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Influence_4

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Influence_4Influe

    Influence

Dec.2008,Volume2,No.12(SerialNo.13)JournalofMaterialsScienceandEngineering,ISS

    N1934-899,us

    ;Influenceofhightemperaturetreatmentofactivatedcarbon ;onperformance0tsupercapaclt0rS?J

    ;SUNGang-weiWANGCan,ZHANLiang,QIAOWenming,LIANGXiao-yi,LINGLicheng

    ;(StateKeyLaborato~ofChemicalEngineering,EastChinaUniversityofScienceandTech

    nology,Shanghai200237,China)

    ;Abstract:Highsurfaceareaactivatedcarbon(AC)was ;heattreatedathightemperaturetoimprovetheelectrochemica1 ;propertiesofdoublelayercapacitors(EDLCs).Effectsofthe ;hightemperaturetreatmentootheporestructureofACwere ;investigatedaswellastheelectrochemicalperformance.After ;theorigina1ACwasheattreatedat1500?fCl500)for2h,its

    ;specificsurfacearea,totalporevolumeandcontactresistance ;decreasedfrom1312m/g,O.87cm,54.9Qto947m/g,

    ;O.57cm/g,l2.7Q,respectively.Thetolerancevoltageofthe ;capacitorassembledwithC.1500couldstabilizeat2.7Vafter ;charged/dischargedfor6000cycletimes.Whentheheat ;treatmenttemperaturewashigherthanl800?.thetolerance

    ;voltagecanmaintainedat35V,moreover,themechanismof ;chargestorageansformedfromtheformationoftheelectric ;doublelaycrate1ectr0de/electrolvteinterfaceintointercalation ;processforthesakeoftheexistenceofthecrystallographic ;strucmre.

    ;Keywords:electricdoublelayercapacitors(EDLCs);activated ;carbon;intercalationcapacitance;hightemperaturetreatment ;1.Introduction

    ;Electricdoublelayercapacitors(EDLCs)arenew

    ;energystoragedeviceswithgreatapplicationprospect ;basedontheformationoftheelectricdoublelayerat ;electrode/electrolyteinterface.Comparingwith ;rechargeablebatteries,EDLCshavemanyadvantages, ;suchashighpowerdensity,remarkablecycling

    ;performance,highsafety,hightemperaturestability,

    ;friendlinesstoenvironmentandsoon[.1.Durin2last

    ;Acknowledgement:ThisworkwassupportedbytheNational ;NaturalScienceFoundationofChinafNos.50672025, ;50730003),TheresearchfundfortheDoctoralProgramof

;HigherEducationfNo.2007025lOO8).

    ;Corresp0ndingauthor:SUNGangwei(1983),male,M.S.;

    ;researchfield:electricdoublelayercapacitors.Email:

    ;zhanliangecust.edu.cn,lchlingecust.edu.ca. ;decades.EDLCshavebeenextensivelydeveloped[3-4] ;tosatisfytheincreasingdemandinthehybridpower ;sourcesforelectricalvehicles,digital ;telecommunicationsystems,pulselasertechniquesand ;otherenergyfields[.

    ;Recently,activatedcarbon(AC)fortheelectrode ;materialsofEDLCshasbeenwidelyresearched ;becauseofitshighspecificsurfacearea,excellent ;electricconductivityandlowcostsoJ.Effectsofthe

    ;specificsurfaceareaandporesizedistributionofAC ;havebeendiscussed,andconsiderableeffortshave ;beenpaidtoimprovethecapacitance.However,these ;attemptsalmostapproachthecapacitancesaturationvs. ;specificsurfaceareaatpresent.Furthermore,the ;oxygencontainingfunctiongroupsofACwithhigh ;surfaceareaarequitesusceptibletoelectrolytes【『J’

    ;whichlimitthetolerancevoltageforthesakeoflong ;cyclestabilityandsafetyrlowerthan2.7Vin ;nonaqueouselectrolytesandthatof1.2Vinaqueous ;electrolytes)[s.So,theenergydensityofanEDLCis ;muchlowerthanLithiumionbattery(LIB)thatits

    ;usageinavehicleislimitedtoonlymomentarypower ;supply.

