ElectrodereactionspossessingasimplemechanismObRb

Masstransfer

MasstransferORne

ElectrodeChargetransferChapter2KINETICSOFCHARGETRANSFERPROCESSESATELECTRODE/ELECTROLYTEINTERFACESAbriefreviewofthekineticsof

homogeneouschemicalreactionReactionratesandchemicalequilibriumABkfkbv=vf–vb=kf

cA-kb

cB

TherateconstantandthefreeenergyofactivationAtequilibrium,

v=0,then:FreeenergyReactantsProductsReactioncoordinateHeterogeneouschargetransferreactionatelectrode/solutioninterfacesChargetransferplaneFeimilevelLUMOHOMOChargetransferplaneFeimilevelPotentialshiftsnegativelyeAz+eAz-1Fe3++e

Fe2+ForExample:Butler-VolmermodelforinterfacialelectrochemicalreactionsEffectsofpotentialsontheactivationfreeenergyofinterfacialchargetransferreactionsO+eR1---Freeenergycurveatj0(initialpotential)2---Freeenergycurveatj3---Distributioncurveofpotentialchange4---Freeenergychangeofanelectron

curve2=curve1–curve4a,b---transfercoefficientO···eEffectsofpotentialsonthereactionrateofinterfacialchargetransferreactionsO+neRkckakc0---rateconstantofreductionreactionat

0

ka0---rateconstantof

oxidationreactionat

0

IfWeset

0=0,then

-

0=

O+neRkckaAsj=j平ic---thecurrentduetoreductionic---thecurrentduetooxidationic0---thereductioncurrentatzeropotentialia0---theoxidationcurrentatzeropotentiali0----exchangecurrentdensitylgi0Thus,atstandardequilibriumpotential:ThekeyparametersinthekineticsofinterfacialelectrodereactionTransfercoefficient

a,b

---Ameasureofthechangingrateoftheactivationfreeenergywiththepotential;alsoameasureofthesymmetryofthetheenergybarriersforreductionandoxidationbranchesofaredoxcouple.Exchangecurrentdensityi0

---Theessentiallyequalcurrentdensityforbothreductionandoxidationbranchesastheequilibriumisreached.Standardrateconstant

k0

---Theessentiallyequalrateconstantforbothreductionandoxidationbranchesatthestandardequilibriumpotential.O+neRkckaProblem:What’stherelationshipbetweenk0andi0

?Pleasegiveanexpressionandthederivation.ProblemTheimplicationsoftheButler-VolmermodelontheelectrodereactionsElectrochemicalequilibriumAsj=jeq

:ic=ia=i0,soNernstequation:Inthelimitofequilibrium,thekineticequationsessentiallycollapsetotherelationsofthermodynamicform;Theequilibriumisnotastationarystate,butadynamicstateTheCurrent–OverpotentialEquationsLineari-

relationshipatsmallAsthepotentialdepartsfromitsequilibriumvalue,therewillbenetcurrentproducedatinterfaceandtheequilibriumbreaks.Wecallthedeviationofelectrodepotentialfromitsequilibriumvalueas“over-potential”.Forverysmallx,theexponentialexcanbeapproximatedas1+x,thus

forsufficientlysmall

,wehaveTafeli-

relationshipatverylarge

Alsocalledsemi-logarithmrelationshipi-

plotslgi0lgiclgiaSlope:F/2.3RT

Slope:-bF/2.3RT

-TafelEquationTheimplicationsofthei0valueonanelectrodereactioni00i0

:smalli0

:largei0

ThepolarizabilityoftheelectrodesystemTheelectrochemicalreversibilityoftheelectrodesystemI-relationIdealpolarizedelectrodeIrreversibleSimilartothechargingbehaviorofaRCcircuitEasilypolarizedelectrodeLessreversibleUsuallysemi-logarithmrelationHardlypolarizedelectrodeRatherreversibleUsuallyLinearrelationIdealnonepolarizedelectrodeReversibleTheelectrodepotentialhardlychangeswiththecurrentTheestablishmentofrelativelysteadyequilibriumpotentialIftheelectrodesystemconsistsofared-oxcouplewithverylargevalueofi0sothatthecurrentiflowingthroughtheelectrochemicalcellisalwaysnegligible,i.e.,i<<

i0,theelectrodepotentialwillalwaysstayatavalueveryclosetoitsequilibriumandhardlychanges.Forasysteminwhichtherearemorethanonered-oxcouples,theelectrodepotentialisdeterminedbytheequilibriumofthered-oxcouplewithlargeri0valueThereferenceelectrodesystemusuallyused:Standardhydrogenelectrode(NHE):Pt/H2(a=1)/H+(a=1,aqueous)Saturatedcalomelelectrode(SCE):Hg/Hg2Cl2/KCl(