Organic Electrolytes for Graphene-Based Supercapacitor: Liquid, Gel or Solid

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Materials

Today

Communications

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

Organic

electrolytes

for

graphene-based

supercapacitor:

Liquid,

gel

or

solid

Evgeniya

Kovalska

∗

,

Coskun

Kocabas

DepartmentofPhysics,BilkentUniversity,06800Ankara,Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received26April2016 Accepted27April2016 Availableonline6May2016 Keywords:

Graphene-basedsupercapacitor Organicelectrolyte

Electricalandelectro-opticalfeatures Opticalmodulator

a

b

s

t

r

a

c

t

Theelectrolyteisanimportantanddecisivefactorinbattery,capacitorandsupercapacitorfabrication. Herewereporthowelectrolyte’sprovenanceandstructureeffectontheelectro-opticalpropertiesof thegraphene-basedsupercapacitor.Thethreeorganicelectrolytesweresynthesized:liquidelectrolyte, whichonthebasisofpropylenecarbonate(PC),gelelectrolyte–polyvinylalcohol(PVA)andsolid electrolyte–polyvinylideneﬂuoride (PVDF).Asanapplication,wedemonstrateanoptical modula-torusingsupercapacitorstructurebuiltbygrapheneelectrodesandpreparedelectrolytes.Allorganic electrolyte-basedsupercapacitorspotentiateopticalmodulationofgrapheneelectrodesoverabroad rangeofwavelengths,underambientconditions.Werevealhighercapacitance(78␮F/cm2₎_for_a

superca-pacitorwithgelelectrolyteduringvariousbiasvoltages.Werepresenttheincreasingoflighttransmission at3timesusingsolidelectrolyte,incomparisonwithliquidandgelelectrolytesandillustratethe super-capacitorpossibilitywithgelelectrolytetooperateundernegativevoltage.Consequently,wesuggest applyingofsolidelectrolyteasamoreappropriateelectrolyteforfabricationofgraphene-based super-capacitor.Weanticipatethatusingofsolidelectrolyteallowsustogetdesiredelectro-opticalproperties, minimizethesizeofthedeviceandvaryitshape.

1. Introduction

Thesupercapacitorhistory hasmorethanadozenyears,but

techniquedemandofsuchdevicesstillmakesscientistslookfor

waystoreducethecostoftheirdevelopmentandproduction[1].

Supercapacitor,whichalsowell-knownasultra-orelectric

double-layercapacitorstores energy through reversibleion adsorption

ontoactivematerialswithhighspeciﬁcsurfacearea[2]:activated

carbon[3],graphene[4,5],carbonaerogel[6,7],orcarbon

nano-tubes[8,9].Exceptcarbonmaterials,researchersuseelectroactive

oxideorhydroxideﬁlmsoftransitionmetals(MnO2[10],RuO2[11],

NiO[12],MoO3[13])andconductingpolymers(polypyrrole[14],

polyaniline[15],andpolythiophene[16]etc.).Thecyclicstability

andperformanceofsupercapacitordependonpropertiesof

elec-trolyteaswell.Electrolytes–theseareliquid,gelorsolidsystems

whichmusthavethehighconcentrationofmobileions,low

resis-tance,lowconcentrationofelectricallyactiveimpurities,andbe

chemicallystable[17,18].Forthepurposetogetquality

superca-pacitorwehavetoﬁndtheoptimalcombinationofelectrodeswith

electrolyte.

∗ Correspondingauthor.

E-mailaddress:ikovalska@bilkent.edu.tr(E.Kovalska).

Liquidelectrolytes whichconsist ofwater and organicchains

haveionicconductivityupto1S/cm,highdielectricconstant[18],

andgivehigherspeciﬁccapacitanceofactivematerialsthanthe

organic-basedelectrolyte.Thecellvoltageofsupercapacitorsbased

onaqueouselectrolyteislower(1V)[19]thanorganicelectrolyte

(2.5V)[20].Thus,wewereabletoobtainsigniﬁcantlybetterresults

withnon-aqueous[21],biologicalsubstances[22]orpolymersolid

electrolyte[23].Toavoidtheshaperestrictions,leakageordrying

ofelectrolyte(thatinherentofliquids)wecanusegelelectrolyte.

