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Electrochimica Acta 109 (2013) 587– 594

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

ynthesis of polyaniline/2-dimensional graphene analog MoS2

omposites for high-performance supercapacitor

e-Jing Huang ∗, Lan Wang, Yu-Jie Liu, Hai-Bo Wang, Yan-Ming Liu ∗, Ling-Ling Wangollege of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China

r t i c l e i n f o

rticle history:eceived 10 June 2013eceived in revised form 13 July 2013ccepted 21 July 2013vailable online 6 August 2013

eywords:olyanilineolybdenum disulfide

lectrode materials

a b s t r a c t

We report a facile strategy to synthesize polyaniline/molybdenum disulfide (PANI/MoS2) nanocompositeby in situ polymerization to achieve excellent electrochemical properties for application as supercapaci-tor electrodes. MoS2 nanocomposite with graphene-like subunits structure is prepared by a hydrothermalmethod and serves as an excellent 2D conductive skeleton that supports a highly electrolytic accessiblesurface area of redox-active PANI and provides a direct path for electrons. The layered nanostructure ofPANI/MoS2 composites provides a larger contact surface area for the intercalation/deintercalation of pro-tons into/out of active materials and shortens the path length for electrolyte ion transport. The structureof the composite is characterized by scanning electron microscope, transmission electron microscope, X-ray powder diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis, and the

upercapacitor electrochemical performances of the composites are evaluated by cyclic voltammogram and galvanostaticcharge–discharge. The maximum specific capacitance of 575 F g−1 at 1 A g−1 is observed at the PANI/MoS2

electrodes. The energy density of 265 W h kg−1 is obtained at a power density of 18.0 kW kg−1. In addition,the PANI/MoS2 composite electrode shows excellent long-term cyclic stability (less than 2% decrease inspecific capacitance after 500 cycles at a current density of 1 A g−1), indicating a positive synergistic effectof MoS and PANI for the improvement of electrochemical performance.

2

. Introduction

Supercapacitor is a new energy storage device, and it hasany advantages such as long service life, great power den-

ity, high energy density, green environmental protection and hasrawn considerable attention in the recent years [1]. Compared toechargeable batteries, supercapacitors carry much higher specificower density (per unit mass) and energy/power efficiency, fasterharge/discharge rate and longer lifetime even in harsh condi-ions, and it has been found broad applications in instant switches,ortable electronics, backup power supply, regenerative brakingystem, motor starter, industrial power and energy management,tc. [2,3].

Supercapacitors can be divided into electrical double-layerapacitor (EDLC) and pseudo-capacitor according to the chargetorage mechanism [4]. The energy storage of EDLCs is the accu-ulation of ionic charges which occur at the interface between

he electrode and electrolyte. For the pseudo-capacitor, it is pro-uced by the fast reversible faradic transitions of active materials,.g., transition metal oxides, conducting electric polymers. Among

∗ Corresponding authors. Tel.: +86 376 6390611.E-mail addresses: kejinghuang@163.com (K.-J. Huang), liuym9518@sina.com

Y.-M. Liu).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.168

© 2013 Elsevier Ltd. All rights reserved.

these materials, polyaniline (PANI) is a potential electrode mate-rial in redox supercapacitors due to its unique properties suchas controllable conductivity, high electrochemical activity, goodbiocompatibility, and low cost combining with the easiness ofpreparation [5,6]. However, a pure electrode of PANI has drawbacksof poor cycling stability and temperature dependence, because theredox sites in the polymer backbone are not sufficiently stable andthe backbone of polymer can be destroyed within a limited numberof charge/discharge cycles. Therefore, it is often hybridized withinorganic materials (carbon materials, metal oxides) or organicmaterials to prepare a composite and used in supercapacitors withbetter cycleability, specific capacitance and mechanical stability[7–9].

