MoS2 nanosheets assembling three-dimensional nanospheres for enhanced-performance supercapacitor

Accepted Manuscript MoS2 nanosheets assembling three-dimensional nanospheres for enhancedperformance supercapacitor Yong-Ping Gao, Ke-Jing Huang, Xu Wu, Zhi-Qiang Hou, Yuan-Yuan Liu PII:

S0925-8388(18)30111-7

DOI:

10.1016/j.jallcom.2018.01.110

Reference:

JALCOM 44574

To appear in:

Journal of Alloys and Compounds

Received Date: 24 November 2017 Revised Date:

6 January 2018

Accepted Date: 8 January 2018

Please cite this article as: Y.-P. Gao, K.-J. Huang, X. Wu, Z.-Q. Hou, Y.-Y. Liu, MoS2 nanosheets assembling three-dimensional nanospheres for enhanced-performance supercapacitor, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.110. 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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MoS2 nanosheets assembling three-dimensional nanospheres for enhanced-performance supercapacitor Yong-Ping Gao a, Ke-Jing Huang b, *, Xu Wu c, Zhi-Qiang Hou d,Yuan-Yuan Liu a

b

College of Science and Technology, Xinyang College, Xinyang 464000, China

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a

College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China

School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000,

d

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China

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c

School of Chemistry and Chemical Engineering, ZhouKou Normal University, ZhouKou, 466001, China

*

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Corresponding author. Tel.: +86-376-6390611

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E-mail address: [email protected] (K. Huang)

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ACCEPTED MANUSCRIPT ABSTRACT: Uniform MoS2 nanosheets assembling three-dimensional nanospheres are prepared by using a facile hydrothermal procedure with SiO2 nanospheres as the

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template. The morphologies and structure of MoS2 nanospheres are thorough characterized and evaluated as electrode material for supercapacitors. Galvanostatic charge/discharge measurements reveal that MoS2 nanospheres deliver a specific

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capacity of 683 F/g at 1 A/g, and the specific capacity retains about 85.1 % after

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10000 cycles. An aqueous asymmetric supercapacitor is fabricated by employing MoS2 nanospheres and activated carbon as the positive and negative electrodes, respectively. The specific capacitance is 65.33 F/g at a current density of 1 A/g. An energy density of 20.42 Wh/kg is obtained at power density of 750.31 kW/kg. The

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results show an attractive performance to take advantage of MoS2 nanosheets assembling three-dimensional nanospheres as electrode material for supercapacitors. Two cells in series can easily light the light-emitting diode lamp brightly, further

Supercapacitor;

MoS2

nanosheets

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Keywords:

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demonstrates the high energy storage capability of the prepared MoS2 material. assembling

three-dimensional

nanospheres; Electrode materials; Aqueous asymmetric supercapacitor

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ACCEPTED MANUSCRIPT 1. Introduction Recently, stable and flexible energy-storage device has been paid tremendous research interest with the enhancing requirement of sustainable energy [1-5].

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Supercapacitor is an updated kind of energy storage device, which possesses multiple strengths including long life-time, excellent power density, fast charge/discharge processes, and environment friendly [6–11]. Generally, supercapacitors are classified

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into two main types on the basis of charge-storage theory: (i) electrical double-layer

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capacitor, usually utilizing carbon material, (ii) pseudocapacitor, utilizing redox-active material. Compared to electrical double-layer capacitors, pseudocapacitors grounded upon transition metal oxide or conducting polymer show superior specific capacitance because of their rapid and invertible electrosorption and Faraday reaction coming up

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at or close to electrode surface, while electrical double-layer capacitors just utilize capacitance increasing from charges separating on electrodes/electrolytes interfaces. Furthermore, carbon materials have some obvious drawbacks, such as low energy

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density and instability in aqueous electrolyte [12–16]. Therefore, a growing attention

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has been paid to the redox-active materials such as metal oxide [17–19] and conducting polymers [21-21] for pseudocapacitors. Transition metal sulfides have recently attracted worldwide attention in various

fields,

such

as

lithium-ion

batteries

[22-24],

sodium-ion

batteries

[25],

electrochemical capacitors (ECs) [26-29], catalysts [30,31] and solid lubricants [32,33], benefiting from their distinct structure and chemical

properties.

