It is demonstrated that NiMoO4 nanosheets were successfully grown on the surface of the Co3O4 nanowires. XPS was used to further study the surface chemical states of Co3O4@NiMoO4 composite sample. The survey spectra (Fig. 3b) indicated that the Co3O4@NiMoO4 sample contained Co, Ni, Mo and O elements. Ni 2p spectra (Fig. 3c) shows four peaks at 855.4, 873.4 and 857.1, 875.6 eV, which are attributed to Ni2+ and Ni3+. In addition, two satellite peaks at 862.1 and 880.4 eV can be assigned to the high oxidation state [24]. Mo 3d peaks can be split into two peaks of Mo 3d5/2 and Mo 3d3/2, as shown in Fig. 3d. The peak binding energy at 231.6 eV belongs to Mo 3d5/2. However, the peak at 234.7 eV is from Mo 3d3/2, which further confirm the existence of Mo6+ oxidation state [25]. In Fig. 3e, O 1 s peaks at 531.8, 530.6 and 529.4 eV correspond to defect oxygen, O2− and OH− , respectively [26]. In Fig. 3f, two spin-orbital doublet peaks are well fitted to Co 2p1/2 and Co 2p3/2, revealing that Co2+ and Co3+ co-exist in the as-prepared composite material. Moreover, the peaks at 786.1 and 804.5 eV present the shakeup satellites [19].

Fig. 3 
figure 3

a XRD patterns of the samples. b Full spectra of XPS spectra of Co3O4@NiMoO4. Samples c Ni 2p, d Mo 3d, e O1s, and f Co 2p

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Figure 4a shows CV curves of the Co3O4, NiMoO4, Co3O4@NiMoO4 samples at scan rate of 50 mV/s. The obvious redox peaks suggest that the samples possess pseudo-capacitive characteristics. The Co3O4@NiMoO4 samples show the largest integral area, which means that hybrid samples present excellent electrochemical performance. Co3O4@NiMoO4 samples present the longest discharge time (Fig. 4b), revealing their maximal specific capacitance. It can be calculated by Eq. (1) that the Co3O4@NiMoO4 sample possesses the specific capacitance of 600 C/g, which is higher than those of Co3O4 (177.9 C/g) and NiMoO4 (315 C/g). The enhanced performance can be attributed to the synergistic effect between two individual materials. On one hand, the electrical conductivity can be improved and the transmission of electrons and ions can be facilitated. On the other hand, Co3O4 is a p-type semiconductor. As the core material, it undergoes inter-band transition to form electron–hole pairs, which result in strong redox ability. A weak electric field is formed between the two composite materials, which can prevent the recombination of electrons and holes, thus greatly improving the electrochemical performance.

Fig. 4 
figure 4

a CV curves at 50 mV/s. b GCD curves at 1 A/g. c CV curves. d GCD curves. e Nyquist plots. f Cycle performance

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Fig. 5 
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a CV curves of Co3O4@NiMoO4//AC ASC device at 50 mV/s. b CV curves at different potential windows. c CV curves at different scan rates. d Charge–discharge curves at different current densities

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Fig. 6 
figure 6

a Digital photos of the flexible device. b CV curves at various bending angles at the same scan rate. c Nyquist plots. d Cycling stability. e Ragone plots

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Figure 4c shows the CV curves of the prepared Co3O4@NiMoO4 sample at different scan rates. The curve shapes further reveal a pseudo-capacitance behavior. Even at scan rates of 50 mV/s, the initial shape of CV curves is still unchanged, which confirms that the composite samples possess an excellent electrical conductivity and high-rate performance. The corresponding GCD curves are shown in Fig. 4d. Even at the current density of 8 A/g, the specific capacitance can reach 97% of initial value.

Figure 4e shows typical Nyquist plots of the three samples. The Co3O4@NiMoO4 electrode possesses the lowest resistance of about 0.5 Ω in the three samples. To evaluate the cycle stability of the samples, the long cycle measurements were conducted at current density of 1 A/g. The results are shown in Fig. 4f. The Co3O4@NiMoO4 composite sample shows the 98.2% capacitance retention after 10000 cycles.

The performances of the fabricated ASC device were also measured. The CV curves of the device are shown in Fig. 5a. Co3O4@NiMoO4 and activated carbon electrodes possess the potential window from 0 to 0.6 V and − 1 to 0 V, respectively. From Fig. 5b, it can be found that the stable voltage window of ASC is 0–1.6 V. The Co3O4@NiMoO4//AC device (Fig. 5c) shows the voltage windows from 0 to 1.6 V at scan rates from 10 to 50 mV/s. GCD measurement (Fig. 5d) is conducted at different current densities with a voltage window of 1.5 V. It clearly shows that the device has a long discharge time.

The mechanical stability of energy storage devices is important for flexible electronic products. The mechanical stability of the fabricated ASC device was further investigated, as shown in Fig. 6a. When the device was folded at 30°, 90° and 120°, the shapes of the CV curves remains unchanged (Fig. 6b), revealing its outstanding stability and flexibility. It could be ascribed to the flexibility of the Ni foam and the tight contact between the electrode material and the Ni substrate. The EIS curves of the device are presented in Fig. 6c, revealing a low equivalent resistance and fast electron transfer rate. The lower inset is local EIS curve and the upper inset shows the corresponding equivalent circuit. Cycle performance (Fig. 6d) is a key performance for the application of SCs. The result demonstrates that 84.4% of the initial specific capacitance can be retained after 10000 cycles, indicating that the Co3O4@NiMoO4//AC device possesses an excellent electrochemical stability. From the CV curves of the first five cycles and the last five cycles, it can be found that the charging and the discharging time are always symmetric, indicating that the device presents very good reversibility and high-rate performance. Moreover, the discharge time of the last five cycles does not reduce compared with that of the first five cycles. which further confirms that there is little drop of the capacitance over 10000 cycles. Figure 6e is a Ragone plot of several devices. Table 1 shows the electrochemical performance of the devices based on different electrode materials. It was found that the Co3O4@NiMoO4//AC device can deliver an energy density of 36.1 Wh/kg at 2700 W/kg, which is higher than those reported in previous literatures [27,28,29,30,31,32,33,34].

Table 1  Electrochemical performance comparison of several SC devices based on transition metal oxides
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4 Conclusion

In summary, the core–shell Co3O4@NiMoO4 samples were successfully grown on Ni foam by a simple hydrothermal route. The synthesized samples presented an excellent specific capacitance (600 C/g at 1 A/g) and cycle stability. After 10000 charge–discharge cycle tests, the capacitance retention of the as-prepared composite still reached 98.2%, which shows long-term charging and discharging behavior. The as-assembled ASC delivered superior electrochemical performance with an energy density of 36.1 Wh/kg at 2700 W/kg and 84.4% initial capacity retention after 10000 cycles.