In-situ embedding MOFs-derived copper sulfide polyhedrons in carbon nanotube networks for hybrid supercapacitor with superior energy density

Journal Pre-proof In-situ embedding MOFs-derived copper sulfide polyhedrons in carbon nanotube networks for hybrid supercapacitor with superior energy density Haoting Niu, Yu Liu, Baodong Mao, Na Xin, Hong Jia, Weidong Shi PII:

S0013-4686(19)32001-8

DOI:

https://doi.org/10.1016/j.electacta.2019.135130

Reference:

EA 135130

To appear in:

Electrochimica Acta

Received Date: 3 September 2019 Revised Date:

17 October 2019

Accepted Date: 21 October 2019

Please cite this article as: H. Niu, Y. Liu, B. Mao, N. Xin, H. Jia, W. Shi, In-situ embedding MOFs-derived copper sulfide polyhedrons in carbon nanotube networks for hybrid supercapacitor with superior energy density, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135130. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

A supercapacitor constructed by HKUST-1-derived copper sulfide polyhedrons and nitrogen-doped carbon polyhedrons interspersed into carbon nanotubes thin film, which shows a superior energy density of 38.4 W h kg-1 at a power density of 750 W kg-1.

In-situ embedding MOFs-derived copper sulfide polyhedrons in carbon nanotube networks for hybrid supercapacitor with superior energy density Haoting Niua, Yu Liua,*, Baodong Maoa, Na Xina, Hong Jiab, Weidong Shia,* a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,

212013, P. R. China. Tel: +86-511-88791800 b

College of Physics and Electronic Information & Henan Key Laboratory of

Electromagnetic Transformation and Detection, Luoyang Normal University, Luoyang 471934, China. E-mail: [email protected], [email protected] Abstract The binder-free flexible electrode materials have greatly potential for portable flexible new energy equipment. Herein, we rationally fabricate a flexible composite electrode by connecting the HKUST-1 derived copper sulfide (CuS) polyhedrons with carbon nanotubes (CNT). Because of the synergistic interaction of the HKUST-1 derived CuS polyhedrons and the CNTs network, many excellent electrochemical properties of the composite thin film are obtained working as supercapacitors electrodes. The specific capacitance of the as-prepared electrode can achieve 606.7 F g-1 at a current density of 1 A g-1. Moreover, the as-fabricated asymmetric supercapacitors by using HKUST-1 derived CuS polyhedrons and nitrogen-doped carbon polyhedrons interspersed into carbon nanotubes films act as positive (CCS) and negative (CNC) electrodes deliver up to a higher energy density of 38.4 W h kg-1 at a power density of 750 W kg-1, which also show a favourable cycle life (87.0% capacity retention after 6000 cycles).

1

It indicates that the electrodes we fabricated are potential electrode materials and our research offers a valuable inspiration for high performance energy storage devices. Keywords: Hybrid supercapacitor, metal-organic frameworks, copper sulfide, N-doped porous carbon, carbon nanotubes 1. Introduction With the increasingly serious environmental pollution and the great demands of new energy, the enormous efforts have been paid to renewable energy sources and advanced energy storage devices [1-4]. Supercapacitors, a unique energy storage device with high power density, rapid charge/discharge rate and good cycling stability, have exerted a positive influence in the fields of hybrid vehicles, energy storage systems and defence military [5-9]. Unfortunately, the practical application of supercapacitor has been limited by their lower energy density (generally not exceeding 10 W h kg-1) [10]. Different kinds of electrode materials have been investigated as potential electrodes to enhance the energy density of supercapacitor, including transition metal sulfides, transition metal oxide and carbonaceous materials [11-12]. Among these materials, the transition metal sulfides have been universally explored, due to its as conversion type materials with satisfactory reversible redox ability and higher theoretical capacities [13-17], Such as CoxSy [18], NixSy [5,9] and CuxS [19]. Recently, copper sulfides (CuxS) with various phases from Cu poor to Cu rich, have attracted tremendous attention as potential electrode materials for numerous energy storage device due to their remarkable physicochemical characteristics [20-21].

