Rationally designed CuCo2O4@Ni(OH)2 with 3D hierarchical core-shell structure for flexible energy storage

Journal of Colloid and Interface Science 557 (2019) 76–83

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Rationally designed CuCo2O4@Ni(OH)2 with 3D hierarchical core-shell structure for flexible energy storage Di Zhu a,b, Xun Sun a,b,c, Jing Yu a,b,⇑, Qi Liu a,b, Jingyuan Liu a,b, Rongrong Chen a,b,c, Hongsen Zhang a,b, Rumin Li a,b, Jia Yu d,⇑, Jun Wang a,b,c,⇑ a

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China Institute of Advanced Marine Materials, Harbin Engineering University, Harbin 150001, PR China d College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, PR China b c

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 9 July 2019 Revised 1 September 2019 Accepted 3 September 2019 Available online 4 September 2019 Keywords: CuCo2O4@Ni(OH)2 High specific capacitance Supercapacitors Flexible device

a b s t r a c t Composite electrodes that possess both rational structures and appropriate integration are needed to deliver high electrochemical performance in energy storage devices. In this paper, a flexible and binder-free electrode material based on a heterogeneous core-shell structure of CuCo2O4@Ni(OH)2 nanosheets grown on carbon cloth was fabricated by a simple method. The unique three-dimensional hierarchical structure gives the electrode a large specific surface area, which enables rapid response and increases of specific capacitance. The CuCo2O4@Ni(OH)2/carbon fiber cloth (CFC) composite electrode exhibited a specific capacitance of 2160 F g
1 at 1 A g
1 and a good rate capability energy of 82.7% at 20 A g
1. A flexible all-solid-state asymmetric supercapacitor (FAASC) was assembled with the CuCo2O4@Ni(OH)2/CFC electrode as the positive electrode, and activated carbon (AC)/CFC as the negative electrode. This device showed both a high energy density and power density (58.9 W h kg
1 at a power density of 400 W kg
1), and good long-term cycling stability. Furthermore, the assembled CuCo2O4@Ni (OH)2/CFC//AC/CFC devices were capable of driving a blue light-emitting diode after a short charge.

⇑ Corresponding authors at: Harbin Engineering University, Harbin 150001, PR China (Jing Yu, Jia Yu, and Jun Wang). E-mail addresses: [email protected] (J. Yu), [email protected] (J. Yu), [email protected] (J. Wang). https://doi.org/10.1016/j.jcis.2019.09.010 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

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The remarkable performance of this CuCo2O4@Ni(OH)2/CFC electrode indicates that this heterogeneous structure has great potential for applications in flexible high-performance energy storage devices. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Demand for portable and wearable electronic products, has motivated researchers into the development of high-performance flexible energy storage devices [1,2]. Flexible supercapacitors (FSCs) have high power densities, long cycle life times, and ultrafast charging/discharging capacities; additionally the devices are light weight, portable, and flexible, suggesting great promise for applications in energy storage devices [3-6]. However, it remains difficult to make FSCs that remain their high energy density without lowering their power density and service life in practical applications [7-9]. Energy density (E) can be improved by increasing specific capacity (C) or expanding the working voltage (V) [10,11]. Therefore, the design of asymmetric supercapacitors (ASCs) that make full use of the operating voltage windows of both positive and negative materials is an effective strategy to improve energy density [12]. However, flexible asymmetric supercapacitors (FASCs) have low energy densities owing to the low specific capacity of the electrode materials, which has yet to be addressed. Transition metal oxides possess a high theoretical specific capacitance and are extensively used as electrode materials [13– 15]. The surface area of the electrodes notably influences the electrochemical performance of FASCS [6,16]. Unfortunately, because of high surface energies, nanosized transition metal oxides reorganize during electrochemical processes, weakening energy storage performance in practical applications. However, it has been found that 3D hierarchical core-shell structures can retain their stability as nanostructured electrodes [17–20]. Furthermore, heterostructures not only combine the properties of multiple materials, but also effectively improve charge transfer processes of the electrode material, which contribute to enhance electrochemical performance. Qiu et al. reported hierarchical CuCo2O4@NiO composite ‘‘nanotrees” with high specific capacitances [21]. Zhao et al. synthesized Ti3C2@Ni-Co-Al-LDH heterostructures with high a specific capacitance [22]. Liang et al. synthesized NiCo-LDH@NiOOH coreshell heterostructure, which exhibited a superior specific capacitance [23]. The selection of appropriate materials for hierarchical coreshell structures is crucial. Generally, materials as a heterogeneous core should have a high degree of crystallization, and good conductivity and electrochemical stability [24]. Because of the higher specific capacity and cycling stability of binary transition metal oxides than those of single component oxides, there has been extensive research on spinel class binary metal transition oxide electrode materials, such as, ZnCo2O4 [25], NiCo2O4 [26], and CuCo2O4 [27]. Among these binary metal oxides, CuCo2O4, has aroused broad interest [28-30], owing to its relatively low activation energy, high electronic conductivity and cycle stability, together with its greater number of accessible oxidation states than simple cobalt and copper oxides. As heterogeneous shell materials, the current response ability, electrochemical reaction rate, and charge transfer number for the electrochemical reaction should be considered. Nickel-based transition metal compounds, particularly nickel hydroxide, have high electrochemical reaction rates and are considered to be promising shell materials [31–33]. Two-dimensional Ni(OH)2 nanosheets produce electrodes with high specific surface areas and abundant electrochemical sites [34]. In this work, we designed and constructed a heterogeneous core-shell structure CuCo2O4@Ni(OH)2 on a carbon cloth as a

