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Page 1: University of Bath · M KOH at room temperature by cyclic voltammetry (CV, BioLogic, SP-50, USA) with a rotating disk electrode (RDE, 3 mm in diameter, RRDE-3A, ALS Co., Japan). The

Citation for published version:Chandrasekaran, S, Kim, EJ, Chung, JS, Bowen, CR, Rajagopalan, B, Adamaki, V, Misra, RDK & Hur, SH 2016,'High performance bifunctional electrocatalytic activity of a reduced graphene oxide-molybdenum oxide hybridcatalyst', Journal of Materials Chemistry A, vol. 4, no. 34, pp. 13271-13279. https://doi.org/10.1039/c6ta05043c

DOI:10.1039/c6ta05043c

Publication date:2016

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Page 2: University of Bath · M KOH at room temperature by cyclic voltammetry (CV, BioLogic, SP-50, USA) with a rotating disk electrode (RDE, 3 mm in diameter, RRDE-3A, ALS Co., Japan). The

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Received00thJanuary20xx,Accepted00thJanuary20xx

DOI:10.1039/x0xx00000x

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Highperformancebifunctionalelectrocatalyticactivityofareducedgrapheneoxide-molybdenumoxidehybridcatalystSundaram Chandrasekarana, Eui Jung Kima, Jin Suk Chunga, Chris R Bowenb, BalasubramaniyanRajagopalana,VaiaAdamakib,R.D.K.Misrac,andSeungHyunHur*a

Theadvances incosteffective,highlyactiveandstableelectrocatalystsfortheoxygenreductionreaction(ORR)andtheoxygenevolutionreaction(OER)remainthemajorissuesforthecommercializationofmetalair-batteriesandalkalinefuelcells. In thisaspect,a facilehydrothermal routewasdevelopedtopreparenonpreciousmetalelectrocatalysts includingpristineMoO3rods,nanospheres,andtheirhybridswithreducedgrapheneoxide(rGO).ThisisthefirstreportoftheuseofrGO coupledwith hexagonalMoO3 nanocrystals to act as bothORR andOER catalysts. The rGO -MoO3 sphere hybridcatalystexhibitedexcellentcatalyticactivity towardboththeORRandOERthanpristineMoO3rods,MoO3spheresandrGO-MoO3rods.Inaddition,therGO-MoO3nanospherehybridexhibitedexcellentcatalyticactivity,long-termdurability,andCO tolerance compared toahighquality commercial Pt/C catalyst. Thismakes theGMShybrid compositeahighlypromisingcandidateforhigh-performancenon-preciousmetal-basedbi-functionalelectrocatalystswithlowcostandhighefficiency for electrochemical energy conversion. The enhanced activity of the rGO - MoO3 nanosphere hybrid is duemainlytotheenhancedstructuralopennessinthetunnelstructureofthehexagonalMoO3whenitiscoupledwithrGO.

1.IntroductionThe advancement of cost effective, naturally abundant,

environmentallybenign,andhighlyactivecatalystsforenergyconversion and storage is of utmost importance and is animportanttopicinrenewableenergyresearch1-4.Inparticular,water splitting5, fuel cells6 and metal-air batteries7 arerequiredforfutureenergyconversionandstorageapplicationsasaresultoftheirhightheoreticalenergydensity.Theoxygenreductionreaction(ORR)andoxygenevolutionreaction(OER)playmajorrolesinthesedevices.Inthiscontext,thediscoveryofrobustmaterialsforboththeOERandORRisimportantbutalso very challenging. Traditional catalysts, such as Pt and itsalloys,performwellfortheORR,buttheirhighcostlimitstheirapplications8-11. Materials such as, iridium and ruthenium-based compounds and their oxides are well known OERcatalysts12-14.Thesemetalsarealsoexpensiveandamongtherarest elements on earth,making them impractical for large-scale applications. Therefore, alternative catalysts based oncost effective, metal-free materials or non-precious metalshaveattractedconsiderableattention4.Asa result,anumberof non-precious nanomaterials, such as Ni0.2Co0.3Ce0.5Ox

15,NiOx-Bi

16 and Ni3S217, have been explored for their high OER

activity.Recently,arangeofuni-functionalcarbonmaterialsormetaloxideswithcarbonmaterialsalsohavebeenreportedto

exhibithighactivitieseitherfortheOERorforORRactivity18-23.However,theuseofsinglecatalystasabi-functionalcatalyst24-32 has rarely been reported because the bi-functionalityrequiressignificantlylowoverpotentialsandhighactivitiesforboth the OER and ORR. Therefore, it remains a considerablechallenge to design a high-performance catalyst capable ofunifying the both OER and ORR for energy conversion andstorage applications with the relevant properties of highactivity, low-cost, carbon monoxide (CO) tolerance, andexcellentstability.

