Emerging Robust Heterostructure of MoS2−rGO for High-Performance SupercapacitorsMohit Saraf,† Kaushik Natarajan,† and Shaikh M. Mobin*,†,‡,§

†Discipline of Metallurgy Engineering and Materials Science, ‡Discipline of Chemistry, and §Discipline of Biosciences and BiomedicalEngineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India

*S Supporting Information

ABSTRACT: The intermittent nature of renewable energyresources has led to a continuous mismatch between energydemand and supply. A possible solution to overcome thispersistent problem is to design appropriate energy-storagematerials. Supercapacitors based on different nanoelectrodematerials have emerged as one of the promising storagedevices. In this work, we investigate the supercapacitorproperties of a molybdenum disulfide−reduced grapheneoxide (rGO) heterostructure-based binder-free electrode,which delivered a high specific capacitance (387.6 F g−1 at1.2 A g−1) and impressive cycling stability (virtually no loss upto 1000 cycles). In addition, the possible role of rGO in thecomposite toward synergistically enhanced supercapacitancehas been highlighted. Moreover, an attempt has been made to correlate the electrochemical impedance spectroscopy studies withthe voltammetric analyses. The performance exceeds that of the reported state-of-the-art structures.

KEYWORDS: heterostructure, supercapacitors, binder-free, synergism, specific capacitance

1. INTRODUCTION

Owing to the rapid depletion of nonrenewable resources, anupsurge has been witnessed in the demand of energystorage.1−16 In this concern, supercapacitors, based on novelelectrode nanomaterials, have shown remarkable progressbecause of their ultrafast charge−discharge rate, high powerdensity, and long cycle life.17−23 However, significant advance-ments are necessary to improve the energy density ofsupercapacitors. The energy and power density of super-capacitors are primarily governed by the elementary charge-storage mechanism and the corresponding kinetics at theelectrode/electrolyte interface.24−28 According to the generalthumb rule, the performance of supercapacitors is largelydependent on electrode materials.25−32 Hence, the design andsynthesis of the electrode materials warrant urgency.2,18

Recent reports show a paradigm shift in employing two-dimensional (2D) materials toward energy-storage applicationsbecause of their striking structural and electronic properties.33

Among them, 2D molybdenum disulfide (MoS2) is a risingcandidate,26 which encompasses a layered structure with stronginterlayer covalent bonds separated by weak van der Waalsforces.25,34 Thanks to its sheetlike morphology, unique atomicstructure, high surface area, and electrical conductivityanalogues with graphene, which improves the energy-storageproperties.22,23 Furthermore, MoS2 reveals higher intrinsic ionicconductivity and theoretical capacitance when compared withoxides and graphite, respectively.35 The layered structure ofMoS2 helps proper insertion of foreign atoms and provides

structural rigidity to the structure,36 which is beneficial forobtaining high cycle life during the cycling test. However, dueto the semi-insulating nature of MoS2, it is not an immediatelyattractive electrode material for energy storage.36 One possibleapproach to avoid this shortcoming is to extend MoS2 into theheterostructure with another 2D material.29 These 2D materialsare known to have high mechanical stability, strength, flexibility,transparency, and chemical stability. Moreover, they tend tohave higher surface areas, which are envisioned to greatlyenhance the performance of electrochemical capacitors.14 Suchheterostructures of 2D materials encompass an interfacialcontact between two individual chemically different 2D units,which can maximize the charge-storage capability and lead tosynergistic effects. Furthermore, the insertion of stabilizingspecies (ions and molecules) at the hetero-interface canenhance electrochemical stability and ion diffusion.37 Hence,the combination of two 2D materials is expected to give rise tosynergistically coupled hybrid materials with escalated proper-ties for supercapacitor applications. The advantages of 2Dheterostructures include additional specific capacitance due tothe synergistic contribution of pseudo and/or electrochemicalcapacitance from the materials involved in the heterostructureand increases in stability, rate performance, and crystallinity.14

