Growth of highly mesoporous CuCo2O4@C core-shell arrays as advanced electrodes for high-performance supercapacitors

Accepted Manuscript Full Length Article Growth of highly mesoporous CuCo2O4@C core-shell arrays as advanced electrodes for high-performance supercapacitors Hailong Yan, Yang Lu, Kejia Zhu, Tao Peng, Xianming Liu, Yunxin Liu, Yongsong Luo PII: DOI: Reference:

S0169-4332(18)30070-9 https://doi.org/10.1016/j.apsusc.2018.01.066 APSUSC 38205

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

26 September 2017 1 January 2018 7 January 2018

Please cite this article as: H. Yan, Y. Lu, K. Zhu, T. Peng, X. Liu, Y. Liu, Y. Luo, Growth of highly mesoporous CuCo2O4@C core-shell arrays as advanced electrodes for high-performance supercapacitors, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.01.066

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Growth of highly mesoporous CuCo2O4@C core-shell arrays as advanced electrodes for high-performance supercapacitors Hailong Yan,a,b Yang Lu,a,b Kejia Zhu, a,b Tao Peng,a,b Xianming Liu,c Yunxin Liu,d and Yongsong Luo a,b a

School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, P. R.

China. b

Key Laboratory of Microelectronics and Energy of Henan Province, Xinyang Normal University,

Xinyang 464000, P. R. China. c

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022,

P. R. China. d

Department of Physics and Electronic Science, Hunan University of Science and Technology,

Xiangtan 411201, China

Abstract A series of CuCo2O4 nanostructures with different morphologies were prepared by a hydrothermal method in combination with thermal treatment. The morphology, structure and composition were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. As the electrode materials for supercapacitors, CuCo2O4 nanoneedles delivered the highest specific capacitance compared with other CuCo2O4 nanostructures. Electrochemical performance measurements demonstrate that the carbon layer can improve the electrochemical stability of CuCo2O4 nanoneedles. The CuCo2O4@C electrode exhibits a high specific capacitance of 1432.4 F g−1 at a current density of 1 A g−1, with capacitance retention of 98.2 % after 3000 circles. These characteristics of CuCo2O4@C composite are mainly due to the unique one 

To whom correspondence should be addressed: Tel./fax: +86 376 6390801, E-mail: [email protected] (Y. S. Luo).

dimensional needle-liked architecture and the conducting carbon, which provide a faster ion/electron transfer rate. These excellent performances of the CuCo2O4@C electrode confirmed the material as a positive electrode for hybrid supercapacitor application. Keywords Core-shell structures; Morphology control; Megnetron sputtering; High specific capacitance; Good cycling stability

1. Introduction The development of alternative sustainable energy sources and high efficient energy-storage devices has become an urgent issue, which have a wide application on hybrid power devices and electric devices

1-5

. Supercapacitors have attracted intense

attention as the energy storage devices for its specific capacitance, power density, cycle stability. Countless studies have shown that the excellent performance of supercapacitors is mainly determined by the type and the structure of the electrode materials. Transition metal oxides have received an upsurge of interest in recent years due to their promising roles in supercapacitors

6-8

. However, most of metal oxide

nanomaterials suffer from their low capacitance and poor conductivity

9-10

. Recently,

spinel cobalties electrode especially NiCo2O4 has been investigated as high performance electrode due to its inherent advantages such as higher electrochemical activity and small environmental impact

11-13

. CuCo2O4 is not only inexpensive and

environmental friendless, but also electrochemically active. Thus, various CuCo2O4 nanomaterials have also been widely synthesized and conceived as a promising and scalable alternative electrode material for Li-ion batteries

14

and supercapacitors

15-17

since it offers many advantages such as low cost, high theoretical capacitance and non-toxicity. For example, Wang et al. reported the synthesis of maguey-liked CuCo2O4 nanowires with a specific capacity of 982 F g-1 at 1.5 A g-1 18. Cheng et al. demonstrated the hydrothermal fabrication of mesoporous CuCo 2O4 nanograsses with high specific capacity of 796 F g-1 at a current density of 2 A g -1

19

. Despite these

obtained progresses, the achieved electrochemical performances are still largely unsatisfactory, it is still highly essential to design and construct core-shell CuCo2O4 nanostructures to further enhance the power density and cyclic performance of the electrode materials 20-24. However, the production processes of the core-shell electrode materials are often very complex. This paper reports the preparation of CuCo 2O4 nanostructures and CuCo2O4@C core-shell arrays using a simple hydrothermal and megnetron sputtering method. And, the cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectra (EIS) measurements were employed in comparing the electrochemical performance of CuCo2O4 nanostructures. With their distinctive feature, the CuCo2O4 nanoneedles delivers the highest specific capacitance and results in the desired hybrid nanostructures with favorable kinetics of ion diffusion and electron transport. After the coating of conducting carbon, CuCo2O4 nanoneedles delivered the specific capacitance of 1432.4 F g−1 at a scan rate of 1 A g−1 with a good cyclic stability.

