武汉大学电化学课件 陈胜利 Chaper 6 -Double layer

Chapter6Thepropertiesandstructuresoftheelectricdoublelayeratelectrode/electrolyteinterfacesTheelectrode/electrolyteinterfacesItisessentialtohaveanelectrode/electrolyteinterfacefortheelectrochemicalreactionstotakeplace.Suchaninterfaceisestablishedbydirectcontactoftheelectrode(electronconductors)withelectrolyte(ionconductors).Theelectricdoublelayeristhecoreoftheelectrode/electrolyteinterface,whichisformedbythechargesresideintheinterfacialregion,andinwhichtheinterfacialpotentialdifferenceisdistributed.TheformationofchargedlayersattheinterfacesbetweentwophasesAstwophasesofdifferentnaturesaremadeincontact,achargedregionmaybeproducedineachphasesnearthephaseboundaryduetoChargetransferbetweentwophase,e.g.,thedissolutionofmetal:AgAg++eAdsorptionofchargedspeciesatinterfacesOrientatedarrangementofdipoleabcArelativelydetailedviewofpotentialsandinterfacialpotentialdifferencesTheequi-potentialnatureofconductorsThepotentialdifferencebetweentwopointswithinaphase——theworkrequiredtobringaunitpositivechargefrompoint1topoint2.ThepotentialisuniformwithinconductorsTheconductorsusuallycontainsabundanceofchargecarriers.Whennocurrentpassesthroughaconductingphase,thereisnonetmovementofchargecarriers.Thisimpliesthattheelectricfieldatallinteriorpointswithinaconductingphasemustbezero.Therefore,thepotentialdifferencebetweenanytwopointsintheinteriorofthephasemustalsobezero.21Thepotentialwithinaphase,f,iscalledtheinnerpotential(orGalvanipotential)ofthephaseThelocationoftheexcesschargesinaconductingphaseTheGausslaw:ifweencloseavolumewithanimaginarysurface(aGaussiansurface),wewillfindthatthenetcharge“q”insidethesurfaceisgivenbyanintegraloftheelectricfieldoverthesurface:ThisimpliesthattheexcesschargeresidesintheregionnearthesurfaceoftheconductingphaseThethermalprocesseswillimpedethecompactaccumulationoftheexcesschargesstrictlyonthesurface.Thenachargedzoneisestablishednearthesurface,whichiscalledaspacechargeregion,thethicknessofwhichdependsonthebulkconcentrationofthechargecarriers.TheinnerpotentialandouterpotentialofaphaseThevalueofthepotentialataparticularpointwithinaphase,f,isrelatedtothework(W)requiredtoovercomethecoulombicinteractionbetweenthetestchargeandthestudyingphaseuponbringingthetestchargefromthelocationofzeropotentialintothephase.“W”consistsoftwocontributions:(a)W1---thatrequiredtobringtheunitchargetotheclosestdistancefromthephaseboundarywherenonon-coulombicinteractionstartstofunction;(b)W2---thatrequiredtoovercomethecoulombicforcewhenbringthetestchargefromoutsideofphaseboundaryintothephase.W1W2+mWhereyiscalledtheouterpotential(orVoltapotential)ofthephase,Whereciscalledthesurfacepotentialofthephase1mmDuetothatmovingthetestchargeintoaphasealwaysinvolveschemicalinteractionsbetweenthephaseandthetestcharge,itisimpossibletomeasurethesurfacepotentialindependently.TheconceptoftheelectrochemicalpotentialInadditiontothecoulombicwork,(Wc),itisalsonecessarytoovercomethechemicalinteractionuponmovingthetestchargeintothephase,thecorrespondingfreeenergychangeisdefinedasthechemicalpotential(m)ofthetestchargeinthestudyingphase.Thus,thetotalfreeenergychangeduetomovingatestcharge(z)fromthepointofzeropotentialintoaphaseis:W1W2+miscalledtheElectrochemicalpotentialofthetestchargeinthephaseTheconceptoftheemersionwork(Wi)andworkfunctionTheworkrequiredtomoveonemolechargedparticleoutofaphaseintothevacuumclosetoitsouterboundaryForanychargedparticlesinaphase:TheFermilevelofelectronenergyTheelectronsinaconductingphasearegenerallydelocalizedanddistributedindifferentenergylevels(energystates).Theprobabilityf(E)forthelevelwithenergyEtobeoccupiedbyelectronsobeystheFermi-Diracdistributionlaw,thatisEF---theenergyoftheFermilevel;T=0，E＜EF：f（E）=1E＞EF：f（E）=0Theenergychangeduetoadditionofanelectrontothephase.T≠0，E=EF：f（E）=1/2T≠0,E＜EF：1/2＜f(E)＜1;E＞EF：0＜f(E)＜1/2IfkT<d,theGouy-Chapmantheoryissatisfied.Thepotentialprofileinthediffuselayerx>d:x|qE|,qDhassamesignasqE.ThepotentialprofilechangesdirectionatIHP.