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1、OriginoftheOverpotentialforOxygenReductionataFuel-CellCathodeJ.K.Norskov,*J.Rossmeisl,A.Logadottir,andL.LindqvistCenterforAtomic-scaleMaterialsPhysics,DepartmentofPhysics,TechnicalUniversityofDenmark,DK-2800Lyngby,DenmarkJ.R.KitchinDepartmentofChemicalEngineering,UniNersityofDelaware,Newark,Delaware
2、19716T.BligaardScienceInstitute,VR-1I,UniNersityofIceland,IS-107Reykjavik,IcelandH.JonssonFacultyofScience,VR-II,UniNersityofIceland,IS-107Reykjavik,IcelandReceived:June18,2004;InFinalForm:September2,2004Wepresentamethodforcalculatingthestabilityofreactionintermediatesofelectrochemicalprocessesonthe
3、basisofelectronicstructurecalculations.Weusedthatmethodincombinationwithdetaileddensityfunctionalcalculationstodevelopadetaileddescriptionofthefree-energylandscapeoftheelectrochemicaloxygenreductionreactionoverPt(111)asafunctionofappliedbias.Thisallowedustoidentifytheoriginoftheoverpotentialfoundfor
4、thisreaction.Adsorbedoxygenandhydroxylarefoundtobeverystableintermediatesatpotentialsclosetoequilibrium,andthecalculatedrateconstantfortheactivatedproton/electrontransfertoadsorbedoxygenorhydroxylcanaccountquantitativelyfortheobservedkinetics.Onthebasisofadatabaseofcalculatedoxygenandhydroxyladsorpt
5、ionenergies,thetrendsintheoxygenreductionrateforalargenumberofdifferenttransitionandnoblemetalscanbeaccountedfor.Alternativereactionmechanismsinvolvingproton/electrontransfertoadsorbedmolecularoxygenwerealsoconsidered,andthisperoxidemechanismwasfoundtodominateforthemostnoblemetals.Themodelsuggestswa
6、ystoimprovetheelectrocatalyticpropertiesoffuel-cellcathodes.IntroductionLow-temperaturefuelcellsareattractingconsiderableinterestasameansofproducingelectricitybydirectelectrochemicalconversionofhydrogenandoxygenintowater.Thereare,however,severeshortcomingsofthepresenttechnology,whichneedtobeovercome
7、tomakelow-temperaturefuelcellsmoreeconomicallyattractive.Oneofthemostimportantproblemsisrelatedtothelowrateofthecathodereactionwhereoxygenisreduced1+2(H+e-)fH2O(l)2O2Ptisthecommonlyusedelectrodematerial,butthereisaconsiderableoverpotentialassociatedwiththisreactionoverPt.Forsomereason,thekineticsoft
8、hecathodereactionmakeitmuchslowerthantheanodereaction,H2f2(H+e)(2)andthereispresentlynoconsensuswhythisisso.”Inthefollowing,weusedensityfunctionaltheory(DFT)calculationstogainsomeinsightintothecathodereactions.DFTcalculationscanprovideinformationaboutthestabilityof*Correspondingauthor.E-mail:norskov
9、fysik.dtu.dk.surfaceintermediatesinthereactions,whichcannotbeeasilyobtainedbyothermeans.WestartbyconsideringthesimplestpossiblereactionmechanismoveraPt(111)surface.Weintroduceamethodforcalculatingthefreeenergyofallintermediatesasafunctionoftheelectrodepotentialdirectlyfromdensityfunctionaltheorycalc
10、ulationsofadsorptionenergiesforthesurfaceintermediates.Onthisbasis,weestablishanoverviewofthethermodynamicsofthecathodereactionasafunctionofvoltage,andweshowthattheoverpotentialofthereactioncanbelinkeddirectlytotheprotonandelectrontransfertoadsorbedoxygenorhydroxidebeingstronglybondedtothesurfaceatt
11、heelectrodepotentialwheretheoverallcathodereactionisatequilibrium.Weintroduceadatabaseofdensityfunctionaltheorycalculationsofenergiesofthesurfaceintermediatesforanumberofmetalsandshowthat,onthisbasis,wecanestablishtrendsinthethermodynamiclimitationsforallthemetalsinquestion.Themodelpredictsavolcano-
12、shapedrelationshipbetweentherateofthecathodereactionandtheoxygenadsorptionenergy.ThemodelexplainswhyPtisthebestelementalcathodematerialandwhyalloyingcanbeusedtoimproveitsperformance.Tomodelthewaterenvironmentoftheelectrochemicalcell,weincludetheeffectofamonolayerofwateronthestabilityofadsorbedOandOH
13、inthecalculation.Forthelow7TheSimplestModelTointroducethebasicconcepts,wefirststudythesimpledissociativemechanism,1021/jp047349jCCC:$27.502004AmericanChemicalSocietyPublishedonWeb10/22/2004O+*fO*21(3)IO*+H+efHO*(4)H0*+H+efH2O+*(5)coverageresults,wehavesimplyaddedwatertofillthesurface,andwehaveaddedb
14、ilayerofwaterontopoftheadsorbedOandOHforthehighcoverageresultsasproposedbyOgasawaraHere,*denotesasiteonthesurface.Later,wewillalsodiscussindetailtheassociativemechanismwhereOdoesnotdissociatebeforeitishydrogenated,andwewillshowthatalthoughthischangesseveralimportantdetailsofthereactionkinetics,itdoe
15、snotaffectthemainconclusions,inparticular,regardingtheoverallvariationsinthereactionratefromonemetaltothenext.ThestabilityoftheintermediatesO*andHO*canbecalculatedonaPt(111)surface.InTable1,weshowthecalculatedbindingenergiesdefinedasthereactionenergiesofthereactionsH2O+*fHO*+1/2H2(6)H2O+*fO*+H2(7)wh
16、ereH2OandH2areinthegasphase.ThestabilityofbothadsorbedOHandadsorbedOdependsstronglyontheoxygencoverage;thereforeinTable1,weincluderesultsfortwodifferentoxygencoveragestoillustratethiseffect.Wenowintroduceourprocedureforcalculatingthefreeenergyoftheintermediatesoftheelectrochemicalreactions(eqs3-5).Itgoesinsixsteps:1.Bysettingthereferencepotentialtobethatofthestandardhydrogene
