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Progress in Polymer Science, 36, 11, pp. 1443-1498, 2011-11-01

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Sulfonated hydrocarbon membranes for medium-temperature and

low-humidity proton exchange membrane fuel cells (PEMFCs)

Park, Chi Hoon; Lee, Chang Hyun; Guiver, Michael D.; Lee, Young Moo

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ProgressinPolymerScience36 (2011) 1443–1498

ContentslistsavailableatScienceDirect

Progress

in

Polymer

Science

jo u r n al h om ep a ge : w ww . e l s e v i e r . c o m / l o c a t e / p p o l y s c i

Sulfonated

hydrocarbon

membranes

for

medium-temperature

and

low-humidity

proton

exchange

membrane

fuel

cells

(PEMFCs)

Chi

Hoon

Park

a,1

,

Chang

Hyun

Lee

b,1

,

Michael

D.

Guiver

a,c

,

Young

Moo

Lee

a,∗ aWCUDepartmentofEnergyEngineering,HanyangUniversity,Seoul133-791,RepublicofKorea

bMacromoleculesandInterfacesInstitute,VirginiaPolytechnicInstituteandStateUniversity,Blacksburg,VA24061,USA cInstituteforChemicalProcess&EnvironmentalTechnology,NationalResearchCouncil,Ottawa,Ontario,K1A0R6,Canada

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received1February2011

Receivedinrevisedform27March2011 Accepted27March2011

Available online 6 June 2011 Keywords:

Protonexchangemembranefuelcell (PEMFC)

Sulfonatedhydrocarbonpolymers Sulfonation

Polymerarchitecture

Physico-chemicaltuningtechnology

a

b

s

t

r

a

c

t

Thisreviewsummarizeseffortsindevelopingsulfonatedhydrocarbonprotonexchange membranes(PEMs) withexcellentlong-termelectrochemicalfuelcellperformancein medium-temperatureand/orlow-humidityprotonexchangemembranefuelcell(PEMFC) applications. Sulfonated hydrocarbonPEMsare alternativestocommercially available perfluorosulfonicacidionomers(PFSA,e.g.,Nafion®)thatinevitablyloseproton

conduc-tivitywhenexposedtoharshoperatingconditions.Overthepastfewdecades,avariety ofapproacheshavebeensuggestedtooptimizepolymerarchitecturesanddefine post-synthesistreatmentsinordertofurtherimprovethepropertiesofaspecificmaterial. Strategiesforcopolymersynthesesaresummarizedandfuturechallengesareidentified. Researchpertainingtothesulfonationprocess,whichiscarriedoutintheinitial hydro-carbonPEMfabricationstages,isfirstintroduced.Recentsyntheticapproachesarethen presented,focusingonthepolymerdesigntoenhancePEMperformance,suchashigh pro-tonconductivityevenwithalowionexchangecapacity(IEC)andhighdimensionalstability. Polymerchemistrymethodsforthephysico-chemicaltuningofsulfonatedPEMsarealso discussedwithintheframeworkofmaximizingtheelectrochemicalperformanceof copoly-mersinmembrane-electrodeassemblies(MEAs).Thediscussionwillcovercrosslinking, surfacefluorination,thermalannealing,andorganic–inorganicnanocompositeapproaches. © 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction... 1445

2. SynthesisofsulfonatedhydrocarbonPEMs... 1447

2.1. Sulfonation... 1447

2.1.1. Polymersulfonation... 1447

2.1.2. Directcopolymerizationofsulfonatedmonomers... 1454

2.2. Recenthigh-performancesulfonatedhydrocarbonPEMs... 1457

2.2.1. Introductionoffunctionalgroups ... 1457

2.2.2. SulfonatedhydrocarbonPEMswithhighfreevolume... 1460

2.2.3. Hydrophilic–hydrophobicmultiblockcopolymers... 1462

Correspondingauthor.Tel.:+82222200525;fax:+82222915982. E-mailaddress:ymlee@hanyang.ac.kr(Y.M.Lee).

1 Theseauthorscontributedequallytothiswork.

0079-6700/$–seefrontmatter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2011.06.001

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2.2.4. Graftedorbranchedsulfonatedhydrocarboncopolymers... 1464

2.2.5. SulfonatedhydrocarbonPEMswithhighIEC... 1467

2.2.6. SulfonatedhydrocarbonPEMsbasedonhighlysulfonatablemonomers... 1472

2.2.7. PropertiesofsulfonatedhydrocarbonPEMs... 1474

3. Physico-chemicallytunedsulfonatedhydrocarbonPEMs... 1480

3.1. TunedsulfonatedhydrocarbonPEMs... 1480

3.1.1. CrosslinkedPEMs... 1480

3.1.2. SurfacefluorinatedPEMs... 1483

3.1.3. ThermallyannealedPEMs. . ... 1484

3.2. Composite-typePEMs. . ... 1487

3.2.1. Organic–inorganiccompositePEMs... 1487

3.2.2. ReinforcedsulfonatedhydrocarbonPEMs... 1490

4. Conclusionsandoutlook... 1492

Acknowledgements... 1493

AppendixA. Supplementarydata... 1493

References... 1493

Nomenclature

6F-BPA 4,4′-hexafluoroisopropylidenebisphenol

6FCN disulfonated poly(arylene ether

benzoni-trile)copolymercontaining6F-BPA

AFM atomicforcemicroscopy

BAPFDS 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonicacid

BDSA 4,4′-diamino-biphenyl2,2-disulfonicacid

BES

N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonicacid

BPS biphenol-based SPAES copolymer in salt

formsynthesizedbyMcGrathetal.