2

Among the reported CuxS materials, CuS exhibits many outstanding properties, such as high electrical conductivity of 103 S-1 and favourable theoretical capacity of 560 mA h g-1, making it as a potential electrode material for high-power electrochemical capacitors [22]. Nevertheless, the large-scale practical application of CuS as an electrode material is mainly hindered by its persistent capacity fading and unsatisfactory cycling stability during the discharge/charge processes [23-24]. Great efforts still needed for the design and fabrication of CuS- based electrodes for supercapacitors. Metal-organic frameworks (MOFs) are a novel class of materials with such intriguing characteristics as high surface area, tailoring pore structures and controlled channels, and have been regarded as effective sacrificial precursors to prepare various kinds of porous nanostructures for electrode materials [25-27]. It is noteworthy that HKUST-1 (Cu3(BTC)2) has been widely studied to synthesize highly effective electrode materials for supercapacitors and lithium-ion batteries, because of its ultrahigh specific surface areas and tunable pore structures [28]. Typically, MOFs derived transition metal chalcogenides and porous carbon materials own ideal specific surface area and remarkable pore volume, showing outstanding electrochemical properties when used as electrodes [29]. For instance, Foley et al. utilized the HKUST-1 to prepare copper sulfide (CuxS) nanowires for Li-ion battery cathodes, showing a specific capacity of 220 mA h g−1 after 200 charge-discharge cycles [14]. In addition, Liu et al. fabricated a unique electrode composed of CNT with carbon nanoparticles derived from ZIF-8, which exhibited an ultrahigh energy density of 21.1

3

W h kg−1 at a power density of 5000 W kg−1 in supercapacitors [30]. However, MOFs derived porous materials act as an electrode material alone makes it difficult to achieve the goal of superior performance supercapacitor, which is restricted by its poor conductivity and cycle stability [31-33]. One effective method that may allow this problem to be mitigated is the construction of composite electrode materials, which composed of MOFs derived materials with carbon-based materials to enhance the conductivity, actual specific capacity and stability [34-36]. For example, Peng’s group combined CNTs with porous carbon material derived from the HKUST-1 to act as an electrode for supercapacitor, which exhibited superior capacitance of 381.2 F g−1 at 5 mV s−1 and 194.8 F g−1 at 2 A g−1, as well as superior cycling stability [37]. To the best of our knowledge, carbon nanotubes (CNT) are one of the most promising carbon-based materials with unique one-dimensional tubular structure and ultra-high conductivity. When forming the unique network structure, CNT not only can effectively improve the electrical conductivity and stability, but also play a critical role to intersperse the HKUST-1 derived porous materials to broaden the specific surface area [38]. Thus, it is of great interest to combine HKUST-1 derived porous materials with CNT networks as electrode materials for supercapacitors. Herein, HKUST-1 derived CuS polyhedrons and nitrogen-doped carbon polyhedrons interspersed into carbon nanotubes films are used as the positive (CCS) and negative (CNC) electrodes for the asymmetric supercapacitor (ASC) for the first time. The carbon nanotubes serve as a flexible conductive substrate, and also bind the HKUST-1 derived CuS polyhedrons and nitrogen-doped carbon polyhedrons to form

4

flexible and binder-free films. This interpenetrating structure not only greatly improves the overall conductivity but also maximizes the ion-accessible surface area that is beneficial for ions to transport through the composite film. Moreover, the porous HKUST-1 derived CuS polyhedrons show larger specific surface area and remarkable pore size distribution, which provide more exposed active sites and abundant pathways for electrons and ions transfer. The binder-free CCS hybrid electrode displays a higher capacity of 606.7 F g-1 at 1 A g-1. Remarkably, the as-assembled ASC device of CCS //CNC shows an ultrahigh energy density of 38.4 Wh kg-1 at a power density of 750 W kg-1. And after 6000 cycles, the device still maintains 87.0% capacitance retention, which shows nice cycling life. 2. Experimental section 2.1 Materials Every chemical was analytical grade and used directly no more purification. Cu(NO3)2·3H2O and thioacetamide (TAA) were bought from Sinopharm Chemical Reagent Co., Ltd.1,3,5-benzenetricarboxylic acid (H3BTC) and 2-aminoethanol (AE) were purchased from Sigma-Aldrich. Carbon nanotube was obtained from XFNANO Company Limited. Sodium formate (HCOONa) was obtained from Alfa Aesar. 2.2 Synthesis 2.2.1 Synthesis of the CNT @ HKUST-1 hybrid thin film [37] In a typical procedure, 2 mM Cu(NO3)2·3H2O was dissolved in 30 mL DI water, and then the AE aqueous solution (1.6 mM) poured into the above solution with continuously stirring overnight. After that, the copper hydroxide nanowires (CHNs)