    ;Theoretically,theenergydensityofEDLCis ;determinedby1/2CV,inwhichCisspecific ;capacitanceandVtolerancevoltage.Itwouldbea ;beueroptiontoobtainahighenergydensityby ;improvingvoltagewindowratherthanspecific ;capacitance.Therefore,focusesofcurrentresearches ;areconcentratingonperceivingthereasonwhythere ;existstolerancevoltagelimitationorhowtoenhance ;thepotentialstability[.

    ;Recently.somegroupsfl0_

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    ;Influenceofhightemperaturetreatmentofactivatedcarb

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    ;Galvanostaticcharge/dischargecycleproperties ;oftheEDLCsassembledwithC1500,C1800,C2000,

    ;C2800weremeasuredinapotentialrangeof0.05to ;3.5VatconstantcurrentdensityOf10mm/cmzon

    controlledArbininstruments,whilethatof ;computer

    ;C0.C.1050,C.1200wereana1)rzedinthepotential ;rangeof0.05to2.7V.Cyclicvoltammetry(cv,was ;carriedoutinthepotentialrangeof0to3Vusinga ;twoelectrodeconfigurationtoevaluatethecapacitive ;characteristicatascanningrateof10mWs.The ;electrochemicalimpedancespectroscopy(EIS)was ;exploitedinthefrequencyrangeof10mnzto100kHz ;withallACamplitudeof?10mV.

    ;3.Resultsanddiscussion

    ;3.1Characterizationofheat-treatedactivated ;carbon

    ;Tabkllistedthemainporecharacteristics

    ;determinedfromtheadsorption/desorptionisotherms ;ofnitrogen.Itrevealedthatboththespecificsurface ;area(SBET)andtotalporevolume(vl0t)decreasedwith ;theheattreatmenttemperatureincreasing.Whenthe ;originalAC(C-0)wascarbonizedat1200?for2h,its

    ;SBET,surfaceareaofmicropores(Smjc),surfaceareaof ;mesopores(Smes),Vtot,Vmic,mesoporevolume(Vmes) ;decreased3.5,2.9,20,10.4,6.2and20.6%,

    ;respectively.Oncetheheattreatmenttemperaturewas

    ;higherthan1500~C,thevaluesofaboveparameters ;decreaseddramatically,whichcouldbealsoconfirmed ;fromtheN2adsorption/desorptionisothermsatrelative ;pressureofP/P0=0.5.asshowninFig.1.AfterC-0 ;samplewasheattreatedat1800?for2h.micropores

    ;disappearedandsomedecreasedfrom269to208m/g, ;theresuhsabovecouldalsobededucedfromitspore ;sizedistributionshowninFig.2.Whentheheat ;treatmenttemperatureroseupto2000or2800?.

    ;micoporcsofC-0woulddisappearcompletely,notto ;mentionmicropores.Xraydiffraction(XRD)was

    ;furtheradoptedtoobservetheformationofcrystalline. ;Fig.3indicatedthatwhenitwasgraphitizedat1800”C,

    ;partialoflamellarmoleculeinthefundamentalmatrix

    ;ofACbegantorearrangeandtendedtographiticlayers, ;i.g.,crystallographicstructure,whichcouldbe ;demonstratedfromtheintensived002peaksappeared ;at20=26..Oncethegraphitizationtemperaturewas ;2800?.XRDrepresentedaverysharpandsymmetry

    akswhichsuggesteditsgraphiticstructurewas ;d002pe

    ;alreadyratherperfect.

