JApplElectrochem(2014)44；RESEARCHARTICLE；Enhancedelectrocatalytic；Ah-ReumPark?Young-WooLee；Received:10April2014/Acc；AbstractThecarbon-suppor；KeywordsCoreCshelláPdCo@；1Introduction；
JApplElectrochem(9C1223DOI10.-014-0738-3
RESEARCHARTICLE
EnhancedelectrocatalyticactivityandstabilityofPdCo@PtcoreCshellnanoparticlesforoxygenreductionreaction
Ah-ReumPark?Young-WooLee?Da-HeeKwak?BumwookRoh?InchulHwang?Kyung-WonPark
Received:10April2014/Accepted:18August2014/Publishedonline:7September2014óSpringerScience+BusinessMediaDordrecht2014
AbstractThecarbon-supportedPdCo@PtcoreCshellnanoparticlesforanoxygenreductionreaction(ORR)werepreparedviaatwo-stepprocessatroomtemperature.Theas-preparedPdCo@Pt/Cwithanaverageparticlesizeof*3.5nmexhibitedawell-de?paredtopurePt,PdCo@Pt/Cshowedahighercurrentdensityinthekineticcontrolledregionandmorepositivehalf-wavepotentialfortheORR.InacyclingstabilitytestofthePdCo@Pt/Celectrocatalyst,noremarkableactivitylosswasseen.
KeywordsCoreCshelláPdCo@PtáElectrocatalystáOxygenreductionreaction
1Introduction
Polymerelectrolytemembranefuelcells(PEMFCs)havebeenapromisingelectrochemicalpowersourcebecauseoftheirhighenergyef?ciencies,reducedpollutionemissions,andlowoperatingtemperatures[1C3].However,thekinet-icallysluggishoxygenreductionreaction(ORR)atthecathode,whereoxygen,astheoxidant,isreducedintowaterbyreactionwithprotonsandelectrons,limitstheelectro-chemicalenergyef?ciencyofPEMFCs[4].TheslowkineticsoftheORRremainsaseriouschallengeformass
A.-R.ParkáY.-W.LeeáD.-H.KwakáK.-W.Park(&)DepartmentofChemicalEngineering,SoongsilUniversity,Seoul156-743,RepublicofKoreae-mail:kwpark@ssu.ac.kr
B.RoháI.Hwang
HyundaiMotorCompany,Mabuk-Ri,
Gyeonggi-Do,Yongin446-912,RepublicofKorea
productionofPEMFCs,becausetheyrequirehighPtloadingatthecathode[5C7].
?aandco-workerssuggestedasimplesynthesisofAbrun
PdCo@PdcoreCshellnanoparticles(NPs)supportedoncarbonusinganadsorbate-inducedsurfacesegregationeffect.ThestabilityofPdCo@PdNPsandtheirelectrocat-alyticactivityfortheORRwereenhancedbydecorationwithasmallamountofPtdepositedviaaspontaneousdisplace-mentreaction[8,9].Adzicandco-workersreportedthatasynthesisofelectrocatalystswithwell-de?nedcoreCshellstructurehasbeencarriedoutviaagalvanicdisplacementandanunderpotentialdeposition.Theas-preparedPd2-Co@Pt/CcoreCshellelectrocatalystwithanaverageparticlesizeof*5.4nmwithabroadsizedistributionshowedexcellentORRactivityandstabilityinanacceleratedpotentialcyclingtest[10C12].Liuetal.[13]representedbimetallicPtCPdcoreCshellNPssuchasPd(core)@Pt(shell)/CandPt(core)@Pd(shell)/Csynthesizedbyco-reductionandsequentialreductionmethodsinethyleneglycol(EG).Herein,wedemonstrateafacilemethodofcarbon-sup-portedPdCo@PtcoreCshellNPsasORRelectrocatalystspreparedviaatwo-steprouteatroomtemperature[14C19].Thestructure,morphology,andcompositionofthecarbonsupportedNPsarecharacterizedbytransmissionelectronmicroscopy(TEM),energy-dispersivespectroscopy(EDS),andX-raydiffractionmethod(XRD).Electrochemicalcharacterizationsarecarriedoutwithcyclicvoltammo-grams(CVs)andlinearsweepvoltammetry(LSV)witharotatingdiskelectrode.
