Towards sustainable solid-state supercapacitors

14 Pages·2016·4.76 MB·English

Journal of
Materials Chemistry A
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Towards sustainable solid-state supercapacitors:
ce. electroactive conducting polymers combined with
n
AM. Lice Citethis:J.Mater.Chem.A,2016,4, biohydrogels†
3:57 orted 1792
12:5Unp MariaM.Pe´rez-Madrigal,abcFrancescEstrany,cdElaineArmelin,abcDavidD´ıazD´ıaz*ae
2023 n 3.0 and Carlos Alema´n*bc
3/14/butio Solid-state organic electrochemical supercapacitors (OESCs) have been fabricated using poly(3,4-
ed on s Attri easthryelednoexdaiodxdyitthivieo.pThheenee)ff(PeEcDtivOitTy)oeflescotdroiudmes,aalgibniaothey,dkr-ocgaerlraagseeelneacntr,oclyhtietossyasnteamn,dagnedlaptoinlyhanydilirnoegefilbsearss
dn
oamo electrolyticmediahasbeenevaluatedconsideringdifferentcriteria.Resultsindicatethatk-carrageenan-
nlm
wo based hydrogel is the most suitable to perform as electrolyte due to the appropriate combination of
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y 2016. DCreative apeslreoscaptyersortciinehsde:imcmaicteeaclhtrhaeanstpicoaOnlEssSetCaobsfilPibtyEa,DseeOdaTseoenloefcPtEproDredOpeaTsr.aeCtilyoeccnlt,ircolavdcoeklstaomafnmwdaektter-yrcaalernradakgigneagel,vnaaannnodshtgyadotirocodgchemlareagdseiu–emdleiscfctorhroalrtyhgteee
nuarer a exhibitsagoodsupercapacitorresponseintermsofspecificcapacitance,cyclingstability,smallleakage
ad
4 Jun current and low self-discharging tendency. On the basis of these good properties, four OESC devices
d on 0ensed wchearreacatsesreismticbsled(e.ign.seelrimiesinaantidonusoefdlitqouipdolweaekrinagreadndLEeDn,hcaonncfiermmeinngt othfatth,eindaedvdiciteioncotmopaadcvtannetsasg),eothues
Article. PublisheThis article is lic RADwceOwccIwee:pi.1vrt0seec.dd1.o024r38gt9h/thM/cJaO5atntecaurt0ioaa8brlsy6eA8r2002a10615 dpwtoerhesbilicigemhnvienaedcarytrbypaioasroshasmyadryiesrsoidnhgogaexvla-aencddbod,enittethiavneienrp.eineOfogrEfroeSOrC,mEfuSdeCtdeuvreleiocxaehwdsiobinpirtgkrseitppshaooertreekiedn-nctuitaaesrldirnfaogtgoreswpeuanrcarahdcnstliohcthaaydlederaiodmpgppbelirilcoowahvtieyitodmhnroeps.nogtOelyloanhfnattilhhvineeeeirobndetaheenesnoirgfifhnoba.uennrdsd,,
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A Introduction althoughtheyhavehigh-energydensities,whereastheopposite
n
pe istrueforconventionalcapacitors.1Inbetween,electrochemical
O
Global concerns on green-chemistry and sustainable develop- capacitors, also called supercapacitors (ESCs), have recently
ment have oriented research and technological developments been designed with advanced functionalities via environmen-
towards more eco-friendly approaches. This is particularly tally-friendlyprocesses.Thesesystemsexhibithighpowerand
importantforenergy-relatedissuesduetoboththedepletionof energydensitieswithoutcompromisingtheircyclingstabilityor
fossil fuels and the negative environmental effects that they other features.2–4 Furthermore, the development of nano-
generate.Withinthiscontext,effortsarebeingmadeto(i)shi structuredelectrodematerialsandthebetterunderstandingof
torenewableenergysources;and(ii)designhigh-performance the energy-storage mechanisms have contributed to improve
energystoragedevices(i.e.batteriesandcapacitors)usingeco- the performance of ESCs and to bridge the gap between elec-
friendly materials. Batteries display low-power densities trochemical capacitors and batteries in terms of energy
density.5–10
Depending on the energy-storage mechanism, electrode
aInstitut fu¨r Organische Chemie, Fakulta¨t fu¨r Chemie und Pharmazie, Universita¨t materialsaredividedintwodifferentmaingroups.Intherst
Regensburg, Universita¨tsstr.31,93053 Regensburg,Germany. E-mail: david.diaz@
one the capacitive behaviour solely relies on the electrostatic
chemie.uni-Regensburg.de
interactions (adsorption) between electrolytic ions and the
bDepartamentd'EnginyeriaQu´ımica,ETSEIB,UniversitatPolit`ecnicadeCatalunya,
electrodesurface(i.e.electricaldouble-layercapacitance),while
Avda.Diagonal647,BarcelonaE-08028,Spain.E-mail:[email protected]
cCenter for Research in Nano-Engineering, Universitat Polit`ecnica de Catalunya, faradaic reactions contribute to the overall capacitance (i.e.