2. Experimental section 2.1 Synthesis of CuCo2O4 nanostructures Different CuCo2O4 nanostructures were synthesized by a simple hydrothermal

method (as shown in Fig. 1). Before depositing CuCo2O4 nanoneedles, the Ni foam was immersed in sequential sonication in acetone, 2 M HCl solution, deionized water for 20 min to remove the possible oxide layer on the surface. 0.24 g of Cu(NO3)2·3H2O

(Sigma-Aldrich,

99.9%)

and

0.58

g

of

Co(NO3)2·6H2O

(Sigma-Aldrich, 99.9%) were dissolved in a 40 ml mixture containing 5 ml of ethanol absolute and 35 ml of deionized water, followed by the addition of 0.3 g of urea (analytical grade). After stirring for about 60 min, a transparent solution was obtained. The resulting solution and the cleaned conductive substrate were transferred into a 50 ml Teflon-lined stainless steel autoclave, followed by heating at 120 °C for 12 h. After reaction for 12 h, the product was taken out from the solution and cooled down to room temperature. Then the product was cleaned by ultrasonication to remove the loose products on the surface. At last, the substrate was dried at 60 °C for further characterization. Similarly, the CuCo2O4 nanosheets were fabricated at 160 °C with Cu2+ concentration of 0.0375 M and the urchin-liked CuCo2O4 microspheres were fabricated at 120 °C without the addition of Ni foam substrate. To obtain high crystallized CuCo2O4, the as-grown precursors were annealed at 400 °C in the air for 2 h with a heating rate of 2 °C min−1. 2.2 Deposition of conducting carbon layers Conducting carbon layers were deposited on CuCo 2O4 nanoneedls by radio-frequency (r. f.) magnetron sputtering (as shown in Fig. S1). A 50-mm-diameter carbon target was employed. Prior to deposition, the base pressure of the deposition chamber was kept below 1×10-3 Pa. During deposition, the pressure of the deposition

chamber was kept at 1.2 Pa, the sputtering power and the distance between substrate and target were kept 200 W and 100 mm, respectively. The samples were prepared at room temperature for 30 min. 2.3 Materials characterization The morphology of the product was examined by a field emission scanning electron microscopy (Hitachi, S4800, Chiyoda-ku, Japan). The chemical composition was characterized by X-ray diffraction (XRD, Bruker, D8-Advance X-ray Diffractometer, Cu Kα, λ=1.5406 Å). X-ray photoelectron spectroscopy (XPS) spectra were measured on a modified K-ALPHA XPS system. High-resolution transmission electron microscopy (HRTEM) images were captured on a JEOL JEM-2010 microscope, and the nanosheets were lightly scraped off from Ni foam and then with an ultrasonic processing. Raman tests were carried out by Renishaw inVia with a 532 nm laser. 2.4 Electrochemical measurements The capacitive performance of the samples was tested on a CHI 660E electrochemical workstation (CH Instruments, Chenhua, Shanghai, YP, China) using a three-electrode cell where Pt foil serves as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The area of the electrode immersed into the electrolyte was controlled to be around 2.8 cm×1.6 cm. The urchin-like electrode was prepared by mixing 85 wt.% active materials, 10 wt.% acetylene black as conductive agent, and 5 wt.% poly (tetrafluoroethylene) as binder to form a slurry and overlay the slurry onto a Ni foam. Solutions containing 2 M NaOH were used as the electrolytes.