Recently, a wide range of molybdenum oxide-basedcompoundswiththemanifoldstructuralmotifsandpropertieshave attracted considerable interest in the field of materialsresearch. These materials have been shown to exhibitexcellent properties for catalysis33, 34, photocatalysis35,batteries36, solar energy conversion37, 38, and sensors39, 40. Incrystallographic view, MoO3 exists in three phases,orthorhombicα-MoO3,monoclinicβ-MoO3,andhexagonalH-MoO3. The hexagonalH-MoO3 system is ametastable phase,which is constructed from the zigzag chains of octahedral[MoO6] as the building blocks attached through adjacentoxygen. Compared to α and β crystal structures, hexagonalMoO3retains a number of interesting properties. The tunnelstructureofH-MoO3canresultinelectron-holeseparationandafford large special locations for cation insertion andextraction41. To date, there are no reports ofmultifunctionalcatalystsbasedonahexagonalmolybdenumoxideembeddedgraphenematerial forORR andOER activity. Importantly thethree essential factors to achieve in commercializing fueltechnologiesincludehighefficiency,stability,andscalability42.In this regard, thepresent study focusedonacatalyst that isnot only highly active but is also facile, robust, highly stable,and easily scalable. This paper reports the first, highly active

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andmultifunctionalreducedgrapheneoxide(rGO)-hexagonalMoO3 hybrid catalysts synthesized via a facile hydrothermalroute. For sample identification, the prepared MoO3 rods,MoO3 nanospheres, rGO-MoO3 rods, and rGO-MoO3

nanospheresamplesaredenotedasMR,MS,GMR,andGMSrespectively. The synthesizedmaterials were tested for boththeORRandOERreactionsandGMShybridshowspromisingperformance compared to a high quality commercial Pt/Ccatalyst, indicating its potential as a cost effective andhighlyactivebi-functionalcatalyst.

ExperimentalSection

2.1CatalystsSynthesis In a typical procedure of MR, 3 mmol of ammoniumheptamolybdate tetrahydrate ((NH4)6Mo7O2.4H2O) wasdissolvedin50mLofdistilledwaterand3mmolHClwasthenadded to the reaction solution with vigorous stirring. Theresulting solution was transferred to a 100 mL Teflon-linedstainless autoclave and sealed tightly, which was kept at180 °C for 14 h. The autoclave was then allowed to coolnaturally to room temperature. The resulting products werefiltered, washed several times with absolute ethanol anddistilledwater,anddriedat250°Cfor4h.TheMSsamplewassynthesized using the same procedure, only the HCl wasreplacedwithammonia.

The graphene oxide (GO) was synthesized from graphitepowders using a modified Hummers’ method. In a typicalsynthesisoftheGMRorGMS,20mgofMRorMSwasaddedgradually to 20mL of the GO solution. Themixture solutionwas then stirred magnetically, and 5 mL of ammonia wasaddedgraduallytothemixture.After2h,theresultingstablesuspensionwas then transferred to a Teflon-lined autoclave,andtreatedhydrothermallyat180°Cfor14h.Finally,theas-preparedproductwasdriedat250°Cfor4h.2.2Instrumentalanalysis

The crystalline phaseswere identified by X-ray diffraction(XRD;Rigaku)usingCu-Kαradiation.Thesurfacemorphologieswereidentifiedbyfieldemissionscanningelectronmicroscopy(FE-SEM; JSM 6500F) and transmission electron microscopy(TEM; JEOL JEM2100F). The vibrational, rotational andotherlow-frequencymodes in thesamplewereanalyzedbyRamanspectroscopy (Thermo Scientific DXR). The materialcompositionandoxidationstatesofcarbonandironatomsinthe GMS nanocomposites were analyzed by X-rayphotoelectron spectroscopy (XPS, Thermo Scientific K-alpha).N2 adsorption–desorptionmeasurementswere carried out at77 K using a Micromeritics ASAP 2020 gas-sorption system.Thermogravimetricanalysis(TGA)wascarriedoutonQ50(TAinstruments) thermo-analyser under air atmosphere at aheating rate of 10 °C min−1. The samples were heated fromroomtemperatureto800°Catalinearheatingrate.2.3Electrochemicalmeasurements