Layered transition-metal dichalcogenides (TMDs), such as WS2,MoS2, and VS2 etc., have been successfully established as a newparadigm in the chemistry of nanomaterials especially for nano-tubes and fullerene-like nanostructures as well as the grapheneanalogs during the past decades [10–12]. From the structural pointof view, MoS2 is a typical family member of TMDs, in that MoS2crystals are composed of the metal Mo layers sandwiched betweentwo sulfur layers and stacked together by weak van der Waals inter-

actions [13]. The unique and complicated electronic structure ofMoS2 inspired the scientist that this layered compound deservesspecific attention as a promising functional material, since the 2Delectron electron correlations among Mo atoms would preferably

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nduce more complicated planar electric transportation properties.ctually, capacitor research has also focused on MoS2 mainly due

o its higher intrinsic fast ionic conductivity [14] (than oxides) andigher theoretical capacity (than graphite) [15]. For example, Soonnd Loh [16] reported that the MoS2 used as an electrode materialor capacitor due to its sheet-like morphology, which provides largeurface area for double-layer charge storage. The results showedhat the supercapacitor performance of MoS2 is comparable to thatf carbon nanotube array electrodes. However, the electronic con-uctivity of MoS2 is still lower compared to graphite/graphene, andhe specific capacitance of MoS2 is still very limited in alone fornergy storage applications. The combination of MoS2 and otheronducting materials may overcome these deficiencies, such asonducting polymers. Rao et al. has reported the preparation ofhe polyaniline-MoS2 composite by in situ polymerization of ani-ine in the presence of few-layer MoS2 and the composite showedvidence of polaronic character [17].

In this paper, we report a facile strategy to synthesizeolyaniline/2-dimensional graphene analog MoS2 (PANI/MoS2)omposites via in situ polymerization of aniline monomer in theresence of 2D MoS2 suspension. MoS2 acts as a polymeriza-ion substrate in the polymerization system and provides a pathor the insertion and extraction of ions within the PANI, andnsures a high reaction rate. As is expected, this highly conduc-ive PANI/MoS2 nanocomposite based electrodes demonstrated aigh specific capacitance and excellent long-term cycling stabil-

ty, showing promising signs for this new material to be utilized innergy storage devices.

. Experimental

.1. Synthesis of MoS2

The MoS2 nanocomposite was synthesized as follows: 0.30 ga2MoO4·2H2O were dissolved in 40 mL deionized water. Afterdjusting the pH value to 6.5 with 12 M HCl, 0.80 g l-cysteine wasdded and the mixture was diluted with water to 80 mL, and thenhe solution was violently stirred for about 1 h. Subsequently, the

ixture was transferred into a 100 mL Teflon-lined stainless steelutoclave and heated at 180 ◦C for 48 h. After cooling naturally, thelack MoS2 composites were collected by filtration, washed withistilled water and absolute ethanol for several times, and thenried in vacuum at 60 ◦C for 24 h.

.2. Synthesis of PANI

PANI was synthesized according to the following steps: 0.92 mLf aniline, 20 mL of 1 M HCl and 10 mL of ethanol were dispersedn 50 mL of deionized water with magnetic stirring in an ice-bath.fter stirring for 10 min, 20 mL of 1 M (NH4)2S2O8 was added into

he above mixture within 1 h drop by drop. The polymerization wasarried out for 12 h in an ice-bath with the maintained magnetictirring. Then, the suspension of PANI was filtered and rinsed sev-ral times with deionized water and ethanol to remove retainedniline monomer and oxidant until the filtrate became colorless.he precipitate was dried under vacuum at 60 ◦C for 24 h to obtainhe PANI.

.3. Synthesis of PANI/MoS2 composites

PANI/MoS2 composites were synthesized by in situ chemical

xidative polymerization directed by molybdenum disulfide. In aypical process, 0.02 g MoS2 was ultrasonic dispersed in 30 ml 1 MCl at ambient temperature. Then, the solution was transferred

nto ice bath, cooled to below 5 ◦C. Then, 0.92 mL of aniline, 20 mL

Acta 109 (2013) 587– 594

of 1 M HCl and 10 mL of ethanol were dispersed in 50 mL of deion-ized water with magnetic stirring in an ice-bath. After stirring for10 min, 20 mL of 1 M (NH4)2S2O8 was added into the above mix-ture within 1 h drop by drop. The polymerization was carried outfor 12 h in an ice-bath with the maintained magnetic stirring. Then,the suspension was filtered and rinsed several times with deion-ized water and ethanol to remove retained aniline monomer andoxidant. The precipitate was dried under vacuum at 60 ◦C for 24 hto obtain the PANI/MoS2 composites.

