應(yīng)用電化學(xué)第一章電化學(xué)基礎(chǔ)

Chapter1

電化學(xué)基礎(chǔ)教學(xué)目的在《物理化學(xué)》課程基礎(chǔ)上，加深對電化學(xué)基本概念和基本原理的理解通過電化學(xué)原理的學(xué)習(xí)，增強(qiáng)分析問題能力基本內(nèi)容與要求（知識點(diǎn)）電化學(xué)基本概念電化學(xué)熱力學(xué)不可逆電極過程電化學(xué)極化和濃度極化課外自學(xué)吸附；電催化本章導(dǎo)學(xué)作業(yè)：概述《物理化學(xué)》中“電化學(xué)”基本內(nèi)容電化學(xué)研究對象

Anelectrochemicalreactionisaheterogeneous（異相）chemicalprocessinvolvingthetransferofchargestakingplaceattheinterfacebetweentheelectronicconductorandionicconductor.

——電化學(xué)反應(yīng)問題：電化學(xué)反應(yīng)具有什么特點(diǎn)？怎么來研究電化學(xué)反應(yīng)？電化學(xué)反應(yīng)特征

異相催化反應(yīng)（Heterogeneouscatalysisreaction）

電極（electrode）------催化劑（catalyst）氧化還原反應(yīng)氧化和還原反應(yīng)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(yīng)熱力學(xué)（Thermodynamics）(reversesystem)動力學(xué)（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(zhì)）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(dǎo)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(zhì)或局外電解質(zhì)）

b)

ConductanceandconductivityConductance（電導(dǎo)）

conductivityκ(specificconductance)（電導(dǎo)率或比電導(dǎo)）,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(dāng)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(rèn)識歷史、測定方法和主要應(yīng)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(xì)法）：界面張力~電位關(guān)系differentialcapacity（微分電容法）：微分電容~電位關(guān)系

基本方法：indirectmethods——模型法？(1)實(shí)驗(yàn)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(rèn)識作業(yè)：Lippmann方程推導(dǎo)電毛細(xì)實(shí)驗(yàn)結(jié)果為雙電層研究提供什么支持？(2)Electrocapillarity(σ~Ecurve)

實(shí)驗(yàn)分析界面張力與電位的關(guān)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(gòu)建1.3Irreversibleelectrodeprocesses

機(jī)理——BasicstepsofelectrodeprocesselectrodereactionAllrealelectrochemicalprocessesareirreversibleelectrodeprocesses

——Kineticsofelectrodeprocess1.3.1CharacteristicsofelectrodeprocessSpecialheterogeneouscatalysisredoxreactionObulkOelectrodemasstransportOelectrodeRelectrode

electrontransferRelectrodeRbulkmasstransportAdditionaltypesofsteps:chemicalchange,adsorption,phaseformation

機(jī)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(xué)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(xué)極化或濃度極化）

★Electrochemicalpolarizationisnotaffectedbyagitating★

Therearerelationshipsbetweenvoltageandcurrentasbellow:Tafel’slawη=a+blogIlinearpolarizationequationη=KI★Theeffectingfactors:realelectrodearea（電極實(shí)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(dǎo)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(xué)表達(dá)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(zhì)方式(1)Electricmigration（電遷移）自學(xué)（復(fù)習(xí)）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(zhì)總通量

J=Jmigration+Jconvection+Jdiffusion如何使問題簡化？——只考慮擴(kuò)散如何實(shí)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.