Effects of electrode layer composition/thickness and electrolyte concentration on both specific capacitance and energy density of supercapacitor

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Electrochimica Acta, 60, pp. 428-436, 2011-11-29

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Effects of electrode layer composition/thickness and electrolyte

concentration on both specific capacitance and energy density of

supercapacitor

Tsay, Keh-Chyun; Zhang, Lei; Zhang, Jiujun

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ContentslistsavailableatSciVerseScienceDirect

Electrochimica

Acta

jo u r n al h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Effects

of

electrode

layer

composition/thickness

and

electrolyte

concentration

on

both

specific

capacitance

and

energy

density

of

supercapacitor

Keh-Chyun

Tsay, Lei

Zhang

∗

_,

_Jiujun

_Zhang

InstituteforFuelCellInnovation,NationalResearchCouncilofCanada,Vancouver,BCV6T1W5,Canada

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28September2011 Receivedinrevisedform 22November2011 Accepted23November2011 Available online 29 November 2011 Keywords: Electrochemicalsupercapacitor Electrodelayer Activecarbon Compositionoptimization Electrolyte

a

b

s

t

r

a

c

t

Inthispaper,theeffectsofseveralexperimentalconditions,suchaselectrodelayerbindercontent, conductingcarboncontent,electrodelayerthickness,aswellaselectrolyteconcentration,onboththe specificcapacitanceandenergydensityofaBP2000carbon-basedsupercapacitorareinvestigatedusing bothcyclicvoltammetryandagalvaniccharging–dischargingcurve.Theelectrodelayerstudiedcontains SuperC45carbonastheconductingadditive,PTFEasthebinder,andNa2SO4astheaqueouselectrolyte,

respectively.Withthepurposeofoptimizingtheelectrodelayerstructure,15wt%ofSuperC45and5wt% ofPTFEintheelectrodelayerwithathicknessof100␮m,arefoundtobethebestcompositionintermsof improvingbothspecificcapacitanceandenergydensity.Regardingtheeffectofelectrolyteconcentration intherangeof0.1–1.0M,0.5MofNa2SO4givesthebestperformance.

1. Introduction

Inrecentyears,electrochemicalsupercapacitors(ESs)havebeen recognizedaskeyenergyefficiencydevicesforrapidenergystorage anddelivery.Thisisduetotheirhighpowerdensity,longlifecycle, highefficiency,widerangeofoperatingtemperature, environmen-talfriendliness,andsafety,aswellasservingasabridgingfunction forthepower/energygapbetweentraditionaldielectriccapacitors (whichhavehighpoweroutput)and batteries/fuelcells(which havehighenergystorage)[1,2].Withtheseadvantages,EShave becomeverycompetitiveforapplicationssuchaselectrichybrid vehicles,digital communicationdevices suchas mobilephones, digitalcameras,electricaltools,pulselasertechnique, uninterrupt-iblepowersupplies,andstorageoftheenergygeneratedbysolar cells[3].However,severalchallengessuchaslowenergydensity, highcost,andhighself-chargeratehavelimiteditswider applica-tions.

Regardinglowenergydensity,oneofthemajorlimitationsis inducedbythelowspecificcapacitanceoftheelectrodematerials used.Atthecurrentstateoftechnology,themostpracticalmaterial forESelectrodelayerisnano-scaledcarbon,suchasactivated car-bon,carbonaerogels,carbonnanotubes,templatedporouscarbons andcarbonnanofibers.Althoughsomehybridmaterialsbetween carbonandmetaloxidessuchasMnOxandRuOxhavebeenused

∗ Correspondingauthor.Tel.:+16042213000x5504;fax:+16042213001. E-mailaddress:lei.zhang@nrc.gc.ca(L.Zhang).

toconstructECelectrodelayers,theyarestillinastageofresearch anddevelopment[3].Therefore,nano-scaledcarbonsarestillthe preferredESelectrodematerials.