binder-free electrode for FASC by a simple method. Twodimensional CuCo2O4 was directly grown as a heterogeneous core, and used as a highly conductive supporting framework to promote electron transfer. Nanosized Ni(OH)2 sheets were subsequently grown in situ on core materials to form an intimate hierarchical core-shell structure. This unique 3D hierarchical structure can markedly enlarge the reaction surface, which promotes rapid reactions at the electrode and increases its specific capacitance. Moreover, the Ni(OH)2 shell can ensure structural integrity during charging and discharging procedures, thus accelerating transmission of the ions between the electrode/electrolyte interface. A well-designed CuCo2O4@Ni(OH)2/carbon fiber cloth (CFC) electrode possesses a high specific capacitance, and rate capability, and high cycling stability. Furthermore, we assembled a FASC based on CuCo2O4@Ni(OH)2/CFC as the positive electrode, and activated carbon (AC)/CFC as the negative electrode. This device had a superior energy storage capacity and flexibility, with good potential for applications in flexible energy storage devices. 2. Experimental 2.1. Synthesis of CuCo2O4/CFC electrode Before synthesis of CuCo2O4 nanosheets, the CFC was cleaned with acetone, anhydrous ethanol, deionized (DI) water for ultrasonic 30 min, respectively. The obtained CFC was treated by 12 M HNO3 overnight, and then washed with deionized water and dried in a vacuum oven for 6 h at 60 °C. In a typical process, 0.5 mmol Cu (NO3)23H2O, 1.0 mmol Co(NO3)26H2O, and 6 mmol urea were dissolved in 40 mL of deionized water under stirring to form a rose pink solution. A CFC with a size of 2 cm  3 cm was immersed into the above solution and hydrothermally treated at 120 °C for 6 h. The Cu-Co precursor nanosheets/CFC was collected, washed with DI water and ethanol several times. The as-prepared precursor after drying was annealed at 350 °C for 3 h with a heating ramp of 5 °C/min in air. 2.2. Preparation of CuCo2O4@Ni(OH)2/CFC electrode The CuCo2O4 nanosheets on the CFC surfaces were used as a conductive skeleton to construct Ni(OH)2 nanosheets through a chemical deposition process. The chemical deposition of Ni(OH)2 nanosheets was performed in an aqueous solution containing 16 mL of 1 M NiSO46H2O, 20 mL of 0.25 M K2S2O8 and 6 mL of concentrated ammonia for 10 min at room temperature. Finally, the CuCo2O4@Ni(OH)2/CFC electrode was collected, rinsed with DI water and ethanol several times, and dried in a vacuum oven. 2.3. Assembly of flexible all-solid-state asymmetric supercapacitors (CuCo2O4@Ni(OH)2/CFC//AC/CFC) For the CuCo2O4@Ni(OH)2/CFC//AC/CFC FASCs, polyvinyl alcohol/KOH (PVA/KOH) gel electrolyte was used as both the separator and electrolyte. In a typical preparation, 6 g of PVA was added into 40 mL of DI water. This suspension solution was vigorously stirred at 85 °C until the solid powder was completely dissolved. Then 20 mL of 0.15 g mL
1 KOH solution was slowly dropped into the above solution. CuCo2O4@Ni(OH)2/CFC, AC/CFC electrodes were immersed in the PVA/KOH solution for 5 min and then dried at

