應用電化學第一章電化學基礎

Chapter1

電化學基礎教學目的在《物理化學》課程基礎上，加深對電化學基本概念和基本原理的理解通過電化學原理的學習，增強分析問題能力基本內容與要求（知識點）電化學基本概念電化學熱力學不可逆電極過程電化學極化和濃度極化課外自學吸附；電催化本章導學作業(yè)：概述《物理化學》中“電化學”基本內容電化學研究對象

Anelectrochemicalreactionisaheterogeneous（異相）chemicalprocessinvolvingthetransferofchargestakingplaceattheinterfacebetweentheelectronicconductorandionicconductor.

——電化學反應問題：電化學反應具有什么特點？怎么來研究電化學反應？電化學反應特征

異相催化反應（Heterogeneouscatalysisreaction）

電極（electrode）------催化劑（catalyst）氧化還原反應氧化和還原反應在空間上可以分離特殊性：doubleelectriclayer（雙電層）surfaceelectricfield（表面電場）特殊的異相催化氧化還原反應熱力學（Thermodynamics）(reversesystem)動力學（Kineticsofelectrodeprocess）(irreversiblesystem)研究方法1.1

Fundamentalconcepts1.2Electricaldoublelayer1.3Irreversibleelectrodeprocesses1.4Electrontransferandelectrochemicalpolarization1.5Masstransportandconcentrationpolarization1

Basicelectrochemicaldevices

2Somecommontypesofelectrodeprocesses

3Electronicconductorsandionicconductors

4.Faraday’slawsofelectrolysis

5.ElectromotiveforceandElectrodepotential

1.1

FundamentalConcepts

1.1.1

Basicelectrochemicaldevices

(Electrochemicalreactors)

Twokindsofelectrochemicaldevices:

Galvaniccell（原電池）

Electrolyticcell（電解池）

Thebasiccomponentsofcells:

anode（陽極）

cathode（陰極）

electrolyte（電解質）Anode:

positivechargefromelectrodeintosolution(oxidation)Cathode:

positivechargefromsolutiontoelectrode(reduction)

GalvaniccellElectrolyticcellPositiveelectrodeCathodeAnodeNegativeelectrodeAnodeCathodeElectrode:

workingelectrode(WE)工作電極Acounterelectrode(CE)對電極Breferenceelectrode(RE)參比電極C問題Electrochemicalcellsinlaboratory:問題：為什么用三電極體系？簡述各電極作用和要求1.1.2Somecommontypesofelectrodeprocesses

1.1.3Electronicconductors

andionicconductors

(1)Electronicconductors(thefirstconductors)(commonlyknownaselectrodes):

metal,carbon,semiconductor

Energybandtheory

valencebandEv（價帶）

conductionbandEc（導帶）

forbiddenband(energygap)Eg（禁帶或能隙）

conductor,semiconductorandinsulator

(2)Ionicconductors(thesecondconductors)Electrolyte:

Asubstancepresentinsolutionorinameltwhichisatleastpartlyintheformofchargedspecies----ions.Propertiesofelectrolyticsolution

(seepp51-54)

a)

Electricmigrationandtransportnumber

Electricmigration（電遷移）:themovementofchargedspeciesduetoapotentialgradient

（電位梯度）Ionicmobility（離子淌度）:

thespeedofionsunderaunitfieldstrength,m2s-1V-1Transportnumberti（遷移數(shù)）:

thefractionofthecurrentcarriedbytheioniti=Qi/∑Qi=Ii/∑Ii

supportingelectrolyte(foreignelectrolyte)（支持電解質或局外電解質）

b)

ConductanceandconductivityConductance（電導）

conductivityκ(specificconductance)（電導率或比電導）,Sm-1areaA,lengthS(seep52Tab.1.4)

c)Activityandactivitycoefficient

activity（活度）

a+=γ+m+ora-=γ-m-whereγ+,γ-aretheactivitycoefficientsforthepositiveandnegativeionsrespectivelyandm+,m-aretheremolalities.

meanionicactivity

a±=γ±m(xù)±m(xù)eanactivitycoefficientγ±(seep54Tab1.5)1.1.4Faraday’slawsofelectrolysis

inelectrolyticprocessestheamountofchemicaldecompositionisproportionaltothequantityofelectricitypassed.FaradayconstantF=96500C/mol=26.8AhElectrochemicalequivalentK（電化當量）,g/AhTheoreticalelectricconsumptionk（理論耗電量）,Ah/gFaraday’slawk=1/K

1.1.5ElectromotiveforceandElectrodepotential

Therearemanytypesofinterfacesincells:Metal/electrolytesolutionMetal/metalElectrolytesolution/electrolytesolutionSolutionoflowerconcentration/semipermeablemembrane/solutionofhigherconcentration(1)ElectromotiveforceEcell（EMF,電動勢）作業(yè)：簡述電動勢認識歷史、測定方法和主要應用EMF：總電勢差Nernstequation(2)ElectrodepotentialE（電極電位）Whatiselectrodepotential？potentialdifferenceattheinterfacebetweenmetalandelectrolytesolution