Theelectrochemicalmeasurementswerecarriedoutin0.1M KOH at room temperature by cyclic voltammetry (CV,BioLogic, SP-50, USA) with a rotating disk electrode (RDE, 3

mm in diameter, RRDE-3A, ALS Co., Japan). The three-electrode cell consisted of an Ag/AgCl electrode as thereferenceelectrode,Ptasthecounter-electrode,andaglassycarbonelectrode/rotatingdiskelectrode(RDE)loadedwiththevarious catalysts as the working electrode. As the workingelectrode,theelectroactivematerialsinethanol(1mg/ml)and10μLofNafionsolution (0.5wt% in isopropanol)asabinderwere mixed by sonication. Subsequently, all the catalystloadinginthesameamountof~0.10mgcm-2wascoatedonaglassy-carbon RDE and dried in air for the electrochemicalcharacterization. Finally, the measured potential vs. Ag/AgClwas converted to the reversible hydrogen electrode (RHE)scaleusingtheNernstequation.3.ResultsandDiscussion

The size,morphology, andhybrid structureof all sampleswere examined by scanning electron microscopy (SEM) andhigh resolution transmission electron microscopy (HR-TEM).Fig. 1 shows the microscopic images of pristine MoO3 rods(MR),rGO-MoO3rods(GMR),andpristineMoO3spheres(MS).Fig. S1 representsFE-SEM imagesofpristine rGOused in thepreparation of GMR andGMS hybrids. From themicroscopicimages(Fig.1(a-d))ofpreparedMRsample,itevidentlyshowsauniquerod-likemorphology.Thelengthofrodsisabout8-12μm and the averagewidth is ~ 400-500 nm as shown inFig.1(a-d). The fact demonstrated that, an additionofHCl in thereaction process was crucial for pure hexagonal-MoO3formation43.Duringthesynthesis,acidicadditives(HCl)turnedout to be most effective in terms of rod formation andmorphologycontrol44.TheH+concentrationinfluencesarapidprecipitationand resulted the formationofH-MoO3. Inotherwords, the H+ is crucial to the formation of H-MoO3 rod likestructures44-46. Fig. 1(e-h) shows the microscopic images ofrGO-MoO3(GMR)rodsstructures,wheretheaveragewidthof100-125nmofMoO3rodswerehybridwiththerGO.Besides,it is to note that the nano spheres ofMoO3 (forMS sample)with theaveragediameterof~10-20nmwereobtainedwithaddition of ammonia in the reactant solvent medium duringsynthesis as shown in Fig. 1(i-l). Generally, the organicadditives during the synthesis play a crucial role both ininducingtheMoO3nucleianddirectingthecrystalgrowth

47,48.Theeffectofammoniumionsonthenucleationandgrowthiscritical in controlling the morphology of the MoO3nanostructures,moreover thecrystallizationof thehexagonalphasewithsphereshapetakesplacethroughtheself-assemblyofMoO6 with the assistance of NH4

+and H2O49-51. Finally the

purityandcrystallinityofsamplesisimprovedbytemperatureeffect.TherGO-MoO3nanospheres(GMS)haveadiameterof~10-20nmandconsistofnumeroussmallerspheres,formingaporousstructure,asshowninFig.2,wherelargeamountsofMoO3 nanoparticles are decorated on rGO. The GMS, Pt/C,GMR,MR, andMS catalysts showed Brunauer-Emmett-Teller(BET) surface areas of 158.185, 133.415, 28.788, 6.817, and6.415m2g−1,respectively,asshowninFig.2fandFig.S2.

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Fig.1FE-SEMimages(a,b);(e,f)and(i,j)oftheMR,GMR,andMSsamples,respectively;(c,d);(g,h)and(k,l)areTEMimagesoftheMR,GMR,andMSsamples,respectively.