BPSH biphenol-basedSPAEScopolymerinproton

formsynthesizedbyMcGrathetal.

CLPE cross-linked high-density polyethylene

substrate

DBSA 4-dodecylbenzenesulfonicacid

DCDPS 4,4′-dichlorodiphenylsulfone

DMFC directmethanolfuelcell

DOE DepartmentofEnergy

DP diphenol

DS degreeofsulfonationorsulfonationdegree

FDA 4,4′-(9-fluorenylidene)dianiline

FPEB 4-fluoro-4-phenylethynylbenzophenone

HC-MEA sulfonatedhydrocarbonPEM-basedMEA

HFR high-frequencyresistance

IEC ionexchangecapacity

IEP isoelectricpoint

MEA membrane-electrodeassembly

NMP N-methyl-2-pyrrolidinone

OBBA 4,4′-oxybis(benzoicacid)

ODADS 4,4′-diaminodiphenylether-2,2-disulfonic

acid

PAA poly(acrylicacid)

PAAVS poly(acrylicacid-co-vinylsulfonicacid)

PAE poly(aryleneether)

PAES poly(aryleneethersulfone)

PATBS poly(acrylamide-tert-butylsulfonicacid)

PBI polybenzimidazole

PC polycarbonate

PEEK poly(etheretherketone)

PEFC polymerelectrolytefuelcell

PEK poly(etherketone)

PEM proton exchange membrane or polymer

electrolytemembrane

PEMFC protonexchangemembranefuelcell

PEO poly(ethyleneoxide)

PFSA perfluorosulfonicacidorperfluorinated

sul-fonicacid

PPh polyphosphazene

PPO poly(phenyleneoxide)

PS polystyrene

PSf polysulfone

PSK poly(sulfideketone)

PSSA-MA poly(styrenesulfonicacid-co-maleicacid)

PTFE polytetrafluoroethylene

PVA poly(vinylalcohol)

PVdF polyvinylidenefluoride

RH relativehumidity

SA-DADPS 3,3′-disulfonic acid-bis

[4-(3-aminophenoxy)phenyl]sulfone

SEBS styrene–ethylene–butylene–styrene

SPAE sulfonatedpoly(aryleneether)

SPAEEN sulfonatedpoly(aryleneetherethernitrile)

SPAEK sulfonatedpoly(aryletherketone)

SPAEN sulfonatedpoly(aryleneethernitrile)

SPAES sulfonatedpoly(aryleneethersulfone)

SPAESK sulfonated poly(arylene ether sulfone

ketone)

SPEEK sulfonatedpoly(etheretherketone)

SPEFC solid-polymerelectrolytefuelcell

SPEK sulfonatedpoly(etherketone)

SPI sulfonatedpolyimide

SPPh sulfonatedpolyphosphazene

SPPSf sulfonatedpoly(phenylenesulfone)

SPPSSf sulfonatedpoly(phenylenesulfidesulfone)

SPPSSfN sulfonatedpoly(phenylenesulfidesulfone

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SPS sulfonatedpolystyrene

SSA sulfosuccinicacid

TEM transmissionelectronmicroscope

Tg glasstransitiontemperature

ZrP zirconiumphosphate

1. Introduction

Many countries are developing eco-friendly energy

sources as an alternative to fossil fuels. Such develop-menthasbeenacceleratedbytheinstabilityofoilprices

and frequent natural disasters causedby globalclimate

change[1–3].Fuelcellsareregardedasapromising

alter-native energy conversion device for both mobile and

stationaryapplications.Thesecellsgeneratewater,heat, and electricity withoutemitting pollutants via an

elec-trochemicalreactionwithhydrogenasafuelandoxygen

intheairasanoxidant[4–6].Althoughhydrogenisstill derived mainly fromfossil fuel, energy circulation

sys-temwithzeropollutant-emissionmaybeaccomplished

by fuel cell using hydrogen produced from renewable

energysourcesinthenearfuture.Amongthevariousfuel

cell types,proton exchange membrane fuelcells

(PEM-FCs),alsocalledpolymerelectrolytefuelcells(PEFCs)or solid-polymerelectrolytefuelcells(SPEFCs),areatypeof fuelcellwhereprotonconductivemembranes(i.e.,proton

exchangemembranes(PEMs))areusedasanelectrolyte

[7,8]. Themainadvantage ofPEMFCsis that theyallow fortheuseofthinpolymerelectrolytes.Such thinPEMs areeffectiveinprovidingashortpathwayforiontransport fromoneelectrodetotheother,whichdirectlyimproves fuelcellperformancebyreducingcellresistance.Thin elec-trolytescanalsoreducethetotalvolumeandweightofa fuelcellstacksystem.Polymerelectrolytescanbeeasily utilizedbecauseoftheabsenceofleakageproblemsthatare encounteredwithliquidelectrolytes.Duetothese

advan-tages,PEMFCshavebeenwidelystudiedasmobilepower

sourcesfortransportationapplicationssuchasafuelcell car,andportableelectronicdevicessuchasmobilephones

andnotebookcomputers.

Despite theirlimited capacity and need for frequent

recharging, lithium ion batteries are currently still the primarypowersuppliesusedforsmall-sizeportable

elec-tronic devices, which only consumea small amount of

power.Accordingly,therehavebeenlessintensiveefforts towardsthecommercializationofportablePEMFCsystems. However,intransportationbatterypowerapplications,a rechargingtimeofabout20minisneeded.Itisthus

impos-sibletoimplementthecurrentbattery-only-typepower

sources such as lithium ion batteries in transportation

applicationsbecausetheyrequirechargingfroman exter-nalelectricsupplysourceforalongperiodoftime(e.g., afewhours).Directfueling-typepowersources,suchas internalcombustionengines,areinsteadneededtoreduce rechargingorfuelingtime.Sincetransportationconsumes

alargeamountofpower,powersourcesshouldhavean

adequatelylongoperatingtimebetweenrefuelingtoallow thecurrentvehiclerangeofdistance.PEMFCsareexpected

tobedesirabletransportationpowersourcesduetotheir highefficiencyandenergydensitypervolumeandweight.

