Mn-modified polypyrrole films for supercapacitor thin electrodes Synthetic Metals

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Synthetic Metals

j ou rn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / s y n m e t

Mn-modified polypyrrole thin films for supercapacitor electrodes

Purnama Ningsih, Clovia Z. Holdsworth, Scott W. Donne

^∗

DisciplineofChemistry,UniversityofNewcastle,Callaghan,NSW2308,Australia

a r t i c l e i n f o

Articlehistory:

Received29April2014

Receivedinrevisedform9July2014 Accepted14July2014

Keywords:

Polypyrrole Manganeseoxides

Supercapacitorelectrodematerials Electrochemicalquartzcrystal microbalance

Atomicforcemicroscopy

a b s t r a c t

ThinfilmMn-modifiedpolypyrrole(PPy)compositeelectrodeshavebeenpreparedbychronoampe- rometricelectrodepositionandcharacterizedintermsoftheirphysico-chemicalandelectrochemical propertiesandperformance.Analysisofthechronoamperometricdatashowsthatelectrodepositionof thethinfilmresultsinarelativeincreaseinelectrochemicallyactivesurfaceareaofupto30times.

Thisfindingwassupportedbytransmissionelectronmicroscopy(TEM),atomicforcemicroscopy(AFM) andprofilometryanalysisofthefilms.Electrochemicalquartzcrystalmicrobalance(EQCM)studieshave allowedforthedirectdeterminationofelectrodemass,bothduringdepositionandelectrochemicalper- formanceevaluation,whichhasenabledanalysisofelectrodeproperties,includingfilmgrowth(upto 26␮g/cm²),density(∼2g/cm³),andthechargestorageduringelectrochemicalcycling,includingtherates ofmassuptake/removalwithcharge.Thecharacteristicsofthecompositeelectrodeswerecomparedwith PPy-onlyelectrodesthroughout.

©2014ElsevierB.V.Allrightsreserved.

1. Introduction

1.1. Energystorageinsupercapacitorsystems

Nowadaysenergyhasemergedasafundamentalfocusformajor worldeconomicpowers and agreat challengefor thescientific community[1].Energystoragesystemshaveinthepast,andwill continuetodosointothefuture,playasignificantroleinconsumer electronics. Theyalso have an emerging role in transportation, wheretheintroductionofelectricandhybridelectricvehiclesis growinginpopularity,andingridenergystorage,wheretheyarean importantcomplementtorenewableenergyproductionsystems.

Inthisworkourfocusisontheuseofsupercapacitorsforenergy storage. Therelative performance of supercapacitors compared tobatteriesandfuelcellscanbesummarizedbyatypicalRagone diagram[2].Supercapacitorsaretypifiedbyahighspecificpower outputbutalowspecificenergydensity,whichhasunfortunately inhibited their widespread commercialization. Another distinct advantagethatsupercapacitorspossesscomparedtoothertypesof energystoragedevicesistheircyclability,whichistypicallyofthe orderof>10⁵cycles,comparedto∼10³forbatteries.Considerable research in recent times has gone into improving the specific energy of supercapacitor electrode materials and devices, with theintentofalsoretainingtheirhighspecificpowerandexcellent

∗ Correspondingauthor.Tel.:+61249215477;fax:+61249215472.

E-mailaddress:scott.donne@newcastle.edu.au(S.W.Donne).

cyclability(e.g.,[3–5]).Such developmentswill providegreater energystorageoptionsforconsumers.

1.2. Supercapacitorchargestoragemechanismsandmaterials

Present day commercial supercapacitors typically employ a symmetricalgeometry,inwhichbothelectrodesarebasedonhigh surfaceareacarbonssuchasactivatedcarbon,carbonfibres,aero- gels,xerogels,fullerenes,andothervariousnanostructures[6,7].

Theelectrolyteinsuchsystemsiseitheranon-aqueousoraque- ouselectrolyte;e.g.,1Mtetraethylammoniumtetrafluoroboratein acetonitrileoraqueous1MH₂SO₄,respectively.Energystoragein suchdevicesoccursbychargeseparationatthesolid–electrolyte interfacewithintheporouselectrodestructure[8].Asaresultof theuseofahighsurfaceareacarbonmaterials,considerablecharge canbestored;i.e.,upto300F/g[8–10].Furthermore,sincenophys- icalorchemicalchangesareoccurringintheelectrodewithsucha process,suchsystemsexhibitexcellentcyclability.

Analternativeapproachtochargestorageinsupercapacitorsys- tems isto employpseudo-capacitance,which is essentiallythe useoffastandreversiblesurfaceornearsurfaceredoxreactions [11].Suchanapproachisacommonstrategyforimprovingspe- cificcapacitanceduetothefactthatmorethanjustthesurfaceof thematerialisbeingutilizedforchargestorage.Classesofmate- rialsthatexhibitpseudo-capacitanceincludeconductivepolymers andmetaloxides,withthislatterclassofmaterialscrossingthe boundarybetweenbatteryandsupercapacitordomains[12,13].An http://dx.doi.org/10.1016/j.synthmet.2014.07.007

0379-6779/©2014ElsevierB.V.Allrightsreserved.

excellentrecentreviewofmaterialsforsupercapacitorsbyNaoiand Simoncanbefoundinreference[13].

Conducting polymers that have been examinedas superca- pacitor electrodematerialsinclude polyaniline, polypyrroleand polythiophene,togetherwiththeirmanyderivatives[14].Thespe- cificcapacitanceofsuchmaterialshasbeenreportedtorangeup to∼300F/g,withenergystorageagainoccurringviachargesep- arationattheconductivepolymer–electrolyteinterface[15].One ofthemaindrawbacksofthesesystems,however,istheirpoor mechanicalstabilityafterrepeatedcycling[16].

Theprototypicalmetaloxidethathasbeenexaminedasasuper- capacitor electrode is hydrated amorphous ruthenium dioxide, whichhasbeenreportedtohavea specificcapacitanceofupto 1200F/g[17,18].Whileanexcellentperformingmaterial,itshigh costandtoxicityhaslimiteditswidespreadcommercialuptake.As reportedbyNaoiandSimon,othermetaloxidesthathavebeen examinedinclude manganese, nickel, tin, and iron oxides [13].

Ofthese,itisapparentthatmanganeseoxideshaveconsiderable potentialassupercapacitorelectrode materials,exhibiting good electrochemicalperformance(upto800F/g[13]),aswellasbeing oflowcostandtoxicity.Recentreportsfromourownlaboratory haveshownthatelectrodepositedthinfilmsofmanganesediox- ideexhibitexcellentspecificcapacitancesofover2000F/ginan aqueous0.5MNa2SO4electrolyte[19].Thislevelofperformance hasbeenascribedtotheuseofahighsurfaceareaactivematerial exhibitingbothpseudo-capacitanceandchargeseparationatthe solid–electrolyteinterface.

