Proton conducting gel polymer electrolytes for supercapacitor applications

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OULOUSE

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_{Łatoszyńska, Anna A. and Taberna,}

Pierre-Louis and Simon, Patrice and Wieczorek, Władysław Proton

conducting gel polymer electrolytes for supercapacitor applications.

(2017) Electrochimica Acta, vol. 242. pp. 31-37. ISSN 0013-4686

Any correspondence concerning this service should be sent to the repository

administrator:

staff-oatao@listes-diff.inp-toulouse.fr

Proton

conducting

Gel

Polymer

Electrolytes

for

supercapacitor

applications

Anna

A.

Łatoszy!

nska

*

,

Pierre-Louis

Taberna

,

Patrice

Simon

,

Władysław

Wieczorek

a

FacultyofChemistry,WarsawUniversityofTechnology,Noakowskiego3,00-664Warsaw,Poland

b

UniversityPaulSabatier,CIRIMAT,UMR-CNRS5085,31062ToulouseCedex4,France

c

RéseausurleStockageElectrochimiquedel’Energie(RS2E),FRCNRSn!_3459,France

Keywords:

protonconductiontypemechanism gelpolymerelectrolytes

phosphoricacidester electrochemicalcapacitor widetemperaturerange,

ABSTRACT

Anon-aqueous,mechanically-stable,protonconductinggelpolymerelectrolytehasbeenpreparedfor ElectrochemicalCapacitor(supercapacitor)applications.Itisbasedon2-hydroxymethylmethacrylate monomermixedwithtwodifferentsolvents(propylenecarbonateandN,N-dimethylformamide).Itwas shown that the capacitiveperformance changeswith thegel electrolyte composition.The proton conduction typemechanismaffectsions mobilityand transport intothe porouscarbon electrode. AdditionofsmallamountsofDMFsolventleadstoachangeintheconductionmechanismfroma vehicle-toaGrotthuss-type,andcapacitanceof90Fg"1_at20!_{Cwasachievedusinga15wt.%DPhHPO4/P}

(HEMA)/30wt. % DMF–70wt. % PC gel composition. Electrochemical tests were done in a large temperaturerange(from"40to80!_{C).Thecelldeliveredacapacitanceof54Fg}"1_at"40!_C,thatis60%

ofthevalueobtainedatroomtemperature,and90Fg"1_at80!_{Cwithinvoltagewindowof1V.}

1. Introduction

Use of semi-solid – rather than liquid – electrolytes in

Electrochemical Capacitors would be of great interest to solve many concerns related to packing, corrosion, self-discharge or leakagecurrentsissues[1].Thekeyissueistomaintainhighionic conductivityaswellasgoodcontactattheelectrolyte-electrode interface.Thisrequirementisofparticularimportanceinthecase ofElectricalDoubleLayerCapacitors(EDLCs),wherehighsurface areacarbonsareusedasactivematerials.

Protonconductingsystemsarevery promisingassolid ionic conductorsbecause of theirsuperior ionic conductivities [2,3]. Theyareusedaselectrolytesorseparatorsinvariousapplications such as photosynthesis [4,5], fuel cells [6–8], sensors [9,10], supercapacitors[11–13]orelectrochromicdevices[14,15].

Severalfamilies of protonconducting-electrolyteshavebeen reportedsofar,with:i)water-basedsystems,wherewatersolvent takespartinprotonconductivity[2,3,16,17],ii)oxoacidsandtheir salts,wheretheprotonconductivityoccursviaself-dissociation [3],iii)blendsoforganiccompounds,containingbasicsiteswith

acids,i.e.H3PO4orH2SO4[3,18],iv)xerogels,thatareamorphous

materialsobtainedbydryingofinorganicgelssynthesizedby sol-gelmethod[3,19,20]andv)high-temperatureconductors[3,21]. Anotherclass ofprotonconductorsaregelpolymerelectrolytes (GPE).GPEsconsistinapolymermatrixswollenwithasolution

