copper sulfide composite as electrode materials for supercapacitors with high energy density

copper sulfide composite as electrode materials for supercapacitors with high energy density

Accepted Manuscript Facile preparation of reduced graphene oxide/copper sulfide composite as electrode materials for supercapacitors with high energy ...

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Accepted Manuscript Facile preparation of reduced graphene oxide/copper sulfide composite as electrode materials for supercapacitors with high energy density Tingkai Zhao, Wenbo Yang, Xin Zhao, Xiarong Peng, Jingtian Hu, Chen Tang, Tiehu Li PII:

S1359-8368(18)31063-1

DOI:

10.1016/j.compositesb.2018.05.058

Reference:

JCOMB 5728

To appear in:

Composites Part B

Received Date: 4 April 2018 Revised Date:

26 May 2018

Accepted Date: 30 May 2018

Please cite this article as: Zhao T, Yang W, Zhao X, Peng X, Hu J, Tang C, Li T, Facile preparation of reduced graphene oxide/copper sulfide composite as electrode materials for supercapacitors with high energy density, Composites Part B (2018), doi: 10.1016/j.compositesb.2018.05.058. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile preparation of reduced graphene oxide/copper sulfide

composite

as

electrode

materials

for

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supercapacitors with high energy density

Tingkai Zhao1,*, Wenbo Yang1,*, Xin Zhao2, Xiarong Peng1, Jingtian Hu1, Chen Tang1, Tiehu Li1

State Key Laboratory of Solidification Processing, Shaanxi Engineering Laboratory for Graphene New Carbon

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Materials and Applications, School of Materials Science and Engineering, Northwestern Polytechnical University,

Xi’an 710072, China. 2

Queen Mary University of London Engineering School, NPU, Northwestern Polytechnical University, Xi’an

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710072, China.

Abstract: Recently, copper sulfide (CuS) quite arouses researchers’ interest due to its high theoretical capacity and excellent electroconductivity. However, poor cycling stability seriously

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limited the application in supercapacitors. In addition to the improvement of cycling performance,

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it is also a challenge to develop electrode materials with energy density. Herein, RGO/CuS composite is prepared successfully by solvothermal reaction methods. By the observation using FESEM and TEM, CuS microstructure displays regular and tiny nanoparticles, which are supported by RGO sheets. After the electrochemical measurements, RGO/CuS composite exhibits a maximum specific capacitance of 946 F·g-1 at 10 mV·s-1 and 906 F·g-1 at 1 A·g-1, respectively. The excellent cycling stability is also achieved and it maintains 89% retention after 5000 cycles at

* Correspondence and requests for materials should be addressed to T.K.Z. (email: [email protected]) These authors contributed equally to this work.

ACCEPTED MANUSCRIPT 5 A·g-1. RGO/CuS composite also possesses high energy density of 105.6 W h·kg-1 at the power density of 2.5 kW·kg-1, which indicates that RGO/CuS composite has a bright future as electrode materials for supercapacitors.

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Keywords: regular polygon CuS nanoparticles; supercapacitor; cycle stability; energy density

1. Introduction

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In the past few decades, the excessive consumption of fossil fuels has caused environment

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pollution badly such as global climate changes, acid rain and so forth. Meanwhile, owing to the shortage of non-renewable energy resource, it is necessary and urgent affair to develop sustainable and environment-friendly energy source. To solve energy crisis, transition metal is extensively investigated and widely used in electrochemical energy storage [1-11]. Supercapacitor or

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ultracapacitor, as an energy storage device, possesses higher specific capacitance compared with conventional capacitor, more excellent cycling stability and higher power density contrasted with batteries [12]. And transition metal oxides and conducting polymers, served as pseudocapacitive

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materials, have been investigated extensively such as MnO2 [13, 14], NiO [15, 16], CoO [17],

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polyaniline [18, 19], polypyrrole [20] and so on. Recently, transition metal chalcogenides attract great attention as electrode materials because of their great chemical and physical properties. For instance, CoS [21], MnS [22], NiS [23] and MoS2 [24] have been employed as electrode materials for supercapacitors. Among numerous transition metal chalcogenides, copper sulfide (CuS) is low cost, abundant and environment friendly, which is widely used in various fields. It has been also regarded as promising electrode materials of supercapacitors due to its high theoretical capacity [25-29]. Many investigations on the morphology of CuS have been reported. For example, Zhu et

