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Layered MoS2egraphene composites forsupercapacitor applications with enhancedcapacitive performance

Ke-Jing Huang*, Lan Wang, Yu-Jie Liu, Yan-Ming Liu*, Hai-Bo Wang,Tian Gan, Ling-Ling Wang

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

a r t i c l e i n f o

Article history:

Received 17 July 2013

Received in revised form

14 August 2013

Accepted 26 August 2013

Available online 17 September 2013

Keywords:

Molybdenum disulfideegraphene

Layered nanocomposites

Electrode materials

Supercapacitor

* Corresponding authors. Tel.: þ86 376 63906E-mail addresses: kejinghuang11@163.co

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.08.1

a b s t r a c t

Layered molybdenum disulfide (MoS2)egraphene composite is synthesized by a modified

L-cysteine-assisted solution-phase method. The structural characterization of the com-

posites by energy dispersive X-ray analysis, X-ray powder diffraction, Fourier transform

infrared spectroscopy, XPS, Raman, and transmission electron microscope indicates

that layered MoS2egraphene coalescing into three-dimensional sphere-like architecture.

The electrochemical performances of the composites are evaluated by cyclic voltammo-

gram, galvanostatic chargeedischarge and electrochemical impedance spectroscopy.

Electrochemical measurements reveal that the maximum specific capacitance of the MoS2

egraphene electrodes reaches up to 243 F g�1 at a discharge current density 1 A g�1. The

energy density is 73.5 Wh kg�1 at a power density of 19.8 kW kg�1. The MoS2egraphene

composites electrode shows good long-term cyclic stability (only 7.7% decrease in specific

capacitance after 1000 cycles at a current density of 1 A g�1). The enhancement in specific

capacitance and cycling stability is believed to be due to the 3D MoS2egraphene inter-

connected conductive network which promotes not only efficient charge transport and

facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/

contraction and aggregation of electroactive materials during chargeedischarge process.

Taken together, this work indicates MoS2egraphene composites are promising electrode

material for high-performance supercapacitors.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction device, and it has many advantages such as long service life,

Recently, energy problem has become the greatest problems

and attractedworldwide attention. It has been proved to be an

important task for scientist to search new materials possess-

ing great performances in dealing with the energy conversion,

storage and usage [1,2]. Supercapacitor is a newenergy storage

11.m (K.-J. Huang), liuym951

2013, Hydrogen Energy P12

great power density, high energy density, green environ-

mental protection and has attracted enormous research in-

terest in the recent years. Compared to rechargeable batteries,

supercapacitors carry much higher specific power density

(per unit mass) and energy/power efficiency, faster charge/

discharge rate and longer lifetime even in harsh conditions.

8@sina.com (Y.-M. Liu).

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Schematic illustration of the preparation of

MoS2eGr composites.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 414028

Supplemental to batteries, supercapacitors have found broad

applications in instant switches, portable electronics, backup

power supply, regenerative braking system, motor starter, in-

dustrial power and energy management, etc. [3e5].

Carbon-based materials are commonly used as electrodes

in electrochemical double-layer capacitors (EDLC) due to their

outstanding long-term electrochemical stability as a result of

high electrical conductivity and extra ordinary chemical sta-

bility [6,7], such as activated carbon [8], carbon nanotubes

[9,10], mesoporous carbon [11], and carbide-derived carbons

[5]. Graphene (Gr) is a novel EDLC-based carbon material with

a one-atom thick layer. It has been used as supercapacitor

electrode material due to its high surface area, excellent sta-

bility, and good conductivity [12e15]. However, because of the

aggregation of hydrophobic Gr nanosheets, the surface area of

Gr is always much lower than the theoretical data, and the

pure Gr is hard to be dissolved or dispersed even after the

long-time ultrasonic sonication [16], leading to relatively low

capacitance performances. Yet, due to the discrete nature of

exfoliated Gr or graphitic nanoflakes, the Gr nanosheets

stacked loosely in porous electrodes may lead to noticeable

electrical contact resistance that may adversely decrease the

energy and power efficiencies of the supercapacitors [17].

Therefore, the surface modification of Gr to improve its

dispersion and processibility is an important technique to

utilize its unique nanostructures and good functionality for

high-performance capacitors.