Inaddition,manycountrieshaveencouragedautomobile

companiestodevelopeco-friendlyfuelcellcarsbyphasing inenvironmentalregulationsrelatedtovehicleemissions andfuelefficiency.

PEMFCs are classified into three categories

depend-ing on their operating temperatures: high-temperature

PEMFCsover120◦C,medium-temperaturePEMFCsfrom

70◦C to 120C, and low-temperature PEMFCs below

70◦C. High-temperature PEMFCs are generally

oper-ated under non-humidified conditions because water

boilsat 100◦C. Low-midtemperaturePEMFCsare

oper-ated either under fully humidified conditions below

100◦Corunderreducedhumidityconditionsover100C.

Underhigh-temperature/non-humidifiedconditions,

cat-alyst poisoning from carbon monoxide (CO) impurities

infuelsbecomeslessseriousbecausecatalystactivityis

improvedatelevatedtemperatures.Thisenablestheuse

offuelswithlessstringentpurityrequirements.

Further-more,thecomplexityandoverallsizeofPEMFCsystems

canbereduced due totheabsenceoflargehumidifiers

and the adoption of smaller coolingsystems [9].

How-ever,highoperatingtemperaturescanleadtothethermal

degradationofPEMFCcomponents.Inaddition,the

elec-trochemicalperformanceofhigh-temperaturePEMFCsis

currently too low for their adoption in commercially

availabletransportationsystems.Incontrast,conventional

PEMFCs operated under low temperature/fully

humidi-fiedconditionsexhibithighelectrochemicalperformance.

Unfortunately,PEMFCsoperatingatlowtemperatureand

high humidity require high-quality CO-free fuels and

additionalwatermanagementsystems,including

humidi-fiers.Therefore,PEMFCsystemsoperatinginthemedium

temperature range and at reduced humidity are being

extensivelystudied,particularlyinthefieldof transporta-tion[10].

ToselectappropriatePEMsforuseinPEMFCs,itis

nec-essary tounderstand the protontransport mechanisms

inPEMs.Ingeneral,protonswithinPEMsaretransported

throughwaterchannelsincomplexformsassociatedwith

water molecules such as H3O+, H5O2+, and H9O4+ (i.e.,

thevehiclemechanism[11]).Theprotonsaretransferred

by “formingand breaking hydrogenbondswith proton

acceptingmediasuchaswater orphosphoric acid”and

by“inter molecularproton transfer” (i.e.,theGrotthuss

mechanismorhoppingmechanism)[12–19].Thevehicle

mechanismisbelievedtobethedominantprotontransport

mechanisminlow-midtemperaturePEMsatlowdegrees

ofhydration.TheGrotthussmechanismcanalsobepresent

in low-mid temperature PEMs at high levels of

hydra-tionsince water molecules can actas proton accepting

media.TheGrotthussmechanismalsoprovidesinsighton

protontransportthroughPEMsoperatingathigh

tempera-turesandundernon-humidifiedconditions,becausesome

liquidswithhydrogenbondingsites(suchasphosphoric

acid, imidazole and triazole) can act as proton

accept-ingmediawithhighdegreeofselfdissociation[9,20–23].

PEMsforeachoperatingconditionhavethusbeen

devel-opedbyconsideringbothtransportmechanisms.Thefocus

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O(CF

2

)

2

CF

CF

3

OCF

2

SO

3

H

CF

2

CF

2

*

X

CF

CF

2 Y n

*

Fig.1.ChemicalstructureofNafion®.

generallydesignedtohavewaterchannelsthatareformed

by the hydrophilic characteristics of polymer matrices.

Hydrophilicityinpolymerscangenerallybeachievedby

introducingacidfunctionalitiessuchassulfonicacid,

car-boxylicacid,andphosphoricacidinpolymerbackbones.

Amongtheacidcandidates,sulfonic acidgroups areby

farmostcommonlyemployed[24–29].Inaddition,dueto theirlowpKavalues(i.e.,highacidity),sulfonicacidgroups

aremost widelyused in PEMs.There aretwo methods

ofintroducing sulfonicacidgroups intopolymer

matri-ces:directcopolymerizationofsulfonatedmonomers(the

monomersulfonationmethod)andthepolymer

sulfona-tionmethod(orpost-sulfonationmethod).Inthisreview, bothmethodswillbediscussedandprogressinPEM mate-rialsdevelopmentwillbedetailed.Inadditiontosulfonic acidgroups,otheracidfunctionalgroupssuchas

phopho-phonicacid[29–31]andsulfonamide[32,33]have been

usedtoimparthydrophilicityinpolymers.

Nafion®, manufactured by DuPontTM (see Fig. 1) is

themostwell-known PEMwithacidfunctional groups.