5

with positively charge were prepared. Subsequently, the negatively charged CNTs solution (2 mL, 0.15 wt%) was mixed with the prepared CHNs solution with constantly stirring for 30 mins and then vacuum filtered into a CNT@CHN thin film. After that, the as-prepared CNT@CHN thin film was immersed into the mixture solution of HCOONa (15 mM) and H3BTC

10 mM

ethanol-water (1:1 in volume

ratio) solution for 1 hour to prepare CNT@HKUST-1 hybrid films. 2.2.2 Synthesis of the CCS thin film Typically, the CCS thin film was obtained via the hydrothermal vulcanization reaction. The prepared CNT@HKUST-1 thin film was transferred into a Teflon-lined autoclave containing the mixture solution of TAA (20 mg) and anhydrous ethanol (20 mL) at 120 ˚C for 2 h. After then, the hybrid films of CCS were washed by deionized water and ethanol alternately. Ultimately, the hybrid thin film was annealed at 350 oC for 2 h in N2 flow. Loading weight of CCS electrode is 0.4 mg cm-2. 2.2.3 Synthesis of CNC thin film The prepared thin film of CNT@HKUST-1 was carried out in N2 flow at 500 ˚C with a heating rate of 1 ˚C/min for 2 h. Then the product was immersed into aqueous solution of HNO3(0.7 M) for 48 h at room temperature. Finally, the obtained thin film was washed by ethanol and deionized water. 2.3 Characterization The morphology and size of the samples were obtained by a field emission SEM (FESEM, Hitachi Japan S-4800). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were acquired by

6

a F20 S-TWIN electron microscope (Tecnai G2, FEI Co.) at 200 kV acceleration voltage. The X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB_250Xi X-ray photoelectron spectrometer with 150WAl Ka X-ray sources. The X-ray diffraction (XRD) was performed on the D/MAX-2500 instrument (Rigaku, Japan) with Cu Kα radiation. 2.4 Electrochemical measurements The capacitance measurement was analyzed with a CHI 760E electrochemical workstation in the 6 M KOH electrolyte. For a three-electrode system, CCS binder-free film was served as working electrode and sandwiched into two sheets foam nickel sheets (1cm2), Pt foil and Hg/HgO electrode were used as counter electrode and reference electrode, respectively. Electrochemical performance from samples were evaluated via cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy tests. The specific capacity of electrode materials could be calculated by the following equation:
=

 × ∆  × ∆

where C is the specific capacitance and its unit is F/g, I is the discharge current and unit is A, ∆t is the discharge time and unit is s, m is the mass of the electrode and its unit is g, and ∆V is the potential window and its unit is V. The ASC device was assembled with CCS film (positive electrode) and CNC film (negative electrode). For the ASC device, the cyclic voltammetry and galvanostatic charge-discharge tests were used to calculate the corresponding specific capacitance

7

and further to calculate energy density and power density, which based on the gross mass of the both electrodes in the 6 M KOH electrolyte. 3. Results and discussion

Fig. 1 Schematic illustration of the preparation process of CCS composite thin film.

The synthetic strategy for the binder-free film of CCS was schematically demonstrated in Fig.1. Firstly, the hybrid flexible film of CNT@CHN was synthesized by vacuum filtration and then a typical solution method was carried out to grow HKUST-1 polyhedrons on the flexible film. Secondly, the subsequent hydrothermal vulcanization and calcination processes will result into a porous structure of HKUST-1 derived CuS polyhedrons. Cu 2p

♦

Intensity (a.u.)

♣

♣

c

b

♣

Cu 2p

2p3/2

Intensity (a.u.)

(103)

♣ CuS ♦ CNT

(105)

Intensity (a.u.)