Ingeneral,theamountofenergystoredinnano-scaled carbon-basedsupercapacitorsisdeterminedbythespecificcapacitanceof thecarbon,theelectrical/ionicconductivityoftheelectrodelayer, theionicconductivityoftheelectrolytefilledseparator,aswellas thevoltagewindowoftheelectrolyte[4].Regardingspecific capac-itanceoftheelectrodelayer,specificsurfacearea,poresize,and electrodestructureofelectrodesaredominatingfactors[5–8].Two carbonsurfacesexistintheelectrodelayer:oneistheexternal sur-faceandtheotheristheinnersurface,andeachsurfacecangive theircorrespondingspecificcapacitances[9–12].Itfollowsboth surfacesmakecontributionstothetotalspecificcapacitance.From ourexperiments withCarbonblack BP2000-based material,we observedthatthecontributionfromtheexternalcarbonsurface tothetotalcapacitanceisaround40%andtheother60% contribu-tioncamefromtheinnermicroporoussurface.Barbierietal.[13]

studiedtherelationshipbetweenspecificcapacitanceofvarious activecarbonmaterialsandtheirspecificsurfaceareas,andfound thatthespecificcapacitancecouldincreaselinearlywithspecific surfaceareawithinaspecificrangeofsurfaceareas.Shietal.[5]

alsofoundalinearrelationshipbetweenspecificcapacitanceand surfacearea.Theseresultsindicatethatcarbonsurfaceareaplays animportantroleinincreasingspecificcapacitance.

Inadditiontousingcarbonmaterialwithhighsurfacearea,for practicalpurposes,typeofbinderanditsconcentration,thickness oftheelectrode layer,aswellaspreparation procedurearealso

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importantinachievinghighspecificcapacitance,andinturnhigh supercapacitorperformance.Thepurposeofthisworkisto opti-mizethesupercapacitor’sperformancebyinvestigatingtheeffects ofthesefactorsontheelectrodelayers.

In this paper,we report ourattempt in theoptimization of carbonbased supercapacitor performance usingin-house fabri-catedelectrode layersand modified test cellapparatus.Several factorsaffectingelectrodepreparationwereinvestigated includ-ingbindercontent,conductingcarboncontent,aswellaselectrode layerthickness.Electrochemicalmethodssuchascyclic voltam-metry(CV)andgalvaniccharging-discharging curve(GCC)were employedfortestingelectrodeperformance.

2. Experimental

2.1. Materialsforelectrodelayer

Forelectrode layerpreparation,Carbon blackBP2000 (Cabot Corporation),conductingcarbonSuperC45andSuperPLi(both fromTIMCAL),and 60wt%polytetrafluoroethylene(PTFE)-water dispersion(Aldrich)wereusedasreceivedwithoutfurther mod-ification.

2.2. Electrodelayerpreparation

Forelectrodelayerpreparation,carbonandconductingcarbon powdersweremixedwithaVortexMixer(ThermoScientific)for 30mintoformauniformlymixedpower.Thispowderwasthen transferredintoabeakercontainingbothPTFEbinderandethanol solutionunderconstantstirringtoformapowdersuspension.Then thetemperatureofthissuspensionwasraisedtoevaporatethe sol-vent.Aftermostoftheethanolwasremoved,thepasteformedwas collectedonaglassplate.Usingaspatula,thispastewas manipu-latedbyrepeatedlyfoldingandpressingtillasufficientmechanical strengthwasachieved.Thenthispastewasrolledtoathin elec-trodesheetwiththerequiredthicknessusingaMTIrollingpress. Finally,thiselectrodesheetwasplacedintoavacuumovenat90◦_C

underactivevacuumforatleast12h.

Forelectrodelayerpreparation,thedryelectrodesheetwascut intotwo2cm×2cmelectrodelayersquares.Thenthesetwo elec-trodelayerswere sandwichedwitha 30␮mthick porousPTFE separator (W.L.Gore &Associates, Inc.) in themiddle, forming anelectrode separator assembly (ESA). Theassembly was then impregnatedin anaqueouselectrolyte containing Na2SO4 with

desiredconcentrationinsideavacuumovenat60◦_C_for_at_least

1h.Thiselectrolyte-impregnatedESAwasthenassembledintothe two-electrodetestcellpurchasedbyIMIandmodifiedinour labo-ratory.

2.3. Two-electrodetestcellassemblingandelectrochemical measurements

Fig.1showsthetwo-electrodecelldesignfortestingand diag-nosingsupercapacitorperformance.Theactiveelectrodesurfaces forbothpositiveandnegativeelectrodesareall4.0cm2_._It_can_be

seenthatthetwoelectrodesareplacedbetweentwoTeflonplates withtwometalplatesoutside.Thesetwometalplatesserveas hold-erstotightentheESAtogether.Ononemetalplate,therearethree screws(M5×0.8mm)withathree-pointgeometry,whichensurea betterpressurebalancethanafour-pointgeometry.Betweeneach Teflonplateandelectrodethereisametalsheetserveascurrent collector.Duringassembly,screwsweretightenedwithatorque screwdriverto80cNm.Thepressurefromthestainlesssteelplates canbedirectlytransferredtotheESAthroughtwostainlesssteel coinsplacedonthebackofeachTeflonplateoftheinternalcell. Thetestcellwasthensetina100mLvolumebeakerfilledwith

about20mLofNa2SO4solution.Thebeakertogetherwiththetest

cellandelectrolytewereplacedinavacuumovenat60◦_C_for_at

leasthalfanhourtoremovetrappedair.