ResearchregardingPEMFCsbeganwiththeGeminispace

program in the 1960s. The first membrane was a

sul-fonated polymer based on polystyrene-divinylbenzene,

which exhibited a very short lifetime due to the

sus-ceptibility of its backbone to radical attack (i.e., poor oxidative stability). Nafion®, a perfluorinated sulfonic

acid(PFSA) membrane, was utilized in PEMFC to

over-comesuchaweakness.WhileNafion® membraneswere

originally developed for the chloroalkali permselective

membranes, they have been widely used in PEMFCs

becauseoftheirexcellentchemicalandoxidative stabil-ity derived fromtheir perfluorinated backbone [27,34]. Despitethelowionexchangecapacity(e.g.,IECofNafion®

112=0.9mequiv.g−1), Nafion® PFSA membranes have a

highaciditydue totheelectron-withdrawing

character-isticsoffluorineatomsaroundtheterminalsulfonicacid

groupsonthe sidechains[16,35–38].PFSAmembranes

alsoformwaterchannelseffectively,despitethelowwater uptake,asaresultofthestrongphaseseparationbetween

extremely hydrophobic backbones and hydrophilic side

chains(Fig.2(a))[12,16,38–40].Inparticular,the flexibil-ityof theacidicside chainsallowsthe readyformation

of water channel [38]. Consequently, PFSA membranes

displayhighprotonconductivityanddimensional stabil-ity [16,38,39,41]. However, a number of issues suchas cost,safety,thedehydrationandconsequentlossof

con-ductivityatelevatedtemperaturehinderthewidespread

commercializationofPEMFCsthatusePFSAmembranes.

Suchissues arise duetothecomplex natureoffluorine

chemistry and environmental pollution associated with

toxicfluorinatedexhaustfumesthatareemittedduringthe incineratingprocess.Furthermore,thelowglasstransition

temperatures(Tg)ofPFSAmembranes(e.g.,broadTgfrom

∼55◦Cto∼130◦C[42])reducetheirmechanicalstrength.

Given the aboveissues, extensive researchhasbeen

devotedtodevelopingalternativestoPFSAPEMs,

partic-ularlyforPEMFCsastransportationpowersources.Note

that PEMsmust satisfytough criteria;theyshould

pos-sesshighdimensionalstability,excellentphysico-chemical durability,lowfrequencyresistancewithelectrodes,and

highproton conductivityeven at low relative humidity

(RH)andhightemperature(e.g.,USDepartmentofEnergy (DOE)target>0.1Scm−1at50%RHand80C).These

prop-ertiesarerequiredsoastoachievehighelectrochemical fuelcellperformanceoveralongperiodtime(currentDOE target>5000h).

The most promising PEM candidates are

sul-fonated hydrocarbon PEMs. As shown in Fig. 3,

sulfonated hydrocarbon PEMs can be classified as

sul-fonated polystyrene copolymers (SPSs), sulfonated

polyimides (SPIs), sulfonated poly(phenylene)s,

sul-fonated poly(arylene)-type polymers, or sulfonated

poly(phosphazene)s(SPPhs)accordingtotheirbackbone

structures[5,24,25,27,28,43–47].Themajoradvantageof

these hydrocarbonPEMs is that it is possibleto design

tailoredpolymerstructureswithdesiredpropertiesusing

variousmonomers.WhencomparedtoPFSAs,sulfonated

hydrocarbonPEMsaregenerallyeasytoproduceand recy-cle,relativelyfreefromenvironmentalpollutionproblems,

andcanbesynthesizedwithrelativelycheapmonomers.

In particular, since many sulfonated hydrocarbon PEMs

have highthermal and mechanical stabilities, they can

maintain their mechanical properties and have high

wateruptakesoverawidetemperaturerange[27,28,34].

Sulfonated hydrocarbon PEMs also have significantly

lowergaspermeabilitythanPSFAmembranes.Forthese

reasons,sulfonatedhydrocarbonPEMshavebeen

recog-nized as promising electrolyte materials, especially for

medium-temperatureandlow-temperaturePEMFCs.

Despite their high water uptake, poor water

chan-nel formation has frequently been observed in

sul-fonated hydrocarbon PEMs due to weak phase

sepa-ration between hydrophilic and hydrophobic moieties

(Fig.2(b))[12,16,38–40].Consequently,sulfonated hydro-carbonPEMsexhibitrelativelylowprotonconductivities

even at highion exchange capacities (IECs) and

exces-siveswellingbehaviorunderhydratedconditions(i.e.,low dimensional stability) [16,38]. Furthermore, sulfonated

hydrocarbon polymersgenerallyexhibit lower chemical

and oxidative stability when compared to hydrocarbon

polymers without sulfonic acid groups. Such behavior

arisesfrom thesusceptibilityofthesulfonated polymer backbones(suchasinpolyimides)tochemialattack[26], ortheirlowmolecularweightduetothelowreactivityof

sulfonatedmonomers[27].Whilesulfonatedhydrocarbon

PEMsgenerallyhaveahighwateruptake,areductionin

theirprotonconductivityisinevitableover100◦Cdueto

theevaporationofwatermolecules.

Many attempts have been made to overcome the

aforementioned issues via the synthesis of copolymers

withdesirablepolymerarchitectures(e.g.,block

copoly-mers, high-free volume copolymers, grafted/branched

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Fig.2.Comparisonofsimulatedwatermoleculesdistributionin(a)Nafion®and(b)sulfonatedblockcopolyimides(DS=80)havingthesimilarvalues.

Colors:red(O),white(H),blueline(hydrogenbonding)[38].(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothe webversionofthearticle.)

[38]Copyright2010,AmericanChemicalSociety.

copolymers) or through polymer chemistry approaches

forthephysico-chemicaltuningofsulfonatedPEMs(e.g., crosslinking,surfacefluorination,thermalannealing,and

organic–inorganicnanocomposites).Thisreview

summa-rizeseffortsindevelopingsulfonatedhydrocarbonPEMs

with properties that make them attractive for durable

medium-temperatureand/orlow-humidityPEMFC

appli-cations.