(dottedlineinthefigure)SummaryonthedoublelayerstructureTheinterfacialdoublelayerconsistsofcompactpartanddiffusepart.Theformationofcompactlayerisduetothefinitesizeofionsandthesolventmolecules.Thefirstlayeradjacenttotheelectrodesurfaceinthesolutionsideisalayeroforientedsolventmolecules.Thetotalthicknessofthecompactpartisbelow1nmaccordingtothesizeofionsthatcounterbalancethechargeonelectrode.Theformationofdiffusedoublelayerisaresultofacompetitionbetweenthecoulombicinteractionbetweenthechargesandthethermalmovementofionsinsolution.ItsthicknesscouldbefromafewAngstromtoseveraltensofnanometersdependingontheconcentrationoftheelectrolyteandthechargedensityontheelectrodesurface.ItspropertiesishardlyaffectedbythenaturesoftheionsandcanbewelldescribedbytheGouy-Chapmantheory.Withdilutesolutionandlowsurfacechargedensity(nearPZC),thepropertiesofthedoublelayerismainlydeterminedbydiffuselayer.Thediffuselayerisnegligiblewithconcentratedsolutionandhighsurfacechargedensity.Theadsorptionofcationandanionintheinterfacialregionhavedifferentmechanism.Thecationsadsorbatinterfacemainlyduetotheelectrostaticinteraction.Whereas,theanionsadsorbthroughshortrangeinteractionwithelectrode.SuchtypeofadsorptioniscalledspecificadsorptionorcontactedadsorptionThestructureandpotentialprofileofdoublelayermaybealteredsignificantlybytheextentofspecificadsorption.OHPIHPOHPd1d2IHPOHPd1d2vsPZC0x0d1d2OHPPotentialofZeroCharge(PZC)ThemethodstomeasurethePZC.ThevaluesofPZCmaybealteredbythecrystalfaces,thenatureandconcentrationofanioninsolution,thesurfaceoxideformation.TheapproximatelylinearrelationshipbetweenPZCandtheEmersionWorkofelectron(We)Ifthereisnonetchargeonthemetalphase,thedifferentmetalshouldhaveverycloseouterpotentialvalues.StudiesatsolidelectrodesDifficulties:Toobtainacleanelectrodesurfaceandtokeepthesurfacestatusunchangedduringthemeasurements;Tohaveatomicallysmoothsurface(alwayshavedefects);Toestablishtheconditionsofanidealpolarizedelectrode;Tomeasuresurfacetensionofsolids.Themostwidelyusedapproachinstudyingthesolidelectrode/solutioninterfacesistoevaluatethepotentialofminimumdifferentialcapacitanceinasysteminvolvingdilutesolutionDifferentcrystalfacesofsolidexhibitdifferentproperties(e.g.,PZCorworkfunction).Thecrystalsurfacewouldsometimesundergochangesinconfiguration.SuchareconstructioncanbefunctionofpotentialandtheextentofspecificadsorptionStudiesatsolidelectrodesFromcapacitancemeasurementswithscrupulouscareinelectrodesurfacepreparationandsolutionpurity,informationaboutthePZCandthenatureofthesolid/electrolyteinterfacemaybeobtained.(VvsSCECd(mFcm-2)ThedifferentialcapacitancecurvesatinterfacebetweenAu(210)andNaFsolutionofdifferentconcentration.Features:Indilutesolution,aminimumoccurs,thepotentialshouldcorrespondtothePZC;Athighchargedensity,thevalueofCdtendstobeconstant;Withconcentratedsolution,however,amaximumcanbeseenDoublelayereffectsonelectrodeprocesses(nospecificadsorption)OHP10sItisapparentthattheconcentrationofelectroactivespeciesatthereactionplaneisaffectedbythepotentialprofileindiffusedoublelayer.Ifweassumethatthemasstransferrateisveryfast,thenthedrivingforcefortheinterfacialchargetransferis(-1)insteadof(-s)Assumingthatthebackreactioncanbeignoredandthats=0Theoveralleffectofthedoublelayerontheelectrodereactionisthattheapparentstandardrateconstant(orexchangecurrentdensity)changeswithpotential,supportingelectrolyteconcentration.Theend