CampusSud,EdiciC',C/PasqualiVilas/n,BarcelonaE-08028,Spain pseudocapacitance) in the second group.11 Electroactive con-
dDepartament d'Enginyeria Qu´ımica, Escola Universita`ria d'Enginyeria T`ecnica ductingpolymers(ECPs)belongtothelattergroupandrender
IndustrialdeBarcelona,UniversitatPolit`ecnicadeCatalunya,Comted'Urgell187, promising electrodes in organic electrochemical super-
08036Barcelona,Spain capacitors (OESCs) due to their cost-effectiveness, lightweight
eIQAC-CSIC,JordiGirona18-26,E-08034Barcelona,Catalonia,Spain
and exibility.12 In fact, ECPs display pseudocapacitance
†Electronic supplementary information (ESI) available. See DOI:
behavior asa result of: (i) the redoxmechanism(i.e. chargeis
10.1039/c5ta08680a
1792 | J.Mater.Chem.A,2016,4,1792–1805 Thisjournalis©TheRoyalSocietyofChemistry2016
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stored by electron transfer to the polymer with ion pairing to the Si/C anode of Li-ion batteries was enhanced by applying
stabilizethecharge);and(ii)thedouble-layerbehaviorofdoped across-linkedalginatehydrogelbinder.35
ECPs, which allows resonance throughout the conjugated This work develops greener and more sustained energy
structureofthepolymericbackbone.Inarecentwork,Mikeand storage devices by replacing conventional liquid electrolytic
Lutkenhaus13extensivelyreviewedtheadvancesofECPsinthe systems with biohydrogels in OESCs. If properly designed,
energy storage eld. Among ECPs, poly(3,4-ethyl- biohydrogelsaresafe(i.e.non-toxic,non-ammable,andnon-
enedioxythiophene) (PEDOT) has been widely used for the leakage), cheap and easy to prepare; they exhibit high-ionic
fabrication of OESCs because of its outstanding capacitive conductivitiesandcanbeobtainedfromrenewablesources.In
e. performance, good electrical conductivity, high doping rate, spite of the advantageous outcome of biohydrogel-based
nc stable charge–discharge response, easy of synthesis and systems as sustainable energy storage devices, caution is
AM. Lice advantagesincost.14–18 required since the electrolyte system plays a crucial role in
53:57 ported incOrenasitnhgeaotttehnetrionh,a1n8,1d9,spEeCcPia-bllaysfeodr thhyedirroagpeplslicaarteionreacseievliencg- dcoentecremiviendi,ngthteheiocna-pdaiffciutosirornesrpaotensien.Thhydurso,gifelnsomtaapypbroepprioaotreelyr
3 12:0 Un trodesinOESCsdevices.20–23Forexample,multifunctionalpol- than that shown by liquid electrolytes, and thus limiting the
202n 3. yaniline (PAni) hydrogels, with high surface area and three- nalapplicationoftheenergystoragedevice.Inthiswork,Na-
ed on 3/14/s Attributio dtcraiompnaeiccnitscoioornsn,daulscupticovhritoyuPasAnnndaienloehscyttdrruoroccghtueerlmse,icedaxilhsppibrlaoitypeeeddrtieehxsic.g2eh2llAesnsptseuecplieecrc-- a(dSlegcvhiinceaemtse.e,MS1ok†r-e)cahsrparveaecgiebeecneaanllnyc,,oamcbheriintaoestdawnwo-isthtaenEpdCsPelsgeetcoltaioftainnbprircohacytedesrOso,EgteShlCes
adon capacitanceandcyclingstability.Similarly,thespeciccapaci- most appropriate biohydrogel constituted the electrolyte in
om
nlm tancedisplayedbynanostructuredconductivepolypyrrole(PPy) a device containing PEDOT electrodes and PAni nanobers
wo
y 2016. DoCreative C helyedcErtcoroog-deflersise.2n3sdulpypionrntoedvatitvheeisrtraatepgpileicsahtiaovnebaesensuunpdeerrctaapkaecnitotor (PAni-nfs)asredoxadditive.
4 Januarunder a iemrinprgodvieffethreenptestrrfaotremgiaensc,2e4–3o0fasenfoerrgeyxasmtoprlaegtehededveisciegsncoofnbsoitdh- MMaetetrihalos ds
d on 0ensed reelescoturrocdees fmorateelreiacltsrodanedmraetderoixalsel(eec.tgr.oPlyAtensi24inussoindgiumnataulrgail- For the synthesis of PEDOT electrodes, 3,4-ethyl-
s Article. Publishe This article is lic seaKnoltaOetltrciHeadt)rc,-2ost5Hetdaad2entSsediOnsba4cuuqrpaetuneaeardsoclisaunNopsga-abb2acSyaitOtstoaee4rnd)ss.t2,osi6owolRinldhueitbgceiaeholecrnadcasrtuirenasosgfeloyetortlhmeefectestherydloesentilcreyottmrtaiocdols(vnySyatlEseyntStbes)amyy,gsseht(oeoealmu.vigdes., bAeAwnelladdfesorrdiiriccpeohhuxury((sc4Atehh8C.ia3oSFs0peo2rhder8ae)t;nghferaeeoncme(stE,tyoDn$nPtO9hiatT5enr.,sir0liee9s%a7(oc)R%fwe)(PaSaAgwsp.naasPisitnhonp)r.;aeuEndrLuocirihCn.ablfOsaeoernr4dsa,ofnfavrroaenolnmimylsianitsSSe,ii7ggA(0AmmCC(cid:3)SaaCS)--
ces characteristics: high specic/volumetric energy density, exi- reagent$99.5%)andammoniumperoxodisulfate(ACSreagent
n Ac bility, light-weight and easy of handling in comparison to $98.0%)werepurchasedfromSigma-Aldrich.
pe conventional capacitors.31,32 Furthermore, SES reduce or The biopolymers used were all purchased from Sigma-
O eliminate leakage of any liquid component and increase the Aldrich.Specically,alginicacidsodiumsaltfrombrownalgae-
devicecompactness. low viscosity (A2158, 100–300 cP, 2% (25 (cid:3)C)), k-carrageenan
Polymeric materials used asSESs can be classied inthree sulfatedplantpolysaccharide(22048,5–25mPas,0.3%inH2O
groups:(1)gel-basedmaterials(e.g.polyacrylamideoragar);(2) (25 (cid:3)C)), gelatin from porcine skin (gel strength 300, type A
polymerswithdissolvedsalts(e.g.lithiumperchlorateinpoly- G2500),andchitosanmediummolecularweight(448877,200–
ethylene oxide); and (3) polyelectrolytes. Interestingly, recent 800cP,1wt%in1%aceticacid,25(cid:3)C).
studiesfocusoneco-friendlyelectrolytesforhigh-performance Otherreagentsusedduringthepreparationofbiohydrogel-
solid-state supercapacitors. For example, Buchtov´a et al.33 based electrolytes included: sodium chloride coarse salt QP
prepared an ionogel based on a chemically cross-linked poly- (Panreac Qu´ımica S.L.U., Spain), potassium chloride GR
saccharidematrixandahydrophobicionicliquidasagreener (Merck), maleic anhydride (Fluka Analytical, puriss., $99.0%
alternativetopetrochemical-basedpolymericSESs.Choudhury NT), formaldehyde solution (ACS reagent, 37 wt% in H2O,
etal.34reviewedthatgel-polymerelectrolytesfrequentlyrequire contains10–15%methanolasstabilizer,Sigma-Aldrich),acetic
using organic solvents during the gelation process, which is acid min. 99.5% (POCH), ethanol 96% v/v (USP, BP, Ph.Eur.
environmentally malignant. Hence, replacement of organic pure-pharmagrade)andsodiumhydroxide(pellets)bothfrom
solvents with water in these electrolytic systems is highly PanreacQu´ımicaS.L.U.,Spain.
