Hierarchical Porous Reduced Graphene Oxide

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Sep 6, 2018 - -1 . The symmetric supercapacitors assembled by two similar S/N co-doped reduced graphene oxide/molybdenum disulfide electrodes deliver ...
Accepted Manuscript Hierarchical Porous Reduced Graphene Oxide Decorated with Molybdenum Disulfide for High-performance Supercapacitors Jinghao Huo, Yujia Xue, Xiaojian Zhang, Shouwu Guo PII:

S0013-4686(18)32188-1

DOI:

10.1016/j.electacta.2018.09.180

Reference:

EA 32770

To appear in:

Electrochimica Acta

Received Date: 24 July 2018 Revised Date:

6 September 2018

Accepted Date: 26 September 2018

Please cite this article as: Jinghao Huo, Yujia Xue, Xiaojian Zhang, Shouwu Guo, Hierarchical Porous Reduced Graphene Oxide Decorated with Molybdenum Disulfide for High-performance Supercapacitors, (2018), doi: 10.1016/j.electacta.2018.09.180 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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Graphical Abstract

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Hierarchical Porous Reduced Graphene Oxide Decorated with Molybdenum Disulfide for High-performance Supercapacitors Jinghao Huo a∗, Yujia Xue a, Xiaojian Zhang b, Shouwu Guo a, c* School of Materials Science and Engineering, Shaanxi University of Science and Technology,

Xi’an, 710021, China b

c

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a

Hubei New Huaguang Information Materials CO., LTD, Xiangyang 441057, China

Department of Electronic Engineering, School of Electronic Information and Electrical

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Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Abstract

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Two-dimensional materials are relevant for supercapacitor applications due to their excellent electrical, thermal, and physical properties. In this study, one-pot solvothermal method with l-cysteine mixed in ethylene glycol is utilized to prepare S/N co-doped reduced graphene oxide, which is decorated with molybdenum Furthermore,

three-dimensional

these

two-dimensional

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disulfide.

hydrogel

architectures.

Field

sheets

are

emission

found scanning

to

form

electron

microscopy and high-resolution transmission electron microscopy show this

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compound gel with hierarchical porous structure. The specific surface area of this compound gel is estimated to 151.41 m2 g-1, which is larger than that of S/N co-doped

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reduced graphene oxide gel (28.98 m2 g-1). This composite is used to fabricate binder-free electrodes. The electrochemical tests reveal high specific capacitance of the electrodes reaching up 400.10 F g-1 at a current density of 1 A g-1. The symmetric supercapacitors assembled by two similar S/N co-doped reduced graphene oxide/molybdenum disulfide electrodes deliver specific capacitance as high as 95.10

∗ Corresponding author E-mail address: [email protected] (Jinghao Huo), [email protected] (Shouwu Guo). 1

ACCEPTED MANUSCRIPT F g-1 at a current density of 1 A g-1 with capacitance retention of 91.32% after 5000 cycles.

These

results

demonstrate

that

S/N

co-doped

reduced

graphene

oxide/molybdenum disulfide composite with hierarchical porous structure is promising binder-free material for high-performance supercapacitors.

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Keywords: S/N co-doped graphene; molybdenum disulfide; hierarchical porous structure; supercapacitors

1. Introduction

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With the growing demand for electrochemical energy storage (EES) devices and

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rapid development of friendly-environment economy, tremendous efforts have been paid for development of novel materials suitable for EES devices with high power and energy densities [1-3]. Supercapacitors (SCs) are good candidates for energy storage because of their rapid charge/discharge performance, superior power density, and good cycling stability [4]. SCs devices require anode and cathode as electrodes

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immersed in an electrolyte. On the other hand, the electrode material plays an important role in performance of SCs. Many types of materials are employed in SCs,

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including oxides [5-8], carbon materials [9, 10], graphene [11], metal hydroxides [12-14], two-dimension (2D) transition metal dichalcogenides (TMDs) [15], and

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conducting polymers [16, 17].