(101)

a

O 1s

2p1/2

C 1s S 2p

20

30

40

50

60

70

80

0

200

400

2 Theta (degree) S 2p

S 2p1/2

C-S

160

e

164

Binding Energy (eV)

800

1000

C-C

1200

930

C-O

950

960

O 1s

f

O-C=O

SO24

168

940

Binding Energy (eV) C 1s

Intensity (a.u.)

S 2p3/2

Intensity (a.u.)

d

600

Binding Energy (eV)

Intensity (a.u.)

10

280

284

288

Binding Energy (eV)

8

292

528

532

Binding energy (eV)

536

Fig. 2 (a) XRD pattern of CCS thin films, XPS spectra of the CCS thin films (b) the survey scan spectrum, (c) Cu 2p spectrum, (d) S 2p spectrum, (e) C 1s spectrum and (f) O 1s spectrum.

Fig. 2a reveals the XRD pattern of CCS thin film. The major diffraction peak at 27.9o, 32.09o and 39.19o can be assigned to the (101), (103), (105) planes of CuS (JCPDS No.75-2234), indicating the sample was successfully synthesized. The intense peak around at 26.0° can be indexed to the CNTs [37]. The composition and valence states of elements of CCS thin film were revealed via XPS and the results were shown in Fig. 2b-f. Typical survey spectrum of CCS thin films was shown in Fig. 2b, indicating the major characteristic peaks for Cu, S, C, O elements without impurity peaks. In Cu 2p spectrum of the composite (Fig. 2c), the major peaks at 932.0 eV and 952.0 eV are attributed to Cu 2p3/2 and Cu 2p1/2 spin-orbit states, respectively, indicating +2 oxidation state for Cu in CCS thin films [39]. The XPS spectrum of S 2p (Fig. 3d) can be divided into four peaks of 161.6 eV, 162.7 eV, 163.6 eV, and 168.5 eV, which are ascribed to the S 2p3/2, S 2p1/2, C-S and SO42-, respectively [40]. It is obviously seen that the existence of C-S is assigned to the interfaces between CNT and CuS, while the existence of SO42- is attributed to partially oxidized sulfur species on composite surface. For the C 1s XPS spectrum (Fig.2e), the fitting peaks at 284.8 eV and 285.9 eV are ascribed to sp2 hybridized carbon and C-O bonding, respectively, while another fitting peaks at 287.7 eV is attributed to carbonylate C (O-C=O) component in CCS thin films [10]. The peak intensity of C-C bonding is so much strong than remaining two peaks, is mainly because of most oxygen containing groups have been removed after annealed process. In Fig. 2f, there is a major peak at 530.9 eV, which is

9

assigned to O 1s.

Fig. 3 Typical SEM images of (a, b) CNT@CHN hybrid film, (c, d) CNT@ HKUST-1 thin film, (e, f) CCS thin film, respectively.

The morphologies of the binder-free hybrid films were characterized by scanning electron microscopy (SEM). Fig. 3a and b show the SEM images of the obtained CNT@CHN hybrid films. It can be clearly seen that CNT and CHNs nanowires are uniform interweaved in both longitudinal and transverse directions contributing to a flexible hybrid film. As shown in Fig. 3c and d, after the immersing process, the HKUST-1 polyhedrons are in-situ grown uniformly into the hybrid film to obtain the bonding free thin films of the CNT@ HKUST-1. Obviously, the inset in Fig. 3d shows that the colour of the as-synthesized sample changed from black to blue, indicating the typical HKUST-1 polyhedrons are successfully compounded. After hydrothermal 10

vulcanization, Fig. 3e and f shows the SEM image of CCS hybrid thin film, from which we can clearly see the surface of HKUST-1 polyhedrons changed from typical smooth to porous, the result will be further verified in the test of transmission electron microscopy (TEM). The unique porous structure can improve the reactivity and maximize the ion-accessible surface area, which is greatly beneficial for transport of the electrons and ions. As we expected, owe to the CNTs interwoven with each other, the hybrid film still exhibits excellent flexibility without any rupturing during the bending (as shows in the inset Fig. 3f). Besides, we further tested the specific surface area of the CCS composites, which is 126.4 m2g-1, such large surface area is conducive to providing more active sites for electrochemical reaction.