TheXRDpatterns(Fig.3a)showthegoodcrystallinenature

of all the prepared samples, as evidenced by the sharp XRDpeaks, which can be indexed to the hexagonal MoO3 phaseJCPDS card (21-0569). In addition, theMR andGMR samplescontained mixed phases of hexagonal MoO3 and triclinicMo13O33,whichisingoodagreementwiththeJCPDScard(21-0569forH-MoO3and82-1930forMo13O33).XRDindicatedthat

for theGMRandGMShybrids, theMoO3particlesarepurelyformed on the graphene surface without the mixing ofimpurityparticles. In theGMRandGMShybridstructure, thepresence of MoO3micro rods andnanoparticles reduces theaggregationofgraphenesheets,whichresults in lessstackingof therGOsheets.Asa result,aweakgraphenerelatedpeakwasobservedintheXRDpatterns.

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Fig.2FE-SEMimages(a,b,c);(d,e)areTEMimagesand(f)representstheBETanalysisoftheGMShybrid.

Fig.3(a)XRDpatterns,(b)RamanspectraofMR,GMR,MS,andGMShybrid(insertedgraphrepresentstherGOpeaksfortheGMSandGMRsamples);(c),(d)and(e)areXPSMo3d,O1sandC1score-levelspectraoftheGMShybrid.

Fig. 3b shows theRamanspectraof thepreparedpristineand graphene-MoO3 nanocomposites of MR, MS, GMR andGMS.TheintensityoftheRamanpeaksvariesaccordingtothecrystal orientation and polarization of the laser source. TheirreduciblerepresentationofMoO3withthespacegroupD2h

16(Pbnm) is known as follows: Γ =8Ag+8B1g+4B2g+4B3g+4Au+3B1u+7B2u+7B3u, where Ag, B1g, B2g,andB3garetheRaman-activemodes40,52.Anarrowbandat~993cm−1istheresultofantisymmetricνMo=O1stretching(Ag),in which the bonding aligns along the b axis direction.Furthermore, theRamanpeakat~818cm−1,which isdue tothestretchingvibrationsfromthedoublycoordinatedbridgingoxygen in the symmetric νMo-O3-Mo stretching (Ag)with thebondingalongthea-axis,isthemostintenseRamanpeak.Thepeakat659cm−1andaweakpeatat466cm−1arefalloutsofthe asymmetric νMo-O2-Mo stretching (B2g) and bending (Ag)modes, respectively53. The Raman peaks at 377 cm−1 wereassignedto theB1gmodedueto theδO2=Mo=O2scissor,andthe 334 cm−1 peak is the characteristic of δO2=M=O2deformation (B1g mode). The broad band at 281 cm−1

corresponds to the wagging vibrations (δO1=Mo=O1 of B2g

mode).Thepeaksat~244,217,194cm−1canbeattributedtothe B3g, Ag, and B2g modes, respectively, due to theδO2=Mo=O2scissor.Thepeaksat~144,120,and111cm

−1arethe results from the deformation ofMoO3

54. In addition, thepeaksat~1610cm−1and1314cm−1correspondtotheGandDbandsof thegraphenesheets for theGMRandGMShybrids,respectively (inserted graph in Fig. 3b); the characteristicpeaksoftheMoO3particlesandgrapheneappearedtogetherintheGMRandGMSsamples,suggestingtheformationofthecomposites.

Inaddition,theelectronicstateandchemicalcompositionof theGMS samplewereexaminedbyXPS. TheXPS full scansurvey spectra revealed thepresenceofMo, C, andO in therGO-MoO3nanocomposite,asshowninFig.S3a.TheFig.3c-efocuses on the XPS core level signatures at the rGO-MoO3interface,with theevolutionof theMo3d,O1s,andC1s corelevels. The GMS hybrid sample features two main peaks at232.18 eV and 235.28 eV,which are related to theMo 3d3/2and 3d5/2 components (Fig. 3c)with an integrated peak arearatio of 3:2 and a binding energy (∆Mo 3d = 3.1 eV),corresponding to anoxidation stateofMo(VI) forMoO3, and

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are indicative of a predominant Mo6+ oxidation state40. TheO1score-levelspectrumoftheGMSsampleshowtwopeaksat530.14eVand532.11eVcorrespondingtoMo-OandMo-O-Cbond formations, respectively, as shown in Fig. 3d. The C1sXPSspectracouldbedeconvolutedintothreeGaussianpeakscentered at 284.08, 285.57, and 287.98 eV, which wereassignedtoC-C/C=C,C-O,C=O,andO=C-Ogroups,respectively(Fig. 3e). The XPS data strongly support the XRD and Ramanresults and confirm the presence ofMoO3on the surface ofrGOintheGMSnanocomposite.