Graphene is a 2D material with one-atom-thick layer of graphite. Since its

discovery in 2004, graphene has been widely studied for supercapacitor use because of its unique properties. Nevertheless, compared to 2D structures, the 3D graphene-like foam, hydrogel, aerogel and sponge have larger specific surface areas, more hole-like structures, outstanding conductivities, and superior mechanical properties [18-22]. In 2010, Shi et al. [23] prepared a 3D self-assembled graphene hydrogel (SGH) from 2D graphene sheets using convenient one-step hydrothermal 2

ACCEPTED MANUSCRIPT method. The obtained SGH was electrically conductive, mechanically strong, thermally stable, and exhibited a high specific capacitance. After that, more studies dealing

with

hydrogels

for

supercapacitors

were

launched

and

hydrothermal/solvothermal method became popular for fabricating 3D graphene. For

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example, Aradilla et al. [24] prepared GHs by one-step process using hydrazine hydrate as gel assembly agent (GH-HD). They found the GH-HD to possess high electrical conductivity (1141 S m-1), elevated specific capacitance (190 F g-1 at 0.5 A

Nevertheless,

oxygen-containing

functional

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g-1), high power capability, and relevant cyclic stability.

groups,

defects,

and

some

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heteroatoms (e. g. N, S, P, B) are inevitably introduced in 3D GHs prepared by hydrothermal/solvothermal methods. Therefore, defect engineering of 3D graphene is of high importance [25-27]. Zhang et al. [28] used a facile self-assembly process to prepare N/S co-doped and hierarchical porous graphene for supercapacitors. They

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demonstrated that doping with both N and S at pH 12 could effectively improve the electrical double layer capacitance (EDLC) and specific capacitance of graphene. However, the contribution of N and S doping in graphene capacitance performance is

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still limited. Therefore, improvements of capacitor energy densities are limited by the

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electrical double layer capacitance. The design of graphene composites and pseudo-capacitive materials are effective

means to improve the capacitance characteristics of supercapacitor electrodes. Common constraint capacitor materials include oxides, hydroxides, conductive polymers and TMDs. Molybdenum disulfide (MoS2) with 2D graphene-like layers has strong covalent bonds between Mo-S and weak van der Waals interaction forces between S-S. In addition, the plane charge transport properties are enhanced due to molybdenum atoms electron correlation in the two-dimensional plane. As

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ACCEPTED MANUSCRIPT supercapacitor electrode materials, MoS2 nanomaterial with large specific surface areas could store more electrical double-layer charge and provide Faraday capacitance. However, the electrical conductivity of pure MoS2 is moderate and structure could easily break after some time. Hence, synergistic effects of MoS2 and graphene

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become important in improving the electrochemical properties of SC electrodes.

Numerous methods are used to fabricate graphene/MoS2 composites, where both morphology and structure of graphene/MoS2 have important impacts on the

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electrochemical performance of SC. For instance, Firmiano et al. [29] used microwave heating to fabricate layered MoS2 covalently bonded (Mo-O-C) to

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graphene. The specific capacitance of the electrode at 10 mV s-1 reached 265 F g-1 with certain concentrations of MoS2. Li et al. [30] used a facile hydrothermal method to fabricate MoS2 nanosheets on 3D graphene foam prepared by chemical vapor deposition. The graphene/MoS2 composites were used as electrodes to form flexible

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all-solid-state supercapacitors with volumetric capacitance of 19.44 F cm-3 combined with high stretchability and stability. Xie et al. [31] fabricated flower-like MoS2/N-doped graphene by one-pot hydrothermal process. The specific capacitance

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of this electrode reached 245 F g-1 at 0.25 A g-1.

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During the hydrothermal process of graphene/MoS2 composites, thiourea, thioacetamide and l-cysteine are widely used as sulfur source and reducing agent [32-35]. L-cysteine is common reductant and dopant for GO due to its -NH2, -SH, and -COOH groups. Many researchers fabricated rGO/sulfide composites in deionized water mixed with l-cysteine, used as sulfur source and reducing agent [35, 36]. Here, S/N-doped reduced graphene oxide (SNG) decorated with MoS2 was prepared by simple one-pot solvothermal method in ethylene glycol. The resulting SNG/MoS2 composite exhibited hierarchical porous structure with macropores and mesopores.