Among such materials, reduced graphene oxide (rGO) holds

Received: March 20, 2018Accepted: April 26, 2018Published: April 26, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 16588−16595

© 2018 American Chemical Society 16588 DOI: 10.1021/acsami.8b04540ACS Appl. Mater. Interfaces 2018, 10, 16588−16595

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great promise owing to its layered structure, remarkablemechanical stability, and conductivity.38,39 Although rGOcomposites usually exhibit enhanced properties, their tediousand time-consuming preparation restricts their advantages. Theproduction of these composite-based electrodes is costly alsobecause of the requirement of binders such as Nafion,poly(vinylidene difluoride), etc.36 Moreover, the hydrophobic-ity and poor dispersion of rGO even after prolongedultrasonication restrict its practical utility.36 Concerning thesepoints, facile hydrothermal/solvothermal approaches have beenproven to be key-enabling techniques by modifying the surfaceof MoS2 with rGO effectively.23 Therefore, by fully utilizing theadvantages of the two structures, the energy-storage efficiencyof the materials can be greatly enhanced. For example, Yang etal. recently developed hydrothermally grown MoS2/graphenenanosheets composite, which was used along with carbon blackand poly(tetrafluoroethylene) to fabricate the workingelectrode, which can deliver high specific capacitance (320 Fg−1 at 2 A g−1).22 Similarly, Thangappan et al. preparedhydrothermally driven graphene-decorated MoS2 nanosheetsand obtained a specific capacitance of 270 F g−1 at 0.1 A g−1.They used Nafion during their electrode fabrication.23

However, studies on MoS2−rGO-based heterostructures forsupercapacitors are yet in their infancy and there is scope forimproving the performance by tailoring the composition andmorphologies. Additionally, the fabrication of electrodes underoptimized conditions without using any conductive additives orbinders may be an alternative cost-effective way to improve theperformance.Keeping the above facts in mind, we synthesized MoS2−rGO

nanoflowers by a simple hydrothermal method under optimizedconditions and investigated their binder-free supercapacitorproperties. The synergistic interplay between layered MoS2 andrGO in the MoS2−rGO hybrid leads to a superior super-capacitor performance over previous reports.

2. EXPERIMENTAL SECTION2.1. Synthesis of MoS2 and MoS2−rGO Composite. Graphite

oxide (GO)2 and MoS2−rGO composite were utilized as prepared inour previous reports.236 Briefly, the appropriate amounts of Na2MoO4·2H2O and NH2CSNH2 were dissolved into the GO solution bystirring for half an hour followed by heating at 180 °C for 24 h in aTeflon-lined steel autoclave. The recovered black precipitates ofMoS2−rGO were thoroughly washed and subsequently dried. MoS2was used as synthesized using the same procedure in the absence ofGO.36,40

2.2. Fabrication of Electrodes. The glassy carbon electrodes(GCEs) were prepared in a manner similar to that described in ourprevious work.36 Briefly, glassy carbon electrodes were thoroughlycleaned and rinsed using deionized (DI) water and dried. A 5 mgsuspension of the powders (MoS2 and MoS2−rGO) was preparedaccording to the procedure described previously, and 5 μL of thesesuspensions was deposited on the working surface of the glassy carbonelectrodes and dried in air, followed by a three-time rinse in DI waterand further drying in air. The prepared electrodes were denotedMoS2/GCE and MoS2−rGO/GCE.

3. RESULTS AND DISCUSSION

3.1. Characterization. The materials were characterized byvarious advanced physicochemical characterization techniquessuch as X-ray diffraction, scanning electron microscopy/transmission electron microscopy (SEM)/(TEM), X-rayphotoelectron spectroscopy (XPS), atomic force microscopy,Brunauer−Emmett−Teller (BET) analysis, etc., which have