It hasfastcharging/dischargingandhighpowerdensity aswell

[24]. Currently,the majorpolymersfor gelelectrolyte

prepara-tionarepolyethyleneoxide[25],polyacrylonitrile[26],polymethyl

methacrylate[27]polyvinylideneﬂuoride[28]etc.Relativelynew

electrolyteforsupercapacitordesignissolidelectrolyte[29,30].This

materialoffersmanyadvantages overtheliquidsandgels:

con-ductionofelectricity,duetotheionmovementthroughvoidsor

defects,inowncrystallattice;leakageresistance,duetodispersion

andﬁxation,intoapolymermatrix;dualfunctionalizationas

sepa-ratorandelectrolyte.Solidelectrolytesimplementinthedesignof

ﬂexibleandnonﬂexiblesupercapacitorsandshowexcellent

elec-trochemicalperformance[9,31,32].

Inanycase, propertiesofelectrolyteand itsstructure

deter-mine applied aspects of the supercapacitor. Thus, we report

syntheses of liquid, gel and solid organic electrolytes and

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growthwascarriedoutcoolingdownwithH2 ﬂowinguntilthe

temperaturewasbelow100◦C.Grapheneoncopperwastakenout

oftheCVD-reactorwhenthetemperaturereached50–60◦C.

2.2. TransferofmonolayergrapheneusingS1813

A photoresist S1813 (Shipley Company) supported transfer

methodwasutilizedtotransfergraphene(withtheareaaround

1×2cm)tothepolishquartzwafersforsupercapacitor

fabrica-tion.TheetchingprocesseswereperformedbyFeCl3 forCu-foil

andusingacetoneforremovingofphotoresist.

2.3. Fabricationgraphene-basedsupercapacitor

Supercapacitorcellwasformedusingtwocoveredbygraphene

glasswafers.Afterward,thevastnessbetweentwoelectrodeswas

ﬁlledwithanelectrolyteaccordingtoitstype.Wehaveusedthe

pipetteforliquidandgelelectrolytesandtweezersforathin

trans-parentﬁlmofsolidone.

2.4. Preparationoftheelectrolytes

Differenttypesofelectrolytesystemswhichbasedonorganic

chemicalswereprepared.

LiquidelectrolytewithactiveLi+_-ions_were_obtained_using_a

sol-ventpropylenecarbonate(PC–C4H6O3,99.7%,Sigma-Aldrich)and

thesourceofpositiveions–lithiumbis(oxalate)borate(LiBOB–

LiB(C2O4)2,powderofcrystals,Sigma-Aldrich).Themixturewas

stirredaround2.Atransparentanddilutedmilkcolorelectrolyte

withaconcentrationofLi+_-ions₁₀_wt.%._were_ﬁnally_obtained.

LiBOBwasusedforgelelectrolytepreparationaswell.Polyvinyl

alcohol (PVA – (CH2CHOH)n, Sigma-Aldrich) with an average

molecularweightof20,000wasdissolvedin10%Li+_-ions_aqueous

solution.Glycerin(C3H8O3,Birpa,Ankara)wasaddedasastabilizer.

Lastly,wehavegotgoodpellucidgelelectrolyte.

A solid electrolyte was obtained using polyvinylidene

ﬂu-oride, with nominal Mn=130,000g/mol, Mw=400,000g/mol,

melting point 140–145◦C (PVDF) – molecular structure is

[CH2 CF2]n [CF2 CF(CF3)]m ,Sigma-Aldrich.Thepolymer

solu-tionwas preparedby dissolving10wt.%of PVDF inacetone at

70◦C under magnetic stirring for at least 30min until a clear

homogeneoussolutionwasobtained.Theionicliquidintheratio

1:4(polymer:ionicliquid,respectively)wasaddedtothepolymer

solutionundermagneticstirring andtheydissolvedcompletely

ina few minutes.Thefollowing ionicliquid whichsupplied by

Sigma-Aldrichwereused:1-butyl-3-methylimidazolium

hexaﬂu-orophosphate–C8H15F6N2P.Theﬁlmswerepreparedbysolution

castingofthepolymer/ionicliquidmixtureinacetoneonaPetri

dishandbysubsequentsolventevaporationatroomtemperature

for24h.Further,theﬁlmsweredriedat70◦Cfor4htoguarantee

completeremovalofthevolatilesolvent.

range between500and 1100nm.Thegraphene-based

superca-pacitorwasbiasedusingKeithley2400sourcemeasureunitthe

transmittancemeasurements.