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superior Cm of 95.10 F g-1 at 1 A g-1 and good cycling performances (5000 cycles, 91.32%).

2. Experimental

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2.1. Chemicals and materials

The graphene oxide (GO) slurry (3.865 wt%) was purchased from Shanghai

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carbon source valley new material technology Co. Ltd, China. The GO slurry was freeze-dried to prepare SNG/MoS2 hydrogels. The sodium molybdate dihydrate (Na2MoO4·2H2O), l-cysteine, ethylene glycol (EG), acetone, 2-propanol, ethanol, potassium hydroxide (KOH), potassium chloride (KCl) were all A. R. grade and

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purchased from Sinopharm Chemical Reagent Co. Ltd, China. The polypropylene /polyethylene (PP/PE) membrane was purchased from Taiyuan lizhiyuan lithium battery technology center Co. Ltd., China.

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2.2. Preparation of SNG/MoS2 electrodes

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Figure 1 shows the process of fabrication of 3D SNG/MoS2 hydrogels with hierarchical porous structure using similar solvothermal method reported previously [37]. Firstly, a 30 mL mixed EG solution containing 2 mg mL-1 GO and 0.8 mmol Na2MoO4·2H2O was stirred for 0.5h at 70 and further stirred for 2h at 70

C. Next, 2 mmol l-cysteine was added

C. The mixture was then transferred to a

hydrothermal reactor and reacted for 12h at 180

C. After cooling to room

temperature, the hydrogels were washed with 500 mL mixed solution of deionized water and ethanol (volume ratio: 9:1) to obtain 3D SNG decorated with MoS2 5

ACCEPTED MANUSCRIPT nanosheets. Nickel foams (NFs) were employed as current collectors, and SNG/MoS2 freeze-dried gel was used as active material. For comparison, the SNG freeze-dried gel and pure MoS2 powder were prepared by the same method with SNG/MoS2. The SNG/MoS2 binder-free electrode was prepared by putting as-prepared gel on

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NF and pressing with pressure machine at 2 MPa. The SNG binder-free electrode was prepared by the same method with SNG gel. However, the pure MoS2 powder was hard to fabricate a binder-free electrode because it is easy to fall off during testing. So

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the MoS2 electrode was prepared by MoS2 powder mixed with acetylene black and PTFE solution (weight ratio was 80: 5: 5) in anhydrous ethanol. The paste was coated C for 12h. At last, a pressure of 2 MPa

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on Ni foams with a brush and dried at 60

was applied on NFs to form a MoS2 electrode.

Figure 1

2.3. Characterization

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The crystallographic structures of as-prepared SNG and SNG/MoS2 freeze-dried gels and pure MoS2 powder were measured by X-ray Diffraction (XRD, D/max2200PC, Cu Ka, λ = 1.54178 Å). Raman spectroscopy (Renishaw-invia,

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England) was used to identify the microstructures of SNG and SNG/MoS2

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freeze-dried gels. Elemental types and valence states of the as-prepared gels were tested by X-ray photoelectron spectroscopy (XPS, Axis Supra, England). The XPS data were calibrated by C 1s (284.8 eV) and fitted by XPSPEAK 4.0 software. The morphologies of the as-prepared gels were identified by field emission scanning electron microscopy (FESEM, SU4800, HITACHI, Japan) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20 S-TWIN, USA). The thermogravimetric analysis (TGA) of as-prepared gels was performed with synchronous comprehensive thermal analyzer (STA409PC, Netzsch, Germany) at a

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C under nitrogen atmosphere.