2. SynthesisofsulfonatedhydrocarbonPEMs

2.1. Sulfonation

Sulfonation is an electrophilic substitution reaction whereasulfonationagentreactsonthearomaticringsand theirprotons aresubstitutedbysulfonic acid. Here,the reactionsiteselectivelyoccursontheelectron-richsiteof benzenerings,suchastheortho-positiontothe

electron-donatinggroups.Electron-withdrawinggroupsdeactivate

benzene rings to electrophilicsulfonation. As such, the positionofsulfonicacidgroupscanbecontrolledbythe

choiceofthemonomerorthepolymerstructure. Sulfona-tionmethodsaremainlycategorizedbythehostmaterials intotwogroups,whicharediscussedbelow.

2.1.1. Polymersulfonation

Thepost-polymerizationsulfonationmethodhaslong

been used for the sulfonation of natural materials in

cationicexchangeresinsforwatersofteningand deminer-alization[48].Duetoitsconvenience,thismethodhasoften beenemployedforPEMsinordertointroducesulfonicacid groupsintovarioustypesofaromaticpolymers.However,

polymerchaindegradationand/orundesirableside

reac-tionsmayoccurduringthesulfonationprocess without

adequateattentiontosuitablesulfonationagents,reaction temperature,andreactiontime.Inaddition,itissometimes difficulttopreciselycontrolthedegreeofsulfonation(DS) andthesiteofsulfonicacidgroupsinthepolymer.

Sulfonation agents for polymers should be carefully

chosenaccordingtothestructureofthehostpolymer,due todegradationandsolubilityproblems[24,27].For exam-ple,Smithaetal.reportedthatpoly(phenyleneoxide)(PPO)

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and polysulfone (PSf) were sulfonated with chlorosul-fonicacid,whereasacetylsulfatewasappropriateforthe

sulfonation of polystyrene (PS) and polycarbonate (PC)

[29].Theconcentrationofthesulfonationagentandthe

reactiontimeandtemperaturearegenerallytheprimary

factorsthatdeterminetheDSoftheresultingsulfonated polymers.Representativesulfonationagentsinclude:

(1) Strongagents:concentratedsulfuricacid,fuming sul-furicacid,chlorosulfonicacid.

(2) Mildagents:acetylsulfate,sulfurtrioxidecomplexes, trimethylsilylchlorosulfonate((CH3)3SiSO3Cl).

Strong sulfonationagents may resultin an

inhomo-geneous reactionorchaindegradationdue totheirhigh

reactivity,andexcessivesulfonationcanrenderthehost polymerwatersoluble.Whilemildsulfonationagentscan

lead to a more homogeneous reaction, with no

degra-dation, and less side reactions occurring, it is often

difficult to achieve a sufficiently high DS with these

agents[29,49]. CH2CH CH2CH * CH2CH CH2CH2 CH2CH CH2CH * x y n m n SO3H SO3H CH2 CH3

(a) Sulfonated styrene cop

olymers (S

PSs) [50

, 51]

.

O N N O O O O SO3H H3OS N N O O O O

(b) Sulfon

ated pol

yimides (SPIs)

[92]

.

*

* n

C O O

SO3H

(c) Sulfonated poly(phe

nylene)s [4

5].

Fig.3.Varioustypesofsulfonatedhydrocarbonprotonexchangemembranes(PEMs):(a)sulfonatedstyrenecopolymers(SPSs)[50,51];(b)sulfonated polyimides(SPIs)[92];(c)sulfonatedpoly(phenylene)s[45];(d)sulfonatedpoly(arylene)typespolymers[27];and(e)sulfonatedpoly(phosphazene)s

[46,47].

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X Y * X X X Y X n* X = O, S Y= a bond, SO2, CH3 CH3 C , CF3 CF3 C , P O Z = SO2, C O , P O 1-X

Hydrophilic Hydrophobic

Z Z

(d) Sulfonated po

ly(arylene) types

polymers [27].

R O P O N * * x HO3S SO3H

(e) Sulfonated poly(phosphaze

ne)s [46,

47].

Fig.3. (Continued).

Since thepost-sulfonationmethodrequires aromatic

rings for the electrophilic substitution reaction, it has

been widely used for polymers containing styrene

units. A representative PEM based on polystyrenes is

thesulfonatedstyrene–ethylene–butylene–styrene(SEBS)

membrane(Fig.3(a)),suchasthatmanufacturedbyDais Analytic.For the sulfonationof SEBSpolymers,a sulfur trioxide/triethylphosphatecomplexsolutionatlow

tem-O C O x O * O O * SO3H C O n O C O O 1-x Sulfonation agent (e.g. conc. H2SO4)

Electron donating group

Electron withdrawing group

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O O * X * R n * O O X * R n SO3H C O C O C O C O C O 95-98% H2SO4 room temperature X: R: -H R: -CH3 Ph-SPEEK Ph-SPEEKK Ph-SPEEKDK Me-SPEEK Me-SPEEKK Me-SPEEKDK

(a) Homopolymer-type sulfonated p

oly(ether ketone)s (SPEKs)

O O C m O * O C CF3 CF3 O C O O O C m O * O C CF3 CF3 O C O SO3H 95-98% H2SO4 1-m

Segment readily sulfonated Segment not readily sulfonated

1-m

(b) Copolymer-type sulfonated pol

y(ether ke

tone)s (SPEKs)

Fig.5. Sulfonationofpoly(etherketone)s(PEKs)havingvariouspendantphenylgroups:(a)homopolymer-typesulfonatedpoly(etherketone)s(SPEKs), and(b)copolymer-typesulfonatedpoly(etherketone)s(SPEKs)[59].