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room temperature. Next, the electrodes were assembled with the PVA/KOH electrolyte and dried in an oven at 40 °C to remove excess water in the electrolyte. 2.4. Characterization The material structure, morphology and chemical composition of the CuCo2O4@Ni(OH)2 electrode was imaged with a scanning electron microscope (SEM, Thermoscientific Apreo S LoVac) and transmission electron microscope (TEM, FEI Talos F200X G2) fitted with an energy-dispersive X-ray spectroscopy (EDS) detector. The crystal phase structures of the CuCo2O4@Ni(OH)2/CFC electrode was measured by an X-ray diffractometer (XRD, Rigaku D/Max2550PC) with Cu Ka radiation. 2.5. Electrochemical measurements The electrochemical properties of the CuCo2O4@Ni(OH)2/CFC electrode were tested in a three-electrode system based on an electrochemical workstation (CHI 660E, Chenhua) and 2 M KOH solution as the electrolyte. The CuCo2O4@Ni(OH)2/CFC electrode, Pt foil and Hg/HgO electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The electrochemical properties of the assembled CuCo2O4@Ni(OH)2/CFC// AC/CFC supercapacitor were tested in a two-electrode system. The capacitance values were determined according to the equation (1). The energy and power densities were calculated from equation (2) and (3), respectively.

C¼

It mV

ð1Þ

E¼

1 C V2 2 3600

ð2Þ

P¼

3600 E t

ð3Þ

where C is the specific capacitance (F g
1), V is the working voltage window (V), I is the current density (A), m is the mass of active materials (g), t is the discharge time (s), E is the energy density (W h kg
1), and P is the power density (W kg
1), respectively. 3. Results and discussion The synthesis process of the CuCo2O4@Ni(OH)2/CFC electrode is illustrated in Fig. 1. CuCo2O4 nanosheets were first grown on the CFC substrate by a hydrothermal synthesis (Step I) and annealing (Step II). Subsequently, Ni(OH)2 nanosheets were anchored on the conductive CuCo2O4 skeleton by a chemical deposition method (Step III). This open 3D hierarchical nanostructure provides a large