2.4. Characterization

X-ray powder diffraction (XRD) pattern was operated on a JapanRigakuD/Maxr-A X-ray diffractometer equipped with graphitemonochromatized high-intensity Cu K� radiation (� = 1.54178 A).Fourier transform infrared spectroscopy (FT-IR) was measured on aBruker-Tensor 27 IR spectrophotometer. The morphologies of thenanocomposite were recorded on a JEM 2100 transmission elec-tron microscope (TEM) and a Hitachi S-4800 scanning electronmicroscope (SEM). Thermogravimetric analysis (TGA, SDTQ600)was performed under a nitrogen atmosphere at a heating rate of10 ◦C min−1.

2.5. Preparation of electrodes and electrochemical measurement

The fabrication of working electrodes was carried out as follows:briefly, the as-prepared materials PANI/MoS2, carbon black andpoly(tetrafluoroethylene) were mixed in a mass ratio of 80:15:5.Then the resulting slurry was coated onto the stainless steel sub-strate (1 cm × 1 cm), which was followed by drying at 60 ◦C for 24 hin a vacuum oven.

All electrochemical measurements were done in a three elec-trode setup: the stainless steel substrate coated with PANI/MoS2 asthe working electrode, platinum foil and Hg/HgO electrode as thecounter and reference electrodes. The measurements were carriedout in a 1 M H2SO4 aqueous electrolyte at room temperature. Cyclicvoltammograms (CV), galvanostatic charge/discharge and electro-chemical impedance spectroscopy (EIS) were measured by a CHI660D electrochemical workstation. CV tests were done between−0.4 and 0.6 V (vs. Hg/HgO) at different scan rates. Galvanostaticcharge/discharge curves were measured in the potential range of−0.4 and 0.6 V (vs. Hg/HgO) at different current densities of 1, 1.5,3, 5 and 10 A g−1. EIS measurements were also carried out in thefrequency range from 100 kHz to 0.1 Hz at open circuit potentialwith an ac perturbation of 5 mV. The specific capacitance of elec-trode material was calculated according to the following equation[7]:

Cs = It

�Vm(1)

where I, t, �V and m are the constant current (A), discharge time (s),the total potential difference (V) and the weight of active materials(g), respectively.

3. Results and discussion

3.1. Material characterization

The morphology of MoS2, PANI and PANI/MoS2 nanocompos-ites was investigated using SEM and TEM. Fig. 1A shows the SEMimage of MoS2, indicating the layered MoS2 is flowerlike with the

overlapped or coalesced sheet-like subunits structure. As shown inFig. 1B, the pure PANI is random short-rod agglomerate which isstacked by spherical particles. It can be seen that, as compared tothe PANI (Fig. 1C), the morphology of the PANI/MoS2 composite is

K.-J. Huang et al. / Electrochimica Acta 109 (2013) 587– 594 589

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Fig. 1. SEM images of MoS2 (A), PANI (B), and PANI/MoS2 composites.

imilar but coating offwhite fluff, which is helpful to increase thepecific area of the nanocomposite.

The morphology and structure of the as-synthesized MoS2,ANI and PANI/MoS2 composites were further characterized usingEM. Fig. 2A shows a typical TEM image of MoS2 nanosheets, whichs thin layers folded and tangled together morphology. From TEMFig. 2B), irregular pure PANI stacked to form layer agglomerates.

s shown in Fig. 2C, the MoS2 nanosheets are embedded in PANI,ith some possessing folded edges corresponding to the different

ayers of MoS2 sheets. The layer agglomerates of PANIs anchorednto MoS2 sheets to form a loose structure, which is desirable for

Fig. 2. TEM images of MoS2 (A), PANI (B), and PANI/MoS2 composites.

supercapacitor application because both surfaces containing thinPANI wafers are effective in contributing pseudocapacitance to thetotal energy storage.