Layer-structured transition-metal dichalcogenides, such

as tungsten sulfide (WS2) and molybdenum disulfide (MoS2),

have attracted lots of interest because of their unique chem-

ical and physical properties. As one of the representative

members, MoS2 has received much attention in many fields

including electrochemical devices, hydrogen storage, catal-

ysis, capacitors, solid lubricant, and intercalation host

[18e20]. It has an analogous structure to graphite, which are

composed of three atom layers: a Mo layer sandwiched be-

tween two S layers, and the triple layers are stacked and held

together by weak van der Waals interactions [21]. Recent

capacitor research has also focused onMoS2 [22]mainly due to

its higher intrinsic fast ionic conductivity [23] (than oxides)

and higher theoretical capacity (than graphite) [24]. For

example, Soon and Lohz [25] reported that theMoS2 used as an

electrode material for capacitor due to its sheet-like

morphology, which provides large surface area for double-

layer charge storage. The results showed that the super-

capacitor performance ofMoS2 is comparable to that of carbon

nanotube array electrodes. In addition to double-layer

capacitance, diffusion of the ions into the MoS2 at slow scan

rates gives rise to faradaic capacitance, which plays an

important role in enhancing charge storage capabilities.

However, the electronic conductivity of MoS2 is still lower

compared to graphite/Gr, and the specific capacitance of MoS2is still very limited in alone for energy storage applications.

The combination of Gr and MoS2 may overcome these de-

ficiencies. Most recently, we have reported a MoS2eGr

nanocomposite-based electrochemical sensor and it showed

excellent electrochemical performance [26].

In this work, for the first time, we report three-dimensional

(3D) sphere-like architecture MoS2eGr composites which ob-

tained from the overlapping or coalescing of layered MoS2eGr

used as a novel electrode material for supercapacitor. The

MoS2eGr nanocomposites possessed a pronounced enhance-

ment of the electrochemical properties, realizing high energy

density characteristics for electrochemical supercapacitor

applications. The MoS2eGr nanocomposites based electrodes

were found to deliver a capacitance of 243 F g�1 at a discharge

current density of 1 A g�1 with excellent long-term cycling

stability over 1000 cycles in 1 mol L�1 Na2SO4.

2. Experimental

2.1. Synthesis of MoS2

The MoS2 nanocomposites were synthesized according to the

reference [22]. In short: 2.2 g Na2MoO4$2H2O and 2.0 g

H2NCSNH2 were added in 70 mLwater with violent stirring for

about 10 min. After adjusting the pH value to less than 1 with

12 M HCl, the mixture was transferred into a 100 mL Teflon-

lined stainless steel autoclave and heated at 200 �C for 24 h.

After cooling naturally, the black MoS2 composites were

collected by filtration, washed with distilled water and abso-

lute ethanol for several times, and then dried in vacuum at

60 �C for 24 h.

2.2. Synthesis of MoS2egraphene composites

Graphene oxide was prepared by the modified Hummers

method [26]. MoS2eGr composites were prepared by a modi-

fied L-cysteine-assisted solution-phase method [26,27]. 0.1 g

graphene oxide and 50 mL water were transferred into a

200 mL breaker. Then, 0.5 g Na2MoO4$2H2O was added and

stirred for 30 min. Subsequently, the pH value of the mixture

was adjusted to 6.5 with 0.1 M NaOH. The mixture and 1.0 g of

L-cysteine were added in 100 mL water and then transferred

into a 100 mL Teflon-lined stainless steel autoclave, and

heated at 180 �C for 36 h. After cooling to room temperature

naturally, the MoS2eGr black precipitates were collected by

centrifugation, washed with water and ethanol, and dried in a

vacuum oven at 80 �C for 24 h. The procedure for MoS2eGr

composites preparation is illustrated in Fig. 1.

2.3. Characterization

X-ray powder diffraction (XRD) pattern was operated on a

Japan RigakuD/Maxr-A X-ray diffractometer equipped with

graphite monochromatized high-intensity Cu Ka radiation

(l ¼ 1.54178 �A). Fourier transform infrared spectroscopy

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 4 14029

(FT-IR) was measured on a Bruker-Tensor 27 IR spectropho-

tometer. The morphologies of the nanocomposite were

recorded on a JEM 2100 transmission electron microscope

(TEM) and a Hitachi S-4800 scanning electron microscope

(SEM). The N2 adsorptionedesorption isotherms of the sam-

ples were measured at 77 K using NOVA 2000 (Quantachrome,

USA) in order to determine the specific surface areas. The

specific surface area was calculated from the Bru-

nauereEmmetteTeller (BET) plot of the nitrogen adsorption

isotherm.