been discussed in detail in our previous work.36 Briefly, the X-ray diffraction spectrum of MoS2−rGO reveals a diploidrelationship among the d-spacings, which represents theformation of a new lamellar structure. Also, a general loss ofsharpness of peaks for MoS2−rGO suggests the presence ofrGO and the formation of composite. Raman spectra showcharacteristic peaks at 377 and 407 cm−1 in the MoS2−rGOcomposite along with two additional D and G bands at 1349and 1582 cm−1 attributed to the defects of rGO and vibrationof sp2 carbon atoms, respectively. According to themorphological investigations by SEM and TEM images, thespherical structures (flowerlike nanoflakes) tend to increase inthe composite, with the implication that this presents a greaternumber of active sites for the electrochemical processes. Someadditional SEM and TEM images of MoS2 have been provided(Figure S1). SEM and TEM images of the MoS2−rGOcomposite have also been given in Figure S2. Such a compositestructure improves electronic conductivity, mechanical robust-ness, surface area, and roughness, which is beneficial forapplications as a supercapacitor electrode. The XPS analysisconfirms the presence of C−C and C−O species in thecomposite, whereas BET analysis confirms increased surfacearea for the MoS2−rGO composite.36

3.2. MoS2−rGO/GCE as a Supercapacitor Electrode.During the sensor study of MoS2−rGO/GCE,36 we noticedsome interesting behavior of these electrodes such as highcyclic voltammetry (CV) current and area under the curve. TheCV and electrochemical impedance spectroscopy (EIS) resultssuggested that the material might be capacitive in nature. Suchobservations along with the unexplored literature on MoS2-based composites motivated us to employ them for energystorage also. It was also considered that in the MoS2−rGOcomposites, the plentiful open sites of MoS2 provide highsurface area and enhance the electrode/electrolyte contactarea.22,23 Additionally, during the charge−discharge process,these nanosheets significantly shorten down the ion diffusionpathways and help in obtaining high specific capacitance. Inaddition, the overlapping or coalescence of the rGO sheetsalong with less-conducting MoS2 forms an interconnectedconducting network to facilitate fast electron transportation.40

The superior mechanical strength of rGO also enhances thelong-term cyclic stability of the MoS2−rGO compositenanostructure. Overall, the robust electrode fabrication canalleviate the mechanical degradation and volume expansion

Scheme 1. Schematic of the MoS2−rGO Composite from theElectrochemical Point of View

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during the charge−discharge process, which is beneficial forsupercapacitors (Scheme 1).23,41−52

The supercapacitor behavior of MoS2−rGO/GCE wasinvestigated using cyclic voltammetry (CV), chronopotentiom-etry/galvanostatic charge−discharge (GCD), and electrochem-ical impedance spectroscopy (EIS) techniques in a broadpotential window (−1 to +1 V) by employing 1 M Na2SO4solution. The performance of MoS2−rGO/GCE was comparedwith that of bare GCE and MoS2/GCE.Figure 1a−c presents the comparison of CV profiles of all

three electrodes, that is, bare GCE, MoS2/GCE, and MoS2−rGO/GCE, which shows the highest charge transfer in the caseof MoS2−rGO/GCE, indicating minimized internal resistance.Moreover, its CV profile tries to achieve a rectangular shape,which is a characteristic of an ideal double-layer capacitor.18 Onthe basis of this observation, it can be concluded that MoS2−rGO/GCE primarily follows the double-layer capacitivemechanism because of the presence of rGO and its synergisticeffects with MoS2. The effect of the presence of rGO along withMoS2 can be seen from Figure 1d, in which MoS2/GCEdisplays poor charge propagation and less CV-integrated areaunder the curve compared to those in MoS2−rGO/GCE. The

effect of scan rates (10−500 mV s−1) on the CV profiles wasalso recorded for both MoS2/GCE and MoS2−rGO/GCE,demonstrating that the area under the CV curves increases withthe increasing scan rate (Figure 1e,f).10,18 Furthermore, MoS2−rGO/GCE exhibits high rate ability, as well as remarkableelectrochemical reversibility as it maintains a good rectangularCV shape even at a higher scan rate (500 mV s−1) without anysignal deformation. The combination of rGO with MoS2substantially improves the charge propagation process. Hence,the resulting capacitance of the MoS2−rGO composite is theresult of synergy between rGO and MoS2. Herein, rGOprovides additional channels for charge transport and increasesthe overall conductivity of the electrode.10,18,38−40