3. Results and discussion The morphologies of different CuCo2O4 nanostructures were observed through field emission scanning electron microscopy (SEM). Fig. 2a shows the low SEM image of CuCo2O4 nanosheetes, in which we can see CuCo2O4 nanosheetes vertically grown over the whole Ni foam with a size of 2-5 μm and a depth of about 10-20 nm. From the high-magnification SEM image (Fig. 2d), it is found that a series of holes uniformly distributed on the surface of CuCo2O4 nanosheetes. Fig. 2b shows a typical SEM picture of urchin-liked CuCo2O4 nanostructures without using any substrate. The detailed morphology demonstrated in Fig. 2e indicates that the urchin-liked CuCo2O4 nanostructures keep a smooth surface after calcination. The morphology of CuCo2O4 nanoneedles is shown in Fig. 2c and Fig. 2f. It can be seen that large-scale and aligned CuCo2O4 nanoneedls vertically grown on the Ni foam with their tops tangled up, which ensure a good mechanical adhesion and electrical connection to the current collector. In addition, with the rough surface of CuCo2O4 nanoneedls, the active sites and transport efficiency of the electron or ion in the electrode are much enhanced. Fig. 3a-c is the low and high magnification SEM images of CuCo2O4@C core-shell structure, the carbon layer was deposited by a megnetron sputtering method. After the deposition of carbon layer, CuCo2O4@C core-shell structures still present a needle-liked morphology (as shown in Fig. 3c). To study the phase and composition of the products, X-ray diffraction (XRD) analyses were performed. XRD pattern (Fig. 3d) shows the crystal structures and diffraction planes of the CuCo2O4 nanoneedles and CuCo2O4@C core-shell composites. Four diffraction peaks positioned at 31.1°,

37 °, 59.3°, 65.2° corresponding to the [220], [311], [511], and [440] diffraction planes. All the diffraction peaks in XRD pattern are readily indexed to the orthorhombic of CuCo2O4 (JCPDS card no. 78-2177). Three additional intense peaks positioned at 44.7°, 52.1°, and 76.6° are assigned to the [222], [400], and [533] diffraction planes of the Ni-foam 25. No residues or contaminants have been detected, indicating the high purity of the sample. After coating a layer of carbon shell, the XRD diffraction peaks of the CuCo2O4 nanoneedles do not show any obvious change. Additionally, no any diffraction peaks that attributable to carbon were observed, indicating that the carbon shell is amorphous or very thin. The chemical compositions and metal oxidation states of the CuCo2O4@C composites were analyzed by XPS (as shown in Fig. 4). A full-survey scan spectrum in Fig. 4a indicates the presence of Cu, Co, C and O elements in the CuCo2O4@C core/shell nanostructures. The deconvolution peaks (Fig. 4b) of the O 1s spectrum are centered at binding energies of 529.1 and 530.9 eV corresponds to the O2- composing oxide with metal elements and defects sites. The Cu 2p spectra region (Fig. 4c) displays two peaks at the binding energies of 933.4 and 953.4 eV, ascribing to Cu 2p3/2 and Cu 2p1/2 spin-orbit peaks, together with two satellite peaks (Sat.) at the binding energies of 941.6 and 961.7 eV. Two major peaks with the binding energies at 779.3 and 794.4 eV are observed by deconvolution of complex Co 2p curve (Fig. 4d), which are assigned to Co 2p3/2 and Co 2p1/2. The resolved peaks at 779.3 and 794.3 eV are ascribed to Co3+, while two peaks at 780.7 and 795.9 corresponds to Co2+, demonstrate the spinel structure of the cobalt

25

. In addition to

two main peaks, there are two shakeup satellite peaks at 788.2 and 803.4 eV. Consequently, these results further confirmed the formation of CuCo 2O4. The detailed microstructures of the CuCo 2O4@C composites are also investigated by the HRTEM. Fig. 5a-c presents the typical TEM images at different magnifications of CuCo2O4@C composites ultrasonic down from the Ni foam. HRTEM image of the CuCo2O4@C composites (Fig. 5d) clearly displays the lattice fringes of 0.249 nm, corresponding well to the (311) planes of CuCo 2O4 phase,

26

respectively. It is

revealed that the carbon coating are firmly anchored on the scaffold of CuCo 2O4, further confirming the formation of CuCo 2O4@C core-shell nanostructures. To reveal the spatial distribution of constituent elements in the CuCo 2O4@C composites, EDS mapping was carried out under TEM. As illustrated in Fig. 5e, one can clearly see that the Co, Cu, O and C signals are identified in the same detected regions, suggesting the uniform deposition of C shell on the CuCo 2O4 nanoneedles. To further confirm the structural properties of CuCo 2O4@C composites, the samples were measured with Raman spectroscopy. As shown in Fig. S2, two scattered peaks located at 1340 cm-1 and 1587 cm-1 correspond to the D band and G band of carbon [27, 28]. In addition, three broad peaks approximately at 476 cm-1, 518 cm-1, and 681 cm-1 can be observed, which are good agreement with that of the CuCo 2O4 phase. Different CuCo2O4 nanostructures were firstly confirmed by the CV, GCD, and EIS in a three-electrode cell. Fig. S3a shows the typical CV curves of CuCo2O4 electrodes, which were recorded at 5 mV s-1 in the potential from 0 to 0.6 V. A pair of peaks located at around 0.178 V and 0.371 V clearly reveals the pseudocapacitive

characteristic of CuCo2O4 nanostrutures, which was attributed to the faradaic redox reactions of Co4+/Co3+ and Cu+/Cu2+ associated with H2O and OH－. The possible reaction equations are as follows: CuCo2O4+2H2O+e－↔2 CoOOH+CuOH+OH－,