Representatively, MoS2 has obtained extremely consideration because of its specific 3

ACCEPTED MANUSCRIPT atomic structure. MoS2 possesses three atom layers, which are piled and combined through weak Van der Waals interactions [34,35]. Therefore, it is easy to peel MoS2 layers from the bulk [36,37]. Nevertheless, MoS2 usually exhibits unsatisfactory

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supercapacitor performance, primarily due to the big volume change during cycling and aggregation of alloy particles. Furthermore, the extremely low conductivity between two adjacent Van der Waals bonded layers would significantly suppress their

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overall electrochemical performance [38]. To solve this problem, adjusting the

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microcosmic morphology or connecting with other materials is an effective way. Researchers have done a lot in this field. For example, carbon-MoS2 nanotube has been synthesized and it delivered a specific capacitance of 210 F/g at 1 A/g [39]. Wang et al. [40] prepared a MoS2@carbon nanofiber which displayed a specific

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capacitance of 355.6 F/g at 5 mV/s. Huang et al. [41] reported a layered MoS2-graphene composite, which almost reserved 41.1% capacitance from 1 to 10 A/g. Yin et al. [42] synthesized a hierarchical MoS2/Mn3O4 hybrid architecture. The

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cycle stability of this material still reserved 119.3 F/g after 2000 cycles, and kept

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about 69.3% of the initial capacitance, which was more than two times higher than that of pure layered MoS2. In this work, uniform MoS2 nanosheets assembling three-dimensional

nanospheres are prepared by a simple hydrothermal route with SiO2 nanosphere as template. The MoS2 nanospheres possess a larger surface area, affording ample active surface for charge transfer and reducing diffusion length of electrolyte. The physical property and electrochemical performance of the as-obtained MoS2 nanospheres are 4

ACCEPTED MANUSCRIPT evaluated in detail. The results show that the as-prepared MoS2 nanospheres have a big specific capacity, high energy density and good cyclic stability, which indicates that the MoS2 nanosphere can be served as a promising electrode material for

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electrochemical supercapacitor.

2. Experimental

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2.1. Synthesis of MoS2 nanosheets assembling three-dimensional nanospheres

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Concentrated ammonia solution, tetraethoxysilane (TEOS), Na2MoO4·2H2O, and L-cysteine were obtained from Aladdin Chemicals Co. Ltd. (Shanghai, China). Synthesis of SiO2 spheres: 90 mL concentrated ammonia solution and 61.8 mL absolute ethyl alcohol were mixed uniformly in 24.8 mL water. Then, 4.5 mL TEOS

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was dropwise added into above solution as Si source under vigorous stirring and maintained for 2 h. The white SiO2 precipitates were then centrifuged, washed thoroughly with water and alcohol, and dried at 50 °C.

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Synthesis of MoS2 nanospheres: 0.55 g SiO2 sphere was first dispersed in 40 mL

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water. 0.6 g Na2MoO4·2H2O was then added and stirred for 10 min. Subsequently, 1.6 g L-cysteine was dispersed into the mixture as S resource. Finally, the solution was added into a Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. Black precipitate of SiO2@MoS2 core-shell sphere was obtained after centrifugation, washed thoroughly with ethanol, and dried at 50 ◦C for 12 hours. Fig. 1 shows the preparation

procedure

of

the

5

MoS2

nanosphere.

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Fig. 1. The synthetic process of the MoS2 nanospheres. 2.2. Materials characterization

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The morphology of MoS2 nanospheres was characterized with JSM-6510 scanning electron microscopy (SEM) (JEOL Ltd., Japan) and transmission electron microscopy (TEM, JEM-2100F). X-ray powder diffraction (X'Pert-Pro MPD) was employed to exhibit the crystalline phase of products. N2 adsorption-desorption

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isotherms were recorded on an ASAP2020 surface area and porosity analyzer (Micrometrics Instrument Corporation). Raman spectra was recorded at ambient

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UK).

