Ndoped carbon nanomaterials are durable catalysts for oxygen

RESEARCHARTICLE

ENERGYRESOURCES

2015?TheAuthors,somerightsreserved;N-dopedcarbonnanomaterialsaredurable

exclusivelicenseeAmericanAssociationfortheAdvancementofScience.DistributedcatalystsforoxygenreductionreactioninacidicunderaCreativeCommonsAttributionNon-CommercialLicense4.0(CCBY-NC).fuelcells

10.1126/sciadv.1400129

JianglanShui,MinWang,FengDu,LimingDai*

Theavailabilityoflow-cost,efficient,anddurablecatalystsforoxygenreductionreaction(ORR)isaprerequisiteforcommercializationofthefuelcelltechnology.Alongwithintensiveresearcheffortsofmorethanhalfacenturyindevelopingnonpreciousmetalcatalysts(NPMCs)toreplacetheexpensiveandscarceplatinum-basedcatalysts,anewclassofcarbon-based,low-cost,metal-freeORRcatalystswasdemonstratedtoshowsuperiorORRperformancetocommercialplatinumcatalysts,particularlyinalkalineelectrolytes.However,theirlarge-scalepracticalapplicationinmorepopularacidicpolymerelectrolytemembrane(PEM)fuelcellsremainedelusivebecausetheyareoftenfoundtobelesseffectiveinacidicelectrolytes,andnoattempthasbeenmadeforasinglePEMcelltest.Wedemonstratedthatrationallydesigned,metal-free,nitrogen-dopedcarbonnanotubesandtheirgraphenecompositesexhibitedsig-nificantlybetterlong-termoperationalstabilitiesandcomparablegravimetricpowerdensitieswithrespecttothebestNPMCinacidicPEMcells.Thisworkrepresentsamajorbreakthroughinremovingthebottleneckstotranslatelow-cost,metal-free,carbon-basedORRcatalyststocommercialreality,andopensavenuesforcleanenergygenerationfromaffordableanddurablefuelcells.

INTRODUCTION

AlongwithintensiveresearcheffortsofmorethanhalfacenturyinThemolecularoxygenreductionreaction(ORR)isimportanttomanydevelopingnonpreciousmetalORRcatalysts,anewclassofmetal-freefields,suchasenergyconversion(forexample,fuelcells,metal-airbat-ORRcatalystsbasedoncarbonnanomaterialshasbeendiscovered(1)teries,andsolarcells),corrosion,andbiology(1,2).Forfuelcellstogen-andattractedworldwideattention(15–33),which,asalternativeORRerateelectricitybyelectrochemicallyreducingoxygenandoxidizingfuelcatalysts,couldmarkedlyreducethecostandincreasetheefficiencyofintowater,cathodicoxygenreductionplaysanessentialroleinproducingfuelcells.Inparticular,itwasfoundthatverticallyalignednitrogen-dopedelectricityandisakeylimitingfactoronthefuelcellperformance(3–5).carbonnanotubeToconstructfuelcellsofpracticalsignificance,efficientcatalystsare?(VA-NCNT)arrayscanactasametal-freeelectrodetocatalyzea4eORRprocesswithathreetimeshigherelectrocatalyticrequiredtopromotetheORRatcathode(6–8).Traditionally,platinumactivityandbetterlong-termstabilitythancommerciallyavailablehasbeenregardedasthebestcatalystforfuelcells,althoughitstillsuffersplatinum/Celectrodes(forexample,C2-20,20%platinumonVulcanfrommultipledrawbacks,includingitssusceptibilitytotime-dependentXC-72R;E-TEK)inanalkalineelectrochemicalcell(1).Thesecarbon-driftandMeOHcrossoverandCOpoisoningeffects(4,9).However,basedmetal-freeORRcatalystsarealsofreefromtheCOpoisoningandthelarge-scalepracticalapplicationoffuelcellscannotberealizedifthemethanolcrossovereffects.

expensiveplatinum-basedelectrocatalystsforORRcannotbereplacedQuantummechanicscalculationswithB3LYPhybriddensityfunc-byotherefficient,low-cost,anddurableelectrodes.

