Graphene-Based Supercapacitors Using Eutectic Ionic Liquid Mixture Electrolyte

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Graphene-Based Supercapacitors Using Eutectic Ionic

Liquid Mixture Electrolyte

Zifeng Lin, Pierre-Louis Taberna, patrice Simon

To cite this version:

Zifeng Lin, Pierre-Louis Taberna, patrice Simon. Graphene-Based Supercapacitors Using

Eutec-tic Ionic Liquid Mixture Electrolyte. Electrochimica Acta, Elsevier, 2016, vol. 206, pp. 446-451.

�10.1016/j.electacta.2015.12.097�. �hal-01466485�

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Eprints ID : 16653

To link to this article : DOI:10.1016/j.electacta.2015.12.097

URL :

http://dx.doi.org/10.1016/j.electacta.2015.12.097

To cite this version :

Lin, Zifeng and Taberna, Pierre-Louis and

Simon, Patrice Graphene-Based Supercapacitors Using Eutectic

Ionic Liquid Mixture Electrolyte. (2016) Electrochimica Acta, vol.

206. pp. 446-451. ISSN 0013-4686

Any correspondence concerning this service should be sent to the repository

Graphene-Based

Supercapacitors

Using

Eutectic

Ionic

Liquid

Mixture

Electrolyte

Zifeng

Lin

a,b

,

Pierre-Louis

Taberna

a,b,

*

,

Patrice

Simon

a,b

a_{UniversitePaulSabatierToulouseIII,CIRIMATUMRCNRS5085,118routedeNarbonne,31062Toulouse,France} b

ReseausurleStockageElectrochimiquedel’Energie(RS2E),FRCNRS3459,France

Keywords:

supercapacitors graphenefilm ionicliquideutectic workingtemperaturewindow

ABSTRACT

Compact graphene filmswerepreparedandelectrochemically testedatvarioustemperatures ina eutectic of ionic liquid mixture (1:1 by weight or mole N-methyl-N-propylpiperidinium bis (fluorosulfonyl)imideandN-butyl-N-methylpyrrolidiniumbis(fluorosulfonyl)imide)electrolyte.Alarge temperaturewindowfrom-30!_Cto80!_{Cwasachievedtogetherwithalargepotentialwindowof3.5Vat}

room temperature and below. A maximum gravimetric capacitance of 175F.g"1 _(85mAh.g"1_{) was}

obtainedat80!_C.130F.g"1_(63mAh.g"1_)and100F.g"1_(49mAh.g"1_{)werestilldeliveredat-20}!_Cand

-30!_{Crespectively.Besides,avolumetriccapacitanceof50F.cm}"3_{wasachievedwithathickgraphene}

film(60mm).Theoutstandingperformanceofsuchcompactgraphenefilmintheeutecticionicliquid mixtureelectrolytemakesitapromisingalternativetoactivatedcarbonforsupercapacitorapplications, especiallyunderextremetemperatureconditions.

1.Introduction

Thehightheoreticalspecificsurfacearea(upto2630m2_g"1₎

combined with low resistivity (10"6

_V

_{.cm) as well as high}

mechanical strength and chemical stability make graphene a

promising active material for supercapacitors [1,2].As a result,

intensive research efforts have been made to characterize

graphene and graphene-based materials in supercapacitors

applications [1,3–5]. Specific capacitance beyond 200F.g"1 _has

beenachievedinbothaqueousand organicelectrolytes[4,6–9], which is similaror even betterto many carbon materials like

porous activated carbon [10]. However, the low density of

graphene often comes with a low volumetric capacitance (F.

cm"3_{),whichisaconcerninmostofapplications,especiallyfor}

micro devices [11]. Besides, restacking of graphene film still remainsanissueandblocksthewaytohighercapacitance[12,13].

Accordingly, efforts have been devoted to synthesize more

compactbutun-restackedgraphenefilms.In2013,D.Li'sgroup

[7] proposed a method to develop a dense graphene film by

capillary compressionofgraphenegelfilminthepresenceofa nonvolatile liquid electrolyte,wherethepresence ofelectrolyte betweenthelayerspreventstherestackingofgraphenelayers.

