NON-FLUORINATED POLYMERMATERIALS

2021-08-28 来源：步旅网

Annu.Rev.Mater.Res.2003.33:503–55doi:10.1146/annurev.matsci.33.022702.154657c2003byAnnualReviews.AllrightsreservedCopyright󰀁

NON-FLUORINATEDPOLYMERMATERIALSFOR

PROTONEXCHANGEMEMBRANEFUELCELLS

JacquesRozi`ereandDeborahJ.Jones

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.LaboratoiredesAgr´egatsMol´eculairesetMat´eriauxInorganiques,UMRCNRS5072,Universit´eMontpellierII,34095Montpelliercedex5,France;email:debtoja@univ-montp2.fr

KeyWordspolyheterocyclic,polyaromatic,sulfonatedmembranes,conductivity,swelling

sAbstractThepast10yearshavewitnessedatremendousaccelerationinre-searchdevotedtonon-ﬂuorinatedpolymermembranes,bothascompetitivealternativestocommercialperﬂuorosulfonicacidmembranesoperatinginthesametemperaturerangeandwiththeobjectiveofextendingtherangeofoperationofpolymerfuelcellstowardthosemoregenerallyoccupiedbyphosphoricacidfuelcells.Importantre-quirementsareadequatemembranemechanicalstrengthatlevelsoffunctionalization(generallysulfonation)andhydrationallowinghighprotonconductivity,andstabilityintheaggressiveenvironmentofaworkingfuelcell,inparticularthermohydrolyticandchemicalstability.Thisreviewprovidesanoverviewofprogressmadeinthedevelop-mentofproton-conductinghydrocarbonandheterocyclic-basedpolymersforprotonexchangeanddirectmethanolfuelcellsanddescribesthevariousapproachesmadetopolymermodiﬁcation/synthesisandsalientpropertiesofthematerialsformed,includ-ingthoserelatingtoprotontransportandprotonconductivity,e.g.,waterdiffusionandelectro-osmoticdrag.Themicrostructure,deducedfromsmallangleX-rayandneu-trondiffractionmeasurementsofrepresentativenon-ﬂuorinatedpolymersiscomparedwiththatofperﬂuorosulfonicacidmembranes.Differentdegradationmechanismsandagingprocessesthatcanresultinchemicalandmorphologicalalterationareconsid-ered,andrecentcharacterizationofmembrane-electrodeassemblies(MEAs)indirectmethanolandhydrogen-air(oxygen)fuelcellscompletesthisreviewofthestateoftheart.Whileseveraltypesofnon-ﬂuorinatedpolymermembranehavedemonstratedlifetimesof500–4000h,onlyalimitednumberofsystemsexistthatholdpromiseforlong-termoperationabove100◦C.1

Listofabbreviationsandacronyms:PEM,protonexchangemembrane;DMFC,di-rectmethanolfuelcell;MEA,membraneelectrodeassembly;PSU,Polysulfone;PES,Poly(ethersulfone);PEK,Poly(etherketone);PEEK,Poly(etheretherketone);PPQ,Poly(phenylquinoxaline);PBI,Polybenzimidazole;P3O,Poly(2,6-diphenyl-4-phenyleneoxide);PPS,Poly(phenylenesulﬁde);RH,Relativehumidity;PPZ,Poly(phosphazene);PPBP,Poly(4-phenoxybenzoyl-1,4-phenylene);PI,Polyimide;IEC,Ionexchangecapac-ity;SAXS,smallangleX-rayscattering;SANS,smallangleneutronscattering.

0084-6600/03/0801-0503$14.00503

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INTRODUCTION

Solidpolymerelectrolytemembrane(PEM)fuelcellshavereachedatechno-logicalstagewheretheyofferarealchallengetoconventionalpower-generating

technologies,inparticularfortransportation(1).Theirefﬁciencyishigher,andtheyofferthepossibilityofzeroemissionatthepointofuse.Fuelcellefﬁciencyisnotasinglenumber,however,itisafunctionofthepowerdensityatwhichthefuelcelloperates,andthustheoptimumnominalefﬁciencydependsonthefuelcellperformanceanditscapitalcost(2).Theelectrolyteisimplicatedinbothoftheseaspects.ThefunctionofthemembraneinPEMfuelcellsistwofold:protonconductionfromtheanodetothecathode,andeffectiveseparationoftheanodeandcathodegases.Naﬁon®isbyfarthemoststudiedprotonelectrolyteforPEMfuelcells.Becauseofitspoly(tetraﬂuoroethylene)backbone,itischemicallyinertinbothoxidizingandreducingatmospheres.Naﬁonmembranesarehighlyacidic,haveexcellentprotonconductivity(9·10−3−12·10−2Scm−1at80◦Cintherange34–100%relativehumidity)(3),andunsurpassedlongevity(>60,000h)inafuelcellenvironment.Despitetheseattributes,muchresearchhasbeencarriedoutoverthepastdecadeonexploringthepossibilitiesofhydrocarbon-basedmembranesascandidateprotonelectrolytesforPEManddirectmethanolfuelcells(DMFC).Mostimportantly,Naﬁonmembranesarelimitedintheirtemperaturerangeofoperationtoaround80◦C,theyhavepoorbarrierpropertiestomethanol,allow-ingmethanolcrossoverfromtheanodetothecathodeinaDMFC,highosmoticdrag,whichmakeswatermanagementathighcurrentdensitiesdifﬁcultand,fromthestandpointofrecyclingofthecomponentsofmembrane-electrodeassemblies(MEAs),theperﬂuorinatedcompositionmightbecomeafutureissue.Thesefea-turesarecommontootherperﬂuorinatedmembranes,suchasthoseproducedbyDow,AsahiGlass,andAsahiChemical.

Consequently,muchefforthasfocusedonthedevelopmentofalternativeprotonexchangemembranesforPEMfuelcellsandDMFC,inparticularwiththeaimofincreasingthetemperatureofoperationofthefuelcell.Increaseintempera-tureisattractiveforanumberofreasons:(a)improvedtoleranceoftheelectrodestocarbonmonoxide,whichenablestheuseofhydrogenproducedbyreform-ingofnaturalgas,methanolorgasoline;(b)simpliﬁcationofthecoolingsystem;(c)possibleuseofcogeneratedheat;(d)increasedprotonconductivity;and(e)inDMFC,improvedkineticsofthemethanoloxidationreactionattheanode.However,increaseintemperatureconferstherestraints.Becauseinthemajorityofproton-conductingpolymermaterialsprotonconductioniswater-assisted,theyexhibitthehighestprotonconductivitywhenfullyhydrated(seebelow).Thisre-quireshumidiﬁcationofgasfeedsbeforeenteringthefuelcellandtheapplicationofpressuretomaintainadequaterelativehumidity.Pressurizingasystemabovearound3atmgivesaworkingupperlimitforawater-saturatedenvironmentofaround135◦C.Workingbeyondthistemperaturerequiresthedevelopmentofnewapproachestoenableprotontransportinawater-freeenvironment.Finally,thehighcostofNaﬁoniscurrentlyincommensuratewithpotentialmassmarketssuchaspersonaltransportforexample.

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.NON-FLUORINATEDPOLYMERMEMBRANES505

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Therearefewnon-ﬂuorinatedmembranematerialsappropriateforfuelcellap-plicationattemperaturesabove80◦C,theprimaryrequirementbeingthetempera-turestabilityofthepolymer.Thisgenerallymeansthatthepolymersaremadeupofpolyaromaticorpolyheterocyclicrepeatunits,andexamplesincludepolysulfones(PSU),poly(ethersulfone)(PES),poly(etherketone)s(PEK),poly(phenylquinox-aline)(PPQ),andpolybenzimidazole(PBI).Developedforhigh-temperatureap-plications,thethermalstabilityofthesetypesofpolymersiswelldocumented.However,theyareelectricallyinsulatinguntilmodiﬁed.Avarietyofapproacheshavebeendevelopedforthepreparationofpolymermaterialsbyderivatizationofthermostablepolymersforapplicationinmedium-orhigh-temperaturePEMfuelcells(seebelow).Thecharacteristicsofthemembranespreparedfromthesepolymersclearlydepend,ataﬁrstlevel,uponthechemicalnatureofthepoly-merbackbone,butsecondarylevelparameterssuchasthepolymermolecularweightandmolecularweightdistribution,thenatureofthesolventusedforcast-ing,andthepossiblepresenceofresidualsolventinthepolymerﬁlmalsoinﬂuencepropertiessuchasconductivityandmembranemechanicalstrength.Theeffectofsuchfactors,sometimesless-wellanalyzed,canimpedecomparisonofproper-tiesofnominallysimilarpolymermembranes.Subsequentsectionsconsidermi-crostructuraldifferencesbetweenNaﬁonandhydrocarbon-basedmembranesandthestabilityofnon-ﬂuorinatedpolymermembranesunderconditionslikelytopre-vailinafuelcell(includingthermohydrolyticstability,stabilityinanoxidizingenvironmentandwithrespecttoradicaldegradation).Waterdiffusionandelec-troosmoticdrag,andfuelcellperformanceanddurabilityarediscussedintheﬁnalsections.

ThenumberofpublishedfuelcelltestsandMEAlongevitydataarenotex-tensive,andinparticularthereislittlepublishedinformationontestingattem-peraturesabove80–90◦C.Theavailabilityofsuchdataisessentialtovalidatetheapproachofusingnon-ﬂuorinatedpolymersasacomponentofPEMFCandDMFCmembranes.

Recentprogressintheareaofnon-ﬂuorinatedprotonconductingmembranesincludesasurveybySavadogo(4),whichcoverstheliteratureuptotheendof1997.ForrecentreviewarticlesonmembranesforPEMfuelcellsthereaderisreferredtoReferences(5–9),aswellasthoseonsolidstateprotonicconductors(3,10).

FROMHIGH-TEMPERATUREPOLYMERSTOPROTONELECTROLYTEMEMBRANES:FUNCTIONALIZATIONBYTHEADDITIONOFPROTOGENICGROUPS

Severalmethodsforthepreparationofthermallystableproton-conductingpoly-mershavebeendevelopedthatincludeacidorbasedopingofathermostablepolymer,directsulfonationofapolymerbackbone,graftingofasulfonatedorphosphonatedfunctionalgroupontoapolymermainchain,graftpolymerizationfollowedbysulfonationofthegraftcomponent,andtotalsynthesisfrommonomerbuildingblocks(Figure1).Inaddition,theproton-conductingpropertiescanbe

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entirelyconferredorenhancedbytheadditionofaninorganicprotonconductor(11,12);seethereviewbyCasciola&Albertiinthisvolume(12a).Theapproachchosendependsontheparticularpropertiesandchemicalreactivityofthepolymerconcerned.

AcidandBaseDopingAromaticpolybenzimidazolesarehighlythermostable,withmeltingpoints>600◦C(13).Thepolybenzimidazolecommerciallyavailableispoly[2,2󰁁-(m-phenylene)-5,5󰁁-bibenzimidazole(PBI)(Figure2).PBIshowssometendencytotakeupwater,thusexplainingtheprotonconductivitythatislow(∼10−7Scm−1)asshownevenbythenon-modiﬁedpolymer(14,15).PBIisbasic(pKvalueof∼5.5),anditreadilyformscomplexeswithorganicandinorganicbases.EarlymentionofstabilizedandplasticizedPBIreferredtoPBItreatedwithsulfuricandphosphoricacid,respectively(13).Suchtreatmentleadstoasigniﬁcantincreaseintheconductivity.Theaciduptakereaches5molH3PO4perPBIrepeatunit,farbeyondthequantityusuallyunderstoodbythetermdopedforothersystems,andyetthisisthetermgenerallyusedtodescribePBI-acidcomplexes.Theprop-ertiesofsuchdopedmembranesandtheirapplicationinPEMfuelcellsandincellsusinghydrocarbonsandmethanolasfuelshavebeensystematicallystudiedbyWainrightetal.(16,17)andothers(18–21)since1994,culminatingintheproductionbyCelaneseVenturesofMEAsbasedonphosphoricacid-dopedPBI.Thisrepresentedalandmarkdiscovery;suchmembraneshavinglowpermeabilitytomethanolandlowelectroosmoticdragopenednewopportunitiesforDMFCandhigh-temperatureoperation(22).Dopingwithotheracids(hydrochloric,per-chloric,nitricacid)leadstomembraneswithsimilarconductionproperties(23).DifferentmethodsareusedfortheformationofPBI–acidcomplexes,includingtheimmersionofaPBImembraneinanacidsolutionofgivenconcentrationfor

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agivenlengthoftime,anddirectcastingfromasolutionofPBIandphosphoricacidintriﬂuoroaceticacid(24).Arecentvariantofthisapproachusespolyphos-phoricacidasthecondensingagentforthepolymerizationandasamembranecastingsolvent.Absorptionofwateraftercastingleadstoinsituhydrolysisofpolyphosphorictophosphoricacidinthemembrane(22).Thelastroutecanbeusedtoproduceﬁlmswithhighacidcontent.Inallcases,ahomogeneouspolymerelectrolytesystemisformedbydissolutionoftheacidinthePBImatrix,anddop-inglevels≥50wt%areachieved.Theconductivitydependsonthisdopinglevel,whichcanalsobeexpressedasthenumberofH3PO4moleculesperPBIrepeatunit.Formembraneswith0.07–0.7H3PO4/PBI,theconductivityis10−5–10−4Scm−1at25◦C,whereaswithcomposition4–5H3PO4/PBI,theconductivityis>10−3Scm−1at25◦C(18)and>3.10−2Scm−1at190◦C(17).Thisconceptcanbeextendedtootherpolymers(e.g.,polybenzoxazole)andfromacidstobases(25).Thus,inorganichydroxidesandorganicbasessuchasimidazolealsoeffectivelyincreasetheconductivityofPBI.ThesesubjectsaretreatedindepthinthechapterbyMeyer&Schusterinthisvolume(25a).

