Graphene-Based Supercapacitors Using Eutectic Ionic Liquid Mixture Electrolyte

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O

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A

rchive

T

OULOUSE

A

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O

uverte (

OATAO

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

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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

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Graphene-Based

Supercapacitors

Using

Eutectic

Ionic

Liquid

Mixture

Electrolyte

Zifeng

Lin

a,b

,

Pierre-Louis

Taberna

a,b,

*

,

Patrice

Simon

a,b

a_Universite_Paul_Sabatier_Toulouse_III,_CIRIMAT_UMR_CNRS_5085,₁₁₈_route_de_Narbonne,₃₁₀₆₂_Toulouse,_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!_C_to₈₀!_C_was_achieved_together_with_a_large_potential_window_of_3.5_V_at room temperature and below. A maximum gravimetric capacitance of 175F.g"1 ₍₈₅_mAh.g"1₎ _was

obtainedat80!_C.₁₃₀_F.g"1₍₆₃_mAh.g"1₎_and₁₀₀_F.g"1₍₄₉_mAh.g"1₎_were_still_delivered_at_-20!_C_and -30!_C_{respectively.}_Besides,_a_volumetric_capacitance_of₅₀_F.cm"3_was_achieved_with_a_thick_graphene

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 _speci

ﬁc

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_within_a_3.5_V_potential_window.

Samegroupreportedareducedgraphenewithenhanced

capaci-tanceof258F.g"1_(measured_at₅_mV/s) _by_{nitrogen-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:[email protected](P.-L.Taberna).

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unusableatsub-zerotemperatures[15–22].Therefore,aeutecticof ionicliquidmixturewithlowviscositywasdesignedbyLinetal.as anelectrolyteforsupercapacitors,whichturnedouttopossessa largeworkingtemperaturerangefrom-50!_C_to₈₀!_C_alongside_a

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 ₁₂_hours,

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 ₄₈_hours, _the _volatile _acetonitrile _was

completely evaporated and nonvolatile ionic liquids remained

insidethegraphenefilm,whichisexpectedtolimittherestacking ofgraphenelayers.Asaresultofthedensification,thethickness ofthegraphenefilmwasgreatlydecreased(fromaround180

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]_._After_reaching_each_test_temperature,

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!

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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!_of_pristine_graphite_(GP)_corresponds_to

an expected interlayer spacing of 0.336nm. For graphite oxide (GO),thepeakshiftsto9.0!_as_a_result_of_the_increased_interlayer

spacingto0.976nmafteroxidation[26].Nopeakswereobserved for graphenefilm filled ornotwithelectrolyte.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"3_cm3₎_to_obtain_the_volumetric_capacitance._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_was_obtained_at_a

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 ₅_mV.s"1_, _which _is _also

comparabletoconventionalactivatedcarbonpowders[10].

Fig. 5 shows the electrochemical characterization at higher temperature,namely40!_C,₆₀!_C_and₈₀!_C._As_expected,_the_high

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!_C_to₈₀!_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).

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

Fig.5.Electrochemicalcharacterizationat40!_C,₆₀!_C_and₈₀!_C_of_a_2-electrode_cell_assembled_with_graphene_ﬁ_lms,_in_eutectic_mixture_electrolyte._Nyquist_plots_(a),_CVs_(b)

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signature(Fig.6b)stillshowsacapacitivebehaviordownto-30!_C

whileitbecomestooresistiveat-40!_C._A_capacitance_of₁₃₅_F.g"1

(63mAh.g"1

)wasobtainedat0!_C_and₁₀₀_F.g"1

(49mAh.g"1

)at -30!_C._When_the_temperature_goes_down_to_-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 ₅₀_F.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 ₍₈₅_mAh.g"1₎ _was _reached

at80!_C._Despite_the_capacitance_at_-40!_C_was_limited_at₆₀_F.g"1

(30mAh.g"1_),₁₃₀_F.g"1₍₆₃_mAh.g"1₎_and₁₀₀_F.g"1₍₄₉_mAh.g"1₎

werestillachievedat-20!_C_and_-30!_C_{respectively.}_The_potential

windowwasincreasedupto3.5Vatroomtemperatureand3.2Vat 80!_C,_thus_leading_to_substantial_improvement_in_the_cell_energy

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).

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