number of electrochemical active sites, and promotes the overall utilization of the electrode material. This unique structure also possesses abundant pores, which can alleviate the volume change caused by ion insertion and releasing processes which is beneficial for improving cycling stability. Fig. 2 shows XRD patterns of CuCo2O4/CFC and CuCo2O4@Ni (OH)2/CFC. The XRD spectrum of the obtained CuCo2O4 sample has a series of characteristic diffraction peaks, which are consistent with a previously reported spinel CuCo2O4 phase (JCPDS No. 011155). No other peaks were observed except for that marked in black at 26°, which is attributed to CFC. Hence, the purity of the as-prepared CuCo2O4 was relatively high. In the XRD spectra of the heterogeneous core-shell CuCo2O4@Ni(OH)2 electrode, aside from diffraction peaks of CuCo2O4, peaks derived from Ni(OH)2 (JCPDS No. 01-1047) were observed at low intensity, which ascribe to a poor degree of crystallization. Furthermore, XRD pattern of the Ni(OH)2 prepared by chemical deposition is shown in Fig. S2. These results reveal that the CuCo2O4@Ni(OH)2 electrode was successfully synthesized. The morphologies of CuCo2O4/CFC and CuCo2O4@Ni(OH)2/CFC are shown in Fig. 3. Fig. 3a shows CuCo2O4 nanosheets, which were almost uniformly grown on the CFC substrate. A high magnification SEM image (Fig. 3b), shows a large gap between the CuCo2O4 sheets, which is advantageous for constructing 3D hierarchical core-shell structures. The average thickness and length of the CuCo2O4 nanosheet are approximately 50 nm and 1.5 lm, respectively. After chemical deposition, the morphology of CuCo2O4@Ni (OH)2/CFC is similar to that of CuCo2O4/CFC (Fig. 3e), indicating that the addition of the Ni(OH)2 shell does not destroy the structure and ordered arrangement of the original CuCo2O4 nanosheets. The CuCo2O4 are densely coated with Ni(OH)2 nanosheets (Fig. 3f), with a total thickness of approximately 200 nm, which confirmed that a 3D hierarchical core-shell structure was successfully constructed. To further investigate the morphology and structure of the CuCo2O4, and CuCo2O4@Ni(OH)2 nanosheets, we used TEM imaging. Fig. 3c shows that the CuCo2O4 nanosheets possessed a porous structure. A HRTEM image of the CuCo2O4 nanosheet (inset Fig. 3c) clearly shows lattice fringes with a crystal plane spacing of 0.243 nm, corresponding to the (3 1 1) crystal plane of the spinel CuCo2O4 phase, which is consistent with the XRD results. TEM images of the obtained CuCo2O4@Ni(OH)2 composite nanosheets (Fig. 3f) exhibited that the Ni(OH)2 nanosheets are interconnected and supported by CuCo2O4 nanosheets, forming a tough core-shell heterostructure. This unique 3D hierarchical structure with CuCo2O4 nanosheets as the core and Ni(OH)2 nanosheets as the shell has abundant pore structure and interconnection between the sheets greatly improved the utilization rate of these two electrochemical active materials. The inset of Fig. 3f shows lattice fringes having a spacing of 0.156 nm, which corresponds to the (1 1 0) crystal plane of Ni(OH)2. The HAADF and cross-sectional EDS (energy dispersive

Fig. 1. Schematic illustration of CuCo2O4@Ni(OH)2 nanosheets on CFC.

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Cu+/Cu2+, and Co4+/Co3+/Co2+ with OH– in the electrolyte [35,36]. Furthermore, the area integrated within the current-potential curve for CuCo2O4@Ni(OH)2 is much larger than that of CuCo2O4 indicating that the CuCo2O4@Ni(OH)2 electrode has a higher electrochemical reaction activity [37]. To further quantify the contribution of the capacitive and diffusion processes, we measured the CV at different scan rates, as shown in Fig. S1b. The capacity effect was analyzed through the relationship between the peak current (i) and the scan rate (v):

i ¼ avb

Fig. 2. XRD patterns of CuCo2O4/CFC and CuCo2O4@Ni(OH)2/CFC.

spectroscopy) mapping results of the CuCo2O4@Ni(OH)2 core-shell heterostructures are shown in Fig. 3g-k. Elemental mapping analysis showed that the elements Cu and Co are mainly distributed in the core section and Ni and O are uniformly distributed across the structure. This pattern suggests the successfully formation of a CuCo2O4@Ni(OH)2 heterogeneous core-shell structure. The electrochemical properties of the prepared samples were tested in a standard three-electrode system with 2 M KOH solution. The CV curves of CuCo2O4, and CuCo2O4@Ni(OH)2 electrodes measured at the different scan rates are shown in Fig. S1. Fig. 4a shows the CV curves of the CuCo2O4, and CuCo2O4@Ni(OH)2 electrodes at the same scanning rate of 20 mV s
1. The CV curves of both the CuCo2O4, and CuCo2O4@Ni(OH)2 electrodes have obvious broad redox peaks, which attribute to the redox reaction of

