J. Phys. Chem. B 2005, 109, 14410-14415-(meida)

14410J.Phys.Chem.B2005,109,14410-14415

Platinum/CarbonNanotubeNanocompositeSynthesizedinSupercriticalFluidasElectrocatalystsforLow-TemperatureFuelCells

YueheLin*andXiaoliCui

PacificNorthwestNationalLaboratory,902BattelleBouleVard,P.O.Box999,Richland,Washington99352

CliveYenandChienM.Wai*

DepartmentofChemistry,UniVersityofIdaho,Moscow,Idaho83844ReceiVed:March21,2005;InFinalForm:May9,2005

Carbonnanotube(CNT)-supportedPtnanoparticlecatalystshavebeensynthesizedinsupercriticalcarbondioxide(scCO2)usingplatinum(II)acetylacetonateasmetalprecursor.Thestructureofthecatalystshasbeencharacterizedwithtransmissionelectronmicrograph(TEM)andX-rayphotoelectronspectroscopy(XPS).TEMimagesshowthattheplatinumparticles’sizeisintherangeof5-10nm.XPSanalysisindicatesthepresenceofzero-valenceplatinum.ThePt-CNTexhibitedhighcatalyticactivitybothformethanoloxidationandoxygenreductionreaction.Thehighercatalyticactivityhasbeenattributedtothelargesurfaceareaofcarbonnanotubesandthedecreaseintheoverpotentialformethanoloxidationandoxygenreductionreaction.CyclicvoltammetricmeasurementsatdifferentscanratesshowedthattheoxygenreductionreactionatthePt-CNTelectrodeisadiffusion-controlledprocess.AnalysisoftheelectrodekineticsusingTafelplotsuggeststhatPt-CNTfromscCO2providesastrongelectrocatalyticactivityforoxygenreductionreaction.Forthemethanoloxidationreaction,ahighratioofforwardanodicpeakcurrenttoreverseanodicpeakcurrentwasobservedatroomtemperature,whichimpliesgoodoxidationofmethanoltocarbondioxideonthePt-CNTelectrode.ThisworkdemonstratesthatPt-CNTnanocompositessynthesizedinsupercriticalcarbondioxideareeffectiveelectrocatalystsforlow-temperaturefuelcells.

Introduction

Directmethanolfuelcells(DMFCs)areconsideredtobeoneofthemostpromisingoptionsforaddressingfutureenergyneeds.1DMFCsprovideacleanandmobilepowersourcewithhigh-energyconversionefficiencyandlowpollutantemissions.ThebasicprincipleemployedinDMFCsinvolvesmethanoloxidationandoxygenreductionoverpreciousmetalcatalysts,suchasplatinumandplatinum-rutheniumalloydispersedoveracarbonsupport.TherearesomeobstaclesinhibitingtheapplicationsofDMFCs;oneofthemainproblemsisthelowcatalyticactivityofelectrodesbothforoxygenreductionreaction(ORR)andformethanoloxidationreaction.Boththecarbonsupportandthedispersionofcatalystsaretwomainconcernstoenhancethecatalyticactivity.

Carbonblack(VulcanXC-72,aregisteredtradenamefromCABOT)hasbeenthemostwidelyusedsupportforpreparingfuelcellcatalystbecauseofitsgoodcompromisebetweenelectronicconductivityandsurfacearea.2However,toincreaseorimproveactivity,anewcarbonsupportwithahighsurfaceareamayprovidebetterutilizationoftheelectrocatalysts.Othercarbontypessuchascarbontubulemembranes,3orderedporouscarbon,4graphitenanofibers,5filmsofC60clusters,6ahardcarbonspherules,7andcarbonnanotubes(CNT)havebeenusedassupports8forDMFCs.Inparticular,CNThaveattractedspecialattentionbecauseoftheirexcellentcatalyticandelectronicpropertiesandextensiveapplicationsinmanyareas,forexample,