containing the conducting species dissolved in an appropriate

solvent.VariousGPEscanbepreparedbychangingthenatureand

the ratio of the chemicals, leading to various mechanical and

electricalaswellasthermalproperties[22–27].Gelelectrolytes canbedividedintohydrogelsandanhydroussystems;hydrogels show the best conductivities, about 10"2 _Scm"1 _{compared to}

nonaqueous systems [28,29]. GPEs combine high conductivity

withhighmechanicalstabilityandcanbeaswellprocessedinto thin films.Accordingly,theyaregood candidatestobeusedas electrolytestoassemblesemi-solidsupercapacitors[11,12,30,31], tosolveissueslistedabove(corrosion,self-dischargeorleakage currents,andpacking)[11].

SolventsusedinprotonicGPEmustfillseveralrequirements, suchashighdielectricconstant,lowviscosityandlargeoperation

temperature range, i.e. low melting and high boiling points.

Additionally, they may offer possibility of solvent molecule

protonationbya protondonor source[22,32,33]. Two different

solvents can be distinguished: protophilic and protophobic.

ProtophilicsolventsleadtoaGrotthuss-typeprotonconductivity

* Corresponding author at: Faculty of Chemistry, Warsaw University of Technology,Noakowskiego3,00-664Warsaw,Poland.

E-mailaddress:aojrzynska@ch.pw.edu.pl(A.A.Łatoszy!nska).

mechanism, where the conductivityoccurs via protonated and

unprotonated solvent molecules. According to their

physico-chemical properties (highdielectric constant, low viscosityand

wide operationaltemperaturerange),PC andDMF(or

dimethy-lacetamide,DMA)areappropriatesolventsforprotonconducting system.

Raduchaetal.havesynthesizedseveral typesofnonaqueous gels plasticized with various polar solvents, such as propylene

carbonate(PC),N,N-dimethylformamide(DMF),

N-methylpyrrro-lidone(NMP)andethyleneglycol[32].Theyusedphosphoricacid asprotondonorsource,which iscrystallineinanhydrousform.

Other polymer matrices were tested, such as various acrylic

monomers (methyl methacrylate, glycidyl methactrylate or

acrylonitrile)and weresynthesizedvia“insitu”polymerization

of monomers. The type of solvent plays a major role in the

conductionmechanism.Inmostofthecases,thepolymermatrixis

an inert component, except GMA monomer, which used in

mentionedexampleistakingpartinprotontransport[34,35]. Wieczoreket al.havestudiedproton conductivityfor H3PO4

dissolvedinvarioussolvents,swolleninapolymermatrix(PVdF) [22].Thehighestconductivityatroomtemperaturewasobtained for DMF based gels (3.16.10"4_Scm"1_{). A decrease in the}

conductivity was observed for PVdF-NMP-H3PO4 systems

(1.26.10"5_Scm"1_{), inagreementwithhigherviscosityandlower}

dielectricconstantoftheNMP.Indeed,thesolventviscositywas

found tobethe key parameter for achievinghighconductivity

sinceitaffectschargecarriersmobility.IncreasingtheDMFcontent (vs.PC)resultsinincreasingtheelectrolyteconductivity,explained bythedifferentprotontransportmechanisminDMFandPC[22].

In DMF-based electrolytes, solvent molecules can be easily

protonated fromprotondonors. In presenceof protonated and

unprotonatedsolventspecies,theprotonconductiontakesplace

by a two-step Grotthuss-type mechanism where protons are

exchangedbetweenprotonatedandunprotonatedsolvent

mole-cule.Thismechanismshows alowactivation energyfor proton

conduction and relatively high ionic conductivity at room

temperature(1.58.10"4_Scm"1_)[22].