ð4Þ

When the value of b is 1, the current is completely surface storage controlled (i.e., capacitance). When the value of b is 0.5, the current is controlled by semi-linear diffusion (i.e., battery) [38– 40]. The value of b determined by taking the logarithm of Eq. (1), as shown in Fig. S2a, the slope is the value of b. The anode b value was 0.7, indicating that the charge storage was affected by diffusion processes. To further determine the ratios of the capacitive contribution and diffusion contribution, Eq. (4) was used:

iðVÞ ¼ k1 vþ k2 v1=2

ð5Þ

where, i is the current at a particular voltage, v is the scan rate, k1v and k2v1/2 represent the surface and diffusion control, respectively. The data in Fig. S3b indicate that the capacitive contribution ratio value increased and the diffusion contribution ratio value decreased as the sweep rate was increased. The capacitive-controlled process contributed 64%, 68%, 74%, 81%, and 89% of the total charge storage at 5, 10, 20, 30, and 50 mV s
1, respectively, indicating a dominant capacitive charge-storage mechanism in the CuCo2O4@Ni(OH)2/CFC electrode. The galvanostatic charge/discharge curves of CuCo2O4, and CuCo2O4@Ni(OH)2 electrode at the same current density of 5A g
1 are shown in Fig. 4b. The discharge time of the CuCo2O4@ Ni(OH)2 composite electrode was longer than that of the CuCo2O4 electrode, demonstrating better electrochemical performance.

Fig. 3. SEM images of (a, b) CuCo2O4/CFC and (d, e) CuCo2O4@Ni(OH)2/CFC. TEM images of (c) CuCo2O4 and (f) CuCo2O4@Ni(OH)2. (g) HAADF-STEM image and (h-k) STEM-EDS mapping images of the CuCo2O4@Ni(OH)2.

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Fig. 4. Electrochemical performance of CuCo2O4@Ni(OH)2/CFC and CuCo2O4/CFC in a three-electrode configuration. (a) Comparison of CV curves at a scan rate of 20 mV s
1. (b) Comparison of galvanostatic charge/discharge curves at a discharge rate of 5 A g
1. (c) Galvanostatic charge/discharge curves of CuCo2O4@Ni(OH)2/CFC at different current densities. (d) Impedance Nyquist plots. (e) Specific capacitance at different scan rates. (f) Cycling performance at a discharge rate of 5 A g
1.

Fig. 4c shows the constant current charge and discharge curves of CuCo2O4@Ni(OH)2 electrode at different current densities. The charge-discharge curves have good symmetry, indicating good electrochemical capacitance characteristics and reversibility of the superior Faraday reaction. Fig. 4d shows the Nyquist plots of CuCo2O4, and CuCo2O4@Ni(OH)2 electrodes. As shown in the Fig. 4d. The impedance curves of both samples have a similar shape and are made up of two parts: one part shows an irregular semicircle in the high-frequency region, and the other part shows an oblique line behind the semicircle in the low-frequency region. The equivalent circuit model is introduced to clarify the performance of the electrodes (Fig. S4), the equivalent series resistance (Rs) of the CuCo2O4@Ni(OH)2 is approximately 0.741 X, which is lower than that of the pristine CuCo2O4 (1.038 X). Hence, there is a strong connection between the CuCo2O4 and Ni(OH)2 nanosheets. The rapid

kinetics of the electrode was demonstrated by its low charge transfer resistance (Rct = 1.1 X, semicircle diameter) The resistance results combined with the CV and GCD data reveal that the CuCo2O4@Ni(OH)2 electrode exhibited better conductivity, more rapid electron transport kinetics, and fast diffusion of ions at the interface of electrode/electrolyte conductivity and electroperformance than those features of pure CuCo2O4. Fig. 4e shows the specific capacitance curves of CuCo2O4, and CuCo2O4@Ni(OH)2 electrode materials at different current densities. The specific capacitance of the CuCo2O4@Ni(OH)2 electrode is higher than that of CuCo2O4 electrode materials, which is consistent with the CV and constant current charge and discharge results. The specific capacitances of CuCo2O4@Ni(OH)2 under current densities of 1, 2, 5, 10, and 20 A g
1 are 2271, 2245, 2160, 2071, and 1878 F g
1, respectively, which represent better performance than that of the

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Fig. 5. (a) CV curves of AC/CFC and CuCo2O4@Ni(OH)2/CFC in 2 M KOH electrolyte. (b) CV curves of CuCo2O4@Ni(OH)2/CFC//AC/CFC device at different scan rates. (c) Charge and discharge curves of CuCo2O4@Ni(OH)2/CFC//AC/CFC device at different current densities. (d) Specific capacitance of CuCo2O4@Ni(OH)2/CFC//AC/CFC device at different current densities. (e) Cycling stability of CuCo2O4@Ni(OH)2/CFC//AC/CFC device at a current density of 5 A g
1. (f) Ragone plots compared with other relevant supercapacitors reported in the literature.