2.4. Preparation of electrodes and electrochemicalmeasurement

The fabrication of working electrodes was carried out as fol-

lows: Briefly, the as-prepared MoS2eGr, carbon black and

poly(tetrafluoroethylene) were mixed in a mass ratio of

80:15:5. Then the resulting slurry was coated onto the stain-

less steel substrate (1 cm � 1 cm), which was followed by

drying at 60 �C for 6 h in a vacuum oven.

All electrochemical measurements were done in a three

electrode setup: the stainless steel substrate coated with

MoS2eGr as the working electrode, platinum foil and Hg/HgO

electrode as the counter and reference electrodes. The mea-

surements were carried out in 1 M Na2SO4 aqueous electro-

lyte. Cyclic voltammograms (CV), galvanostatic charge/

discharge and electrochemical impedance spectroscopy (EIS)

were measured by a CHI 660D electrochemical workstation.

CV tests were done between �1.0 and 0 V at different scan

rates. Galvanostatic charge/discharge curves were measured

in the potential range of �1.0 to 0 V at different current den-

sities of 1, 2.5, 5 and 10 A g�1. EIS measurements were also

carried out in the frequency range from 0.1 Hz to 100 kHz at

open circuit potential with an ac perturbation of 5 mV.

3. Results and discussion

3.1. Material characterization

The morphology of Gr, MoS2, and MoS2eGr was investigated

using SEM and TEM. Fig. 2A shows the SEM image of Gr sheets,

illustrating the flake-like shapes of Gr. Fig. 2B shows the SEM

image of the as-prepared MoS2eGr composites, illustrating a

3D sphere-like architecture. The 3D architecture is helpful to

increase the specific area of the nanocomposite. In the

Fig. 2 e SEM images of Gr (A) and MoS2eGr (B); the photograph

two days (C).

MoS2eGr composite, the overlapping or coalescing of the Gr

will form an interconnected conducting network, and facili-

tate rapid electronic transport in electrode reactions.

Furthermore, this 3D sphere-like structure also enhances the

stability of the MoS2eGr composites due to superstrength of

Gr. Gr and MoS2eGr dispersed in water by vigorous shaking,

producing the homogenous black solution. However, Gr sub-

sides to the bottom after left to stand for two days and

MoS2eGr still keep well disperse in the water (Fig. 2C), indi-

cating good dispersibility of MoS2eGr composite in water.

Fig. 3A shows the typical TEM image of the synthesized Gr

composite, showing the layered platelets. Fig. 3B shows the

TEM image of MoS2eGr composites. The image reveals a

general trend with the sheets of MoS2 homogeneously

embedded in Gr. TheMoS2 (black stripes) layers are visible and

appear to be in intimate contact with the Gr layers. The high-

resolution TEM (HRTEM) image of MoS2eGr in Fig. 3C shows

that the interlayer spacing between the MoS2 sheets in the

composites is estimated to be 0.62 nm.

Fig. 4A shows the XRD patterns of Gr and MoS2eGr com-

posites. For Gr, the appearance of the (002) diffraction line in

the XRD pattern gives evidence that the graphite oxide is

reduced to Gr. For MoS2eGr composites, the presence of (002),

(100) and (110) reflections suggests a few-layered structure for

MoS2. The diffraction peaks display very weak, indicating the

poor crystallinity of MoS2. This is attributed to the incorpo-

ration of the Gr inhibiting the growth of the layered MoS2crystal during the hydrothermal process.

To obtain further information on the structure and topol-

ogy of as-prepared nanocomposite, Raman spectroscopy was

carried out. In Fig. 4B, the Raman spectrum of Gr exhibits the

presence of D and G bands. The D band at 1342.2 cm�1 arises

from sp3-hybridized carbon, and the peak at 1593.1 cm�1

represents theE2g zone centermodeof the crystalline graphite.

The characteristic bands of MoS2 observed at 375.4 cm�1 and

408.6 cm�1 correspond to the E2g and A1g modes, respectively.

The MoS2eGr composites show the peaks at 1342.2 cm�1 and

1593.1 cm�1, which are assigned to the D and G peaks of the Gr

and 375.4 cm�1 and 408.6 cm�1 corresponds to the MoS2,

respectively, thus confirming the presence of the Gr and MoS2in the composites and complete correspondence with the

findings from the XRD diffraction studies.