InsystemscontainingonlyPCassolvent,PCmoleculescannot

be easily protonated and the charge transport occurs via

polyatomicspeciesofprotondonors,sothattheprotontransport takesplaceviaavehicle-typemechanism;solventmoleculesare

not involved in proton transport. This type of mechanism is

characterized with high activation energy for conduction and

lowerroomtemperatureconductivities.ThehigherviscosityofPC resultsin a lowerionic chargemobility, whichcan alsobethe

reason for the lower ionic conductivities measured for GPEs

containingonlyPCassolvent[22].

Thereisalargevarietyofchoiceforgelelectrolytesprecursors

that can be mixed together in various ratios to prepare GPEs.

However, besides the ionic conductivity, the reactivity of the

different components has also to be taken into account. For

instance,inprotonconductingGPEs,somemonomerscanreact

withprotondonors[32].Inthepresenceofstrongacidssuchas H2SO4orH3PO4,thedegradationoftheC""Obondsinpolyethers

orpolyalcoholsusedassolventsmayoccur.Anotherissuearethe solvent-polymermatrixinteractions,suchasthewettability;for

instanceMMAorHEMAmonomersarenotcompatiblewithpure

DMForDMAsolution.

Inthispaper,wehavepreparedseveralprotonconductingGPEs withvarioussolvent ratios, tobeusedaselectrolytesin super-capacitorapplications.Theinfluenceofthegelcompositiononthe

ionic conductivity, temperature behavior and electrochemical

propertieshasbeenstudiedinalargetemperaturerange("40to

80!_{C). Phosphoric acid ester (diphenyl phosphate) has been}

chosen as theproton donor sourcedue to itsacidic properties (someofphosphoricacidestersareknownasstrongeracidsthan

orthophosphoricaciditself),whichallowsreachingionic conduc-tivityoneorderofmagnitudehigherthanthatoforthophosphoric acid-basedelectrolytes.Thisissuewasmorepreciselydescribedin ourpreviouspaper[11].

2. Experimental

Unless mentioned, all the chemicals were purchased from

Aldrich.Gelpolymerelectrolyte(GPE)waspreparedinaglove-box underAratmospherebymixingpropylenecarbonate(PC)with

2-hydroxyethyl methacrylate (HEMA), a radical poce:para

id="par0075">Unless mentioned, all the chemicals were pur-chasedfromAldrich.Gelpolymerelectrolyte(GPE)wasprepared

in a glove-box under Ar atmosphere by mixing propylene

carbonate (PC) with 2-hydroxyethyl methacrylate (HEMA), a

radicalpolymerizationinitiator(benzoylperoxide,BP)andacross linkingagent(triethyleneglycoldimethacrylate,TEGDM). Appro-priateamountsofBP(1wt.%)andTEGDM(5wt.%)wereusedwith the respect to the polymer content. After stirring for 1h, the solutionwascastasathinfilm(#200

m

soakingthefilmwithasolutionofDPhHPO4inDMFandleftfor1

week.Allthepreparedelectrolytescontained12.5wt.%polymer matrix,withvariousconcentrationsofDPhHPO4(from5to40wt.%

ofthefinalGPEs)andvarioussolventmixtureratio(PC:DMF).

Ionic conductivity was measured using electrochemical

im-pedancespectroscopytechnique(EIS)inatemperaturerangefrom "40to80!_{C.Samplesweresandwichedbetweentwo stainless}

steelblockingelectrodesandplacedinatemperature-controlled

thermostat. Electrochemical measurements were done using a

VMPMultichannelPotentiostat(VMP,BiologicScienceInstrument, France),inafrequencyrangefrom10to500kHz.

Differential Scanning Calorimetry (DSC) studies were

per-formedinthe"150to150!_{CtemperaturerangeusingaDSCQ200}

V24.2Build107systemequippedwithlow-temperature

measur-ingheadandliquidheliumcoolingelement.Sampleswereloaded

into aluminum pans and stabilized by cooling from the room

temperature down to "150!_{C. Samples were then heated at}

20!_Cmin"1 _{rate up to 150}!_{C; an empty pan was used as a}

reference.