ACCEPTED MANUSCRIPT al. reported that CuS nanoneedles displayed 114 F·g-1 at 2 mV·s-1 and 122 F·g-1 at 1.2 A·g-1. Hsu et al. showed that CuS nanowires array 305 F·g-1 at 0.6 mg·cm-2. Zhang et al. presented that CuS microspheres exhibited 237 F·g-1 at 0.5 A·g-1. However, it is not satisfactory and disappointing to

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be served as electrode materials for supercapacitor owing to its inferior conductivity and poor cyclic stability like other chalcogenides. In addition, low energy density of supercapacitor compared with batteries is a severe problem, which hinders its application. Therefore, it is

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challenging and expectant to prepare CuS composites with high specific capacitance, energy

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density and excellent cycle life for energy storage applications.

As electrode materials, carbon materials have outstanding advantages such as high power density and excellent cycling stability because of their large specific surface area, high chemical stability and electrical conductivity [30-34]. Graphene, a novel carbon material, has captured

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scholars’ attention due to its fascinating mechanical, physical and chemical properties [35-40]. As a kind of electrode material, it has been investigated widely. Hence, it is a nice idea to introduce graphene in CuS for preparing a kind of CuS/graphene hybrid and the relevant

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researches have been reported. For example, Xiao et al. reported that flower-like copper sulfide/

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reduced graphene oxide composite presented the specific capacitance of 368.3 F·g-1 at 1 A·g-1 and 88.4% retention at 3 A·g-1 after 1000 cycles [41]. Chen et al. showed that copper sulfide microspheres/nitrogen-doped graphene displayed the specific capacitance of 379 F·g-1 at 1 A·g-1 [42]. Although the incorporation of graphene can efficiently improve the specific capacitance of CuS, it is a challenge to prepare CuS/graphene composites with excellent electrochemical performance such as higher specific capacitance, energy density and longer cycle life. In our work, reduced graphene oxide (RGO)/CuS composite was prepared successfully by

ACCEPTED MANUSCRIPT solvothermal reaction method. CuS microstructure presents regular polygon and tiny nanoparticles, which are dispersed uniformly onto RGO nanosheets. Owing to the unique structures, RGO/CuS composite exhibits high specific capacitance, favorable cycle stability and

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ultrahigh energy density, which exceeds CuS composites mostly reported before. Due to the excellent electrochemical performance, RGO/CuS composite should be used as potential

2. Experimental

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2.1. The preparation of graphene oxide (GO)

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electrode material for supercapacitors in the future.

GO was prepared successfully by a modified Hummers’ method in previous literature [43]. Firstly, 1 g graphite powder and 1 g NaNO3 were mixed and added in 50 ml concentrated sulfuric acid and the reaction temperature was kept at 0 °C by ice bath. Then 6 g KMnO4 was added

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slowly keeping reaction temperature below 20 °C. And then the mixture was maintained at 35 °C for 1 h under continuous stirring. 80 ml distilled water was added dropwisely in the above solution and the temperature was increased to 95 °C and maintained for 30 mins. Lastly, 200 ml distilled

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water and 6 ml 30% H2O2 solution was added to obtain yellow GO solution. The mixture was

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centrifuged and washed several times with distilled water, hydrochloric acid, and ethanol to make pH=7. GO was obtained by filtering and drying overnight at 60 °C. 2.2. The synthesis of RGO/CuS composite 20 mg GO was dispersed in 50 ml distilled water by sonication for 30 mins. 20 ml CuCl2

solution (1 mmol) was added in 50 mL highly dispersed aqueous GO solution (2 mg/mL) under constant stirring for 30 mins. Then 20 mL thiourea solution (2 mmol) was added dropwisely in the above mixture under magnetic stirring for 30 mins. The obtained mixture was washed several

ACCEPTED MANUSCRIPT times in order to remove residual CuCl2 or thiourea. The resultant mixture was then heated on a water bath at 90°C for 5 hrs. The black mass was washed several times with distilled water followed ethanol. Finally, the separated black product was dried at 60°C in vacuum oven for 24

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hrs and employed as active materials. The schematic diagram of the whole preparation process is

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presented in Fig 1.