3. Resultanddiscussion

The choice of graphene as an electrode caused by its high

absorptioncoefﬁcientinbroadspectralrange,excellenttransport

propertiesandgate-tunablecarrierdensity.Theabilitytoproduce

high-qualitysingle-layergraphene[33,34],andtransferitonto

sub-stratemakespracticalapplicationofgrapheneforoptoelectronic

devices.Nowadaysgrapheneintegratedwithsiliconphotonicsis

especiallyinteresting[35]andwouldbeofuseforoptical

commu-nicationapplications[36].Forthispurposegraphene-on-graphene

optical modulator with parallel plate geometry was described

[37,38]. However, we would like to consider broadband

opti-calmodulatorwhichconsistsoftwographeneelectrodesonthe

glass/PVCwafersandelectrolytebetweenthem.Thedeviceshave

simpleparallelplategeometryandbasedonthesupercapacitor

structure.

3.1. Graphene-basedsupercapacitor

We present graphene-based supercapacitor which was

fab-ricated using glass wafers, covered with single-layergraphene

(Fig.1),andliquidorgelelectrolytes(Fig.2a);ﬂexible

supercapac-itor,whereissolidelectrolytebetweentwographeneelectrodes

(Fig.2b).

Graphene-based device works on the principle of a typical

supercapacitor(Fig.2c),whichstoreselectricchargedirectlyacross

theinterface.Themechanismofsurfacechargegenerationcanbe

enumeratedassurfacedissociationandionadsorptionfromthe

electrolytesolution.Thecapacitancearisesfroman

electrochemi-caldoublelayeranditsthicknessdependsontheconcentrationof

theelectrolyteandsizeofactiveions.

3.2. Electrolytesforgraphene-basedsupercapacitor

Wetestedelectrolytesofvariouscompositionandstate:theﬁrst

electrolyteisliquid,thefollowingﬁve–gelsandremaining–are

solids(Table1).Toavoidleakageofelectrolyteandshape

limita-tionofthedeviceweusepolymericmatrixes:polyvinylalcohol

(PVA)andpolyvinylideneﬂuoride(PVDF);asasourceofionswe

uselithiumsaltorinorganicacids.Herein,wedistinguishand

com-parethreeelectrolytes,whichinouropinionaremostsuitablefor

supercapacitorsasanopticalmodulators[39].

3.3. Liquidelectrolyte–PC/LiBOB

We prepared the solutionof lithium bis (oxalate) borate in

propylene carbonate (PC/LiBOB) [40]. The choice fell onLiBOB

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Fig.1.SEMimage(a)andtypicalRamanspectra(b)ofsinglelayergrapheneﬁlmcoatedonSi/SiO2substrate.

Fig.2.Schematicexplodedviewofthenon-ﬂexible(a)andﬂexible(b)opticallytransparentsupercapacitorsformedbytwoparallelgrapheneelectrodestransfer-printed onglass(a)orPVC(b)substratesandtheelectrolytemediumbetweenthem;schematicdrawingofbehaviorofthesupercapacitor(c).

Table1

Electrolytesforgraphene-basedsupercapacitor.

N TypeofEL Ratio Voltage(V) Modulation(%)

1 PC/LiBOB 1/1.1 0–2.0 1.0 2 PVA/H3PO4 1/1.5 0to−3.0/0to3.0 1.6/1.8 3 PVA/H2SO4 1/1.5 0to−3.0/0to3.0 2.3/2.5 4 PVA/LiBOB 3/1 0to−2.8/0to2.8 0.4/2.9 5 PVA/LiBOB/Eth/Ac 1/1 0to−2.4/0to3.0 2.7/3.2 6 PEO – 0to−3.0/0to3.0 1.0/2.3 7 PVDFinacetone/ionicliquid 1/4 0to2.8 3.2 8 PVDFinacetonitrile/ionicliquid 1/4 0to−2.8/0to3.0 1.3/1.5 9 PVDF/PEO 9/1 0to−3.5/0to3.0 2.3/1.3

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Fig.3.Schematicbehaviorbetweenpropylenecarbonateandlithiumbis(oxalate) borate:lithiumcationﬁndsitselfsurroundedbynegativelychargedoxygenatomsof propylenecarbonate;[BOB]−anionsarelocatedintheelectrolytesolution,thereby determiningitsmobility.

andgoodcharging/dischargingcycles[41],wideelectrochemical

stability window, no erosionto manganese and iron’s cathode

materials,ﬂuorine-free, non-toxic, etc. Furthermore, LiBOB has

weakestcoordinatingability(incomparisonwithcommonlyused

LiPF6[42]),caneffectivelystabilizethegraphiteanodesurfaceeven

inpurePC-basedelectrolytes,and cantolerateashighasabout

100ppmwatercontent[43,44].AproticPChasahighmolecular

dipolemoment(4.9D,duetothiswewereabletoobtain

solu-tionswithvarietysaltsconcentration)andexcellentperformance

atlowtemperatures.Highpolaritypropylenecarbonateallowsto

createaneffectivesolvationshellaroundlithiumcations(Fig.3),

therebyobtainingaconductiveelectrolyte.Accordingly,the

mobil-ityofelectrolytewilldeterminethe[BOB]−anions.