    ;Table1PorestructureparametersofAC

    ;Relativepressure

    ;Fig.1Adsorption/desorptionisotherms ;ofnitrogendifferentAC

    ;Poresize()

    ;Fig.2PoresizedistributioncurvesofdifferentAC ;3.2Capacitivepropertiesofactivatedcarbon ;43

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    ;treatmentofactivatedcarbononperformanceofsupercapacitors

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    ;Fig.3X-raydiffractionpatternsofdifferentAC ;Theefficientchargepotentialanditsstability ;affectsignificantlytheenergydensityaswellascycle

    ;stabilityperformance.Fig.4showedthetolerance ;voltagestabilityofthecapacitorsassembledwith ;differentACforcharged/discharged6000cycletimes ;ataconstantcurrentof10mm/cm2.Thecharge ;potentialoftheEDLCusingC0aselectrodematerial

    ;decreasedfxom2.7to1.95V.especiallyhadasharp ;declineinthefirst1000cycles.However,thecharge ;potentialcouldbemaintainat2.7Vevenitwas ;charged/dischargedfor6000cycletimes,whenC1500

    ;wasusedaselectrodematerialwhichsuggestedthatthe ;potentialstabilityincreasedwiththeheattreatment

    ;temperatureincreasing.

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    ;Fig.4ChargepotentialstabilityofdifferentEDLCs ;Toclarifytheessentialreasonsofresultsabove,the ;componentandsurfaceoxygencontainingfunction

    ;groupsoftheelectrodematerialswerefurther ;investigatedthroughtheelementalanalysisandXPS. ;Table2presentedthattheoxygencontentofC-0was ;23.88wt%andthatofO/Cratio32.63wt%,suggesting ;theexistenceofabundantsurfaceoxygenfunction ;groups,suchascarboxyl,hydroxylandSOon.However, ;afterC0wassubjectedtohJghtemperature,the ;oxygencontentandO/Cratiodecreasedsignificantly. ;Additionally,theoxygencontainingfunctiongroupsof

    ;C0andC1500samplewerefurtheranalyzedwith ;X-rayphotoelectronspectroscopy(xPs)asshownin ;Fig.5.TherewerefourcurvesofOlspeakswim ;bindingenergiesat530.9,532.4,533.8and535.2eV ;assignedtoC=O,CO,OC=Oandchemisorbed02(or

    ;H20),respectively,thedeclineoftheratiosof0C=O,

    ;C0andC=Ohadagoodagreementwiththeelemental ;analysisresults.Theresultsdemonstratedthatthe ;improvementofthepotentialstabilityshouldbe ;attributedtothedeclineoftheoxygenelementand ;oxygencontainingfunctiongroupsduetofollowing ;reasons.Ontheonehand,majorityof

    ;oxygen-containingfunctiongroupshadanegative ;effectonthepenetrationoftheelectrolyteionsinto ;microporesduetothesterichindrance.Ontheother ;hand,theoxygencontaininggroupsmighthave

    ;catalyticfunctionforthedecompositionofthe ;electrolyte(Et4BF4/PC)undertheexistenceofminute ;waterwiththeresiduecontentofabout10ppmin

    ;electrolyte.Therefore,thestabilityofchargepotential ;hadsignificantimprovementifthesurfacechemical ;performancewasmodifiedbyheattreatlectrochemicalimpedancespectroscopy ;(EIS)

    ;ElectrochemicalpropertiesofEDLCcouldbe ;investigatedwithalternatingcurrentimpedance ;spectrum.Fig.6showedtheNyquistplotsofthe ;capacitorsusingactivatedcarbonassymmetry ;electrodemateria1.Generally,theplotsofcapacitor ;withporouscarbonelectrodewascharacterizedbya ;semicircle,alinearpartwithanangleof45.,andan ;almostverticaIlinetotheZaxiscorrespondingtothe ;regionofhigh,middleandlowfrequencies,

    ;respectively.ThehighfrequencyinterceptontheZ ;axiscorrespondedtotheresistanceoftheelectrolyte ;(Rs)andthevalueofthediameterofthesemi.cycle ;representedthatofthecontactresistance()among ;carbonparticlesorbetweenelectrodesandcurrent ;collector.TheoveralIresistance(Rt)couldbededuced ;fromtheextrapolationofthecapacitivelinetoZaxis. ;Table3suggestedthatRslocatedalmostintheregions ;of1.6to2.0Qbecauseofthesameelectrolyte ;employed,whilebothRcandRtdecreaseddramatically ;withtheheattreatmenttemperatureincreasing.