[59]Copyright2007,AmericanChemicalSociety.

peraturesbetween−5◦Cand0◦Coracetylsulfateat50◦C isusedasasulfonationagent[50,51].However,the appli-cabilityofSEBSpolymersinPEMFCsislimitedduetothe pooroxidativestabilityofsulfonatedaliphaticpolystyrene units[52].Sulfonatedpolystyreneshavethusbeen

stud-ied as a grafted side chain so as to introduce sulfonic

acidgroups onto a non-sulfonated polymer main-chain

[53,54].

NoshayandRobesonoriginallyreportedonthe

poly-mersulfonationofbisphenolApolysulfone[55].Thiswork

wasfollowedbymanystudieswhereotherpoly(arylene

ether)-typepolymerswereemployed.Amongthese

poly-mers,poly(etheretherketone)(PEEK),suchascommercial Victrex®,ismostextensivelyusedasahostpolymer.In

PEEK, the sulfonic acidgroup is introduced tothe ring

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electron-OCONHPr PrNHCOO F5 F5 K2CO3 DMAc O O n F F F F F F F F ClSO3H, CHCl2 KOHaq, DMSO HClaq SO3H m O O n F F F F F F F F + 160 ºC 1. 2. 3.

Fig. 6.Synthesisand sulfonationprocess ofsulfonated poly(arylene ether)scomposedoftetraphenylphenyleneetherand perfluorobipheny-leneunits[60].

donating character. Concentrated sulfuric acid is more

appropriateasasulfonationagentthanchlorosulfonicacid orfumingsulfuricacidduetopolymerchaindegradation issues(Fig.4)[56–58].

Guiverandco-workersreportedonamethodto

con-trol the sulfonation sites by adjusting the molecular

structuresofthehostpolymers[59].PEEKwiththe

var-ious side substituents, such as phenyl, methylphenyl,

trifluoromethylphenyl, and phenoxyphenyl groups, was

sulfonatedunderrapidandmildreactionconditionsusing

concentrated sulfuric acid (95–98%) at room

tempera-ture. It was observed that while PEEKs with selected

pendant groups, such as phenyl and 4-methylphenyl,

haddramaticallyshortersulfonationtimesthan

commer-cialPEEKundermildreactionconditions,otherpendant

groups could not be readily sulfonated. In addition, a

seriesofsulfonatedpoly(etherketone)s(SPEKs)with

dif-ferent DSwaspreparedbycontrollingthelengthofthe

O C CH3 CH3 O S O O n-BuLi THF O C CH3 CH3 O S O O Li O C CH3 CH3 O S O O S O LiO O C CH3 CH3 O S O O HO3S SO2 H 2O2/OH H+/H2O -65oC 1. 2. Electron withdrawing group -65oC

Fig.7.Sulfonationprocessvialithiationofsulfonatedpoly(sulfone)s[62].

ketonemonomersinthehomopolymer(Fig.5(a))andthe

composition of sulfonatable/non-sulfonatable segments

(Fig.5(b)).Homopolymer-typeSPEKswerefoundtoexhibit

alowerdimensionalswellingratiothancopolymer-type

SPEKswithsimilarIECvalues.

In2001,Hayetal.suggestedinterestingPEMdesigns

thatincludedhighly sulfonatablemonomerswithmany

pendantphenylrings,whereahydrophilicsegmentcould

O C CH3 CH3 O S O O n O C CH3 CH3 O S O O n Li O C CH3 CH3 O S O O n C O HO3S BuLi O S O O O H+ 1. 2.

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O C CH3 CH3 O S O O n-BuLi THF O C CH3 CH3 O S O O Li O C CH3 CH3 O S O O O C F H+ F C Cl O SO3Na O H SO3Na O H S O O C O O SO3H S O O C O O SO3H -65oC THF -78oC K2CO3, DMAc or or

Fig.9.Synthesisofsulfophenoxybenzoylpolysulfoneandsulfonaphthoxybenzoylpolysulfone[67].

havemorethanthreesulfonicacidgroupsafter sulfona-tion[60,61].Fig.6 shows thesynthesis and sulfonation

processes for sulfonated poly(arylene ether)s (SPAEs)

composedoftetraphenylphenyleneether and

perfluoro-biphenylene units [60]. After sulfonation, sulfonic acid

groupswerelocatedattheparapositionofthependant

phenylring;theirDSperrepeatunit(m)couldbecontrolled bythereactionstoichiometry.However,sincethe

result-inghighly sulfonatedpolymers withm=3 weresoluble

in methanol,copolymerswith non-sulfonatable

bis(3,5-dimethylphenyl)sulfonemonomerswerealsosynthesized.

TheresultsattainedbytheHayresearchgroupwerelater

appliedtotheconceptofPEMswithhighlysulfonatable

monomers,whichwillbediscussedlaterinSection2.2.6. Anotherapproachtopolymersulfonationismetalation,

or lithiation, without the use of conventional

sulfona-tion agents. Thisapproach wasfirst reported byKerres

etal.,whoappliedathree-stepreactionprocessof lithia-tion,sulfonation,andoxidationtocommercialpolysulfone Udel®,asshowninFig.7[62].Inaconventional

sulfona-tionreaction,sulfonicacidgroupsareintroducedontothe ortho-positionofelectron-richbenzeneringsactivatedby electron-donatinggroupssuchasetherlinkages,whichcan giverisetoinstabilityduetothelowreactionenergybarrier

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O H C CF3 CF3 OH K2CO3 S O O F F O H C CF3 S F OH DMAc CF3 C CF3 O S O O CF3 C O * S O O S O O x O O F K2CO3, DMAc CF3 C CF3 O S O O CF3 C O * S O O S O O x O O O HO3S SO3H CF3 C CF3 O S O O CF3 C O * S O O S O O x F 1-x + + 1-x formic acid 30% H2O2 2N HCl 1-x

Fig.10.Syntheticrouteofpoly(aryleneethersulfone)swithgraftingcapability[68].