ThechemicalpurityandthermalstabilityofMR,MS,GMRand GMS samples were evaluated by TGA under airatmosphere as shown in Fig. S4.When the samples heatedfrom ambient temperature to 800° C, a prololongeddecomposition starts at 200°C and extends upto 720°Cleadingtoatotalweightlossof8.37,9.27,18.74and19.99% for MR, MS, GMR and GMS samples respectively. Whichindicates the highly stable nature of the prepared samples.These TGA results are in good agreement with previouslyreportedresults55-59.AsshowninFig.S4,MRandMSsamplesshow a similar thermal decomposition trend under airconditionswithathenearbysameweightlossof~7wt%upto100-350° C, which was attributed to the removal of oxygencontainingfunctionalgroups.Usually,theweightlossesinthetemperature range of 150° C - 300° C and 300° C - 600° C

correspond to the removal of water and organic moietyrespectively 51, 60 . For GMR and GMS nanocomposites, anobviousmasslossappearedbetween230and600°C,owingtothe complete oxidation of graphene to carbon dioxide in airatmosphere. Then the curves showed thermal stabilityplateaus from600,until 720 °C followedby anobviousmassloss due to melting and then evaporation of MoO3 beyond720°C.

To assess the electrochemical oxygen reduction reaction(ORR) activity, the synthesizedMR,MS, GMR, GMS catalystsand a high quality commercial Pt/C (HiSPEC™ 4000, JohnsonMatthey, 53wt% Pt) catalysts with the same loading wereinitially loadedonto glassy carbon electrodes to examine thecyclic voltammetry (CV) behavior in a 0.1M KOH electrolytesaturated with either nitrogen (N2) or oxygen (O2) at apotentialscanrateof10mVS-1usingathree-electrodesystemuntil reproducible CVs were obtained. Compared to thefeatureless CV profile in the N2-saturated electrolyte (dottedred line in Fig. 4a), a strong reduction current peak wasobserved when the electrolyte was saturated with O2 (solidblack line inFig. 4a), indicating theelectrocatalyticactivityoftheas-synthesizedsamplestowardoxygencathodicreduction.Therotatingdiskelectrode(RDE)techniquewascarriedouttofurtherinvestigatetheelectrocatalyticORRbehaviorofallthesynthesizedcatalystsandhighpuritycommercialPt/Csamples.

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Fig. 4 (a)CVcurvesofMR,GMR,MS,andGMShybrid inanO2-saturated(solidblack line)orN2-saturated(dottedred line)0.1MKOH. (b)Rotating-diskvoltammograms of the GMS hybrid in an O2-saturated 0.1M KOH with a sweep rate of 10 mV s−1 at various rotation speeds. The insets in (b) showcorresponding Koutecky-Levich plots (J−1 versus ω−0.5) at different potentials. (c) ORR polarization curves of the MR,GMR, MS, GMS, and high qualitycommercialPt/CcatalystsinanO2-saturated0.1MKOHatroomtemperaturewithasweeprateof10mVs−1atarotationspeedof1600rpm.(e)TafelplotsofallthesamplesincludingcommercialPt/Cderivedbyamass-transportcorrectionofthecorrespondingRDEdata.

The polarization curves were obtained by scanning thepotentialsatascanrateof10mVs−1atdifferentrotationrates,asshowninFig.S5forMR,MSandGMRcatalysts,andinFig.4bforGMRcatalyst.ThelinearityoftheKoutecky-Levichplotsandthenearparallelismofthefittinglines(Fig.S5andinsetofFig. 4c) suggest first-order reaction kinetics toward theconcentrationofdissolvedoxygenandsimilarelectrontransfernumbersfortheORRatdifferentpotentials.