1.3. Thiswork

Inthevastmajorityofworkreportedpreviouslyintheliterature, thematerialsbeingexaminedhave beenpreparedasfree flow- ingpowders,whichwerethencastintoelectrodesonasuitable conductingsubstrate.Itisrelativelyrarethatthinfilmsofman- ganesedioxide,oranyelectro-activematerialforthatmatter,are depositedontoasubstratewhichwasthenusedimmediatelyasthe supercapacitorelectrode.Thisapproachhasconsiderableadvan- tages,notonlyintermsofperformance,butalsointermsofeaseof processing.Anumberofresearchershaveusedelectrodepositionto producepolypyrrole,manganesedioxide,andcompositesthereof (e.g.,[20–25]);however,relativelythickdepositswerestudiedin thesereports which hasultimately influenced theperformance results,makingtheelectrodepositedfilmscomparableinperfor- mancetopowderedmaterials.

Conductingpolymerssuchaspolypyrrolecanalsobeprepared viaelectrodeposition(e.g.,[26]);however,similartotheelectrodes preparedfrompowdered materials,theirmechanicalinstability causesabreakdowninelectrodeperformance[27].Inthiswork we will examine theelectrochemical depositionof polypyrrole thathasbeenmodifiedbythepresenceofMn²⁺,withtheintent ofstabilizingthepolypyrroleduringcycling.Additionally,wewill alsodemonstratethatenhancedelectrodeperformancefromthese compositescanbeachievedthroughtheuseofthinfilmelectrode- position.

2. Experimental

2.1. Materials

Thematerialsusedinthisstudyforthesynthesisoftheelec- trodecompositeswere thepyrrolemonomer(Py; Merck; 99%), MnSO₄·H2O (Sigma–Aldrich; >99%)and H₂SO₄ (Sigma–Aldrich;

98%).Experiments toevaluatetheelectrochemicalperformance of our electrodes employed an aqueous solution of K2SO4

(Sigma–Aldrich;>99%).Allaqueoussolutionspreparedinthisstudy madeuseofMilli-Qultrapurewater(resistivity>18.2M.cm).

2.2. Electrochemicalprotocols

AllelectrochemicalexperimentswereconductedusingaStan- ford Research Systems QCM200 as an electrochemical quartz crystalmicrobalance(EQCM).Inthiscasetheworkingelectrode wasa1.325cm² platinum disksputtercoatedona5MHzreso- nantfrequencyquartzcrystal.Priortouse,thisplatinumelectrode wascleanedbyimmersionina0.1MH2SO4+5%H2O2 solution.

Thiswasusedinconjunctionwithasaturatedcalomelreference electrode(SCE),againstwhichallpotentialsweremeasuredand reported,andagraphiterodcounterelectrode(6.3cm²).Allelec- trochemicalexperimentswerecontrolledusingaPerkinElmerVMP multi-channelpotentiostat/galvanostatwithECLabsoftware.

Electrodepositionexperimentswerecarriedoutusingaqueous solutionscontainingcombinationsofPy(0,0.001,0.01or0.1M), MnSO₄·H2O(0,0.001,0.01or0.1M)and0.1MH₂SO₄.Allelectrode- positionexperimentswerecarriedoutusingchronoamperometry, although prior to this a linear sweepvoltammetry experiment wasconductedoneachsolutionbeingstudiedsoastoidentify anappropriatesteppotential.Oncethispotentialhadbeeniden- tified,andaftertheplatinumsubstratehadbeencleanedagainin 0.1MH2SO4+5%H2O2,eachchronoamperometryexperimentwas carriedoutfor30s.

Aftereachchronoamperometricdeposition,thecoatedquartz EQCM electrode was rinsed thoroughly with Milli-Q water to removeanyassociatedplatingelectrolyte.Withoutbeingdriedthe electrodewasthenimmersedintoa0.1MK₂SO₄solution,together withthesameSCEreferenceandgraphitecounterelectrodesas usedpreviously.Thiselectrodewasthencycledforatleast50cycles between0.0and1.0VvsSCEatarateof5mV/s.

The Pt working electrode was used for electrochemical impedancespectroscopywork(EIS)work.Aftereachelectrodepo- sition,thePtworkingelectrodewasrinsedH₂Oandwithoutbeing driedtheelectrodewasthenimmersedintoa0.1MK2SO4 solu- tion,togetherwiththesameSCEreferenceandgraphitecounter electrodesinanEISsystembyusingthecombinationofaSolartron 1254FrequencyResponseAnalyzerandaSolartron1287Electro- chemical InterfacecontrolledbyZPlot software.From theopen circuitvoltageofthethinfilmselectrode(0.300V)thevoltagewas steppedintheanodicdirection25mV,afterwhichitwasallowed toequilibrateforaperiodof10min.Afterthis,animpedancespec- trumonthethinfilmselectrodewasmeasuredusingthefrequency range20kHzto0.1Hzanda10mVexcitationsignal.Thissequence wasrepeatedtotheuppervoltagelimit(1.0VvsSCE),downtothe lowervoltagelimit(0.0VvsSCE),andthenoncemorebackupto theuppervoltagelimit.Theresultantimpedancespectrawerethen interpretedusinganappropriateequivalentcircuit.

2.3. Morphologicalcharacterization

Owingtothenatureoftheverythinfilmsofelectrodeposited material,morphologicalcharacterizationisquitechallenging.Tra- ditionalmethodsofanalysissuchasX-raydiffractionandscanning electron microscopy (SEM) are not possible due to thelimited amountofmaterialpresent.Despitethis,lengtheningthedeposi- tiontimetomakeathickerdeposithasshownthatthematerial is stillamorphous toX-raydiffraction[19].Therearehowever, concernsaboutthisapproachbecauseofchangesinthenatureof thedepositusingthechronoamperometricmethod.Nevertheless, formorphologicalcharacterizationinthisstudythesampleswere examined by atomic force microscopy (AFM; Asylum Research CypherScanningProbeMicroscope)inACmodewitha2.44Hzscan rate,transmissionelectronmicroscopy(TEM;JEOLJEM-1200EXII)

onsamplesscrapedofftheplatinumsubstrate,andsurfacepro- filometry(KLATencorAlfaStep-500)afterscratchingthefilmina smallareatoexposetheplatinumsubstrate.

2.4. Chemicalcomposition

Separatefilmswerealsopreparedforcompositionalanalysis todeterminetherelativeamountsofpolypyrrole(PPy)andman- ganeseineachfilm.Toachievethis,eachfilmwasfirstthoroughly washedwithMilli-Qultra-purewatertoremoveanyresidualelec- trolyteand thendigestedin 30mLof0.1MH₂SO₄+5%H₂O₂ to essentiallyextractthemanganesefromthefilm.Eachextractsolu- tionwasthentransferredtoitsown100mLvolumetricflask,which wasmadeuptovolumeusingMilli-Qwater.Thesesolutionswere thenaspiratedintoaVarianLibertySeriesIIICP-OESinstrumentto determinethemanganeseconcentration.Matrixmatchedstandard solutionswerepreparedfromMnSO4·H2OandH2SO4.

3. Resultsanddiscussion

3.1. Linearsweepvoltammetry

Fig.1 comparestheanodic voltammetricbehaviourof solu- tionscontaining0.1MPy+0.1MH2SO4,0.1MMn²⁺+0.1MH2SO4

and0.1MPy+0.1MMn²⁺+0.1MH₂SO₄ scannedat5mV/s.The behaviourofthe0.1MMn²⁺+0.1MH₂SO₄ solutionconsistsofa voltammetricwavestartingat∼1.08V,whichisfollowedbyoxy- genevolutionathigherpotentials.Thisbehaviouristypicalofthat expectedforsuchanelectrolyte[19].OxidationofMn²⁺inthiscase occursviathereaction

Mn²⁺+2H2O→ MnO2+4H⁺+2e⁻ (1) However,themechanismofoxidation isnotsostraightforward.