tionaltheoryandsubsequentexperimentalobservationsindicatedthatCobaltphthalocyaninewasreportedastheORRelectrocatalystinal-thecarbonatomsadjacenttonitrogendopantsintheNCNTstructurekalineelectrolytesin1964(5,10).Sincethen,thesearchfornonprecioushadasubstantiallyhighpositivechargedensitytocounterbalancethemetalcatalysts(NPMCs)withtransitionmetal/nitrogen/carbon(M-Nx/C,strongelectronicaffinityofthenitrogenatom(1).Aredoxcyclingpro-typicallyx=2or4,M=Co,Fe,Ni,Mn)complexcatalyticsitesaslow-cessreducedthecarbonatomsthatnaturallyexistinanoxidizedform,costalternativestoPtforelectrochemicalreductionofoxygeninfuelcellsfollowedbyreoxidationofthereducedcarbonatomstotheirpreferredhasattractedlong-terminterest.AlthoughtremendousprogresshasbeenoxidizedstateuponOmadeandafewrecentlyreportedNPMCsshowelectrocatalyticper-2absorptionandreduction,leadingtoareducedORRpotential.Furthermore,theNdoping–inducedchargetransferformancecomparabletothatofPt(11–14),mostoftheNPMCsarestillfromadjacentcarbonatomscouldchangethechemisorptionmodetooexpensiveand/orfarawayfromsatisfactionindurabilityforpracticalofOapplications.ThoseNPMCsofhighcatalyticactivitiesoftenexhibitfast2fromanusualend-onadsorption(Paulingmodel)atthepureCNTsurfacetoaside-onadsorption(Yeagermodel)ofOdecayundersomewhatchallengingtesting/operationconditions,suchastoeffectivelyweakentheO-Obondingfor2ontotheNCNTelectrodeefficientataconstantvoltageof0.5Vwithpureoxygenascathodefuel(11,13),andORR(1).Hence,dopingcarbonnanomaterialswithheteroatomsasrelativelygooddurabilityhasonlybeenobservedunderlessefficientintheNCNTelectrodescouldefficientlycreatethemetal-freeactiveworkingconditionswitharelativelylowpotential(forexample,0.4V)sitesforelectrochemicalreductionofO2.

and/ordilutedoxygen(air)asthecathodefuel(14).

Recentworldwideresearchactivities(15–33)inthisexcitingfieldhavenotonlyconfirmedtheabovefindingsbutalsofurtherprovedthatCenterofAdvancedScienceandEngineeringforCarbon(Case4Carbon),Departmentthedoping-inducedchargetransferhaslargeimpactonthedesign/ofMacromolecularScienceEngineering,CaseSchoolofEngineering,CaseWesterndevelopmentofnewmetal-freeelectrocatalyticmaterials,includingvar-ReserveUniversity,10900EuclidAvenue,Cleveland,OH44106,USA.iousheteroatom-dopedCNTs(1,19),graphene(16,20,21),andgraph-*Correspondingauthor:E-mail:liming.dai@case.edu

ite(22–28)forfuelcellandmanyotherapplications(29–31).High

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electrocatalyticactivitycomparableorevensuperiortocommercialPt/CetchingoffthepurifiedVA-NCNTarrayfromtheSiwafersubstrateinelectrodesandexcellenttolerancetoMeOHcrossoverandCOpoi-aqueoushydrogenfluoride[10weightpercent(wt%)],rinsingitcopiouslysoningeffectshavebeendemonstratedformanyofthecarbon-basedwithdeionizedwater,transferringitontoagasdiffusionlayer[GDL;Car-metal-freeORRcatalystsinelectrochemicalhalf-cellswithalkalineelec-bonMicro-porousLayer(CMPL),ElectroChemInc.],anddrop-coatingtrolytes.Nevertheless,thelarge-scaleapplicationsofthecarbon-basedwithasulfonatedtetrafluoroethylene-basedionomer“Nafion”(DuPont)metal-freeORRcatalystsinpracticalfuelcellscannotberealizediftheyasbinderandelectrolyte,whichwasthenassembledwithaPt/C-coateddonothaveanadequatelong-termdurabilityandhighORRperformanceGDLastheanodeandanintermediatelayerofproton-conductiveinacidicpolymerelectrolytemembrane(PEM)fuelcells,whichcurrentlymembrane(NafionN211,DuPont)astheseparator(seetheSupple-serveasthemainstreamfuelcelltechnologyofgreatpotentialforlarge-mentaryMaterialsfordetailedpreparationandfig.S4fortheMEAscaleapplicationsinbothtransportandstationarysystems(9).Asyet,cross-sectionimages).AscanbeseeninFig.1(AtoC)andfig.S4,thehowever,theperformanceevaluation(forexample,electrocatalyticac-NCNTORRcatalystwithintheMEAthusproducedlargelyretaineditstivityandlong-termoperationalstability)ofcarbon-basedmetal-freeverticalalignment.