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Fig.1. The schematic diagram of the preparation process of RGO/CuS composite

2.3. Characterization technique

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The chemical compositions of CuS, RGO, RGO/CuS composite were performed by X-ray

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diffraction (XRD, D/max 1200, Cu Ka). The characteristic peaks of materials were measured by a RM 2000 Microscopic Confocal Raman Spectrometer (Renishaw PLC, England) fitted with an Ar laser at a wavelength of 532 nm. Fourier transform infrared (FTIR) spectra were recorded with Avatar 370 FTIR spectrometer. X-ray photoelectron spectra (XPS) were recorded by VG Scientific ESCALAB 250 spectrometer. The morphologies were observed by a field emission scanning electron microscope (SUPRA 55, German ZEISS) at 15 kV and field emission transmission electron microscope (Tecnai F30 G2, American FEI) at 300 kV.

ACCEPTED MANUSCRIPT 2.4. Electrochemical measurements RGO/CuS composite, conductive graphite, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 8:1:1 under magnetic stirring for 24 hrs to obtain a slurry. And then the slurry

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was coated onto a nickel foam (1×1 cm2) and dried at 60℃ for 12 hrs to get the RGO/CuS composite electrode. The electrochemical performance of RGO/CuS, including CV, GCD and EIS measurements was tested in 6 M KOH solution by a CHI 650D electrochemical workstation

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(Shanghai, Chenhua Company). A two-electrode system was used in all the electrochemical

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measurements. The RGO/CuS electrodes were straightway served as a working electrode and a counter electrode, respectively, which were separated by a diaphragm in electrochemical test. CV curves were performed in the potential range from 10 to 100 mV·s-1. The GCD curves were measured in the potential window of 1 V at different current densities. EIS was tested in the

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frequency range from 0.01 Hz to 100 kHz at open circuit.

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3. Results and discussion

(a)

(b)

(103)

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(102) (006)

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RGO/CuS

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composite

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Fig.2. XRD patterns (a), Raman spectra (b), FTIR spectra (c) and XPS spectrum of RGO, CuS and RGO/CuS

In order to confirm the composition analysis of as-prepared samples, X-ray diffraction was performed and corresponding XRD patterns are shown in Fig.2(a). Clearly, the characteristic diffraction peaks of RGO/CuS composite appear at 10.8°, 27.2°, 27.7°, 29.3°, 31.7°, 32.8°, 48°,

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52.8° and 59.4°, which are indexed to (002), (100), (101), (102), (103), (006), (110), (108) and (116) planes, respectively. The XRD patterns of RGO/CuS composite match well with that of CuS,

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which indicates the good crystallinity of CuS. Fig.2(b) exhibits Raman spectra of CuS, RGO and RGO/CuS composite. As shown in Fig.2(b), apparently, RGO exhibits three characteristic bands at

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1350, 1580 and 2730 cm−1, which correspond to D, G and 2D bands of carbon materials, respectively. The D band results from the structural disorder and defects on the carbon basic plane such as the attachment of oxygen functional groups, G band is ascribed to in-phase vibration of sp2-bonded carbon atoms and 2D band represents intrinsic peak of graphene [44, 45]. And in Raman spectra of CuS, a distinct and sharp peak appears at 474 cm-1, which is representative of CuS crystal [44-46]. The intensity ratio of D and G bands of RGO/CuS (ID/IG=0.947) is higher than that of RGO (ID/IG=0.727), which reveals the increase of defect during the reduction of GO.

ACCEPTED MANUSCRIPT According to Raman spectra of as-prepared RGO/CuS composite, the characteristic peaks of both CuS crystal and RGO become strong, which proves the synthesis of CuS crystal successfully. Fig.2(c) presents FTIR spectra of GO and RGO/CuS composite. The FTIR spectra of GO

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shows many peaks, which are ascribed to numerous function groups including O-H stretching mode (~3414 cm−1), C=O stretching mode (~1735 cm−1), C=C stretching mode (~1625 cm−1), epoxy stretching mode (1226 cm−1), and alkoxy stretching mode (1053 cm−1) [47, 48]. Moreover,

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the FTIR spectra of RGO/CuS composite demonstrate the removal of oxygen functional groups

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after the heat treatment. More detailed information on the chemical composition of RGO/CuS composite was acquired through XPS analysis, which is shown in Fig.2(d). The signals of Cu, S, C and O elements are observed in the range of 0~1200 eV. The presence of O may be caused by surface-adsorbed CO2 or O2. No additional peaks of any impurities other than C, O, Cu and S can

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be found. The Auger line of Cu (Cu LMM) at 569.1 eV is the typical binding energy for CuS, which demonstrates that Cu element is existent in the form of bivalent state.