The charge-storage efficiencies of MoS2/GCE and MoS2−rGO/GCE were further investigated by studying their charge−discharge properties20,30 in a wide potential window (−1 to +1V). Figure 2a,b presents the GCD curves of MoS2/GCE andMoS2−rGO/GCE at different current densities (1.2−4 A g−1).A potential drop (iR) was observed in the case of MoS2/GCEat each current density, which kept on increasing withincreasing current density. Moreover, its charge−dischargecurves are not symmetrical and at around −0.2 V, the discharge

Figure 1. (a) CV profiles of bare GCE, MoS2/GCE, and MoS2−rGO/GCE at a scan rate of 100 mV s−1, (b) comparison of CVs of bare GCE andMoS2/GCE at a scan rate of 100 mV s−1, (c) comparison of CVs of bare GCE and MoS2−rGO/GCE at a scan rate of 100 mV s−1, (d) comparison ofCVs of MoS2/GCE and MoS2−rGO/GCE at a scan rate of 100 mV s−1, (e) CV profiles of MoS2/GCE at different scan rates (10−500 mV s−1), and(f) CV profiles of MoS2−rGO/GCE at different scan rates (10−500 mV s−1), in 1 M Na2SO4.

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curve becomes almost parallel to the x axis. In contrast, thecharge−discharge curves of MoS2−rGO/GCE are somewhatmore linear and, interestingly, no potential drop was observed,which can be attributed to the synergistically enhancedconductivity and minimized internal resistance because of thepresence of rGO along with MoS2. The discharge times ofMoS2−rGO/GCE were also found to be greater than those of

MoS2/GCE, suggesting a higher specific capacitance of theMoS2−rGO composite than that of bare MoS2 because of ahigher diffusion of ions from the electrolyte to the electrode.53

The values of specific capacitances calculated at currentdensities of 1.2, 1.6, 2, 2.8, 3.2, 3.6, and 4 A g−1 are 387.6, 252,171, 134.4, 128, 115.2, and 104 F g−1, respectively, for MoS2−rGO/GCE and 196.2, 127.2, 21, 10.5, 9.28, 8.1, and 7 F g−1,respectively, for MoS2/GCE. A graph between the calculatedspecific capacitance and current density has been plotted inFigure 3, which demonstrates that MoS2−rGO/GCE has acomparatively good rate ability than MoS2/GCE because of thehealthy alliance between MoS2 and rGO.It can be observed that specific capacitance kept on

decreasing on increasing current density in both the cases.The reason of this observation can be associated with the factthat the electrolyte ions occupy almost all of the available poresof the electrode at lower current densities, leading to acomplete insertion, hence inducing a higher specific capaci-tance.10,34,35 In contrast, the effective interaction between theions and the electrode is substantially reduced at higher currentdensities; as a result, the capacitance decreases. A probablemechanism for the observed dominant electrical double-layercapacitor (EDLC) behavior in the composite MoS2−rGO/GCE electrode is based on the favorable kinetics for the non-Faradaic proton adsorption on the interface of MoS2 andrGO54 (according to the below-mentioned equation), which isenhanced by the presence of hydroxyl groups present on rGOsheets, acting as proton donors.55

+ ↔ −+ +MoS (interface) H MoS H (interface)2 2

Figure 2. (a, b) GCD profiles of MoS2/GCE and MoS2−rGO/GCE atdifferent current densities (1.2−4 A g−1), respectively.

Figure 3. Specific capacitance as a function of current density forMoS2/GCE and MoS2−rGO/GCE.

Figure 4. EIS data of bare GCE, MoS2/GCE, and MoS2−rGO/GCE.The inset shows the equivalent circuit used to fit the EIS data.