    ;Specifically,ifC0wasreplacedwithC1500,C-2800,

    ;&woulddecreasefrom54.9to12.7,4Qrespectively. ;Theimprovementofelectrochemicalresistanceshould ;beascribedtothemodificationofsurfacechemical ;performanceandtheformationofcrystallinestructure ;ofACintheprocessofhightemperaturetreatment. ;Theimmigrationrateoforganicionswasrelatedtothe ;surfacechemicalfunctiongroups.Thedecreaseofthe ;amountoftheoxygencontaininggroupswouldbenefit

    ;totheenhancementofthehydrophobicenvironment ;whichcouldimprovethewettabilityoforganic ;electrolytetothesurfaceofcarbonmaterials.Sincethe ;presenceofoxygencontainingfunctiongroupsmight

    ;retardthemobilityofions,leadingtotheincreaseof ;thecontactresistancebetweenelectrolytesand ;electrodes,moreover,thefunctiongroupswouldalso ;actaspolarsitesmakeitdifficultfortheelectrolyteto ;diffuse.Therefore,themodificationofthesurface ;chemicalpropertieswasconsideredtobeaneffective ;measurementfortheimprovementoftheresistance

    ;performance.Besides,withthedegreeofgraphite ;increasing,theintrinsicelectricconductivityof ;activatedcarbonimproved,andthecontactresistance ;betweenactivatedcarbonparticlesandcurrent ;collectorwoulddecrease.Furthermore,the ;transformationofchargestoragemechanismhada ;positiveeffectontheimprovementofthe

    ;electrochemicalresistance.Inthiscase,organicions ;andsolventoftheelectrolytewereconsideredto ;intercalateintographite

    ;tmmlgratlngintoporous

    ;dramaticdecreaseofions

    ;detaildiscussionaboutthe

    ;wasexhibitedsubsequently

    ;layerseasilyinsteadof

    ;carbon,leadingtothe

    ;disionresistance.The

    ;chargestoragemechanism

    ;45

    ;InfluenceofhightemperaturetreatmentofactivatedcarbonOilperformanceofsupercapac

    itors

    ;

    ;:

    ;Z’(Ohm)

    ;Fig.6Nyquistplotsofdifferentcapacitors ;Table3lsistancevaluesofdierentEDLCs

    ;3.4Chargestoragemechanism

    ;Hightemperaturetreatmenthadexertedgreat ;effectonelectrodematerials,meanwhile,charge ;storagemechanismwouldalsobeaffected, ;simultaneously.Cyclicvoltametry(CV)curvewasan ;effectivemethodtorevealcapacitivecharacteristicas ;wellasthechargestoragemechanismsofEDLC. ;Generally,ifporouscarbonmaterialwasusedas ;EDLCelectrode,thevoltammogramsweresupposedto ;exhibitatypicalrectangularshape.However,CV ;curvesofthecapacitorsassembledwithC-0,C1050,

    ;C.1200weredistortedandcurvesbecamerectangular ;shapewithheattreatmenttemperatureincreasing,as ;showninFig.7(a).Theresultsshouldbeascribedto ;highcontactresistanceReatelectrode/electrolyte ;interfaceduetotheexistenceofoxygencontaining

    ;functiongroups.Additionally,thepresenceoffaradic ;reactionswouldalsoplayanimportantroleinthe

    ;distortionofthevoltammograms.IfC-0wasfurther ;carbonizedat1500~Cfor2h,anapproximate

    ;rectangularshapeofCVcurvewithalittleof ;pseudocapacitanceinthesweeprangeof0-0.5and

    3Vwasobserved,whichsuggestedthatC1500 ;2.5

    ;samplehadthesuitablesurfaceoxygencontaining

    ;functiongroupsandporesizecharacteristicsforthe ;wettabilityofelectrolyteandmigrationoforganicions ;andsolvent.