275

280

285

Binding Energy (eV)

290

295

(b)

Cu 2p3/2

Intensity (a.u.)

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Experimental data Fitted data C=O C=C C-O C-N

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Intensity (a.u.)

(a)

920

Cu 2p1/2

930

940

950

Binding Energy (eV)

960

970

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(d)

S 2p3/2

Experimental data Fitted data C=C C-O C-N

158

160

162

164

Binding Energy (eV)

166

168

275

280

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Intensity (a.u.)

Intensity (a.u.)

S 2p1/2

285

290

295

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Fig.3. High-resolution XPS C 1s spectra of GO (a), Cu 2p (b), S 2p (c) and C 1s (d) XPS spectra of RGO/CuS

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composite, respectively

In the high-resolution XPS C 1s spectra of GO shown in Fig.(a), the peak located at 284.9 eV is ascribed to C=C bonding in graphitic structure and the other three peaks indicate the presence of various function groups including C=O (290.2eV), C-O (287.3eV) and C-N (288.6eV). Fig.3(b),

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(c) and (d) show the high resolution XPS spectra of Cu 2p, S 2p and C 1s of RGO/CuS composite. As is shown in Fig.3(b), two major peaks appeared at 931.7 and 951.7 eV are the binding energy

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of Cu 2p3/2 and Cu 2p1/2, respectively. Furthermore, a strong peak is easily observed at 944 eV in the Cu 2p XPS spectrum which was contributed to the characteristic shakeup satellite peaks for

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Cu2+ and proves the presence of the paramagnetic chemical state of Cu2+. The high-resolution S 2p spectrum of RGO/CuS composite reveals two typical peaks at 162.6 and 163.4 eV for S 2p3/2 and S 2p1/2, which illustrates the presence of S2− in the composites. These binding energies are all accordance with the values of CuS crystal reported previously [41, 49]. Fig.3(d) is the XPS spectra for C 1s region. Except for the peak at 284.9 eV for C=C bonding, no additional peaks reveal that most of function groups have been absolutely removed during the hydrothermal process. Though the above analysis about XPS spectra, it is affirmed that RGO/CuS composite has been

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successfully prepared.

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Fig.4. (a) (b)The SEM images of RGO/CuS composite at two scales

The morphology and structure of RGO/CuS composite are characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). Fig.4 shows SEM images of RGO/CuS composite at two different scales. It is obvious that CuS

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microstructure presents clusters and nanoparticles. And thin and wrinkled RGO sheets are located between CuS clusters and support CuS nanoparticles, which is attributed to great mechanical property of RGO. Further, the detail of CuS nanoparticles could be acquired by TEM analysis,

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which is shown in Fig.5. As can be seen from Fig.5(a) and (b), CuS clusters and nanoparticles are

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uniformly dispersed onto RGO nanosheets. And CuS nanoparticles show the shape of pentagon, hexagon and near-sphere, and they have the same size with radius around 20-30 nm in Fig.5(c). So the dispersive and tiny particles sufficiently indicate large active sites and electrochemical reaction. Fig.5(d) shows the high-resolution TEM (HRTEM) image of RGO/CuS composite, the space of lattice fringes is determined to be 0.32 nm, which corresponds to the (101) plane of covellite CuS, further confirming the formation of CuS crystal.

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Fig.5. TEM (a) (b) (c) and HRTEM (d) images of RGO/CuS composite

3.1 Electrochemical analysis

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Fig.6. (a) CV curves of CuS at different scan rates (b) CV curves of CuS, RGO and RGO/CuS composite at 10

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mV·s-1 (c) CV curves of RGO/CuS composite at different potential windows (d) CV curves of RGO/CuS

composite at various scan rates (e) GCD curves of RGO/CuS composite at different current densities and (f) the

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specific capacitances of RGO/CuS composite at different current densities