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temperature on a Renishaw Raman system model 1000 spectrometer (Gloucestershire,

2.3. Electrochemical measurements Electrochemical tests were performed on a CHI660E electrochemical

workstation (Shanghai Chenhua Instruments Co.) with platinum and Hg/HgO as counter electrode and reference electrode, respectively. The working electrode was assembled with active material, carbon black and polytetrafluoroethylene in a mass ratio of 80:10:10. The slurry was pasted upon the Ni foam and dried at 65 ℃ for more 6

ACCEPTED MANUSCRIPT than 6 h. The prepared SiO2@MoS2 loaded on the electrode was in a range of 3-5 mg per electrode, and the thickness of the electrodes was approximately 100 mm and an area of about 1.0 cm2.

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Cycle voltammetry (CV) was recorded between -0.2 V and 0.7 V vs. Hg/HgO. Electrochemical impedance spectroscopy (EIS) was detected in the range of 0.01-100,000 Hz with the AC voltage amplitude of 5 mV and voltage frequencies

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from 100 kHz to 0.1 Hz at the applied potential of 0.2 V. Galvanostatic

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charge/discharge (GCD) and CV test were all executed in KOH solution (2.0 M). For quantitative considerations, the specific capacity was calculated from the GCD values with the equation as follows:

Cs=It/∆Vm

(1)

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where I, ∆V, t, and m are the constant current, the total potential difference, discharge time and the weight of active materials, respectively. The energy density (E) and power density (P) of asymmetric supercapacitor

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(ASC) were calculated by follows equations: 1 1 CV 2 × 2 3 .6 E P = 3600 × ∆t

(2)

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E=

Where

∆t

(3)

is the discharge time and V is the voltage range applied during the

charge-discharge measurements.

3. Results and discussion 3.1. Characterization of MoS2 nanospheres 7

ACCEPTED MANUSCRIPT X-ray diffraction (XRD) pattern provides the crystallinity and phase information, confirming the formation of obtained materials. Fig. 2A shows XRD patterns of pure SiO2 and MoS2 nanospheres. It displays well-resolved diffraction lines and the

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diffracted peaks of SiO2 and high-intensity peaks of MoS2 are observed clearly. The diffraction peaks at 14.7°, 33.26° and 54.52° on the diffractogram of MoS2 corresponds to (002), (100) and (110) planes, respectively, which agrees well with the

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standard data file (JCPDS: 29-0914). No characteristic peaks from other impurities

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are observed in the XRD pattern, manifesting the sample is highly pure. Fig. 2B shows the Raman spectra of the MoS2 nanosphere. As expected, the Raman spectra of SiO2 has four characteristic regions. The peaks at approximately 220, 847 and 1016 cm−1 correspond to symmetric Si-O stretching vibrations. A peak at

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687 cm−1 associates with Si-O-Si bending modes of bridging oxygens [43]. In addition, the sample also exhibits characteristic E2g and A1g Raman modes of MoS2 locating at 362 cm−1 and 403 cm−1, indicating the presence of MoS2.

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In order to further characterize the chemical composition of MoS2 nanospheres,

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the X-Ray photoelectron spectroscopy (XPS) spectra is recorded, as shown in Fig. 2C. A full survey of MoS2 nanospheres reveal the presence of four elements, which are attributed to the existence of Mo, S, Si and O elements. The element content of Mo, S, Si and O are 9.17%, 18.92%, 24.13% and 47.78%, respectively. The results of XPS indicate the atomic ratios of Mo to S and Si to O are about 1:2, approaching the theoretical stoichiometric value of SiO2 and MoS2.

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Fig. 2. XRD patterns of SiO2 and MoS2 nanospheres (A), Raman spectra (B) and XPS

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spectrum of MoS2 nanospheres.

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The morphologies of as-prepared SiO2 templates and MoS2 nanospheres were identified by SEM and TEM. The SEM images of SiO2 templates are shown in Fig.