(1)

CoOOH+OH－↔CoO2+H2O+e－,

(2)

CuOH+ OH－↔Cu(OH)2+e－,

(3)

For comparison, the CD curves of different CuCo2O4 nanostrutures were characterized at a current density of 5 A g−1, as shown in Fig. S3b. The specific capacitance of CuCo2O4 nanoneedles is as high as 501 F g−1 at 5 A g−1, much larger than that of CuCo2O4 nanosheets and urchin-liked CuCo2O4 nanostructures. Fig. S3c shows the Nyquist plots of CuCo2O4 electrodes. It is obvious that CuCo2O4 nanoneedles with their tops tangled up exhibit the lowest charge storage resistance and diffusive resistance. Highly conducting carbon was further immobilized onto the nanoneedles based on a magnetron sputtering method using a 50-mm-diameter carbon target as carbon source. To evaluate the electrochemical performance of the CuCo2O4@C composites, CV and GCD measurements were conducted in a three-electrode system, as shown in Fig. 6. Fig. 6a shows the CV curves of CuCo2O4 and CuCo2O4@C core/shell electrode at a scan rate of 10 mV s-1. It can be seen that the CV curve of CuCo2O4@C electrodes exhibits a larger integral area than the pristine CuCo2O4, indicating the larger capacitance of CuCo2O4@C electrodes. In order to investigate the influence of scan rate on the electrochemical performance, the CV curves of the CuCo2O4@C

electrodes were performed at different scan rates ranging from 5 to 100 mV s-1, as illustrated in Fig. 6b. Due to the electrode polarization generated during the charge-discharge process, the anodic peaks shift towards positive position, and the cathodic peaks shift towards negative position. All of the redox peaks are symmetrical at different scan rates, demonstrating that the redox reaction reversibility of CuCo2O4@C electrodes is excellent, which are important for power devices. The excellent electrochemical performance of the CuCo2O4@C electrodes was confirmed by GCD measurements, under a potential window of 0 to 0.5 V, as shown in Fig. 6c. The CuCo2O4@C electrode exhibits excellent pseudocapacitance of 1432.4 F g−1, 926.2 Fg-1, 763.4 Fg-1, 514.5 Fg-1, and 452.7 Fg-1 at the scan rates of 1, 2, 5, 10, and 20 Ag−1, which are about 1.5 times of CuCo 2O4 electrodes, as shown in Fig. 6d. Fig. 7a further displays the CD curves of the CuCo2O4@C composite and pristine CuCo2O4 nanoneedle at a current density of 5 A g-1. As expected, the CuCo2O4@C composite demonstrates a much longer discharging time than CuCo2O4 nanoneedle. The cyclic performance of CuCo2O4@C and CuCo2O4 electrodes at progressively increasing current density was recorded, as shown in Fig. 7b. During the first 100 cycles with a current density of 2 A g-1, the CuCo2O4@C electrode presented a specific capacitance of 920.3 F g−1. When the current density decreased back to 2 A g-1 again, a capacitance of 926.2 F g−1 was recovered without degradation. Long-term cycling stability of CuCo2O4@C and CuCo2O4 electrode was also evaluated at the current density of 2 A g-1, as shown in Fig. 7c. The results in Fig. 7c presents that the CuCo2O4@C electrode exhibited an excellent long-term stability with only 1.8 %

capacitance loss after 3000 cycles, which is much better than 7.9 % capacitance loss for the CuCo2O4 electrode. To further understand the influence of carbon coating on the charge-discharge kinetics, EIS of the CuCo2O4@C composites and the pristine CuCo2O4 electrodes were measured in the frequency range from 0.01 Hz to 100 KHz. The arc in high-frequency region is associated with the interfacial properties of the electrodes and corresponds to the charge transfer resistance (Rct), and the straight line in low frequency region is ascribed to the diffusive resistance related to the diffusion of electrolyte