DirectModiﬁcationofaPolymerBackbone

Polyarylenepolymers,andpolymerscontainingphenylpendantgroupssuchasstyrene/ethylene-butylene/styrene,aswellasheterocyclicsystems,canbe

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.sulfonatedbydirectreactionwithanappropriatereagent.Themostattractivesiteforsulfonationdependsbothonthepolymerstructureandthedirectingef-fectofsubstituentgroups,andadvantagesofsimplicityandreproducibilityofthesulfonationreactionunderdeﬁnedconditionsareassociatedwiththisinherentre-activity.Sulfonatingreagentsusedincludesulfuricandchlorosulfonicacids,sulfurtrioxide,andcomplexformationwithtrimethylsilylchlorosulfonicacid,etc.Whentheacidisbothsolventandreagent,thesulfonationreactiontakesplaceatthesametimeasdissolution,andthedissolutionstepisthusfullyimplicatedinthehomogeneityofsulfonationandreproducibilityofthepolymermicrostructureandassociatedproperties,inparticularforpolymersoflowdegreesofsulfonation(26).Thefunctionalizedpolymermostreadilyformedisnotnecessarilythatwhichisthemoststabletowarddesulfonationunderthehigh-temperature,highrelativehumid-ityconditionsofthefuelcell,however,andalternativemeansenablingsulfonationatelectron-poorsiteshavebeendevelopedinrecentyears.Ionomerspreparedbydirectsulfonationshowstatisticalsubstitutionalongthepolymerchain;otherapproaches(describedbelow)allowtheassemblyofcopolymerswithadeﬁnedsulfonationpattern.

InCelazole®PBI,theproximityoftheimidazoleringtothe

fusedbenzeneringactivatesthelattertowardelectrophilicattack.However,PBIdissolvedinsulfuricaciddoesnotreactappreciably(27),andthermalactivationisrequired.Thiscanbeachievedbyheattreatmentofsulfuricacid–dopedPBImembranesforshortperiodsoftimeat450–500◦Cleadingtolevelsofsubstitutionupto∼0.6sulfonicacidgroups/PBI(27–29)(Figure2).However,suchmembranesarenotamenabletofurtherprocessingbecausethesolubilityofPBIinsolventssuchasdimethylacetamideislost,andthesulfonatedmembranescanberecastonlyfromsulfuricacid.Inaddition,theconductivityisbarelyhigherthanthatofnon-substitutedPBI,evenathighestdegreesofsulfonation.Thesepropertiesareincontrasttothoseobservedforbenzylsulfonate-graftedPBI(seebelow)andtendtosuggestthatcrosslinkingofsulfonatedPBIchainshasoccurred,eitherbystronghydrogenbondingorthroughsulfonelinkages,asmightbeexpectedgiventhehightemperatureneededtoinducethesulfonationreaction.

POLYBENZIMIDAZOLEFollowingthesameapproach,poly(phenylquinox-aline)(PPQ)(Figure3)canalsobesulfonatedasacastﬁlmbybrieﬂytreatingsulfuricacid–dopedmembranesathightemperature(30).ThusimmersionofPPQmembranesin50%H2SO4for2hleadstoaPPQ-sulfuricacidcomplexinwhichthehydrogensulfategroupsareconvertedtocovalentlybondedsulfonicacidat300◦C.Thesulfonationlevelispoorlyreproducibleusingthismethod,withnon-homogeneoussulfonationoccurringthroughoutthemembrane(30).UnlikethesulfonatedPBImembranesoflowconductivitypreparedusingtheimmer-sion/thermaltreatmentapproach,sPPQdisplaysprotonconductivityvaluesupto0.1Scm−1,andfurtherinvestigationofsPPQmembranespreparedusingthismethodisjustiﬁedonthebasisoftheseresults.However,theﬁrstmodiﬁcationof

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Figure3line).

Sulfonatedpoly(phenylquinoxa-

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.PPQbydirectsulfonationforfuelcellmembraneapplicationwascarriedoutbyBallardAdvancedMaterials(31)initsearlyeffortstodeveloplow-costpolymersforuseasprotonexchangemembranes.TheoutstandingquestioniswhethersPPQmembranespreparedbyimmersion/thermalgraftingshowanadvantagewithre-specttoMEAlifetimesinafuelcelloverthisﬁrstgenerationofBallardAdvancedMaterials(BAM1G)membranes.

SulfonationofvariousPPQs,preparedeitherbycondensationoftetraaminesandtetraketones,orbyself-condensationpolymerizationofabifunctionalmono-merofequivalentweight(EW)400–800gmol−1canbecarriedoutbyreactionofdissolvedPPQinchlorosulfonicacid(31).Thesulfonatedpolymerhasaglasstransitiontemperatureof220◦Candisstableatupto300◦Cinair.Thesiteofsulfonationisorthototheelectrondonatingetherlinkage,aswhensulfonationiscarriedoutbythermalgrafting.TheperformanceofMEAspreparedusingsPPQsinahydrogen/airfuelcellatacurrentdensityof0.538Acm−2(500Aft−2)at70◦Cisintherange0.50V(EW,796gmol−1,thickness110µm)to0.66V(EW427gmol−1,47µm).However,inlong-termtests,theaverageMEAlifetimewasonly350h,withfailurebeingattributedtotransferofreactantgasesowingtoincreasedpermeabilitycausedbyprogressivemembraneembrittlement.ThesecondgenerationofBAMmembraneswas

concernedwithmorethanonetypeofpolymerandincludedpoly(substituted-phenyleneoxide).Oxidativecouplingof2,6-diphenylphenolgivespoly(2,6-diphenyl-4-phenyleneoxide)(P3O),whichissulfonatedinchlorosulfonicacidsolutionsatthemorereactivebackbonearylgroup(Figure4c).PolymerswithEW450molg−1[ionexchangecapacity(IEC)2.22meqg−1]providehighercellvoltageat0.538Acm−2(500Aft2,0.68V)thantheBAM1GsPPQofsimilarequivalentweight.However,lifetimewaslimitedto500hbyinternaltransferofreactantgasesacrosstheMEA,withacontributionofbothphysicalandchemi-caldegradationprocessestoultimatemembranefailure;sP3Omembranesofsuchlevelofsulfonationshowhighswellingandpoormechanicalresistancetotearingandpoortensilestrength.

Thecentralarylgroupcanbedeactivatedbysubstitutionofbromideorcyanidefunctionalitiesonthisring.Harsherconditionsofsulfonationarethenrequired,

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure4(a)Sulfonatedpoly(phenyleneoxide);(b)crosslinkedsulfonatedpoly(phenyleneoxide);(c)sulfonatedpoly(2,6-diphenyl-4-phenyleneoxide);(d)re-actionschemeforthesulfonationofpoly(3–bromo-2,6-diphenyl-4-phenyleneoxide).Redrawnfrom(31).

andsulfonationtakesplaceononeoftheperipheralphenylsubstituents(Figure4d).Despiteapredictedincreaseinoxidativestability,thefuelcelllifetimeoftheseBAM2Gvariantswasalso450–500h,althoughthesP3OBrionomersgavethehighestinitialperformanceatthestart-upofanyoftheBallardAdvancedMa-terials(31).Sulfonatedpoly(phenyleneoxide)(Figure4a)thermallycrosslinkedviaallylphenolfunctionalgroups(Figure4b)canbepreparedbyelectropolymer-ization(32).

Poly(phenylenesulﬁde)(PPS)canbesulfonatedin

concentratedsulfuricacid(33).Higherlevelsofsulfonationareachievedus-ingapolysulfoniumcationin10%SO3/H2SO4(34).Here,thestrongelectron

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withdrawingpropertyofthesulfoniumgroupinthemainchainsuppressesthecrosslinkingreactionandpromotessulfonationat120◦C.Demethylationandacidi-ﬁcationstepsleadtoformationofapoly(phenylenesulﬁdesulfonicacid)(Figure5).Althoughthehighcarrierconcentrationensureshighprotonconduc-tivity(>10−2Scm−1at20◦C,94%RH),thesystemiswatersolubleatadegreeofsulfonation>30%.However,theremaybeapplicationforPPSinanon-aqueousenvironment;blendsofhighlysulfonated(85%)poly(phenylenesulﬁdesulfonicacid)withpoly(oxoethylene)displayaconductivityunderdryargonatmosphereof1.3·10−3Scm−1at130◦C(35).

Poly(aryleneethersulfone)s(PES)arethermoplas-ticshavingexcellentthermalandmechanicalproperties.Thebasicrepeatunitsinthisfamilyofpolymersconsistofphenylringsseparatedbyalternateetherandsulfone(-SO2-)linkages,suchasinthePESpolymersthatarecommerciallyavail-ablefromVictrex(Figure6a).Polysulfones(PSUUdel®,BPAmoco)contain2-propylidenespacersinadditiontotheetherandsulfonegroups(Figure6b,d).ThesespacerswerehexaﬂuorinatedintheﬁnalmemberoftheBAM2Gpoly-mers(31),asshowninFigure6c.Thearomaticether(bisphenol)part,commontopoly(etherketone)sandsomepolyimides,confersﬂexibility(asdoestoacertaindegreethe2-propylidenelink),whereasthesulfonegroupisstablewithrespecttooxidationandreduction.

Modiﬁcationofpoly(aryleneethersulfones)byadditionofsulfonicacidus-ingvariousreagentshasbeeninvestigatedextensively.Directsulfonationus-ingsulfuricorchlorosulfonicacidasbothsolventandsulfonatingagentcanleadtosomepolymerchaindegradation(36)thatresultinpolymerswithpoormembrane-formingproperties(27).sPESofionexchangecapacity(IEC)1.8–3.0(∼100%sulfonation)hasbeenpreparedusingSO3indichloromethane(37).PolymersofIECbetween2.5and3.0meqg−1haveconductivityclosetothatofNaﬁon,althoughtheyswellsigniﬁcantlyat80◦C.Analternativemethodforsulfonation,reportedtominimizesidereactions,isbasedonreactionwithan

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure6Sulfonatedpoly(ethersulfone)(a)andpolysulfonessulfonatedorthototheethergroup(b),andtothesulfonegroup(d).Structureofhexaﬂuorinatedsulfonatedpolysulfone[BallardAdvancedMaterialssecondgeneration(BAM2G)sPSUF6](c).

SO3-triethylphosphatecomplexindichloroethanesolution.Complexedchlorosul-fonicacidalsoallowsmilderdirectsulfonation.Reactionoftrimethylsilylchloro-sulfonatewithPSUgivesasilylsulfonatepolysulfonefromwhichthetrimethylsi-lylmoitiescanbecleavedtogivetheacidformofthealkalinesulfonate(37–39).AsshownbythedataofFigure7a,theeffectivedegreeofsulfonationusingthisrouteisalwayslowerthanthattheoreticallypossiblebasedonreagentsto-ichiometry,buttheefﬁciencyofsulfonationisgreateratlongerreactiontimes.Sulfonationisrapidovertheﬁrst5hofreactionandreachesamaximumof1.35molSO3Hgroupperpolymerrepeatunitafter48h(Figure7b),suggestingaprogressivedeactivationofthearomaticringselectrophilicsubstitutionasthereactionproceeds.

AlltheaboveelectrophilicsubstitutionreactionslocatethesulfonicacidgroupatthemostactivatedsiteorthotothearomaticetherbondinthebisphenolApartofthemolecule(Figure6b).Suchsulfonatedpolysulfonesareexpectedtobelessstabletodesulfonationthanthosewheresulfonationisdirectedtotheelectrondeﬁcientringoftherepeatunit.Kerresetal.havedevelopedadirectsulfonation

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure7(a)Sulfonationofpolysulfone(PSU)using(CH3)3SiSO3Cl.Reactioncon-ditionsare24hat35◦C(uppercurve)4hat40◦C(lowercurve).Allotherconditionsareidentical.(b)ConversionwithtimeofPSUtosPSUexpressedasexperimentaldegreeofsulfonationat35◦C.Redrawnfrom(36)withpermissionoftheauthors.

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.methodthatproceedsviaalithiatedintermediateformedbydeprotonationwithn-butyllithiumofthediphenylenesulfonesegment(40).ReactionofthelithiatedPSUwithSO2andoxidationofthesulﬁnatethusformedleadstosPSU.Alter-natively,lithiatedPSUcanbereactedwithSO2Cl2toyieldPSUsulfochlorideandthencePSUsulfonate.SuchpolysulfonesulfonatedatapositionorthotothesulfonegroupisshowninFigure6d.

ThegreattendencyofsPESandsPSUtoswellinhotwaterhasledtothedevelopmentofmethodsforcrosslinkingwiththeaimofimprovingmechanicalstability.sPESmembraneshavebeenpartiallycrosslinkedthroughactivationofthesulfonicacidgroupsviaconversiontothesulfonicacidchloride/bromideortothesulfonicacidimidazolidewithsubsequentreactionwitharomatic/aliphaticdiamines(Figure8a)(37).Thissulfonamidecrosslinkingisnotsufﬁcientlysta-bletohydrolysisforfuelcellapplications,anduseofthesulfonicacidgrouptocrosslinkpolymerchainsreducestheionexchangecapacityandhencetheprotonconductivity.Indeed,topreserveadequateelectricalpropertiesaftercrosslinking,polymersofhigherdegreeofsulfonationthathavepossiblysufferedsomedegra-dationthroughtheharsherreactionconditionsneededmustbeused,andsuchapossibleeffectonmembranelifetimemayoffset,tosomeextent,theadvantagesformechanicalstrengthconferredbyreticulation.

Orthosulfonesulfonatedpolysulfonehasbeencrosslinkedintwodifferentap-proaches:viasulﬁnatealkylation,Figure8b(41)andviadisproportionationofsulﬁnicacidgroups(42).ThewateruptakeoforthoethersulfonatedPSUandcrosslinkedorthosulfonesulfonatedPSUisshowninFigure9asafunctionofthemeasuredionexchangecapacity.Theseresultssuggestthatcrosslinkingex-tendstheIECrangeoverwhichthehydrationnumber(λ)ofsPSUremains<25,extensivehydrationandswellingoccurringaboveanIECof1.3and1.6meqg−1fornon-crosslinkedandcrosslinkedmembranes,respectively.Membraneswellingalsodependsonthelengthofthecrosslinkingspecies.