Electrical conductivity values of gels were obtained using a four-probe conductivity tester. The surface areas and pore size distributions of as-prepared gels were

analyzer (ASAP 2460, Micromeritics, USA). 2.4. Measurements

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calculated from N2 adsorption/desorption data using an automatic volumetric sorption

The electrochemical properties of the as-prepared electrodes and SSC devices

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were measured by working station (CHI660E, CH Instrument, China) at room temperature. A three-electrode cell was employed to study the electrochemical

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properties of the electrodes. The prepared materials were used as working electrode, a Pt sheet (1.0×1.0 cm2) as counter electrode, and Hg/HgO as reference electrode. The aqueous electrolyte solution insisted of mixture of 1.0 M KOH and 0.5 M KCl. The cyclic voltammetry (CV) curves were recorded from -1 V to 0 V at scan rates of 1, 5,

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10, 20, 50, and 100 mV s-1. The galvanostatic charge/discharge (GCD) profiles were measured at current densities of 1, 3, 5, 7 and 10 A g-1 at voltages ranging from -1 V to 0 V. All electrochemical impedance spectroscopy (EIS) data were recorded at

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frequencies from 100 kHz to 0.01 Hz with corresponding amplitude of 5 mV. The Cm

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(F g-1) was calculated using Eq. (1), Cm =

I × ∆t m × ∆V

(1)

where I (A) is the discharge current, ∆t (s) is discharge time, m (g) is the mass of active material on the electrode, and ∆V (V) represents the discharge potential range. The CR2032 coin-type SSC devices were assembled by two SNG/MoS2 electrodes and a PP/PE separator between them. Before assembled, the electrodes were immersed in aqueous electrolyte of 1.0 M KOH and 0.5 M KCl for 12 h to fully

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ACCEPTED MANUSCRIPT absorb electrolyte. The CV curves were recorded from 0 V to 1 V at scan rates of 1, 5, 10, 20, 50, and 100 mV s-1. The GCD properties were measured at current densities of 1, 3, 5, 7 and 10 A g-1 in the voltage window from 0 V to -1 V. The performance of SSC was tested for 4500 cycles at a current density of 1 A g-1. The Cm was calculated

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by Eq. (1), where m is the total mass of two electrode active materials. The values of energy density (E, Wh kg-1) and power density (P, W kg-1) were calculated by means

1 1 E = × C ×V 2 × 2 3 .6 E × 3600 ∆t

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P =

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of Eqs. (2) and (3), respectively.

(2)

(3)

where C (F g-1) is the capacitance of SSC device, V (V) is the discharge potential range, and ∆t (s) is the time spent in discharge.

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3. Results and discussion

3.1. Fabrication mechanism of SNG/MoS2 hydrogels The fabrication mechanism of graphene/MoS2 composite was already reported in

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many papers. A number of studies have shown that, GO sheets could be used as a novel substrate for the nucleation and subsequent growth of MoS2 [38, 39]. In this

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work, GO, Na2MoO4 and l-cysteine were uniformly distributed in EG solution. The abundant oxygenic groups of GO could tightly adsorb MoO42- on GO sheet surface. Afterward, l-cysteine may thermally be decomposed into H2S and NH3 at 180

C to

form S/N co-doped rGO by C-S and C-N bonds. Meanwhile, both MoO42- and H2S fabricated MoS2 nanosheets were anchored on SNG surface, and stacking of SNG sheets was inhibited. Furthermore, SNG/MoS2 sheets had self-assembled 3D hydrogel, where water vapor created pores in the sheets under the solvothermal conditions. The obtained hierarchical porous structure provided rich transmission channels, high 8

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3.2. Composition and morphology of SNG/MoS2 hydrogels The crystal phase and structure of SNG/MoS2 were investigated by XRD and Raman analysis. As shown in Figure 2a, the XRD patterns of SNG showed a broad

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diffraction peak at ~ 25 °, matching with the (0 0 2) plane of graphite [37]. However, with introduction of MoS2 to SNG, the graphene characteristic peak was decreased, and there is a wide drum between 30° and 40° associated with MoS2. The reason

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for this may be due to poor crystallinity and small amounts of MoS2. The Raman spectra of SNG and SNG/MoS2 are depicted in Figure 2b. Two prominent peaks at

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1350 (D band) and 1590 cm-1 (G band) were observed, caused by defects and sp2 hybridization of carbon atoms, respectively [40]. The intensity ratio of D band to G band (ID/IG) is useful to reveal defects in graphene. The obtained ID/IG values of SNG (1.12) is smaller than that of SNG/MoS2 (1.21), indicating edges/defects increasing on

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SNG with introduction of MoS2 [40, 41]. Meanwhile, two small peaks at 2680 (2D band) and 2920 cm-1 (D+G band) appeared, and all peaks were associated with carbon nature of 3D SNG. The MoS2 also showed little effect on Raman spectrum of

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SNG/MoS2, which agreed well with XRD data.