Fig.2summarizesthepossibleMn²⁺oxidationpathwaystoform MnO2thathavebeenreportedintheliterature[19].Thefirststepin theprocessisthesingleelectronoxidationofMn²⁺toformasoluble Mn³⁺intermediate.Theacidityoftheelectrolytethendetermines whichpathwaypredominates.Inmoreconcentratedacidicelec- trolytes(PathC)theMn³⁺intermediatehasareasonablestability,

Fig.1. Linearsweepvoltammetrydatacollectedonthe0.1MPy+0.1MMn²⁺(0.1M H2SO4)electrolyte.Platinumsubstrate;scanrate=5mV/s.

Fig.2. Mechanisticpathwaysfortheelectrodepositionofmanganesedioxide.

meaningthatitcandisproportionatewithanotherMn³⁺speciesto formsolubleMn²⁺andMn⁴⁺,thelatterofwhichhydrolyzesimme- diatelytoprecipitateMnO₂ontheelectrodesubstrate.However, iftheelectrolyteisnotsoconcentratedinacid(PathA),thenthe Mn³⁺intermediateisnotsostable,hydrolyzingtoprecipitateasolid Mn(III)species(suchasMnOOH)ontheelectrodesurface,which canbesubsequentlyoxidizedinthesolidstatetoformMnO₂.There isthepossibilityofadirecttwo-electronoxidationofMn²⁺toMnO2

(PathB);however,thereisnoevidenceintheliteraturetosupport theoccurrenceofthisprocess.Certainlynooneprocessoccursindi- vidually,butratherthecombinationofprocessesisshiftedoneway ortheotherdependingontheexactelectrolyteconditions.

TheanodicpolymerizationofPyinFig.1beganatapotentialof

∼0.54V,andexhibitedasharpincreaseincurrent,somuchsothat diffusionlimitedconditionswerenotencounteredbeforeacurrent maximumwasreachedwiththepotentiostat.It wasalsonoted thatincreasingtheconcentrationofPyintheelectrolytecausea cathodicshiftintheonsetpotentialforPPyformation.Thiswasto beexpectedbasedonNernstequationconsiderations.Inthiscase, oxidationofPyoccursviatheprocess

Py→ Py^+•+e⁻ (2)

withthecationicradicalactingastheinitiatorforthepolymeriza- tionprocess;i.e.,

n(Py^+•)→(Py)_n+nH⁺ (3)

wheretheresultantpolymer(polypyrrole;PPy)isdepositedonto theanodicsubstrate.

For the mixed electrolyte containing both Py and Mn²⁺ the voltammetricbehaviourappearsessentiallyasa combinationof theindividualPyandMn²⁺i–Vprofiles,particularlyintermsofthe mainonsetpotentialsforoxidation.Forthiselectrolytethereisalso significantcurrentflowingatpotentialsmorecathodiccompared tothemainonsetpeakforPyoxidation;i.e.,<0.54V.Thecurrent flowinginthisregionismuchhigherthaneitheroftheotherelec- trolytes,whichmayindicatesomefavourableinteractionsbetween thePymonomerandtheMn²⁺ionsleadingtoeffectivelyunder- potentialdepositionof thecomposite electrode material.Asan additionalpoint,fromthelowercurrentresponseinthecombined electrolyte,itwouldseemasthoughthepresenceofMn²⁺hashad aneffectontheoxidationofPy,possiblythroughsurfaceadsorp- tion,thathasinhibitedthecontinuedoxidation ofthePy.From thedatainFig.1,apotentialof0.75VvsSCEwaschosenasan appropriatepotentialtocarryoutchronamperometryexperiments todepositthinfilmsofPPywithandwithoutthepresenceofMn²⁺. Thispotentialwaschosenbasedonpreviousworkfromourlabo- ratory[19]whichhasdemonstratedthatrelativelylowpotentials, correspondingtonon-diffusion limited conditions,gives riseto superiorperformingelectrodes.Furthermore,thispotentialisalso

Fig.3. ChronoamperometricdataforeachofthePy+Mn²⁺electrolytesconsidered inthisstudy.Platinumsubstrate;steppotential=0.75VvsSCE.

nottoohighsuchthattheirreversibleover-oxidationofPPyoccurs [27].

3.2. Chronoamperometricdepositionandmassanalysis

Fig.3comparesthechronoamperometricresponsesobtained fromthe0.001Py+0.001MMn²⁺,0.01MPy+0.01MMn²⁺ and 0.1MPy+0.1MMn²⁺electrolytesin0.1MH2SO4.Thedatashows quiteinterestingbehaviour,verydifferentfromwhatisexpected fromatypicalchronoamperometricdeposition;i.e.,Cottrellequa- tionbehaviour,i∝t^−½[28].Depositionfromsolutionscontaining lowconcentrationsofPyandMn²⁺exhibitedbehaviourthatcould beregardedasbeingtheclosesttoexpected;however,eveninthese circumstancesanincreaseincurrenttowardsthelatterstagesof depositionwasobserved. Asthesolutionconcentrationofelec- troactivespeciesincreased,thisnon-Cottrellbehaviourwasalso enhanced. The origin of this behaviour is due to anincreasing electrodeareaasa resultofongoing electrodeposition,through nucleationandgrowthphenomena,ashasbeendescribedinearlier literature(e.g.,[29]).Inthisearlierliteraturethefocushasbeenon extractingrateconstantsforbothcrystalnucleationandgrowthfor metals,saltsandpolymers,albeitintheabsenceofmasstransport considerations,whichisofconsiderableimportancehere.Thefocus oftheworkpresentedhereisontheincreaseinelectrodeareaasa resultoftheelectrodepositionofPPy.

Toexaminethis further,let usfirstconsiderthemechanism ofelectrodepositionongoinginthissituation.Inthetimeframe immediatelyafterapplicationofthepotentialstepthePymonomer isoxidizedtoproduceasolublereactionintermediate,asdescribed previously.InthiscasewewouldexpectconventionalCottrelli–t behaviour;i.e.,

i=nFACD^1/2

(t)^1/2 (4)

where i is the current (A), n is the stoichiometric number of electronsflowing in theredox reaction,F is Faraday’s constant (96,486.7C/mol),Aistheelectrochemicallyactiveelectrodearea (m²),Cis theelectroactivespeciesconcentration(mol/m³),Dis thediffusioncoefficientofelectroactivespeciesintheelectrolyte (m²/s),andtisthetimesinceonsetofthepotentialstep.Fig.4

Fig.4.FittingoftheCottrellequationtotheearlystagesoftheexperimentalchro- noamperometrici–tdataforthe0.001Py+0.001MMn²⁺(0.1MH2SO4)electrolyte.

Platinumsubstrate;steppotential=0.75VvsSCE.Inset:i–t^−1/2forthesamedata set.

demonstratesthefittingoftheCottrellequationtotheearlystages ofthechronoamperometricdeposition.