ORRcatalystsinactualPEMfuelcellshasbeenlargelyignored.ThisisTheresultingMEAcontainingtheVA-NCNTmetal-freeORRelec-presumablybecausecarbon-basedmetal-freeORRcatalystsareoftentrocatalystswasevaluatedinanacidicPEMfuelcelloperatingwiththefoundtobelesseffectiveinacidicelectrolyteswithrespecttoalkalineNafionelectrolyteandpureH2/O2gases.Tostartwith,thePEMfuelcellmedia,anditisquitechallengingtomakethemsufficientlyeffectivewasactivatedafter100scanningcyclesfromopencircuitpotentialforsingle-celltestinginacidicPEMfuelcells.

(OCV)to~0.1V(Fig.1D).Tooursurprise,aconsistentpolarizationper-Inspiteofthis,wedemonstratedherethatboththeVA-NCNTarrayformancewasobservedformorethan5100scanningcycles,indicat-andarationallydesignednitrogen-dopedgraphene/CNTcompositeingastableelectrocatalyticperformanceeveninacidundertheharsh(N-G-CNT)asthecathodecatalystsinacidicPEMfuelcellsexhibitedworkingcondition.Thus,N-Ccentersinthecarbon-basedmetal-freecat-remarkablyhighgravimetriccurrentdensitycomparabletothemostalystsseemtobemorestablethanthetransitionmetalactivesitesinactiveNPMCs.Becausecarbonismuchmoreanti-corrosivetoacidsNPMCsinPEMfuelcells(34,35).Therelativelypoorpolarizationperform-thanmosttransitionmetals,theVA-NCNTarrayandN-G-CNTcom-anceseeninFig.1Dforthefirst10cyclesis,mostprobably,duetotheweakpositefurthershowedasignificantlydurableperformance,evenwithelectrode-electrolyteinteractionontheas-preparedhydrophobic

pureH2/O2gases,inacidicPEMfuelcells,outperformingtheirNPMCcounterparts.Therefore,carbon-basedmetal-freecata-lystsholdgreatpotentialaslow-cost,effi-cient,anddurableORRcatalyststoreplacePtinpracticalPEMfuelcells.

RESULTS

VA-NCNTarrayshavebeenpreviouslyreportedtoshowexcellentORRperform-ance(1),evensuperiortothecommerciallyavailablePt/Celectrodes,inelectrochem-icalhalf-cellswithalkalineelectrolytes,asalsoconfirmedbytheVA-NCNTsusedinthisstudy(figs.S1toS3).TocarryouttheperformanceevaluationofVA-NCNTsinPEMfuelcells,wemadetheVA-NCNTarrays(80mminheight,?2asurfacepackingdensityof0.16mgcm)intoamembraneelectrodeassembly(MEA)atthehighestal-lowablecatalystloadingof0.16mgcm?2.Figure1schematicallyshowsproceduresfortheMEApreparation(Fig.1A),alongwithatypicalscanningelectronmicroscopic(SEM)imageofthestartingVA-NCNTar-ray(Fig.1B)andaphotographicimageofthenewlydevelopedMEA(Fig.1C),whereasFig.1.FabricationofMEAofVA-NCNTarraysanditsperformanceinaPEMfuelcell.(A)SchematictheMEAfabricationdetailsaregiveninthedrawingsforthefabricationofMEAfromVA-NCNTarrays(0.16mgcm?2)andtheelectrochemicaloxidationSupplementaryMaterials.Briefly,wefirsttoremoveresidueFe.C.E.,counterelectrode;R.E.,referenceelectrode;W.E.,workingelectrode.(B)TypicalperformedtheelectrochemicaloxidationSEMimageoftheVA-NCNTarray.(C)DigitalphotoimageoftheusedMEAafterdurabilitytestwiththecross-inHsectionSEMimagesshownintheinserts.(D)Polarizationcurvesasthefunctionofthearealcurrentdensity2SO4toremoveFeresidue,ifany,intheVA-NCNTsmadefrompyrolysisofafteraccelerateddegradationbyrepeatedlyscanningthecellfromOCVto0.1Vattherateof10mAs?1.(E)Polarizationandpowerdensityasthefunctionofthegravimetriccurrentdensity.Cathodecatalystloadingiron(II)phthalocyanine(1),followedby

0.16mgcm?2,Nafion/VA-NCNT=1/1.H2/O2:80°C,100%relativehumidity,2-barbackpressure.