Thepoly(etherketone)sareafamilyofpolyarylenes

linkedthroughvaryingsequencesofether(E)andketone(K)unitstogiveether-rich:PEEK(Victrex®PEEK,GatoneTMPEEK,GhardaChemicals)andPEEKK(Hostatec®),orketone-richsemicrystallinethermoplasticpolymers:PEK(AmocoKadel®,FuMA-Tech)andPEKEKK,Ultrapek®,BASF)(Figure10).Oxidativeandhydrolyticstabilityisexpectedtoincreasewithincreasingproportionofke-tonesegments,andexperimentally,PEKK(Declar®,DuPont)undergoeslowerweightlossat400◦Cunderwater/oxygenthaneitherPEKEKKorPEEK.Someofthese(Hostatec,Ultrapek)arenolongercommerciallyavailable.Thesul-fonatedpoly(etherketone)familyofpolymershasprobablybeenmorebroadlyandextensivelystudiedinrecentyearsthananyothernon-ﬂuorinatedsystem,withcon-tributionsfromKreuer,Kerres,Bauer,Rozi`ere,andtheirco-workersandothers,tostudiesrangingfrommodelingofthemicrostructure,protontransportproper-ties,applicationinlow-andmedium-temperaturePEMFCandDMFC,andasacomponentofpolymerblendandhybridinorganic-organicmembranes.

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure8Crosslinkingofsulfonatedpolyethersulfoneandpolysulfone.(a)Sulfon-amidecrosslinkingviareactionofsulfonicacidimidazolidewitharomaticoraliphaticdiamines.Redrawnfrom(37).(b)CrosslinkingbyS-alkylationofthesulﬁnategroupofpartiallyoxidizedsulﬁnatedPSUwithdiiodobutane.Redrawnfrom(41).

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure9Dependenceofhydrationnumberofnon-crosslinkedandcrosslinkedsul-fonatedpolysulfonesonionexchangecapacity.Membranesimmersedinwaterat80◦C.(󰁓),non-crosslinked,cycle1(39);×,non-crosslinked,cycle2(39);◦,non-crosslinked(36);󰀩,crosslinkedbysulfonate/sulﬁnatedisproportionation(42);᭿,butanecrosslinked(92).Redrawnwithpermissionoftheauthors.

Figure10Sulfonatedpoly(etheretherketone)andstructuresofrepresentativemembranesofthepoly(etherketone)family.

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure11ReactionofVictrex®poly(etheretherketone)(PEEK)withconcentratedH2SO4atroomtemperature.ComparisonoftheincreaseofIECofsPEEKwithreac-tiontimereportedby(44)󰀂;(45)󰁓;(26)᭿;J.Rozi`ere,D.J.Jones&M.Marrony,unpublishedresults,󰀃.

Thepoly(etherketone)currentlymostreadilyavailablecommerciallyisVictrexPEEK,andthedatareportedbelowrefertoapolymerfromthissource.Thepresenceofadjacentortho-directingethergroupsconfershighestreactivitytothefourequivalentsitesonthehydroquinoneunitsituatedbetweentheethersegments.Ortho-ethersubstitutionbysulfonicacidgroupscanbecarriedoutinconcentratedsulfuricacidoroleum,theextentofsulfonationbeingafunctionofthereactiontimeandtemperatureandSO3concentration(26,43–45).DataontheIECofsulfonatedPEEKasafunctionofreactionconditions(Figure11)showex-cellentagreementbetweenauthorsandunderlinethereproducibilityofthedegreeofsulfonationunderdeﬁnedconditions.TheinitialPEEKconcentrationinsulfuricaciddoesnotinﬂuencetheprogressofthereaction(46).ThesolubilityofsPEEKinvariousmediaasafunctionofIECisshowninTable1.Thesedataindicatethatdis-solutioninN-methylpyrrolidone(neededformembranecasting)becomespossibleaboveanionexchangecapacityaround0.9meqg−1,whereassolubilityinhotwa-teroccursaboveanIECofaround1.8meqg−1.ThisrangedeﬁnestheappropriatedegreeofsulfonationforfuelcellmembranesbasedonsPEEKunlessthepolymeriscrosslinked,blended,orotherwisemodiﬁed.1HNMRspectroscopyisonemeansofestimatingthedegreeofsulfonation,sinceprotonsαtoasulfonicacidgroupare

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TABLE1SwellingofsulfonatedPEEKmembranesIECMeqg−100.50.81.01.1

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.N-methylpyrrolidone−−++++++++++++++++++++

Ethanol−−−−−sw+++++++++++

Water−−−swswsw+++++++

1.551.82.02.13.0

−stableinboilingsolvent;swswollen.

+incompletedissolution;++solutionafterheating;+++solutionat25◦C.

shifteddownﬁeldcomparedwiththepositionoftheothertwoprotonsinthehydro-quinonering.Thedegreeofsulfonationsoapproximatedcanbelowerthantheionexchangecapacitydeterminedbytitrationthroughaccessibilityorcrosslinking,forexample.

PropertiesofsPEEKaregenerallyafunctionofthedegreeofsulfonation.Underdynamicconditions,sPEEKstartstoloseweightbetween240and300◦C(44,45)dependingontherateofheatingand,toalesserextent,thedegreeofsulfonation.ThesulfurcontentoftheresiduerecoveredafterheatingsPEEKat400◦Cdecreasedby∼90%,indicatingthatthermaldecompositionbeginsbydesulfonation(47).Combustionofthepolymeroccursabove450◦C.TheTgincreasesfrom150◦C(PEEK)toaround230◦CinsPEEKof60%sulfonation.

Asforotherpolyaromaticpolymers,theprotonconductivityofsPEEKde-pendsnotonlyonthedegreeofsulfonationandambientrelativehumidityandtemperature,butalsoonthethermalhistoryofagivenmembraneandthepossiblepresenceofresidualsolventfromthecastingstage.Ashortsummaryofvariousstudiesispresentedhere.WhereasthenumericaldataarethoseforsPEEK,thegeneralconclusionsalsoapplytoothersulfonatednon-ﬂuorinatedpolyaromatics.Theincreaseofconductivitywithtemperatureat100%relativehumiditydependsonthepre-treatmentimposedonthemembrane(Figure12a).Forexample,forasPEEKmembraneofIEC1.6meqg−1andthickness60µmpre-treatedinboilingwaterfor4hpriortomeasurement,theconductivityshowedweaktemperaturede-pendencefrom0.03–0.07Scm−1overthetemperaturerange25–150◦C,whereasthatofanon-treatedmembranemaintainedat100%relativehumidityincreasedbymorethanafactor10overthesamerange(44).Thedegreeofhydrationattainedbythemembraneafterprolongedimmersioninboilingwaterismaintainedwhenthemembraneisremoved,anditisgreaterthanthatofamembraneconditionedinan

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environmentof100%relativehumidity.Thisisfurtherdiscussedbelow.AsshownbythedataofFigure12b,theconductivityofnon-treatedsPEEKmembranesshowsgreaterdependenceontherelativehumiditythandoesthatofNaﬁon;theformerincreasesbyanorderofmagnitudebetween66and100%relativehumidityat100◦Ctoreach0.02Scm−1,whereastheconductivityofNaﬁonincreasesbyafactorof4(44).Protonconductivitythusreliesmoreheavilyontheamountofwa-terbeyondthatintheﬁrstandsecondhydrationspheresinsPEEKthaninNaﬁon(6,48).Measurementsmadeasafunctionoftemperatureatarelativehumidityof75%(Figure12c)showthathighlysulfonatedPEEK(IEC2.48meqg−1,degreeofsulfonation∼90%)displaysthesameconductivityasNaﬁonat160◦C,whereasthatofsPEEKatlowerdegreeofsulfonation∼60%(IEC1.6meqg−1)isanor-derofmagnitudelowerundertheseconditions(49).Interactionbetweenpolymerchainsdecreaseswithincreasingtemperature,favoringgreaterhydrationofthepolymeratagivenvalueoftherelativehumidity.Thediffusioncoefﬁcientsandelectroosmoticdraginsulfonatedpoly(etherketone)isdiscussedfurtherbelow.Althoughdeterminationofconductivityofmembranesrepresentskeyexsitucharacterization,adifferenceinvaluesbetweenreportsintheliteraturecanbeattributed,rightlyorwrongly,toadifferentmethodofmeasurement.Formem-branesdestinedforfuelcellapplication,insitumeasurementinaworkingfuelcell(50)isthemostrelevantandcanalsoservetovalidateexsituevaluation.Forex-ample,usingthelinearregionofthepolarizationcurveofsPEEK(IEC1.6meqg−1)at100◦C,orusingthecurrentinterruptmethod,leadstoaconductivityof5·10−2Scm−1,inagreementwiththatderivedfromimpedancespectroscopyat100◦Cand100%relativehumidity(44).AppraisaloftheliteratureshowsthatsPEEKmembranesdrawnfromN-methylpyrrolidonesolution(6,44,49)havehigherconductivity(≥10−2Scm−1)thanthosecastfromdimethylformamide(∼10−5Scm−1)ordimethylacetamide(45,47).Suchamide-basedsolventscom-monlyusedforsolutioncastingofsPEEKmembranescaninteractwiththesulfonicacidgroupsandhavedetrimentalinﬂuenceonprotonconductionpropertiesifnotcompletelyeliminatedfromtheformedmembrane.Thisprovidesaplausibleex-planationforthesigniﬁcantdiscrepancyinconductivitydatabetweenauthorsforsPEEKofsimilardegreesofsulfonation.

SulfonatedPEEKcanalsobechemicallycrosslinked,eitherbyreactionwithasuitablearomaticoraliphaticamine,orbythermaltreatmentundervacuumtoinduceintra-/inter-chainpolymerizationofthesulfonicacidgroups(51).Inprinciple,twogeneralapproachescanbeusedforthe

preparationofsulfonatedpolyphosphazenes(sPPZ).Intheﬁrst,anaryloxide,alkoxideorarylaminethatalreadybearsaterminalsufonicacidorsulfonategroup,replacesthechlorineatomsinpoly(dichlorophosphazene).Thesecondapproachinvolvesthesynthesisofphosphazeneswithunsubstitutedaryloxysidegroups,followedbysulfonationofthesesidegroups.PolyphosphazenesofferavarietyofdifferentpolymercompositionsthroughthenatureofthesidechainsontheP=N-backbone,andsomeofthemostthermallyandchemicallystablepolyphosphazenes

POLY(PHOSPHAZENE)S

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.ylno esu lanosrepr oF .70/02/30 no anihC fo ygolonhceT & ecneicS fo tyisrevinU ybAnnu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgNON-FLUORINATEDPOLYMERMEMBRANES521

beararyloxysidegroupsthatcanbefunctionalizedtointroduceacidicunits,includ-ingcarboxylic,sulfonic,andphosphonicacidgroups.Sulfonatingagentshavein-cludedSO3,andconcentratedandfumingsulfuricandchlorosulfonicacids.Usingarangeofaryloxy-andarylamino-phosphazenes(monomersandthecorrespond-ingpolymers),Allcock&Fitzpatrick(52)showedthatClSO3H/dichlorethanegavecrosslinkedinsolubleproducts,whileuseofsulfuricacidledtopolyphosphazenessulfonatedprimarilyatthepara-position.Adecreaseinmolecularweightbyafactorof4to70indicatesthatsomeskeletalcleavageoccurred,predominantlyinfumingsulfuricacid,enhancedathighertemperature,andessentiallyduringtheﬁrst10minsofreaction.Thesepolymersaresolubleinwaterabove30%sul-fonation.Filmsofethane-crosslinkedpolyphosphazenecanbesulfonatedfromthesurfaceinward.Crosslinkingpreventsthesurfaceoftheﬁlmfromdissolvinginwater,thusallowingsulfonationtoproceedthroughthematerial;however,theoutersurfaceabsorbswatertoformhydrogelregions.

Poly[bis(phenoxy)phosphazene]shavealsobeensulfonatedindichloroethanewithSO3(53,54).TheionexchangecapacityincreasesgraduallywithSO3/PPZmolratioforpoly[(3-methylphenoxy)(phenoxy)phosphazene]andpoly[(4-methylphenoxy)(phenoxy)phosphazene],whereasthecorrespondingethyl-substitutedpolymersareunstableunderthereactionconditionsemployed(53).TheﬁrststageofsulfonationinvolvescomplexationofSO3tonitrogenoftheP=N-backbone,withsubsequentarenesulfonationtakingplaceonthemethylphe-noxy,ratherthanthephenoxy,sidegroup.Non-crosslinkedsulfonatedpoly[bis(3-methylphenoxy)phosphazene](Figure13)softensanddeformsabove76◦Candissolubleinliquidmethanol.Figure14representsthedependenceoftheprotonconductivityonmembraneswellingforthreesulfonatednon-crosslinkedsamplesofdifferentionexchangecapacity(55).Thedatashowthatconductivityabove10−2Scm−1isattainedonlybysamplesthatundergoavolumechangeinwaterat25◦Cof>50%,i.e.,thosewithIEC>1.2meqg−1.Thehighprotonconductivityundertheseconditionsisattributedtoclusteringofsulfonatedpolymerdomains

˚(55).Pho-andthecloseproximityofsulfonicacidgroups,estimatedas4.7–4.9A

tocrosslinkingcanbeinitiatedbybenzophenoneviaahydrogenabstractionmech-anismwiththemethylphenoxysidechain(56–58),givingcrosslinkedmembranesthatarethermomechanicallystableto173◦C.Althoughcrosslinkingthroughsuchsitesisclearlywithoutconsequenceonthenumberofsulfonicacidgroups(ion

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

Figure12ConductivityofVictrexsulfonatedpoly(etheretherketone).(a)Tem-peraturevariationat100%RH.sPEEKofIEC1.6meqg−1.(•)nopre-treatmentofthemembrane.(󰀃)membranewater-swollenbyboilinginwaterfor4hbeforemountingintheconductivitycell(44).(b)Variationwithrelativehumidityat100◦C.󰀃Naﬁon®-117;•sPEEKofIEC1.6meqg−1.Nopre-treatmentofthemembrane(44).(c)TemperaturevariationofNaﬁon-117(󰀃)andsPEEKofIEC2.48(᭡)and1.6(•)meqg−1at75%relativehumidity.Redrawnfrom(49)withpermissionoftheauthors.

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Figure13Sulfonatedpoly[bis(3-methylphenoxy)-phosphazene].