Theenergy densitybeingproportional tothevoltagesquare (E¼1=2$CV2_{where E is the energy density, C is the}

specific capacitanceandVistheworkingpotentialwindow),maximizing thecellvoltageisofhighimportancefordesigning highenergy

devices. Today, most of the graphene and graphene-based

materials have been characterized in aqueous or conventional

organicelectrolytes[11,12].Asaresult,maximumvoltageupto3V werereported[2,14].

Ionic liquid electrolytes provide a large working potential window(morethan4Vinsomecases)[15,16].Inaddition,high chemicalstability,negligiblevaporpressureandnon-flammable propertiesmake ionicliquidsverypromisingaselectrolytesfor bothsupercapacitorsandbatteries[15,17].Oneofthefirstreport

about the use of ionic liquid electrolytes in graphene-based

supercapacitorswasdonebyVivekchandandetal[18],whofound aspecificcapacitanceof75F.g"1_{withina3.5Vpotentialwindow.}

Samegroupreportedareducedgraphenewithenhanced

capaci-tanceof258F.g"1_{(measuredat5mV/s) bynitrogen-doped[19].}

Ruoff's group synthesized activated graphene materials with

extremelyhighspecificsurface area–upto3100m2g"1_–able

delivering 200F.g"1 _{(0.7 Ag}"1_{) in EMIMTFSI ionic liquid [20].}

Despite efforts were devoted to improve the performance of

graphene-basedsupercapacitorsin ionicliquidelectrolytes, less attentionwas paid to theoperation temperaturerange

perfor-mance of supercapacitors. Limited by high viscosity and low

meltingpointnearroomtemperature,neationicliquidsarestill

*Correspondingauthor.

E-mailaddress:taberna@chimie.ups-tlse.fr(P.-L.Taberna).

unusableatsub-zerotemperatures[15–22].Therefore,aeutecticof ionicliquidmixturewithlowviscositywasdesignedbyLinetal.as anelectrolyteforsupercapacitors,whichturnedouttopossessa largeworkingtemperaturerangefrom-50!_Cto80!_Calongsidea widepotential window(up to3.7V)[21,23]. Thiseutecticionic liquidmixtureiscomposedofthesameanion(bis(fluorosulfonyl) imide (FSI)) but two different cations (pyrrolidium (PYR) and

piperidinium (PIP)) with the same molecular formula. The

differenceofmolecularstructure andtheassociatedasymmetry amongthecationswerethoughttohinderlatticeformation,thus

lowers the melting point while maintaining good miscibility

withinalargeworkingtemperaturewindow.Conductivityvalues

of28.9mS/cmand4.9mS/cmweremeasuredat100and20!_C,

respectively,asreportedearlier[23].

Inthispaper,weproposetouseadensifiedgrapheneelectrode suchasproposedbyLi’sgroup[7].incombinationwithanionic liquideutecticmixtureaselectrolytetoimproveboththevoltage

window – that is the energy density – and the operating

temperaturerange. 2.Experimental 2.1.Electrodespreparation

Commerciallyavailablegraphitepowder(KS44graphite, pur-chasedfromIMERYSGraphite&CarbonCorporation)wasusedas

the raw material. Graphite oxide was prepared by a modified

Hummersmethod[24].Briefly,amixtureofgraphiteflakes(1.0g,

1wt equiv) and KMnO4 (6.0g, 6wt equiv) was added to a

9:1 mixture of concentrated H2SO4/H3PO4 (120:13mL). The

reaction was then heated to 50!_{C and stirred for 12hours,}

followingbyanaddition of200mLicewith1mL 30wt%H2O2.

Afterwashingbywater,30%HClandethanol,thegraphiteoxide dispersionwasobtained.Grapheneoxidedispersionwasachieved by1hoursonicationand20minutescentrifugationat10,000rpm ofgraphiteoxidewaterdispersion.

Inafirststep,graphenedispersionwasobtainedthroughthe reduction of graphene oxide dispersion by hydrazine solution. 0.2mLhydrazine(35wt%inwater)and0.35mLammonia(28wt%

in water) solution was added into graphene oxide dispersion

(0.5mgmL"1_{, 100mL)inaglassvialforstirringover1hour(100}!_C). 60mL 0.5 mgmL"1 _{reduced graphene dispersion was used for}

vacuum filtration, and an Anodiscã membrane (Whatman

company)withan averagepore sizeof 0.1

m

mwas used. After

vacuum filtration, graphene film was carefully peeled off and

immersedindeionizedwaterovernighttoremovetheremaining hydrazineandammonia[7].