*Authorstowhomcorrespondenceshouldbeaddressed.Tel:509-376-0529;fax:509-376-5106;e-mail:yuehe.lin@pnl.gov(Y.L.).Tel:208-885-6787;fax:(208)885-6173;e-mail:cwai@uidaho.edu(C.M.W.). Permanentaddress:DepartmentofMaterialsScience,FudanUnivesity,Shanghai,200433,China.E-mail:xiaolicui@.inchemicalsensors,9forhydrogenstorage,edCNTasaplatinumsupportforprotonexchangemembranefuelcellsasawaytoreducethecostoffuelcellsthroughincreasedutilizationofplatinum.8dItisalsoreportedthatCNTwith12wt%Ptdepositioncangive10%highervoltagethancarbonblackwith29%Ptdepositioninpolymerelectrolytefuelcells.8cThus,CNTshowgreatpotentialforuseindesigningelectrodesforminiaturefuelcells.

Itiswell-knownthattheparticleshapeandsize,aswellasdispersion,ofPt-basedcatalystsarekeyfactorsthatdeterminetheirORRactivityandcellperformanceforDMFCs.Severalapproaches,includingimpregnationandchemicalreduc-tion2a,2c,8b,8e,15andelectrodeposition,6,8d,8g,8hhavebeendevelopedtoloadPtorothermetalcatalystparticlesonthesurfaceofsupports.PlatinumnanocatalystssupportedonVulcanXC-72carbonhavebeensynthesizedthroughthereductionofchloro-platinicacidwithformicacid,usingsurfactanttetraoctylam-moniumbromideasthestabilizerinthesolventtetrahydrofuran.2cThroughchemicalreductioninH2at580°C,Cheetal.wereabletofilltheverynarrowsizePt-RualloyparticlesinCNTmembranes.3Liuetal.8eloadedPtnanoparticlesontheCNTbyanelectrolessplatingmethodthroughtwo-stepsensitizationandactivationprocesses.However,conventionalpreparationtechniquesbasedonwetimpregnationandchemicalreduction

10.1021/jp0514675CCC:$30.25©2005AmericanChemicalSociety

PublishedonWeb07/12/2005

Platinum/CarbonNanotubeNanocomposite

ofthemetalprecursorsoftendonotprovideadequatecontrolofparticleshapeandsize.Inaddition,theseprocedurescanbetime-consumingandlabor-intensive.Consequently,continuingeffortsareunderwaytodevelopalternativesynthesismethodstogeneratecolloidsandclustersonthenanoscalewithgreateruniformity.Electrodepositionofplatinumparticleshastheadvantageofhigh-puritydepositsandasimpledepositionprocedure.6,8d,8g,8hOneof+theproblems,however,isthelikelyconcurrentreductionofHduringtheelectrodepositionprocess.Theloadingmassofthemetalliccatalystisnoteasytoestimateaccordingtothedepositioncharge.ThecurrentefficiencyforPtdepositionisnotequalto100%.6eThesynthesisofnanoscaleparticleswithgooddispersionoverthecarbonaselectrocatalyticmaterialsstillremainsachallenge.

Inrecentyears,theuseofsupercriticalfluids(SCFs)forthesynthesisandprocessingofnanomaterialshasproventobearapid,direct,andcleanapproachtodevelopnanomaterialsandnanocomposites.11c,16Inarecentpaper,platinum/carbonaerogelnanocompositesweresynthesizedusingasupercriticaldeposi-tionmethod.17SCFsareidealsolventstosynthesizeandprocessmanytypesofnanomaterials,includingnanoparticales,nano-crystals,nanotubes,nanowires,andnanocomposites.Theap-plicationofsupercriticalfluidtechnologycanresultinproducts(andprocesses)thatarecleaner,lessexpensive,andofhigherqualitythanthosethatareproducedusingconventionaltech-nologiesandsolvents.Throughhydrogenreductionofmetal- -diketonecomplexesinsupercriticalcarbondioxide(scCO2),CNTcanbedecoratedbymetalnanoparticlessuchasPd16g,16jandRh16fwithuniformitytoachievenanocomposites.11c,16hInthispaper,platinumnanoparticlesweredecoratedonCNTsurfacesinscCO2andwerecharacterizedbytransmissionelectronmicrograph(TEM)andX-rayphotoelectronspectro-scopy(XPS).ThePt-CNTpowderwasloadedontheglassycarbon(GC)electrodethroughacastingprocess,andtheelectrocatalyticactivityformethanoloxidationandoxygenreductionwasinvestigatedatroomtemperatureusingcyclicvoltammetry(CV)andchronoamperometry(CA).

