应用电化学第一章电化学基础

Chapter1

电化学基础教学目的在《物理化学》课程基础上，加深对电化学基本概念和基本原理的理解通过电化学原理的学习，增强分析问题能力基本内容与要求（知识点）电化学基本概念电化学热力学不可逆电极过程电化学极化和浓度极化课外自学吸附；电催化本章导学作业：概述《物理化学》中“电化学”基本内容电化学研究对象

Anelectrochemicalreactionisaheterogeneous（异相）chemicalprocessinvolvingthetransferofchargestakingplaceattheinterfacebetweentheelectronicconductorandionicconductor.

——电化学反应问题：电化学反应具有什么特点？怎么来研究电化学反应？电化学反应特征

异相催化反应（Heterogeneouscatalysisreaction）

电极（electrode）------催化剂（catalyst）氧化还原反应氧化和还原反应在空间上可以分离特殊性：doubleelectriclayer（双电层）surfaceelectricfield（表面电场）特殊的异相催化氧化还原反应热力学（Thermodynamics）(reversesystem)动力学（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（原电池）

Electrolyticcell（电解池）

Thebasiccomponentsofcells:

anode（阳极）

cathode（阴极）

electrolyte（电解质）Anode:

positivechargefromelectrodeintosolution(oxidation)Cathode:

positivechargefromsolutiontoelectrode(reduction)

GalvaniccellElectrolyticcellPositiveelectrodeCathodeAnodeNegativeelectrodeAnodeCathodeElectrode:

workingelectrode(WE)工作电极Acounterelectrode(CE)对电极Breferenceelectrode(RE)参比电极C问题Electrochemicalcellsinlaboratory:问题：为什么用三电极体系？简述各电极作用和要求1.1.2Somecommontypesofelectrodeprocesses

1.1.3Electronicconductors

andionicconductors

(1)Electronicconductors(thefirstconductors)(commonlyknownaselectrodes):

metal,carbon,semiconductor

Energybandtheory

valencebandEv（价带）

conductionbandEc（导带）

forbiddenband(energygap)Eg（禁带或能隙）

conductor,semiconductorandinsulator

(2)Ionicconductors(thesecondconductors)Electrolyte:

Asubstancepresentinsolutionorinameltwhichisatleastpartlyintheformofchargedspecies----ions.Propertiesofelectrolyticsolution

(seepp51-54)

a)

Electricmigrationandtransportnumber

Electricmigration（电迁移）:themovementofchargedspeciesduetoapotentialgradient

（电位梯度）Ionicmobility（离子淌度）:

thespeedofionsunderaunitfieldstrength,m2s-1V-1Transportnumberti（迁移数）:

thefractionofthecurrentcarriedbytheioniti=Qi/∑Qi=Ii/∑Ii

supportingelectrolyte(foreignelectrolyte)（支持电解质或局外电解质）

b)

ConductanceandconductivityConductance（电导）

conductivityκ(specificconductance)（电导率或比电导）,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±meanactivitycoefficientγ±(seep54Tab1.5)1.1.4Faraday’slawsofelectrolysis

inelectrolyticprocessestheamountofchemicaldecompositionisproportionaltothequantityofelectricitypassed.FaradayconstantF=96500C/mol=26.8AhElectrochemicalequivalentK（电化当量）,g/AhTheoreticalelectricconsumptionk（理论耗电量）,Ah/gFaraday’slawk=1/K

1.1.5ElectromotiveforceandElectrodepotential

Therearemanytypesofinterfacesincells:Metal/electrolytesolutionMetal/metalElectrolytesolution/electrolytesolutionSolutionoflowerconcentration/semipermeablemembrane/solutionofhigherconcentration(1)ElectromotiveforceEcell（EMF,电动势）作业：简述电动势认识历史、测定方法和主要应用EMF：总电势差Nernstequation(2)ElectrodepotentialE（电极电位）Whatiselectrodepotential？potentialdifferenceattheinterfacebetweenmetalandelectrolytesolution

为什么存在电极电位？Theobservedpotentialsareproducedbytheelectricaldoublelayerarisingfromanexcessofchargesattheinterfacewhichmaybeions,electronsororienteddipoles.E=Eq+Es+EdipEq----ionicdoublelayer（离子双层）

Es----adsorptiondoublelayer（吸附双层）

Edip----dipolardoublelayer（偶极双层）referenceelectrode:standardhydrogenelectrode(SHE),EHsaturatedcalomeleletrode(SCE),ESCEEH=ESCE+0.2438

Eo—standardelectrodepotential

Nernstequation如何测定电极电位？相对电位（Relativeelectrodepotential）测定Reversiblepotential(orequilibriumpotential)Ee