Inasecond step,thegraphenediscfilmswerepunched out

(7mmdiameter)andtransferredintoacetonitrileformorethan 48htototallyreplacethewaterwithacetonitrile.Filmswerethen immersedin10wt%(0.27M)ionicliquidmixtureinacetonitrile formorethan72hours,toexchangetheacetonitrileingraphene filmwiththeionicliquidsolution.Sampleswerethencollected,

clippedbetweentwoglassplatesandmovedtoavacuumoven

under 80!_{C. After 48hours, the volatile acetonitrile was}

completely evaporated and nonvolatile ionic liquids remained

insidethegraphenefilm,whichisexpectedtolimittherestacking ofgraphenelayers.Asaresultofthedensification,thethickness ofthegraphenefilmwasgreatlydecreased(fromaround180

m

m

to60

m

m), whilethediameter wasmeasured closeto5.4mm.

Herein, the graphene film was well prepared and ready for

electrochemicaltests.

Theweightoftheelectrodeswasmeasuredafter electrochemi-caltests.Specifically,theelectrodeswerewashedbyacetonitrile and ethanol to eliminate the trapped electrolyte and then the

weightwasmeasuredaftervacuumdrying.

2.2.Electrochemicaltests

Graphenefilmswereusedasworkingelectrodesdirectlyafter vacuumdryingwithoutanybinders.Aeutecticionicliquidmixture

composed of (1:1 by weight or molar ratio)

N-methyl-N-propylpiperidinium bis(fluorosulfonyl) imide (PIP13-FSI) and

N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (PYR14

-FSI) was prepared and used as the electrolyte [21]. Bothionic liquidswereboughtfromSolvionicSA(France).Two25mm-thick porousAl2O3separatorswereusedtogetherwithgold(forRHD

cell)orplatinum(forSwagelokcell)asthecurrentcollector. Forhighandlowtemperaturetests,aRHDcell(Figure.S1)was used,whichenablesfinelyprecisecontrolofthecelltemperature from"40upto100!_{C[25].Afterreachingeachtesttemperature,} thetemperaturewasheldatleasttwohoursbeforemeasurements toensurethecellhasreachedthedesiredtemperature.Testswere

performed by using an Autolab PGSTAT128N (Metrohm,

Switzerland).Besides,three-electrodeSwagelokcellwere assem-bledandtestedatroomtemperature,usingasilverwireas

quasi-reference electrode and tested by using a VMP3 potentiostat

(Biologic, S.A.).All supercapacitorscells mentioned abovewere

assembled in a glovebox under argon atmosphere (waterand

oxygencontentslessthan1ppm).

Electrochemicalimpedancespectroscopy(EIS)measurements

werecarriedoutatopencircuitvoltage(OCV)withanamplitudeof 10mVatthefrequencyrangefrom0.01Hzto200kHz.Capacitance wascalculatedfromcyclicvoltammogramsbymeasuringtheslope oftheintegrateddischargechargeQversusdischargetime(s)plot

and divide by scan rate (V/s). Gravimetric capacitance was

calculated by dividingthe electrode capacitance bythe weight ofgrapheneononeelectrode,accordingtoequation(1)

C¼

D

Q

D

t$#$m ð1Þ

WhereCisthespecificcapacitance(F.g"1_);DQ

Dt istheslopeofthe

integrateddischargechargeQversusdischargetimeplot(A);#_is thescanrate(V/s)andmisthemassofgrapheneononeelectrode (g).

2.3.Characterization

X-raydiffractionpatternswererecordedbyanX-ray diffrac-tometer(D4ENDEAVOR,Bruker,Germany)usingCuK

a

radiation (

l

=0.154nm)withanoperationvoltageof40kVand currentof 40mA.

SEMobservationsofthegraphenefilmsweremadeusinga

JSM-6510LV Scanning Electron Microscope after completion of

Fig.1.X-Raydiffractionpatternsofgraphite(GP),graphiteoxide(GO),graphene film(GF)andgraphenefilmfilledwithelectrolyte(GF-IL).Inset:zoominthe8!_-35!

electrochemicaltests,forthesakeofaccuracyinthefilmthickness measurement.

3.Resultsanddiscussion

X-Ray diffraction patterns of graphite, graphite oxide and graphenefilmswithorwithoutelectrolytearepresentedinFig.1.