Fig.4dcomparesthepolarizationcurvesofMR,GMR,MS,GMS,andhighqualityPt/CcatalystinanO2-saturated0.1MKOHatroomtemperaturewithrotationat1600rpm.Remarkably,theORRonsetpotential was 0.819 V vs. RHE for the GMS hybrid, and morepositivethanthatofMR(0.738),MS(0.762V),andGMR(0.767V)catalystasshown inFig. S6a.Significantly, theMR,GMR,MS,andGMS catalysts, exhibited ORR activity with an electron transfernumberof3.1, 2.91,3.2, and3.3at0.2V vs.RHE, respectively asshown in Fig. S4 and Fig. 4c, suggesting a dominant reductionprocess. Compared to the prepared samples, the GMS hybridstructure showed enhanced ORR activity (Fig. 4d) because theaddition of graphene, small sized particles, morphology, andexposed active faces have significant impacts on the catalyticactivityintheGMShybrid.Inaddition,theenhancedactivityoftheGMShybrid is due to thehigher structureopenness in the tunnelstructure of hexagonal MoO3, which was not only coupled withgraphene, but also provide an effective highway for thetransportationofchargesforanactivecatalyticactivity41.Thiscouldprovide more active sites in the GMS hybrid. Similarly, grapheneprevents theagglomerationofMoO3nanospheres,which canalsoenhancetheelectrochemicalperformance.Moreover,theGMRandGMShybridsaffordedanORRcurrentdensityof1.26mAcm-2and2.27mAcm-2, respectively (Fig. 4d). Theperformanceof theGMShybridcatalystwasenhancedgreatlycomparedtoothercatalysts.In addition, the diffusion-limiting current of the GMSnanocomposite was close to that of a commercial Pt/C catalyst(onsetpotential~0.915V)and thehalf-wavepotentialwas~114mVlowerthanthatofPt/C.

On the other hand, theGMS hybrid catalyst has a highercurrent density (2.27 mA cm-2) that that obtained using thecommercialPt/Ccatalyst(1.98mAcm-2)seeFig.4d,whichcanbeduetothehigherelectronacceptingcapabilityoftheORRactivesitesandthelargesurfaceareaoftheGMShybrid.Theexcellent ORR activity of the GMS hybrid catalyst was alsoobservedfromthemuchsmallerTafelslopeof56mV/decadeatlowover-potentials(Fig.4e)thanthosemeasuredwithMR(110mV/decade),GMR(76mV/decade),MS(89mV/decade),and high quality commercial Pt/C (61 mV/decade) in the O2saturated0.1MKOHelectrolyte.

During durability testing (Fig. 5a), the GMS hybridexhibited superior long termdurability compared to thePt/Ccatalyst inO2 saturated 0.1MKOHwith less decay (∇15% -

29%)oftheORRactivitythanthatofthePt/Ccatalyst(∇35%-48%) over 10,000 - 20,000 s of continuous operation with arotationof1600rpmat0.65Vvs.RHE(Fig.5a).AlthoughtheGMR showed slightly higher activity, it suffered a 39%decrease in current density over 20,000 s of continuousoperationinO2saturated0.1MKOH.Inparticular, inalkalineelectrolytes (for alkaline fuel cells), the Pt catalyst degradesgradually over time due to surface oxides, aggregation, andparticle dissolution. Long term durability is one of the mostimportant properties for potential ORR active catalysts aselectrocatalysts since theymust operate for a long time in aharsh environment61. In this regard, the durability of GMR,GMS,andhighqualitycommercialPt/CwastestedagainusingaglassycarbonelectrodebycontinuouslyperformingtheORRat0.65Vvs.RHE inanO2saturated0.1MKOHsolution.Theresults were compared with those obtained for commercialPt/Ccatalystunderthesameenvironment,asshowninFig.6b.These results suggest that the GMS hybrid structure has alarger long term durability than GMR and high qualitycommercialPt/Ccatalysts.

In previous reports, the long-term stability of other ORRcatalysts, such as Ag, Au, Pd, and bi-metallic nanocrystals, inalkaline solutions are improved relative to Pt, but they stillsufferfromdeactivationandarebelowthetargetsforenergystoragedeviceapplications42,61.Poorcatalystdurabilityisoneof the major challenges for alkaline fuel cells. Hence, theexcellentstabilityofthisGMShybridmakesitfavorablefortheORR and further important catalytic kinetic reactions inalkalinesolutions.

