Co₃O₄@NiMoO₄ composite electrode materials for flexible hybrid capacitors

Abstract

Co₃O₄ nanomaterials as electrodes have been studied widely in the past decade due to their unique structural characteristics. However, their performance does not yet reach the level required for practical applications. It is, nevertheless, an effective strategy to synthesize hybrid electrode materials with high energy density. Herein we prepare Co₃O₄@NiMoO₄ nanowires by a two-step hydrothermal method. The as-obtained sample can be directly used as cathode material of supercapacitors; with specific capacitance of 600 C/g at 1 A/g. An assembled capacitor delivers an energy density of 36.1 Wh/kg at 2700 W/kg, and retains 98.2% of the initial capacity after 8000 cycles.

Graphical Abstract

Introduction

The shortage of fossil energy resources leads to urgent requirement for the exploration of sustainable energy conversion and storage equipment [1–3]. 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 [6–8]. 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 [9–13]. 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 [14–16]. Co-based materials have been used as cathodes for SCs [17–20]. It is still crucial to tailor their shapes and structures to improve the electrochemical performance by constructing a Co₃O₄-based hybrid structure [21–23].

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.

Methods

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.

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.

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).

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.

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 (C_s) of the samples can be obtained by applying discharge time (Δt):

$$C_{\text{s}}=I\mathrm{\Delta }t/m,$$ 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:

$$E=1/2C_{\text{s}}{\left(\mathrm{\Delta }V\right)}^2,$$ 2

$$P=3600E/t.$$ 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 .

Schematic of the structure of Co₃O₄@NiMoO₄ composite samples

Fig. 2 .

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

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 3d_5/2 and Mo 3d_3/2, as shown in Fig. 3d. The peak binding energy at 231.6 eV belongs to Mo 3d_5/2. However, the peak at 234.7 eV is from Mo 3d_3/2, 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 2p_1/2 and Co 2p_3/2, 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 .

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

Figure 4a shows CV curves of the Co₃O_4, 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 .

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

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.

Fig. 5 .

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

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 [27–34].

Fig. 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

Table 1 .

Electrochemical performance comparison of several SC devices based on transition metal oxides

Material	Energy density/(Wh⋅kg−1)	Power density/(W·kg−1)	Reference
NiCo2O4@NiMo2S4	30.7	374.9	[27]
Co3O4	26.6	189.5	[28]
Co3O4 nanohorn	31.7	16.7	[29]
NiCo2O4	27.4	493.2	[30]
Co3O4 nanocages	19.8	90.2	[31]
Mn0.4Ni0.1Co-OA	32.2	770.2	[32]
Co3O4 thin sheets	8.0	0.8	[33]
ZnCo2O4 sheets	9.78	33.98	[34]
Co3O4@NiMoO4	36.1	2700	This work

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.

Biographies

Yongli Tong

works as a lecturer in Shenyang Ligong University, China. In 2018, she joined Prof. Xiang Wu’s group at Shenyang University of Technology, China for pursuing her Ph.D. degree in Materials Science and Engineering. Her research interest focuses on transition metal oxide electrode materials for hybrid capacitors. She has published several papers in Chinese Chemical Letters, Acta Physico-Chimic Sinica and Frontiers in Materials.

Tengxi Zhang

received his B.S. degree in Materials Science and Engineering from Shenyang Ligong University, China in 2020. Now he is a Master candidate in Materials Science and Engineering at Shenyang University of Technology, China. His thesis is electrochemical capacitor.

Yuchen Sun

received her B.S. degree in New Energy Materials and Device from Huanghuai University, China in 2020. Then she came to Prof. Xiang Wu’s group at Shenyang University of Technology, China for pursuing her master degree in Materials Science and Engineering. Her research interest focuses on energy storage mechanism of electrochemical capacitor.

Xiaowei Wang

graduated from Huanghuai University, China in 2020. After that he studied at Shenyang University of Technology, China for pursuing his master degree in Materials Science and Engineering. His research interest focuses on energy storage mechanism of electrochemical capacitor.

Xiang Wu

received his Ph.D. degree in Materials Science and Engineering from Harbin Institute of Technology, China in 2008. After that he joined Harbin Normal University, China until Sept. 2016. He ever worked as a visiting scientist in National Institute for Materials Science (NIMS), Japan, and Taiwan University, China. He is now a full professor of materials science at Shenyang University of Technology, China. His research interests focus on synthesis, characterization of semiconductor nanomaterials and their applications in environment and energy fields. He has authored and co-authored over 160 peer reviewed journal articles. Cited time of the publications is over 6500 and H-factor is 47.

Author contributions

YT made experiments, analyzed the data and wrote the original draft. XW, TZ and YS analyzed the data by Software. XW designed the project and polished the manuscript. All authors read and approved the final manuscript.

Declarations

Competing interests

The authors declare that they have no competing interests.