FT-IR spectra of Gr and MoS2eGr composites were

compared between 4000 and 400 cm�1. As shown in Fig. 4C, in

the FT-IR spectra of Gr, the bands around 3435, 1410, and

1100 cm�1 are attributed to the oxygen containing functional

of Gr and MoS2eGr dispersed solution after left to stand for

Fig. 3 e TEM images of Gr (A), and MoS2eGr composites (B); HRTEM image of MoS2eGr composites.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 414030

groups on Gr, which are the residual oxygen functionalities

after the reduction of graphene oxide. The bond at 3435 cm�1

appears at both Gr and MoS2eGr FT-IR spectra, which is

mainly assigned to stretching vibrations of the OeH bonds.

The difference on the intensity of OH vibration indicated that

the free hydroxy groups increase after MoS2 loading at the Gr.

This is helpful to increase the dispersibility of MoS2eGr

composites in water. The weak peaks at about 500 cm�1 are

assigned to MoeS vibration [28].

Furthermore, the specific surface areas calculated by the

BET method of Gr and MoS2eGr composites are 48.1 m2 g�1

and 102.8 m2 g�1, respectively. Obviously, the BET specific

surface area of MoS2eGr composites is much higher than that

of pure Gr, which means that MoS2 can effectively decrease

the stacking of Gr, resulting in the high electrochemical uti-

lization of Gr layers.

Energy dispersive X-ray analysis (EDAX)was also applied to

determine the composition of the as-prepared layered

MoS2eGr composites. The results indicated the samples

contain C (5.82), Mo (62.1), S (29.35), and a small quantity of O

(2.73). The calculated atomic ratio of S toMo elementwas 2.12,

approaching the theoretical value of MoS2. These values sug-

gested the products were stoichiometric MoS2. The C was

provided by Gr, while a small quantity of O came from a few

parts of the Gr that were not completely reduced during the

hydrothermal process.

3.2. Electrochemical properties

In order to evaluate the electrochemical properties of the

MoS2, Gr and MoS2eGr composites, cyclic voltammetry (CV)

and galvanostatic charge/discharge tests were performed. The

CV curves for the MoS2, Gr and MoS2eGr composites elec-

trodes at a scanning rate of 20 mV s�1 in the potential window

Fig. 4 e XRD patterns of Gr and MoS2eGr composites (A); Rama

spectra of Gr and MoS2eGr composites (C).

of �0.1 to 0 V (vs. SCE) in 1 M Na2SO4 electrolyte are presented

in Fig. 5. By comparison, the area of the CV curve for the

MoS2eGr electrode is larger than those of the MoS2 and Gr

electrodes, indicating higher specific capacitance and the

synergistic effect of MoS2 and Gr.

For further research the properties of the MoS2eGr com-

posites electrode, Fig. 6 shows the CV curves of the composites

electrode at various scan rates. The CV curves of the MoS2eGr

electrode at different scan rates from 2 to 100 mV s�1 exhibit

quasi-rectangular shapes, indicating all samples have a good

capacitive behavior. No peak is observed at different scan

rates in the CV curves, suggesting that the electrode is charged

and discharged at a pseudo-constant rate over the complete

voltammetric cycle. The CV curve at the faster scan rate has a

larger area than the lower scan rate one, but does not indicate

a greater charge capacitance at the higher scan rate. Mean-

while, with the scan rate increasing, the effective interaction

between the ions and the electrode is greatly reduced because

of the resistance of metal oxide and the deviation from rect-

angularity of the CV becomes obviously.

Fig. 7 shows the constant current charge/discharge curves

of MoS2eGr composites at different current densities (1, 2.5, 5

and 10 A g�1). During the charging and discharging steps, the

charge curve of binary composites is almost symmetric to its

corresponding discharge counterpart with a small internal

resistance (IR) drop, indicating the pseudocapacitive contri-

bution along with the double layer contribution.

The specific capacitance (Csp) of the electrode is obtained

from the following equation:

Csp ¼ It=Dvm (1)

where I, t,Dn andm are the constant current (A), discharge time

(s), the total potential difference (V) and the weight of active

materials (g), respectively. Fig. 8A shows rate performance of

n spectra of MoS2, Gr and MoS2eGr composites (B); FT-IR

Fig. 7 e Galvanostatic charge/discharge curves of MoS2eGr

composites at different current densities (1, 2.5, 5 and

10 A gL1).

Fig. 5 e CV curves of MoS2, Gr and MoS2eGr composites at

20 mV sL1 in 1.0 M Na2SO4.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 4 14031

the sample electrodes at various current densities. The

capacitance decreases with the increasing current density.