Typically,carbonmonoxide (CO)moleculesareoneof themain poisons of fuel cell catalysts because of their strongcoordination toactivemetal surfaces,whichreduces therateof the catalytic reaction. Importantly, the present hybridexhibits superior CO tolerance to Pt/C catalyst due to theinjectionofa3.0MmethanolsolutioninthemiddleoftheRDEatarotationof1600rpm.AsshowninFig.5c,arapiddecreasein the normalized currentwas observed for the Pt/C catalystimmediately after the methanol injection. In contrast, therewas almost no change in the GMS and GMR catalyst, whichindicates the excellent CO tolerance of the graphene-MoO3hybrids fabricated in thisstudy.ThemorepositiveORRonsetpotential, the more positive half-wave potential, and thesmallerTafelslopemeasuredbyCV,RDE,longtermdurability,and CO tolerance indicates the excellent electrocatalyticperformance of the GMS hybrid structures fabricated in thisstudy. The enhanced ORR activity of the GMS hybrid mayresult from the small particle size, high surface area, higherstructureopennessinthetunnelstructureofhexagonalMoO3,andthethermalreductionofGOduringthesynthesisprocess.ThisisaresultofthehigherelectricalconductivityoftherGO,

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which can lead to better ORR activity of the resultingnanocomposites due to the effective charge transfer fromMoO3torGO

25,30,62.ToexaminetheOERelectrocatalyticactivity,CVandlinear

sweep voltammetry (LSV) were performed with the samecatalystsloadedontheglassycarbonelectrodesandarotatingdisk electrode in an N2-saturated 0.1 M KOH. For a direct

comparison,theCVcurvesoftheGMShybrid,GMR,MR,andMSweremeasuredtoinvestigatetheircatalyticactivityfortheOER under the same conditions, as shown in Fig. S3b. Fromthis CV curves, it shows that GMS has the lowest onsetpotentialcomparedtotheothercatalysts.Further,toexamine

Fig.5Durabilitytestbythechronoamperometricresponses(percentageofcurrentretainedversusoperationtime/I–ttest)oftheMR,GMR,MS,GMS,andhigh-qualitycommercialPt/Ccatalystskeptat0.65VversusRHEinanO2-saturated0.1MKOHelectrolyte(a)at1600rpmand(b)ChronoamperometricresponsesofGMR,GMS,andahigh-qualitycommercialPt/Ccatalystsonglassycarbonkeptat0.65Vvs.RHE.(c)RDEmeasurementsat1600rpmontheCOtoleranceeffect.

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Fig.6(a)OxygenevolutioncurrentsofMR,GMR,MS,andGMShybridmeasuredinanN2-saturated0.1MKOHwithasweeprateof10mVs−1.(b)TafelplotsoftheOERcurrentsin(a).Durabilitytestbythechronoamperometricresponses(percentageofcurrentretainedversusoperationtime/I–ttest)oftheMR,GMR,MS,andGMScatalystskeptat1.55VversusRHEina0.1MKOHelectrolytewithasweeprateof10mVs−1 in(c)glassycarbonelectrodeandthedigitalimagesin(c)showthegradualaccumulationofevolvedO2bubblesontheglassycarbonelectrodeat10,40,and60minutesduringtheI-ttest.(d)RDE(withrotationof1600rpm)andthedigitalimagesin(d)showthattherewasnoO2bubbleaccumulationontherotatingdiskelectrodeat10(initial)and90(final)minutesduringtheI–ttest.

theonsetoftheelectrocatalyticOER,theLSVsofthecatalystswereperformedatasweeprateof10mVs−1,asshowninFig.6a.AsshowninFig.6atheLSVcurves,asmall(~20microamppercm2)catalyticcurrentwasobservedatapproximately1.23Vvs.RHE (standardpotential forwater splitting/OER) for theGMS.Intheanodicscan,theGMSelectrodeexhibitedalowerOERonsetpotentialandhigherOERpeakcurrent(0.12mAcm-

2 at an over potential of ~ 0.54 V as shown in Fig. 6a) thanthoseoftheothersynthesizedcatalysts.ThesmallTafelslopewasaslowas47mV/decade,whichiscomparabletothebestperformance ever reported using other OER catalysts, asshowninFig.6bandTableS1.Asanefficientoxygenevolutionanode, GMS showed an onset potential of 1.768 V, which ismuch smaller than those ofMR (~ 2.095 V),MS (~ 1.923 V),andGMR(~1.901V),asshowninFig.S6b.