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure14Non-crosslinkedsulfonatedpoly[bis(3-methylphenoxy)phosphazene]:protonconductivityasafunctionofmembranewateruptakeat25◦C(55)forthreemembranesofionexchangecapacity:•,1.6;󰀩,1.2:᭢,0.8meqg−1.Redrawnwithpermissionoftheauthors.

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.exchangecapacity1.4meqg−1),thewateruptakefromliquidwaterislower,withthehydrationnumberdecreasingfrom19to13H2O/SO3Hinnon-crosslinkedandcrosslinkedpoly[bis(3-methylphenoxy)phosphazene],respectively.Nevertheless,theprotonconductivityofbothtypesofmembraneisidentical,0.04–0.08Scm−1overtherange30to65◦C(58)at100%relativehumidity.Membrane-electrodeas-sembliespreparedfromtheabovesPPZblendedwithpolyacrylonitrilehavebeentestedinDMFCat60◦C(59).Conductionpropertiesathighertemperatureshavenotbeenreported,and,indeed,thetemperaturerangeofapplicationofmodiﬁedpolyphosphazenesisnotwell-deﬁnedatpresent.

Phosphonicacid–substitutedPPZhasbeenpreparedfromaprecursorpolymerbearingabromophenoxysidegroupbytreatingﬁrstwithbutyllithiumandthenwithdiphenylchlorophosphonate.Theresultingphosphonateesterisconvertedtophenylphosphonicacidgroupsthroughbasichydrolysisandsubsequentacidiﬁ-cation(60,61).AsdepictedinFigure15,thenumberofphosphonicacidgroupsincorporatediscontrolledbythelengthofthe(bromophenoxy)(methylphenoxy)

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.segmentofthecopolymer,relativetothatofthebis(methylphenoxy)segment,andpolymerswithIECsof1.17,1.34,and1.43meqg−1havebeenprepared,corre-spondingtoP-OHcontentsof37,45,and50%perpolymerrepeatunit,respectively.Waterswellingatroomtemperatureofdriedphenylphosphonicpolyphosphazenemembranesisintherange11–32wt%,lowerthanthatofsPPZoflowerIEC(1.07meqg−1)andofNaﬁonmeasuredunderthesameconditions(38and30%,respec-tively)(61).Theroomtemperatureprotonconductivity(∼0.05Scm−1)andthelowmethanoldiffusioncoefﬁcients(12timeslowerthanthatofNaﬁon-117)alsopointtopossibleapplicationofthesemembranesinDMFC,althoughconductionpropertiesathighertemperatureandvariablerelativehumidityhavenotyetbeenreported.Polyphosphazeneswithsulfonimidesidegroups(62)haverecentlybeenreportedtogiveapowerdensityof0.47Wcm−2at80◦Cinahydrogen/oxygenfuelcell(63).

GraftingofaFunctionalGroup

Graftinganacidicfunctionalgroupontoapolymermainchainprovidestheoppor-tunityofcontrollingtheionexchangecapacityandthesiteofsulfonation,which,however,remainsrandomoverthedesignatedsites.Polybenzimidazole(PBI)canbederivatizedbyreplacingtheimidazolehydrogenwitharyloralkylsubstituents(14,64).ThismethodwasdevelopedforfurtherimprovingthechemicalstabilityofPBIbyintroductionintotheimidazoleringofgroupslessreactivethantheimidazolehydrogen,anditprovidestheopportunityoftuningthepropertiesofthepolymerbythechoiceofthesubstituent.Inthissynthesis(Figure16),hydrogenabstractionwithLiHisfollowedbyreactionofthePBIpolyanionwithafunc-tionalizedgraftingspecies,whichincludesbothsulfonatedandcarboxymethyl(65)groups.Thesynthesis(14,66,67)andelectrochemicalcharacterizationofbenzylsulfonate(14)andsulfopropylN-substituted(67)PBIhaverecentlybeen

Figure16Synthesisroutetobenzylsul-fonatedpolybenzimidazoleviapolyanionformation(14,66).

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.described.Controlovertheextentoffunctionalizationcanbeexertedatoneorbothofthereactionsteps,i.e.,bylimitingthenumberof–NHsitesionizedand/orbylimitingtheratioofthesulfonatedsidechaingrouptoPBI.Thermogravimetricanalysisshowsonsetofweightlossataround360◦C,higherthaninpolyarylenepolymersinwhichthesulfonicacidisdirectlysubstitutedontothepolymermainchain.SolubilityandtexturalpropertiesofPBIareincreasinglysigniﬁcantlymod-iﬁedasthedegreeofsulfonationincreases.ThewateruptakeandconductivityofsulfonatedPBIofdifferentdegreesofsulfonationarerepresentedinFigure17(68).Theshapeofthetwocurvesissimilar,wateruptakecorrespondingtoahy-drationnumberof∼7watermoleculespersulfonicgroup,almostindependentlyofthedegreeofsulfonation.ThisisonlyabouthalfthewateruptakeofNaﬁonorsulfonatedpoly(etherketone)membranes.Thehydrationnumber(under90%relativehumidity)isslightlyhigher,∼11,insulfopropylatedPBI(67).Figure18showstheconductivityofbenzylsulfonate-graftedPBI,non-modiﬁedPBI,andNaﬁon-117derivedfromresistancemeasurementsat25◦Cinaqueousphosphoricacid.Membraneswereallowedtoequilibratefor8hpriortomeasurement.Un-dertheseconditionsofmeasurement,theconductivityofbenzylsulfonategraftedPBIisintherange3·10−3to2·10−2Scm−1(concentrationrangeofH3PO4:0–10moldm−3),muchhigherthanthatofnon-graftedPBI(∼10−5Scm−1).At140◦C,theconductivityofsulfopropylatedPBIis10−3Scm−1;thepresenceofthealkylsidechainraisesthequestionofthestabilityofthissystemundercondi-tionsoffuelcelloperation.

Figure17Conductivityandwateruptakeofbenzylsulfonategraftedpoly-benzimidazole(68).

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure18Conductivityat25◦Cof(a)Naﬁon-117,(b)benzylsulfonate-graftedpoly-benzimidazole,and(c)polybenzimidazole.Conductivityderivedfromresistancemea-surementsat25◦CinaqueousphosphoricacidasafunctionoftheH3PO4concentration.Membranesequilibratedfor8hpriortomeasurement(5).

Benzylsulfonate-graftedPBIdisplaysahighconductivityaslongasthemem-branesarekeptfullyhydrated.Oncehighlysulfonatedsystemsareexposedtoadryenvironment,theydryoutandbecomebrittle.Theoriginalﬂexibilitycanberecoveredcompletelyonlybyimmersingthesulfonatedmembranesinadiluteaqueoussolutionoforganicorinorganicbasesforashorttime(15–60min);suchtreatmentdisruptsthehydrogenbondnetworkbetweendonorandacceptorsitesonthegraftedPBIchains.Thiscanbeclearlyidentiﬁedbyinfraredspectroscopy(Figure19)(69),wherethestrongandbroadabsorptioninthewavenumberre-gionbetween2200and3700cm−1inthespectrumofbenzylsulfonate-graftedPBIischaracteristicofastronglyhydrogenbondedsystem.Thisabsorptionismuchreducedinbreadthandintensityafterimmersioninasolutionofalkalimetalhydroxide,tetramethylhydroxide,imidazole,etc.

Benzylsulfonate-graftedPBIisremarkablystableinsolutionsof3%H2O2/Fe(II),showingneitherdegradationofphysicalintegritynorlossofconductivity,refutingthepossiblescissionatthebenzyllinkthroughfreeradicalattack.Unfor-tunately,insitufuelcellexperimentaldataarenotyetavailable.Thesesystemsmeritfurtherstudyaselectrolytesinbothhydrogenanddirectmethanolfuelcells,inparticularinapplicationswherelow-electroosmoticdragisrequired(7).

Thesamehydrogenabstraction/graftingroutehasbeenusedtopreparephospho-thylatedPBI,buttheresultingpolymerwasinsolubleinorganicsolvents,andthe

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure19IRspectraofprotonformbenzylsulfonategraftedpolybenzimidazolebefore(a)andafter(b–e)immersionininorganichydroxides(1moldm−3,1h,25◦C);(b)LiOH,(c)NaOH,(d)KOH,and(e)CsOH.Notethelossofintensitybetween3700and2200cm−1in(b–e).

materialcouldnotberecastasamembrane.Theconductivityofapressedpelletis∼10−3Scm−1(8).

Otherpolymershavealsobeendevelopedinwhichthesulfonicacidgroupisbornebyapendantgroupdistantfromthemainchain.Althoughstrictlynotagraftedsystem,aninterestingrecentexampleissulfonatedpoly(4-phenoxyben-zoyl-1,4-phenylene)(sPPBP),producedfrom2,5-dichloro-4󰁁-phenoxybenzophen-one(70)asParmaxTM-2000membranesbyMaxdemInc.,wherethepolyphenylenebackbonebearspendantketoneandether-linkedunitsinastructuresimilartoaPEK(Figure20).Thesependantgroupsaresulfonatedbysulfuricacidattheparapositionoftheterminalphenoxyunit.sPPBPissolubleinDMF,DMSO,andNMPabove30%sulfonation,and,asshowninFigure21,itsprotonconductivity

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Figure20

phenylene).

Sulfonatedpoly(4-phenoxybenzoyl-1,4-

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure21Conductivityofsulfonatedpoly(4-phenoxybenzoyl-1,4-phenylene)(sPPBP)asafunctionofrelativehumidityatroomtem-perature.Redrawnfrom(47)withpermissionoftheauthors.

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isstronglydependentontherelativehumidity,increasingby8to9ordersofmagnitudebetween0–100%relativehumidityatroomtemperature.sPPBPof65%sulfonationcastfromDMFhasaprotonconductivityof∼10−2Scm−1at100%relativehumidity,suggestingthatthecastingsolventhaslessinﬂuenceonprotonconductionpropertiesofsPPBPthansPEEK.Thependantgroupslikelyinﬂuencepolymermicrostructure,withgreaterseparationbetweenhydrophobicregionsandclusteredsulfonicacidgroups.

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.DirectPolymerizationfromMonomerUnits

Greatertailoringofpolymer

compositionispossiblebythedirectsynthesisofanewpolymerfrommonomerbuildingblocksfunctionalizedwithsulfonicacidorsulfonategroupsthancanbeattainedbysimplepolymermodiﬁcation.Controloftheposition,number,anddis-tributionofprotogenicfunctionsalongthepolymerbackbonecouldprovideaccessinthefuturetomorethermohydrolyticallystablesulfonatedpolymersandallowtuningofthemicrostructureandconcomitantsalientpropertiessuchasconductiv-ityanddegreeofswelling.Inreality,therangeofpolymersthatcanbeproducedislimitedbythecommercialavailabilityor,atleast,theeaseofpreparationofthecorrespondingmonomers,andeventualmultistagesynthesesthatwouldtendtoincreasecostscouldrenderthepolymerlessattractiveforindustrialusethanonepreparedbydirectsulfonation.Nevertheless,forcertainclassesofpolymer(poly-imides,polybenzimidazole),thetotalsynthesisrouteismoreappropriatethandi-rectsulfonation,andithasbeenlargelyusedforthedevelopmentofsulfonatedpol-yimides.Morerecently,andwiththeaimofplacingthesulfonatedmoietyatthemorestableortho-ketoneorortho-sulfonesites,respectively,sulfonatedpoly(etheretherketone)andpoly(ethersulfone)havealsobeensynthesizedabinitio.Forexample,nucleophilicsubstitutioncondensationpolymerizationof4,4󰁁-dichlorodiphenylsulfone,4,4󰁁-biphenol,and3,3󰁁-disulfonate-4,4󰁁-dichlorodiphen-ylsulfoneleadstoarandomsulfonatedpoly(aryleneethersulfone)withtwosul-fonicacidgroupsperrepeatunit,andinwhichthesulfonicacidgroupsaresitedonthedeactivatedsulfone-linkedrings(Figure22)(71).Locationoftheionomergrouponthedeactivatedphenylringshouldimprovestabilitytowarddesulfonationbecausetheintermediatecarbocationrequiredfordesulfonationismoredifﬁculttostabilizeonsuchasulfone-deactivatedring.Thereisachangeinthepolymermicrostructureasthecontentofdisulfonatedmonomercomponentincreases:Thesizeofthehydrophilicionicdomainsincreasesfrom10to25nmandbecomesinterconnectedtoformacocontinuousmorphologyabove50mol%.Atthisperco-lationlimit,twoglasstransitiontemperaturesdevelopandwateruptakeincreasesdrastically,tendingtosolubility.Polymermembraneswithionexchangecapacityof0.41to2.2meqg−1preparedbyvaryingthemonomermolarratiohaveacon-ductivityof0.01–0.16Scm−1at30◦C.Materialswithadegreeofsulfonationof40to50%couldbeadaptedforfuelcellapplicationwhenthequestionisnotonlytheperformance,butinparticularthehydrolyticstabilityandlifetimeofthisdirect

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure22Directpolymerizationsynthesisofrandomsulfonatedpoly(aryleneethersulfone)s.Redrawnfrom(71)withpermissionoftheauthors.

polymerizationsulfonatedpoly(ethersulfone)comparedwiththatofcongenerspreparedbydirectsulfonation.

Sulfonatedpoly(ethersulfone)shavingthesulfonicacidgroupatthelesssta-blesiteorthototheetherlinkcanbepreparedfrom4,4󰁁-diﬂuoro-diphenylsulfoneandhydroquinone2-potassiumsulfonate/hydroquinone(72).Usingthesameap-proach,sulfonatedpoly(etherketone)homo-andcopolymershavebeenpreparedfrom5,5󰁁-carbonylbis(2-ﬂuorobenzenesulfonate)usingthereactionshowninFigure23(72).Thebestmechanicalpropertiesareexhibitedforapolymerwithonesulfonicacidgroupperthreearomaticunits[i.e.,correspondingtoasulfonatedpoly(etheretherketone)with100%sulfonation],whentheexperimentallydeter-minedIECofapolymermembraneswithmolecularweight53000gmol−1was2.29meqg−1.Noconductionpropertiesontheseorthoether—sulfonated,directpolymerizationpoly(etherketone)sandsulfones—arereported.