Forelectrochemicalmeasurements,cyclicvoltammogramsand galvanic charging–discharging curves were recorded using a Solartron1287potentiostat.Forcellinternalresistance measure-ments, a Tsuruga 3566 AC m-ohm Tester was used at an AC frequencyof1kHz.Allmeasurementsweremadeatroom tem-peratureandambientpressure.

2.4. Specificcapacitancecalculationbasedontheexperiment measurements

Sincethetwoelectrodesareidenticalandtheelectrodematerial is Carbonblack BP2000, which is non-faradic, the supercapaci-tors we studiedin this papershould besymmetrical and pure double-layer supercapacitors. For a symmetrical double-layer supercapacitor,thecapacitancemeasuredfromcyclic voltammo-gram(CV)orgalvaniccharging–dischargingcurve(GCC)inafull cellhardwareshouldbecalculatedaccordingtoEqs.(1)–(4)[14]:

1 CT = 1 Cp + 1 Cn (1)

where CT is the total capacitance (F)measured usingthe

two-electrode cell apparatus, Cp is the capacitance of the positive

electrodelayer,andCnisthecapacitanceofthenegativeelectrode

layer.Forasymmetricsupercapacitor,CnshouldequaltoCp.Ifwe

candefinetheindividualelectrodecapacitance(C):

C=Cp=Cn (2)

Eq.(1)willbecomeEq.(3):

CT=1

2C (3)

Forapositiveornegativeelectrode,thespecificcapacitanceofthe activematerial(CS)ofthiselectrodecanbedefinedas:

CS= C mact

= 2CT mact

(4)

wheremactisthemassweightofactivematerialinthepositiveor

negativeelectrodelayer.NotethatthisEq.(4)isonlyapplicable forasymmetricsupercapacitor.FromEq.(4),itcanbeseenthat thespecificcapacitancecanbeobtainedbymeasuringthetotalcell capacitance(CT).

ThetotalcapacitanceCTcanbecalculatedusingthemeasured

chargequalityQ,whichiseitherthetotalchargeatforward charg-ingdirection(Qf)orthatatthebackwardchargingdirection(Qb)in

thecellvoltagerangeofV1toVcell.IfV1istheinitialcellvoltage,

andsummingitequalsto0,andVcellisthevoltageatafullycharged

state.Normally,Qf=Qb.Therefore,CTcanbeexpressedasEq.(5):

CT= Q Vcell = Qf Vcell = Qb Vcell (5)

NotethatfromtheCVcurve,thechargesQ_fandQ_bcanbeobtained throughintegration,asexpressedbyEqs.(6)and(7):

Qf = V

_cell V1 I(V )dt= V

_cell V1 I(V )

v

dV (6) Qb= V₁

V_cell I(V )dt= V₁

V_cell I(V )

v

dV (7)

where

v

isthevoltagescanrate,I(V)isthecurrent,whichisa

func-tionofcell voltage(V),dV anddt arethecellvoltageand time changeswiththevoltagescanning,respectively.Inasimilarway,

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Fig.1. Schematicillustrationofthetwo-electrodesupercapacitortestcell.

fromthegalvaniccharging–dischargingcurve(boththecharging current,If,anddischargingcurrent,Ib,areconstant),thechargesQf

andQbcanalsobeobtainedthroughintegration,asexpressedby

Eqs.(8)and(9): Qf = t

_end 0 Idt=I_ft_end (8) Qb= t

end 0 Idt=Ibtend (9)

wheretendisthechargingordischargingtimewhenthe

superca-pacitorisfullychargedordischarged.