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3A and B. It is clearly observed that the SiO2 templates possess a uniformly spherical structure. The low diameter distribution of SiO2 spheres can be well served as template to support the subsequent growth of the hierarchical MoS2 shell. From Fig. 3C, it can be observed that MoS2 spheres interlace together. The magnified SEM image of MoS2 spheres (Fig. 3D) reveals that the shell of MoS2 spheres is composed of many ultrathin MoS2 nanosheets with a few nanometers in thick, emerging a hierarchical structure. The surface of the nanospheres is highly wrinkled, which would 9

ACCEPTED MANUSCRIPT increase the contacting area between the electrolyte and electrode material. It is no doubt that this morphology endows the material a large specific surface area which accelerates the transportation of ions and electrons, leading to an enhanced

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electrochemical performance. From the comparison of TEM images of SiO2 (Fig. 3E-F) and MoS2 (Fig. 3G-H), it can be more clearly seen that the SiO2 is evenly wrapped by MoS2 nanosheets, and the diameters of these hierarchical MoS2 spheres

which are folded, curved and assembled on the

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exposed MoS2 ultrathin nanosheets

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are less 250 nm. Fig. 3H shows a magnified TEM image of MoS2. A large quantity of

surface of spheres can be observed. The hierarchical surface and sphere structure would offer a huge surface area for electrochemical reactions. From the HRTEM image in Fig. 3I, lattice fringes are found on the MoS2 nanosheets. The d-spacing of

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MoS2 belonging to the (002) lattice plane is 0.62 nm. These SEM and TEM images reveal the successful fabrication of MoS2 spheres. Brunauer-Emmett-Teller (BET) experiments of MoS2 spheres were studied by nitrogen adsorption–desorption analysis.

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Fig. 3J shows the sample can be classified as one reversible type II isotherms. The

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results show the BET specific surface area of MoS2 sphere is 101.6 m2/g and pore diameter is about 3.2 nm. The BET specific surface area of MoS2 sphere is much higher than that of MoS2 (21.1 m2/g) [44]. The high surface area and pores of MoS2 spheres can provide a large space for pseudocapacitor applications, and enhance electrochemical surface reactions. Meanwhile, it can offer a higher concentration of electrochemically active sites, larger interfacial area between the electroactive materials and the electrolyte ions, and shorter diffusion paths for fast ionic diffusion. 10

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I

d=0.62 nm 10 nm

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Fig. 3. Low and high magnification SEM images of SiO2 (A, B) and MoS2 nanospheres (C, D), low and high magnification TEM images of SiO2 (E, F) and

isotherm of MoS2 nanospheres (J).

3.2. Electrochemical performance

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MoS2 nanospheres (G, H), HRTEM image of MoS2 (I), N2 adsorption-desorption

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The electrochemical performance of MoS2 electrodes was researched by applying CV, GCD and EIS technologies. Meanwhile, the electrochemical

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measurements were achieved in 2 M KOH electrolyte solution. As shown in Fig. 4A, one pair of well-distinct and stronger redox peaks, which represents cation

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intercalation and reversible redox reactions between different valence states of Mo (+4 and +3). The data indicates pseudocapacitance induced by intercalation, and redox reactions contributed to the supercapacitor performance. From the study, the MoS2 nanosphere electrode possesses a bigger capacitance than that of pure MoS2, illustrating that once charge carriers conduct effectively and rapidly, the intimate interaction between SiO2 and MoS2 will accelerate electrochemical activity simultaneously. 12

ACCEPTED MANUSCRIPT The CV curves of as-obtained samples were recorded at different scan rate of 10-200 mV/s from -0.2 to 0.7 V in 2.0 M KOH (Fig. 4B). At low scan rates, the anodic and cathodic peaks can be obviously identified, which indicates that the

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capacitance of the MoS2 materials mainly come from faradaic redox reactions. With the increasing of sweeping rates, the CV curve area enhances with the increase of scan rate. Fortunately, when the scan rate reaches 200 mV/s, the redox peaks can still be

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discerned. This result reveals that MoS2 nanospheres have a reversible redox reaction

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together with good rate ability.