GF stands for graphene films without electrolyte, obtained by

vacuum filtrationthat is containingwater inside. GF-IL arethe graphenefilmsobtainedafterelectrolyteimmersionthatcontain ionic liquid mixture electrolyte after acetonitrile removal. The (002)peakaround26.0!_{ofpristinegraphite(GP)correspondsto} an expected interlayer spacing of 0.336nm. For graphite oxide (GO),thepeakshiftsto9.0!_{asaresultoftheincreasedinterlayer} spacingto0.976nmafteroxidation[26].Nopeakswereobserved for graphenefilmfilledornotwithelectrolyte.It wasexpected sinceforgraphenefilm/GF(withoutelectrolyte),whichcontains morethan90wt%percentofwaterinside,theexfoliatedgraphene

flakes were separated from each other by water, therefore no

restackinghappens[13].Ontheotherhand,whenelectrolytefilled graphenefilms/GF-IL(waterreplacedbynonvolatileelectrolyte), thepresenceoftheelectrolyteisassumedtopreventrestacking. SEMobservationsofthefilmsareshowninFigs.2.Ascanbe

seen in Fig. 2a graphene films/GF present a homogeneous

thicknessof60

m

mwhichenablesaccuratedeterminationofthe volume(1.374' 10"3cm3_{)toobtainthevolumetriccapacitance.A}

layer-by-layerstructurewas showninFig.2b,asaresultofthe filtrationprocess.Suchalayeredstructureisassumedtobeatthe originofthedensificationofthefilmsascomparedtoconventional

three-dimensional disordered structure, resulting in a high

volumetricenergydensity[7].

Fig.3showsCyclicVoltammograms(CVs)ofanasymmetric 3-electrode Swagelok cell at roomtemperature,ata scan rate of 20mV/s,intheionicliquidmixtureelectrolyte.Therectangular shape of theCVreveals acapacitive behavior. Highgravimetric capacitance (175F.g"1_{) was achieved, superior to most of the}

reducedgrapheneoxidesreportedsofarinionicliquidelectrolytes orconventionalorganicelectrolytes(Table.S1)[7–28].Inaddition, alargepotentialwindowof3.5V(from-2Vto1.5V(vsAgwire)) makethesematerialsmorepromisingfordesigninghighenergy supercapacitors.

Asymmetricaltwo-electrodeSwagelokcellwasassembledand testedatroomtemperature.Thehighfrequencyloopobservedon theNyquistplot(Fig.4a)isassumedtooriginatefromthecontact resistancebetweenthecurrentcollectorsandthefilm[29].The highfrequencyseriesresistancewasmeasuredat40

V

.cm2_,which

isinthesamerangeaspreviouslyreportedforthiselectrolyteat roomtemperature[23].Whengoingdowntolowfrequencies,the fastincreaseoftheimaginarypartoftheimpedancerevealsthe capacitive behavior of the electrode. Fig.4b shows theCVs at differentscanrates.Highcapacitanceof170F.g"1_{wasobtainedata}

scanrateof5mV/swithina3.5Vpotentialrange,whichisbeyond mostof theconventional carbons reported sofar [10–31]. The changeofthecapacitancewiththepotentialscanrateisshownin

Fig. 4c. More than 75% of the maximum capacitance was still

preserved at 50mV/s, thus highlighting the decent power

performanceofthecelldespiteusinganeationicliquidelectrolyte.

The associated volumetric capacitance, calculated from the

electrode thickness, was 50F.cm"3 _{at 5mV.s}"1_{, which is also}

comparabletoconventionalactivatedcarbonpowders[10].

Fig. 5 shows the electrochemical characterization at higher temperature,namely40!_C,60!_Cand80!_{C.Asexpected,thehigh} frequencyseriesresistancedecreaseswithincreasingtemperature (Fig. 5a), due tothe decrease of the electrolyte viscosity. The improvedcapacitivebehavioratelevatedtemperatureascanbe

seen by the sharp increase in the imaginary part at lower

resistance; it is associated withthe increase of theelectrolyte conductivitywiththetemperature[21,23].Theimproved capaci-tive behavior with the temperature is also visible on the CVs (Fig.5b),withcapacitiveelectrochemicalsignatures.However,a limitationinthepotentialwindowfrom3.5Vdownto3.2Vwas observedwhenthetemperaturewasincreasedfrom40!_Cto80!_C. Such a behavior is associated withtheelectrochemical activity (oxidation) of the ionic liquid electrolyte at high potential, as already reported elsewhere [21]. Specific capacitance was in-creasedfrom160F.g"1_upto175F.g"1_{withincreasingtemperature}

becauseoftheviscosityfalloff.