Electricalcharacterizationofa supercapacitorwithPC/LiBOB

electrolyte shows proportional voltage dependence of

capaci-tance/resistance(Fig.4a).Thecapacitance-voltagecurveindicates

highercapacitanceofthedevicearound40␮F/cm2 _and_minimal

pointat −0.34V.Theminimum valueof thecapacitance

corre-spondsthecharge neutralitypointsofgrapheneelectrodes and

dependsontheresidualchargedensity.Whilethealmost

asym-metricvoltagedependenceoftheresistancerepresentstwodistinct

peaksat9.4and12.7k.Thetotalresistanceofdevicevariedfrom

13to5.5kasabiasvoltagechangefrom−4.4to4.5V.

investigations.

3.4. Gelelectrolyte–PVA/LiBOB

Whereasexistingaproblemofliquidelectrolyteleakagewe

pro-posetousepolymer-basedgelelectrolyte.Obvious,thatgelplays

theroleofacontainerwhichholdssolventand,asaresult,possesses

thecharacteristicsofboth–liquidsandsolids.

TheadditionofLiBOBintopolyvinylalcohol(PVA)increasesthe

amorphousnessoftheelectrolyte[46].Ithappensduetothe

inter-actionofLi+_cation_of_salt_with_the_oxygen_atom_of_the_hydroxyl

groupinPVA(Fig.5).ThePVAisabletosolvatealargeamount

ofsaltandprovidesareasonablyhighconductivity.Owingtothe

decompositiontemperatureofPVA(230◦C)andLiBOB(>290◦C)

theelectrolytecanbemorestableinabroadtemperaturerange.

The electrical characterization of a supercapacitor with

PVA/LiBOB shows symmetric voltage dependence of

capaci-tance/resistancecurves(Fig.6a)withabiasvoltagechangefrom

−2.0to1.0V. Thisoccursdue totheinteractionbetweenmore

activeions(Li+₎_and _graphene_surface._The_graphene_electrodes

are neutralityat −1.2V that conﬁrmed by one minimum on a

capacitance-voltagecurve.Thehigher capacitanceofthedevice

observesat78␮F/cm2_._Analysis_of_the_{resistance-voltage}_curve_(in

modefrom7.5to12k)showsemergingofthesolepeakat12k.

Theelectro-opticaltestdetectsthegradualgrowinginthe

trans-missionat3and0.4%,duringavarietyofthevoltagebetween0to

+2.8Vand−2.8to0V,respectively(Fig.6b).Thereby,the

superca-pacitorisactiveinoppositedirectionsduetoreactivityofbothions

(Li+_and_BOB−₎_in_the_dependence_on_supplied_voltage._The

modu-lationofeachelectrodeisnegligiblewithawavelengthfrom600

to1100nmanditnormalizesat0V.

Fig.4.Thedependenceofthecapacitanceandresistance(a)andnormalizedchangeofthetransmission(b)ofsupercapacitorwithPC/LiBOBelectrolyteforvariousbias voltages.

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Fig.5. Schematicinteractionbetweenlithiumbis(oxalate)borateandpolyvinyl alcoholviainteractionoflithiumcationsofsaltwiththeoxygenatomsofthe hydroxylgroupinPVA.

Thus,weachievedconductionwhichprovidesbyfreemobile

lithium cations and obtained following beneﬁts: the polymer

matrixprovidesmechanicalstabilityandhelpstoavoidtheleakage

anddryingproblemsofelectrolyte;thesolventactsasaconducting

medium;thesupercapacitorwithPVA/LiBOBelectrolyteissuitable

forvisibleandnear-IRfrequencyinvestigationsinoppositeregions.

3.5. Solidelectrolyte–PVDF/ionicliquid

Toavoidchallengesofliquidandgelelectrolytes,during

super-capacitorfabricationanditstesting,weproposesolidelectrolyte–

polyvinylideneﬂuoride(PVDF)withionicliquid(Fig.7).Itisa

per-fectcombinationforthepurposeofthefabricationsolid-stateand

ﬂexibledevice.