desirableintermsofsustainability,alsominimizingthedevice
cost substantially. However, water-containing gel-polymer
electrolytes–hydrogel-polymerelectrolytes–limittheoperating Materialspreparation
potential-window(i.e.(cid:1)1.23Vonly).34Withinthiscontext,the PEDOT electrodes were prepared by chronoamperometry (CA)
performance of some ESCs has been improved using bio- under a constant potential of 1.40 V and using LiClO as sup-
4
hydrogels (e.g. sodium alginate, hydroxycellulose, gelatin and portingelectrolyte.Detailsaboutthepreparationprocedureare
chitosan).34–37Forexample,theelectrochemicalperformanceof providedintheESI.†Thepolymerizationchargewasadjustedto
Thisjournalis©TheRoyalSocietyofChemistry2016 J.Mater.Chem.A,2016,4,1792–1805 | 1793
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JournalofMaterialsChemistryA Paper
2.67C(445 mCcm(cid:4)2), whichcorrespondedtoamassofpoly- Electrochemicalcharacterization
merizedPEDOTof1.69(cid:5)0.23mg. Electrochemicalcellsinthree-andtwo-electrodecongurations
AbriefdescriptionofNa-alginate,k-carrageenan,gelatinand
were assembled for electrochemical characterization: the
chitosanaswellasoftheproceduresusedforthepreparationof
formerwassuitableforthefastcharacterizationoftheelectrode
theirhydrogelsisprovidedintheESI.†
material(i.e.characterizationofthePEDOTworkingelectrode),
whereas the latter was used to evaluate the supercapacitor
PreparationofOESCenergystoragedevices performanceofOESCdevicesunderspecicworkingconditions
As a general procedure, OESC devices with biohydrogel-based (i.e.theresponseofthewholecell).
ce. electrolytes were prepared 24 hours before testing, and the Accordingly, the electrochemical response of PEDOT in
M. cen electrolytegelationproceededwiththeelectrodesplacedinsitu different biohydrogel electrolytic media was rst studied using
3:57 Aorted Li thoydarcohgieelvaetbthoetheleactbroedttee/relceoctnrtoalcyttebienttwerefeance,PaEnDdOtToapnredsetrhvee awatshrmeea-edleectorfodbearceelsltacionnlesgsursatteieoln,.wThhielecaonunAtegr|AegleCcltreoldeectr(oCdEe)
3/14/2023 12:5bution 3.0 Unp ttochhafeertrhiganeegtleeahegtynirdoiatnrnyosogatfeneltpd)h.ewgMeEolouCartlPedinod,vlaeimnrm,s(eait.orgete.ieoitnnnhsesouofErmetChePaeccPaolEsmreDrseOd,cauTtsefefutoloenrcctethtrxieooadmnrieigpnailgdeioektyrf- sce(etolaelnecncttdatrroiaondraidenctghiv(yisRdtayErto)ua.grneadntTehedeleelecKctCretorlleoscdtaatqerbouiacelihtotyeu2,ms5reiscs(cid:3)oapClleu)ctwtiaoicvanetsilvy(iu)Etsy0oefd¼PanEa0sDd.2Or2eTs2fteairnVbeintlvhictseey.
ed on s Attri tbhioepboiloymhyedrrosogleultiaosneplerciotrrotloytgee,l0a.t1ionM. NaCl was mixed in the dtriyff(eCreVn)taeleerctarpoplyltyiicnmg5e0dicaowneserecudteivteeromxiidnaetdiobny–creydcluicctvioonltacmycmlees-.
dn
oamo PEDOT electrodes were correctly xed inside the OESC The initial and nal potential was (cid:4)0.2 V, whereas the reversal
nlm
wo container(i.e.glassvialorplasticcupfordeviceswithtwo-or potential was 1.0 V. All measurements were performed at room
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nuary 2016. Der a Creative eutlehxsaprevedoiesn-tuegorle1elicmcotrmfiotsdottehefeeslecaaoprnenaadragoitutfiosrPnacEtobDioneOnttawT,cet(re6inencsgmpthwe2eci)mtt,hiav.netTlhdyee,thhoSunycsd®hrpeormgelemvele.snOSwt2nteh†cr)eee, wteamspqeuraantutireeudsiansgLthEaeAslco¼asnsDQroQaftee(cid:6)leo1cft0r50o0a¼cmtQiVvi2tsQy(cid:4)(cid:4)(1QL.ET#Ah;eine%lec):trostabil(i1ty)
4 Jaund the biopolymer solution was prepared (ESI†), the container II 2
Article. Published on 0This article is licensed mwrepdelruaegeersccapithnrardaogrliendlntedihegdcsefa.otlwghlpelEieotlrxahowgctpieieanoeplpanrgtttpiisserouftoseoncxprohitfmaescccmathhcetiopipettreloosydr,ssaiaaa2ntnnlu0gb,–rhete2oyiat5d(leEsrlamloSicggIOLh†ehE)lpt.tS(ordsCDoeicfuffeueedendlrleeutyevoxnritcecctso,eehesvnseceatarmipwroooteenhosrl)eeeyr, wo(ePQafEhc#DteS)hh.reOeecsDToysDa)snetQtucdaeoslmiyinwsn,.detgthrh(edeQei2offd)pbieffaetranerefinrdonetrentmbhdcieeaoanhebscvyeedatthlwruooeeafgetaeneOvldeEetrhlSoaeeCxgciteodrdxoaoieltdfyviotaiticntchei–orsmrenee(edWcdsuhiEaacamtrwi¼gopeanlseC(icsenEyvfcaCol¼ler)-
ss for chitosan-containing devices, PEDOT electrodes and the uated in a two-electrode conguration. Galvanostatic charge–
ce discharge(GCD)cycleswererunbetween0.0and0.8V.Different
c reference electrode (when required) were inserted with
A
en extremely caution into the biohydrogel electrolyte aer gela- current densities (i.e. charge and discharge rates, which are
Op tion. Finally, OESC devices were maintained at low tempera- speciedinunitsofcurrentperelectrodemass)wereconsidered:
ture (4–6 (cid:3)C, fridge) before their electrochemical 0.276, 0.692, 1.382, 2.764 or 5.529 A g(cid:4)1 that corresponded to
0.467, 1.168, 2.336, 4.672 and 9.344 mA, respectively. All elec-
characterization.