20

. The internal resistance (Rs) is the sum of the ionic resistance of the

electrolyte, the intrinsic resistance of the active material and the contact resistance at the active material/current collector interface, and can be obtained from the intercept of the plots on the real axis. The inset of Fig. 7d gives an equivalent circuit used to fit the EIS curves to measure Rs and Rct, where Zw and CPE are the Warburg impendence and the constant phase element, respectively. The steeper shape represented the smaller ion diffusive resistance of CuCo2O4@C composites than that of pristine CuCo2O4 nanoneedles. The Rct of CuCo2O4@C (0.87 Ω) arrays is also smaller than that of pristine CuCo2O4 arrays (1.76 Ω) (Table 1). Besides this, the real axis intercept of CuCo2O4@C reduce to about 40 % of the pristine CuCo2O4. These analyses revealed that the carbon layer anchored on the surface greatly reduced the electrical conductivity and contact resistance between CuCo2O4@C and the electrolyte. For comparison, Table 2 summarizes some key parameters of CuCo2O4@C composites, CuCo2O4 nanostructures and CuCo2O4-based composites

[19, 28-36]

. It is

clear that our sample delivers a higher specific capacitance than that of CuCo2O4

nanostructures,

which

can

be

compared

with

CuCo2O4@MnCo2O4

and

CuCo2O4@Graphite. Moreover, the cyclic performance is better than pristine CuCo2O4, CuCo2O4@MnCo2O4 and CuCo2O4@Graphite. The high performance was attributed mainly to the following factors. Firstly, the directly grown arrays with their tops tangled up, which can ensure good mechanical adhesion and electrical connection to the current collector. Secondly, the high specific capacitance and cycle performance delivered by the CuCo 2O4@C core-shell nanostructures can be put down to the synergistic effect between CuCo 2O4 and high conductive carbon layer. Meanwhile, the CuCo2O4@C core-shell structures can not only effectively facilitate liquid electrolyte diffusion into the active materials but also decrease the contact resistance between active materials and the electrolyte.

4. Conclusions In summary, we demonstrated the direct growth of three CuCo2O4 architectures on Ni foam using a simple hydrothermal method. Among these nanostructures, the CuCo2O4 nanoneedles delivered the maximum specific capacity. In addition, the optimized CuCo2O4@C core-shell electrodes delivered a high specific capacitance of 1432.4 F g−1 at 1 A g−1 with a well cyclic performance. The excellent performance can be attributed mainly to the unit needle-liked structures with their tops tangled up and the high conductive carbon layer, which would further decrease the charge transfer resistance, ion diffusive resistance and contact resistance at the interface. The excellent properties of the CuCo2O4@C core-shell electrodes make it a promising candidate for high performance supercapacitors.

Acknowledgments This research was financially supported by the National Natural Science Foundation of China (Nos., 61574122, 51502257, 61675175, 51572233, and 61574121), the Science and Technology Key Projects of Education Department Henan Province (No. 2016GGJS-095). The authors are indebted to Dr J. B. Cheng, Z. Yang for their technical assistances and kind help.

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Fig. 1 Growth schematic illustration of three types of CuCo2O4 nanostrutures.

Fig. 2 Low and high magnification SEM images and the structure schematic illustrating of CuCo2O4 nanostructures prepared by hydrothermal method.

Fig. 3 a) Low and high magnification SEM images of CuCo2O4@C nanoneedles, b) the XRD patterns of CuCo2O4@C and CuCo2O4 nanoneedles.

(b) Intensity (a.u.)

C 1s

0

200

400

600

800

1000

529.1 eV 530.9 eV

525

1200

530

Cu 2p

Cu 2p3/2

535

540

Binding Energy (eV)

Binding Energy (eV)

(c)

O 1s

O 1s

O 1s

Intensity (a.u.)

Co 2p Ni 2p Cu 2p

(a)

(d)

Co 2p

Cu 2p1/2

Sat.

Sat.

930

940

950

960

Intensity (a.u.)

Intensity (a.u.)

Co 2p3/2

3+ Co 2p3/2

2+

3+

2+

Sat.

Sat.

780

Binding Energy (eV)

790

800

810

Binding Energy (eV)

Fig. 4 (a) XPS spectra of as synthesized CuCo2O4@C nanoneedles; (b-d) XPS survey scan of O 1s, Cu 2p and Co 2p regions, respectively.

Fig. 5 Core-shell structure of CuCo2O4@C arrays: (a-c) low and high magnification TEM images; (d) HRTEM image and the magnification image (inset); (e) EDS mapping of Co, Cu, O, and C elements.