The composite carbon electrodes were prepared by mixing

YP80-Factivatedcarbon(KurarayChemicalCo.,LTD)with5wt.% poly(vinylidene fluoride " hexafluoropropylene) (PVdF " HFP) binderin acetone.YP-80F isamicroporouscarbon,witha high specificsurfaceareaof2145$ 16m2_g"1_{.Argassorption}

measure-ments have shown that the carbon structure contains mainly

micropores(porediameterlowerthan2nm),accountingfor87%of

thetotalporevolumeandmesoporesinthe2to7nmdiameter

range.50% ofthe microporessizeis lowerthan 1nm[11].The

slurry was casted onto Au disks (thickness,#60

m

area,#1.29cm2_{),placedinvacuumovenanddriedat120}!_C.The

carbon mass loadings of the electrodes are given in Table 1.

Symmetric cells were assembled using 2-electrode Swagelok1

cells[11].

Electrochemicalmeasurementsweremadeatvarious

temper-atures (from "40 to 80!_{C) using a climatic chamber. Before}

startingelectrochemicalmeasurements,thecellswerekeptfor4h

Table1

Activatedcarbonmassloading(mgcm"2_{ofelectrode).}

GPEtype Electrodecoating(mgcm"2₎

90wt.%PC–10wt.%DMF 3.6 80wt.%PC–20wt.%DMF. 3.4 70wt.%PC–30wt.%DMF 4.1 60wt.%PC–40wt.%DMF 4.4

untilthedesiredtemperaturewas reached,whichwasfoundto ensurethatthecelltemperaturewasthesametemperatureasthat

ofthechamber.Electrochemicalimpedancespectroscopy

meas-urementsof2-electrodesupercapacitorcellsweremadebetween

100kHzand1mHz.Cyclingvoltammetry(CV)experimentswere

performedatscanratesof1,5and10mVs"1_{andconstantcurrent}

charge/discharge tests (GCPL) were carried at 0.075Ag"1_{. The}

gravimetric capacitance Cam of active material (in F g"1) was

estimatedfromthecellcapacitanceCcellaccordingtoEq.(1):

C¼2xCcell

mac ð1Þ

whereCistheelectrodecapacitance(Fg"1_ofcarbon),C cellisthe

cellcapacitance(F),andmactheweightofactivatedcarbonatone

electrode[11].

3.ResultsandDiscussion

2-hydroxyethylmethacrylate(HEMA)monomerwasselected

as the polymer matrix, as already proposed in previous work

[11,32,36].HEMA-basedpolymergelelectrolytesaretransparent

andfreestandingmembranesthatshowhighflexibilityandgood

mechanicalstability.Theelasticpropertiesdependonthesolvent content[22,37]andthemainroleofpolymermatrixistoensure good mechanical properties [32]. All the prepared electrolytes contain12.5wt.%ofHEMA.

Ionicconductivityingelisachievedinthesolventphase,by protonationofsolventmoleculesbytheprotondopant[3,22,38].

Theselectedmethacrylicmonomeriscompatiblewithpropylene

carbonate and the gel synthesis is possible by simple radical

polymerization.However, when PCis only usedas solvent,the

solventmoleculescannotbeeasilyprotonatedandtheformation ofchargecarriersinPCoccursmostlyviatheself-dissociationof protondonor,leadingtolowconductivityvalues.Theadditionofa protophilicsolventsuchasDMF,contributestoprotonconduction byaGrotthus-typemechanism,whichisprotontransportviaan

exchange of protons between protonated and unprotonated

solventmolecules.