Fig.6showstheelectrochemicalcharacterizationsatlowand

negative (sub-zero) temperature. Owing to the high viscosity

underlowtemperature,aslowscanrateof1mV/swasapplied.The Nyquistplot(Fig.6a)showsanimportantincreaseoftheseries

resistance at negative temperature, in agreement with the

decreaseoftheelectrolyteconductivity[23].Theelectrochemical

Fig.2. SEMimagesofcross-sectionmorphologyofgraphenefilmafterelectrochemicaltests.Scalebar:20mm(a),5mm(b).

Fig.3.CyclicVoltammogramsatroomtemperaturein N-Methyl-N-propylpiper-idiniumbis(fluorosulfonyl)imide(PIP13-FSI):N-Butyl-N-methylpyrrolidiniumbis (fluorosulfonyl)-imide(PYR14-FSI)ionicliquidmixture(1:1byweightormolar ratio).Thepotentialscanratewas20mV/s,withapotentialwindowfrom-2Vto 1.5V/Reference(usingasilverwireaspseudo-referenceelectrode).

Fig.4.Electrochemicalcharacterizationatroomtemperatureofa2-electrodecellassembledwithdensegraphenefilms,ineutecticmixtureelectrolyte:EISNyquistplot(a), CVsatvariousscanrates(b),changeofthespecificcapacitancewiththepotentialscanrate(c).

Fig.5.Electrochemicalcharacterizationat40!_C,60!_Cand80!_{Cofa2-electrodecellassembledwithgraphenefilms,ineutecticmixtureelectrolyte.Nyquistplots(a),CVs(b)}

signature(Fig.6b)stillshowsacapacitivebehaviordownto-30!_C whileitbecomestooresistiveat-40!_{C.Acapacitanceof135F.g}"1

(63mAh.g"1_{)wasobtainedat0}!_Cand100F.g"1_(49mAh.g"1_)at

-30!_{C.Whenthetemperaturegoesdownto-40}!_{C, the} electro-chemicalbehaviorisgovernedbytheohmicdropsinsidethebulk oftheelectrolyte.

These results showthat thecombination ofdensegraphene

filmswithaeutecticionicliquidmixturecanbepromisingasan alternative to conventional porous carbons for supercapacitor applications.Beyondthehighgravimetricandvolumetric capaci-tance obtained at room temperature (165F.g"1 _{and 50F.cm}"3_,

respectively),thelargepotentialwindow(3.5V)andtemperature operation range (from -30!_{C to 80}!_{C) make them suitable for} designinghighenergydensitysupercapacitors.

4.Conclusions

Using vacuum filtration, electrolyte immersionand selective evaporation,compactgraphenefilmscharacterizedbya layer-by-layerstructurewaspreparedandusedaselectrodematerialsfor supercapacitorswithoutbinders.Thankstotheuseofaeutectic ionic liquidmixtureaselectrolyte,a largeworkingtemperature

window from -40 to 80!_{C could be explored. A maximum}

gravimetric capacitance of 175F.g"1 _(85mAh.g"1_{) was reached}

at80!_{C.Despitethecapacitanceat-40}!_{Cwaslimitedat60F.g}"1

(30mAh.g"1_),130F.g"1_(63mAh.g"1_)and100F.g"1_(49mAh.g"1₎

werestillachievedat-20!_Cand-30!_{Crespectively.Thepotential} windowwasincreasedupto3.5Vatroomtemperatureand3.2Vat 80!_{C,thusleadingtosubstantialimprovementinthecellenergy} density.Additionally, avolumetriccapacitanceof50F.cm"3_was

alsoachievedwithathickgraphenefilmof60

m

m.Thesematerials couldbeapromisingalternativetoactivatedcarbonoperatingin

conventional electrolytes for supercapacitorapplications, espe-ciallyunderextremetemperatureconditions.

Acknowledgements

ZifengLINissupportedbyChinaScholarshipCouncil(CSC).P.S. acknowledgesthesupportfromEuropeanResearchCouncil(ERC, AdvancedGrant,ERC-2011-AdG,Projectno.291543-IONACES). References

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