(a)

(b)

0.16

0.54

5

0.08

-1

mV s

-1

10 mV s

0.36

CuCo2O4

-1

20 mV s

CuCo2O4@C

Current (A)

Current (A)

0.12

0.04 0.00 -0.04

-1

50 mV s

0.18

-1

100 mV s 0.00 -0.18

-0.08 -0.36 0.6

0.2

0.3 0.4 Potential (V)

20 A g-1 10 A g-1

0.5 Potential (V)

0.5

0.4

5

A g-1

2

A g-1

1

A g-1

0.0

0.6

0.1

(d) 1600 -1

(c)

0.1

Specific Capacitance (F g )

0.0

0.3 0.2 0.1 0.0

0.2 0.3 0.4 Potential (V)

0.5

0.6

CuCo2O4@C

1200

CuCo2O4

800

400

0 0

400

800 1200 Time (s)

1600

2000

0

4

8 12 16 -1 Current density (A g )

20

Fig. 6 (a) CV curves of the CuCo2O4 and CuCo2O4@C core-shell nanostructure at a scan rate of 10 mV s−1. (b) CV curves of the CuCo2O4@C electrode at different scan rates. (c) CD curves of the CuCo2O4@C electrode at various current densities. (d) The capacitances of CuCo2O4 and CuCo2O4@C electrodes at different current densities.

0.6

(b) 1200 CuCo2O4

-1

0.5

Specific Capacitance (F g )

(a)

Potential (V)

CuCo2O4@C

0.4 0.3 0.2 0.1

CuCo2O4

800 600

2Ag

-1

2Ag 5Ag

400

-1

-1

10 A g

200

0.0

-1

20 A g

-1

0

0

40

80 120 Time (s)

160

200

(c)1000

0

(d)

200

Time (s)

400

600

5 CuCo2O4@C

98.2 % 92.1 %

800

CuCo2O4

4 - Z'' (ohm)

Specific Capacitance (F g-1)

CuCo2O4@C

1000

600 400

Simulated Simulated

3 2 1

200

0

0 0

600

1200 1800 Time (s)

2400

3000

0

1

2

3 Z' (ohm)

4

5

Fig. 7 (a) CD curves of the CuCo2O4 and CuCo2O4@C nanostructures at a scan rate of 5 A g-1. (b) Cycliing stability of the CuCo2O4 and CuCo2O4@C nanostructures at progressively various current densities. (c) Cyclic performance of the CuCo2O4 and CuCo2O4@C nanostructures at 2 A g-1. The inset is the SEM image of the CuCo2O4@C nanoneedles. (d) Electrochemical impedance spectra (EIS) and the simulated curves of the CuCo2O4 and CuCo2O4@C electrodes and the equivalent circuit (inset).

Samples

Rct (Ω)

Rs (Ω)

CuCo2O4@C

1.76

0.023

CuCo2O4

087

0.054

Table 1 Charge transfer resistance (Rct) and internal resistance (Rs) of the CuCo2O4 and CuCo2O4@C electrodes.

Samples

Specific capacitances

Cycles

Retain rates

Ref.

CuCo2O4@C

1432.4 F g

3000

98.2 %

This paper

CuCo2O4

796 F g

3000

94.7 %

19

CuCo2O4@MnCo2O4

1434 F g

10000

88.4 %

28

CuCo2O4

982 F g

-1

3000

100 %

29

CuCo2O4

809 F g

-1

1800

127 %

30

CuCo2O4

611 F g

-1

8000

94.8 %

31

CuCo2O4@Graphite

1131 F g

-1

7000

80 %

32

Graphene@CuCo2O4

1813 F g

-1

6000

95.2 %

33

CuCo2O4@Ni(OH)2

439 μAh cm

2000

84 %

34

CuCo2O4@NiMn2O4

2207 F g

-1

5000

95.6 %

35

CuCo2O4

1210 F g

-1

250

100 %

36

-1

-1

-1

-2

Table 2 The key performance parameters of CuCo2O4@C composites, CuCo2O4 nanostructures and other CuCo2O4-based composites.

Highlights CuCo2O4@C core-shell arrays were prepared by a simple hydrothermal and megnetron sputtering method. The performances of different morphology CuCo2O4 structures and CuCo2O4@C core-shell arrays were compared. High specific capacitance of 1432.4 F g−1 at current density of 1 A g-1. After 3000 cycles, 98.2 % of the initial specific capacity was retained.