Fig. 3A displays X-ray powder diffraction (XRD) patterns of MoS2and PANI/MoS2 nanocomposites. It can be seen that diffractionpeaks of the pure MoS2 show at 2� = 14◦, 33◦, 40◦ and 59◦, whichcan be assign to the (0 0 2), (1 0 0), (1 0 3), and (1 1 0) planes of MoS2

(JCPDS No. 37-1492), respectively [18]. For the PANI/MoS2 compos-ite, it has a similar XRD patterns to MoS2. However, the peak ofPANI/MoS2 composite shows another two broad peaks at 2� = 20◦

and 25◦, which is different from the peak of MoS2. This is attributed

590 K.-J. Huang et al. / Electrochimica

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ig. 3. XRD patterns of MoS2, PANI, and PANI/MoS2 composites (A); FT-IR spec-ra of MoS2, PANI, and PANI/MoS2 composites (B); TGA plots of MoS2, PANI, andANI/MoS2 composites at a heating rate of 10 ◦C min−1 under N2 atmosphere (C).

o the diffraction patterns of PANI, indicating the periodicity par-llel and perpendicular characteristics of the polymer chain [19].he results confirm that the PANI was successfully attached intohe MoS2 nanosheets layer.

The FT-IR measurement was carried out in order to obtain

he bending and stretching vibrations of functional group presentn the samples. As shown in Fig. 3b, the peak at 1580 cm−1

nd 1550 cm−1 were consistent with C C stretching deforma-ion of quinoid and benzene rings, respectively [20]. The bands at

Acta 109 (2013) 587– 594

1310 cm−1, 1250 cm−1 and 1120 cm−1 can be attributed to C Nstretching of secondary aromatic amine and the aromatic C Hin-plane bending. The peaks 790 cm−1 and 500 cm−1 in PANI andMoS2-PANI spectra can be assigned to the out-of-plane deforma-tion of C H in the 1,4-disubstituted benzene ring [21]. The weakpeaks at about 590 cm−1 at both MoS2 and PANI/MoS2 FT-IR spec-tra are assigned to Mo S vibration [22]. The bond at 3420 cm−1

appears at all MoS2, PANI and PANI/MoS2 FT-IR spectra, which ismainly assigned to stretching vibrations of the O H bonds. The dif-ference on the intensity of the OH vibration indicated that the freehydroxy groups decrease after PANI coating at MoS2. Therefore, theresults from FTIR spectra indicated that the polyaniline has beentightly incorporated in MoS2 backbone.

The thermal stability of MoS2, PANI, and PANI/MoS2 compos-ites was studied by thermogravimetric analysis (TGA) (Fig. 3C).All the materials have an initial mass loss around 100 ◦C, which isattributed to the evaporation of surface absorbed water molecules.For the MoS2, the weight lose of 27% from 100 ◦C to 395 ◦C associ-ated with the dopant HCl and water in the MoS2 nanosheets. For thePANI, the gradual weight loss between 100 ◦C and 350 ◦C is ascribedto the deprotonation of the PANI through the loss of the dopantHCl. The major weight loss before 639 ◦C is related to the degra-dation and decomposition of PANI with different polymerizationdegrees. For the PANI/MoS2 composite, the initial decompositiontemperature is about 295 ◦C, which is attributed to the loss of co-intercalated water molecules and the liberation of co-intercalatedHCl. The obviously weight loss of 68% in the temperature rangeof 295–457 ◦C is related to the degradation and decomposition ofPANI. It is obvious the thermal stability of the PANI/MoS2 compos-ites is better than PANI.