CuCo2O4 electrode (697, 687, 653, 590, and 454 F g
1 at 1, 2, 5, 10, and 20 A g
1, respectively). Hence, the synergistic effect of the heterostructure can effectively improve the charge transfer process of the electrode materials, and thus increase the energy storage efficiency. When the current density was increased 20 times from the 1 to 20 A g
1, the CuCo2O4@Ni(OH)2 electrode maintained a high specific capacitance of 1878 F g
1 (82.7% specific capacitance retention compared with at a current density of 1 A g
1), whereas the pure CuCo2O4 electrode retained only 65.1% of its original specific capacitance, demonstrating a good rate performance of CuCo2O4@Ni(OH)2. Cycling performance is also an important measure of electrode quality. The cycle performances of the CuCo2O4@Ni(OH)2, and CuCo2O4 electrodes were tested and evaluated by repeatedly charging and discharging under a current

density of 5 A g
1, as shown in Fig. 4f. After repeated charging and discharging, the specific capacitance of the CuCo2O4@Ni(OH)2 electrode gradually decreased. After 5000 cycles, its specific capacitance decreased from 2160 to 1987 F g
1 (i.e., 92% capacitance retention), which is considerably higher than that of the CuCo2O4 electrode (86% capacitance retention). The three-dimensional hierarchical core-shell structure enhanced the integrity of the internal structure of the composite electrode and gave it better stability. To further evaluate the real electrochemical performance of the CuCo2O4@Ni(OH)2 electrode and its potential practical application value, we used a two-electrode system. Asymmetric supercapacitors were assembled based on CuCo2O4@Ni(OH)2 as the positive electrode, AC as the negative electrode, and PVA/KOH as both the electrolyte and separator. The electrochemical performances of