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VA-NCNTelectrode,whichbecamehydrophilicuponelectrochemicalEmmett-Teller)surfacearea,1270m2g?1;KetjenblackEC-600JD]andactivationduringthesubsequentpolarizationcycles(36).FortheVA-NCNT375mgofNafionsolution(5%)in1.5mlofdeionizedwaterandiso-MEA,significantlyhighgravimetriccurrentdensitieswereobserved:35Ag?1propanolmixture(volumeratio=1:2).Thereafter,theinkwassonicatedat0.8V,145Ag?1at0.6V,and1550Ag?1at0.2V(Fig.1E).Ascanalsofor10minandstirredovernight,thenpaintedontoa5-cm2GDLasthebeseeninFig.1E,thepeakpowerdensitywas320Wg?1forourVA-NCNTcathodeelectrode,andassembledintoaMEAwithaPt/C-coatedGDLMEA,outperformingorcomparabletoeventhemostactiveNPMCcat-astheanodeandanintermediatelayerofproton-conductivemembranealysts(Table1)(11).

(NafionN211,DuPont)astheseparatorforsubsequenttesting(fig.S6).AsabuildingblockforCNTs,thetwo-dimensional(2D)singleSeveralsynergisticeffectscanarisefromtheabovefabricationprocesstoatomiccarbonsheetofgraphenewithalargesurfaceareaandpeculiarmaximizetheutilizationofcatalystsitesintheN-G-CNTcomposite:(i)electronicpropertiesisanattractivecandidateforpotentialusesinN-GcanpreventN-CNTsfromtheformationofthebundlestructuremanyareaswhereCNTshavebeenexploited.Thus,heteroatom-dopedtofacilitatethedispersionofN-CNTsbyanchoringindividualN-graphenehasquicklyemergedasanotherclassofinterestingcarbo-CNTsonthegraphenesheetsviathestrongp-pstackinginteractionnaceousmetal-freeORRcatalysts(16)soonafterthediscoveryof(fig.S5,AtoD);(ii)N-CNTscanalsoeffectivelypreventtheN-GsheetselectrocatalyticactivityofVA-NCNTs(1).SuperiortoCNTs,theone-fromrestackingbydispersingCNTsonthegraphenebasalplanetoatom-thickgraphenesheetshaveallconstituentcarbonatomsatthemakemorerigidcurvedN-G-CNTsheetsthantheN-Gsheets(fig.S5,surfacetoenhancethesurfaceareaanda2DplanargeometrytofurtherCtoF);and(iii)theadditionofcarbonblack(Ketjenblack)cannotfacilitateelectrontransport(37),andhenceveryeffectiveelectrocat-onlyfurtherseparateN-G-CNTsheetsinthecatalystlayerbutalsoin-alysis.Althoughgraphenesheetswithalargesurfaceareaandexcel-ducecontinuedporousmultichannelpathwaysbetweentheN-G-CNTlentchargetransportpropertiesareidealelectrocatalyticmaterialsforsheetsforefficientO2diffusion(Fig.2).Acomparisonoffig.S6FwithORRafterdopingwithappropriateheteroatoms(forexample,B,S,N,fig.S6Cindicatesthattheintroductionofcarbonblackparticlesledtoand/orP)(16,20,38),muchofthegraphenesurfaceareaandtheas-aporousnetworkstructurefortheN-G-CNT/KBcatalystlayer,facili-sociatedcatalystsitesarelostbecauseofrestackingviathestrongp-ptatingtheO2diffusion(seealso,Fig.2,AtoD).BETmeasurementsoninteractionifthegraphenesheetsarenotphysicallyseparatedtopreservetheelectrodesshowedthata5-cm2porouscathodeN-G-CNT/KB@GDLthehighsurfaceareaintrinsicallyassociatedwithindividualgraphenehasasurfaceareaof155m2g?1(or1161m2g?1aftertakingofftheweightsheets.Alongwithothers(37,39),wehaveprepared3Dgraphene-CNTofGDLandNafion)andasignificantnumberofporesfrommicro-toself-assemblies(dopedwithorwithoutheteroatoms)oflargesurface/macrosizes(Fig.2,EandF).Incontrast,adensecathodeN-G-CNT@GDLinterfaceareasandwell-definedporousnetworkstructuresaselectrodewithoutinterspersedcarbonblackparticleshasasurfaceareaaslowmaterialswithfastiondiffusionandefficientelectrontransportforen-as16cm2g?1withnegligibleporevolume.Thepresenceofporesinergyconversionandstorage(31,40–42),includingmetal-freeORRcat-Fig.2(CandD)couldfacilitatethemasstransferofO2gasintheporousalysts(43).