The PVDFmatrix is promising and suitable polymer due to

itshighdielectric constant (11.38 D), low crystallinity and low

glass transition temperature [47–49]. The (VDF)-phase makes

polymerchemicallystableandplastically[50],thestrong

electron-withdrawingfunctionalgroups( C F)–highlyanodicallystable

[25].

As a doping agent, we use ionic liquid

(1-butyl-3-methylimidazolium hexaﬂuorophosphate), which have been

recognized as an ideal candidate to substitute the traditional

Fig.7. Schematicmolecularstructureandprotonhoppingmechanismbetween polyvinylideneﬂuorideand1-butyl-3-methylimidazoliumhexaﬂuorophosphate.

electrolytes.Itpossessesuniqueproperties:low vaporpressure,

non-ﬂammability,excellentelectrochemical/thermalstabilityand

unlikeaqueouselectrolyteshaswider electrochemicalwindows

(>1V)[51].

Theprofounddependenceofcapacitance/resistanceof

super-capacitorwithPVDF/ionicliquidelectrolytedemonstrateshigher

capacitanceofadeviceat15.5␮F/cm2_and_total_resistance_varied

from4.5to13.5k.(Fig.8a).Applyingvoltagefrom−3to3V,we

obtaintwoclearlyneutralitychargedpointsofgrapheneelectrodes

onthecapacitance-voltagecurve(−0.5and0.5V)andtwoadjacent

peaks(13and11k)ontheresistance-voltagecurve.

Thenopticaltransmissionshift(from500to1100nm)appears

inapositiveregionfrom0to2.8V(Fig.8b).Thesearesymmetrically

changesin3.2%thatindicateamodulationoftwoelectrodes;the

transmissionconditionofsupercapacitornormalizedat0V.

Thus,dopingofpolymerwithionicliquidisagoodapproachto

developelectrolyteintheﬁlmform.Namely,thenonpolarnature

ofthePVDFprovidesstructuralintegrity and,atthesametime,

formshighionicallyconductivechannels,offeringitssuitabilityas

anelectrolyteinsupercapacitors.

4. Conclusions

Insummary,wereportelectro-opticalfeaturesofa

graphene-basedopticalmodulatorwithsupercapacitorstructureusingliquid,

gel,andsolidelectrolytes.Weestablishedthebestcapacitanceat

Fig.6.Thedependenceofthecapacitanceandresistance(a)andnormalizedchangeofthetransmission(b)ofsupercapacitorwithPVA/LiBOBelectrolyteforvariousbias voltages.

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Fig.8.Thedependenceofthecapacitanceandresistance(a)andnormalizedchangeofthetransmission(b)ofsupercapacitorwithPVDF/ionicliquidelectrolyteforvarious biasvoltages.

78␮F/cm2_for_gel_PVA/LiBOB_electrolyte_and_showed_its

possibil-itytooperateintwodirectionsaswell,duetothesimultaneous

activityofLi+ _and_[BOB] _ions._We _demonstrated_capacitance _at

15.5␮F/cm2 _and _increasing_of _light _transmission_in ₃_times _for

solidPVDF/ionicliquidelectrolyte.Wenotedfollowingchallenges:

leakage/dryingunderanairofliquidPC/LiBOBelectrolyte,

impossi-bilitiesofusingofaqueousgelPVA/LiBOBelectrolyteinmicrowave

frequencyinvestigation.Therefore, wewouldliketodistinguish

mostsuitable electrolyte for graphene-based supercapacitors –

solidPVDF/ionicliquidelectrolytewithsatisfactoryelectro-optical

results,chemical/thermalstability,andﬂexibility.

We anticipate that application of the proposed electrolytes

togetherwithsimplicityandvarietyofthegraphene-baseddevice

geometrywillenableavarietyofadvancedopticaldevices

rang-ingfromplasmonicstooptoelectronics.Graphenesupercapacitor

can be used as a saturable absorber due to graphene

ultra-widebroadbandcapabilityandlowersaturationintensity.Anovel

supercapacitorwithtransparentgrapheneelectrodesfabricatedon

flexiblesubstratecouldapplyaselectricallyreconfigurableflexible

coatingsorsmartwindowsaswell.

Acknowledgement

This work was supportedby the Scientiﬁc and

Technologi-calResearchCouncilofTurkey(TUBITAK)grantno.114F052and

113F278.

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