Co3O4@NiMoO4 composite electrode materials for flexible hybrid capacitors

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][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 Co 3 O 4 -based hybrid structure [21][22][23].
Herein, we report Co 3 O 4 @NiMoO 4 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 Co 3 O 4 @NiMoO 4 as cathode and activated carbon as anode (Co 3 O 4 @NiMoO 4 // AC). The device shows an energy density of 36.1 Wh/kg and long cycle stability.

Synthesis of Co 3 O 4 nanowires
First, the Ni foam (2 cm × 1 cm, as substrate) was washed three times with absolute ethanol and deionized (DI) waterby ultrasonic cleaner (SK7200H, Shanghai Kedao). Then, 5 mmol CoCl 2 ·6H 2 O, 10 mmol NH 4 F 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 Co 3 O 4 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 Co 3 O 4 @NiMoO 4 composite
Briefly, as-prepared Co 3 O 4 nanowires were utilized as the core structure for the growth of Co 3

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 Co 3 O 4 @NiMoO 4 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 (Co 3 O 4 , NiMoO 4 , and Co 3 O 4 @NiMoO 4 ) 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): where I stands for current density, m represents the mass of the electrode. An ASC was assembled by using the Co 3 O 4 @NiMoO 4 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: (1) C s = IΔt∕m, Schematic of the structure of Co 3 O 4 @NiMoO 4 composite samples Page 3 of 9 25 The XRD patterns of Co 3 O 4 @NiMoO 4 samples are shown in Fig. 3a [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 Mo 6+ oxidation state [25]. In Fig. 3e, O 1 s peaks at 531.8, 530.6 and 529.4 eV correspond to defect oxygen, O 2− 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 Co 2+ and Co 3+ co-exist in the asprepared composite material. Moreover, the peaks at 786.1 and 804.5 eV present the shakeup satellites [19]. Figure 4a shows CV curves of the Co 3 O 4, NiMoO 4 , Co 3 O 4 @NiMoO 4 samples at scan rate of 50 mV/s. The obvious redox peaks suggest that the samples possess pseudo-capacitive characteristics. The Co 3 O 4 @NiMoO 4 samples show the largest integral area, which means that hybrid samples present excellent electrochemical performance. Co 3 O 4 @NiMoO 4 samples present the longest discharge time (Fig. 4b), revealing their maximal specific capacitance. It can be calculated by Eq. (1) that the Co 3 O 4 @ NiMoO 4 sample possesses the specific capacitance of 600 C/g, which is higher than those of Co 3 O 4 (177.9 C/g) and NiMoO 4 (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, Co 3 O 4 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. Figure 4c shows the CV curves of the prepared Co 3 O 4 @ NiMoO 4 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 Co 3 O 4 @NiMoO 4 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 Co 3 O 4 @NiMoO 4 composite sample shows the 98.2% capacitance retention after 10000 cycles.

Results and discussion
The performances of the fabricated ASC device were also measured. The CV curves of the device are shown in Fig. 5a. Co 3 O 4 @NiMoO 4 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 Co 3 O 4 @NiMoO 4 // 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) 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 Co 3 O 4 @ NiMoO 4 //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 Co 3 O 4 @  Wh/kg at 2700 W/kg, which is higher than those reported in previous literatures [27][28][29][30][31][32][33][34].

Conclusion
In summary, the core-shell Co 3 O 4 @NiMoO 4 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.   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 included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.