电极反应动力学燃料电池

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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