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Fig. 6. (a) Photographs of CuCo2O4@Ni(OH)2/CFC//AC/CFC ASC device exhibiting good flexibility. (b) CV curves of the flexible all-solid-state ASC device under different bending conditions. (c) Demonstration of a blue LED powered up by two devices connected in series at different times. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the supercapacitors are shown in Fig. 5. The CV curves of the positive and negative electrodes at a scanning speed of 20 mV s
1 in a three-electrode measuring system indicate that the voltage window ranges of the AC and CuCo2O4@Ni(OH)2 electrodes were
1.0–0 V and 0–0.6 V, respectively (Fig. 5a). The stable operating voltage of flexible energy storage devices should be approximately 1.6 V. Fig. 5b shows CV curves of the CuCo2O4@Ni(OH)2/CF//AC/CFC device at scan rates from 5 to 50 mV s
1 at a voltage window of 1.6 V. There were no obvious differences in the shape of the CV curves; however the area of the curve became larger as the scan rate was increased from 5 mV s
1 to 50 mV s
1. The galvanostatic charge/discharge curves of CuCo2O4@Ni(OH)2/CFC//AC/CFC devices at different current densities are shown in Fig. 5c. The obtained GCD curves were nearly symmetric, indicating good capacitive performance within the voltage range of 0–1.6 V. The specific capacitances of the CuCo2O4@Ni(OH)2/CF//AC/CFC device were calculated according to the discharge time of these galvanostatic charge/discharge curves, as shown in the Fig. 5d. The specific capacitances of the CuCo2O4@Ni(OH)2/CF//AC/CFC device were 165.9, 145.1, 121, 108.4, and 101.4 F g
1, respectively, (under various current densities of 0.5, 1, 2, 5, and 10 A g
1). When the current density was increased from 0.5 to 10 A g
1, the specific capacitance of the CuCo2O4@Ni(OH)2/CF//AC/CFC device maintained 61.1% of its initial capacitance, indicating a good rate capability. The cyclic stability of the CuCo2O4@Ni(OH)2/CF//AC/CFC device was evaluated by 5000 cycles of galvanostatic charge/discharge testing at a current density of 5 A g
1. As shown in Fig. 5e, the capacitance retention of the CuCo2O4@Ni(OH)2/CF//AC/CFC device remained at 91.2% after 5000 cycles, demonstrating an excellent cyclic stability. The energy density and power density, which are important indicators of the electrochemical characteristics of CuCo2O4@Ni(OH)2/CF//AC/ CFC asymmetric supercapacitor, are shown in Fig. 5f. The maximum energy density was 58.9 W h kg
1 when the power density of the CuCo2O4@Ni(OH)2/CF//AC/CFC asymmetric supercapacitor was 400 W kg
1. The performance of the CuCo2O4@Ni(OH)2/CF// AC/CFC device was superior to that of many previously reported asymmetric supercapacitors, such as CuCo2O4@NiO//AC (38.9 W h kg
1 at a power density of 750 W kg
1) [41], CCONS//HCP-CNF (21.5 Wh kg
1 at a power density of 400 W kg
1) [42], CuCo2S4/CuCo2O4//GA (33.2 Wh kg
1 at a power density of 800 W kg
1) [43], Co3O4@Ni(OH)2//AC (40 W h kg
1 at a power density of 346.9 W kg
1) [44], Ni(OH)2/MnO2//AC (29.9 W h kg
1 at a power density of 1900 W kg
1) [45], and NiCoP@C@Ni(OH)2//AC (49.5 Wh kg
1 at a power density of 399.8 W kg
1) [46]. To verify the flexibility of the CuCo2O4@Ni(OH)2/CFC//AC/CFC device, the CuCo2O4@Ni(OH)2/CF//AC/CFC device was measured under different bending conditions (Fig. 6a) and Fig. 6b shows CV curves of the CuCo2O4@Ni(OH)2/CF//AC/CFC measured under nor-

mal, bent, twisted, recovered conditions. There was no notable change in the shape of the CV curves, indicating excellent functionality when flexed, which is useful for applications to flexible energy storage devices. To demonstrate the real application potential of the proposed device, we assembled two CuCo2O4@Ni(OH)2/CFC//AC/ CFC devices connected in series, which could light up a blue lightemitting diode (LED) for more than 15 min, as shown in Fig. 6c. 4. Conclusion In summary, we prepared a flexible and binder-free electrode material with a heterogeneous core-shell structure of CuCo2O4@ Ni(OH)2 nanosheets grown on a carbon cloth by a simple method. The as-prepared electrode showed a superior specific capacity (2160 A g
1) at a current density of 1 A g
1. After 5000 continuous charge and discharge cycles at 5 A g
1, only an 8% capacity drop was observed on the CuCo2O4@Ni(OH)2 electrode. In addition, the assembled CuCo2O4@Ni(OH)2/CFC//AC/CFC FASC exhibited a high energy density and energy density (58.9 W h kg
1 at a power density of 400 W kg
1), excellent cyclic stability (91.2% capacitance retention), and remarkable flexibility. This binder-free CuCo2O4@Ni(OH)2 electrode with a heterogeneous core-shell structure holds great promise for promoting the electrochemical properties of supercapacitors. The synthesis strategy of electrode materials with various hierarchical structures has considerable importance for energy storage and conversion materials. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 51872057, 51672054, 51901055), Natural Science Foundation of Heilongjiang Province (LH2019E025), Fundamental Research Funds of the Central University (3072019CF1003), China Postdoctoral Science Foundation (2019M651260), and International Science & Technology Cooperation Program of China (2015DFR50050). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.010. References [1] D.P. Dubal, N.R. Chodankar, D.H. Kim, P. Gomez-Romero, Towards flexible solid-state supercapacitors for smart and wearable electronics, Chem. Soc. Rev. 47 (2018) 2065–2129.

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