3.2. Electrochemical properties

In order to evaluate the electrochemical properties of the MoS2,PANI and PANI/MoS2 composite, cyclic voltammetry (CV) testswere performed. The CV curves for the MoS2, PANI and PANI/MoS2composite electrodes are presented in Fig. 4A at a scanning rateof 20 mV s−1 in the potential window of −0.4–0.6 V (vs. SCE) in1 M H2SO4 electrolyte. From the CV curves, it can be observed thatthere are distinct redox peaks at PANI and PANI/MoS2 compositeelectrodes. The CV curve of the MoS2 electrode shows a rectangu-lar shape without obvious redox peaks, indicating MoS2 possessof a typical electrical double-layer capacitance. For the PANI andPANI/MoS2 composites electrodes, the capacitance characteristicis distinct from that of the electric double-layer capacitance. Boththe CV curves are close to the ideal rectangular shape with pseudo-capacitance characteristics, and the shape does not have obviouschange except for the output current of PANI/MoS2 nanocompos-ite is higher than that of pure PANI. It was obvious PANI/MoS2nanocomposite electrode possessed a high capacitance than purePANI and MoS2 electrodes since the current for the same activeweight represented the capacitance at the same sweep rate. Theresult could be attributed to the intimate interaction between thePANI and MoS2 substrates layers through in situ oxidation polymer-ization method, which was facilitated to MoS2 electrochemicallyactive since charge carriers could be effectively and rapidly con-ducted back and forth from MoS2 nanosheets layers edges to insideand outside of PANI. Therefore, the excellent electrochemical prop-erties of the PANI/MoS2 composites are attributed to the integralcomposite structure and the synergistic effect between PANI andMoS2 substrates.

For potential supercapacitor applications, the capacitorbehaviors of the electrodes were examined. The galvanostaticcharge–discharge plots which were measured at a constant cur-rent density of 1 A g−1 with voltage between −0.4 and 0.6 V are

K.-J. Huang et al. / Electrochimica Acta 109 (2013) 587– 594 591

Fig. 4. CV curves of MoS2, PANI, and PANI/MoS2 composites at 20 mV s−1 in 1.0 MHc

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ions and electrons must transfer during the charge/discharge pro-

2SO4 (A); galvanostatic charge/discharge curves of MoS2, PANI, and PANI/MoS2

omposites at 1 A g−1.

iven in Fig. 4B. For PANI and PANI/MoS2 composite electrodes,t shows two variation ranges in the charge–discharge curves, in

hich a perfect variation of potential vs. time dependence (belowbout 0 V) parallel to the potential axis indicates pure double-ayer capacitance behavior from the charge separation at thelectrode|electrolyte interface. By contrast, a sloped variation ofotential vs. time (0.2–0.4 V) indicates typical pseudocapacitanceehavior, caused by electrochemical adsorption/desorption or aedox reaction at the electrode|electrolyte interface. Obviously, its a typical double-layer capacitance behavior for MoS2 electrode.rom the discharge curves, the gravimetric capacitance of theaterials can be calculated according to the Eq. (1). It can be

btained that the PANI/MoS2 composite electrode exhibits a spe-ific capacitance of 575 F g−1, while the specific capacitances of theure PANI and MoS2 electrodes are only 289 and 98 F g−1, respec-ively. The greatly increased specific capacitance of the PANI/MoS2omposite can be ascribed to the large pseudocapacitance of PANInd high specific surface area of MoS2. Additionally, it is notablehat the specific capacitance of the composite is larger than theummation of those of pure PANI and MoS2. This obviously resultsrom the effect of the combination of PANI and MoS2. In otherords, the MoS2 sheets in the composite can not only offer a highly

onductive path, but also serve as a high surface area support

aterial for PANI, which provides enhanced electrode/electrolyte

nterface areas and facilitates the rapid transport of electrolyte ionsn the electrode during charge–discharge processes. Therefore,

Fig. 5. CV curves of PANI/MoS2 composites at different scan rates (2 mV s−1,5 mV s−1, 10 mV s−1, 20 mV s−1) in 1.0 M H2SO4.

the electrochemical property of the composite was improvedeffectively.

Fig. 5 presents the representative CV curves of the PANI/MoS2composite at different scan rates. It is noted that the cathodicpeaks shift positively and the anodic peaks shift negatively withthe increase of the scan rate from 2 to 20 mV s−1, which is possiblydue to the resistance of the electrode. It can be calculated the spe-cific capacitance values decreases with increase of the scan rates.This may be because at high scan rates, the movement of electrolyteions is limited and only the outer active surface is utilized for chargestorage due to the time constraint.