Fig. 4 (a) TEM image, (b) enlarged TEM image and (c) HRTEM image of the as-prepared CCS thin film, (d-i) TEM image and EDS mapping images of C, Cu, S, N and O elements.

The internal features of the composite thin film are further analysed via TEM and HRTEM measurements. Fig. 4a shows the CCS thin film TEM image, both of these CNTs and CuS polyhedrons are observed overtly, which is accord with the above-mentioned SEM image. Fig. 4b clearly reveals the magnified image on the 11

edge of the CuS polyhedrons, which can be seen that CuS polyhedrons consist of many small nanorods. The HRTEM image in Fig. 4c displays 0.27 nm lattice distance, which is indexed as the (103) plane of CuS polyhedrons calculated from XRD (Fig. 2a). Besides, the calculated lattice spacing of 0.33 nm can be indexed as the (002) plane of CNTs phase. The homologous SAED pattern (Fig. S4) revealing polycrystalline nature of the CuS. Moreover, EDS elemental mapping images of CCS thin film are exhibited in Fig. 4d-i, further indicating that all elements are uniformly distributed.

Fig.5 (a) CV curves of the CCS electrode at different scan rates; (b) GCD curves of the CCS 12

electrode at different current densities, (c) Specific capacitances of the CCS electrode at various current densities, respectively, (d) Nyquist plots of impedance for the CCS and CNC electrodes, (e) Capacitive and diffusion-controlled contributions of the CCS electrode scan rates of 2 mV s-1, (f) The capacitive contribution from surface and di usion-controlled contribution for CCS electrode at di erent scan rates.

With the purpose of further investigating the electrochemical properties of the CCS and CNC electrodes, both of which were analysed in a three-electrode configuration. A series of CV curves of CCS thin film at various scan rates with the potential window from 0 to 0.5 V are exhibited in Fig. 5a. We can see that all CV curves show clear redox peaks and tend to shift to higher and lower potentials respectively with ascending the scan rate, indicating a reversible faradaic reactions process connected with CuS polyhedrons and fast electronic and ionic transport. Fig. 5b records the GCD curves of CCS electrode under various current densities, which are almost symmetrical indicate outstanding reversible electrochemical performance [41]. Fig. 5c exhibits the specific capacitance of CCS electrode, which are obtained based on the homologous GCD curves. Obviously, the specific capacitances of CCS electrode are 606.7, 516.2, 429.4, 360.5, 302.2 and 270.0 F g-1 at the corresponding current densities of 1, 2, 3, 5, 8, and 10 A g-1. The maximum specific capacitances of CCS electrode can achieve 606.7 F g-1 at the relevant current densities of 1 A g-1 that is superior to those of previous reports, as shown in Table 1. There are several factors contribute to the superior specific capacitances of CCS electrode. Firstly, the HKUST-1 derived CuS polyhedrons with large specific surface area can enhance the

13

reactivity and further improve specific capacitances. Secondly, the in-situ grown CuS polyhedrons can maximize the integral specific surface area that furnishes nice transport environment for the electrons and ions of electrolyte. Finally, the interweave network of CNTs can further boost the electrical conductivity and stability of thin film. Additionally, the morphology of the CNC thin film is also studied by SEM, as shown in Fig. S1a and b. It is clear that the carbon polyhedrons with favourable specific surface area still maintain, which shows superior electrochemical performance and is advantage for the as-assembled ASC. The CV and GCD curves of CNC thin film and their respective specific capacitances are also investigated in the potential window of -0.8-0 V. The maximum specific capacitances can achieve to 361.9 F g-1 at 2 mV s-1 and 553.3 F g-1 at 1 A g-1, as shown in Fig. S2. Table 1 The specific capacitances of various CuS based electrodes presented in literature and the present work. Materials

Specific capacitance

Scan rate

Current density

Ref.