TheconcentrationofPyintheinitialsolutiondeterminesthe concentrationoftheintermediatePy^+• intheimmediatevicinity oftheelectrodesurface (noteatthis steppotentialMn²⁺isnot electroactive),whichwithtimeandcontinuedoxidation,increases toacriticalsaturationlevel,atwhichpointdepositionofthesolid productsbegins;i.e.,thePy^+• radicalinitiatespolymerizationto formpolypyrrole(PPy).Themechanismofthispolymerizationis asfollows:

Py→ Py^+• +e⁻ Initiation (5)

Py^+• +Py^+• → Py⁺ Py⁺→ Py Py+2H⁺ (6)

Py Py→ Py Py^+• +e⁻ Initiation (7)

Py Py^+• +Py Py^+• → Py Py Py Py+2H⁺ (8) n(Py) →(Py)_n+nH⁺+ne⁻ Overall (9) ThepresenceofMn²⁺,asindicatedinthelinearsweepvoltam- metry data, serves tomodify PPy depositionprobably through adsorption.Theelectrodeareabeginstoincrease,thusgivingriseto anincreasedcurrenteventhoughmasstransporttotheelectrode surfaceisconstant(diffusionlimitedcurrent).Bycomparingthe actualcurrentmeasured(iexp)intheexperimenttothatpredicted fromtheCottrellequation(i_pred), anestimateoftheincreasein electrodeareacanbemade;i.e.,iexp/ipred,asshowninFig.5.This comparisonis validbecauseallotherparametersintheCottrell equationremainconstant,exceptthearea.

FromFig.5itisquiteobviousthatadramaticincreaseinelec- trodeareahasoccurredabove and beyondtheinitialplatinum electrodearea(determinedbylinearsweepvoltammetry[28]on thewell-behaved andreversible[Fe(CN)₆]^3−/4−redox couplein KNO₃ electrolytes).Furthermore,asexpected,thisareaincreases withanincreasingconcentrationofelectroactivespecies;e.g.,for themostconcentratedelectrolytestudied,theelectrodeareawas foundtoincreasebyupto30timesoverthecourseofthe30sdepo- sition.Theoriginofthisincreasedelectrochemicallyactiveareais

Fig.5.Relativeareachangeforeachforeachofthethinelectrodefilmspreparedin thisstudy.

acombinationofthehighnumberofnucleationsitesforthecom- posite,aswellasporosityintheelectrodepositedfilm.Irrespective oftheoriginoftheincreasedelectrodearea,thisisa promising signintermsoftheelectrochemicalactivityofthecompositeelec- trodesbecauseoftheinherentincreaseinspecificcapacitywith electrochemicallyactivearea.

Since all theelectrodeposition experiments were conducted usingtheEQCM,themassofmaterialdepositedontotheplatinum substrateisimmediatelyavailable.Fig.6(a)showsasamplemas- sogram,ascomparedtoitschronoamperometricdata,inthiscase for0.01MPy+0.01MMn²⁺+0.1MH2SO4.Themassograminthis figureshowsacontinualincreaseinmass,withlittleinductiontime evident,indicatingthatpolymerizationanddepositionwereessen- tiallyinstantaneousaftertheonsetofthesteppotential.Table1 liststhefinalelectrodemassaftera30sdepositionasafunction oftheelectrodepositionconditions.Asexpectedfromthechrono- amperometricdata,higherelectrodemasseswereobservedwhen theelectrolytewasmoreconcentratedinelectroactivespecies(as indicatedbythetotal chargepassed duringthechronoampero- metricdeposition).Table1alsocontainsthecorrespondingdata forelectrodepositionexperimentscarriedoutwithjustPyasthe electroactivespecies(noMn²⁺present).Acrossallexperimentsthe electrodemasswaslesswhenMn²⁺wasincludedintheelectrolyte.

TheimplicationofthisisthatthepresenceofMn²⁺affectstheelec- trodepositionprocess,aswasnotedpreviouslyinthelinearsweep voltammetrydata.Anexplanationforthisisthattherelativearea ofthePPyonlyelectrodesis greaterthanthosepreparedinthe presenceof Mn²⁺,withthegreater areafacilitating thedeposi- tionofmoreactivematerial.Table1alsocontainsthepredicted areaincreaseforalltheseelectrodes,andapartfromthelowest concentrationelectrolyte,therelativeincreaseinareawassimilar irrespectiveofwhetherMn²⁺waspresentornot.Thisinfersthatthe Mn²⁺influencesthedepositionmechanismofPPydirectly,rather thanthroughmorphologicalchanges.

Fig.6(b)combinesthemassogramdatawiththechargecalcu- latedbyintegrationofthechronoamperometrici–tdata.Ascanbe seen,thiscomparisonisessentiallylinearforeachofthedifferent electrolytesconsidered.Thisdataalsoallowsdeterminationofthe rateofmass(m;␮g)accumulationasafunctionofcharge(Q;C)

Fig.6.Chronoamperometricandmassogramdatacollectedduringelectrodeposi- tionfromthe0.01MPy+0.01MMn²⁺(0.1MH2SO4)electrolyte.Platinumsubstrate;

steppotential=0.75VvsSCE.

passed(i.e.,dm/dQ),whichcanprovideinsightintothenatureof thedepositedmaterial.Table1liststhevaluesfordm/dQforthevar- iousdepositsconsideredhere.Inallexceptoneinstance,thevalue ofdm/dQwasconsistentat∼320␮g/C,withaslighttrendtowards ahighervalueforthemorediluteelectrolytes.Theexceptionwas thecasefor0.001MPy+0.001MMn²⁺,inwhichcasedm/dQwas 943␮g/C,almosttriplethat obtainedfromtheotherelectrodes.

Thisresult,while surprising,wasconsistent, withthreesimilar experimentsrepeatedtogivethesameresult.Unfortunately,the originofthismuchhighervalueisunknownatthistime.

Themagnitudeofthedm/dQdatacanalsobeusedtoexamine thenatureofthedepositedproduct.Forinstance,electrodeposition ofPPyviaEqs.(2)and(3)isexpectedtoproduceadm/dQvalueof 674␮g/C,whichismuchlargerthanthatobtainedforthemajority ofexperiments.Fortheelectrodepositionof manganesedioxide we expect dm/dQ to be 451␮g/C, which could contribute to

Table1

PropertiesofthePPyandMn-modifiedPPyfilms.Chronoamperometricsteppotential=0.75VvsSCE,steptime=30s,and0.1MH2SO4asthesupportingelectrolyte.

Electrolyte EQCMmass(␮g/cm²) Charge(C/cm²) Relativeareaincrease [Mn]^a(␮g/cm²) dm/dQ(␮g/C) Capacitance^b(F/g) [Py](M) [Mn²⁺](M)

0.001 0 1.1±0.6 2.1×10⁻³ 8.8 – 383 892

0.001 0.001 0.7±0.1 8.0×10⁻⁴ 3.9 0.17±0.05 943 2213

0.01 0 12±2 0.036 19.6 – 302 72

0.01 0.01 8±1 0.027 19.6 1.7±0.2 290 139

0.1 0 33±4 0.092 32.5 – 334 45

0.1 0.1 26±1 0.079 30.2 14.3±1.9 323 60

aEmployedICP-OES.

bElectrodeswerecycledin0.1MK2SO4between0.0and1.0VvsSCEatarateof5mV/s.

loweringtheoverallvalue;however,aswehavealreadyindicated, manganesedioxideisnot beelectrodepositedherebecausethe potentialwehaveusedistoolow,andassuchanymanganeseinclu- sionisbyadsorption,whichshouldhavetheeffectofincreasingthe valueofdm/dQbecausethemassshouldincreasewithoutanypas- sageofcurrent.Infact,theonlypossiblereasonhowthemeasured valueofdm/dQislessthantheexpectedvalueisifafractionofthe electrochemicallygeneratedspeciesislosttotheelectrolyte,and doesnotcontributetomassbuildupontheelectrode.Thiscould occuras a result of the electro-generated intermediate species Py^+• diffusing away from the electrode surface into the bulk electrolyte,orperhapsmorelikely, thePy^+• speciesundergoing a decomposition reactionwiththe solventwater toreform Py.