2.5. Energydensitycalculationbasedonthemeasuredcell voltageandspecificcapacitance

Inordertoobtaintheexpressionofenergyforasupercapacitor,

Fig.2schematicallyshowsthechargingcurveofadouble-layer supercapacitorataconstantchargingcurrentofIf.Assumingthat

t dt end V Vcell 0 Time Cell Voltage Charging

S

₁ Vdt

Charging at a constant current (I )t

S

₂

Fig.2.Schematicillustrationofthechargingcurveofapuredouble-layer super-capacitor(theequivalentseriesresistance(ESR)andparallelleakageresistanceare notconsideredhere).

thecellisfullychargedatatimeoftend,givingacellvoltageofVcell,

asshowninFig.2,theenergy(ESC)canbeexpressedas:

ESC=QfV= t

_end 0 If dtV=If t

_end 0 Vdt (10)

FromFig.2,itcanbeseenthattheareaunderthecharginglineis markedbyS1,andtheareaabovethecharginglineismarkedbyS2.

Thetermof

t

end

0

VdtinEq.(10)isequaltoS1,whichcanbeexpressed

asEq.(11): t

_end 0 Vdt= S1 S1+S2 Vcelltend (11)

CombiningEq.(11)withEq.(10),wecanobtain:

ESC= S1 S1+S2

IftendVcell= S1 S1+S2

QfVcell (12)

CombiningEq.(5)withEq.(12),ageneralexpressionforenergycan beobtained:

ESC= S1 S1+S₂CTV

2

cell (13)

For a puredouble-layersupercapacitor, thechargingcurveis a straightline,soS1=S2,suggestingthatS1/(S1+S2)=1/2.Therefore,

Eq.(13)canbewrittenasEq.(14):

ESC= 1 2CTV

2

cell (14)

Thisequationhasbeenwildlydiscussedandusedinliterature[15]. ItshouldbenotedthatthisEq.(14)isonlyapplicabletothecaseof idealdouble-layersupercapacitorswithoutequivalentseries resis-tanceandparallelleakageresistance.Iftheelectrodematerialis Faradaicorhybridizedbetweendouble-layerandFaradaic materi-als,thechargingcurvemaynotbeapproximatedtoastraightline, sothatS1isnotequaltoS2.Inthiscase,thefactorS1/(S1+S2)will

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-0.02 -0.01 0 0.01 0.02 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

Cell Voltage, Volt

Current Density, A/cm

2 100mVps 50mVps 20mVps 10mVps 5mVps 2mVps 40 60 80 100 120 120 100 80 60 40 20 0 Scan Rate, mV/s Specific Capacitance, F/g

(b)

(a)

Fig.3. (a)Cyclicvoltammograms,recordedatvariousvoltagescanratesusinga car-bonBP2000-basedsupercapacitorwithanelectrodecompositionofBP2000:Super C45:PTFE=80:15:5(wt%),electrodethicknessof100␮m,andactivecarbonloading of3.0mg/cm2_._(b)_Specific_capacitance_as_a_function_of_voltage_scan_rate.

Furthermore,theenergydensityoftheactiveelectrode

mate-rial(ESC-act)canbeobtainedbydividingCTinEq.(13)bythemass

weightofactiveelectrodematerialmact:

ESC-act=1 2 CT mactV 2 cell (15)

Iftheindividualelectrodecapacitance(C)isobtainedbyCVmethod, theenergydensityoftheactiveelectrodematerialcanbeexpressed alternativelybasedonEqs.(3)and(15):

ESC-act=1 4 C mact V2 cell= 1 4CSV 2 cell (16)

Again,thisequationisonlyapplicabletothecaseofpure double-layersupercapacitors.ForaFaradaicorhybridsupercapacitor,Eq.

(13)shouldbeused.

3. Resultsanddiscussion

3.1. Specificcapacitancemeasuredbycyclicvoltammetryandthe effectofpotentialscanrateonspecificcapacitance

Fig. 3(a) shows the cyclic voltammograms of a symmetri-calsupercapacitorcellwhosebothelectrodeswerecomposedof BP2000carbonparticles.TheelectrodelayerconsistedofBP2000 carbon,SuperC45and PTFEwithawt%ratioof 80:15:5.It can beseenthatatlowscanrates,theCVsdisplayanidealcapacitive behaviour(rectangularshape),butwhenthescanrateisincreased, thisidealbehaviourisdistortedwithagraduallossincellspecific capacitancewhichismeasuredand calculatedaccordingtoEqs.