N-G-CNT/KBcatalystlayer(Fig.2G)withrespecttothedenselypackedBecauseexcellentORRperformance,particularlyinalkalinemedia,N-G-CNTsheets(Fig.2,AandB)withouttheintercalatedcarbonblackhasalsobeendemonstratedforgraphene-basedmetal-freeORRcata-(Fig.2H).

lysts(17,43),itishighlydesirabletoalsoevaluatetheirperformanceinBeforethesingle-cellperformanceevaluation,wecarriedoutthero-actualPEMfuelcellsinacidicmedia.Forthispurpose,wefirstpreparedtatingdiscelectrode(RDE)androtatingring-discelectrode(RRDE)metal-freegrapheneoxide(GO)suspensionbythemodifiedHummers’testsforthenewlydevelopedN-G-CNTmetal-freecatalystinamethod(31),whichwasthenmixedwithoxidizedCNTsuspension,three-electrodeelectrochemicalcell.Figure3AreproducestypicalcyclicpreparedfromcommerciallyavailablenonalignedmultiwalledCNTsvoltammetric(CV)curvesoftheN-G-CNT,showingalargecathodic(BaytubesC150HP,BayerMaterialScience)afterpurificationtore-peakat0.8VinO2-saturated0.1MKOHsolution,butnotN2-saturatedmovemetalresidues,toproducemetal-freeporousN-dopedgrapheneelectrolyte.TheonsetpotentialoftheN-G-CNTisashighas1.08V,andCNTcomposites(N-G-CNT)throughfreeze-drying,followedbynearly80mVhigherthanthatofPt/C(Fig.3B).Half-wavepotentialannealingat800°CinNHoftheN-G-CNTis0.87V,30mVhigherthanthatofPt/C.Therefore,detailsandfig.S5).3for3hours(seetheSupplementaryMate-rialsforTheN-G-CNT–basedcatalystinkforMEAstheN-G-CNTshowsexcellentelectrocatalyticperformancein0.1Mwasthenpreparedbymixing2.5mgofN-G-CNTcatalystwith10mgKOH,evenbetterthanthecommercialPt/Celectrode(C2-20,20%ofcarbonblackparticles[primaryparticleradius,34nm;BET(Brunauer-platinumonVulcanXC-72R;E-TEK),viaaone-step4e?ORRprocess

Table1.Thegravimetricactivitiesofvarioustransitionmetal–derivedNPMCscomparedwiththemetal-freeVA-NCNTandN-G-CNT+KBinPEMfuelcells.Allthedatainthetablehavealsobeenscaledbytheelectrodesurfacearea.Materials

Currentat0.8V

Currentat0.2V

Peakpowerdensity

Catalystloading-H2back(Ag?1)

(Ag?1)

(Wg?1)

(mgcm?2O2)

pressure(bars)

Reference

FeCo/N/C1570020021.0(14)Fe/N/C8/100800/2500233/4003.9/0.90.5(11)Fe/N/C153258041.3(45)VA-NCNT3515503200.161.5ThisworkN-G-CNT+KB

30

1500

300

0.5

1.5

Thiswork

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Fig.2.MorphologicalfeaturesoftheN-G-CNTelectrodeswithandwithoutvolumedistributions(F)ofapieceof5-cm2GDL,GDLwithKB(2mgcm?2theadditionofKetjenblack.(AtoD)Cross-sectionSEMimagesof(AandB)?2),GDLwithN-G-CNT(0.5mgcm),andGDLwithN-G-CNT/KB(0.5/2mgcm?2)thedenselypackedcatalystlayerofN-G-CNT/Nafion(0.5/0.5mgcm?2)andasindicatedinthefigures.(GandH)SchematicdrawingsoftheMEAcatalyst(CandD)theporouscatalystlayerofN-G-CNT/KB/Nafion(0.5/2/2.5mgcm?2).layercrosssection,showingthatO2efficientlydiffusedthroughthecarbonPurplearrowsin(D)indicatetheparallellyseparatedN-G-CNTsheetswithin-blackseparatedN-G-CNTsheets(G)butnotthedenselypackedN-G-CNTterdispersedporousKBagglomerates.(EandF)BETsurfaceareas(E)andpore

sheets(H).