Unfortunately,thepreparationofgelsbasedonlyonDMFand

PHEMA polymer matrix is impossible since HEMA is not

compatiblewith DMF.Enrichment of methacrylicbased matrix

with DMF is possible by combining PC and DMF solvents, by

soakingDMFintoPCbasedgel.ThemaximumofDMFcontentthat

canbeaddedtoHEMA-based gelsis 40wt. %in solvent phase;

beyondthisvalue,aphaseseparationoccurs.Fig.1showstheionic conductivity of GPEcontaining15wt. %of diphenyl phosphate, withPC:DMFratiosrangingfrom9:1to6:4.Thepossibilityofthe

protonation of solvent molecules by the protondopant in the

systembeingveryimportant.Thelowestconductivityvaluewas obtainat20!_{CfortheDMF-freeGPE(1.6.10}"4_Scm"1_);itfurther

increasesupto5.3.10"4_Scm"1_{for30wt.%DMFcontent.Below}

0!_{C,highestvalueswerereachedforthe10wt.%DMF}

composi-tion.Fortemperaturesbeyond0!_{C,thehighestconductivitywere}

obtainedfor30wt.%ofDMF.Theprotophilicsolvent(DMF)itself

contributetoprotonconductionbyaGrotthus-typemechanism

(protontransportviaanexchangeofprotonsbetweenprotonated andunprotonatedsolventmolecules).

Electrolyte thermal stability was assessed by Differential

Scanning Calorimetry (DSC). Table 2 shows DSC data obtained

for gel polymer electrolytes PHEMA-DPhHPO4-PC-DMF gels

containing various PC-DMF ratio. The only phase transition

occurringinthe"150to150!_{Ctemperaturerange(Fig.2)isthe}

glasstemperaturetransitionofthesamples.AslightdecreaseofTg

isobservedwhenadding10wt.%ofDMFcomparedtoDMF-free

electrolyte,inagreementwithpreviousresults;thisisascribedto thechangeofthesolventcomposition[39].ThelowestvalueofTg

wasobtainedforthesamplecontaining10wt.%ofDMF;further

increase in DMF concentration results in strengthening of the

system(assumedtobelinkedwithimprovedinteractionsbetween thesolventandthepolymermatrix),thusleadingtoadecreaseof Tg.

Basedonresults,the(PC:DMF)ratiowasfixedat(70:30).The

decrease of the activation energy Ea (see Table 2) of

DMF-containingGPEsuggeststhatprotonconductionismainlyachieved

Fig.1.Ionic conductivity as a function of inverse temperature for PHEMA-DPhHPO4-PC-DMFsystem,whereesterconcentrationwas15wt.%andvarious

PC-DMFratio.

Table2

Conductivity,activationenergyandTgvaluesobtainedfromDSCmeasurements,for

protonconductinggelsbasedonPHEMA,containingdifferentsolventsratioand 15wt.%diphenylphosphate. Proton Donor Solvent s(Scm"1 ) at20!_C Ea(kJmol"1) Tg(!C) (20–80!_C) DPhHPO4 (15wt.%) 100wt.%PC 1.6(10"4 23.4 "103.2 90wt.%PC–0wt.%DMF 4.5( 10"4 16.1 "107.5 80wt.%PC–20wt.%DMF 3.9(10"4 17.9 "105.5 70wt.%PC–30wt.%DMF 5.3(10"4 _16.7 "102.3 60wt.%PC–40wt.%DMF 3.6(10"4 _18.0 "87.0 Fig.2.DSCplotsforPHEMAbasedelectrolyte(where12.5wt.%ispolymermatrix and15wt.%ofdiphenylphosphate)preparedwithvariousPC"DMFsolventratio.

by a Grotthuss-type mechanism, moving to a vehicle-type mechanismforDMF-freeelectrolytes.

Fig.3 showsthechangeoftheionicconductivityversusthe

reverseofthetemperatureplotsforaPHEMA-DPhHPO4-PC-DMF

system containing (70:30) PC:DMF wt. % ratio, for various

DPhHPO4 contents. Two temperatureregions can be identified

on the plot: above #–10!_{C, the conductivity increases with}

increasingamountofDPhHPO4; below#–10!C, theconductivity

decreaseswithincreasingDPhHPO4concentrations.