Figure23Directpolymerizationsynthesisofsulfonatedpoly(etheretherketone).Redrawnfrom(72).

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Sulfonatedpolybenzimidazolecanbepre-pared,forexample,bythepolycondensationof2-sulfoterephthalicacidwith1,2,4,5-tetraaminobenzenetetrahydrochlorideat190◦C(73,74).Furtherderiva-tizationbygraftingofsulfopropylgroupsleadstoawater-solublepolymer(75);inprinciple,however,thenumberofpendantsulfonicacidgroupsgraftedinthesecondstagecouldbelimitedinordertoavoidsolubility.Noconclusionscanyetbemadeconcerningelectricalpropertiesonthebasisofthepartialconductivitymeasurementsreportedtodate,butthenumberofchargecarriersavailablebysuchdoublesulfonationisexpectedtoleadtohighprotonconductivity.

Sulfonatedpolyimideshaverecentlybeensuccessfullydevelopedinseverallaboratories.Mercierandco-workers[Besseetal.(76),Cornetetal.(77),Faureetal.(78),andGeniesetal.(79)]ﬁrstsynthesizedvariousrandomandsequencedsulfonatedcopolyimidesfromnaphthalene-1,4,5,8-tetracarboxylicdianhydride(NTCD)or4,4-oxydiphthalicanhydride(OPDA),4,4󰁁-diaminobiphenyl-2,2󰁁-disulfonicacid(BDSA),andcommonnon-sulfonateddiaminemonomerssuchasoxydianiline(ODA).Theincorporationofdiaminecomonomersadjustsprop-ertiessuchasﬂexibilityandwateruptakebypushingaparttherigidrodbackboneofthepolymer,therebycreatingfreevolumeforwater.Two-stagepolycondensationofBDSA-OPDA-ODAandBDSA-NTCD-ODAgivesphthalicandnaphthalenicsulfonatedpolyimide(sPI)copolymers,respectively,thathavebeenextensivelystudied(76–79).Sulfonatedphthalicpolyimidesdegradehydrolyticallyandchainscissionoccursrapidlyinwaterat80◦C(79,80).ThemolarratioofBDSA-NTCDorBDSA-OPDAintheﬁrststageofpolycondensationgivesthelengthoftheionicsequence,whilethesubsequentintroductionofODAandtheremainingdianhy-dridespacestheionicblockswithhydrophobicsequences.Theselattersequenceshavesomeresidualﬂexibility,whereasthephenylenebondsinthesulfonatedmoi-etyconferaglassycharacter.Polymersofvariousequivalentweightanddifferentratios(x/y)ofsulfonated(x)toneutral(y)diamine(Figure24)havebeenpre-paredthatcanbecastintomembranesfromm-cresol.Thelengthoftheblocksequenceforaconstantequivalentweighthasgreaterimpactonwateruptakeandconductionpropertiesthandoesavariationoftheequivalentweightforpolymerswiththesameblocklength.Formembranesswolleninliquidwater,thehydrationnumberincreaseswiththelengthoftheionicblocksequence,buttheconductivity

Figure24Naphthalenicsulfonatedpolyimides(77,78).

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decreases(from1.8·10−2to4·10−3Scm−1forsPIofequivalentweight793g

mol−1whenthelengthoftheionicblockincreasesfrom3to9units)(81).Thisobservationseemsincontrastwiththegeneralideathatconductivityincreaseswiththehydrationnumberandsuggeststhattheconductivityalsodependsonmicrostructuralchangesaccompanyingtheincreaseinblocklength.However,foragivensPIcomposition,theconductivityfollowsthegenerallyobservedtrendofincreasingwiththedegreeofhydration(Figure25).Forthesameblocklength(5)

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure25Protonconductivityofnaphthalenicsulfonatedpolyimideswithx/y=30/70(ionexchangecapacity=1.26meqg−1,seeFigure24forstructure).(a)Variationofconductivitywithionicblocklength;(b)variationofconductivitywithhydrationnumberforapolymerofionicblocklength(x)=5.Redrawnfrom(77)withpermissionoftheauthors.

NON-FLUORINATEDPOLYMERMEMBRANES533

anddifferentEW,thehydrationnumberformembranesimmersedinwateriscon-stant(λ=19H2O/SO3H).AsalreadynotedforothersulfonatedpolymerssuchassPEEK,thewateruptakebysPIissigniﬁcantlyhigherformembranessoakedinwater(λ∼17–30H2O/SO3HforsPIwithx/y=30/70)thanformembranesexposedtoasaturatedwatervaporenvironment(λ∼10–11H2O/SO3H)wherethehydrophilic/hydrophobicblockratiotosomeextentinﬂuencesthehydrationnumber.Inafuelcell,naphthalenicsPImembranesofIEC1.26meqg−1(60µmthickness)provide0.5Vat0.5Acm−2(H2-O2,4barabs.,70◦C)(76).HowevermembranedurabilityinanH2-O2fuelcellisdirectlyrelatedtothehydrolyticsta-bilitydeterminedexsitu(82,83),whichcloselydependsonthenumberofsulfonicacidgroupsinthe4,4󰁁-diaminobiphenyl-2,2󰁁-disulfonicacid/naphthalene-1,4,5,8-tetracarboxylicdianhydride/oxydianiline(BDSA/NTCD/ODA)copolymer.Thussulfonatedpolyimideswithionexchangecapacitiesof0.86,1.26,and1.98meqg−1havefuelcelllifetimesundersteadystateconditionsat0.3Acm−2/80◦Cof600,300,and20h,respectively(83).

Othergroupshavereportedthatsomecopolyimidemembranescontainingbulkyand/orangledcomonomers(84,85)haveproducedhigherprotonconductivitiesthanNaﬁon,possiblybyincreasingthefreevolumeavailabletowater.Polymer

˚forinterchainspacingsderivedfromX-raydiffractionarehigherby∼0.1–0.4A

sulfonatedpolyimides[incorporatinganangled(e.g.,oxydianiline,ODA)oralin-earbutbulkycomonomer,e.g.,bis(aminophenyl)polyphenylbenzenes]thanthoseforthecorrespondingBDSA-NTCDhomopolymer(84,85).Thesestudiescon-centrateonvaryingthenon-sulfonateddiaminemoieties.Polyimideshavealsobeenpreparedusingothercommercial(86,87)andnon-commerciallyavailablesulfonateddiamines(80,88–90).SomeoftheresultingsulfonatedpolyimidesaresolubleinDMSO(88),andtheirconductivitiescanexceed0.1Scm−1atroomtemperature/100%relativehumidity(84,88,90).However,themostrel-evantadditionalquestioninthecontextofapplicationasfuelcellmembranesistheirthermohydrolyticstability.Limiteddataareavailable:AmembraneofBDSA/NTCD/1,4-bis(4-aminophenyl)-2,3,5,6-tetraphenylbenzenesoftenedafter500hinwaterat≥90◦C(85),whereasasulfonatedpolyimidesynthesizedfromBDSA/NTCD/bis[3-(aminophenoxy)-4-phenyl]isopropylidenebecamebrittleaf-ter∼200hinwaterat80◦C(80).Otherdataindicatehigherhydrolyticandoxidativestabilityforrandompolyimidesthanforthecorrespondingsequencedpolyimidemembranes(88).Therelationshipbetweentheseexsitutestsandinsitufuelcellcharacterizationhasnotbeenreported.

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.PolymerBlends

Polymerblendingisapotentiallyversatilewayoftuningthepropertiestothosedesirableforfuelcellapplication.Themiscibilitybetweentwopolymerscanbeeffectivelyimprovedbyfavoringspeciﬁcinteractionsbetweenthepolymerchains,suchasionicinteraction,hydrogenbonding,orion-dipoleinteractions,whichalsoacttocrosslinktheblendandmodifymechanicalandswellingproperties.Ionically

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crosslinkedpolymerblendsarepreparedbyassociatingasulfonatedpolymer(for

exampleinitstriethylammoniumform)withabasicpolymer,followedbyacidwashingtoregeneratetheprotonicmaterial.Thedegreeofformationofioniccrosslinksdependsupontherelativeacidandbasestrengthsofthecomponents.Weaklybasicpolymersactashydrogenbondacceptorsfortheprotonsontheacidicpolymer;formorebasicpolymers,protontransferfromtheacidiccomponentproduceanionicallycrosslinkedmembrane.Kerresetal.(91–95)haveexploredthisapproachextensivelybyassociatingarangeofcommercialandspeciﬁcallysynthesizedpolymerblendcomponentsandhasidentiﬁed,inparticular,thesuit-abilityofacid-basemembranesforapplicationinDMFC.

Forexample,sulfonatedPEEKandPSUhavebeenusedinblendswithPBI,aswellaswithmoreweaklybasiccomponentssuchaspolyetherimine,poly(4-vinylpyridine),diaminatedPSU(91,92),andothermodiﬁedPSUscontainingpendantbasicaromaticgroups(93).Theeffectiveionexchangecapacityisagoodmeasureoftheextentofprotontransfertothebasicpolymer.EveninsPSU-PBIandsPEEK-PBIblends,theexperimentallydeterminedIECishigherthanthevaluecalculatedonthebasisofcompleteprotontransfer(91).Theionicinteractionin-creasestheglasstransitiontemperature,Tg,oftheblendmembranescomparedwiththatofeitherofthecomponentsby15to>50◦CinsPSU-PBI.Sucheffectsdependupontherelativeproportionofsulfonatedandbasicpolymerandthebasestrengthofthelatter.Blendswithalowioniccrosslinkingdensity(weaklyba-siccomponents,fewsulfonicacidsites)aremicrophaseseparated.ThismaybeseeninFigure26(92),wheretheimages(a)and(b)ofamicrophase-separatedmembranehavebeenpreparedusingsPSUofionexchangecapacity1.7meqg−1;image(c)isofanionicallyassociatedmembranepreparedwithsPEEKofIEC2.95meqg−1.Swellinginwaterandbrittlenessinthedrystatearebothlowerthanforthecorrespondingnonblendedsulfonatedpolymermembranetotheextentthatawater-solublesPEEKbecomesvirtuallyinsolubleonblendingwithPBI.Thesead-vantagesarelostathighertemperatureasthedegreeofioniccrosslinkingisseverelyreducedabove70◦Cforsulfonatedpolymer-aminatedPSUandabove110◦Cforsulfonatedpolymer-PBI;abovethesetemperatures,reverseprotontransferoccurswithanassociatedincreaseinsolubilityofthesulfonatedcomponent.Therefore,achallengeremainstoassociatebothcovalentandioniccrosslinkinginordertocombinethereducedswellingoftheformerwiththeﬂexibilityandmechanicalstabilityofthelatter.ThespeciﬁcprotonresistanceisplottedinFigure27asa−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→Figure26TEMmicrographsofpolymerblendmembranes(a,b)95wt%sulfonatedpolysulfone(ionexchangecapacity1.7meqg−1)and5wt%polybenzimidazoleatmagniﬁcation(a)6000×and(b)250,000×.(c)Polymerblendmembraneof79wt%sulfonatedpoly(etheretherketone)(IEC2.93meqg−1)and21wt%polybenzimidazole.Themeasuredeffectiveionexchangecapacitiesofmembranes(a,b)and(c)are1.19and0.96meqg−1,respectively.ReprintedfromReference(92).J.Membr.Sci.Copyright2001withpermissionfromElsevier.

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only..ylno esu lanosrepr oF .70/02/30 no anihC fo ygolonhceT & ecneicS fo tyisrevinU ybNON-FLUORINATEDPOLYMERMEMBRANES535

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Figure27Speciﬁcprotonresistancesofrepresentativepolymerblendmembranesat25◦C.Redrawnfrom(91).sPEEK:sulfonatedpoly(etheretherketone);PSU-NH2,orthosulfoneaminatedpolysulfone;sPSU,sulfonatedpolysulfone;P4VP,poly(4-vinylpyridine);PEI,polyethyleneimine;PBI,polybenzimidazole.

functionoftheionexchangecapacityforarangeofacid-baseblendmembranes.ThelowestresistanceatmoderateIECvaluesisshownbysPEEK-aminatedPSU,sPEEK-PBI,andsPSU-aminatedPSUblends(91).Thesecompositionsareofpar-ticularinterestfordirectmethanolfuelcellapplicationbecausethepermeabilityofmethanolisafactorof10–20(at40◦C)lowerthanthatofNaﬁon(94).Thisismostrelevanttofuelcelloperationatlow-currentdensities,whereelectroosmoticdragplaysalesserrole.Atacurrentdensityof300mAcm−2,aMEAofsPEEK-PBIgave500mVat110◦C,withlowmethanolcrossover(95).Higherpowerden-sities(∼0.25Wcm−2)areprovidedbyotherblends(sPEK-PBI-aminatedPSU,sPEK-PBI),butmethanolcrossoverisalsohigher.

ThePBIcomponentofblendmembranescanalsobeaciddoped.Aciddopingincreasestheconcentrationofprotonchargecarriers,butthebeneﬁtswithregardtomechanicalpropertiesconferredbytheionicinteractionarelostatlowtemperature.Infact,theconductivityofphosphoricacid–dopedPBIandsPSU-PBImembranes(dopinglevel500mol%)isessentiallyidenticalinadryatmosphereat175◦C(0.02Scm−1)(96).Indeed,thecurrentdensityversuscellvoltagecurveat190◦Conhydrogen/air(1barabsolutepressure)showssimilarbehaviortothatgivenbyaH3PO4-dopedPBIalone.Thelong-termadvantageofimprovedhigh-temperaturetensilestrengthconferredbythepresenceofthesulfonatedcomponentstillrequiresdemonstrationunderthesehigh-temperature/low-relativehumidityconditions.