(1)–(4).Formoreclarity,Fig.3(b)showsthedependencyof spe-cificcapacitanceonpotentialscanrate.Theobserveddecreasein

0 2 4 6 8 10 120 100 80 60 40 20 0 Discharge Current mA Energy Density, Wh/kg

(c)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 250 200 150 100 50 0 Time, Sec

Cell Voltage, Volt

100mA 50mA 20mA 10mA 5mA 20 40 60 80 100 120 120 100 80 60 40 20 0 Discharge Current mA Specific Capacitance, F/g

(a)

(b)

Fig. 4.(a) Charging–discharging curves, recorded at various charging rates usingacarbonBP2000-basedsupercapacitorwithanelectrodecompositionof BP2000:SuperC45:PTFE=80:15:5(wt%),electrodethicknessof100␮m,andactive carbonloadingof3.0mg/cm2_;_(b)_specific_capacitance_as_a_function_of_charging_rate;

and(c)energydensityasafunctionofchargingrate.

specificcapacitancewithincreasingthepotentialscanrateisdue tothelimitedtransferofionstothecarbonparticlesurface, result-ingininaccessibleporeportionsoftheelectrodelayerathighscan rates(chargingrates).Thisphenomenonistypicalforalltypesof supercapacitors,reflectingthelimitedmasstransferkineticswithin theporouselectrodelayer[16–18].

3.2. Specificcapacitanceandenergydensitymeasuredbycell charging–dischargingcurves,andtheeffectofchargingrateon theperformance

Fig.4(a)showsthecharging–dischargingcurvesatthecharging ratesfrom5mAto100mAinthecellvoltagerangebetween0and 1.0V.Thischarging–dischargingcurrentrangecoversnearlythe samerangeincyclicvoltammogrammeasurementsexcept2and

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60 65 70 75 80 85 90 95 12 10 8 6 4 2 0

PTFE Binder Content, wt%

Specif ic Capacitance, F/ g 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 Energy Density, Wh/kg

Fig.5.SpecificcapacitanceandenergydensityofcarbonBP2000electrodematerial asafunctionofPTFEcontents.DatafromTable1.

5mV/swhichhavecharging/dischargingcurrentslessthan1mA. Thespecificcapacitancesatvariouscharging–dischargingcurrents werecalculatedbasedonthemeasuredCT,Vcell,andQusingEq.(5),

andplottedasafunctionofcharging–dischargingcurrent,asshown inFig.4(b).Basedonthesespecificcapacitancevalues,theenergy densitycanbecalculatedaccordingtoEq.(16),andplottedasa functionofcharging–dischargingcurrent,asinFig.4(c).Specific capacitanceobtainedfromlowchargingratesareslightlyhigher thanthosefromhighrates.Thisisduetolimitedionaccessible timetothemicropore[6].Theaverageparticlesizereportedis about12–15nm[19,20]andsomemicroporeswithsizelessthan 4nmcontributeasignificantportionofporevolumeaccordingto ourBETmeasurements.Smallermicroporesaremoredifficultto accessandrequirethecharging–dischargingratetobelowerthan acriticalvalue.Whenthecharging–dischargingrateishigherthan thiscriticalvalue,currentperturbationisunabletoreachinsidethe pore.

3.3. Effectofbindercontent

Themainrolesofabinderinside theelectrodelayerare:(1) to holdcarbon particles togetherforming a compacted porous layer;and(2)tohelpthiselectrodelayertoadhereuniformlyonto thecurrentcollector.Materialswithathree-dimensionalnetwork structurelinkedbylongmolecularchains,andgood electrochemi-calstabilityaswellasgoodadhesiontometalplatearedesiredfor electrodelayerbinders.Withrespecttothis, polytetrafluoroethy-lene(PTFE)hasbeenrecognizedasoneofthemostpopularchoices. However,PTFEisahydrophobicagent,andiftoomuchPTFEisused,

60 65 70 75 80 85 90 95 25 20 15 10 5 0

Conducting Carbon Content, wt%

Specific Capacitance, F/g 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 Energy Den s ity, Wh/kg

Fig.6.SpecificcapacitanceandenergydensityofcarbonBP2000electrodematerial

asafunctionofconductingcarboncontent.DatafromTable2. Table

1 Performance data of Carbon black BP2000-based supercapacitor, measured by both cyclic voltammetry (at a potential scan rate of 20 mV/s), and galvanic charge–discharge curves (at a charging–discharging rate of 5 mA/cm 2). The electrolyte: 0.5 M Na 2 SO 4 aqueous solution (Room temperature); conducting carbon (Super C45) content in the electrode layer: 15 wt%; and electrode layer thickness: 100 ␮ m. Binder content (wt%) Active carbon loading, mact mg/cm 2 Specific capacitance from CV, CS (F/g active carbon) Electrode geometric capacitance from CV, C ′(F/cm 2) Specific capacitance from GC, CS (F/g active carbon) Electrode geometric capacitance from GC, C ′(F/cm 2) Average specific capacitance, CS (F/g active carbon) Cell voltage, Vcell (V) Energy density, Espc-act (Wh/kg active carbon) 3.0 2.95 80.0 0.118 82.3 0.121 81.2 1.00 5.64 5.0 3.00 86.1 0.129 88.2 0.132 87.2 1.00 6.05 8.0 2.73 80.6 0.110 83.2 0.114 81.9 1.00 5.69 10.0 2.70 75.9 0.102 77.5 0.105 76.7 1.00 5.33