Ontheglassycarbonelectrode,theGMScatalystsuffereda huge current drop in the durability test (Fig. 6c); the GMScatalyst lost as much as ∼38% of its activity after 5000 soperation. The rapid decay in current ismainly due to theobstructionofsomeactivesitesonthecatalystbythegradualaccumulationofevolvedoxygen(O2)bubbles,asevidencedbythe digital images of the electrode taken at 10, 40, and 60minutes during the test, which clearly show that O2 bubblesaccumulated gradually on the catalyst surface. Consequently,allcatalystsexhibitedpronouncedcurrentdecay,whichcouldbe caused by the accumulation of gas bubbles that partiallyblocktheactivesitesoftheelectrode.Theotherreasonfortheactivitydrop is thatbubble formationon the catalyst surfacemayleadtopeel-offanddelaminationofthecatalystfromtheelectrode.

To avoidbubble formationon the electrodes, a durabilitytestwasperformedusing theRDEwithahighrotationspeed(1600 rpm). The RDE durability test results (Fig. 6d) alsoconfirmthatthelongtermstabilityofthecatalystisimprovedgreatly(only∼26%activitylossafter6000soperation)ontheRDEandthedigital imagesshowthattherewasalmostnoO2bubble accumulation on the electrode. This is due to theevolved O2 bubbles being removed from the catalyst andmaintains a clean catalyst surface, thereby providing amorereliable way to examine the stability of the OER catalysts.These results make this GMS hybrid material the foremost bi-functional catalyst for both oxygen reduction and evolution. Thestrong interaction between the Mo oxide species andgraphene/rGO during the formation of rGO-MoO3 is crucial forimproving the OER activity63. The present GMS catalystoutperformed previously reported catalysts with smaller over-potentials for both ORR and OER, indicating that it is one of thehighest performance non-precious metal-based bi- functionalcatalysts; Table S1 provides a detailed comparison. The excellent

catalyticactivitiesoftheGMSnanocompositeforboththeORRandOERmaybeattributedto threemain factors.The first is thewell-dispersed rGO/MoO3 nanospheres with a heterogeneousnanocomposite structure (Fig. 2). The second is the high specificsurfaceareaofthenanocomposite,whichcanprovidemoreactivesitesforbothORRandOERcatalysis64-67.Thirdly,theformationofatunnel structure of MoO3 results in effective electron-holeseparation and affords large special locations for cation insertionandextraction.4.Conclusions

Afacilehydrothermalroutehasbeendevelopedtopreparepristine MoO3 rods, nanospheres, and their hybrids withreduced graphene oxide. While the MoO3 rods and MoO3spheres alone have lower catalytic activity for the OxygenReduction Reaction (ORR) and Oxygen Evolution Reaction(ORR), their graphene hybrid materials exhibit, surprisinglyhighORRactivitiesinalkalinesolutions,whicharecomparableto high quality commercial Pt/C catalysts and exceed theperformance of Pt/C in terms of stability, durability, and COtolerance.GraphenecoupledwithMoO3enablesthecompleteuse of the catalyst surface area by minimizing theagglomeration/restacking of graphene sheets, which greatlyreducestheaccessiblesurfaceareaofthecatalyst.Inaddition,theenhancedactivityof theGMShybridcanbeattributedtothe higher structural openness in the tunnel structure ofhexagonal MoO3 when it is coupled with rGO. This workpresents a highly promising catalyst for alkaline fuel cells forwhich there has been a recent resurgence in interest as asolutiontoelectrolytecarbonation.Moreimportantly,thelowcost of the syntheticmethod combinedwith their promisingbifunctional catalytic activity, stability, durability, and COtolerance effect makes graphene-MoO3 a new class ofelectrocatalystforthenextgenerationfuelcells(bothORRandwater splitting/OER). The GMS catalyst offers newopportunities for the development ofORR andOER catalystswith carbon and non-precious metal-based materials, and isbelieved to be a promising candidate for advanced catalystsforenergyconversion.Acknowledgements

This research was supported by Basic Science ResearchProgram through theNational Research Foundation of Korea(NRF) funded by the Ministry of Education(2013R1A1A2A10004468).

Notesandreferences ‡ Footnotes relating to the main text should appear here.Thesemightincludecommentsrelevanttobutnotcentraltothe

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matterunderdiscussion,limitedexperimentalandspectraldata,andcrystallographicdata.

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