In case of the three-electrode conguration, the reference trochemical experiments were run in triplicate and results pre-
sentedinthisworkcorrespondtotheaverageddata.
electrode was not placed in the container before the gelation
Thespeciccapacitance(SC,Fg(cid:4)1),whichisthecapacitance
step,butaglasstubewithopenendswasusedinstead(Scheme
S2b†). 1–2 mL of phosphate buffered solution (PBS, pH 7.4) perunit ofmass for one electrode, was derivedfrom the GCD
wereintroducedintheglasstube,andlettodiffusethroughthe assaysandcalculatedbyapplyingthefollowingexpression:
electrolyte hydrogel overnight to provide a good contact C
SC¼4 (2)
between the biohydrogel-based electrolyte and the reference m
electrode when introduced in the glass tube prior the electro-
whereC(inF)isthemeasuredcapacitanceforthetwo-electrode
chemicalassays.
cell and m the total mass of the active material in both elec-
Hereaer, the OESC devices with biohydrogel-based elec- trodes(i.e.3.38(cid:6)10(cid:4)3g).ThemassofPEDOTdepositedduring
trolyte using Na-alginate, k-carrageenan, chitosan and gelatin polymerization was determined as the weight difference
willbereferredasOESC-NaAlg,OESC-kC,OESC-ChiandOESC-
betweencoatedanduncoatedsteelsheets(ESI†).Themultiplier
Gel,respectively.
4adjuststhecapacitanceofthecellandthecombinedmassof
the two electrodes to the capacitance and mass of a single
Characterization electrode.38 The cell capacitance was obtained from the GCD
TheESI†providesdetailsoftheequipmentandconditionsused curvesusingthefollowingequation:
forcharacterizationbyopticalmicroscopy,UV-visspectroscopy, I
C¼ (3)
scanning electron microscopy (SEM) and atomic force micros- ðdV=dtÞ
copy(AFM),aswellasforelectricalconductivitymeasurements.
1794 | J.Mater.Chem.A,2016,4,1792–1805 Thisjournalis©TheRoyalSocietyofChemistry2016
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whereIisthedischargingcurrentappliedtothedevice.Asthe
pseudocapacitancevarieswiththevoltage,dV/dtwascalculated
as (V (cid:4) 1/2V )/(t (cid:4) t ), where V corresponds to the
max max 2 1 max
highestvoltageintheGCDcurveaerthevoltagedrop(V )at
drop
thebeginningofthedischargingprocess.
Although GCD evaluation is the procedure most commonly
usedtodeterminetheSCvaluesofapseudocapacitor,CVcanalso
provide this information. Cyclic voltammograms were recorded
e. from0.0to0.8Vaerveconsecutiveredoxcyclesatscanratesof
nc 10,25,50,75,100,150and200mVs(cid:4)1.Again,byapplyingeqn(2)
M. ce
ALi and(3)theSCoftheOESCdevicecanbedetermined.Inthiscase,
53:57 ported I(frisomcon0.s8idVerteod0aVs)t;haendavderVa/gdeticsutrhreenstcdanurriantgetuhseedd.ischargestep
3 12:0 Un Thecoulombicefficiency(h,%)ofOESCdeviceswasevalu-
202n 3. atedastheratiobetweenthedischargingandchargingtimes(td
aded on 3/14/ons Attributio aVnadntdc,0r.e8spVe:ctively)fortheelhec¼trottcdhemicalwindowbetween0(4.0) SPcrheepmarea1tioPnreapnadractihoanraocftOerEiSzCat-ikoCn:PoAfnOi-EnfSsCdeevniceergs.ystorage
wnloomm c deviceswithPAninanobers-containingbiohydrogels
oC
d on 04 January 2016. Densed under a Creative caasratuputcrpiTudrorerhionenrexedtifnmcrsbtoyhaycmdtoleeiaunlnp0ylgdsp.0il2tsybyt0itaeno–ob2gfai03ld15i.t8j.0ysu30e8sVoct2,aoefndnwAtdhdhtgseoi3(cid:4).c0ah1pI0stri-0cnopoivrcsrtiehdhrpeareeaserrpgctcewoeohd–mnoad-drOmeigsslEeeceStchnoaCtadrnroetddgcddeeeadvctniichcsydoeccanshltetadswtrghgaouaeestf- puPpc17a.5rArlteork–nrrpc8-aaiCea0sgdaronee(cid:3)runeaCrdrinna.cegiaoanOendtein2beohns5entyacrhdnmrs(eir1bowL0(oegPatmodeAshflniediun0nrii-s.)s0n,histn1bfoahsgonlMe)v,ttdehhE,dwHeSsahioCIpn,isll†rcu.owohtlAwciauoetetwdneieroreuesrnr(rwde2ieno5iedsptfrpmreer0oesemL.drcp5s)ruaiiiaxgcnbrteeeegPd4dddA%PainaAunntiwnsd-Sinib/n-vcsfnugthsaiflerktswrtmchebakadeesy-.
Article. PublisheThis article is lic tdi(s5iemt0piF0ceesisncnywdaicnleallnesys,ctdh)te.heeeteevrmroamlnauxigtnieimoednbuemaotwfeeetnhreene1rgOw5yEedSaeenCkndsbeilyt6ey0cr(teErpsoeeccahot,einWnmdgsihc.t3ahk8lgeT(cid:4)cGhh1)Cae,rDwathicmttieecserht-- fvwOo)aErwsS1aC0fsomearnmdiender.edgdTy,haasettnnord,roteothhmdeeecsvotoierclmrueestpsipoewroneanrtsdeutiirrnpergreefdvoporvalirugsemoedvreoearuosasfldl1yehMasogcuraKriibsCn.el.d(FT1ihp0ne%raelgvlevyi/l-,
ess istheamountofenergystoredperunitofmmaxass,andthepower ously, even though in this case the hydrogel-based electrolyte
n Acc density(Pmax,Wkg(cid:4)1),whichdescribesthespeedatwhichthe wasInloitaiadlelyd,wthitehPelAencit-rnofcsh(e1m%icwal/wr)e.sponse of PEDOT in bio-
e energy stored can be delivered, can be derived from the GCD
p
O hydrogels loaded with PAni-nfs was preliminary examined in
curvesbyapplyingthefollowingequations:39
athree-electrodecongurationusingbarestainlesssteelasCE
1
E ¼ (cid:6)V 2(cid:6)SC (5) and a Ag|AgCl electrode containing saturated KCl aqueous
max 2 max solution(E0¼0.222Vvs.standardhydrogenelectrodeat25(cid:3)C)
asRE.Theelectroactivityandelectrostability(eqn(1))ofPEDOT
1 V 2
P ¼ max (6) were determined by CV, applying 50 consecutive oxidation–
max 4m R
i reduction cycles. The initial and nal potential was (cid:4)0.4 V,
V while the reversal potential was 1.0 V. All measurements were
Ri ¼I drop (7) performedatroomtemperatureusingascanrateof50mVs(cid:4)1.