(13)

[63].However,sulfonicacidgroupsintroducedby lithia-tionarepotentiallymorestableandexhibitahigheracidity becauselithiationoccursonelectron-poorbenzenerings

thataredeactivated bytheelectron-withdrawing group

suchasasulfonegroup[34,64,65].Despitethispotential advantage,thefinaloxidationstepthatfollowslithiation andsulfinationcanresultinareductionintheexpected IEC,andchaindegradation[62].

Thegraftingofsulfonationgroupsontopolymerside

chainsisalsoanattractivemethodtopreparenovelflexible

PEMs.AsdemonstratedwithNafion®,sulfonatedgroups

attheendofasidechaininducestrongphaseseparation

betweenthehydrophobicmainchainandthehydrophilic

sidechain.Thisphaseseparatedmorphologycan deceler-atepolymermainchaindegradationbyradicalspeciesthat arepresentinthewaterchannel[38].Inpreviousresearch

by the Jannasch group, a sulfophenyl ring was

intro-ducedtopolysulfone usingtheabove lithiationprocess

developedbyGuiveretal.[65],followed bysubsequent anionicreactionwithsulfobenzoicacidcyclicanhydride, asshowninFig.8[66].Thesamegroupalsosynthesized

novel poly(sulfone)s with pendant sulfonated aromatic

sidechains; theDS wasconveniently controlledby the

degreeof lithiationinthefirststep,becausethesecond stepproceededwithfullconversion(Fig.9)[67].Despite theirnovelsyntheticapproachandgraftingstructure, unfa-vorabletransetherificationreactionslimitedanincreasein theDS.Zhaoetal.reportedonpoly(ethersulfone)switha graftedsulfonatedgroup,wherereagentswithtwosulfonic acidgroupswereattachedattheendofa4-fluorophenyl sulfidependantgroupviaanucleophilicsubstitution

reac-tion [68]. In the study, the authors suggested a novel

graftingstrategytointroducedisulfonatedphenolatesinto thependantsidechain.Thestrategyinvolvedthe activa-tionofparafluorineonapendantgroupviatheoxidation ofsulfidetosulfone(seeFig.10).

2.1.2. Directcopolymerizationofsulfonatedmonomers

Variousapproacheshavebeenproposedtoovercome

the issues of polymer chain degradation and sulfonic

acidgroup instability that are inherent in the polymer

sulfonation method. One approach is to introduce

sul-fonicacidgroupsintomonomersandthencopolymerize

non-sulfonatedmonomerswithsulfonatedmonomersto

controlIEC.Thismonomersulfonationmethodavoidsthe

polymerchaindegradationduringthesulfonationprocess andenablestheintroductionofsulfonicacidgroupsinto

a varietyof polymer backbonesthat have sensitivity to

strongacidicconditions.Forexample,whilemany

stud-ieshavebeencarriedouttopreparesulfonatedpolyimides (SPIs),therearefewreportsonthefabricationofSPIsvia polymersulfonationbecausethePIbackbonesare suscep-tibletodecompositionduringsulfonationprocess[69,70].

Furthermore,theuseofsulfonatedmonomersallowsthe

designofthepolymertobeeasilytailoredfordesirable

propertiessuch asthe DS and the choiceof sulfonated

sites(e.g.,inactivatedsitesforhighstability andacidity, asshowninFig.11[71]).Inparticular,specialarchitecture forsulfonatedpolymers(e.g.,multiblockcopolymers)can bedesignedbycontrollingthetopologyand/ormolecular weightofsulfonatedandnon-sulfonatedoligomersand/or

Fig.11.Introductionofsulfonicacidgroupsviapost-sulfonation(upper) andmonomersulfonation(below).

polymers[72–75].However,therearerelativelyfew

com-mercially available sulfonated monomers [34]. Another

factoristhatthesterichindranceofsulfonatedmonomers maydecreasethereactivityandlimitthehighmolecular weightoftheresultingsulfonatedpolymers[27].

Alistofwidelyusedsulfonatedmonomersisshownin

Fig.12.Sulfonateddihalomonomers,whicharemainly uti-lizedfor polyarylene-typepolymers,werefirstreported

on by Robesen and Matzner. The monomers were not

initially used for fuel cell membranes, but for flame

retardingmaterials[76].Tenyearslater,sulfonated4,4′

-dichlorodiphenyl sulfone was synthesized and purified

by Ueda et al. for aromatic poly(ether sulfone)s [77].

Wangetal.reportedontheuseof3,3′-disulfonated4,4

-difluorodiphenylketonemonomerforpoly(aryleneether

ketone)s(Fig.13)[78].However,researchonfuelcell

appli-cations using sulfonated dihalo monomersbegan when

theMcGrathresearchgroupsynthesized3,3′-disulfonated

4,4′-dichlorodiphenylsulfonemonomersand prepareda

BPSH seriesbasedonsulfonatedpoly(arylene ether

sul-fone)(SPAES)copolymers(Fig.14)[79,80].Thesamegroup

and coworkersalsostudiedstructure–property

relation-ships using various monomers [81–84], which will be

discussedinSection2.2.1.Naandco-workersdetailedthe

useof1,4-bi(3-sodiumsulfonated-4-fluorobenzoyl)

ben-zeneasa sulfonatedmonomerforsulfonatedpoly(ether

ether ketone ketone)s (SPEEKs) [85,86]. Various types

of SPAEs are listedin Fig. 3(d). Sulfonated commercial monomers(Fig.12(b))werealsoexaminedforthe

poly-merization of PEMs due to their availability and low

price.Guiverandco-workersusedcommercialsulfonated

bisphenol monomers to prepare SPAE-type polymers.

