Study of polyaniline doped with trifluoromethane sulfonic acid in gas-diffusion electrodes for proto

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JournalofPowerSources155(2006)

138–144

Studyofpolyanilinedopedwithtrifluoromethanesulfonicacidin

gas-diffusionelectrodesforproton-exchangemembranefuelcells

HusseinGharibia,∗,MohammadZhiania,AliAkbarEntezamib,

RasolAbdullahMirzaiec,MehdiKheirmanda,KarimKakaeia

aDepartmentofPhysicalChemistry,FacultyofScience,TarbiatModarresUniversity,P.O.Box14115-175,Tehran,IranbFacultyofChemistry,TabrizUniversity,Tabriz,IrancDepartmentofChemistry,FacultyofScience,ShahidRajaeeUniversity,Tehran,Iran

Received6February2005;accepted9May2005Availableonline28June2005

Abstract

Polytetrafluoroethylene(PTFE)-bondedgas-diffusionelectrodes(GDEs),modifiedwithpolyanilineasanelectronandprotonconductorinthecatalystlayer,arepreparedandevaluatedforuseinproton-exchangemembranefuelcells(PEMFCs).PolyanilineiscoatedontheGDEbyelectropolymerizationofanilineandtrifluoromethanesulfonicacidastheproton-conductivemonomer.Theelectrodesarecharacterizedbycyclicvoltammetry,current–potentialmeasurements,electrochemicalimpedancespectroscopy,andchronoamperometry.Thepolyanilineisfoundtobehomogenouslydispersedinthecatalystlayer,makingitagoodcandidateprotonandelectronconductor.UseofpolyanilineinsteadofNafioninthecatalystlayer,increasestheutilityoftheelectrocatalystby18%.TheresultsareconsistentwiththepresenceofpolyanilineasaconductivepolymerinthereactionlayerreducingthepolarizationresistanceoftheelectrodeincomparisonwiththatofacorrespondingelectrodecontainingNafion.Thus,thepresentresultsindicatethatPEMFCsusingpolyaniline-containingelectrocatalystsshouldgivesuperiorperformancetothoseusingcatalystscontainingtraditionalionomers.©2005ElsevierB.V.Allrightsreserved.

Keywords:Polyaniline(PANI);Nanofibular;Modifiedelectrode;Gas-diffusionelectrode;Proton-exchangemembranefuelcell;Nanostructure;Oxygenreductionreaction

1.Introduction

Platinumisthemostimportantcatalystusedingas-

diffusionelectrodes(GDEs)[1,2]forproton-exchangemem-

branefuelcells(PEMFCs).Thediscoveryinthe1980s

thatintroducinganionomer(liquidNafion)intoGDEs[3]

extendsthethree-dimensionalreactionzoneoftheelec-

trodeandleadstoasignificantloweringoftheplatinum

loadings,i.e.,fromabout4–10mgcm−2(platinumblack)to

about0.4mgcm−2orevenless(carbon-supportedplatinum).

Nafionmaybeappliedbyimpregnation,brushing,spraying,

orfloating/dippingtheelectrodeinasolutionoftheionomer.

∗Correspondingauthorat:DepartmentofChemistry,TarbiatModarresUniversity,P.O.Box14115-4838,Tehran14155,Iran.Tel.:+98216462548;fax:+98216462254.E-mailaddress:gharibi@irandoc.ac.ir(H.Gharibi).Severalstudieshaveexaminedtheinfluenceoftheionomer

contentinthecatalystlayeronGDEperformance.Inareview

ofdatafromvariousstudiesontheoptimumNafioncontent

ofthecatalystlayer,Antolini[4]concludedthatintroduction

oftheionomerintothecatalystlayerbyimpregnationgavea

loweroptimumNafioncontentthanintroducingtheionomer

byplacingitinthecatalystink.

Matsubayashietal.[5]reportedthatthevariationinelec-

trodeperformancewiththeamountofNafiondependsonthe

methodusedtointroducetheelectrolyte.Hsuetal.[6]stud-

iedtheeffectofNafionimpregnationofadualelectrodeby

cyclicvoltammetryanda.c.impedancemeasurements.The

maximumelectroactivesurfacearea(ESA)andtheminimum

ohmicresistanceofthecelloccurredat0.67mgNafionper

cm2.

Inallofthemethodsusedtodate,theNafionsolutionis

introducedintothecatalystlayerasaphysicalorchemical

0378-7753/$–seefrontmatter©2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.jpowsour.2005.05.016H.Gharibietal./JournalofPowerSources155(2006)138–144139

mixture[7–9].Theuseofthisapproachhasthedisadvantages

thatitisdifficulttocontroltheamountofNafionapplied

andimpossibletoachieveahomogeneousdistributionofthe

ionomerinthereactivelayer.Asaresult,themassactivity

isdecreasedandhencethecostoffabricationisincreased.

TheidealcatalystlayerforaGDEwouldbegas-andwater-

permeable,andwouldconductbothprotonsandelectrons.

Electron–protonconductivepolymers,especiallypolyani-

line(PANI)dopedwithtrifluoromethanesulfonicacidasa

proton-conductingmonomer,aregoodcandidatesforresolv-

ingtheproblemsoutlinedabove.PANIanditsderivatives

havebeenconsideredpromisingmaterialsforbatteriesand

thecatalysisofelectrochemicalreactions[10–27].

Severalelectrocatalyticreactionshavebeeninvestigated

insystemscomprisedofnoblemetalsdispersedinelectron-

conductingpolymers.Thesereactionsincludethereduction

ofprotons[10,11]oroxygen[12–16],andtheoxidationof

hydrogen[17,18]orsmallorganicmoleculessuchasformic

acid[19,20],methanol[21–26],andethyleneglycol[27].In

addition,studieshavebeencarriedoutontheeffectofvary-

ingthecatalystcomposition(PANIorNafionwithPt)on

theoxygenreductionreaction(ORR)[28]andonthemodi-

ficationofGDEs[29].Inmostcases,thereactionrate(i.e.,

thecurrentdensity)wasincreasedandtheovervoltagewas

greatlyreducedbyintroducingPANIintotheelectrodes.