discharge
Aertheseelectrochemicalmeasurements,theapplicabilityof
whereVmaxcorrespondstothehighestvoltageintheGCDcurve thePAni-nfs-containingOESCwasexamined.
aer the voltage drop at the beginning of the discharging
process, m is the total mass of the active material in both
electrodes(i.e.3.38mg)andR istheinternalresistance. Results and discussion
i
Moreover, the evaluation of the leakage current and
PreparationofPEDOTelectrodes
self-dischargingcurvesofselectedOESCdeviceshasbeencarried
out.Intherstcase,aerchargingthedeviceto0.8Vat2.334mA PEDOTelectrodes wereobtained bychronoamperometry (CA).
(i.e.1.381Ag(cid:4)1),itwaskeptat0.8Vfor5minwhilerecordingthe Inthiselectrochemicaltechniquethepotentialoftheworking
current data through the OESC (i.e. leakage current). In the electrode is stepped and the current resulting from faradaic
secondcase,theOESCwaschargedto0.8Vat2.334mAandkept processes at the electrode (caused by the potential step) is
at1(cid:6)10(cid:4)11mAfor10min(self-discharging).Aerthattime,the monitored as a function of time. For this study the employed
devicewasdischargedto0.2Vat(cid:4)1.0mA. potentialwas1.40Vandapolymerizationchargeof2.67Cwas
imposed (details provided in the ESI†). Under these
Thisjournalis©TheRoyalSocietyofChemistry2016 J.Mater.Chem.A,2016,4,1792–1805 | 1795
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electrochemicalconditions,aspreparedPEDOTlmsexhibited In a very recent study Zhang and Suslick46 reported the
athicknessof4.25(cid:5)0.36mm,anelectricalconductivityof0.52 preparation of PEDOT microspheres by polymerizing nebu-
(cid:5)0.16Scm(cid:4)1,andamassof1.69(cid:5)0.23mg.Themorphology lizedmicrodropletsofsolutionscontainingEDOTmonomers
and surface topography of thin lm electrodes (i.e. those with andoxidants,usinganultrasonicspraypolymerization(USPo)
athicknessbelow10mm)affecttheSCandiondiffusionchar- technique.46TheBETsurfaceareaoftheporousmicrospheres
acteristicsofnanostructuredcapacitors.17,38,40Forexample,the obtained in presence of FeCl was 49 m2 g(cid:4)1, which was
3
SCofPEDOTelectrochemicallypolymerizedthinlmsisca.100 comparabletothesurfaceareaofnanoporousPEDOTmono-
Fg(cid:4)1,41decreasingto<20Fg(cid:4)1,<60Fg(cid:4)1or<100Fg(cid:4)1when liths (26 m2 g(cid:4)1) reported in a previous work.47 It should be
e. designed as tracked blocks,42 nanoparticles43 or micro/nano- emphasizedthatZhangandSuslickusedsuchsprayingtech-
nc rods,44 respectively. Within this context, it should be empha- nologytocoatatsurfacesofsiliconwafer,themorphologyof
AM. Lice sized that the SC of ECP-based electrodes can be signicantly the resulting PEDOT coating being similar to the one dis-
53:57 ported eenlehcatrnocdeed(ew.igt.hrerpeslapceicntgtothtehsetseeevlaelluecetsrobdyechuasinngginingtthhieswwoorrkkinbgy wpleayceodnsbiydeorutrhPaEtDaOBTETelseuctrrfoadceesarineaFvigal.uSe1.r†epBoerctaeudsbeyoZfhthanatg,
3 12:0 Un nickel, copper, gold or platinum), optimizing the oxidizing and Suslick46 (ca. 30–50 m2 g(cid:4)1) may be extrapolated to our
202n 3. agent (LiClO4 in this work), adding metallic or ceramic nano- system.
ed on 3/14/s Attributio wmpaoerrdtkiiculiemssf,obccyaursabeodsneolnencatntehodetubrbeieopshlayocdrermoggreaenplthaoennfdet,,hteehtceli.rq4e5ufHoidroewe,leeavcetsrrio,mltyphtliiecs ocanpOagncriattphohresiot.etThhoeexrihpdraeens(deG,nOHc)eanoafnetGdaOlP.4wE8DapsOreeTpxpacreoecmdtepcdootmsoitpreoesisniatfesosrscbueapsteherde-
adon PEDOTelectrodehasbeenusedduringthewholework. stability of PEDOT and maximize its pseudocapacitance. In
om
nlm Fig. S1 and S2† display details on the morphological and thiscase,PEDOTwasobtainedusinginsituchemicaloxida-
wo
y 2016. DoCreative C tIlionkperoesgluaratrifpoahnciectaowlittchhheapmroarocetresprhtihzoaalottigofyan,cPiolEiftDaptOreeTptharelemadcscPpeEsrDsesOaeTnndtelaeescsctparoopndegeoesf-. treietvsaeul.l4pt9ionclogymmGepOra/irPzeaEdtDitoOhneT.rcTeodhmoexpBpoEssieTtuedsspowecacasipa1cc9istmiuvr2efagbc(cid:4)ee1h.a4a8vriMeoaruokofkfNatbihSlae2
4 Januarunder a e(Fleigc.trSo1ly†t)e. Hieonncse,dthuirsinmgoreplhecotlroogcyheremsuiclatsl inredthoixn-plmroscewssitehs mtinicerPoEspDhOeTre.sTehmebeleedcdtreoddienswaePrEeDpOreTplaaryeedrewleitchtrtohcahteomfipcarillsy-
d on 0ensed aincoltnrtaitneudoubsyotpheen sbtirouhcytdurroegaenl-dbavsoeidd sepleacctersoltyhteat,diufrpinrogpetrhley umnidcreorgpraoptehnstieovsidtaetniccecdonthdeitpiornesseunscienogfainrreiogunliacrlipqauritdic.leSsEoMf
s Article. Publishe This article is lic ggOSaepElteaSAectCsFiioModcnfeaivlmsvliytacea,erpgy.cei,rnsowgsasor-suesielzidcentiegaonnonhoddpanrhaocegeirgleehteshtmee(evbpniedstteewwnueicdetehonctSah1EpaMatacnPiodtEabDns3OecremTvmaoatf)giogtanrhresee-. tatttrohhorafbcotehe,uwtegvhmheemroiyBcuiaEcltolrTtowphnsreu(se2rspfumaianrrcf2aeatgsciao(cid:4)ierzn1eeo)a.f4oo9tfphfTrPapihstEretiDisrwnetOiefenoTrPereeEeP,lDdEeeciODvsteTOtrrnoTidbtleuhmsltomesuid.ssgBhwsreiaatmchnsaiedufloaoseuremlentocldoyf-
ces distributed throughout the surface. In consequence, the root ours,themorphologyoftheresultinglmsisnotcomparable
c
n A meansquareroughness(Rq)ofthesurfacevariesdependingon and, therefore, our PEDOT electrodes are expected to show
pe theanalysedarea:from487(cid:5)82nmforareas(5(cid:6)5mm2)with greater surface area values. However, the scarce number of
O
arelativelyhomogenoustopographyto635(cid:5)79nmforthose publicationsreportingthisparameterobligestotakeitunder
areas (20 (cid:6) 20 mm2) in which bigger clusters are identied consideration. Mukkabla et al.50 have also reported the
(Fig.S2†). design of Bi S /GO composites enwrapped by a PEDOT
2 3
It is worth noting that the pseudocapacitance response of coating as the anode in Li-ion batteries, Bi S /GO/PEDOT
2 3
ECPs electrodes with porous morphology and large specic compositelmsbeingpreparedbychemicalpolymerization.