1.2

Electricaldoublelayer1.2.1

研究方法electrocapillarity（电毛细法）：界面张力~电位关系differentialcapacity（微分电容法）：微分电容~电位关系

基本方法：indirectmethods——模型法？(1)实验方法讨论：选什么电极？——researchsystem分析：Twokindsofcurrentandtwokindsofelectrode——idealpolarizedelectrode如Hg/KCl2HgHg22++2eE>0.1VK++eKE<-1.6V选什么电极？

FaradaycurrentorreactioncurrentiFidealpolarizedelectrodeiF=0ChargedcurrentiC（充电电流）idealnon-polarizedelectrodeiC=0

totalcurrenti=iF+iC

提出概念：零电荷电位EZLippmann’sequation

electrocapillarycurveq—chargedensity客观描述寻求解释规律性认识作业：Lippmann方程推导电毛细实验结果为双电层研究提供什么支持？(2)Electrocapillarity(σ~Ecurve)

实验分析界面张力与电位的关系。如何分析？拓展阅读：零电荷电位1.2.2

ThemodelsofelectricaldoublelayerGCSmodel

(Gouy-Chapman-Stern)compactlayer（紧密层）φ-ψ1diffusivelayer（分散层）ψ1

拓展阅读：双电层模型构建1.3Irreversibleelectrodeprocesses

机理——BasicstepsofelectrodeprocesselectrodereactionAllrealelectrochemicalprocessesareirreversibleelectrodeprocesses

——Kineticsofelectrodeprocess1.3.1CharacteristicsofelectrodeprocessSpecialheterogeneouscatalysisredoxreactionObulkOelectrodemasstransportOelectrodeRelectrode

electrontransferRelectrodeRbulkmasstransportAdditionaltypesofsteps:chemicalchange,adsorption,phaseformation

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

v=dN/AdtaccordingtoFaradaylaw

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

I=i/A=nFvA/m2

1.3.2Polarization（极化）

(1)

Overpotential（过电位）η

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

reasonsofpolarizationelectrochemicalpolarization（电化学极化）

concentrationpolarization（浓度极化）

thedeviationofelectrodepotentialEfromthereversiblepotentialEe什么是极化？

(2)

Polarizationcurveη(E)~IMeasuringmethodsGalvanostaticmethod（恒电流法）

E=f(I)Potentiostaticmethod（恒电位法）

I=f(E)

Seep13Fig.1.4

拓展阅读：极化曲线测定方法1.4Electrontransferandelectrochemicalpolarization

Whenelectrontransferstep(infact,alsoincludingchemicalchange)isslow,thepolarizationofelectrodeisknownaselectrochemicalpolarizationoractivationpolarization(电化学极化或浓度极化）

★Electrochemicalpolarizationisnotaffectedbyagitating★

Therearerelationshipsbetweenvoltageandcurrentasbellow:Tafel’slawη=a+blogIlinearpolarizationequationη=KI★Theeffectingfactors:realelectrodearea（电极实际面积）

surfacestate（表面状态）

temperature（温度）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.拓展阅读：B-V方程推导过程: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方程简化得到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

交换电流的数学表达atE=Eo

交换电流影响因素a)

rateconstantsb)

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

交换电流意义WhenIo→∞,idealnon-polarizedelectrodereferenceelectrodeIo→0,idealpolarizedelectrode

(2)

Transfercoefficient(ortransferfactororsymmetryfactor)Transferfactorsαindicatetheeffectsofelectricfieldontheactivationenergies.(3)Rateconstant

课堂PPT：速率常数（什么是速率常数？如何得到速率常数？速率常数有什么作用？）下次课请1-A组讲解1.5Masstransportandconcentrationpolarization

Electricmigrationisthemovementofchargedspeciesduetoapotentialgradientanditisthemechanismbywhichchargepassesthroughtheelectrolyte.1.5.1传质方式(1)Electricmigration（电迁移）自学（复习）什么是电迁移？Electricmigrationis,however,notnecessarilyanimportantformofmasstransportfortheelectroactivespecies.Why?Theforcesleadingtomigrationarepurelyelectrostaticand,hence,donotdiscriminatebetweentypesofions.Asaresult,iftheelectrolysisiscarriedoutwithalargeexcessofaninertelectrolyte(orsupportingelectrolyte)inthesolution,thiscarriesmostofthecharge,andlittleoftheelectroactivespeciesOistransportedbymigration,i.e.thetransportnumberofOislow.(2)Convection（对流）

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

传质总通量

J=Jmigration+Jconvection+Jdiffusion如何使问题简化？——只考虑扩散如何实现？如何消除电迁移和对流的影响？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.