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Fig. 4. CV curves of MoS2 nanospheres and MoS2 nanosheets (A); CV curves of MoS2 nanospheres at various sweeping rates ranging from 10 to 200 mV/s (B); GCD curves

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of MoS2 nanospheres ranging from 1 to 15 A/g (C); the specific capacity at different current densities (1, 2, 3, 5, 7, 10 and 15 A/g) (D); the cyclic performance of MoS2 nanospheres at a current density of 1 A/g (E).

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The GCD is a popular way in estimating electrochemical capacitance of the materials. GCD curves of MoS2 nanospheres (Fig. 4C) are investigated at various

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current densities (1, 2, 3, 5, 7, 10 and 15 A/g). It shows that all curves exhibit good supercapacitive behaviors. Fig. 4D shows the change of the specific capacity of MoS2

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nanospheres at varying current densities. Specific capacities of MoS2 nanosphere at 1, 2, 3, 5, 7, 10, 15 A/g are 683, 575, 564, 474, 427, 373 and 310 F/g, respectively. More importantly, although at the high discharge current density of 15 A/g, the specific capacitance of MoS2 nanosphere invariably keeps up as high as 310 F/g. The capacity decreases when the current density increases, because the electrode together with the insufficient Faradaic redox reaction of the active material shows the resistance property under the higher discharge current densities. The Specific capacity of MoS2 14

ACCEPTED MANUSCRIPT nanosphere is much high than that of MoS2 nanosheets (129.2 F/g) [9]. Fig. 4E shows the variation of specific capacity at different cycles at 1 A/g. About 85.1% of specific capacity retains over 10000 cycles at 1 A/g, which is much higher than that of pure

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MoS2 nanosheets (retention of 85.1% after 500 cycles). This reveals that the MoS2 nanosphere has excellent cyclic properties as a good electrode material for supercapacitors. A comparison of the properties of MoS2 based electrode materials for

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supercapacitor is listed in Table 1 [39,41,46-48], and it shows superior performance of

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the MoS2 nanospheres. The excellent electrochemical performance of the MoS2 nanosphere is due to smart structure of the material. It effectively improves electrode/electrolyte contact area and favors ion transfer, which leads to good rate

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capability.

Table 1 Comparison of the properties of MoS2 based electrode materials for supercapacitor.

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Scan

Material

rate/current Specific capacity (F/g)

Ref.

1 A/g

210

39

MoS2@C

5 mV/s

355.6

40

MoS2-graphene

1 A/g

243

41

MoS2

1 A/g

168

45

MoS2/Polyaniline@C

1 A/g

678

46

MoS2/Mo

5 mV/s

192

47

MoS2 nanospheres

1 A/g

683

This work

density

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C/MoS2

15

ACCEPTED MANUSCRIPT Fig. 5 shows Nyquist plots of the MoS2 nanosphere electrode in 2.0 M KOH after the 1st and 1000th cycles from 0.01 to 100 000 Hz. An equivalent circuit is used to fit their Nyquist plots, which includes the internal resistance (Rs), the charge

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transfer resistance (Rct), the pseudo capacitance (CPE) and the Warburg impedance (Zw). The Rs of MoS2 composite after the 1st and 1000th cycles are 0.7 and 0.8 Ω, respectively. The Rct of MoS2 composite after the 1st and 1000th cycles are 1.3 and

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1.5 Ω, respectively. The as-prepared electrode displays little Rs and Rct due to good

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conductivity of MoS2 nanospheres. The Rs value is little difference to that after 1000 cycles, suggesting good stability of MoS2 nanosphere. The trivial enhancement of the Rct after 1000 cycles is presumably because of the active material falling off from

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current collector.

Fig. 5. Nyquist plots of the MoS2 nanosphere electrode in 2.0 M KOH after the 1st and 1000th cycles. The inset shows the electrical equivalent circuit used for fitting impedance spectra. Rs: solution resistance, Rct: charge-transfer resistance, CPE: pseudo capacitance, ZW: Warburg impedance resulting from the diffusion of ions.