    ;Potential(,,)

    ;Fig.7Thecyclicvoltammetry(Cv)curves

    ;ofdiIferentEDLCs

    ;Ifthetreatmenttemperaturewasfurtherincreasedto ;1800,2000,2800?,theCVcurvesweredistorted

    ;exhibitingnoanyrectangularshapecharacteristicsand ;thespecificcapacitancealsodecreasedsharplyinthe ;voltagerangeof0-2.5V,whilethecapacitanceincreased ;dramaficaHyinthevoltagerangeof2.53.5Vasshownin

    ;Fig.7(b).Theresultssuggestedthatchargestorage ;mechanismWaSdifferentfromtheformingof

    ;convenfionMelectricdoublelayeratelectrode/electrolyte ;interface.Tofurtheranalyzethemechanisms,the ;v昌口JnUl-昌一苗0.I.IIIU

    ;Influenceofhightemperaturetreatmentofactivatedcarbononperformanceofsupercapaci

    t0rs

    ;Galvanostaticcharge/dischargecurvewasapplied. ;Obviously,thechargedischargeprofileofC-1800shown

    ;inFig.8wasalsodifferentfromthatofthetypicalEDLC. ;Itcanbeseenthatthechargecurvecouldbedividedinto ;twosections,linearandnonlinearpart.Theformer

    ;representedthetypicalcharge/dischargecurveof ;doublelayercapacitanceinthepotentialrangeof ;0.052.7V.whilethelatterappearedbatterycharge ;behaviorwiththecharacteristicofagentleslopeinthe ;chargeprocess.Thistransformationofthecharge ;behaviorwasattributedtotheresultofintercalation ;capacitanceinthehighpotentialrangeof2.53.5V.

    ;Briefly,thechargeintercalationmechanismcouldbe ;consideredthatorganicionsandsolventsofthe ;electrolytewouldbeco-intercalatedintothegraphitic ;layersbytheforceofhighpotentialandmakethespecific ;capacitanceincreasesignificantly20_”.Moreover.they

    ;couldbeeasilyextractedalongtheorientedionic ;diffusionpathwayinthedischargeprocess.

;Time(s)

    ;Fig.8Thecharge/dischargecurveof

    ;C--1800--basedcapacitor

    ;4.Conclusion

    ;HightemperaturetreatmentofAChada

    ;significanteffectontheporestructure,surface ;chemicalperformanceaswellastheelectrochemicaI ;properties,suchastolerancepotential, ;chargepotential

    ;stabilityandchargestoragemechanisms.Ononehand, ;theintrinsicelectricconductivityofACcouldbe ;improvedafterheattreatedinhightemperature,

    ;while

    ;specificareawoulddecreasesimultaneously.Onthe ;otherhand,theefficientchargepotentialandstability ;couldbeenhancedcontributedtomodificationofthe ;surfaceoxygencontainingfunctiongroupsofAC. ;Additionally,thechargestoragemechanismwas ;dif.ferentfromtheconventionaImechanismsofEDLC ;duetotheformationofthecrystallographicstructure. ;Furtherinvestigationshouldfocusontheionic ;diffusionprocess,thematchbetweencrystalline ;structureofelectrodematerialandthetypeofion ;species,theconditionsofchae/dischargeorother

    ;factors.

    ;References:

    ;[1Burke,AUltracapacitors:Why,how,andwhereisthe ;technology.JoumalofPowerSources,2000,9,(1):3750.

    ;[2]Conway,B.E..ElectrochemicalSupercapacitorsNew ;York:KluwerAcademicPublisher,1?

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