surfaceisfavouredsincethesestructurespromotefastionand TheBETspecicsurfaceareasofpristineBi S ,Bi S /GOand
2 3 2 3
electron transportation, which in turn allow for higher charge Bi S /GO/PEDOT were 12 51 and 46 m2 g(cid:4)1, respectively,
2 3 ,
storage and release by faradaic reactions. Although both SEM whiletheporevolumeswere0.008,0.034,and0.035cm3g(cid:4)1,
and AFM characterization techniques have provided reliable respectively.
information of the morphology and surface topography of Finally,althoughthestudyofChaoetal.53doesnotinvolve
PEDOT electrodes, Brunauer–Emmett–Teller (BET) analyses theuseofPEDOT,itdeservesspecialconsiderationbecauseof
would allow to report specic values for the surface area and themorphologyofthestudiedsystem.Morespecically,porous
porosity. Unfortunately, in this case the BET analysis was not SnS and SnS architectures were produced with a morphology
2
a feasible technique because of the limitations in the mass of that closely resembles the one displayed by the PEDOT elec-
PEDOT produced by the electrochemical synthetic route. trodes designed in this work: a porous ower-like microstruc-
Indeed,itisworthnotingthattheamountofworksspecifying ture.BETanalysesrevealedthatthesurfaceareasofporousSnS
quantitatively the surface area and porosity is relatively andSnS structuresare66m2g(cid:4)1and33m2g(cid:4)1,respectively.53
2
scarce46–50probablyduetothelimitationsoftheBETtest.Thus, In summary, although the surface area of the PEDOT elec-
the morphology ofPEDOT electrodesfor energystorage appli- trodes used in this work has not been determined due to the
cationsisfrequentlydescribedqualitativelybyexaminingSEM operational restrictions of the BET test, the revised literature
micrographs, especially when the polymer is generated by described above allows us to suggest that this value would be
anodicpolymerization.51,52 foundbetween30and50m2g(cid:4)1.
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O Fig.1 Cyclicvoltammetryusingathree-electrodeconfiguration(Fig.S5†)havebeenusedtodeterminethefollowingelectrochemicalprop-
erties for PEDOT electrodes using different electrolyte systems: (a) variation of the stored charge, where Q (left) and Q (right)refer to
ox red
oxidationandreductioncharges;(b)electrochemicalactivity,whichisdefinedbytheQ /Q ratio;and(c)lossofelectrochemicalactivity(LEA)
red ox
againstthenumberofconsecutiveredoxcycles.Dataaretheaverageofthreesamplesforeachsystem.Galvanostaticcharge–dischargeassays
usingatwo-electrodeconfiguration(Fig.S6†)havebeenemployedtoevaluatethevariationof(d)thecoulombicefficiencyand(e)thespecific
capacitance(SC)againsttheappliedcurrentdensityforOESCdeviceswithdifferentbiohydrogel-basedelectrolytes.
Preparationandselectionofbiohydrogelsaselectrolyte electrolyte.Accordingtotheseresults,chitosanhydrogelswere
system discarded as electrolytic system. At this point, it should be
clariedthattheapplicationofthebiohydrogelsaselectrolytic
Fig. S3a and S3b† display images of the prepared hydrogels
mediadoes not requirecareful evaluation oftheir mechanical
(bulk) and of the OESC-biohydrogel devices in a two-electrode
conguration,respectively.Hydrogel-basedelectrolytesderived properties but only visual inspection to ensure that these
materials are robust enough to keep the electrodes at their
from Na-alginate and chitosan are opaque and exhibit poor
positions.
mechanical integrity (i.e. they disintegrate when submitted to
stress) in comparison to those obtained using k-carrageenan The morphology of k-carrageenan, Na-alginate and gelatin
hydrogels was characterized by SEM (Fig. S4†). Solid-state
andgelatin.Infact,theextremelypoormechanicalintegrityof
electrolytes are expected to present an open porous structure
chitosan required careful handling, limiting drastically its
able to exploit at maximum the pseudocapacitance of PEDOT
applicabilityasbiohydrogel-basedelectrolyte.Inaddition,water
electrodes, favouring ion movement and transfer. Both k-
release was observed for Na-alginate and chitosan hydrogels,
whichalsorepresentedanimportantdrawback.Incontrast,k- carrageenan and Na-alginate hydrogels accomplish this
requirement, while gelatin exhibits a compact morphology in
carrageenan and gelatin hydrogels are transparent and more
whichtheinterconnectednetworkofporesisabsent.