CuS nanoplatelets

72.85 F g-1

5 mV s-1

___

[24]

CuS-CNTs@NF

467.02 F g-1

___

0.5 A g-1

[42]

Cu2S (Cu:HMT)

470 F g-1

5 mV s-1

___

[43]

Cu2S (Cu:AMM)

761 F g-1

5 mV s-1

___

[43]

CuS

773 F g-1

___

2 Ag-1

[44]

PPy/CuS/BC

580 F g-1

___

0.8 mA cm-2

[45]

graphene/CuS

497.8 F g-1

___

0.2 A g-1

[46]

247 F g-1

___

3D CuS-AC

14

0.5 A g-1

[47]

CCS

606.7 F g-1

1 A g-1

___

This work

As shown in Fig. 5d, the charge transfers behaviour of CCS and CNC electrodes are further explored by the electrochemical impedance spectroscopy (EIS). In the low frequency region, the CNC electrode exhibits a more ideal straight and almost vertical line, indicating its non-Faradic charge storage mechanism behaviour. After in-situ grown the HKUST-1 derived CuS polyhedrons, the CCS electrode shows a more inclined curve, revealing the pseudocapacitance predominance for the hybrid electrode of the CCS [10]. The semicircle diameter represents the charge transfer resistance (Rct) from the electrodes at the high frequency region[48,49]. Apparently, the CCS electrode shows lesser diameter than the CNC electrode, manifesting the CCS electrode exhibit higher conductive. It is attributed to the HKUST-1 derived CuS polyhedrons uniformly in-situ grown into the hybrid thin film to constitute a unique structure which is advantageous for the electric transport during charge-discharge process [40]. To gain more insights into the energy storage mechanism of the CCS electrode, the capacitive contribution (h1v) and the diffusion-controlled contribution (h2v1/2) are quantitatively investigated for charge storage. On the basis of the principle of Dunn’s method, CV results at 2 to 100 mV s-1 are analysed to quantitatively separate the above-mentioned two contributions, the current (i) can be investigated by the following equation: [50,51] i = h1v+ h2v1/2

(1)

where i and v represents current response and the scan rate in sequence, h1 and h2 are

15

constants. The diffusion-limited and capacitive processes are specifically revealed at the scanning rates of 2 mV s-1 in Fig. 5e. It is measured that the capacitive contributions proportions of the total capacities at the corresponding scanning rates are 31.7%, 34.7%, 38.6%, 42.5%, 54.0% and 73.1% exhibited in Fig. 5f. It can be observed that the capacitive contribution becomes increasingly pronounced with the increasing scanning rates, implying the surface redox reaction processes of the HKUST-1 derived CuS are dominant for charge storage on high scan rate, which is because of the short distance from ion diffusion and fast electronic transmission [52].

Fig.6 (a) schematic illustration of the ASC device composed by the CCS positive electrode and CNC negative electrode, (b) CV curves of the CCS and CNC electrodes tested at 2 mV s-1 in a three electrode configuration; (c) CV curves of the CCS//CNC device collected in different voltage window, (d) CV curves of the CCS//CNC device at different scan rates, (e) GCD curves of the CCS //CNC device at different current densities, (f) the change of the cyclic performance of the CCS //CNC device at the cycling test.

For further purpose of exploring the electrochemical properties of the CCS

16

electrode, an asymmetric supercapacitor (ASC) is assembled, utilizing the CCS electrode as the positive electrode and the CNC electrode as the negative electrode (Fig. 6a). From the Fig. 6b, the comparing CV curves of both the CCS electrode and CNC electrode are studied at the scan rate of 2 mV s-1. The potential window of the CCS electrode is range of 0 V - 0.5 V, and the CNC electrode is range of -0.8 V - 0 V. The CV curves of the CCS//CNC ASC under various incremental voltage windows at 100 mV s-1 are recorded on Fig. 6c, from which we can clearly see that the optimal operating voltage window can arrive at 1.5 V without visible pronounced polarization. Base on the suitable potential window of 1.5 V, the CV curves of the as-assembled device are measured at varous scan rates from 2 to 100 mV s-1 (Fig. 6d). It is found that the curves shape is almost maintained even the scan rates to ascend to 100 mV s-1 and no obvious noticeable distortion, revealing nice interfacial kinetics and high rate performance. Fig. 6e displays the GCD curves of the ASC device at diverse current densities within the voltage window 1.5 V. Additionally, the corresponding specific capacitances are 122.9, 89.4, 77.4, 69.6, 63.4 and 60.0 F g-1 at 1, 2, 3, 5, 8, and 10 A g-1 (Fig. S3), respectively. Fig. 6f displays the cycling performance of the as-fabricated device is recorded on 5 A g-1. Impressively, the capacitance still maintained 87.0 % of the initial capacitance even the cycles are up to 6000, implying the good cycling stability of the device which is beneficial for the practical application, which is attributed to the highly reversible Faradic reaction of the unique composite of CCS.