2Experimental
WesynthesizedcarbonsupportedPdCo@PtNPspreparedviaatwo-steproute.First,PdCo3asacoredepositedon
1220JApplElectrochem(9C1223
Fig.1TEMimages(a),histogramofsizedistribution(b),andEDSdata(c)ofPdCo/Casacore.TEMimages(d),histogramofsizedistribution(e),andEDSdata(f)ofPdCo@Pt/Casanelectrocatalyst
carbonblackwassynthesizedbyNaBH4reductioninanhydrousethanol(99.5%,SigmaAldrich).Sodiumtet-rachloropalladate(29.7mg,Na2PdCl4,SigmaAldrich)andcobaltchloridehexahydrate(72.3mg,CoCl2á6H2O,SigmaAldrich)weredispersedin100mLofanhydrousethanolwithcarbonblack(113mg,VulcanXC-72R).Afterstir-ringandsonicatingthemixturesolutionfor1h,75.7mgofsodiumborohydride(NaBH4,SigmaAldrich),asareduc-ingagentin10mLofanhydrousethanol,wasquicklyaddedtothemixturesolutionwithcontinuoussonication.Second,fortheelectrocatalsytPdCo@Pt/C,theas-syn-thesizedPdCo/Cwasredispersedin100mlofanhydrousethanol.ThepHofthedispersionsolutionwasadjustedto*7byadding10mLof2MNaOHinanhydrousethanoltostabilizethePdCoasacore.1mMofchloroplatinicacidhydrate(H2PtCl6,SigmaAldrich)inEGasastabilizerwasaddedinthemixturesolution.Inthemixturesolution,PdCo@Pt/CwasformedbyagalvanicdisplacementreactioninEGat25°Cfor24h.Theresultingpowderwaswashedwithwaterandthendriedat50°Cinanovenfor12h.
XRDpatternsofPdCo@Pt/CwereobtainedusingaBrukerD2PHASEsystemwithCuKasource(k=0.15,406nm)radiationat30kVand10mAbetween20°and80°.Thesize,morphology,andcompositionofthecata-lystswereanalyzedbyTEMusinganFEITecnaiF30
systemoperatedat300kVandEDScombinedwiththeTEManalysis.TEMsampleswerepreparedbyplacingdropsofcatalystsuspensiondispersedinethanolonacarbon-coatedcoppergrid.Theweightratioofmetallicelementswascon?rmedbyinductivelycoupledplasma-atomicemissionspectrometer(ICP-AES,Optima-4,300DV,PerkinElmer).
Theelectrocatalyticpropertiesweremeasuredinathree-electrodecellusingapotentiostat(MetrohmAutolab,PGSTAT302)atroomtemperature.PtwireandHg/HgO(insaturated1MNaOH)wereusedasthecounterandreferenceelectrode,respectively.Theglassycarbonasaworkingelectrodewascoatedwith5lLofcatalystinkpreparedbymixingcatalyst(10mg),deionizedwater,isopropylalcohol,and5wt%As-4(TOKUYAMA).Afterdryinginanovenat50°C,thetotalloadingofcatalystwas20lgmetalcm-2.CVsofthesampleswereobtainedinAr-purged0.1MNaOHat50mVs-1atroomtemperature.TheORRpolarizationcurvesofthesampleswereobtainedinO2-saturated0.1MNaOHwith1,600rpmand5mVs-1.
3Resultsanddiscussion
Figure1showsPdCo/CasacorepreparedbyNaBH4reductioninanhydrousethanolandPdCo@Pt/Cas
JApplElectrochem(9C1223Fig.2XRDpatternsofPdCo/CandPdCo@Pt/CincomparisonwithXRDreferencedataofPd(JCPDSNo.
electrocatalystformedbyagalvanicdisplacementreactioninEG.AsindicatedinFig.1aandb,PdCo/Cexhibitsthewell-dispersionofPdCoNPswithanaverageparticlesizeof*3.0nmandatomicratioofPd:Co=(36.4:63.6).ThroughthegalvanicdisplacementprocessofPdCo/C,PdCo@Pt/CrepresentscoreCshellNPswithanaverageparticlesizeof*3.5nmandatomicratioofPd:Co:Pt=(67.7:4.4:27.9)(Fig.1C).Theincreasedpar-ticlesizeandvariationofcompositionofPdCo@Pt/C,comparedtoPdCo/C,indicateawell-de?nednanostructurewithaPd-richcoreandaPtshellformedbydisplacingtheCocorebyPtatroomtemperature.AsindicatedintheXRDresultsforthesamples(Fig.2),PdCo/Cexhibitscharacteristicpatternsfortheface-centered-cubicstructureofPd.ThediffractionpeaksforPdCo@Pt/Cremainedatthesame2hpositionsasfoundforPdCo/C,suggestingthatPtoverlayersareformedontopofthePdCoNPs.