Assumingthislastscenariotobethecase,theimplicationisthat only∼33%ofthechargepassedleadstopolymerization.

3.3. Chemicalandmorphologicalcharacterization

3.3.1. Chemicalcomposition

Asacompositeelectrodepositedmaterialitisimportanttoknow theamountofmanganesewithinthefilm.Toextractthisinforma- tioneachcompositefilmwasfirstwashedtoremoveanyresidual electrolyteandthenextractedwithacidifiedH₂O₂todissolveand extractanymanganeseassociatedwiththefilm.Themanganese liberatedintosolution(asMn²⁺)wasthenanalyzedbyICP-OES, withtheresultthenexpressedastotalmanganeseintheelectrode film(␮g/cm²;Table1).Overall,theincorporationofmanganese intothePPyfilmscloselymatchedtheMn²⁺concentrationinthe startingelectrolyte,witha10-foldincreaseinMn²⁺leadingtoan approximate10-foldincreaseinmanganeseinclusioninthefilm.

Theonlyexceptionisthemostconcentratedelectrolyte,inwhich caseaslightlylowerlevelofmanganeseinclusionwasnoted,pos- siblyduetosaturationofthePPysurface.

3.3.2. Transmissionelectronmicroscopy(TEM)

Conventionalscanningelectronmicroscope(SEM)analysisis notpossiblefortheelectrodespreparedinthisstudyduetotheir verythinnature.EvenaTEManalysisisnotstraightforwarddue tothefactthattheactivematerialisstronglyadherenttotheplat- inumsubstrate.Nevertheless,ourapproachtotheTEManalysisof theseelectrodefilmswastodepositanelectrodefromthe0.1M Py+0.1MMn²⁺+0.1MH₂SO₄electrolyteforalongertime(2min), afterwhichtheresultantfilmcouldbeprisedoffthesubstratesur- face(carefully)usingascalpel.Thismaterialwasthendriedinairat ambienttemperaturebeforebeingmountedintotheTEMforanal- ysis.Fig.7showsatypicalimageofthethinfilm.Itisclearthatmost oftheparticleunderexaminationissemi-transparenttotheelec- tronbeamindicatingthatthebulkofthedepositisPPy.However, therearesomemuchsmaller(<50nm)darkerregionsdispersed randomlythroughoutthefilmthatarenottransparent,whichare crystalsofamanganeseoxide.Thenatureofthemanganeseoxide presentisunknown,butislikelytobeahighervalentmanganese

oxidesuchasMn₃O₄orMnO₂,asaresultofpartialoxidationofthe Mn²⁺presentbytheoxygeninair.Theappearanceofthisdeposit impliesthatthecrystalsofmanganeseoxidearemuchdenserthan thatofthePPy.

3.3.3. Atomicforcemicroscopy(AFM)andprofilometry

Asmentionedabove,directSEManalysisoftheelectrodeposited thin filmswasnot possibledue totheirthin nature,andhence thedifficultyindifferentiatingtheirmorphologyfromthatofthe platinumsubstrate.Therefore,AFMwasusedtocharacterizethe morphology of thethin electrode filmsprepared in this study.

Fig.8showstypicalAFMimages(insemi-profileandplanviews) obtainedfromeachofthecompositefilms.Notetheverticalscale shownineachoftheimages.Itshowsquiteclearlythatasthecon- centrationofelectroactivespeciesintheelectrolyteincreased,so toodidtheroughnessandthicknessofthefilm.Themorphologyof eachofthefilmsappearstoconsistofsmallcrystallitesextending fromthesubstrate,withtheoccasionallargercrystalliteprotrud- ingfurtherfromthesurface.Whatisalsoapparent,particularlyfor themostconcentratedelectrolyte,isthatthesizeofthecrystal- liteshasincreased.Theimplicationherebeingthatunderthemost concentratedelectrolyteconditionsused,crystallitenucleationhas beensupercededslightlybycrystallitegrowthduringdeposition.

Therelativesurface areadatainTable1somewhatreflectsthis throughthediminishingincreaseinsurfaceareaastheelectrolyte concentrationisincreased,asshowninFig.9.Theinsetinthisfig- ureshowsthattherelativeelectrodeareaincreasesapproximately logarithmicallywithelectrolyteconcentration.

FurtherdigitalanalysisoftheAFMimagesallowsforthedeter- minationofsurfaceroughness.Table2comparestherootmean

Fig.7. TransmissionelectronmicrographofaMn-modifiedPPycompositeelectrode preparedfromthe0.1MPy+0.1MMn²⁺(0.1MH2SO4)electrolyte,withthedark regionsindicatingcrystallitesofmanganeseoxide.

Fig.8. TypicalAFMimagesofthefilmspreparedinthiswork.Notetheverticalscaleineachoftheimages.

square(RMS)roughnessofthecompositeelectrodefilmsprepared.

Essentiallythisisameasureofthefluctuationinthesurfaceofthe deposit.ThedatainTable2,insupportoftheAFMimagesinFig.8, showsthatthereisgreaterroughnessinthedepositsproducedfrom themoreconcentratedelectrolytes.

Thethicknessofthefilmspreparedinthisstudywasexamined byprofilometry,withtheresultantdataalsoshowninTable2.Note thatthethinnestfilmwasunabletobeexaminedusingthismethod.

Asexpected,thethickestdepositwasproducedfromthemostcon- centratedelectrolyte.Usingthethicknessdatafromprofilometry, andthemassdatafromTable1,thefilmdensitycanbecalculated,

Table2

Roughnessandthicknessdataforthecompositeelectrodematerials.

[Py](M) [Mn²⁺](M) RMSroughness (nm)

Thickness (nm)

Density (g/cm³)

0.001 0.001 5.2±0.5 NA NA

0.01 0.01 7.4±0.5 50±12 2.00

0.1 0.1 8.6±0.5 183±21 1.91

asalsoshowninTable2.TheliteraturesuggeststhedensityofPPy is1.48g/cm³[30],whichimpliesthatthepresenceofmanganesein thefilmshasincreasedtheoverallelectrodedensity,likelyviathe factthatmanganeseoxidesareinherentlydenserthanPPy[31].

ThisconclusionissupportedbytheTEMimageshowninFig.7, wherethemuchdensermanganeseoxidecrystallitesareapparent, asistheabsenceofanyappreciableporosityinthePPyfilm.