Table 1. Circuit Parameters Generated after the Fitting tothe Given Equivalent Circuit

parameter bare GCE MoS2/GCE MoS2−rGO/GCE

Rs (Ω) 1.5439 × 101 2.0343 × 101 5.2438 × 101

Cgb 5.37164 × 10−6 2.60427 × 10−5 2.1495 × 10−4

αgb 0.84 0.676 0.7375Rgb 5.031 × 103 5.86435 × 102 8.2577 × 103

Rbulk (Ω) 2.37128 × 10−5 6.71477 × 10−4 2.25147× 10−4

Cbulk (Ω) 1.40553 × 10−5 1.08633 × 10−4 2.3621× 10−4

αbulk 0.78 0.6658 0.94τgb (s) 27.02 × 10−3 15.26 × 10−3 17.75 × 10−1

τbulk (s) 3.3329 × 10−10 7.294 × 10−8 5.318 × 10−8

Figure 5. Cycle stability analysis for MoS2−rGO/GCE at a currentdensity of 4 A g−1, where the inset shows the first 20 cycles for bothelectrodes.

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Therefore, an enhanced EDLC behavior is observed for thiselectrode despite MoS2 being typically reported as showingpseudocapacitive properties56,57 because of the synergisticeffects of rGO.3.3. EIS Analysis. The charge transfer kinetics and

capacitive components of all three electrodes, that is, bareGCE, MoS2/GCE, and MoS2−rGO/GCE, were investigated byelectrochemical impedance spectroscopy (EIS). In EIS, the datais usually expressed as Nyquist plots.58,59 In the present work,EIS analysis was carried out in 1 M Na2SO4 solution in thefrequency range of 0.01−100 000 Hz. The obtained circuitmodel is idealized and fitted on the basis of a discrete-component-based circuit model, with the real and imaginaryparts being represented by a resistance and a compleximpedance represented by a constant phase element becauseof the obtained semicircular graph having a depressed shapewith center below the real axis. Hence, a circuit model isadapted on the basis of our previous work.60 The EIS results areportrayed in Figure 4, and the equivalent circuit is shown in theinset.The parametric values obtained from the fitting are shown in

Table 1. The equivalent circuit is therefore represented by aseries resistance arising from the test setup and leads, and twoimpedances in series, each comprising a resistive and acapacitive component, representing bulk (depletion layer)and grain-boundary (interfacial) impedances. The constantphase element is represented again by the following equation asdescribed previously

= · α− −Z A sCPE1

It is clear from Table 1 that there is a marked increase in bothbulk and grain-boundary capacitances, when comparing theMoS2/GCE to the bare GCE. Both capacitances increasefurther in the case of the MoS2−rGO/GCE, indicating strongsynergistic effects because of the combination of the twomaterials, that is, MoS2 and rGO. This result is validated by theCV and GCD curves presented earlier. The values of αbulk andαgb suggest good crystallinity

61,62 of the MoS2−rGO composite.In MoS2−rGO/GCE, the interfacial/grain-boundary resistanceis found to increase in comparison to that of MoS2/GCE. Thisis likely related to the elongation of bond length between Catoms63 under strain because of the penetration of electrolyte-associated species in the porous rGO layer(s)64,65 and thelarger number of smaller grains contributing to the grain-

boundary resistance.66 However, the bulk resistance shows adecrease, which is indicative of improved charge-transferproperties in the MoS2−rGO/GCE. The time constantsincrease progressively among bare GCE, MoS2/GCE, andMoS2−rGO/GCE, which is indicative of a higher energydensity,67 a result confirmed by other tests in this work.The cycling stability test for MoS2−rGO/GCE at a current

density of 4 A g−1 in a potential window (−1 and +1 V) showsthat MoS2−rGO/GCE does not show any noteworthy decay inspecific capacitances up to 1000 cycles (Figure 5).The high specific retention capability of the MoS2−rGO