The variation of the specific capacitance (farad per unit mass)

with the current density is shown in Fig. 8B. The specific ca-

pacitances of theMoS2eGr electrode at 1, 2.5, 5 and 10A g�1 are

243, 158, 130, and 100 F g�1, respectively. Significantly, the

specific capacitance of MoS2eGr composites still remained as

high as 100 F g�1 even at a high discharge current density of

10 A g�1. The results indicate that the MoS2eGr composites

have a high rate of capacitance, which is recognized as one of

the most important electrochemical properties in the appli-

cation of electrodes and batteries. The Csp value of the

MoS2eGr composites electrode from Fig. 9 is about 243 F g�1 at

a current density of 1 A g�1, corresponding to a specific

capacitance of 120, 35 F g�1 for MoS2 and graphene alone.

These values aremainly consistentwith the order indicated by

the CVs. The advantages of MoS2eGr composites electrode

over the MoS2 and Gr electrodes are salient and the excellent

electrochemical performances of MoS2eGr composites are

attributed to their unique microstructure: (1) Layered MoS2coating the surfaces of Gr nanosheets accumulate to form

pores for ion-buffering reservoirs to improve the diffusion rate

of ions within the bulk of the preparedmaterials; (2) The large

Fig. 6 e CV curves of MoS2eGr composites at different scan

rates (2 mV sL1, 5 mV sL1, 10 mV sL1, 20 mV sL1,

50 mV sL1, 100 mV sL1) in 1.0 M Na2SO4.

specific surface area and the nanoscale size of MoS2 phase of

the MoS2eGr composites greatly reduce the diffusion length

over which both ions and electrons must transfer during the

charge/discharge process. This ensures a high utilization of

the electrode nanoscale size of MoS2 sheets; (3) Gr in the

composites not only acts as supports for the loading of MoS2sheets, but also constructs a 3-D highly conductive current

collector. The excellent interfacial contact between MoS2 and

Gr facilitates fast transportation of electrons throughout the

whole electrode matrix. This unique architecture enables the

MoS2eGr composites electrode to have a large specific surface

and fast electron and ion transport simultaneously, thus pre-

senting the best electrochemical capacitive performance.

The EIS analysis has been recognized as one of the principal

methods examining the fundamental behavior of electrode

materials for supercapacitors. For further understanding,

impedance of all products is measured in the frequency range

of 100 kHze0.1 Hz at open circuit potential with an ac pertur-

bation of 5 mV (Fig. 9). Each impedance spectrum has a semi-

circular arc and a straight line. The high-frequency arc

corresponds to the charge transfer resistance (Rct) caused by

the Faradaic reactions and thedouble-layer capacitance (Cdl) at

the contact interface between electrode and electrolyte solu-

tion. Rct can be directly measured as the semicircular arc

diameter. The 45� slope portion of the curve is the Wurburg

resistance (Zw), which is a result of the frequency dependence

of ionicdiffusion/transport in the electrolyte and to the surface

of the electrode. The values of Rct for MoS2, Gr, and MoS2eGr

composites electrodes are 0.65, 1.61, and 0.61 U, respectively.

The equivalent series resistance (Re) can be obtained from the

X-intercept of the Nyquist plots, which is a combined resis-

tance comprising ionic resistance of electrolyte, intrinsic

resistance of substrate, and contact resistance at the active

material/current collector interface. Re, as shown in Fig. 9, is

1.61 U for MoS2, 2.75 U for Gr, and 1.46 U for MoS2eGr com-

posites, respectively. Clearly, the Rct and Re of MoS2eGr com-

posites electrode are smaller than those of MoS2 and Gr

electrode, which demonstrates that the addition of graphene

enhances conductivity and improves charge transfer perfor-

mance of MoS2eGr composites electrode. At the same time,

anchored MoS2 nanowafers prohibit aggregation of graphene

Fig. 8 e (A) Specific capacitance of the MoS2eGr composites at different current densities of 1, 2.5, 5 and 10 A gL1 in 1 M

Na2SO4. (B) Galvanostatic charge/discharge curves of MoS2, Gr and MoS2eGr composites at 1 A gL1.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 414032

sheets and the loose-formed open structure facilitates fast

electron transfer between the active materials and the charge

collector.