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Fig.7 Ragone plots of the CCS //CNC ASC. (Inset shows glowing ten parallel heart-shaped blue and white LEDs lighted by two serious connected the ASC derives.)

Ragone plot plays a worthy role to assess the electrochemical properties of device, which exhibits both of the relationships of energy density and power density (Fig. 7). From that the as-fabricated CCS//CNC device achieves to a high energy density of 38.4 W h kg-1 at the corresponding power density of 750 W kg-1, which is outstanding and even

is comparable with most of the devices

reported,

such as

NiONSs@CNTs@CuO NWAs/Cu//AC@CF (26.32 W h kg−1 at 219.03 W kg−1) [53], meshtype CuO@MnO2//activated graphene (29.9 W h kg−1 at 269.6 W kg−1) [54], m-CuO120/FSS//h-CuS100/FSS

(22.8

W h

kg−1

at

2.3

kW kg−1) [55],

Ti3C2/CuS//Ti3C2 MXene (15.4 W h kg−1 at 750.2 W kg−1) [56], CuS- AC//AC (24.88 W h kg−1 at 800 W kg−1) [47]. Furthermore, it is the most valuable that the practical application of the as-fabricated CCS//CNC device, two devices are connected in series, which can power 10 parallel heart-shaped blue and white LED bulbs (the inset in Fig. 18

7). The results implied that the composite thin film of CCS shows superior electrochemical properties and potential for the energy storage. 4. Conclusions In summary, we have successfully synthesized a unique composite thin film of CCS by a simple in-situ growing and hydrothermal vulcanization strategy. The composite thin film shows favourable conductivity, large specific surface area, and outstanding mechanical flexibility, and has been testified for potential applications in supercapacitors. The in-situ grown HKUST-1 derived CuS polyhedrons not only contributes to the higher pseudocapacitance, but also maximizes specific surface area of the composite film, which provide rich reaction sites and benefit the transfer of ions and electrons in the film. It all benefits from the synergistic action of the CNT and the porous CuS polyhedrons derived from HKUST-1. Furthermore, the CNTs network greatly facilitate the improvement of conductivity, stability and flexibility. Our as-fabricated flexible materials have a promising prospect and can provide a significant reference value for flexible multifunctional energy storage device. Acknowledgements Our research is supported via Foundation from Jiangsu Province Natural Science (BK20160526), Foundation from China National Natural Science (51602131 and 21477050), Training Engineering from Youth Backbone Teacher of “Young Talent Cultivation Program”. Program from Jiangsu Province Innovation/Entrepreneurship (Surencaiban [2016] 32) and Foundation from Henry Fok Education (141068). References

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1. MOFs-derived copper sulfide polyhedrons were successfully interspersed into carbon nanotubes. 2. CCS film provides more exposed active sites and rich transport pathways for electrons and ions. 3. The binder-free CCS electrode achieve a high capacitance of 606.7 F g-1 at 1 A g-1. 4. The ASC devise of CCS//CNC shows outstanding electrochemical property.

1. We rationally fabricated two flexible composite electrodes by connecting the HKUST-1 rived copper sulfide (CuS) polyhedrons and nitrogen-doped carbon with carbon nanotubes (CNT). Both of which can be used as the corresponding positive and negative electrodes for the asymmetric supercapacitor (ASC). 2. The unique thin film of CuS/CNT acts as a positive electrode for supercapacitors, which shows a high capacitance of 606.7 F g-1 at a current density of 1 A g-1. 3. An asymmetric supercapacitor of CuS/CNT//CNC achieves a superb energy density of 38.4 W h kg-1 at a power density of 750 W kg-1, as well as long cycle life (87% capacity retention after 6000 cycles).