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MICROSTRUCTUREOFNON-FLUORINATEDPOLYMERS

Descriptionofthemicrostructureofsulfonatedhydrocarbon–basedandperﬂu-orinatedsystemsismadeinsamegeneralterms,althoughthereareimportantdifferencesofdetail.Sulfonatedpolyimidesshowsomeparticularmicrostructuralfeaturesthatarediscussedbelow.Inalltypesofpolyelectrolytes,thesulfonicacidgroupstendtoclustertoagreaterorlesserextentintohydrophilicregionsthatarehydratedorsolvatedinthepresenceofwater,orwater/methanol,andthepolymerbackbonesegregatestoagreaterorlesserextentintohydrophobicregions.Thehydrophilicdomainsinterconnectandareresponsibleforthetransportofwaterandprotons;thehydrophobicregionsprovidemorphologicalstabilityandpre-ventdissolutioninwater.AsforNaﬁon,athree-regionmodelhasbeenproposedinwhichtheionicclusterscoexistwithamorphousandcrystallinehydrophobicdomains.InNaﬁon,thecharacteristicseparationlengthis5nm(solventvolumefraction0.4).Inhydrocarbon-basedpolymers,theloweracidityofthesulfonicacidgroups,theabsenceofﬂuorinatedgroups,andthegreaterrigidityofthepolymerbackboneconferredbythepresenceofaromaticgroupsallcontributetoreducingthenanoseparationofhydrophobicandhydrophilicdomains,whichinﬂuencestheprotontransportproperties,inparticularbyastrongdependenceofprotoncon-ductivityonwatercontent.Intheblockcopolymerapproachforthesynthesisofasulfonatedpolymer,theionicgroupsaregatheredinblocksalongthepolymerchainandareseparatedbylonghydrophobicsequences.Suchmolecularstructuralphaseseparationallowstheformationoflargerionicdomainsthanthoseinwhichsulfonicacidgroupsaredistributedrandomly.

Amodelbasedonacubicsystemofhydrophilicchannelsinahydropho-bicmatrixparameterizedusingdatafromsmallangleX-rayscattering(SAXS)andwaterself-diffusioncoefﬁcients(obtainedbypulsedﬁeldgradientNMR)ledtoadescriptionofthemicrostructureofsulfonatedpolyetherketones(sPEEKK).sPEEKKismadeupofhighlybranched,narrowchannelsthathavemoredead-endcomponentsthanthewider,less-branchedchannelsofNaﬁon.Themoreextensivehydrophobic–hydrophilicinterfaceregioninsPEEKKresultsingreaterseparationbetweensulfonicacidfunctionalgroups(6,7).Forbothtypesofpolymer,boththecharacteristicseparationlengthandthechanneldiameterincreasewithsolventvol-umefraction,whereasthedimensionoftheinternalinterfaceregionpassesthroughamaximumvaluewhetherinwater,methanol,orwater/methanol(M.Schuster,M.Ise,K.D.Kreuer,G.Gebel&JMaier,inpreparation).Suchobservationsarehighlyrelevanttothevariationofprotonconductionpropertiesofsulfonatedpoly(etherketone)swithhydration,aswellasotherfactorsrelatedtowaterandmethanolpermeationsuchaselectroosmoticdrag.Thereissimilaritybetweenob-servedconductionandswellingpropertiesofsulfonatedpolyphosphazenesandpoly(etherketone)sthatisreﬂectedatthemicrostructurallevel,withnanosepara-tionincreasingwiththedegreeofhydration.However,inaddition,diffractionin-tensityinsulfonatedpolyphosphazenesinthewide-anglerangepointstosometwo-dimensionalshort-rangeorderinthatisnotcompletelydisruptedbyswelling(55).

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Sulfonatedpolyimides(sPI)preparedbyblockcopolymerizationusingnaph-thalenicdianhydride,presentnowell-deﬁnedionomerpeakinsmall-angleneutronscattering(SANS),onlyabroadshoulderonanintensesmall-angleupturninin-tensity(77).ThisindicatesthatthedistributionorshapeoftheionicdomainsisdifferentfromthatofsPIpreparedfromaphthalicdianhydride(78)andfromthatofNaﬁonandsuggeststhatthepolymerchainrigiditystronglyinﬂuencesthemi-crostructure.ThisionomerpeakinnaphthalenicsPIissituatedatverylowangles,correspondingtoaseparationlengthof25nm,suchthatthemicrostructurecorre-spondstohydrophobicdomainsembeddedinahydrophiliccontinuousphase(77).TheevolutionofscatteredintensityathigherangleisnottypicallyPorod-type,pos-siblyduetotheexistenceofaninternalstructureintheionicdomainsorresultingfrominterdomainconnections.SAXSspectraofsulfonatedpolyimidesrecordedacrossthethicknessofthemembraneandparalleltotheplaneofthemembraneprovideevidenceforstructuralanisotropy(lamellarordisc-like),whichincreasesastheblockcharacterincreases(77).Suchmorphologymayarisefromaprefer-enceofthesolventforoneoftheblocks.Polymerblendsandgrowthofinorganicparticlesornetworksinsituinapolymersolutionprovideameansofmodifyingthepolymermicrostructurebecausethemembranestructurereformsatthesametimeastheprecipitationoftheinorganicand/ororganiccomponents(12).

STABILITYCONSIDERATIONSANDSWELLING

Sulfonationofthermostablepolymersisgenerallyaccompaniedbyanincreaseinglasstransitiontemperature,Tg.ThepresenceofpendantSO3Hgroupsproducessterichindrancetointersegmentalmotionandreducesmobilityandﬂexibility,andthesulfonicacidgroupsinteractandformstronghydrogenbondsthatincreasechainrigidity.Forexample,theTgofpolybenzimidazoleincreasesfrom417to447◦ConadditionofonebenzylsulfonicacidgroupperPBIrepeatunit;thatofsulfonatedpoly(ethersulfone)increasesasafunctionofthesulfonationlevelfrom227◦Cfornon-modiﬁedPESto333◦Cforamaterialinwhichessentiallyallrepeatunitsaresulfonated(37).

TheparticularlyaggressiveenvironmentinaPEMFCorDMFCcaninitiatedifferenttypesofdegradationmechanismsandagingprocessesthatresultinei-therchemicalormorphological/texturalalteration.Suchmodiﬁcationmightarisefromdesulfonation,chainscissioncausedthermohydrolytically,orbyfreeradi-calsgeneratedattheelectrodesorlossofmechanicalpropertiesowingtoexcessiveswelling.

ThermalandThermohydrolyticStability

Oneoftheﬁrstconsiderationsisthatofthethermalstabilityofhigh-temperaturepolyelectrolytesbasedonthePEKfamily,PSU,PI,PBI,etc.Thermogravimetricanalysesofsulfonatednon-ﬂuorinatedmembranesgenerallyshowaweightlossabovearound250◦C(inair,usingaramprateof1◦C/min),whichresultsfrom

NON-FLUORINATEDPOLYMERMEMBRANES539

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.condensationof–SO3HgroupsfollowedbylossofSO2,whereascombustionofthepolymeroccursatasigniﬁcantlyhighertemperature(>500◦C).Inadryoxidizingenvironment,thetemperaturerangeofapplicationofsuchmembranesisthuslimitedbytheonsetofalterationofthesulfonicacidfunctions.

However,asthesepolymerswereoriginallydevelopedforapplicationsinen-vironmentsdifferentfromthoseprevailinginaPEMfuelcell,i.e.,extremesoftemperatureandcorrosivity,butwater-free,theirstabilityunderthermohydrolyticconditions,inparticularofthesulfonatedform,isofprimeimportance.Poly-phenylenesulﬁde,poly(ethersulfone),poly(etherketone),andpolyimidefamiliesshowreasonablestabilityat300◦C,whilenoneofthepolymersisstableunderair/waterat400◦C(97).Underinertandsaturatedwatervaporconditionsatvar-ioustemperatures,thestabilityofsulfonatedderivativesofpoly(etheretherke-tone),poly(ethersulfone),polybenzimidazole,andpoly(phenylquinoxaline)canberepresentedbythetemperatureof5%weightlossbythepolymers(Figure28)

Figure28Temperatureof5%weightlossofnon-sulfonatedandsulfonatedpoly(etheretherketone),PEEK;poly(ethersulfone),PES;polybenzimidazole,PBI;poly(phenylquinoxaline),PPQ;polyimide,PI;poly(tetraﬂuoroethylene),PTFEandNaﬁonin(a)heliumand(b)saturatedwatervapor.ReprintedfromReference(98).Polym.Degrad.Stab.Copyright2000withpermissionfromElsevier.

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.(98).Whereasinadryenvironmentallofthepolymersretaintheirintegritytoatleast330◦C,theirstabilityisradicallydifferentundersaturatedwatervapor.Underinertconditions,thedecompositiontemperatureishigherforthesulfonatednon-ﬂuorinatedpolymersthanforNaﬁon.Undersaturatedvaporconditions,allmaterials(bothsulfonatedandnon-sulfonated)showsomedecompositionbe-tween150and200◦C,althoughthethermohydrolyticstabilityofthesulfonatedpolymersisalwayslowerthatthatofthecorrespondingnonmodiﬁedmaterial.Asageneralobservation,sulfonatedpolyaromaticpolymersareslightlymorestablethansulfonatedpolyheterocyclicsystems.Sulfonatedsamplessubmittedtother-mohydrolytictreatmentat225◦Callshowlossofsulfonicacidgroupstosomeextent.Notsurprisingly,desulfonationoccursforsPES,sPPQ,andsPEEKundersaturatedvaporconditions,andevidenceforhydrolysisofthediarylketonegroupisseenbyinfraredspectroscopyforhighlysulfonatedPEEK(98).TherelativeeaseofthedesulfonationreactionisacorollaryofthereadydirectsulfonationofPES,PEEK,andPPQforexampleusingsulfuricacid(seeabove).

Swelling

Whereasthegenerallywater-assistedmechanismofprotontransferrequiresadegreeofmembraneswellingforadequateprotonconductivity,uncontrolledwa-teruptakecanleadtomechanicaldegradation.Thehydrationnumber(λ,nH2O/nSO3H)ofsulfonatedpolyelectrolytemembranesdependsuponwateractivityandtemperature,anditstronglyinﬂuencestheirprotonconductivityanddimensionalstability.Thewateruptakeofasulfonatedpolymermembraneofagiventypeofpolymeratagiventemperatureincreaseswiththedegreeofsulfonation.Kreuer(6)hascarriedoutadetailedstudyofthehydrationbehaviorofsulfonatedpoly(etheretherketoneketone)(sPEEKK)ofionexchangecapacityintherange0.78–1.78meqg−1andofNaﬁon(IEC0.9meqg−1).Allthemembranesshowthreeregimes:oneinwhichthehydrationnumberisinvariantwithtemperature,oneinwhichthehydrationnumberincreasesveryrapidlyoveranarrowtemperaturein-terval,andatransitionalregionbetweenthesetworegimes.Thewateruptakeinliquidwaterisgenerallysigniﬁcantlyhigherthanthatobtainedbyconditioningunderhighrelativehumidity.Naﬁonshowssimilarbehavior(Schroeder’sparadox)athightemperatures,interpretedasthedifﬁcultyincondensingvaporwithintheporesofthemembrane.TheonsetofrapidswellinginliquidwateroccursatlowertemperaturesforsPEEKK(at65,85,and∼120◦CforsPEEKKofIEC1.78,1.4,and0.78meqg−1,respectively)thanforNaﬁon(145◦C),anditisirreversibletosomeextent,thusallowingacertaincontrolofthemaximumwateruptakeatalowertemperaturebyappropriatepreconditioning.Theprimaryhydrationsphereismadeupof3watermoleculesinNaﬁonand5insPEEKK,withmorelooselyboundwaterrepresentingafurther11and5molecules,respectively;thereafterwaterispresentasasecondphase(withλ>55).Attheonsetofextendedswelling,themembranesoftensandbecomesmechanicallyweak,andthisoccurrenceisoftenamajorcontributortomembranefailureinafuelcell.Athighwateruptake,profound

NON-FLUORINATEDPOLYMERMEMBRANES541

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.deformationofthehydrophilicregionsofthepolymerleadstoanirreversiblyswollenstatethatisaccompaniedbymarkeddecohesionanddisentanglementofthepolymerchains,exacerbatedbythelackofcrosslinkingandtherigidnatureofthesefamiliesofpolymer.

SwellingofsulfonatedpolymermembranesindilutemethanolsolutionshasalsotobeconsideredinthecontextofapplicationinDMFC.Sulfonatedpoly(ether-ketone)sofhighdegreeofsulfonationaresolubleinconcentratedaqueoussolu-tionsofmethanol.Evenatlowerconcentrations,thehydrophobicandhydrophilicmoietiesofmethanolinteractwiththecorrespondingregionsofsulfonatednon-ﬂuorinatedpolymers,tendingtomodifythemicrostructurebydistendingthechan-nelsandleadingtoincreasedswelling.Thewater/methanolcontentofsPEEKmembranesisthesameasthatofthesolutioninwhichthemembranewasim-mersed,showingtheabsenceofanyconcentrationordilutioneffectandsuggestingthatbothwaterandmethanolarelocatedinthesameregionofthepolymer(7).Differentapproachestolimitingswellinghavebeendevelopedthatincludecross-linkingandblendingandinclusionofinorganiccomponentsabletoreinforcethepolymerthroughionicinteraction(99,11).

PossibleDegradationbyRadicalSpecies

SpeciessuchasHO•andHO2•couldarisefromoxygendiffusionthroughthemembraneandincompletereductionattheanode,andpossibledegradationmech-anismsinvolvingoxidizingspeciesandhydroxyradicalsmustalsobeconsidered.Comparativeassessmentofmembranestabilitytowardoxygen-containingradicalscanbeapproachedusingtheFentonreactionmedium[H2O2/Fe(II)]andcharacter-izingmembranephysicalintegrity,mechanicalproperties,ionexchangecapacityandconductivity,etc.withtime.ThemechanicaldegradationandpartiallossofIECofsPEEKinsuchamediumdependstronglyontheconcentrationofhydro-genperoxideand,toalesserextent,onthedegreeofsulfonationofthepolymer.Non-sulfonatedPEEKmembranesareunaffectedbyH2O2/Fe(II).Therealprob-lemofevaluatingthestabilityofmembranestosuchradicalattackliesinthedifﬁcultyinrelatingsuchexsitu“acceleratedaging”teststoinsituaginginafuelcell,insofarastheconditionsunderwhichsuchradicalspeciesareformedinthefuelcell,andtheirconcentrationareunknown(82).ThuswhereassPEEKmem-branescanfunctionforhundredstothousandsofhoursinafuelcell(J.Rozi`ere,D.J.Jones,B.Bonnet&B.Bauer,unpublisheddata;51),ithasbeenshownthatsimilarmembranesbecomebrittlewithpartiallossoftheirIECafter4to8hinFentonreactionsolutionsof3%H2O2/1ppmFe(II)at68◦C(82).Forsulfonatedstyrene(ethylene-butene)styrene(DAISprotonexchangemembrane),exsitutestswereusedtopredictalifetimeat60◦Cof2500h(100).