(fig.S7)withabetterstabilityaswellasahighertolerancetoMeOHcrossoverandCOpoisoningeffectsthanthePtcatalyst(fig.S8).Asfarasweareaware,thesere-sultsarethehighestrecordsformetal-freegrapheneandCNTORRcatalysts.Asex-pected,theN-G-CNTcompositealsoexhib-itedmuchbetterORRperformancethanthatofN-CNTandN-Gcatalystsinboththealkaline(Fig.3C)andacidicmedia(Fig.3D)becauseofitsuniquefoam-like3Darchitectureformedinthethincom-positelayerontheRDEelectrodeevenwith-outtheadditionofcarbonblackintheabsenceofmechanicalcompression(fig.S9,videinfra)because3Dcarbonnetworkshavebeenpreviouslydemonstratedtofa-cilitateelectrocatalyticactivities(31,43).MoredetailedORRperformanceoftheN-G-CNTinacidicmediawithrespecttoFe/N/CandPt/Ccanbefoundinfig.S10.TheaboveresultsindicatethatN-G-CNTholdsgreatpotentialforoxygenre-ductioninpracticalfuelcells.Therefore,wefurthercarriedouttheperformanceeval-Fig.3.Electrocatalyticactivitiesofthecarbon-basedmetal-freecatalystsinhalf-celltests.(A)CVsoftheuationonMEAsbasedontheN-G-CNTN-G-CNTinO2-orN2-saturated0.1MKOH.(B)Linearsweepvoltammetry(LSV)curvesoftheN-G-CNTcom-ina5-cm2PEMfuelcellwithpureHparedwithPt/C(20%)electrocatalystbyRRDEinO2-saturated0.1MKOHsolutionatascanrateof10mVs?1asfuelgasesat80°C.andarotationspeedof1600rpm.(CandD)LSVcurvesoftheN-GandN-CNTcomparedwiththeN-G-CNTin?2Atatypicalcatalyst2/O2loadingof2mgcm(11–14,44),thecellO2-saturated0.1MKOH(C)and0.1MHClO4(D).

limitingcurrentwasaslowas700mAcm?2,

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althoughthecellOCVreached0.97V(fig.S11A).Wefoundthattheadditionofcar-bonblack(KB,2mgcm?2)intotheN-G-CNTcatalystlayerintheMEAcaused~85%improvementonthedeliveredcur-rentdensityatalowvoltagerange(<0.4V),althoughKBitselfhadnegligibleelectro-catalyticactivity(fig.S11A).Theaboveob-servedenhancementinthecurrentoutputcanbeattributedtotheKB-inducedporousnetworkformationtoenhancetheO(Fig.2,DandG,andfig.S6F)2dif-fusionbe-causetheporosityseeninfig.S9Ffortheas-castN-G-CNTsingleelectrodehasbeensignificantlyreducedwithinthecorre-spondingMEA(fig.S6C)preparedundermechanicalpressing(seetheSupplemen-taryMaterialsfortheMEApreparation).Theimprovedelectrocatalyticperform-ancewasalsosupportedbythereducedcellimpedancefortheN-G-CNT+KBwithrespecttoitsN-G-CNTcounterpart(fig.S11B).

ThecellperformancesattheN-G-CNTloading?of20.5and2mgcm?2plus

KB(2mgcm)arecomparable(Fig.4A),indicatingamarkedactivitysuppressionatFig.4.PoweranddurabilityperformanceofN-G-CNTwiththe?2additionofKBinPEMfuelcells.(A)highcatalystloadingevenwithcarbonPolarizationcurvesofN-G-CNTwithloadings:2,0.5,or0.15mgcmplusKB(2mgcm?2the)foreachcathode.blackdispersing.Whenthecatalystload-Theweightratioof(N-G-CNT/KB)/Nafion=1/1.(B)Cellpolarizationandpowerdensityasthefunctionofingwasfurtherreducedto0.15mgcm?2,gravimetriccurrentfortheN-G-CNT/KB(0.5/2mgcm?2)withtheweightratioof(N-G-CNT/KB)/Nafion=1/1.(C)Durabilityofthemetal-freeN-G-CNTinaPEMfuelcellmeasuredat0.5VcomparedwithaFe/N/Ccat-however,thecatalyticsitesinthecathodealyst(seetheSupplementaryMaterialsforpreparationdetails).CatalystloadingofN-G-CNT/KB(0.5mgcm?2)