TheCVsofthesamplesinAr-saturated0.1MNaOHareshowninFig.3a.Theas-preparedPdCo@Ptwithaverysmallsizeishardtoanalyzethemappingorline-pro?leanalysisbyTEM.However,asshowninFig.3a,theoxygenreductionpeakofPdCo@Pt/Cshiftstoahigherpotentialby0.14mVincomparisonwithPdCo/C,indi-rectlyimplyingaformationofPtshellonPdCocorestructure.Especially,inhydrogendesorptionregionsofCVsfortheNPs(Fig.3b),PdCo@Pt/CexhibitstypicalelectrochemicalcharacteristicsofpolycrystallinePtsimilartothoseofPt/C(E-TEKCo.)comparedtoPdCo/Casacore,indicatingthecompletesurfacedisplacementbyPtforPdCo@Pt/C.TheelectrochemicalactivesurfaceareaofthePdCo@Pt/Cis*76.7m2g-1,exhibiting2.4-foldenhancementcomparedtothatofPt/C(*32.1m2g-1).Figure4aCcdisplaystheORRpolarizationcurvesforthesamplesinO2-saturated0.1MNaOH,usingarotating
Fig.3CVs(a)ofPdCo/CandPdCo@Pt/Candhydrogenoxidationregions(b)ofPdCo/C,PdCo@Pt/C,andComm-Pt/CinAr-saturated0.1MNaOHwithascanrateof50mVs-1atroomtemperature
diskelectrode(RDE)at1,600rpmatroomtemperaturebeforeandafterstability.Thecurrentdensitywasnor-malizedtothegeometricsurfaceareaoftheelectrodes.ThePdCo@Pt/CshowedhigherORRcurrentdensityof-0.410mAcm-2at0.05Vi.e.thekineticcontrolledregion,andmorepositivehalf-wavepotentialof-0.016V,com-paredtoPt/C.Thearea-basedcurrentdensities(jarea)ofthePdCo@Pt/CandPt/Ccatalystsat0.05Vare0.097and0.060mAcm-2,respectively.Furthermore,thekineticcurrentdensity(jk)ofthePdCo@Pt/C,PdCo/C,andComm-Pt/Cis0.447,0.074,and0.418mAcm-2at-0.85V,respectively.Also,theelectrontransfernumbers(n)ofas-preparedcatalystswerecalculatedonthebasisofKoutecky-LevichplotsasshowninFig.5.ThenvalueforthePdCo/Pt/CtowardORRis*4.0,whichshowsimprovedelectrochemicalactivitycomparedtoComm-Pt/C(*3.8).TheenhancementoftheORRactivityforPdCo@Pt/CmightbeattributedtotheloweroxygenbindingenergyonaPtshellofPdCo@Pt/CthanthatonpurePt[20
1222JApplElectrochem(9C1223
Fig.4ORRpolarizationcurves(aCc)forthesamplesbeforeandafterthestabilitytestinO2-saturated0.1MNaOHsolution,1,600rpm,5mVs-1.Thestabilitytestofthesampleswasperformedbysweepingbetween-0.3and0.1Vfor2,000cyclesatroom
<parisonofORRcurrentdensity(d),half-wavepotential(e),andmassactivity(f)forthesamplesat0.05Vbeforeandafterthestability
Tocon?rmthelong-termstabilityoftheelectrocata-lysts,thestabilitytestwasperformedbycyclingbetween-0.3and0.1Vfor2,000cyclesinO2-saturated0.1MNaOH(Fig.4aCc)[20].AsshowninFig.4dCf,PdCo@Pt/Cexhibitsnoremarkablecurrentdensitylossandnodegradationinthehalf-wavepotentialfortheORRafterthecyclingstabilitytestcomparedtoPt/C,representingmuchenhancedORRstabilityofPdCo@Pt/Casanelectrocatalyst.
4Conclusions
WehavedemonstratedthecarbonsupportedPdCo@PtcoreCshellNPspreparedviaatwo-steproute.ThecoreCshellNPsweresynthesizedbycoreductionofPdCo/CasacoreandagalvanicdisplacementreactionforPdCo@Pt/C.Theas-preparedPdCo@Pt/CelectrocatalystwithacoreCshellstructureshowedmuchimprovedelectrochemicalpropertiessuchhighspeci?cmassandkineticactivity,andenhancedstabilityinoxygenreductionreaction.
AcknowledgmentsThisworkwassupportedbyNew&RenewableEnergyR&DProgram(7A)oftheMinistryofKnowledgeandEconomy.
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