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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[.

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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)