ExperimentalSection

Reagents.Multiwalledcarbonnanotubes(>95%purity,diameter20～50nm,length1～5µm)werepurchasedfromNanoLab,Inc.(Newton,MA).Nafion-perfluorinatedion-exchangeresin(5wt%solution)waspurchasedfromAldrich,andH2SO4waspurchasedfromFischerChemicals.Ultrapurewater(～18.3M cm)wasusedtopreparethesolutions.Platinumprecursor,platinum(II)acetylacetonate,Pt(acac)2,97%,purchasedfromAldrich,wasusedasreceived.High-purityhydrogen,carbondioxide,nitrogen,andoxygengaswereusedinallexperiments.Apureoxygenflowwasintroducedintothesolutiontosupplyoxygenfortheoxygenreductionexperimentsandwaspassedoverthetopofthesolution.Allmeasurementswereconductedatroomtemperature.

DecoratingPlatinumNanoparticlesonCarbonNanotubes.ThesupercriticalfluidreactionsystemforthedepositionofPtnanoparticleonCNTwasdescribedinapreviousreport.16fThePtnanoparticlesweresynthesizedusingthefollowingproce-dures.TheCNT(20mg)andthemetalprecursorPt(acac)2(50mg)withasmallamountofmethanolasamodifierwereloadedintoahigh-pressurereactioncelllocatedinanovenheatedto200°C.CO2gaswasintroducedintothereactioncellandpressurizedto80bartoproduceasupercriticalfluid.H2gasat10barwasinitiallyintheH2+CO2mixercellandwasthenaddedtotheCO2gasof120bar.After1h,whentheprecursorwascompletelydissolvedinthescCO2,theH2+CO2gaswasintroducedintothereactioncellbypressurizingitto160bar.

J.Phys.Chem.B,Vol.109,No.30,200514411

ThereductionofthePt2+toPt0wasfast,occurringwithinonly～powder15min.wasAfterrecovered,thereactionwashedcellwithwasmethanol,depressurized,andsonicatedPt-CNTfivetimes.

ElectrodePreparationandModification.A0.5wt%Nafionsolutionwaspreparedbydilutingthe5wt%Nafionsolutionwithwater.Catalystpowderwasdispersedultrasoni-callyinthe0.5wt%Nafionsolutiontoobtainahomogeneousblacksuspensionsolutionwith1mg/mLPt-CNT,anda5-µLaliquotofthissolutionwaspipettedontothesurfaceofa3-mm-diameterglassycarbon(BAS,WestLafayette,IN)electrode.Beforethesurfacemodification,theGCelectrodewaspolishedwith0.3-µmand0.05-µmaluminaslurries,washedwithwaterandacetone,andthensubjectedtoultrasonicagitationfor1mininultrapurewateranddriedunderanairstream.Thecoatingwasdriedatroomtemperatureintheairfor1h.Themodifiedelectrodesurfacewasthenwashedcarefullywithultrapurewaterbeforemeasurement.

Apparatus.TheTEMimagesofthedecoratedCNTweretakenusingaJEOLJEM2010microscopeequippedwithanOx-fordISISsystem.Theoperatingvoltageonthemicroscopewas200keV.Allimagesweredigitallyrecordedwithaslow-scanCCDcamera(imagesize1024×1024pixels),andimagepro-cessingwascarriedoutusingaDigitalMicrograph(Gatan).ToobtainTEMimages,theas-synthesizedplatinum-modifiedCNTpowderwasdispersedinethanolsolutionunderultrasonicagita-tionfor1minandthenwasdepositedonacopper-carbongrid.FortheXPSanalysis,aKratosAXIS165multitechniqueelectronspectrometerwasusedtoconfirmthepresenceofzero-valenceplatinum.TheloadingofPtismeasuredbyenergy-dispersiveX-rayspectroscopy.TheinstrumentationisLEOSUPRA35VP(FESEN).