Fig. 6A presents the galvanostatic charge–discharge curves ofPANI/MoS2 electrode examined in 1.0 M H2SO4 electrolyte at differ-ent current densities (ampere per unit mass), respectively. Specificcapacitance can be calculated according to the Eq. (1). The vari-ation of the specific capacitance (farad per unit mass) with thecurrent density is shown in Fig. 6B. It shows that the capacitance isdecreased with the increasing current density. The decrease in spe-cific capacitance with increasing current is due to the fact that onlythe outer layers of PANI can contribute to charge–discharge pro-cesses at higher current densities. The specific capacitances of thePANI/MoS2 electrode at 1, 1.5, 3, 5 and 10 A g−1 are 575, 560, 547,526, and 500 F g−1, respectively. Significantly, the specific capaci-tance of PANI/MoS2 composites still remained as high as 500 F g−1

even at a high discharge current density of 10 A g−1. The results indi-cate that the PANI/MoS2 composites have a high rate of capacitance,which is recognized as one of the most important electrochemi-cal properties in the application of electrodes and batteries [23].The specific capacitance value of the PANI/MoS2 nanocompositeelectrode is about 575 F g−1 at a current density of 1 A g−1, corre-sponding to a specific capacitance of 98, 289 F g−1 for MoS2 andPANI alone. These values are mainly consistent with the orderindicated by the CVs. The advantages of PANI/MoS2 compositeselectrode over the MoS2 and PANI electrodes are salient and theexcellent electrochemical performances of PANI/MoS2 compositesare attributed to their unique microstructure: firstly, PANI coatingthe surfaces of MoS2 nanosheets accumulate to form pores for ion-buffering reservoirs to improve the diffusion rate of ions withinthe bulk of the prepared materials; secondly, the large specific sur-face area and the nanoscale size of MoS2 phase of the PANI/MoS2nancomposite greatly reduce the diffusion length over which both

cess; thirdly, MoS2 in the composites not only act as supports forthe loading of PANI, but also construct a highly conductive currentcollector. The excellent interfacial contact between MoS2 and PANI

592 K.-J. Huang et al. / Electrochimica Acta 109 (2013) 587– 594

Fig. 6. Galvanostatic charge/discharge curves of PANI/MoS2 composites at differentcurrent densities (1, 1.5, 3, 5 and 10 A g−1) (A); specific capacitance of the MoS2,P1

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pared to other electrodes.The Ragone plots for the MoS2, PANI and PANI/MoS2 composites

are shown in Fig. 8. The energy and power densities were derivedfrom galvanostatic charge/discharge at various current densities.

ANI, and PANI/MoS2 composites at different current densities of 1, 1.5, 3, 5 and0 A g−1 in 1 M H2SO4 (B).

acilitates fast transportation of electrons throughout the wholelectrode matrix. This unique architecture enables the PANI/MoS2anocomposite electrode to have a large specific surface and fastlectron and ion transport simultaneously, thus presenting the bestlectrochemical capacitive performance.

The electrochemical impedance spectroscopy (EIS) analysis haseen recognized as one of the principal methods examining theundamental behavior of electrode materials for supercapacitors24]. For further understanding, impedance of all products was

easured in the frequency range of 100 KHz–0.1 Hz at open cir-uit potential with an ac perturbation of 5 mV (Fig. 7). It should beoted that the Nyquist plot of an ideal supercapacitor is comprisedf a vertical line, while appearing a semicircle at high frequencyegion is indicative of interfacial charge transfer resistance. Largeremicircle shows poor electrical conductivity of the material. Basedn the Nyquist plots (Fig. 7), the equivalent series resistance (ESR)f MoS2, PANI and PANI/MoS2 obtained from the intersection pointf the curves with the axis of real impedance is 2.81 �, 2.05 �, and.25 �, respectively. The difference in the ESR of electrodes can be

ttributed to the different conductance of electrode materials. Theelative low conductivity of MoS2 resulted in the significant chargeransfer resistance among its particles. In comparison, PANI has the

Fig. 7. Nyquist plots of the MoS2, PANI, and PANI/MoS2 composites electrode in 1 MH2SO4 in the frequency range from 100,000 to 0.1 Hz at open circuit potential withan ac perturbation of 5 mV. Inset: magnified high-frequency regions.