DifferentialScanningCalorimetrymeasurementsofgelsamples containingvariousprotondonorcontentshowssimilartrendsas previouslyobserved(seeTable3).Onlyonephasetransitionwas observed[11],associatedwiththeglasstemperaturetransition(Tg)

which increases with the acid concentration. Increasing the

diphenyl phosphate concentration resulted in mechanical

strengthening ofthegel(associatedwithimprovedinteractions betweensolvent and polymer matrix);as a result,increasedTg

values werefound.Thecalculatedactivation energyvalues (see Table3)fallintherangeof15.6–21.6kJmol"1_{andincrease.The}

lowerEavaluessuggestsaGrotthuss-typeconductivitymechanism

atlowesterconcentrations(5and15wt.%),movingtoa vehicle-type mechanismathigheresterconcentration(30and40wt.%) [32].

3.1.ElectrochemicalCharacterizations 2-electrodeSwagelok1

cellswereassembledwithgel

electro-lyteactingaswellasseparator.GPEsbasedonPHEMApolymer

matrix doped with 15wt. %of diphenyl phosphate wereused.

VariousPC-DMFsolventratioswerestudied.Theactivatedcarbon weight loading is given in Table 1. Fig. 4a shows the Cyclic

Voltammogramms (CVs) of 2-electrode cells assembled with

carbon electrodes (see experimental section) using DMF-free

GPE (15wt. % DPhHPO4/PHEMA/PC). The CVs show a resistive

behavior,associatedwiththelowconductivityoftheelectrolyte.

When pure PC (DMF–free) is used as solvent phase, proton

transportoccursviapolyatomicspeciesofprotondonorwhichare trappedinpolymernetwork.Thechargecarriersmobilityinthe electrolyteislow,resultinginhighresistance.

Whensmall amountof DMF(10wt.%in solventmixture) is

addedtotheGPE,therectangular-shapedCVsignaturesevidencea typicalcapacitive(seeFig.4a).Thiscanbeattributedfirsttothe

change in the proton conducting mechanism from vehicle- to

Grotthuss-typepreviouslymentioned,thatincreasesthe electro-lyteconductivity.Additionally,thedecreaseintheviscosityofthe solventphasefollowingDMFadditionmayleadtoaswellingofthe GPEelectrolyte,thusimprovingthesurfacecontactareabetween

thehighly porouscarbonand the GPE[11].Room temperature

measurementshaveshownthataconcentrationof10wt.%ofDMF wasenoughtoreachdecentcapacitiveofabout80Fg"1_(Table4).

ThecapacitancedoesnotchangedratisticlywithincreasingDMF contents.

Fig. 4b shows the capacitance change versus scan rate,

calculatedfromCVexperiments.Highestcapacitancewasobtained at1mVs"1_(80Fg"1_{)andappearstobeindependentfromtheDMF}

contentatlowpotentialscanrate.However,increasingtheDMF contentintheelectrolyteimprovesthecapacityretentionbehavior

Fig.3.Ionicconductivityasafunctionoftheirreversetemperaturefor PHEMA-DPhHPO4-PC-DMFsystem,where30wt.%insolventphaseisDMF.

Table3

Conductivity,activationenergyandTgvaluesobtainedfromDSCmeasurementsfor

protonconductinggelsbasedonPHEMA,differentprotondonorcontentand PC-DMFratio7:3. Proton Donor Conc. (wt.%) s(Scm"1₎ at20!_C Ea(kJmol"1) Tg(!C) (20–80!_C) DPhHPO4 5 3.0(10"4 15.6 "109.1 15 5.3(10"4 _{16.6 "102.3} 30 6.2(10"4 20.4 "80.3 40 8.0(10"4 21.6 "63.3

Fig.4.a)CyclicvoltammogramsofasymmetricalsupercapacitorwithGPEdopedwithphosphoricacidester(15wt.%DPhHPO4/PHEMA/solvent).CVwasrecordedatroom

temperatureatascanrateof1mVs"1

.b)Capacitancechangewiththepotentialscanrateandwithvariousscanratesatroomtemperature.C)Nyquistplots2-electrode supercapacitorcells.

athighrates;thiscanbeattributedtotheimprovedconductivityof thegelelectrolyteandtheassociateddecreaseintheviscosity.