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The XPS spectra of SNG/MoS2 were recorded to identify the elemental compositions, valence states, and combined ways of MoS2 and SNG. As shown in Figure 2c, besides C, O, S and N, small peak related to Mo appeared for SNG/MoS2 hydrogel. The high-resolution scan of C 1s peak showed main peaks at 284.8 eV, 285.6 eV, 286.7 eV, 287.8 eV and 288.8 eV (Figure 2d), corresponding to C-C/C=C, C-O/C-S/C-N, -C-O, -C=O and O-C=O in SNG/MoS2, respectively [42]. The presence of Mo 3d (228.5 eV, 231.7 eV), S 2s (226 eV) and S 2p (162.4 eV, 161.2 eV) regions confirmed the existence of MoS2 (Figures 2e-f) [43]. Furthermore, the small 9

ACCEPTED MANUSCRIPT peak at 164.9 eV in S 2p region could be ascribed to presence of S-C issued from S-doped SNG [42]. Figure 2 As illustrated in Figure 3a, the SNG hydrogel was composed of large

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micron-sized sheets. And in Figure 3b-c with high resolution, the SNG sheets had a smooth surface. With the introduction of MoS2, the SNG/MoS2 sheets became smaller and more porous structures appeared (Figure 3d). In Figure 3e-f, SNG sheets clearly

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depicted many wrinkles with MoS2 films, so the specific surface area of SNG/MoS2 hydrogel was larger than that of SNG hydrogel. And some large-pores and mesopores

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formed on SNG/MoS2 sheets, which then were connected to build 3D porous structures. This hierarchical porous structure was beneficial to improve the structural stability and provide ions transport channels. This rendered the active material to fully contact the electrolyte to induce good capacitance properties.

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Figure 3

The TEM measurements agreed well with FESEM. In Figure 4a, numerous

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wrinkles and some holes appeared on SNG/MoS2 sheets. Figure 4b shows the MoS2 sheets have a mean interlayer lattice spacing of 0.62 nm for (002) lattice planes with

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defects and disorder structure and distribute on the surface of SNG sheets, which lattice spacing is 0.34 nm for (002) plane. The elemental mapping analysis in Figure 4c evidently confirmed the presence of elements C, Mo, S, O and N with homogeneous distribution in SNG/MoS2 sheets. Figure 4d is the EDX curve and atomic % of element. Due to small Mo content (0.58 %), the elemental mapping of Mo was also small. These data were consistent with XRD and Raman results. Figure 4e is the TGA curves of SNG and SNG/MoS2 freeze-dried gels from 40 to 800 °C. 10

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hybridized carbon network of SNG and SNG/MoS2 gels degraded and the weight had a dramatic loss [46]. While SNG exhibited a weight loss of 92.52 wt% and the

MoS2 in tthe composite is no more than 4.83 wt%.

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Figure 4

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SNG/MoS2 lost much less weight (87.69 wt%), indicating the loading amount of

The N2 adsorption-desorption isotherms of the as-prepared SNG and SNG/MoS2 were recorded to characterize the specific surface areas and porosity. Figure 5 indicated isotherms with typical IV hysteresis loops, according to classification of

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International Union of Pure and Applied Chemistry (IUPAC). The Brunauer-Emmett Teller (BET) surface areas of SNG and SNG/MoS2 were estimated to 28.98 and 151.41 m2 g-1, respectively. Obviously, the surface area of SNG/MoS2 was five times

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higher than that of SNG, caused by introduced MoS2 to form porous composite

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structure and accumulated reduced nanosheets. The pore size distributions are gathered in Figure 5b. Compared to SNG, more macropores and mesopores were found in SNG/MoS2 hydrogels, providing more transport channels for the electrolyte. Hence, large surface areas and hierarchical porous structures were useful to contact and infiltration of the electrolyte to SNG/MoS2 electrode, increasing charge storage. Figure 5