smaller ESR because of its better conductivity. As is seen, the ESR ofthe PANI/MoS2 composite is much lower than the MoS2 and PANI,indicating good conductivity of the material may be due to intactcontact between the MoS2 and PANI. The semicircle in the high fre-quency range is corresponded to the charge transfer resistance andit can be seen from the Nyquist plots that the PANI/MoS2 compositehas the smallest diameter among all of the other materials, revea-ling the reason of its lower charge transfer resistance. Obstructionof ion movement or increased ion diffusion path lengths will resultin increased impedance in low frequency range. It can be seen thatthe straight line of the PANI/MoS2 composite is more vertical com-pared to the other materials, more closely to an ideal capacitor.These observations may be attributed to that both MoS2 and PANIhave high charge density at the electrolyte solution which resultsin high resistance of ion transfer and therefore low capacitance.This is while that the composite material shows low resistance incapacitive part. All the above mentioned reasons, the PANI/MoS2composite showed much better supercapacitive performance com-

Fig. 8. Ragone plots (power density vs energy density) of the MoS2, PANI, andPANI/MoS2 composites.

imica Acta 109 (2013) 587– 594 593

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Fig. 9. Cyclic performance of the MoS2, PANI, and PANI/MoS2 composites electrodesat 1 A g−1 in 1.0 M H2SO4 electrolyte (A); cyclic performance of the PANI/MoS2

composites electrode at 1 A g−1 in 1.0 M H2SO4 electrolyte, the inset shows

K.-J. Huang et al. / Electroch

he specific energy density (E) and power density (P) are evaluatedccording to equations [25]:

= 12

CV2 (2)

= E

�t(3)

here C is the capacitance of the two-electrode capacitor, V is theoltage decrease in discharge, E is the energy and �t is the timepent in discharge. As seen from the Ragone plots, as the powerensity increases from 1800 W kg−1 to 18,000 W kg−1, the energyensity of MoS2 decreases from 61.3 W h kg−1 to 2.5 W h kg−1,nd the energy density of PANI decreases from 145.4 W h kg−1 to19.0 W h kg−1, respectively. Comparatively, the energy density ofhe PANI/MoS2 composite can reach 287.7 W h kg−1 at a power den-ity of 1800 W kg−1, and still remains 265.0 W h kg−1 at a powerensity of 18,000 W kg−1, which exhibited a large power range thatan be obtained while maintaining a relatively high energy den-ity. The results illustrate that the PANI/MoS2 composite materialsave excellent electrochemical properties of high energy densitynd power output.

The length of cycle life of a supercapacitor is a crucial param-ter for its application. Fig. 9A shows the cyclic performance ofhe MoS2, PANI and PANI/MoS2 composite electrodes examinedy galvanostatic charge/discharge tests for 500 cycles. The spe-ific capacitances of PANI/MoS2 composite electrodes are far higherhan that of pure PANI and MoS2 during 500 cycles. We attributehese improved qualities to the flexibility of MoS2 in the compos-te not only forming an open, loose structure but also improvinglectrical conductivity of the overall electrode due to the goodonductivity of MoS2. The connection between the active materialnd electrolyte is improved, and full use is made of pseudocapac-tive PANI in the composite electrode. For PANI/MoS2 compositelectrode, each charge/discharge cycle approximately has a similarotential-time response behavior (the inset of Fig. 9B), implyinghat the charge/discharge process of the PANI/MoS2 compositelectrode is reversible. Interestingly, a small increase of capacitances observed at three electrodes (Fig. 9A) during the first 140 cycles.he initial increase of capacitance can be explained as follows: athe initial stage, active materials have not been fully used. Afterepetitive charge/discharge cycling, the electrochemical active sitesnside the stainless steel substrate electrode will be fully exposed tohe electrolyte. Therefore, an increasing capacitance was displayedn the cyclic tests. For PANI/MoS2 composite electrodes, there isnly 2% decay in the specific capacitance after 500 cycles, indicat-ng a good cycle performance of the composite material. For pureANI and MoS2, the specific capacitance maintains only 87.3% and0.9% after the same cycles, respectively. The enhanced cycling sta-ility of PANI/MoS2 composite electrodes is mainly because of theighly dispersed MoS2 sheets in the composite and the synergisticffect between MoS2 and PANI. MoS2 sheets intercalated by PANIndertake some mechanical deformation in the redox process ofANI, which avoid destroying the electrode material and leading ton outstanding stability. The decreasing deformation as well as themprovement of capacitance retention implies that the PANI/MoS2omposites are good candidates as a material for supercapacitorlectrodes.