The improvement of the electrochemical properties of the

electrolytewiththeDMFcontentisclearlyobservedinFig.4cthat

shows the impedance spectroscopy measurements at room

temperature(20!_{C).Theverticalincreaseof theimaginary part}

atlow frequencyconfirms thecapacitive natureof the electro-chemicalstorageforallcells.Theequivalentseriesresistance(ESR) calculatedfromthehighfrequencyvalueatnullimaginarypart,

decreasesforincreasedDMF content(seeTable 4).Considering thatalltheGPEsampleshavethesamethickness(200

m

results are in good agreement with the change of the ionic

conductivity. The observed decrease in capacitance at "40!_C

originatesfromthereducedionmobilityatsuchlowtemperature; however54%and70%oftheinitialroomtemperaturecapacitance werestill measuredat"40!_{Cfor10and30wt.%DMFcontent,}

respectively;thishighlightsthecapabilityofourproton conduct-inggelelectrolytesforoperatingatlowtemperature.

Table4

Equivalentseriesresistance(ESR),specificcapacitancevaluepergramofaveragemass(Cam)forthesymmetricalsupercapacitorsobtainedatdifferenttemperature,testedin

15wt.%DPhHPO4/PHEMA/PC-DMFwithvariousPC/DMFcontentsGPEs.

T(!_{C) Cut-offpotential(}DEinV) 90wt.%PC–10wt.%DMF 80wt.%PC–20wt.%DMF 70wt.%PC–30wt.%DMF 60wt.%PC–40wt.%DMF ESR(Vcm2 ) Cam (Fg"1 ) at74mAg"1 ESR(Vcm2 ) Cam (Fg"1 ) at74mAg"1 ESR(Vcm2 ) Cam (Fg"1 ) at74mAg"1 ESR(Vcm2 ) Cam (Fg"1 ) at74mAg"1 "40 0–1.3 1870 43 2350 45 800 54 850 57 "20 0–1.3 720 57 850 60 255 62 365 63 RT 0–1.3 190 76 180 70 82 94 90 84 40 0–1.2 110 80 110 80 37 86 46 85 60 0–1.1 80 81 75 82 25 90 32 86 80 0–1.0 54 80 50 84 19 89 24 85

Fig.5.Electrochemicalcharacterizationofsymmetricalsupercapacitorwithpolymergelelectrolyte(15wt.%DPhHPO4/PHEMA/DMF-PCdifferentratio):CVscollectedat

1mVs"1

(a)andcorrespondingaNyquistplots(b)collectedat"40!_{C.CVscollectedat1mVs}"1

The temperature changealsoaffects thevoltagewindow.At highertemperatures(80!_{C,seeFig.5c),themaximumvoltageis}

1.0V, while at room temperature (20!_{C) it reaches 1.3V. The}

voltagewindowishigherthanforpreviouslyanalyzedcellswith

phosphoric acid (1.0V) or for aqueous proton conducting GPE.

Fig. 5 shows the electrochemical characterization of the cell

assembled withGPEcontaining10and 30wt.%ofDMF.Fig.5a showscomparisonofvoltammogramsatscanrate1mVs"1_and5b

thecorrespondingtheNyquistplotsat"40!_{C.AlthoughtheESR}

werefoundtobehigh(1870

V

_V

30wt.%ofDMF,respectively),theNyquistplotssignatureat"40!_C

is still capacitive. The electrochemical signature is mainly

controlledbyohmicdrops(seeFig.5a);however,itiscapacitive ascanbeseenfromtherectangularshapeoftheCVs.

Fig. 5c and d show the CVs and Nyquist plots of the same

supercapacitorscellscycledat80!_Cat1mVs"1_{.TheNyquistplots}

showsacleardecreaseoftheseriesresistancereaching54

V

(DMF 10wt. %) and 19

V

capacitanceincreasethankstoimprovedelectrolyteconductivity.