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Fig.6(a) displays the CV curves of CuS between 0 and 0.5 V from 10 to 100 mV·s-1. The

couple of redox peaks can be observed, which indicates the pseudocapacitive characteristic of CuS. Fig.6(b) shows the CV curves of CuS, RGO and RGO/CuS composite between 0 and 0.5 V at 10 mV·s-1. The CV curves of RGO/CuS composite present the rectangle shape similar with that of RGO, which is distinct difference from that of CuS. The addition of RGO makes CV curve of CuS changed, which means that the electrochemical characteristics of CuS are influenced. Fig.6(c) shows the CV curves of RGO/CuS composite at different potential windows. Owing to the

ACCEPTED MANUSCRIPT presence of RGO sheets, the potential window of CuS could be enlarged, which indicates that RGO/CuS composite has higher energy density. Hence, the CV curves of RGO/CuS composite are measured at various scan rates in large potential window, which is shown in Fig.6(d). The inserted

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picture shows the specific capacitances at various scan rates. The CV curves of RGO/CuS composite presented a symmetrical shape, which means the excellent reversibility of supercapacitor. The specific capacitance can be calculated from CV curves according to the

(1)

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following equation (1):

where Cm (F·g-1) is the specific capacitance calculated from CV curves, I (A) is the charge/discharge current, v (mV·s-1) is the scan rate, m (g) is the total mass of active materials on the electrode, and ∆V (V) is the potential window during CV measurements. As a result, the

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specific capacitances of CuS, RGO and RGO/CuS composite is 230.8, 134.5 and 375.1 F·g-1 at 10 mV·s-1 in the potential window of 0.5 V, respectively. And the specific capacitances of RGO/CuS composite is 946, 908, 876, 820, 793 and 775 F·g-1 at different scan rates in the stable potential

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window of 1 V, respectively. Significantly, the specific capacitance of RGO/CuS composite is

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much higher than the bare CuS owing to the existence of RGO. With the scan rates increasing, the shape of the CVs almost not changes, which suggests that the rapid redox reactions occurred. And the specific capacitance of RGO/CuS composite drops slowly, which is ascribed to the insufficient electrolyte ion diffusion. Fig.6(e) shows the GCD curves of RGO/CuS composite at different current densities. The specific capacitance could be deduced according to the following equation: (2) where Cs (F·g-1) is the specific capacitance calculated from GCD curves, I (A) is the discharge

ACCEPTED MANUSCRIPT current, ∆t (s) is the discharge time, m (g) is the mass of active material on the both electrodes and ∆V (V) is the potential window during GCD measurements. Consequently, the specific capacitances of RGO/CuS composite are 906, 860, 828, 792 and 760 F·g-1, respectively. And the

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corresponding specific capacitances of RGO/CuS composite at different current densities are shown in Fig.6(f). The specific capacitance of RGO/CuS composite shows a slight decline while the current density increases, which originates from inadequate Faradaic redox reaction. (a) 25

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Fig.7. (a) Cyclic stability of RGO/CuS composite electrodes after 5000 cycles at the constant charge/discharge current density of 5A·g-1 and (b) Nyquist plots of CuS, RGO and RGO/CuS composite electrodes in the frequency

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range of 0.01 Hz-100 kHz at open circuit potential

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Fig.7(a) shows cyclic stability of RGO/CuS composite electrodes after 5000 cycles at 5A·g-1

and the first and last GCD curves are inserted. The specific capacitance maintains 89% after the finishing of 5000th cycle. Such excellent cycle performance was owing to the large specific surface area of RGO. In order to further investigate capacitive behaviors of RGO/CuS composite, the impedance was carried out at open circuit potential in the frequency range of 0.01 Hz-100 kHz and Nyquist plots of CuS, RGO and RGO/CuS composite electrodes are shown in Fig.7(b). At high frequency, the diameter of the semicircle stands for charge transfer resistance of the electrode

ACCEPTED MANUSCRIPT at the electrode/electrolyte interface [50, 51]. The linear part at low frequency corresponds to the diffusive resistance or Warburg impedance and its slope is large, it means that the capacitor has the lower diffusive resistance [52]. Obviously, RGO/CuS composite displays much lower charge

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transfer resistance and diffusive resistance than the bare CuS. Hence, the incorporation of RGO is beneficial for improving the superior electrochemical performance of CuS and RGO/CuS, which attributes to the nanosized effect of CuS particles.