3.4. Electrodeperformance 3.4.1. Voltammetry

Theultimatetestofanyenergystoragematerialisitsability tostorechargeundertypicalcellconditions.Assuch,eachofthe electrodematerialspreparedinthisstudywerecycled between thepotentiallimitsof0.0–1.0VvsSCEinanelectrolyteof0.1M K₂SO₄atarateof5mV/sforatleast50cycles.Voltammetricdata obtainedfromtheMn-modifiedPPyelectrodesandthePPy-only electrodesareshowninFig.10,inthiscaseforthe50thcycle.What isapparentfirstlyisthatthereisalargecathodicpeakpresentin thepotentialrange0.13–0.08V,withthepeakshiftingtolower

Fig.9.RelativeMn-modifiedPPyelectrodeareaincreaseasafunctionofthecon- centrationofelectroactivespeciesintheelectrolyte.Inset:Evidenceforthisasa logarithmicrelationship.

potentialswithanincreasingdepositionelectrolyteconcentration ofPyandMn²⁺.Thelikelyoriginofthiscathodicpeakisreduction ofthePPyfilm,withtheinjectedchargebeingrecoveredduringthe anodicpotentialsweep;i.e.,

PPy+ne⁻↔PPyⁿ⁻ (10)

Fortheelectrodepreparedfromthe0.1MPy+0.1MMn²⁺elec- trolyte,thereisalsoasmallcathodicpeakat∼0.59V.Thispeak istypicalofmanganesedioxideelectrochemicalbehaviour[19],

Fig.10.CyclicvoltammetrydatacomparingthebehaviouroftheMn-modifiedPPy (solidline)andPPy-only(dashedline)electrodefilmsforeachconcentrationof Py(+Mn²⁺)usedinthisstudy.Platinumsubstrate;0.1MK2SO4electrolyte;scan rate=5mV/s.

Fig.11.Cyclestabilityofthethinelectrodefilmspreparedinthisstudy.Platinum substrate;0.1MK2SO4electrolyte;scanrate=5mV/s.Inset:Comparisoninspecific capacitancebetweentheelectrodespreparedfrom0.001MPy+0.001MMn²⁺(solid squares)and0.001MPy(opensquares).

withthedifferencebetweenthisandthecorrespondingPPyonly electrodeconfirmingthis.Asidefromthesecathodicpeaks,there arenoothersignificantprocessesoccurring,exceptathighpoten- tialswheretheanodiccurrentbeginstotailupwards.Otherwise,in theintermediatepotentialrange,theelectrodeexhibitsbehaviour typicalofacapacitor;i.e.,arelativelyconstantcurrentover the potentialrangescannedduetoionicadsorptionanddesorption phenomenaoccurringonthePPyfilmsurface.

3.4.2. Cyclestabilityandperformance

ThespecificcapacitanceoftheMn-modifiedPPyelectrodesover thecourseof50cyclesisshowninFig.11.Thefocusofthispart ofthestudyistomoreunderstandtheintrinsicpropertiesofthe deposits,andassuchthatiswhywehaveusedarelativelyslow scanrate.Higherscanratestendtofocusmoreontheelectrodecon- structionitself,whichisnottheintentofthiswork.Furthermore, wehavealsodemonstratedpreviouslyforapowderedmanganese dioxideelectrode,thataslowscanratealsoexacerbatesanyfail- uremechanismswithintheelectrode[32],andsoenablesamuch earlierindicationastowhethertheelectrodewillbestableover extendedcycling.FromthedatainFig.11itisclearthatelectrodes produced fromthemostdilute 0.001MPy+0.001MMn²⁺ elec- trolyteexhibitedanexcellentperformanceofover2000F/gduring thecourseofelectrodecycling.Thespecificcapacitancewasmuch lowerwhentheelectrodesweremadefrommoreconcentrated electrolytes,mostlikelyduetothepreparationofafilmwithrel- ativelylessoftheactivematerialexposedtotheelectrolyte,and hence lessof it availablefor useasanelectrochemically active material.Additionally,thespecificcapacitancefromtheelectrodes remainedessentiallyconstantafterasmallinitialdecrease.Asseen intheinsetinFig.11,thestabilityofboththeMn-modifiedPPy andPPy-onlyelectrodesissimilar,althoughthepresenceofMn²⁺

duringelectrodepositiondramaticallyincreasesthespecificcapac- itanceoftheresultantelectrodematerial.Table1alsocontainsthe averagespecificcapacitanceoverthelast30cyclesasafunctionof theelectrolytefromwhichtheelectrodewasmade.Whatisclear fromthedatainthistableisthatthepresenceofMn²⁺whenthe

Fig.12.(a)ComparisonbetweenthecyclicvoltammetryandEQCMmassogramdata foreachofthethinelectrodefilmspreparedinthiswork.(b)Massvschargedatafor thesameelectrodes.Platinumsubstrate;0.1MK2SO4electrolyte;scanrate=5mV/s;

datafromthe50thcycle.

electrodeisprepared,andultimatelythepresenceofamanganese oxidesuchasMn₃O₄orMnO₂intheresultantelectrode,leadstoa significantenhancementinperformanceoftheelectrode.

3.4.3. EQCManalysis

Themassofeachoftheelectrodeswasalsomonitoredduring thecourseofperformanceevaluation.Fig.12(a)comparesthemea- suredvoltammetricbehaviourwiththeelectrodemassduringthe courseofa cycle,inthiscase the50thcycleforeach electrode.

Notethatthesearetypicalresultsforeachoftheelectrodes.The firstpointtonoteisthatthetotalmasschangetheeachelectrode undergoesisrelativelysmall,beingatmost15,50and75ng/cm² forthe0.001,0.01and0.1Mdepositionelectrolytes,respectively.

Duringthecourseofthedischarge–chargecycleitisapparent thatfor allof theelectrodesthemassbegins toincrease when themainpeakincathodiccurrentisobtained.Thisisinteresting

becausethemassdoesnotincreasewhenthiscathodicpeakbegins, butratherwhenthecathodiccurrentmaximumisachieved.This isalsoreflected bythehysteresisloop apparentin themassvs chargedataforthesameelectrodes,asshowninFig.12(b).The maximumelectrodemassisachievedwhenthereversalpotential (1.0VvsSCE)isreached.Afterthisthemassdecreasesessentially backtoitsstartingpoint,exceptinthecaseoftheelectrodemade from0.001MPy+0.001MMn²⁺,inwhichcaseitsmasswassignif- icantlylowerthanwhereitstarted.Thismaybeevidenceforthe degradationofthecompositeelectrodematerial.Thereisalsoevi- denceinthecaseoftheelectrodemadefromthe0.1MPy+0.1M Mn²⁺electrolyteoftheelectrodemassincreasingabouthalfway throughthedischargehalfcycle(∼0.5VvsSCE).Basedonprevious EQCMstudies[33],thisistheresultofmanganesedioxidewithin thecompositeelectrode.

Thesourceofthesemasschangesarisesfromtheelectrochemi- calprocessesthecomponentsofthecompositeelectrodeundergo.