robust, facilitating the assembly of PEDOT electrodes into the
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Article. Published on 0This article is licensed SFcioegll.ue2mctinoC)nahneodmfPitchEaDelOsstuTrusectlateuicnrtearobadlneedsOm(rEiogSrhCpthcuoosloliungmgynb(S).iEoMhymdricorgoeglrsaapshs) of OESCel-ekcCtro(mchidedmlei)ceallemacetnivtist:yk(-LcEarAra,geeqenna(n1)h)ywdarosgselligahstelyleclotrwoelyrtein(lekft-
s
es electrolytesystem carrageenan (13%) than in Na-alginate and gelatin (18% and
c
en Ac Aer discarding chitosan hydrogels as electrolyte due to its 1P6E%DO, rTesepleeccttriovdeleys).vMaroierseodveepre,ntdhienagmonoutnhteoeflecchtraorlgyetes(tFoirge.d1ab)y:
p extremely poor mechanical integrity, the electrochemical
O k-carrageenan>Na-alginate>PBS>gelatin.
responseofPEDOTineachoftheremainingthreebiohydrogels
wasinvestigatedtodeterminetheireffectivenessaselectrolytic Results indicate that the compact morphology of gelatin
media. Fig. S5a–S5c† display cyclic voltammograms recorded hydrogelhinderstheaccessandescapeofdopantionsduring
aer several consecutive redox scans for k-carrageenan, Na- theoxidationandreductionprocessesofPEDOT,whichreduces
both the amount of charge stored and the current density.
alginate and gelatin electrolyte systems, respectively, using
a three-electrode conguration. PBS was used as control elec- Furthermore,theresponseofPEDOTinbothk-carrageenanand
Na-alginate hydrogel-based electrolytes is similar, indicating
trolyte system (Fig. S5d†). These voltammograms have been
that the interconnecting network of both systems enables ion
used to evaluate the electroactivity of PEDOT (Fig. 1a and b),
transport. Finally, the electrochemical performance of PEDOT
which corresponds to the similarity between the cathodic and
inPBSelectrolyteisworseincomparisontok-carrageenanand
anodic areas in the voltammograms (i.e. ratio between the
Na-alginatebaseddevices.Morespecically,theelectroactivity
reduction and oxidation charges, Q /Q ). The electroactivity
red ox
and LEA values obtained in PBS are lower and higher, respec-
increases with the number of redox cycles indicating that the
tively, than those reached using such twobiohydrogels. These
ability to exchange charge reversibly improves and stabilizes
aer y consecutive oxidation–reduction cycles. Specically, features indicate that the swelling and shrinkage of PEDOT
aerthetenthcycle,theQ /Q ratioishigherthan0.90forthe electrodes during redox processes are enhanced in PBS elec-
red ox
trolyte, inducing a faster and more severe mechanical degra-
three biohydrogel-based electrolytes (Fig. 1b). Comparison of
dation of the ECP that, in turn, affects its electrochemical
thecurrentdensitiesreachedatthereversalpotential(i.e.1.0V
in Fig. S5a–S5d†) indicates that the amount of oxidized mole- properties.
culesissimilarfork-carrageenan,Na-alginateandPBS,whereas Fig.1dandeshowthevariationofboththecoulombiceffi-
ciency(eqn(4))andtheSC(eqn(2)and(3)),respectively,against
it is lower for gelatin-based electrolyte. Moreover, the electro-
the current density for the three biopolymer hydrogel-based
chemical stability of PEDOT also changes with the electrolyte
system (Fig. 1c). Aer ve-hundred redox cycles, the loss of OESCsystems.DataderivefromthecorrespondingGCDcurves
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Article. PublisheThis article is lic rFk1e0i-gcc,.o2a3rr5dr,ae5Egd0lee,efc7rnto5raom,n1c0hh00ye.0d,m1rtoi5co0gae0lalc.n8ahdsaV2rea0laec0tctetdmrrioiffzVleaystrt(cid:4)ieeo1.nn((tab()ct)wCuSrCoyrc-evleniaclteluvcdeotesrlntodasdmeietrimiecvseoo.dngSfifroragolmiudmrsalittrnhieoeecnsoc)cryodocferlOidrceEfsvrSpooColmt-nakmd0C.mt0dooetotvghir0ceae.m8fis,rVsswdtahifcstiphcehlararfipygvereeed–sciednonisn(tacsb)heo(acstreuhgeteCiveeEcqynarcen(ld2edo),WxawnEchdymilc(e3lae)dt)sh.eae(cot)dfsGcaPsaCEhnDDerOdactTuoerasnvneeodssf
ss correspondtothetwenty-fifthcycle.(d)SCvaluesderivedfromtheGCDcurvesdisplayedin(c)(seeeqn(2)and(3)).SCvaluesin(b)and(d)are
e
cc comparedwiththoseobtainedfortheOESC-PBScontroldevice(Fig.S7†).In(d)emptysymbolsrefertotheSCvaluesdeterminedforthetwenty-
n A fifthcharge–dischargecycle.
e
p
O
(Fig. S6†). As it can be seen, the triangular shape of the GCD PBSaselectrolyticmedium(OESC-PBS),whichhasbeenusedas
curvesandSCvaluesarebetterdenedandhigher,respectively, acontrol(Fig.S7†).
for OESC-kC than for OESC-NaAlg and OESC-Gel, indepen- Cyclic voltammograms recorded for the OESC-kC device
dentlyofthecurrentdensityused.Inaddition,aer25charge– (Fig. 3a) from 0.0 to 0.8 V display an almost ideal rectangular
dischargeprocesses,theSCdeterminedforOESC-kCremained shape for scan rates up to 200 mV s(cid:4)1 (Fig. 3a), which is
higher than 96% for all the considered current densities indicative of a good capacitive performance and low contact
(Fig.S6†).Forexample,forthecurrentdensityof1.38Ag(cid:4)1,the resistance. This effect is less pronounced for OECS-PBS
SConlydecreased0.98%,evidencingexcellentcyclingstability. (Fig.S7a†).Furthermore,thecurrentdensityatthereversaland
Overall, these results reect that k-carrageenan is the most initial potential (i.e. anodic and cathodic current density,
suitable biohydrogel to perform as electrolyte due to the respectively) varies linearly with the scan rate (Fig. S8†),
appropriate combination of properties: mechanical stability, evidencingthattheelectrochemicalprocessesatOESC-kCand
easeofpreparation,lackofwaterleaking,andgoodmediumfor OESC-PBS are not controlled by diffusion.54,55 On the other
the electrochemical response of PEDOT. Accordingly, investi- hand,exceptforthelowestscanrate,theSCvalues(eqn(2)and
gations displayed in next sections are exclusively focused on (3))obtainedforOESC-kCareslightlyhigherthanthosedeter-
OESC-kCdevices(Fig.2). minedforOESC-PBS(Fig.3bandTableS1†).