werenotsufficienttosupportanormalandFe/N/C(0.5and2mgcm?2).Testcondition:H2/O2:80°C,100%relativehumidity,2-barbackpressure.polarizationcurve.Figure4Bshowsthegravimetricpolarizationandpowerden-sitycurvesfortheN-G-CNTinthepresenceofcarbonblack(N-G-theN-G-CNT+KBcatalystatbothlowandhighloadings(Fig.4CCNT/KB/Nafion=0.5:2:2.5mgcm?2),fromwhichacurrentof30Ag?1andfig.S14).at0.8V,alimitingcurrentof2000Ag?1at0.1V,andapeakpower

densityof300Wg?1wereobtained.Althoughmetal-freecatalystsusuallyexhibitedalowercatalyticactivitythandidNPMCsinRDEmea-DISCUSSIONsurements(45),theobservedgravimetricactivityoftheN-G-CNT+KBThefastperformancedropatthefirst20hoursfortheFe/N/Ccatalystiscomparabletohigh-performanceFe(Co)/N/Ccatalysts(Table1andwastypicalforNPMCs(5,11,45,46)becauseofdetrimentaleffectsoffig.S12,AtoC),attributabletothefullutilizationofcatalyticsitesintheacidicandstrongreductionenvironmentsonthemetalactivecen-therationallydesignedN-G-CNT+KBcatalystlayerwiththeenhancedtersatthePEMfuelcellcathode(34).BecausetheN-G-CNT+KBcat-multichannelO2pathways(Fig.2,DandG,andfig.S6F).The3Dmulti-alyticsitesarefreefrommetalnanoparticle(fig.S15),nosignificantchannelporousstructure,togetherwiththeuniquematerialshybrid-acidiccorrosionisenvisionedforthecarbonelectrodebecausecarbonization,makesthePEMfuelcellbasedontheN-G-CNT+KBcathodeismuchmoreanti-corrosivetoacidsthanmosttransitionmetals.There-toshowamuchbettercellperformancethandoitscounterpartswithfore,theobservedexcellentstabilitiesforbothN-G-CNT+KBandVA-thecathodemadefromeitheroftheconstituentcomponents(thatis,N-G+NCNTcathodesinPEMfuelcellsshouldbeanimportantintrinsic

KBandN-CNT+KB,respectively)(fig.S13).

characterforthecarbon-basedmetal-freecatalysts,facilitatingthemFinally,theN-G-CNT+KBwasfurthersubjectedtothedurabilityforalargevarietyofpracticalapplications.Theseresultsshowgreatpo-testintheacidicPEMfuelcellsataconstantvoltageof0.5Vwithpuretentialforcarbon-basedmetal-freecatalyststobeusedaslow-cost,H2/O2asfuelgases(Fig.4C)incomparisonwiththeFe/N/CNPMCefficient,anddurableORRcatalystsinpracticalPEMfuelcells.Further-(seetheSupplementaryMaterialsforpreparation).LikeVA-NCNT,more,theVA-NCNTandN-G-CNT+KBcatalystsusedinthisstudytheN-G-CNT+KBexhibitedanexcellentstabilitywitharelativelysharedsimilarfeaturesinthatN-dopedcarbonnanomaterialsweresmallcurrentdecay(~20Tcayover100hours;Fig.4C).Incontrast,usedforthehighORRelectrocatalyticactivities,andthattheporoustheFe/N/Ccatalystshowedaninitialsharpcurrentdecaywithatotalstructureswithalargesurfaceareawererationallydesignedforofabout75Tcayover100hoursatboththehigh(2mgcm?2)andenhancedelectrolyte/reactantdiffusion.Themethodologydevelopedlowloadings(0.5mgcm?2).Excellentdurabilitieswereobservedforherecanberegardedasageneralapproachforthedevelopmentof

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alargevarietyofhigh-performance,low-cost,metal-freecatalystsforvariouspracticalenergydevices,particularlyinPEMfuelcells.