Whendischarging(reducing)thecathode-activePPy,which isa conductingpolymer,weareessentiallyintroducinganexcessof negativechargeintothepolymerstructure,whichinitselfisan interwovenarrayofindividualpolymerchains.Whilethesechains donotformamonolithicstructure,theydoapparentlymakeavery porousstructure(cf.Fig.5)intowhichtheelectrolyteionsfrom thecyclingelectrolytecanpermeate.Asaresultofthis,underdis- chargetheelectrolytecationsareelectrostaticallyattractedtothe polymersurface,wheretheystorechargeandcontributetoincreas- ingthemassoftheelectrode.Themanganeseoxideformedinthe compositeelectrode,ifindeeditisMnO2,undergoesdischargeby intercalation,ashasbeenreportedinthestudiesonthismaterial inthebatteryliterature[34];i.e.,

MnO2+M⁺+e⁻↔ MnOOM (11)

whereinaqueouselectrolytes,M⁺ismostlyH⁺,withotherionssuch asLi⁺,Na⁺andK⁺alsoplayingarole.Thisprocessisaredoxreac- tion(enablingpseudo-capacitance),inwhichMn(IV)isreducedto Mn(III)whenM⁺andtheelectronareinsertedintothemanganese dioxidestructure.Ofcoursewiththeintercalationofforeignmetal ionsintothemanganesedioxidestructure,theelectrodemasswill alsoincrease.

Fig.12(b)showstherateofchangeofmass(m)withrespectto charge(Q)data(dm/dQ)forselectedregionsofthevoltammetric data.Fortheelectrodespreparedfromthe0.1and0.01Melec- trolytesthereisverylittlemasschangeduringtheinitialstagesof discharge(dm/dQ≈0)suggestingtheelectrodeisstoringcharge withinthestructurewithouttheneedforelectrolyteionassocia- tion.Atlowerpotentialsthereisclearlyamassincreaseatarate of592␮g/C(0.01M)and261␮g/C(0.1M).Intheelectrolytebeing usedforcycling(0.1MK₂SO₄)theadditionofnegativechargeto thePPy shouldlead toK⁺ ions electrostaticallyadsorbing onto thePPysurfaceatatheoreticalrateof405␮g/C.Fortheelectrode madefrom0.01MPy+0.01MMn²⁺thehigherexperimentalvalue indicatesthatotherspeciesarealsoassociatedwiththeadsorption.

ThehydrationsheatharoundtheK⁺ionsisalikelysourceofadded mass.The much lower dm/dQ valueobtained forthe electrode made from 0.1M Py+0.1M Mn²⁺ indicates that not all of the compositeelectrodepositedmaterialisbeingutilizedduringthe cyclingprocess.Theelectrodepreparedfrom0.001MPy+0.001M Mn²⁺exhibited behaviourthat wasquitedifferentcomparedto thosemadefromtheotherelectrolytes.Inparticular,itshowed considerable hysteresis with an electrode mass increase only becomingapparentafterthereversalpotentialhadbeenreached.

Itsvaluefordm/dQwas229␮g/C,althoughcautionisnecessary interpreting this value given the differences in behaviour. As mentioned, all electrodesshowed considerablehysteresis upon charge,withthemasschangeassociatedwiththechargebeing extractedfromtheelectrodealwaysbeinglessthanthedischarge;

Fig.13. Chargeefficiency,expressedastheratiobetweentheelectrodemasschange andthestartingmass,foreachofthethinfilmelectrodespreparedinthiswork.

i.e.,155␮g/C(0.1M),226␮g/C(0.01M)and71␮g/C(0.001M).This mayresultfromthemorphology(porosity)ofthedepositinhibit- ingtheremoval ofelectrolyte ions,oralternativelythat charge extractionfromthedischargedelectrodeisintrinsicallymoredif- ficultthanchargeinsertionintotheunchargedelectrode.Despite thehysteresis,theinitialelectrodemasswasessentiallyrecovered.

Themasschangesthatthesecompositeelectrodesundergocan beusedtoexaminethechargeefficiency;i.e.,theamountofcharge accommodatedin theelectrode asa functionof theamountof material present. Fig.13 takes themass changes the electrode undergoesduringcycling(Fig.12(a))andnormalizesitbasedonthe amountofactivematerialpresentintheelectrode.Hereitbecomes clearthattheelectrodemadefrom0.001MPy+0.001MMn²⁺has thegreatestutilizationofitsactivematerial.Thisismostlikelydue tothemorphologyoftheelectrode,inwhichcasetherelativelylow amountofmaterialpresent,formedduringtheearlystagesofelec- trodeposition,presentsthegreatestsurfaceareatotheelectrolyte, andhenceresultsinthegreatestmassuptakebytheelectrode.Such aheavyutilizationmayalsobethecauseofthelossinmassduring cyclingdemonstratedbythiselectrodematerial.

3.4.4. EISanalysis

EIS hasbeenappliedto thethin filmelectrodes toexamine theirpropertiesandperformance.Inthiscaseasequenceof25mV potentialsteps (withan intermediate resttime of 10min)was appliedtoeachelectrode tocyclefromtheopencircuitpoten- tial(0.30V)totheupperanodiclimit(1.00V),thentothelower cathodiclimit(0.00V),andthenbackuptotheupperlimit.Thisvery slowcyclingrate(equivalentto0.042mV/s)enablesthecharacter- izationofthequasi-equilibriumnatureofeachelectrodewithin thepotentialwindowbeingused.Thisapproachalsoexacerbates anyinstabilitytheelectrodemightexperienceduringcyclingthat wouldotherwiseonlybecomeapparentafteranextendedcyclelife [32].

Fig.14(a)showsanexampleoftheEISdatacollected,inthis caseforthethinfilmelectrodepreparedfrom0.001MPy+0.001M Mn²⁺in0.1MH₂SO₄.EachoftheEISdatasetsshowsbehaviourthat istypicalofaboundedporouselectrode[35].Athighfrequencies

Fig.14.(a)SampleoftheEISdatacollected,inthiscaseforthethinfilmelec- trodepreparedfromthe0.001MPy+0.001MMn²⁺in0.1MH2SO4electrolyte.

Inset:ExpandedviewofthehighfrequencyEISdata.(b)Equivalentcircuitused formodellingtheexperimentaldata.

thereisalinearregionintheEISdata(insetinFig.14(a)),which is the resultof transport of the ionicelectrolyte into the elec- trodepores,withadecreasingfrequencyleadingtogreateraccess of thepores by the electrolyte. As the frequency is decreased even furtherthe electrode behaviourchanges to indicatemore conventional charge transfer and double layerprocesses at the porouselectrode–electrolyteinterface.Theequivalentcircuitused tomodelthisEISdataisshowninFig.14(b).Theseriesresistance (Rs)takesintoaccountthecombinedresistanceofthethinfilm electrodeand theelectrolyteinthecellgeometrybeingused.A constantphaseelement(CPE(D))wasthenusedtocharacterize masstransport(diffusion)oftheelectrolyteionsintotheporous electrode.TheimpedanceofaCPE(Z_CPE)isgivenby

ZCPE= 1

T(jω)^P (12)

whereTisacompositevaluecontainingtermsrepresentingpri- marilythediffusionofelectro-activespeciesintothepores,jisthe imaginarynumber,ωistheangularfrequency,andPisaparame- terindicatingthedivergenceoftheinterfacefromidealbehaviour.