composite can be assigned to the synergistically inducedmechanical robustness between MoS2 and rGO, which providesit stability on the electrode surface. The GCD profiles ofMoS2−rGO/GCE before and after cycling have been presentedin Figure S3, which hardly indicate any significant distortion/variation, further suggesting the high cycling stability of MoS2−rGO/GCE. The SEM and TEM images after cycling have alsobeen presented in Figure S4, which further shows insignificantchanges in the morphology of composite nanostructures aftercycling. The supercapacitor performance obtained in thepresent work has been compared with that of previouslyreported state-of-the-art structured electrodes based on theMoS2 and rGO combination and is summarized in Table 2.After careful analysis, it can be concluded that the MoS2−rGOcomposite can also be a potential energy-storage material. Theresults are superior to all of the previous reports, and the facilebinder-free preparation of MoS2−rGO/GCE highlights its edgeover related electrodes. A Ragone plot has been demonstratedto compare the energy density versus power density of presentwork with some related reports (Figure S5).68 The comparisonof Ragone plots clearly describes that the MoS2−rGOcomposite as demonstrated in this work has an edge oversimilar reports. However, further improvements can be done toenhance the power density by tailoring the compositions andmorphologies of such composites.

4. CONCLUSIONS

In summary, the synergistic activities between MoS2 and rGOhave been demonstrated wrt the supercapacitor application.The important role of conductive additive rGO was highlightedin conjunction with the overall improvement in the charge-storage efficiency of the MoS2−rGO composite. It was alsoconcluded that the presence of rGO tends to transform the

Table 2. Comparison of the Supercapacitor Performance in the Present Work with that in Previous Reports Based on the SameMaterial Composition

materialelectrodeused

electrolytemedium

specific capacitance (F g−1) at rate(A g−1/mV s−1) cyclic stability reference

MoS2/graphene nanosheets Ni foam 1 M Na2SO4 320 at 2 A g−1 22graphene decorated with MoS2 nanosheets GCE 1 M Na2SO4 270 at 0.1 A g−1 89.6% after 1000

cycles23

two-dimensional MoS2 on reduced graphene oxide GCE 1 M HClO4 265 at 10 mV s−1 70% after 5000cycles

41

molybdenum disulfide nanostructure intothree-dimensional porous graphene

Cu foil 1 M LiPF6 88.3 at 0.1 A g−1 78% after 2000cycles

42

layered molybdenum sulfide/N-doped graphene hybrid Ni foam 6 M KOH 245 at 0.25 A g−1 91.3% after 1000cycles

43

rGO−MoS2 2D sheets carbonfelt

1 M KCl 387 at 1 A g−1 88% after 1000cycles

44

MoS2−graphene hybrid films FTO glass 1 M Na2SO4 282 at 20 mV s−1 93% after 1000cycles

46

MoS2−rGO composite (binder-free) GCE 1 M Na2SO4 387.6 at 1.2 A g−1 no loss up to 1000cycles

presentwork

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storage characteristics of the MoS2−rGO composite toward anideal double-layer capacitor by providing much more sym-metrical charge−discharge curves than those of bare MoS2.Also, the remarkable cyclic stability feature was assigned to theformation of the robust heterostructure of 2D materials. Thepresent work opens up an avenue of building novelheterostructures of 2D materials for greatly improving theproperties for the next-generation cutting-edge energy-storagesolutions.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b04540.

Experimental information, SEM and TEM images ofmaterials, charge−discharge profiles before and aftercycling, and Ragone plot (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: xray@iiti.ac.in. Tel: +91 731 2438 762.ORCIDMohit Saraf: 0000-0002-2566-5581Shaikh M. Mobin: 0000-0003-1940-3822NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from the SERB-DST (Project No. EMR/2016/001113), New Delhi, and IIT Indore is gratefullyacknowledged by S.M.M. We thank Sophisticated Instrumenta-tion Centre (SIC), IIT Indore, for providing all of thecharacterization facilities. M.S. and K.N. thank MHRD, NewDelhi, India, for providing fellowships. We are grateful to theAdvanced Imaging Centre at IIT Kanpur for TEM facility.

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