CyclicvoltammetricandchronoamperometryexperimentswereperformedwithaCHI660electrochemicalworkstation(CHInstrumentsInc,Austin,TX).Allexperimentswereconductedinaconventionalthree-electrodesystematroomtemperature.TheworkingelectrodewasGCcoatedwithPt-CNTcompositefilms.AAg/AgCl(saturatedbyKClsolution)referenceelectrodewasusedforallelectrochemicalmeasure-ments,andallthepotentialswerereportedversusthisreferenceelectrode.Aplatinumwirewasusedasacounterelectrode.Toobtainreproducibleandreliableresults,afreshmethanolsolutionwasusedineverymeasurement.

ResultsandDiscussion

CharacterizationofCarbonNanotubesDecoratedwithPlatinumNanoparticles.Figure1showsthetypicalTEMimageoftheCNTdecoratedwithplatinumnanoparticlessynthesizedinscCO2.Nanoparticlesofplatinumwithasizeof5～10nmcanbeobservedclearlyonthesurfaceofCNTwithrelativeuniformity.TheloadingplatinumonthesurfaceofCNTwas25%,whichwasestimatedbyenergy-dispersiveX-rayspectroscopy.

ThechemicalcompositionoftheplatinumnanoparticlesdepositedontofunctionalizedCNTwasalsoanalyzedbyXPS.Figure2displaysatypicalXPSspectrumofCNTcoatedwithplatinumnanoparticles.ThebindingenergyforametallicPt4f7/2peakis71.2eV,accordingtotheliterature.18OursamplehasaPt4f7/2peakof71.1eV,whichisveryclosetothatvalue.Wealsofindasmallshoulderpeaknear72-73eV,whichmaycorrespondtotheplatinumoxides.ThespectrashowninFigure2indicatethatmostoftheplatinumparticlesinourcatalystarezero-valence.

CharacterizationofPlatinumfromCyclicVoltammetry.CyclicvoltammogrammsforPt-CNTelectrodesatdifferent

14412J.Phys.Chem.B,Vol.109,No.30,2005Figure1.AtypicalTEMimageofCNTdecoratedbyplatinumnanoparticlessynthesizedinsupercriticalcarbondioxide.

Figure2.XPSspectraofCNTdecoratedwithplatinumnanoparticles.

scanrangesin0.5MH2SO4solutionsaturatedbynitrogenarepresentedinFigure3.Finestructuresofhydrogenabsorption/desorptionpeaksclearlyappeared(Figure3A).Areductionpeakcenteredat0.45Vcanbeobservedduringthenegative-goingpotentialsweep(Figure3B).Thisreductionpeakcanbeattributedtothereductionofplatinumoxide.ThisfeatureofthecurveisconsistentwiththoseofthecyclicvoltammogrammcurvesforPtelectrodes.19Thus,itmayalsobeconcludedthattheplatinumnanoparticleshaveaverycleanactivesurface.Thisisfurtherevidenceofthepresenceofplatinumontheelectrodesurface.

ElectrocatalyticActivityofOxygenReductionReaction.TheORRisespeciallyimportantforrealizinghighlyefficientfuelcells,batteries,andmanyotherelectrodeapplications.Platinumandplatinumalloyparticlesonavarietyofcarbonsupportsarethemostwidelyusedandefficientcatalystsforthefuelcellcathode.13,20

FortheoxygenreductionexperimentswiththePt-CNTelectrode,asolutionof0.1MH2SO4waspurgedwithultrapureoxygenfor～15min.Thesolutionbecamecompletelysaturatedwithoxygen.Theelectrodewasscannedoverapotentialrangefrom0.7Vto0V,involvingfivecyclesatdifferentscanrates

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al.

Figure3.Cyclicvoltammogramin0.5MH2SO4solutionsaturatedbyN2forPt-CNTelectrodeat20mV/s.(A)shortpotentialrangebetween-0.2and0.6Vand(B)longpotentialrangebetween-0.22and1.3V.

