QuaternizedpolyethersulfoneCardoanionexcha

QuaternizedpolyethersulfoneCardoanionexchangemembranesfordirectmethanolalkalinefuelcellsLeiLia,b,∗,YuxinWanga,∗aChemicalEngineeringResearchCenter,SchoolofChemicalEngineeringandTechnology,TianjinUniversity,Tianjin300072,ChinabKarl-Winnacker-InstitutderDECHEMAe.V.,Theodor-Heuss-Allee25,D-60486FrankfurtamMain,GermanyAbstractAnovelanionexchangemembrane,quaternizedpolyethersulfoneCardo,wasprepared.PolyethersulfoneCardowaschloromethylatedwiththecomplexsolutionofchloromethyletherandchloridezinc.Subsequentreactionwithtrimethylamineandionexchangewithsodiumhydroxideyieldsalkalineanionexchangemembrane.ThequaternizedpolyethersulfoneCardo(QPES-C)membranemaybesuitableforuseinlowtemperaturedirectmethanolalkalinefuelcells.Ionicconductivityandmethanolpermeabilityofsuchmembraneasafunctionoftemperaturewereinvestigated.IonicconductivityofQPES-Cmembranein1MNaOHsolutionwas4.1×10−2S/cmatroomtemperatureand9.2×10−2S/cmat70◦C.MethanolpermeabilityofQPES-Cmembranewasfrom5.72×10−8to1.23×10−7cm2/soverthetemperaturerange25–70◦C.©2005ElsevierB.V.Allrightsreserved.Keywords:PolyethersulfoneCardo;Anionexchangemembrane;Directmethanolfuelcell;Methanolpermeability1.IntroductionDirectmethanolfuelcell(DMFC)hasattractedconsider-ableattentionasanalternativetothepresentpowersources,sinceithasnumerousbenefits,includinghighefficiency,highpowerdensity,loworzeroemissionsandeasyfuelcarriage.DMFCisexpectedtohaveawideapplicationasaportableormobilepower,exemplifiedbybatteryforcellularphonesorengineforelectricvehicles.Nafion®membraneappearsthepopularmembraneelectrolyteforDMFCapplications.However,atpresent,twomajortechnicalproblemsinDMFCstillrestrictitsperformanceandapplication.Oneproblemisthecrossoverofmethanolthroughtheelectrolytemembrane.Theotherisslowmethanoloxidationkineticsontheanodecatalyst.Ithasbeenknownforlongthattheanodicoxidationofmethanolinalkalinemediaismorefeasiblethaninacidic∗Correspondingauthors.E-mailaddresses:[e-mail](L.Li),[e-mail](Y.Wang).media[1–3].Then,whilemostresearchersaretryingeverypossiblewaytoimprovetheperformanceofDMFC,increas-ingnumberofresearchersareturningtheirinteresttodirectmethanolalkalinefuelcell(DMAFC)[2–8].ComparedtoDMFCinacidicmedia,DMAFChasanumberofpoten-tialadvantages[4–9].First,itiswellknownthatmethanoloxidationinalkalinemediaiskineticallyfasterthanthatinacidicmedia.Second,themethanoloxidationcatalystsarelesssensitiveinalkalinemediathaninacidicmedia,whichcouldleadtotheuseoflessexpensivenon-preciousmetalcatalysts,suchasNiorAg.Inaddition,thechargecarrierinDMAFCisanionionanditmovesfromthecathodetoanodeduringfuelcelloperation,oppositetothemovementofprotoninacidicmembrane.Assuch,thedirectionoftheelectro-osmoticdragisreversed,reducingthemethanolper-meationrate.AnionexchangemembraneisrequiredasafirststepforDMAFCapplications.Thereare,however,fewreportsaboutsynthesizinganionexchangemembranesforDMAFCapplications.So,inthiscommunication,anewkindofanionexchangemembranebasedonpolyethersulfoneCardo(PES-C)polymerwaspreparedandionicconductivityandmethanolpermeabilitywerestudied.2.Experimental2.1.MembranepreparationThepreparationofquaternizedpolyethersulfoneCardomembranewasinthethreesteps(seeScheme1).First,20gpolyethersulfoneCardopolymer(PES-C,averagemolec-ularweight117,000)wasdissolvedin100mlof1,2-dichloroethane(DCE)atroomtemperature.ThesolutionofPES-Cwasheatedto60◦Cwithrefluxcondensationunderstirring.Thenthecomplexsolutionofchloromethyletherandzincchloride(ZnCl)wasprepared,which1.5gZnClweredissolvedinto20gchloromethylether.ThetotalamountofthepreparedcomplexsolutionwasaddedintothePES-Csolution,andthereactionwasprocessedfor6hat60◦Candcooledtoroomtemperature.Then,thepolymersolutionwasgraduallyprecipitatedintohotwaterundermechanicalagitation.Chloromethylatedpolymerwasprecipitatedfromsolution,andfiltered,washedseveraltimeswithdistilledwateranddriedundervacuumat60◦Cfor24h.Then,thechloromethylatedPES-Cwasdissolvedindimethyldormamidetomakea10wt%solution,whichwasthencastontoaflatglass.Thecastmembranewasdriedat60◦Cfor6h.Aftercoolingtoroomtemperature,theresultantmembranewaspeeledfromtheglassindistilledwater.Thismembranewasimmersedinto30wt%trimethylaminesolu-tionfor48htoinductquaternarygroupsintothemembrane.Thenthemembranewasputinto1MNaOHsolutionfor24h.Atlast,thequaternizedPES-C(QPES-C)membranewaswashedseveraltimeswithdistilledwater,andkeptindistilledwaterbeforetesting.Ion-exchangecapacity(IEC)ofQPES-CmembranewasdeterminedbythebacktitrationmethodfollowingRef.[10].Fig.1.Schematicdiagramofthefour-electrodeglasscellusedforionicconductivityandanionictransportnumbermeasurements.2.2.IonicconductivityIonicconductivityofmembraneswasmeasuredusingthepotentiometricmethod[9].Theexperimentcell(Fig.1)wascomposedoftwoidenticalcompartments.EachonehadtwoopeningstouseoneplatinumelectrodeandoneAg–AgClreferenceelectrode.AlltheelectrodeswereconnectedtoanEG&GPrincetonAppliedResearchModel273Poten-tiostat/Galvanostat.Ionicconductivityofmembraneswasdeterminedbythepotentialdifferencebetweenthereferenceelectrodes;thefirstwithoutmembrane,andthesecondwiththeme