3.3. Electrochemical properties of SNG/MoS2 electrodes The electrochemical properties of SNG/MoS2 electrodes were measured by CV, 11

ACCEPTED MANUSCRIPT GCD, and EIS tests. It is obvious to see that in Figure 6a, the inside area of CV curve for MoS2 electrode at the scan rate of 10 mV s-1 is very small due to the non-ideal conductivity and poor adhesion with NFs. Compared with MoS2 electrode, the CV curves of SNG and SNG/MoS2 electrodes exhibited larger areas and these two

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electrode revealed similar rectangular shapes because of EDLC property. Furthermore, the SNG/MoS2 electrode exhibited excellent specific capacitance with larger inside area of CV curve than that of SNG. The CV profiles of SNG/MoS2 electrode at

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different scan rates are displayed in Figure 6b. It will be noted that as scan rate rose, the inside areas of CV curves increased, suggesting that SNG/MoS2 electrode had

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outstanding rate capability. The reason for this phenomenon is that the large specific surface areas and hierarchical porous structure of SNG/MoS2 made electrode and electrolyte have a full contact and provide more transport channels for K+ ions. Meanwhile, the MoS2 anchored on SNG sheets can effectively prevent the stack of

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SNG nanoparticles. Furthermore, the electrical conductivity performed with four-point probe method clearly showed that SNG/MoS2 gel has a higher conductivity (1188 S m-1) than that of SNG (869 S m-1). Hence Abundant active sites and excellent

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conductivity made SNG/MoS2 electrode process good specific capacitance.

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The GCD curves of MoS2, SNG and SNG/MoS2 electrodes at the current density of 5A g-1 were shown in Figure 6c. The time of charge and discharge of MoS2 electrode both is too short due to it’s easy to fall off in electrolyte. With the introduction of MoS2 in SNG hydrogel, the value of Cm increased from 163 F g-1 to 189.5 F g-1. Figure 6d represents the GCD curves of SNG/MoS2 electrode at different current densities. The time of charge and discharge both decreased when current density increased. This was caused by difficult ions diffusion at higher scan rates. The

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SNG/MoS2 electrode are all larger than that of SNG electrode. However, the capacitance improvement is not obvious due to the small amount of MoS2. The GCD test further indicated the synergistic effect of MoS2 and SNG improve the

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electrochemical performance of electrode.

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To further analyze the interface characterization of SNG/MoS2 electrode, EIS analyses were performed and the data are illustrated in Figure 6f. The EIS curves depicted semicircles in high-frequency regions and straight lines in low-frequency regions. The smaller semicircle of SNG/MoS2 electrode meant smaller series and

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charge-transfer resistance values, confirming that MoS2 improved the conductivity of SNG/MoS2 electrode. Meanwhile, the higher slope of the straight line indicated

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hierarchical porous structure conducive to electrolyte diffusion. Figure 6

3.4. Electrochemical properties of SNG/MoS2//SNG/MoS2 SSC

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Figure 7 represents the electrochemical tests of symmetric supercapacitors

composed of two SNG/MoS2 electrodes. The CV curves of SSC at different scan rates from 1 to 100 mV s-1 showed similar rectangular shapes (Figure 7a), and the inset areas of the CV curve rose as scan rate increased. This suggested SSC with typical EDLC and excellent rate capability. Figure 7b depicts the GCD curves of SSC at current densities from 1 to 10 A g-1. The value of Cm increased as current density decreased, and Cm at 1 A g-1 was estimated to 95.10 F g-1. The cycling performance of

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ACCEPTED MANUSCRIPT SSC at 1 A g-1 was tested for 4500 cycles and the results are gathered in Figure 7c. After 5000 cycles, the capacitance retained 91.32%. Figure 7 Energy density (E) and power density (P) are two important parameters in AAC.