Figure4.TypicalcyclicvoltammogramsofPt-CNTelectrodeforoxygenreductionreactionin0.1MH2SO4saturatedwithoxygenat20mV/s.Alargerreductioncurrentcanbeobservedinthefirstcycle.Thecyclicvoltammogramsatotherscanrateshaveasimilarphenom-enon.

toensurereproducibility.Figure4showstypicalcyclicvolta-mmogramsfortheORRatPt-CNTelectrodesinasolutionof0.1MH2SO4saturatedbyoxygen.Itisinterestingthatthefirstcyclealwaysinvolvesalargerreductioncurrentatallthescan

Platinum/CarbonNanotubeNanocompositeFigure5.CyclicvoltammogramsatafreshPt-CNTelectrodeforoxygenreductionreactionatvariousscanrates,0.01,0.02,0.04,0.06,0.08,and0.10V/s,frominsidetooutside.Theelectrolytewas0.1MH2SO4saturatedwithoxygen.Thescaninvolvedfivecyclesateachscanrate,andthefifthcycle(lastcycle)isshownhere.Thedependenceofpeakcurrentonscanratesisshownintheinset.

ratesandbecomesstablefromthesecondcycle.OtherelectrodessuchasPd-CNTonglassycarbonsurfacealsoexperiencethisphenomenon,whichmayberelatedtotheabsorptionofoxygenontheelectrodesurfaceandtothereductionofplatinumoxidewhichisformedwhentheworkingelectrodeispolarizedtothestartingpotential.Moreexperimentalstudiesarenecessarytogivesuitableexplanationsforthisphenomenon.

Thecathodiccurrentflowingduringthereductionofoxygenshouldalsocontainthecathodiccurrentofthereductionofplatinumoxide.Itisimpossibletoseparatethesetwocontribu-tionsaccurately.However,thecathodiclimitingcurrentsofoxygenreductionunderpresentconditionsweremuchlargerthanthecurrentofplatinumoxidereduction,whichhasbeenconfirmedfromthecathodiccurrentinnitrogen-saturatedsolution.Thus,itisreasonabletoconcludethatthemajorcontributionofthecathodiccurrentinoxygen-saturatedsolutionresultedfromoxygenreduction.

Figure5showsthecyclicvoltammogramsofthePt-CNTelectrodeatdifferentscanrates.Thescanwasperformedwithsuccessivefivecyclestoobtainthestableresponseandthelastcycle(fifth)isshowninFigure5.ThepeakcurrentincreaseslinearlywiththesquarerootofthescanratesasshownintheFigure5inset.ThisfactindicatesthattheORRprocessonPt-CNTiscontrolledbythediffusionofoxygentotheelectrodesurface.SimilarresultswereobservedonaglassycarbonelectrodemodifiedbyPd-CNTnanocomposite14andahybridthinfilmcontainingplatinumnanoparticlesand[tetrakis(N-methylpyridyl)porphyrinato]cobaltmodifiedCNTonaglassycarbonelectrodesurfacefabricatedbyDongandcolleagues.13ATafelplotwasrecordedfrom1.0Vto0Vin0.1MH2SO4solutionsaturatedbyoxygen.AplotoflogiversuspotentialforafreshPt-CNTelectrodeisshowninFigure6.Theexchangecurrentdensitycanbeobtainedbyextrapolatingthelinearregiontozerooverpotential.WefindtheTafelslopeforPt/CNTtobe-21mV/decadeinthepotentialrangefrom0.63to0.76V.TheresultingTafelslopeandexchangecurrentdensityarelistedinTable1.ATafelslopeof-38mV/decadeatOTE/SWCNT/PtwasreportedbyKamatetal.8hTheexchangecurrentdensityonPt-CNTelectrodeisabout1orderlargerthanthatofcommercialPt/Ceventhoughtheloadingofcatalystsismuchlessinoursystem.12aThedifferencebetweenGC/Pt-CNTandOTE/SWCNT/Ptmaybecausedbythe

J.Phys.Chem.B,Vol.109,No.30,2005

14413

Figure6.TafelpolarizationcurveforPt-CNTelectrodein0.1MH2SO4solutionsaturatedbyoxygenat1mV/s.

differentloadingofplatinum,differentcarbonnanotubes,ordifferentmethodsofpreparingcatalystsandelectrodes.