ElectronparamagneticresonancespectroscopycouldbeapromisingtoolforidentifyingdegradationproductsresultingfromHO•radicalattack.Ithasbeenusedwithavarietyofsulfonatedaromaticmolecules,modelsforsulfonatedhy-drocarbonpolymerssuchasPEEK,polystyrene,orPES(101).Thedominating

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reactionistheadditionofHO•tothearomaticrings,preferentiallyintheorthopositiontoalkylandRO-substituents(sincetheparapositionissubstitutedandthusblocked).Forpolymerswithbenzylicα-Hatoms,acid-catalyzedeliminationofwateraccordingtoFigure29canleadtochainscission,inamechanismrelevantforanybenzyl-graftedpolymer.MethoxybenzenesulfonicacidunderwentlossofOCH3,believedtobeinitiatedbyipsoattackofHO•.Althoughthismechanismwouldbeofrelevanceforpolymerssuchaspolysulfoneorpoly(etherketone)hav-ingphenoxybenzeneetherbridgeswhereitcouldleadtobondbreakingwithintheC-O-Clinkages,phenoxyradicalswerenotobservedinelectronparamagneticres-onanceexperimentsonmediaofpHvalueslowerthan5.Theapplicationofsuchanapproachtorealprotonelectrolytemembranesisofobviousinterest,aswellasthedevelopmentoftechniquesenablingthedetectionofradicalspeciesinsituinordertobeabletoassessthepotentialgravityofradicalattackinaworkingfuelcell.

WATERDIFFUSIONANDPROTONCONDUCTIVITY,ELECTROOSMOTICDRAG

Forsulfonatedpolyaromaticmembranessuchaspoly(etherketones)(aswellasthecorrespondingionicallyandcovalentlycrosslinkedcongeners),thewaterself-diffusioncoefﬁcientDH2OforagivenwatercontentislowerthanthatofNaﬁon,

NON-FLUORINATEDPOLYMERMEMBRANES543

Figure30Waterself-diffusioncoefﬁcientsasafunctionofwatervolumefractionin(a)Naﬁonand(b)sulfonatedpoly(etherketone)membranes(M.Schuster,M.Ise,K.D.Kreuer,G.Gebel&J.Maier,inpreparation).(

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.Kdrag,isdeﬁnedasthenumberofwatermoleculestransferredthroughthemem-braneperprotonand,forhigh-temperatureoperationofPEMFCandDMFC,Kdragshouldbesmallinordertoavoidanincreaseinmembraneresistancecausedbyde-pletionofwaterattheanodeside.ItsmagnitudecanbedeterminedindifferentwaysincludingelectrophoreticNMR(105).TheKdragofacid-dopedpolybenzimidazoleisclosetozero,thusenablingaPBIfuelcelltooperateathightemperatureandlowgashumidiﬁcationwithoutdryingoutofthemembrane(106).However,theparticularmicrostructureofpolyaromatichydrocarbonpolymersconfersadistinctadvantagewithregardtoelectroosmoticdragandwaterpermeation,parametersthatdependdirectlyonthesizeofthehydrophilicchannels.Evenatlowdegreesofhydration,Kdragisnolowerthan1,sincethiscorrespondstothelowestprotonhydrate,H3O+.Undertheseconditions,theprotontransferproceedsthroughdis-placementofthehydroniumion[vehiclemechanism(107)].Ingeneral,theKdragincreaseswithincreasingmembranewatercontent(102).Thenarrowchannelsofsulfonatedpoly(etherketone)slimitcollectiveﬂowofwaterthroughthemem-brane,andasaresult,theKdraginsPEEKKislowerthaninNaﬁonbyafactorof2to3(Figure31).Thisdiscriminationislostatveryhighwatercontent,whenKdragforNaﬁonandpoly(etherketone)-basedmembranesarealmostidentical.Wa-terpermeationdecreasesmorestronglywithdecreasingwatervolumefractioninsulfonatedpoly(etherketone)sthaninNaﬁon,butathighwatercontentsitisa

Figure31Electroosmoticdragcoefﬁcientof(a)Naﬁonand(b)sulfonatedpoly(etherketone)membranesobtainedfromelectrophoreticNMRasafunctionofthehydrationnumber(M.Schuster,M.Ise,K.D.Kreuer,G.Gebel&J.Maier,inpreparation).

NON-FLUORINATEDPOLYMERMEMBRANES545

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.factor2lowerthanthatofNaﬁon.Waterpermeationisaffected,amongotherfactors,bychanneldiameterandthestrengthofthepolymer-waterinteraction(7).Thestronglyincreasedswellingofsulfonatedpoly(etherketone)sinwater/methanolmixturesmodiﬁestheKdrag,whichincreaseswithincreasingmethanolconcentration,furtherjustifyingtheapproachesofpolymerblendingandcrosslink-ing.Thewaterandmethanoldiffusioncoefﬁcientsofsulfonatedpoly[bis(3-meth-ylphenoxy)phosphazene]membranesmeasuredbydesorptionkineticsandtracerdiffusionNMRaremorethananorderofmagnitudelowerthanthoseofsulfonatedpoly(etherketone)sandalmosttwoordersofmagnitudelowerthanthoseofNaﬁon(58,108).Ontheotherhand,asthemicrostructureofsulfonatedpolyphosphazenesandpoly(etherketone)sshowmanysimilarities,furtherexperimentalworkisstillneededtoclarifytheoriginofanydifferencesinthetransportbehaviorbetweensPPZandothersulfonatednon-ﬂuorinatedpolymers.

FUELCELLPERFORMANCEOFNON-FLUORINATEDMEMBRANES

Theevaluationofsulfonatedhydrocarbonpolymersinfuelcellsisstillinanearlystage,butresultspublishedbyvariousgroups,inparticularonsulfonatedpoly(etherketone)sandsulfonatedpolysulfonesandblendsthereof,arestartingtoconstituteaperformancedatabasethatcanbecomparedwiththeearlierpublishedresultsontheBallardAdvancedMaterialsﬁrstandsecondgeneration(BAM1GandBAM2G)membranes,correspondingtosulfonatedpoly(phenylquinoxaline)andpoly(2,6-diphenyl-4-phenyleneoxide).Eachtypeofsulfonatedmembranewillhaveanoptimaloperatingwindowoffuelcelltemperatureandpressurewithinwhichthemembranelifetimeishighest,andevenasmalldeviationfromtheseconditionscanleadtodrasticreductionofmembranedurability.Itshouldbeborneinmindthatcellassemblyparameterssuchasgasketdesignandcompression,forexample,canbethecauseofprematuremembranefailure.Inthisrespect,muchoffuelcellhardwarewasdevelopedforNaﬁonmembranesandrequiresadaptingforoptimallifetimeofnon-ﬂuorinatedsystemsinordertotakeintoaccounttheirparticularproperties.

SulfonatedsPEEKmembraneshavebeentestedinbothhydrogenandoxygen(air)indirectmethanolfuelcells.Usinghydrogen,foramembraneofionex-changecapacity1.6meqg−1and18µmthickness,thecellvoltageat0.5mAcm−2at90◦Cis0.8Vand0.72Vwithoxygenandair,respectively(44).Suchthinmembranesthatareneithercrosslinkednorreinforcedwillprobablynothavesuf-ﬁcientmechanicalstabilityinthefuelcell.sPEEKmembranesof70µmthicknessgavehigherperformanceat85◦CthanNaﬁon-115underthesametestconditions,inparticularathighcurrentdensitieswhenthemembranecandehydrate(44).Asdescribedabove,theelectroosmoticdragofsulfonatedpoly(etherketone)sislowerthanthatofNaﬁonatthistemperature,whichshouldreducedrying-outattheanodeside.Patentreportsclaim4000hoffunctioningofsPEEKwithIEC

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Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.1.47meqg−1atamodestcelltemperatureof50◦C(51),givingamaximumpowerdensityof0.386Wcm−2at0.52V,whereasat90◦Catconstantcurrentden-sityof0.5mAcm−2,sPEEKhasattained1000hoffunctioningwithoutfailure(J.Rozi`ere,D.J.Jones,B.Bonnet&B.Bauer,unpublisheddata).Insingle-celltest-ing,theParmaxTM-2000membranegives0.8Vat0.8mAcm−2onhydrogen–airat90◦C,withalifetimeof200h(V.J.Lee,privatecommunication).

Undersimilaroperatingconditions,acellvoltage≤0.3Visgivenbysulfonatedpolysulfoneat0.5mAcm−2,forwhichpoorelectrode/electrolyteinterfacemaybeatleastpartiallyresponsible(39)bycontributingahighohmicresistance.Indeed,forsulfonatedpolyimidemembranes,someimprovementinfuelcellperformancewasobservedwhenimpregnatingtheelectrodeswithamixtureofNaﬁonandsulfonatedpolyimidesolutionstoimprovecompatibility.At70◦CinanH2−O2fuelcell,naphthalenicsPIwithIEC1.26and1.98meqg−1provided0.5and0.6V,respectively,at0.5mAcm−2(76),andalifetimeof3000hhasbeenobtainedat60◦C,0.25mAcm−2,withanoutputofabout0.63V(78).However,membranesofhighIECarehydrolyticallyunstable.

InDMFC,themostpromisingfuelcellperformanceisbeingobtainedonmem-branesbasedonsulfonatedpoly(etherketone)sandtheirblends,andrecentresultsindicatepowerdensityasgoodasthatgivenbyNaﬁon,withthedifferencethatpol-yaromaticmembranescanberestartedoverseveralweekswithoutdegradationattemperature>100◦C.Thusnon-blendedsPEEK(60µmthickness)givesapowerdensityof150mWcm−2at120◦C,whereasansPEEK-PBI-PSU-NH2blendhasgiven250mWcm−2at110◦C(95).TheDMFCoperatingat120◦CwithsPEEKmembranescanbeshutdownandstartedupwithoutachangeinperformanceoveraperiodofmorethantwomonths(M.Dupont,J.Rozi`ere,D.J.Jones&B.Bauer,unpublisheddata).

CONCLUSIONSANDPERSPECTIVES

Manysulfonatednon-ﬂuorinatedpolymershavebeenreported,andmuchoftheavailabledatapointtohighprotonconductivityforthemajorityofthesystemswhenhydrated.Forfuelcellapplication,however,andaspointedoutabove,thesatisfactoryprotonconductivityconferredbyanadequatelevelofsulfonationmustnotbetothedisadvantageofmembranemechanicalstrengthandswelling,sincethesepropertiesareessentialforlong-termfuelcelloperation.Compari-sonbetweensystemsisdifﬁcultbecausemeasurementsareoftennotmadeun-derthesameexperimentalconditions.Thisisparticularlytrueforsingle-cellfuelcelltestingbecauseofthemanyvariablesinﬂuencingfuelcellperformance.Tables2and3summarizerecentdatareportedonarangeofcandidatefuelcellmembranes,intermsofconductivity,hydrationnumberandfueltestperformance,andlifetime.Mostsystemsarewell-characterizedfortheirprotonconductivityunderarangeoftemperatureandrelativehumidityconditions,withmostfre-quentexperimentationonfullyswollenmembranes.Undertheseconditions,most

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.TABLE2Summaryofconductivitypropertiesandhydrationnumberofnon-ﬂuorinatedpolymermembranesConductivityat100◦C,100%RH(Scm−1)ApplicationtypePolymerIEC(meqg−1)Hydrationnumberat25◦C(membranesswolleninwater)molH2O/SO3H2015N/A9cN/A17N/A5–135N/AN/A0.04N/A0.07–0.10N/AN/AN/AN/AefHydrationnumberat100◦C(membranesswolleninwater)molH2O/SO3H5237N/A0.010.01(80◦C)0.08–0.170.01–0.05H2-O2DMFCN/AaReference(6,44)(27,42,45)(49)(47)dSulfonatedpoly(etheretherketone)1.61.30.6–2.22.48H2-airDMFCH2-O2H2-O2H2-O2H2-airH2-O20.040.010.02–0.040.09b2.0155–405–10569Sulfonatedpoly(4-phenoxybenzoyl-1,4-phenylene)dSulfonatedpolysulfone1.10.41–2.20.92–1.61(39)(71)(38)(100,109)(110)(111)(112)Sulfonated(styrene/ethylene-butylene/styrene)1.78Sulfonated(butadienestyrene)1.85Sulfonated(ethylenestyrene)18–30191910cN/A20c12c,g33–51mol%sulfonationSulfonatednaphthalenicpolyimides1.261.631.98N/A1.6–2.70.004–0.020.0140.0370.8(75◦C)0.09–0.21(80◦C)0.09(65◦C)0.06(65◦C)H2-O2(76–78)(77)(76)(85)(88,90)DMFC(55,59)(Continued)NON-FLUORINATEDPOLYMERMEMBRANES