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K.-C. Tsay et al. / Electrochimica Acta 60 (2012) 428– 436 433

PerformancedataofCarbonblackBP2000-basedsupercapacitor,measuredbybothcyclicvoltammetry(atapotentialscanrateof20mV/s),andgalvaniccharge–dischargecurves(atacharging–dischargingrateof20mA/cm).

Theelectrolyteis0.5MNa2SO4aqueoussolution(roomtemperature).PTFEbindercontentintheelectrodelayer:5wt%;electrodelayerthickness:100␮m.

Conductingcarbon content(wt%) Activecarbon loading,mactmg/cm2 Specificcapacitance fromCV,CS(F/g activecarbon) Electrodegeometric capacitancefromCV, C′ (F/cm2₎ Specificcapacitance fromGC,CS(F/g activecarbon) Electrodegeometric capacitancefromGC, C′ (F/cm2₎ Averagespecific capacitance,CS(F/g activecarbon) Cellvoltage, Vcell(V) Energydensity, Espc-act(Wh/kg activecarbon) 5 3.11 80.0 0.126 92.5 0.144 86.3 1.00 6.00 10 3.49 86.1 0.150 88.7 0.155 87.4 1.00 6.07 15 3.00 80.6 0.129 88.2 0.132 84.4 1.00 5.86 20 2.68 75.9 0.107 87.0 0.117 81.5 1.00 5.66 Table3

PerformancedataofCarbonblackBP2000-basedsupercapacitor,measuredbybothcyclicvoltammetry(atapotentialscanrateof20mV/s),andgalvaniccharge–dischargecurves(atacharging–dischargingrateof5mA/cm2_).

Theelectrolyteis0.5MNa2SO4aqueoussolution(roomtemperature).Conductingcarbon(SuperC45)contentintheelectrodelayer:15wt%;PTFEbindercontent:5wt%.

Electrodelayer thickness,␮m Activecarbon loading,mactmg/cm2 Specificcapacitance fromCV,CS(F/g activecarbon) Electrodegeometric capacitancefromCV, C′ (F/cm2₎ Specificcapacitance fromGC,CS(F/g activecarbon) Electrodegeometric capacitancefromGC, C′ (F/cm2₎ Averagespecific capacitance,CS(F/g activecarbon) Cellvoltage, Vcell(V) Energydensity, Espc-act(Wh/kg activecarbon) 50 1.46 84.8 0.062 80.7 0.059 82.8 1.00 5.75 100 3.00 86.1 0.129 88.2 0.132 87.2 1.00 6.06 200 5.64 84.1 0.237 87.0 0.245 85.6 1.00 5.94 300 7.94 78.9 0.313 84.8 0.337 81.9 1.00 5.69

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Table 4 Performance data of Carbon black BP2000-based supercapacitor, measured by both cyclic voltammetry (at a potential scan rate of 20 mV/s), and galvanic charge–discharge curves (at a charging-discharging rate of 5 mA/cm 2). Conducting carbon (Super C45) content in the electrode layer: 15 wt%; PTFE binder content: 5 wt%; electrode thickness: 100 ␮ m. Na 2 SO 4 concentration, M Active carbon loading, mact mg/cm 2 Specific capacitance from CV, CS (F/g active carbon) Electrode geometric capacitance from CV, C ′(F/cm 2) Specific capacitance from GC, CS (F/g active carbon) Electrode geometric capacitance from GC, C ′(F/cm 2) Average specific capacitance, CS (F/g active carbon) Cell voltage, Vcell (V) Energy density, Espc-act (Wh/kg active carbon) 1 3.12 78.1 0.122 81.1 0.126 79.6 1.00 5.53 0.5 3.00 86.1 0.129 88.2 0.132 87.2 1.00 6.06 0.2 3.02 74.7 0.113 76.9 0.116 75.8 1.00 5.26 0.1 2.80 74.2 0.104 77.4 0.108 75.8 1.00 5.26

theporouselectrodelayerwillbecomemorehydrophobic,

lead-ingtoadifficultyinbothelectrolytepenetrationandionmobility

insidethematrixstructurewhenaqueouselectrolyteisemployed.