為什么存在電極電位？Theobservedpotentialsareproducedbytheelectricaldoublelayerarisingfromanexcessofchargesattheinterfacewhichmaybeions,electronsororienteddipoles.E=Eq+Es+EdipEq----ionicdoublelayer（離子雙層）

Es----adsorptiondoublelayer（吸附雙層）

Edip----dipolardoublelayer（偶極雙層）referenceelectrode:standardhydrogenelectrode(SHE),EHsaturatedcalomeleletrode(SCE),ESCEEH=ESCE+0.2438

Eo—standardelectrodepotential

Nernstequation如何測定電極電位？相對電位（Relativeelectrodepotential）測定Reversiblepotential(orequilibriumpotential)Ee

1.2

Electricaldoublelayer1.2.1

研究方法electrocapillarity（電毛細法）：界面張力~電位關系differentialcapacity（微分電容法）：微分電容~電位關系

基本方法：indirectmethods——模型法？(1)實驗方法討論：選什么電極？——researchsystem分析：Twokindsofcurrentandtwokindsofelectrode——idealpolarizedelectrode如Hg/KCl2HgHg22++2eE>0.1VK++eKE<-1.6V選什么電極？

FaradaycurrentorreactioncurrentiFidealpolarizedelectrodeiF=0ChargedcurrentiC（充電電流）idealnon-polarizedelectrodeiC=0

totalcurrenti=iF+iC

提出概念：零電荷電位EZLippmann’sequation

electrocapillarycurveq—chargedensity客觀描述尋求解釋規(guī)律性認識作業(yè)：Lippmann方程推導電毛細實驗結果為雙電層研究提供什么支持？(2)Electrocapillarity(σ~Ecurve)

實驗分析界面張力與電位的關系。如何分析？拓展閱讀：零電荷電位1.2.2

ThemodelsofelectricaldoublelayerGCSmodel

(Gouy-Chapman-Stern)compactlayer（緊密層）φ-ψ1diffusivelayer（分散層）ψ1

拓展閱讀：雙電層模型構建1.3Irreversibleelectrodeprocesses

機理——BasicstepsofelectrodeprocesselectrodereactionAllrealelectrochemicalprocessesareirreversibleelectrodeprocesses

——Kineticsofelectrodeprocess1.3.1CharacteristicsofelectrodeprocessSpecialheterogeneouscatalysisredoxreactionObulkOelectrodemasstransportOelectrodeRelectrode

electrontransferRelectrodeRbulkmasstransportAdditionaltypesofsteps:chemicalchange,adsorption,phaseformation

機理研究的重要方法：rate-determiningstep（thesloweststep）RateofelectrochemicalreactioncurrentdensityIindicatestherate問題：why?therateofheterogeneousreactionv,mol/sm2

v=dN/AdtaccordingtoFaradaylaw

i=dQ/dt=(dN/dt)nF=vAnFhence

I=i/A=nFvA/m2

1.3.2Polarization（極化）

(1)

Overpotential（過電位）η

η=E-EeanodicpolarizationηA=EA-EecathodicpolarizationηC=Ee-EC

reasonsofpolarizationelectrochemicalpolarization（電化學極化）

concentrationpolarization（濃度極化）

thedeviationofelectrodepotentialEfromthereversiblepotentialEe什么是極化？

(2)

Polarizationcurveη(E)~IMeasuringmethodsGalvanostaticmethod（恒電流法）

E=f(I)Potentiostaticmethod（恒電位法）

I=f(E)

Seep13Fig.1.4

拓展閱讀：極化曲線測定方法1.4Electrontransferandelectrochemicalpolarization

Whenelectrontransferstep(infact,alsoincludingchemicalchange)isslow,thepolarizationofelectrodeisknownaselectrochemicalpolarizationoractivationpolarization(電化學極化或濃度極化）

★Electrochemicalpolarizationisnotaffectedbyagitating★

Therearerelationshipsbetweenvoltageandcurrentasbellow:Tafel’slawη=a+blogIlinearpolarizationequationη=KI★Theeffectingfactors:realelectrodearea（電極實際面積）

surfacestate（表面狀態(tài)）

temperature（溫度）1.4.1

Characteristicsofelectrochemicalpolarization1.4.2Butler-Volmerequation

whenE=Ee,nonetcurrentflows,thereisadynamicequilibriumattheelectrodesurfaceelectrodereactionByconvention,anodiccurrentsaretakenaspositive.Io

isknownastheexchangecurrentdensity.Theexperimentalcurrent(ornetcurrent,orpolarizedcurrent)提示：seep9fig.1.3

TherelationshipbetweenIandE?