MATERIALSANDMETHODS

VA-NCNTwassynthesizedbypyrolysisofiron(II)phthalocyanineac-cordingtoourpreviouslypublishedprocedures(1).N-G-CNTcompositewassynthesizedbysequentiallycombiningamodifiedHummers’methodfortheGOfabrication(31),freeze-dryingamixtureofGOandoxidizedCNT,followedbyannealingat800°CinNHbefoundintheSupplementary3for3hours.ThepreparationdetailscanMaterials.ThetransitionmetalFe-derivedcontrolsample(Fe/N/C)wassynthesizedaccordingtoliteratures(11,46).Specifically,100mgofzeoliticimidazo-lateframeworks(ZIF8),togetherwith10mgoftris(1,10-phenanthroline)iron(II)perchlorateion,wasball-milledfor1hourandheatedinArat1000°Cfor1hourandthenat900°CunderNH3for15min.

TheelectrochemicalperformancesoftheaboveORRcatalystswerecharacterizedthrough(i)half-celltestsin0.1MKOHor0.1MHClO4electrolytesbyanRDEmethodand(ii)single-celltestswitha5-cm2MEAandpureHpressure.2/ODetailed2asfuelsat80°C,100%relativehumidity,and2-barbackelectrodefabricationandtestprocessesaredescribedintheSupplementaryMaterials.ThemorphologyandcompositioncharacterizationofthematerialsarealsogivenintheSup-plementaryMaterials.

SUPPLEMENTARYMATERIALS

Supplementarymaterialforthisarticleisavailableathttp://advances.sciencemag.org/cgi/content/full/1/1/e1400129/DC1

Fig.S1.CharacterizationofVA-NCNTs.

Fig.S2.ElectrocatalyticactivitiesoftheVA-NCNTcatalystinalkalineelectrolyte(O2-saturated0.1MKOH)byhalf-celltests.

Fig.S3.ElectrocatalyticactivitiesoftheVA-NCNTcatalystinacidicelectrolyte(O2-saturated0.1MHClO4)byhalf-celltests.

Fig.S4.Typicalcross-sectionSEMimagesoftheGDLwiththeMEAofVA-NCNTsasthecath-odecatalystlayer,Nafionmembrane(N211)astheseparator,andPt/Castheanode.Fig.S5.SEM(A)andTEM(B)imagesofN-CNTbundles.

Fig.S6.Typicalcross-sectionSEMimagesoftheGDLswiththeMEAsof(AtoC)N-G-CNT(2mgcm?2)and(DtoF)N-G-CNT+KB(0.5+2mgcm?2)asthecathodecatalystlayers,respectively.

Fig.S7.Tafelplot(A)andelectrontransfernumber(B)fortheN-G-CNTandPt/C(20%)asthefunctionofelectrodepotentialbyRRDEinoxygen-saturated0.1MKOHsolutionatascanspeedof5mVs?1andarotationspeedof1600rpm.

Fig.S8.Long-timestabilityandtolerancetomethanol/carbonmonoxideofmetal-freecatalystN-G-CNT.

Fig.S9.SEMimagesofcatalystlayercrosssectionsusedinRDEmeasurements.

Fig.S10.Electrocatalyticactivitiesofthecarbon-basedmetal-freeN-G-CNTcatalystsinacidicelectrolyte(O2-saturated0.1MHClO4)byhalf-celltests.

Fig.S11.Optimizationofcathodecatalystlayercomposition.

Fig.S12.Single-cellperformancecomparisonbetweenN-G-CNTandFe/N/Ccatalystsatthesamecatalystlayercomposition:catalyst(0.5mgcm?2)/KB(2mgcm?2)/Nafion(2.5mgcm?2).Fig.S13.PolarizationcurvesoftheN-G-CNTandindividualcomponentsofN-GorN-CNT.Fig.S14.Durabilityofthecatalystlayercomposedofmetal-freeN-G-CNT(2mgcm?2)+KB(2mgcm?2)inaPEMfuelcellmeasuredat0.5V.Fig.S15.Themetal-freecharacterofN-G-CNTcatalyst.

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Submitted20November2014Accepted24January2015Published27February201510.1126/sciadv.1400129

Citation:Shuietal.,N-dopedcarbonnanomaterialsaredurablecatalystsforoxygenreductionreactioninacidicfuelcells.Sci.Adv.1,e1400129(2015).

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Downloaded from http://advances.sciencemag.org/ on January 28, 2016N-doped carbon nanomaterials are durable catalysts foroxygen reduction reaction in acidic fuel cells

Jianglan Shui, Min Wang, Feng Du and Liming Dai (February 27,2015)

Sci Adv 2015, 1:.

doi: 10.1126/sciadv.1400129

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