Asaresult,theelectrodeperformancewillbereduced.Therefore,

thePTFEcontentshouldbeoptimized.Inthispaper,several

typi-calPTFEcontentsweretestedwiththepurposeofelectrodelayer

optimizationwithrespecttotheperformance.

Table1showsthedataobtainedatvariousPTFEbindercontents intheelectrodelayerbytwomethods(cyclicvoltammetry(CV)and galvaniccharging–dischargingcurve(GCC)).Itcanbeseenthatthe specificcapacitancedifferencesbetweenthosedataobtainedbyCV andthosebyGCCareinsignificant(∼2%difference),suggestingthat thedataobtainedhereisreliable.

Toseethetrend moreclearly, bothspecific capacitanceand energydensityoftheelectrodeactivematerialswerealsoplotted asafunctionofPTFEcontent,asshowninFig.5.Itcanbeseenthata maximumspecificcapacitanceexists(∼86–88F/g)ataPTFEbinder contentof5%.WithanyfurtherincreaseinPTFEcontent,thespecific capacitanceisdecreased.Thisismainlybecausethehydrophobicity oftheelectrodelayercanbeincreasedwithincreasingPTFEbinder content,preventingaqueouselectrolyteionsfromenteringpores intheactivecarbonparticles,leadingtolesschargeaccumulation inthedouble-layer,resultinginreducedspecificcapacitance.The trade-offbetweenmoreporeformationandincreasing hydropho-bicitywithincreasingPTFEcontentshouldgiveanoptimizedPTFE content,whichis5wt%inthispaper.Intheliterature,hydrophilic Nafion®_ionomer_was_used_as_the_binder_[21]_,_where

hydrophobic-itydidnotseemtobeanissueintheelectrodelayer.IntheNafion®

bindercontentrangefrom10to30%,nosignificantinfluenceon specificcapacitancecouldbeobserved.

3.4. Effectofconductingcarboncontent

Normally,highsurfaceareaactivatedcarbonsuchasBP2000 giveshigherspecificcapacitancebuthasapoorelectronic conduc-tivityandthuscausesahighercellinternalresistance.Conducting carbonhasahigherelectronicconductivitythanactivatedcarbon and is normallyused toimprove electronicconductivityof the supercapacitorelectrodelayer.Becausetheconductingcarbonhas alowsurfaceareacomparedtoactivatedcarbon,itscontributionto electrodecapacitanceisinsignificant.Inthispaper,both conduct-ingcarbons,SuperC45andSuperPLiwereusedintheelectrode layerstoimprovetheelectronicconductivityoftheelectrodelayer. ItwasfoundthatthesupercapacitorcellwithSuperC45electrodes hadalowerinternalresistancecomparedtothatwithSuperPLi, andbothconductingcarbonshadamaximumspecificcapacitance iftheircontentsareatarangeof10–15wt%.Themaximumspecific capacitancesobservedarenearlythesame,whichisintherangeof 88–92F/g.Table2showsthetypicalsupercapacitorperformance datawhenSuperC45isusedastheconductingcarbon.Itcanbe seenthatbothmethods(CVandGCC)couldgivethesimilar val-uesofspecificcapacitance.Fig.6showsbothspecificcapacitance andenergydensityasafunctionofconductingcarboncontent.The maximumvaluesforspecificcapacitanceandenergydensitycould beachievedwhentheconductingcarboncontentisintherangeof 10–15wt%.

Drops in both specific capacitance and energy density with increasingconductingcarboncontentseeninFig.6couldbemainly causedbyreducedactivesurfaceareainsidetheelectrodelayer whenmoreconductingcarbonwithsmallersurfaceareaisputinto theelectrodelayer.