The Ragone plots for theMoS2, Gr andMoS2eGr composites

are shown in Fig. 10. The energy and power densities were

derived from galvanostatic charge/discharge at various cur-

rent densities. The specific energy density (E) and power

density (P) are evaluated according to equations:

E ¼ ð1=2ÞCV2 (2)

P ¼ E=Dt (3)

where C is the capacitance of the two-electrode capacitor, V is

the voltage decrease in discharge, E is the energy and Dt is the

time spent in discharge. As seen from the Ragone plots, as the

power density increases from 7600 W kg�1 to 19,710 W kg�1,

the energy density of Gr decreases from 24.3 Wh kg�1 to

3.2 Wh kg�1, and the energy density of MoS2 decreases from

34.2Wh kg�1 to 11.5Wh kg�1, respectively. Comparatively, the

energy density of the MoS2eGr composites can reach

85.7 Wh kg�1 at a power density of 7600 W kg�1, and still re-

mains 73.5Wh kg�1 at a power density of 19,710Wkg�1, which

Fig. 9 e Nyquist plots of the MoS2, Gr and MoS2eGr

composites electrode in 1.0 M Na2SO4 in the frequency

range from 0.1 to 100,000 Hz at open circuit potential with

an ac perturbation of 5 mV. Inset: magnified high-

frequency regions.

exhibited a large power range that can be obtained while

maintaining a relatively high energy density. The results

illustrate that the MoS2eGr composites materials have excel-

lent electrochemical properties of high energy density and

power output.

The cycle stability of MoS2eGr composites was evaluated

by repeating the constant current charge/discharge test be-

tween�1.0 and 0 V (vs. SCE) at a current density of 1.0 A g�1 for

1000 cycles. From Fig. 11, a small increase of capacitance is

observed during the first 100 cycles, and the capacitance only

decreases by about 7.7% of the initial capacitance after 1000

cycles, indicating a good cycling life of the composites mate-

rials. The initial increase of capacitance can be explained as

follows: at the initial stage, active materials have not been

fully used. After repetitive charge/discharge cycling, the

electrochemical active Mo sites inside the stainless steel

substrate electrode will be fully exposed to the electrolyte.

Therefore, an increasing capacitance was displayed in the

cyclic tests. The capacitance retention upon electrochemical

cycling and higher specific capacitance of the composites are

attributed to the flexibility of Gr in the composites. The Gr can

not only form an open structure to improve the connection

between active material and electrolyte and make full use of

electrochemical active MoS2 during the charge and discharge

processes, but also improve the electrical conductivity of the

overall electrode due to the high conductivity of Gr.

Fig. 10 e Ragone plots (power density vs. energy density) of

MoS2, Gr and MoS2eGr composites.

Fig. 11 e Cyclic performance of MoS2eGr composites

electrodes at 1 A gL1 in 1.0 M NaSO4 electrolyte; the inset

shows charge/discharge curves of the MoS2eGr electrodes

in potential range from L1.0 to 0 V at 1 A gL1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 4 0 2 7e1 4 0 3 4 14033

4. Conclusions

We synthesized layered MoS2eGr composites with three-

dimensional sphere-like architecture by a versatile and facile

L-cysteine-assisted solution-phase method. The maximum

specific capacitance is 243 F g�1 at a discharge current density

1 A g�1 compared to 120 F g�1 for MoS2 and 35 F g�1 for Gr. The

integration of Gr into the composites provides relatively large

areas to loading MoS2 sheets, leading to three-dimensional

nanostructures. Thus MoS2eGr composites enable an easy ac-

cess for both charge-transfer and ion transport throughout the

electrode. Furthermore, the capacitance retention is still over

92.3% of initial capacitance after 1000 cycles. These results

suggest that the MoS2eGr composites are quite a suitable

and promising electrode material for high-performance

supercapacitors.

Acknowledgments

This work was supported by the National Natural Science

Foundation of China (U1304214), Program for University Inno-

vative Research Team of Henan (2012IRTSHN017), Foundation

of Henan Educational Committee (No. 13A150768) and Natural

ScienceFoundationofHe’nanProvinceofChina (132300410060).

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