Nyquist plots from the impedance analysis were used to char-cterize the changes to the electrode structure before and after00 cycles of operation. As indicated in Fig. 9C, these two Nyquistlots almost overlap, where the electrode exhibits a semicircle in

he high frequency region followed by a straight line in the lowrequency region. After 500 cycles of operation, the intercept ofhe Nyquist plot, which is indicative of the equivalent series resis-ance, only increased from ∼1.7 to ∼2.1 �, while the knee frequency

charge/discharge curves of the PANI/MoS2 composites in potential range from −0.4to 0.6 V (B). Nyquist plots of the PANI/MoS2 composites electrode before cycling andafter 500 cycles. Inset: magnified high-frequency regions (C).

remained the same, suggesting a good contact formation betweenMoS2 and PANI that is retained even after extended cycling.

A comparison of the properties of the as-prepared PANI/MoS2composite with other electrode materials for supercapacitor is

listed in Table 1. It can be seen that the PANI/MoS2 composite showshigh specific capacitance, high energy density, and acceptable cyclestability [1,7,13,26–29].

594 K.-J. Huang et al. / Electrochimica Acta 109 (2013) 587– 594

Table 1Comparison of properties of different electrode materials for supercapacitor.

Electrode materials The maximum specificcapacitance

Energy density Final capacitance compared to the initialcapacitance

Ref.

Copper oxide/graphene oxide 245 F g−1 at 0.1 A g−1 – 79% (after 1000 cycles at 0.25 A g−1) [1]Self-doped

polyaniline/functionalizedcarbon cloth

408 F g−1 at 1 A g−1 7.8 W h kg−1 at power densityof 1 kW kg−1

95% (after 1000 cycles at 2 A g−1) [7]

Polypyrrole/MoS2 553.7 F g−1 at 1 A g−1 49.2 W h kg−1 at power densityof 0.4 kW kg−1

90% (after 500 cycles at 1 A g−1) [13]

Porous tubular C/MoS2 210 F g−1 at 1 A g−1 – 105% (after 1000 cycles at 4 A g−1) [26]Polyaniline

nanowhiskers/mesoporouscarbon

470 F g−1 at 1 A g−1 – 90.4% (after 1000 cycles at 1 A g−1) [27]

Reduced graphene oxide/Co3O4 458 F g−1 at 0.5 A g−1 47.2 W h kg−1 at power densityof 0.2006 kW kg−1

95.6% (after 1000 cycles at 2 A g−1) [28]

NiCo2O4 nanowires/reduced 737 F g−1 at 1 A g−1 – 94% (after 3000 cycles at 4 A g−1) [29]

−1 at

g−1

4

pMpciaecip(impa

A

HId

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

grapheme oxidePANI/MoS2 575 F g−1at 1 A g−1 265 W h kg

of 18 kW k

. Conclusion

We report a facile process to fabricate a new PANI/MoS2 com-osite containing short-rod PANI anchored onto the surfaces ofoS2 for supercapacitor electrode material. The surface mor-

hology, structure, and capacitive behaviors of the PANI/MoS2omposite were thoroughly investigated. The embedding PANInto the MoS2 nanosheets prevents the restacking of MoS2 sheetsnd improves the capacitance of the composite electrode, thelectrolyte/electrode accessibility as well as conductivity, indi-ating a positive synergistic effect for MoS2 and PANI on themprovement of overall electrochemical performance. The pre-ared PANI/MoS2 composite exhibited a high specific capacitance575 F g−1 at 20 mV s−1) and excellent long cycle life, suggest-ng a highly promising prospective for supercapacitors. The facile

ethod of synthesis can be readily adapted to prepare other high-erformance electrode materials containing MoS2 as a conductingdditive.

cknowledgments

This work was supported by the Natural Science Foundation ofe’nan Province of China (132300410060), Program for University

nnovative Research Team of Henan (2012IRTSHN017) and Foun-ation of Henan Educational Committee (no. 13A150768).

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