GCD curves for OESC-kC devices exhibited a typical trian-
gular shape with a small voltage drop at the beginning of the
OESC-kCenergystoragedevice:capacitiveperformance
dischargingstep(Fig.3c).Ingeneral,PEDOTelectrodesdisplay
Specic capacitance. The capacitive behavior of OESC-kC symmetric charge–discharge curves that correspond to almost
was investigated by CV and GCD curves (Fig. 3). Results have idealcapacitiveproperties:highcoulombicefficiencyandgood
been compared with those obtained for an OESC device with reversibility.14,16,18 These features are observed for OESC-kC
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Fig.4 CyclingstabilityofOESC-kCdevices,whichpresentbothCEandWEmadeofPEDOTandk-carrageenanhydrogelaselectrolyte.(a)GCD
curvesrecordedfrom0to0.8Vat1.38Ag(cid:4)1(tworepetitions).Solidlinescorrespondtothesecondcycle,whilethedashedonescorrespondto
thethree-thousandthcycle.(b)SCvaluescalculatedwiththedataderivedfrom(a).(c)GCDcurvesrecordedfrom0to0.8Vat1.38Ag(cid:4)1for
devicestestedjustasprepared(t )andafteroneweekoftheirpreparation(t ).Solidlinescorrespondtothefirstcycle,whilethedashedones
0 1w
correspondtothefive-hundredthcycle.(d)SCvaluescalculatedwiththedataderivedfrom(c).(e)LeakagecurrentcurveforOESC-kCcharged
at2.334mA(i.e.1.38Ag(cid:4)1)andkeptat0.8Vfor300seconds.(f)Self-dischargecurvesforOESC-kCdevices.Theprocedureemployedforthese
assaysisdescribedintheMethodssection.
devices,whichresultsinSCvalues(eqn(2)and(3))thatrange inthe biohydrogel-basedelectrolyte.Incontrast, SCvalues for
from 70 to 81 F g(cid:4)1 for the second charge–discharge cycle the OESC-PBS device do not present signicant changes with
(Fig.3dandTableS2†).AlthoughtheseSCvaluesaresimilarto the current density (Fig. 3d). On the other hand, aer 25
thoseobtainedfortheOESC-PBSdeviceatlowcurrentdensities consecutive charge–discharge cycles, OESC-kC devices dis-
(Fig. S7†), they tend to decrease for higher current densities, played an excellent SC retention (i.e. higher than 92%) for all
possiblyduetoslowerionmovementandelectrolytediffusion testedcurrentdensities(Fig.3d).Forexample,thesystemwas
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Article. PublisheThis article is lic sFceiogcn.on5nedcs(ta)e)dfoFionrusorenraeiesssaentmodpbfoloewudreOraasEsSreeCmd-bkLClEeDdd(eOvvidEiceSeoCs-,dkiwsCphlidacyehevdipcrienessteh(bneltaEcbSkoI†ta)h.n(dbC)EbGlauCneDd,crWeusrEpveemscatriedvceeolyor)df.e(PcdE)aDStOe1l.Tf3-8daniAsdcgh(cid:4)ka1-rc(gcaehrracaruggreinveegnsaanondfhfdyoidsucrrohOgaerEglSinaCsg-ektiClmecedtsreoovlifyc3tee0s,
s connectedinseries.(d)LEDpoweredusingthesystemdescribedin(a).
s
e
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p able to retain ca. 95% of the highest SC value, which was ob- and reduction processes, respectively. Thus, consecutive
O
tainedat1.38Ag(cid:4)1(TableS2†).Asitwasexpected,thestability doping–dedoping steps induce structural changes in PEDOT
oftheOESC-kCdeviceisslightly lowerthanthatoftheOESC- electrodes, which evolve from a porous morphology toa more
PBSone(Fig.3dandTableS2†).Thisshouldbeattributedtothe compact one. The increasing compactness of PEDOT hinders
degradative effects produced by consecutive charge–discharge theaccessandescapeofdopantionsduringtheoxidationand
cyclesinthehydrogelstructure. reduction processes, respectively, until the equilibrium is
Overall,resultsdisplayedinthissub-sectionconrmthatthe reached.
OESC based on PEDOT electrodes and k-carrageenan as elec- Moreover,todeterminethelifetimestabilityofOESC-kC,the
trolyteexhibitsagoodsupercapacitorresponse.Thepresenceof sameassaywasrepeatedaeroneweekofthedeviceprepara-
the solid electrolyte does not affect negatively the PEDOT tion. Fig. 4c and d compare the results obtained for as as-
capacitive behavior, bringing advantageous characteristics to prepared OESC-kC devices (t ) with those obtained for devices
0
thenalpackagedcellforpracticalapplications. prepared one week before (t ). In this series of as prepared
1w
Lifetime stability. Cyclability (i.e. from hundreds to thou- devices,theSCdecreasefrom85(cid:5)7Fg(cid:4)1to80(cid:5)6Fg(cid:4)1aer
sands of cycles) is an important requirement for practical consecutiveGCDcyclesataconstantcurrentdensityof1.38A
supercapacitorapplications.Inordertoevaluatethisproperty, g(cid:4)1 from0to0.8V,which representsa variation ofonly(cid:1)6%
as-preparedOESC-kCdevicesweresubmittedtothree-thousand (i.e.94%ofretention).Forthedevicesthat werecharacterized
consecutiveGCDcyclesataconstantcurrentdensityof1.38A oneweekaertheirpreparation,theSCvaluescorrespondingto
g(cid:4)1from0to0.8V,resultsbeingsummarizedinFig.4aandb. the second (80 (cid:5) 9 F g(cid:4)1) and ve-hundredth (79 (cid:5) 8 F g(cid:4)1)
Aersuchamountofcycles,theSCdecreasefrom79.1(cid:5)0.4F charge–discharge cycle differ by only (cid:1)1% (i.e. 99% of reten-
g(cid:4)1 to 71.1 (cid:5) 2.2 F g(cid:4)1, which represents a variation of only tion) and, more interestingly, they resemble to those deter-
(cid:1)10% (i.e. 90% of retention). This slight reduction in the SC minedfortheas-prepareddeviceaerve-hundredcycles(79(cid:5)
values for OESC-kC has been associated to the swelling and 6Fg(cid:4)1),conrmingthatthestructureofPEDOTisstabilized.In
shrinkagephenomenaexperiencedbyPEDOTduringoxidation addition to this excellent cycling stability, OESC-kC devices,
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