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ID:37460767
大小:1.70 MB
页数:33页
时间:2019-05-12
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1、2011SpringFuelCellandElectrochemistryLecture9KineticsofelectrodereactionsDynamicEquilibriumDynamicEquilibriumThekinetictheorythereforepredictsaconstantconcentrationratioatequilibrium,justasthermodynamicsdoes;Kineticsdescribetheevolutionofmassflowthroughoutthesystem,i
2、ncludingboththeapproachtoequilibriumandthedynamicmaintenanceofthatstate.Thermodynamicsdescribeonlyequilibrium.Exchangevelocity:equilibriumfeaturesnonzeroratesofconversionofAtoВ(andviceversa),butthoseratesareequal.v0TheArrheniusEquationandPotentialEnergySurfacesAisthep
3、re-exponentialfactorkisthereactionratecoefficientEAactivationenergyTransitionStateTheorywefocusonthespecialconditioninwhichtheentiresystem—A,B,andallotherconfigurations—isatthermalequilibriumEssentialsofelectrodereactionswealwayssawthatcurrentisoftenlimitedwhollyorpar
4、tiallybytherateatwhichtheelectroreactantsaretransportedtotheelectrodesurface.Thiskindoflimitationdoesnotconcernatheoryofinterfacialkinetics.Moretothepointisthecaseoflowcurrentandefficientstirring,inwhichmasstransportisnotafactordeterminingthecurrent.Instead,itiscontro
5、lledbyinterfacialdynamics.TafelequationButler-volmermodelofelectrodekineticsSupposetheelectrodepotentialisequaltoE°.ThecathodicandanodicactivationenergiesareWhereα,thetransfercoefficientStandardRateConstantItsimplyisameasureofthekineticfacilityofaredoxcouple.Asystemwi
6、thalargek0willachieveequilibriumonashorttimescale,butasystemwithsmallk0willbesluggish.Thelargestmeasuredstandardrateconstantsareintherangeof1to10cm/s,andareassociatedwithparticularlysimpleelectron-transferprocesses.Molecularrearrangementuponelectrontransferormultistep
7、processcanbeverysluggishTheTransferCoefficientInmostsystemsaturnsouttoliebetween0.3and0.7,anditcanusuallybeapproximatedby0.5intheabsenceofactualmeasurements.Thetransfercoefficientashouldgenerallybeapotential-dependentfactor;Inatypicalchemicalsystem,thefreeenergiesofac
8、tivationareintherangeofafewelectronvolts,butthefullrangeofmeasurablekineticsusuallycorrespondstoachangeinactivationenergyofo
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