ElectrocatalyticActivityforMethanolOxidationReaction.Cyclicvoltammetryisavaluableandconvenienttoolforstudyingmethanoloxidationcatalysts.TheelectrocatalyticactivityformethanoloxidationofPt-CNTpreparedinscCO2wascharacterizedbycyclicvoltammetryinanelectrolyteof1MH2SO4and2MCH3OHat50mV/s,andtheresultingvoltammogramsareshowninFigure7.Thecurrentfrommethanoloxidationbecomesapparentasthepotentialrisesabove0.35V.Intheforwardscan,methanoloxidationproducedaprominentsymmetricanodicpeakaround0.70V.Inthereversescan,ananodicpeakappearedataround0.53V.Thisanodicpeakinthereversescancouldbeattributedtotheremovaloftheincompletelyoxidizedcarbonaceousspeciesformedintheforwardscan.2aThereactionmechanismofelectrooxidationmethanolonthesurfaceofplatinumisacomplexonewhichinvolvesmanycarbonaceousspeciesasintermediates.Thereareseveraldifferentversionsofthereactionmechanism.However,itisgenerallyagreedthatthemostabundantsurfaceintermediateischemisorbedcarbonmonoxide.20cThisfeatureofthecyclicvoltammetriccurveisinagreementwiththereportsforPt/Ccatalysts.2a,2cTheratiooftheforwardanodicpeakcurrent(If)tothereverseanodicpeakcurrent(Ib)canbeusedtodescribethecatalysttolerancetocarbonaceousspeciesaccumulation.2a,2cAhighIf/IbvalueimpliesgoodoxidationofmethanoltoCO2.Inourexperiments,theratiowasestimatedtobe1.4forthePt-CNTelectrodefromthefirstcycleand1.6and1.3forthesecondandthirdcycles.SuchahighvalueindicatesthatmostoftheintermediatecarbonaceousspecieswereoxidizedtoCO2intheforwardscan.Forcomparison,theratio0.87wasreportedwithananosizedPtonXC-72synthesizedbyamicrowave-assistedpolyolprocess.2aTheseexperimentalresultshighlightedthehighactivityformethanoloxidationofPt-CNTpreparedfromscCO2.ItalsoimpliesthatthemajordeficiencyofallPtcatalysts,thatis,theaccumulationofintermediatecarbonaceousspeciesonthecatalysts’surfaceleadingto“catalystpoisoning”,canbepartlyovercomeusingthisnovelPt-CNTindesigningfuelcellelectrodes.ThehighactivitymaybearesultofthehighsurfaceofCNTandthenanostructureofplatinumparticles.Theoxidativecurrentincreaseswithrepetitivescansintheinitialperiodandreachesaplateauafterthreecycles(Figure7).Incontrast,thepeakcurrentinthereversescanincreaseswiththe

14414J.Phys.Chem.B,Vol.109,No.30,2005

TABLE1:ElectrochemicalParametersforOxygenReductionReactionatPt-CNTElectrodes

electrodes

Pt/CNT(25%Pt/CNT,0.07mg/cm2)

Pt/C/Nafion(commercial,20%Pt/C,0.4mg/cm2)OTE/SWCNT/Pt

slope/mV-21-38

exchangecurrent/A/cm2

8.9e-71.09e-76.3e-4

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refsthiswork12a8h

cycle,whichreflectstheaccumulationofintermediatecarbon-aceousspeciesonthecatalysts’surface.

TheeffectofpotentialscanlimitonthereverseanodicpeakcurrentisshowninFigure8.ThisfeatureisconsistentwiththatreportedforPt/C.2aAsshowninFigure8,thereverseanodicpeakcurrentdecreaseswithincreasingtheanodiclimitintheforwardscan.ThisbehavioralsoindicatesthatthereverseanodicpeakcurrentisprimarilyassociatedwithresidualcarbonspeciesonthesurfaceofthePt-CNTelectrode.ItisreasonablethattheIf/Ibratioincreaseswiththeanodiclimit.

Chronoamperometriccurvesweremeasuredatdifferentpotentials0.3(a),0.6(b),and0.8V(c)atthePt-CNTelectrodefor10min,asshowninFigure9.Forthisexperiment,thepotentialwassteppedfromtheopen-circuitpotential(～0.3V)to0.8V.After2s,thepotentialwassteppedto0.3Vfor2s,thensteppedtothedesiredpotential,andthecurrent-timecurve

Figure7.Cyclicvoltammogramsofroom-temperaturemethanoloxidationonPt-CNTelectrodecycledbetweenpotentials0Vto1.0VvsAg/AgClat50mV/sin1MH2SO4,2MCH3OH.Resultsforcycles1-5(fromdownsidetoupside)correspondtosuccessivescansshowingthestabilizationofthecurrentpeak.