Pfallsintherange0–1,butidealbehaviourinthiscaseiswhen P=0.5,whichisforalinear,non-tortuouspore.Atlowerfrequen- ciesthesemicirculararc is typicalofinterfacial chargetransfer anddoublelayerbehaviouratanelectrifiedinterface.Assuchwe haveaccountedforthisportionoftheEISdatausingamodified Randlescircuit[36]thatconsistsoftheparallelarrangementof a chargetransfer resistance(Rct)anda constant phaseelement (CPE(DL))torepresentthedoublelayercapacitanceofaroughand porouselectrode–electrolyteinterface.Thechargetransferresis- tance essentially representsthekinetics of anycharge transfer processesoccurringattheelectrode–electrolyteinterface;i.e.,Eq.

(10),whilethedoublelayercapacitancerepresentstheamountof chargestoredatthisinterface.Theyareinaparallelarrangement becausetheyarecompetingprocesses.Theconstantphaseelement

Table3

Equivalentcircuitparameters.

Electrolyte Potential Rs CPE(masstransport) Rct CPE(capacitance) Cdl

[Py](M) [Mn²⁺](M) (VvsSCE) () T(×10³) P () T(×10⁴) P (␮F)

0.001 0

0.30 18.95 8.21 0.396 148,400 1.99 0.882 313

1.00(A) 19.01 10.65 0.378 31,273 2.01 0.880 258

0.00(C) 18.80 2.81 0.510 8112 1.94 0.932 200

1.00(A) 18.68 6.58 0.437 23,042 2.03 0.905 238

0.001 0.001

0.30 16.32 16.49 0.341 11,331 3.11 0.942 336

1.00(A) 16.16 13.98 0.326 8628 1.86 0.966 189

0.00(C) 15.77 17.73 0.309 5985 2.11 0.937 214

1.00(A) 15.55 14.15 0.319 10,573 1.80 0.967 184

0.01 0

0.30 1.00(A)

0.00(C) 17.27 1.04 0.607 7311 2.20 0.978 218

1.00(A)

0.01 0.01

0.30 6.82 11.91 0.146 63,428 2.82 0.836 496

1.00(A) 18.42 21.91 0.325 83,742 1.93 0.842 325

0.00(C) 16.97 2.28 0.524 7493 2.02 0.944 207

1.00(A) 17.51 5.33 0.489 21,145 2.07 0.897 246

0.1 0

0.30 18.28 23.48 0.291 1114 11.08 0.703 1212

1.00(A)

0.00(C) 20.07 1.05 0.592 7614 2.21 0.984 220

1.00(A) 22.72 1.39 0.606 18,225 2.39 0.943 261

0.1 0.1

0.30 17.89 2.54 0.520 1330 13.73 0.693 1792

1.00(A) 21.63 5.89 0.416 22,855 1.67 0.861 208

0.00(C) 18.59 1.33 0.580 6132 1.92 0.950 194

1.00(A) 20.32 3.65 0.507 20,981 2.03 0.896 240

usedhereisalsodefinedbyEq.(12)above;however,inthiscase Trepresentsthecapacitanceoftheinterface,whilePagainrep- resentsthedivergencefromaflatsurface.Foranidealcapacitor P=1,whichmeansthatT=C.Typicallyforroughsurfaces,Phasa valuegreaterthat∼0.8.Thedoublelayercapacitance(C_dl)canbe calculatedfromtheCPEusingtheexpression[28]:

Cdl=T(ωmax)^P−1 (13)

where ωmax is the angular frequency when the imaginary impedance(−Z^)isamaximum.Todeterminetheoptimumfitting parameters for theequivalent circuitto match theexperimen- taldata,complexnon-linearleast squaresregression(CNLS), as definedbyBoukamp,wasused[37,38].Acomparisonofthesefit- tingparametersforeachoftheelectrodespreparedinthisstudyis showninTable3.

Thefirstcommenttobemadeabouttheequivalentcircuitfit- tingparametersishowtheyreflectonthereversibilityofthethin filmelectrodesasa resultof thepotentialstepelectrochemical cyclingusedhere.Thevalueofeachofthereportedparameters,but particularlyC_dl,indicatesthattheas-preparedelectrodeexhibits thebestperformance,butaftercyclingtheperformancedegrades toanapproximatelysteadystatevaluethatisdependentonthe potential.Certainlythisobservationisconsistentwiththeextended cyclingshowninFig.11,wherethespecificcapacitancefadeswith cyclingtoanapproximatelyconstantvalue.

Theseriesresistance(Rs)incorporatedinthemodelcompen- satesfortheresistanceofboththeelectrolyteandelectrode.Now given the relatively large volume of electrolyte used in these experiments,changes intheelectrolyte resistanceareexpected tobenegligible,andsoanychangesobservedinRs likelyreflect thebehaviourofthethinpolymerfilm.Asanaside,itisdifficultto compareR_sfordifferentelectrodesbecauseitsvalueisdependent onthegeometryoftheelectrochemicalcellusedandourability to reproduciblyplace the working and reference electrodes in exactlythesameplaceforeveryexperiment.Whilethevaluesof Rsweresimilaracrosstheexperimentsconsidered,thedifferences weremostlikelyduetoirreproducibilityintheplacementofthe

electrodes rather than anychanges in the electrode. However, for anindividual electrode, changes in Rs represent changes in theresistance of theelectrode, since, asmentioned above,the electrolytecontributionforanindividualelectrodeshouldremain thesame.Therefore,foreachelectrode,whatwasnotedwasthe relativelylowvalueofRsfortheinitiallypreparedelectrodeatthe opencircuitpotential(0.30VvsSCE).Uponoxidationto1.00VR_s increased,butuponreductionthevalueofRsdecreased,therelative magnitudeofwhichwasfoundtodependontheamountofpoly- merpresent;i.e.,therelativechangesinRswerethesmallestforthe thinelectrodespreparedfromthediluteelectrolytes,comparedto thethickerelectrodespreparedfromthemoreconcentratedelec- trolytes.Certainlythiswastobeexpectedgiventhedependence ofRs()ontheresistivity(cm)andthicknessofthefilm.

Thechargetransferresistancedataextractedfromthefitting providesan indicationof the kineticsof electron injectionand extractionintoandoutofthePPystructure.Foralloftheelectrodes studies,Rct wasonaveragequitelargeindicating the difficulty associated withPPy solid stateredoxprocesses. Itis important torecognize thatR_ct isalsoanareaspecificterm,which means thatitisdependentonboththeintrinsicchargetransferresistance (i.e.,cm),aswellastheareaover whichthat chargetransfer takesplace.ForeachoftheindividualelectrodesthetrendinRct

withappliedpotentialwassimilartothat ofRs,indicating that itisintrinsicallyeasiertoinsertchargeintothePPystructureat lowerpotentials,orthattheelectrodemorphologyischangingso astolowertheelectrode–electrolyteinterfacialarea.Themagni- tudeofthesechangesinRctwithappliedpotentialwouldindicate thattheseeffectsarequitedramatic.

PerhapswhatisofmostimportanceindiscussingthesePPyfilms arethechangesindoublelayercapacitance(C_dl),whichisagain anareaspecificterm.Foranindividualelectrodethechangesin C_dl arecloselyrelatedtoR_ct,whichwasexpectedsincetheyare competingparallelprocessesforchargestoragewithintheelec- trode.Forinstance,ahighvalueforRctindicatesthatlittlecharge isstoredviafaradaicprocesses,withthemajorityinsteadbeing storedinthedoublelayer.For alloftheelectrodesthevalueof