3.5. Effectofelectrodethickness

Electrodelayerthicknessdirectlyrelatestovolumetric capac-itance. Thicker electrodes containing more active carbon offer

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60 65 70 75 80 85 90 95 350 300 250 200 150 100 50 0

Electrode layer thickness, um

Specific Capacitance, F/g 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 Energy Density, Wh/kg

Fig.7.SpecificcapacitanceandenergydensityofcarbonBP2000electrodematerial asafunctionofelectrodelayerthickness.DatafromTable3.

highervolumetriccapacitance,asseeninTable3.Notethatthe SuperC45conductingcarbonandthePTFEbindercontentused hereare15wt%and 5wt%,respectively,whichhavegiven opti-mizedperformance,asshowninTables1and2.However,itcan beseen from Table 3 that withincreasing electrode thickness, thespecificcapacitanceisdecreasedbecausethetransportofthe electrolytic ions into/from active layerbecomes more difficult. Therefore, an optimized electrode layerthickness exists. Fig. 7

showsboththespecificcapacitanceandenergydensityasa func-tionofelectrodelayerthickness.Themaximumvaluesforspecific capacitanceandenergydensitycouldbeachievedwhenthe elec-trodelayerthicknessis∼100␮m.

3.6. Effectofelectrolyteconcentration

Intheliterature[22],electrolyteconcentrationshowedastrong effect on supercapacitor capacitance. For example, if the elec-trolyteconcentrationishigh,theiontransportwithintheelectrode layerwillbeeasier,leadingtoaneffectivebuilding-upfordouble layer.However,iftheelectrolyteconcentrationistoohigh,theion activitymaybereducedduetolesswaterhydration,resultingin decreasesionmobility. Therefore,anoptimizedelectrolyte con-centrationshouldexist.Ingeneral,foraqueouselectrolytebased supercapacitors,themostpopularelectrolyteisaqueousNa2SO4.

Therefore, in this work,we employed Na2SO4 asa

representa-tive system to investigate the electrolyte concentration effect.

Table4 displaystheperformancedataof Carbonblack BP2000-basedsupercapacitor,collectedusingbothcyclicvoltammetryand

60 65 70 75 80 85 90 95 1.2 1 0.8 0.6 0.4 0.2 0 Electrolyte Concentration, M Specific Capacitan c e, F/g 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 Energy Density, Wh/kg

Fig.8.SpecificcapacitanceandenergydensityofcarbonBP2000electrodematerial asafunctionofNa2SO4electrolyteconcentration.DatafromTable4.

galvaniccharging–dischargingcurveatdifferentNa2SO4

concen-trations.Inthesemeasurements,thethicknessofelectrodelayer was 100␮m, the conducting carbon (Super C45) content was 15wt%,andthePTFEbindercontentwas5wt%.Thiscompositionof theelectrodelayerhasbeenidentifiedasthefavouritecomposition asshowninTables1–3.

Fig.8 shows both the specific capacitance and energy den-sityofcarbonBP2000electrodematerialasafunctionofNa2SO4

electrolyteconcentration.Itcanbeclearlyseenthatbothspecific capacitanceandenergydensityarethehighestwhentheelectrolyte concentrationis0.5M.Whentheelectrolyteconcentrationislower than0.5M,bothofthemarelow,whichmaybedueaninsufficient numberofionsfordouble-layerbuilding-up.Thisobservationis consistentwiththeliterature[22].However,whentheelectrolyte concentrationishigherthan0.5M,bothspecificcapacitanceand energydensityarereduced, whichcouldbedue toreduced ion activity(lesswaterhydration)athigherelectrolyteconcentrations.

4. Conclusions

Inordertooptimizesupercapacitorperformance,theeffectof severalexperimentalconditions, suchaselectrode layerbinder content,conductingcarboncontent,electrodelayerthickness,as well as electrolyte concentration, on both the specific capaci-tanceandenergydensityofanelectrodeactivematerial(BP2000), were investigated using both cyclic voltammetry and galvanic charging–dischargingcurves.Inelectrodelayerfabrication,Super C45 carbon was used as the conducting additive, PTFE as the binder, and Na2SO4 as the aqueous electrolyte, respectively. It

wasfound that 15wt% of Super C45 and 5wt% of PTFE in the electrode layerwitha thickness of 100␮m couldgivea better performance in terms of both specific capacitance and energy density.Regardingtheeffectofelectrolyteconcentration,it was observedthat0.5MofNa2SO4 couldgivethebestperformance

within the tested electrolyte concentration range from 0.1 to 1.0M.

Acknowledgements

This work is supported by Transport Canada and National Research Council of Canada’s Institutefor Fuel Cell Innovation. DiscussionwithDr.LucieRobitalille,Dr.AlexisLaforgue,Dr. Dong-fangYang,Dr.YvesGrincourt,Dr.YonghongBing,Ms.JennyKim, Mr.WeiQuandRyanBakerarehighlyappreciated.Theauthors would also like to thank TIMCAL for providing materials and information.

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