Figure8.Cyclicvoltammogramsofroom-temperaturemethanoloxidationonPt-CNTelectrodeat50mV/sin1MH2SO4,2MCH3OHfordifferentforwardpotentialscanlimits.

wasrecorded.Asshown,thelargestcurrentwasobservedat0.6V,andthecurrentdecaywithtimewasobserved.Thisresultisinagreementwithitsbehaviorinthecyclicvoltammogram.DMFCsBasedonElectrocatalystsfromScCO2.TheuseofscCO2canthusprovideanenvironmentallysoundalternativetootherconventionalsolvents.ScCO2hasbeenusedinmanyareas,includingmaterialcleaning,naturalproductextraction,chemicalreactions,samplepreparation,andenvironmentalremediation.ItisalsopossibletopreparecatalystsinSCFsappliedtotheactualminiaturizedfuelcells.

TheresultsaboveshowthatPt-CNTsynthesizedinscCO2haveahighactivitybothformethanoloxidationandoxygenreduction.Theexperimentalconditionsdiffersignificantlyfromthoseusedinanactualfuelcell.Theexperimentsinthisstudywerecarriedoutatroomtemperature,whilecommercialDMFCsareoperatedat40-100°C.ThecatalystloadingsontheelectrodesurfacedonotmatchthosetypicallyusedforDMFCs,whichresultedinthesmallercurrentdensity.However,thehighIf/IbvalueformethanoloxidationindicatedtheadvantagesofthecatalystspreparedinscCO2andtheuseofcarbonnanotubes.Furthermore,manymetalprecursorscanbeusedasstartingmaterialsforthenanoparticleproductioninscCO2.AbetterelectrocatalyticperformanceofbimetallicalloyPt-RuonthesurfaceofCNTsynthesizedinscCO2isexpectedforthemethanoloxidationreaction.Moredetailedstudiesareunderwayinthislaboratory,andtheresultswillbereportedinduecourse.

Figure9.Current-timecurvesat0.3,0.6,and0.8VforthePt-CNTelectrodein1MH2SO4,2MCH3OH.

Conclusions

Usingasupercriticalfluidtechnique,wehavesuccessfullydepositedplatinumnanoparticlesoncarbonnanotubesurfaces.Thisapproachprovidesanewwaytodevelopcatalystswithnanostructureanduniformity.Platinumdepositedoncarbonnanotubesinsupercriticalfluidshasbeenshowntopossessahighercatalyticactivity,bothformethanoloxidationandforoxygenreductionreaction.Thehighercatalyticactivityhasbeenattributedtothelargersurfaceareaofcarbonnanotubesandthedecreaseintheoverpotentialformethanoloxidationandoxygenreductionreaction.TheresultspresentedinthispaperdemonstratedtheuseofscCO2tobeanefficientwaytoprepare

Platinum/CarbonNanotubeNanocomposite

electrocatalystsandthefeasibilityofusingcarbonnanotubebasedelectrocatalystsforthedevelopmentoflow-temperaturefuelcells.

Acknowledgment.ThisworkissupportedbyalaboratorydirectedresearchanddevelopmentprogramatPacificNorthwestNationalLaboratory(PNNL)andagrantfromtheElectricityInnovationInstituteandtheElectricPowerResearchInstitute(E2-P261/C8273).TheresearchdescribedinthispaperwasperformedpartiallyattheEnvironmentalMolecularSciencesLaboratory(EMSL),anationalscientificuserfacilitysponsoredbytheU.S.DepartmentofEnergy’sOfficeofBiologicalandEnvironmentalResearchandlocatedatPNNL.PNNLisoperatedbyBattellefortheU.S.DepartmentofEnergyunderContractDE-AC05-76RL01830.TheauthorswouldliketothankMs.SueGano(PNNL)foreditingthemanuscript.ReferencesandNotes

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