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(b) Our work

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Our work Double-shell CuS nanocages Nanoporous CuS nanospheres Flower-like CuS [email protected] composite GN/CuS composite Flower-like CuS/RGO composite Hierarchical CuS microspheres Nanostructured CuS network [email protected] CuS nanoplatelets

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[email protected]

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composite electrodes

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Fig.8. (a) The specific capacitances of various CuS composite reported and (b) the Ragone plots of RGO/CuS

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For evaluating integrated electrochemical performance, the specific capacitance of various

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CuS composite reported before are shown in Fig.8(a). By comparison, the specific capacitance has been improved significantly and is much higher than that of double-shell CuS nanocages (843 F·g-1) [53], nanoporous CuS nanospheres (814 F·g-1) [54], flower-like CuS (597 F·g-1) [55], [email protected] composite (427 F·g-1) [29], GN/CuS composite [42], flower-like CuS/RGO composite (368.3 F·g-1) [41], hierarchical CuS microspheres (216 F·g-1) [28], nanostructure CuS network (157 F·g-1) [56], [email protected] (103 F·g-1) [49] and CuS nanoplatelets (73 F·g-1) [57]. In our work, the size of CuS crystal is much smaller than previous works, which means more sufficient

ACCEPTED MANUSCRIPT electrochemical reactions. Besides, due to the addition of RGO nanosheets, the potential window is enlarged, which has a significant influence on optimization of electrochemical performance of CuS composite. Hence, the specific capacitance of RGO/CuS composite is significantly improved.

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Furthermore, energy density and power density are important parameters for supercapacitors, which are used to evaluate electrochemical performance. The energy density (W, W h·kg-1) and

(3)

(4)

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power density (P, W·kg-1) were described by the following equations (3) and (4):

where ∆t (s) is the discharge time. The Ragone plots of RGO/CuS composite electrode and other CuS composite reported are given in Fig.8(b). RGO/CuS composite electrode has a high energy density and can achieve a maximum energy density of 125.8 W h·kg-1 at the power density of 500

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W·kg-1. So the high energy density for RGO/CuS composite is ascribed to uniform dispersion of CuS nanoparticles and the large specific surface area of RGO. With the power density increasing, RGO/CuS composite maintained a high energy density about 105.6 W h·kg-1 at 2.5 kW·kg-1,

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which is much superior to that of previous reported materials such as CuS nanoplatelets (6.23 W

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h·kg-1 at a power density of 1750 W·kg-1) [57], nanostructured CuS network (17.7 W h·kg-1 at a power density of 504 W·kg-1) [56], hierarchical CuS microspheres (15.06 W h·kg-1 at a power density of 392.9 kW·g-1) [28], nanoporous CuS nanospheres [54] and [email protected][49]. The superior energy density is owing to sufficient electrochemical reaction and the existence of RGO nanosheets which enable the potential window to amplify. The high specific capacitance and energy density fortell RGO/CuS composite as energy conversion material have a bright prospect in the future.

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Conclusions In this work, a kind of RGO/CuS composite with excellent electrochemical performance was

prepared successfully by solvothermal reaction method. After the observation using FESEM and

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TEM, regular polygon CuS nanoparticles are dispersed uniformly onto RGO nanosheets. After the electrochemical measurements, RGO/CuS composite exhibits a high specific capacitance of 946 F·g-1 at 10 mV·s-1 and 906 F·g-1 at 1 A·g-1, respectively. Compared to the previous reports, the

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stability performance of CuS was significantly improved and RGO/CuS composite maintains 89%

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retention after 5000 cycles at a current density of 5 A·g-1. Additionally, RGO/CuS composite remains also high energy density of 105.6 W h·kg-1 even at the power density of 2.5 kW·kg-1. Consequently, owing to the remarkable electrochemical properties, RGO/CuS composite would

Acknowledge

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have a promising prospect as electrode materials for high-performance supercapacitors.

This work was financially supported by the Natural Science Foundation of China (51672221),

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the Key Industrial Chain Project of Shaanxi Province (S2018-YF-ZDLGY-0030), the China

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Aeronautical Science Fund (2014ZF53074), the Seed Foundation of Innovation and Creation for Graduate Students in Northwestern Polytechnical University (Z2018006) and National College Students Innovation and Entrepreneurship Training Program (201810699094).

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