Butler–Volmerequation

(basicequationofelectrochemicalpolarization)TheequilibriumpotentialEeandtheexchangecurrentdensityIotogethertotallycharacterizetheequilibriumsituationatanelectrode.如何理解？wherenisthenumberofelectrons,αA,αCaretheanodictransfercoefficientandcathodictransfercoefficientrespectively.拓展閱讀：B-V方程推導過程:seepp10-11B-Vequationisaveryusefulequationinexperimentalandappliedelectrochemistryandshowsthatthemeasuredcurrentisafunctionof(a)

overpotential(b)

exchangecurrent(c)

thetransfercoefficientsDiscussionThetransfercoefficientsare,atleastforsimpleelectrontransferprocesses,notindependentvariables.IngeneralαA+αC=1

Overpotentialηatη=0I=0η↑,I↑

so,wesaidthattheoverpotentialisthedriveforceofelectrodeprocesses.SimplificationofB-VequationA)Highoverpotentialregion（Tafelregion）>120mV(52mV)ThelimitingformsofB-Vequation----Tafellaw

η=a+blogI拓展：B-V方程簡化得到Tafel方程

thecathodicTafellaw

b=2.3RT/αCnF,a=-blogIo

theanodicTafellaw

b=2.3RT/αAnF,a=-blogIo

B)Lowoverpotentialregion(linearlypolarizedregion)<10mV

WhenαA=αC=0.5

I=Io(nF/RT)η

η=KI

K=RT/nFIo

(4)Polarizationcurve

TafellawsarethebasisofasimpleexperimentalprocedurefordeterminingI0,αA,αC.1.4.3Basickineticparametersof

electrochemicalpolarization

(1)

Exchangecurrentdensity

交換電流的數(shù)學表達atE=Eo

交換電流影響因素a)

rateconstantsb)

electrodematerialseletrocatalysisc)concentrationExchangecurrentdensityisaveryusefulparameterinthedescriptionofthekineticsofelectrodereactionsbutisprimarilyameasureoftheamountofelectron-transferactivityintheequilibriumsituation.Ahighvalueindicatesthatmuchsimultaneousoxidationandreductionistakingplaceandisindicativeofaninherentlyfastreaction.Asmallvaluesuggeststhatonlyasmallamountofelectron-transferoccursattheequilibriumpotentialandisasymptomofaslowelectrodereaction.

交換電流意義WhenIo→∞,idealnon-polarizedelectrodereferenceelectrodeIo→0,idealpolarizedelectrode

(2)

Transfercoefficient(ortransferfactororsymmetryfactor)Transferfactorsαindicatetheeffectsofelectricfieldontheactivationenergies.(3)Rateconstant

課堂PPT：速率常數(shù)（什么是速率常數(shù)？如何得到速率常數(shù)？速率常數(shù)有什么作用？）下次課請1-A組講解1.5Masstransportandconcentrationpolarization

Electricmigrationisthemovementofchargedspeciesduetoapotentialgradientanditisthemechanismbywhichchargepassesthroughtheelectrolyte.1.5.1傳質方式(1)Electricmigration（電遷移）自學（復習）什么是電遷移？Electricmigrationis,however,notnecessarilyanimportantformofmasstransportfortheelectroactivespecies.Why?Theforcesleadingtomigrationarepurelyelectrostaticand,hence,donotdiscriminatebetweentypesofions.Asaresult,iftheelectrolysisiscarriedoutwithalargeexcessofaninertelectrolyte(orsupportingelectrolyte)inthesolution,thiscarriesmostofthecharge,andlittleoftheelectroactivespeciesOistransportedbymigration,i.e.thetransportnumberofOislow.(2)Convection（對流）

Convectionisthemovementofaspeciesduetoamechanicalforce.Itisarelativeflowoffluids.Inpractice,convectionisusuallyinducedbystirringoragitatingtheelectrolytesolutionorbyflowingitthroughthecell.Sometimestheelectrodeismoved.Whensuchformsofforcedconvectionarepresent,theyarenormallythepredominantmodeofmasstransport.Itispossibletocarryoutelectrochemistryintheabsenceofconvectionbyusingastillsolutioninathermostat,butonlyonashorttimescale,saylessthan10s.(3)DiffusionDiffusionisthemovementofaspeciesdownaconcentrationgradientanditoccurswheneverthereisachemicalchangeatasurface.1.5.2Analysisofmasstransportinacell

傳質總通量

J=Jmigration+Jconvection+Jdiffusion如何使問題簡化？——只考慮擴散如何實現(xiàn)？如何消除電遷移和對流的影響？withsolutionscontainingalargeexcessofbaseelectrolyte，i.e.sothatthemigrationofelectroactivespeciesisunimportantUsingunstirredsolutionandashorttimescaleinlaboratoryexperimentssothatnaturalconvectiondoesnotinterfere

idealsteady-statediffusionSteady-statediffusioncanbedescribedwithFick’sfirstlaw

ci=f(x)Fick’sfirstlaw

Ji=-Didci/dxDi----diffusioncoefficient,cm2/s

Ji=Di(co-cs)/lrealsteady-statediffusionJi=Di(co-cs)/δ

1.5.3Concentrationpolarization

(1)BasicdiffusionequationselectrodereactionIfthediffusionprocessistherate-determiningstep,thentherateofdiffusion=therateofelectrodeprocess

Discussion:

whenI=0,=currentoccuring,decreaseswhile=0,currentreachesitsmaximumvalueIL,whichisknownaslimitingcurrentdensity.

wherekLisamasstransportcoefficient.