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ACCEPTED MANUSCRIPT For purpose of evaluating the application of as-prepared material, an aqueous ASC was fabricated by using MoS2 together with activated carbon (AC) as positive and negative electrodes severally. Fig. 6A exhibits CV curves of AC and MoS2 electrodes

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at 50 mV/s. The MoS2 and AC work in -0.2 ~ + 0.5 V and −1 ~ 0 V potential window, respectively, therefore the operation voltage of the ASC device can reach to approximately 1.5 V. Fig. 6B shows the CV curves of the ASC at different scan rates

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ranging from 10 to 50 mV/s. Tiny change is perceived for shapes of the CV curves,

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which illustrates a fast charge-discharge performance of the ASC. The GCD curves of the ASC are measured at the current densities from 1 to 7 A/g (Fig. 6C). Based on the total mass of the positive and negative electrodes, the calculated capacitances are 65.33, 22.80, 13.60, 8.67 and 6.07 F/g at current densities of 1, 2, 3, 5 and 7 A/g,

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respectively. Ragone curve plotted in Fig. 6D shows an energy density of 20.42 Wh/kg at power density of 750.31 kW/kg. The device still retains 1.90 Wh/kg even at 5250.17 W/kg, indicating its great potential for practical energy storage applications.

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When two MoS2//AC ASC were assembled in series, it easily lighted up one green

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light-emitting diode (LED 0.5 W) after being charged for only 120 s (Fig. 6E). These attractive results indicated the practicality and efficiency of the MoS2 electrode.

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Fig. 6. (A) CV curves of AC and MoS2 electrode at 50 mV/s; (B) CV of MoS2//AC

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ASC at different scan rate; (C) GCD curves ofMoS2//AC ASC at different current densities; (D) Ragone plot of MoS2//AC ACS, (E) LED indicator lighted up by MoS2//AC ACS.

4. Conclusion Hierarchical MoS2 nanospheres are successfully synthesized by a hydrothermal route. The excellent electrochemical performance is mainly due to the synergistic 18

ACCEPTED MANUSCRIPT interaction of SiO2 and MoS2, which can be summarized as follows: (1) the morphology of the MoS2 is changed due to the confinement effect of SiO2, which possesses nanosphere structure with a larger surface area, affording ample active

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surface for charge transfer and reducing diffusion length of electrolyte during the GCD process; (2) with the sphere skeleton of SiO2 and nanosheet structure of MoS2, it is simple and easy to obtain MoS2 nanospheres that significantly promote the

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formation of interconnected and increase the special surface area, resulting in ion

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diffusion from the external electrolyte to the interior surfaces; (3) MoS2 is embedded on the surface of SiO2, which can prevent effectively collapse of active materials. Based on the unique structures and properties, the synthesized nanospheres have

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remarkable potential to be used as electrode materials in energy storage devices.

Acknowledgments

This work was supported by the National Natural Science Foundation of China

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(21475115), Program for University Innovative Research Team of Henan Henan

Provincial

Science

and

technology

innovation

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(15IRTSTHN001),

team (C20150026), Nanhu Scholars Program of XYNU, Nanhu Scholars Program for Young Scholars of XYNU, Henan Science and Technology Cooperation Project (172106000064), Natural Science Foundation of Henan Province (162300410230), Key Scientific Research Project of Henan Province (18B150024), Key Project of Xinyang College (2017zd03), Xinyang College Students' Innovative Entrepreneurial Training Program (CX20171002). 19

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[45] X. Wang, J. Ding, S. Yao, X. Wu, Q. Feng, Z. Wang, B. Geng, High

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electrode for supercapacitors, Electrochem Acta 190 (2016) 305-312.

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Research highlights

► Uniform MoS2 nanosheets assembling three-dimensional nanospheres are prepared

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by using a facile hydrothermal procedure. ► MoS2 nanospheres are evaluated for supercapacitor electrode material application. ► MoS2 nanospheres exhibit high specific capacitance and good cycling stability.

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► MoS2//AC ASC is fabricated and shows good performance.