Sulfonatedpoly[bis(3-methylphenoxy)phosphazene]1.2547

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ecneref))))e1322R6699((((noitaciC2r2lpeFOiaOpp---AyM222tDHHH)%C◦y0)5t01.2yiv1−()l,nit6CocCm0◦.5 ◦ceu0S02sd0(–(un14633 lo0000a....nCtHaR0000osresper rnoeaFbr) .7mbre0umtH3/neaO02nmw(S/o/3itCniO0a◦n2 nr0eHod0lll y1ooAAAAaHt///nawsm/NNNNihC sfore enyba)gromrbetHloua3nnmewOhnmco(nS/eiiTtnO2 aCe&r◦lH3l d5ol2ey2wo.AcnHtaa/smcN9eicS)13 f−4og. 1–ty7986iCqes1952rEm....eI(1011vinUed ynnbeeldzbnaeheldpnbesozofellilhuoapsznyyaolldiooit))csppmidneddzeuneenufeztndziaealnbiiaiytchnnapsomloocoiapt--Cinhc(opnKKhyuEEfEEpx2sePPooldErhyiddLeprmeettlaiaaBmy(nnnynyloooleoffflllTAohPpuuuPSSS)59),)3261991(((lCC2FFMOM-2DDHQ)k2H.0R5–12%000..008(AA//NN.)remrofer.l.2do8)tnn1jeeal(Nbht-feeomnenloa.ayitnimit1aeso78dhorppf..-00de4mylth,ocs1cei.lebC=nriodnu◦op0Rsnde5fn4,ng9l)12eAus%u,oRoprrt.)ed-y0eyel5utCHdy.ilo◦op–aBdi0OtChz58.(n◦d.CCe5ead7tde(Bmua◦2m5–notritmae&h%2ppua.ofnle.2tomiszooeivn2aro%gneirnfzlantaoi19(.gceudolt–%eee1pllJeabsirc62–oe.i3nnn0dcyJ–ezoofl14l.ldm1ahfOeoeiD%n55uf1htuisoPu3fppz,7mo))p)-onetKdebrmwwswylowHloe`ao//o/fiEdyzgcwwhdppedwona.lEilcoonie%yﬁilet%eheltebPapRttaa%((oda(-evJe.dc)eponelmali,hHiverkdmoktaeir%taeflancvtnrRpkatttoeaaho0oo,ipkUupsritndh5dpr%epuunSsidlPfuonp–nup0erorndo-nfes:sei:les5eDre0Lretulolt1.Jtbh2ao5Cetps(bSPMfataaV.wabcdewr9eaAcAEaIwdu/fghijkqlNAnnu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgAnnu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.TABLE3Summaryoffuelcellperformancepropertiesofnon-ﬂuorinatedpolymermembranesPolymerTemperature/MembranepressureatIECApplicationthicknessanode/cathode−1(meqg)type(µm)(◦C/bar/bar)Currentdensityat0.7V(mAcm−2)Maximumpowerdensity(Wcm−2)Conditionsofcurrentdensity/temperature/pressureatLifetimeanode/cathode(h)forlifetestReferenceSulfonatedpoly(etheretherketone)0.05V,50/1/1a1.61.61.343001440a25–5025–5020–100N/A60607070/4/41486080040(at0.5V)70/4/4500N/A45/1/1135e0.580/4/4dN/A80400N/A200N/AN/A500N/AN/AN/AN/A0.05N/AN/A300011090H2-O2(6,44)(51)bDMFC70406085/2.6/2.645/1/1120/1/3600410135(at0.5V)0.6N/A0.14(120◦C)Sulfonatedpoly(4-phenoxybenzoyl-1,4-phenylene)2.0H2-airDMFC0.8V(atN/A200−20.8Acm)360(at0.5V)0.30(110◦C)N/A1Acm−2at80◦C,(47)cH2-O2cN/AN/A0.25Acm−2at80/4/4N/AN/A0.25Acm−2at60/3/3(39)(71)(38)(100,109)(110)(111)(78),(76,77)(77)(76)(55,59)Sulfonatedpolysulfone1.1H2-O20.41–2.2H2-O20.92–1.61H2-O2Sulfonated(styrene/ethylene-butylene/styrene)1.78H2-airSulfonated(butadienestyrene)1.85H2-O2327(at0.5V).164fSulfonatednaphthalenicpolyimides1.261.631.98H2-O2NON-FLUORINATEDPOLYMERMEMBRANES

Sulfonatedpoly[bis(3-methylphenoxy)phosphazene]N/AN/A1.2DMFCgPhenylphosphonicacid1.17–1.43DMFCfunctionalizedpoly(aryloxyphosphazenes)N/AN/AN/AN/A(61)549

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N/AN/AN/AN/A(63)(92)(92)(92,95)Conditionsofcurrentdensity/temperature/pressureatanode/cathodeforlifetestReference󰀁

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N/A(96)N/A(113)g−1..ylno esu lanosrepr oF .70/02/30 no anihC fo ygolonhceT & ecneicS fo tyisrevinU yb)deunitnoC(3ELBTAqeemmi5tef)AAAAAA1.1i//////Lh(NNNNNNyticam)pau2cm−erytgiimxescnawn735aMoe4AA535h.//2..cpdW(0NN.000xe-n)oi)Ve2−5iv.ttcn0yVmee0ttffrica5s7e.(r1un0A0005–0hCet200702tidam(264453wdn/eeedlro)1but/eltahrat51iaab.5/r/trec1.01ierr/9ea5/2/npu52/sdb/./411olms–o/312//ye00rnC///0Ter050130capa◦(867112y.l)orepen–m]rasesrnofben00eemk)11zral11h.2oecim0––p)tnMhµ097000soea(ht(123688hpNte-)enymoxnea.ioltj.aCadinymtenocacherfi2r2C2FdalpeOiaOFOMphldcpyiy-ltpp----en222M2Doh4ct,AytHHHDHQhse1erilmibtmi-=d)un1pa3Rne−no(s,gguhipb2,Rorrs[ieoy)dlCqe9861uhoHy.Em95278app.....BOth(nI(01100.%dCedeBtewk–nomel&n7ipprluos.somofengsroerddznonoiirgc(eedaotnceellztJaiian5dcadnne.itlne9iaJoocfllil..nne.tnnodahtueombAde-enmieepofzoDuceisui,miteﬁnoiwyflltea-d-l-lolnaermncflcnodueraeudooneKneKozKz–see`ior.epn.uzElEaEaybpzcbnpsﬁdaefabddlydiiolnooemosaiitdehlttbehEeEdiEsRtaetondpPnPmPmcapoepliuoaeaisdodidcd-b))ioJiv,rmacnomnmliotaroefezezretutpﬁeaeirtemhtltntnnnt,UflcviduaitmitoesidSueanpauaeaehoa%%sdhn00npeomarPsritfdoyoynynbnbnpo55ouLpueC:o-inflooyoyf––flflflclD.5ololololoesl55uulosJmhpFi9Cs:aPppupupbhu27SSSSPs((dMV.oomMceAEuA/abcdnetDfghIqijNAnnu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgNON-FLUORINATEDPOLYMERMEMBRANES551

ofthepolymersystemsofTable2displayaconductivity>10−2–10−1Scm−1.Theconductivityissigniﬁcantlymoredependentonhydrationnumber/relativehumiditythanontemperature.Thedecreaseinconductivitywithdecreasingwatercontentisadirectresultoftheinherentlyloweracidityandnanoseparationofpolyaromaticpolymerscomparedwithperﬂuorosulfonicacidsystems.Fragilityinthedrystateandincreasingsoftnessanddeformabilityinthewetstatecantosomeextentbecircumventedbycrosslinkingorbyotherapproachessuchaslaminationandblendingwithotherpolymers,orbyincorporatingaphysicalsup-port.Advantageoverperﬂuorosulfonicacidsystemsisprovidedinfunctionalizedpolybenzimidazoleandinsulfonatedpoly(etherketone)sbytheirlowerelectro-osmoticdragcoefﬁcient,whichactstodiminishtheproblemofmembranedry-outinanoperatingfuelcell.Littledataareavailableonconductivitymeasurementsabove100◦C,althoughthoseonacid-dopedpolybenzimidazoleandonsulfonatedpoly(etheretherketone)areimportantexceptions.Tables2and3alsohighlightthelackofpublishedinformationonlong-termfuelcelloperation,inparticularabove80◦C.Suchmeasurementsaretime-consuming,andthestabilityofotherfuelcellcomponentscanbecomeadominantsourceofperformancedegradation.Thede-velopmentofageneralizedandviablemeansofexsituprescreeningexperimentswouldbebeneﬁcialinassessingthesuitabilityofamembraneforfuelcelluse.ThedataofTables2and3identifythosesystemsinwhichmostcompletechar-acterizationhasbeenmade.ApartfromNaﬁonandsimilarperﬂuorosulfonatedpolymermembranes,long-termfuelcellfunctioningat50–80◦Chassofarbeenpublishedonalimitednumberofpolyaromaticandpolyheterocyclicsystems:3000hat70◦Cforasulfonatedpolyimide,and>4000hforsulfonatedpoly(etheretherketone)at50◦C.Foroperationabove80◦C,membranesystemsbasedonsulfonatedpoly(etherketone)sandfunctionalizedpolybenzimidazoleareamongthemostpromising.Thelifetimesmentionedaboverefertohydrogen–oxygen(air)operation;untilrecentlythelackofmembranessuitablefordirectmethanolfuelcelloperationprecludedanylifetesting.Theencouragingperformancetestsrecentlyobtainedwithacid-baseblendmembranesinDMFCshouldbecompletedbydurabilitytesting.Start-up/shutdownovertwomonthsofdiscontinuoustestingat120◦C,usingamembraneelectrodeassemblybasedonsulfonatedpoly(etheretherketone)ofactivearea400cm2,providedasetofencouragingdataintheﬁeldofDMFC.

Reportedlife-testexperimentsareperformedatdifferenttemperaturesandpres-suresanddifferentcurrentdensities(orcellvoltages).Ultimately,oneoftherolesofthelife-testistoidentifytheoperationconditionsgivingoptimalper-formance/lifetimeforagivenMEA;aspeciﬁcpolymermembranewillbebestappliedwithinaparticulartemperaturerangeandassociatedwithaparticulartypeofapplication(residential,transport,portable)andfuel(hydrogen,methanol).Thusaformofacceleratedagingissimplytouseexperimentalconditionslyingout-sidethedeﬁnedoptimaloperationwindow,butunfortunately,noneofthesystemsdescribedinthisreviewhasbeensufﬁcientlycomprehensivelystudiedtoenablelegitimateuseofthisapproach.

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Polymersfunctionalizedotherthanwithsulfonicacidgroups(phosphonic,sul-fonamide),aswellassulfonatedpolymersbuiltfrommonomerunitswithsulfonic

acidgroupplacedsuchastolimithydrolyticattack,areallrecentandsigniﬁcantdevelopments.Finally,itisexpectedthatthemostdurable,performing,andvi-ablemembraneswillresultfromﬁne-tuningthechemicalcompositionandthearchitectureofthepolymer,andtailoringofthemembranemicrostructure.Thedevelopmentofalow-costandreliablehigh-temperaturemembranerepresentsatechnologicalbreakthroughwithimportantimplicationforstationaryand,morecritically,transportapplications.

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgby University of Science & Technology of China on 03/20/07. For personal use only.ACKNOWLEDGMENTS

StimulatingdiscussionandfruitfulcollaborationovermanyyearswithBerndBauerofFuMA-TechGmbH,EnricoRamunniandRubenOrn´elas(NuveraFuelCells),GiulioAlbertiandMarioCasciolaoftheUniversityofPerugia,andKlaus-DieterKreueroftheMax-Planck-Institutf¨urFestk¨orperforschung,Stuttgart,areacknowledgedwithfriendlythanks.

TheAnnualReviewofMaterialsResearchisonlineat

http://matsci.annualreviews.org

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CONTENTS

MATERIALSFORFUELCELLS

.ylNewMaterialNeedsforHydrocarbonFuelProcessing:Generatingno eHydrogenforthePEMFuelCell,R.Farrauto,S.Hwang,L.Shore,su lW.Ruettinger,J.Lampert,T.Giroux,Y.Liu,andO.IlinchanosOxide-IonElectrolytes,JohnB.Goodenough

rep rCompositeMembranesforMedium-TemperaturePEMFuelCells,oF .G.AlbertiandM.Casciola

7/00Methanol-Resistant,Oxygen-ReductionCatalystsforDirectMethanol2/30FuelCells,A.K.ShuklaandR.K.Raman

no aSupportedElectrolyteThinFilmSynthesisforSolidOxideFuelCells,inhLutgardC.DeJonghe,CraigP.Jacobson,andStevenJ.Visco

C foRecentAdvancesinMaterialsforFuelCells,N.P.Brandon,S.Skinner,and ygB.C.H.Steele

loonhAnhydrousProton-ConductingPolymers,MartinF.H.SchusterandceTWolfgangH.Meyer

& eProtonConductionMechanismsatLowDegreesofHydrationinSulfoniccneiAcid-BasedPolymerElectrolytes,S.J.Paddison

cS foConversionofHydrocardonsinSolidOxideFuelCells,MogensMogensen tyandKentKammerisrevProton-ConductingOxides,K.-D.Kreuer

inU ySolidOxideFuelCellCathodes:PolarizationMechanismsandModelingboftheElectrochemicalPerformance,J¨urgenFleig

Non-FluorinatedPolymerMaterialsforProtonExchangeMembraneFuelCells,JacquesRozi`ereandDeborahJ.Jones

NewElectrocatalystsbyCombinatorialMethods,EugeneS.SmotkinandRobertR.D´ıaz-Morales

UnderstandingMaterialsCompatibility,HarumiYokokawa

CURRENTINTEREST

InterfaceFracture,MichaelLane

Solid-StateReactivityatHeterophaseInterfaces,MonikaBackhaus-Ricoultvi

191129155169183233289321333361503557581

2955

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.orgCONTENTSAtomScaleInvestigationofImpuritySegregationtoCrystalDefects,EmmanuelCadel,AnnaFraczkiewicz,andDidierBlavetteLow-EnergyElectronMicroscopyofSurfacePhaseTransitions,JamesB.HannonandRuudM.TrompMaterialsDesignfortheNextGenerationThermalBarrierCoatings,DavidR.ClarkeandC.G.Levi

ScienceandTechnologyoftheTwenty-FirstCentury:Synthesis,

Properties,andApplicationsofCarbonNanotubes,MauricioTerrones

.yINDEXES

lno SubjectIndex

esu CumulativeIndexofContributingAuthors,Volumes29–33lanoCumulativeIndexofChapterTitles,Volumes29–33

srep roERRATA

F .7AnonlinelogofcorrectionstoAnnualReviewofMaterialsResearch/002chapters(ifany,1997tothepresent)maybefoundat/30http://matsci.annualreviews.org/errata.shtml

no ainhC fo ygloonhceT & ecneicS fo tyisrevinU ybvii

215263383419

611645647

Annu. Rev. Mater. Res. 2003.33:503-555. Downloaded from arjournals.annualreviews.org

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