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According to the Ragone plot shown in Figure 7d, the AAC composed of two SNG/MoS2 electrodes delivered elevated value of E (13.54 Wh kg-1) at power density of 500 W kg-1. At higher power density of 5002.11 W kg-1, the energy density reached

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10.56 W h kg-1. After 4500 cycles, E was estimated to 10.85 Wh kg-1 at power density of 500.13 W kg-1. These data indicated the relevance of SNG/MoS2 binder-free

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electrode for high-performance SCs.

4. Conclusions

Hierarchical porous SNG/MoS2 hydrogel was successfully fabricated by simple one-pot solvothermal method with l-cysteine in EC. The SNG/MoS2 composite

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showed several benefits: 1) porous structure of SNG provided transmission channel for charge and ions, 2) large specific surface area of the composite may facilitate the

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contact between the electrode and electrolyte, and 3) SNG sheets decorated with MoS2 nanosheets effectively reduced accumulation and increased structure stability.

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Meanwhile, the conductivity of SNG/MoS2 was improved with the introduction of MoS2. The SNG/MoS2 electrode prompted SC with high capacitance and excellent cycling stability. Overall, the as-prepared SNG/MoS2 hybrid material looks promising in terms of facile preparation and good performance as binder-free electrodes of SCs.

Acknowledgement The authors gratefully acknowledge the financial supporting by the Scientific Research Fund of Sanqin Scholars (BJ11-26), the Natural Science Foundation of

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ACCEPTED MANUSCRIPT Shaanxi University of Science and Technology (2016BJ-49), and the Scientific Research Program Funded by Shaanxi Provincial Education Department (17JK0109).

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Figure captions Figure 1 Schematic diagram of fabricating SNG/MoS2 hydrogels by solvothermal method.

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Figure 2 (a) XRD patterns of MoS2, SNG and SNG/MoS2, (b) Raman spectra of SNG and SNG/MoS2, XPS spectra: (c) survey scan of SNG and SNG/MoS2, high resolution scan of SNG/MoS2: (d) C 1s, (e) Mo 3d, (f) S 2p.

Figure 3 FESEM images of as-prepared gels: (a-c) SNG and (d-f) SNG/MoS2.

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Figure 4 (a, b) HRTEM images of SNG/MoS2 gels, (c) EDX elemental mappings of

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C, Mo, S, O and N, (d) EDX curve and element content (Atomic %) and (e) TGA of SNG and SNG/MoS2 gels.º

Figure 5 (a) Nitrogen adsorption/desorption isotherms and (b) the corresponding pore size distribution curves of SNG and SNG/MoS2 gels.

Figure 6 Electrochemical properties of as-prepared electrodes: (a) CV curves of

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MoS2, SNG and SNG/MoS2 electrodes at a scan rate of 10 mV s-1, (b) CV curves of SNG/MoS2 electrodes with different scan rates (1, 2, 5, 10, 20, 50 and 100

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mV s-1), (c) GCD curves of MoS2, SNG and SNG/MoS2 electrodes with a current density of 5 A g-1, (d) GCD curves of SNG/MoS2 electrodes with different

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current densities (1, 3, 5, 7 and 10 A g-1), (e) specific capacitance of SNG and SNG/MoS2 electrodes calculated from the GCD curves at different current densities and (f) EIS data of SNG and SNG/MoS2 electrodes.

Figure 7 Electrochemical properties of SNG/MoS2//SNG/MoS2 SSC: (a) CV curves at different scan rates of 1, 2, 5, 10, 20, 50 and 100 mV s-1, (b) GCD curves with different current densities (1, 3, 5, 7 and 10 A g-1), (c) cycle performance test at current densitie of 1 A g-1 and (d) Ragone plot of cycle 1 and cycle 5000.

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Research Highlights 

Hierarchical porous S/N co-doped reduced graphene oxide (SNG) decorated with MoS2. The 3D porous structure improved the electrochemical property of SNG/MoS2

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hydrogels.

The SNG/MoS2 bind-free electrode has a capacitance of 400.10 F g-1 at 1 A g-1.



The symmetric supercapacitor with two SNG/MoS2 electrodes had superior

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The symmetric supercapacitor showed good cycling performances (5000 cycles,

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91.32%).

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capacitance.