However, the temperature increase decreases the maximum

voltage window from 1.3V (20!_{C) down to 1.0V at 80}!_{C, in}

agreement withincreased electrochemicalreactions kinetic for protonreductionandsolventoxidation[36].

Fig.6 summarizesthecapacitance changewithtemperature

calculated from previous cyclic voltammetry experiments. As

expected,acapacitancedecreaseisobservedwithdecreasingthe temperature,associatedwiththelowerionicconductivityofthe GPEs.OnlythecellsassembledwithGPEscontaining30and40wt. % of DMFwere able to reachcapacitance higher than 50Fg"1

withinamaximumvoltagewindowof1.3V(seeTable4). 3.2.Long-termcyclingtests

To control the capacitance value stability of carbon based

capacitors with protonconducting GPEwith different PC-DMF

ratio,longtermgalvanostaticcharge/dischargeexperimentswere performedinthepotentialbetween0and1.3V.Fig.7showsthe

change of the capacitance with the cycle number, measured

duringcharge/dischargetestsataconstantcurrentof74mAg"1_.

Thelongtermcyclingtestswereperformedupto1000cycles forfoursolventsratios.Theelectrolytewith30wt.%ofDMFand 70wt.%ofPCaccordingtoourresultsgavethehighercapacityand

the highest capacity retention after 1000 cycles (an initial

capacitance of 94Fg"1 _{and a capacity retention of 96%). The}

reasonofitisthehighestionicconductivityofthissample(see Fig.1)comparetoothersampleswithdifferentDMFcontent. 4.Conclusion

Thisworkfocusedonthepreparationofprotonconductinggel polymerelectrolytes (GPE) for applications in supercapacitors.

Itwasshownthatthecapacitiveperformancechangeswiththe

gel electrolyte composition. Firstly, the solvent type, mostly throughthedifferentprotonconductingmechanism,affectsthe

electrochemical performance of the carbon. The vehicle type

mechanismforprotonconductioninPCbased GPE,whichdoes

notinvolvesolventmoleculesintransport,stronglyaffectsions mobilityandtransportintotheporouscarbonelectrode.Addition ofsmallamountsofDMFsolventleadstoachangeofconduction

mechanism from a vehicle- to a Grotthuss-type mechanism,

where proton transport is achieved via solvent phase. DMF

additionallowsimprovedporouscarbonstructureaccessibilityto theionsandappearanceofdoublelayereffect.

Supercapacitor cells were assembled with different gel

compositionsand tested at varioustemperatures (from "40 to 80!_{C).It wasshown thatthebestpropertieswereobtainedfor}

EDLC using a gel electrolyte containing 15wt. % DPhHPO4/P

(HEMA)/30wt. % DMF–70wt. % PC. The capacitance was about

90Fg"1_(at20!_{C),withinamaximumvoltagewindowof1.3V.The}

use of organic solvents (PC, DMF) allowed increasing the

temperature range from "40!_{C up to 80}!_{C. At "40}!_{C, the}

capacitance reached 54Fg"1_{, that is 60% of thevalue obtained}

atroomtemperature(90Fg"1_{).Athighertemperatures(80}!_C),the

capacitancewas90Fg"1_{withanoperationvoltagewindowof1V.}

TheresultsobtainedinthisworkconfirmthatGPEsareinteresting candidates for replacing liquid electrolytes in supercapacitors devicesforspecificapplications.

Acknowledgments

ThisworkwassupportedwithintheframeoftheMPD/2010/4

ProjectbelongingtotheMPDProgramofFoundationfor Polish

Science(co-financedbytheEuropeanUnion, Regional

Develop-mentFund).

WewouldliketothanktoPhDGra_zynaZofia _Zukowskaforher helpinthiswork.

Fig.6. Capacitancechangewiththetemperatureforthe2-electrodesupercapacitor cellsassembledwithGPEelectrolytescontainingvariousPC:DMFratios. Capaci-tancewerecalculatedfromcyclicvoltammetryexperiments.

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