The researchers also performed a comparative study

of sulfonated poly(arylene ether ether nitrile)(SPAEEN)

copolymers with different sulfonic acid bonding sites

(Fig.15)[87–90],whichwillbefurtherdiscussedinSection

2.2.1.

For SPIs, a sulfonated commercial monomer, 4,4′

-diamino-biphenyl2,2′-disulphonicacid(BDSA)wasused

inearlyresearchbyMercierandPineri’sgroup(Fig.16)

[72,75,91,92].However,BDSA-based SPIshaveexhibited a relatively low hydrolytic stability despiteattempts to

improvethisthroughtheintroductionofasix-membered

(14)

problem,Okamotoandco-workerssynthesizedtwotypes ofsulfonated diamines,4,4′-diaminodiphenylether-2,2

-disulfonic acid (ODADS) with a flexible structure [94]

and 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid

(BAPFDS)witharigidandbulkystructure[95],asshown

in Fig. 17. When compared to BDSA-based SPIs, SPIs

incorporating ODADS exhibited much better hydrolytic

stability duetotheirflexibility.Thisfindingwasfurther

supported by the reduced stability of SPIs when

flex-ible non-sulfonated diamines were replaced with rigid

diamines.However,despitetheirrigidity,SPIswithBAPFDS displayed a hydrolyticstability that wassimilarto that exhibitedbyODADS-basedSPIs.Suchstabilitywasdueto thehighbasicityofBAPFDS,whichfavorsimidoring stabil-ity.TheOkamotogroupalsoattemptedtoimpartintrinsic

hydrolyticstabilitytoSPIsbyincorporatingvarioustypesof monomers[26].Inparticular,SPIswithsulfonicacidgroups onthesidechains(graftedsulfonicacidgroups)exhibited asignificantimprovementintheirhydrolyticstabilitydue tothehigherbasicityof thediaminemoieties andtheir microphase-separated structure [96,97].Using a similar concept,Asanoetal.synthesizedhighlystablesulfonated copolyimideswithaliphaticgroupsinboththemainchains andthesidechains.Thestructuresdidnotdisplaya sig-nificantopencircuitvoltage(OCV)dropduring5000hof operation(Fig.18)[98].The McGrathgroup researched

a novel sulfonated diamine monomer, 3,3′-disulfonic

acid-bis[4-(3-aminophenoxy)phenyl]sulfone(SA-DADPS),

where sulfonated groups were introduced to aromatic

ringsotherthantheamino-phenylringinordertoreduce

Cl S O O Cl SO3Na NaO3S

3,3′-disulfonated 4,4

′-dichlorodip

henyl

sulfon

e

C O F NaO3S F SO3Na

3,3′-disulfonated 4,4

′-difluorodiphenyl ketone

O C F SO3Na F NaO3S C O

1,4-bi(3-so

dium sulfonated-4-fluorobenzoyl)

benzene

(a)

SO3K OH O H NaO3S OH OH O H OH SO3Na O H NaO3S OH SO3Na

(b)

(15)

SO3H HO3S NH2 N H2 NH2 SO3H HO3S N H2 O N H2 NH2 SO3H HO3S C CF3 CF3 O N H2 HO3S O NH2 SO3H N H2 NH2 HO3S(CH2)3O

4,4' -diamino-2,2'-biphenyl disulfonic acid

9,9'-Bis(4-aminophenyl)flouorine-2, 7-disulfonic acid

4, 4'-diamonodiphenylether-2, 2'-disulfonic acid

2, 2-bis[4-(4-aminophenoxy)phenyl] hexafluoropropane disulfonic acid

(2', 4'-diaminophenoxy)propane sulfonic acid

Fig.12. (Continued).

theelectron-withdrawing effectof sulfonic acidgroups

that cause poor hydrolytic stability (Fig. 19) [99,100]. Pertaining to this stability issue, Li et al. suggested an

interestingdesignofanovelsulfonatedmonomer,where

sulfonicacidgroupswereintroducedonthedianhydride

monomer rather than the diamine monomer [101]. In

this study, 4,4′-binaphthyl-1,1,8,8-tetracarboxylic

dian-hydride (BTDA) was sulfonated using fuming sulfuric

F C O F H2SO4/SO3 F C O F HO3S SO3H NaOH NaCl F C O F NaO3S SO3Na

(16)

Cl S O O Cl SO3 Cl S O O Cl HO3S SO3H

NaCl OH2 NaOH NaCl

Cl S O O Cl NaO3S SO3Na Cl S O O Cl OH O H K2CO3 Toluene NMP S O O O O * O S O O SO3K * O KO3S n y S O O O O * O S O O SO3H * O HO3S n y H2SO4 (28%) 110 ºC pH=6-7 1-n 1-n

Fig.14.Syntheticrouteof3,3′-disulfonated4,4-dichlorodiphenylsulfonemonomerandsulfonatedpoly(aryleneethersulfone)[79,80].

acid, andthenpolymerizedwithvariousnon-sulfonated

diamines.Sincetheresultingsulfonated PIshadsulfonic acidgroupsonthedeactivatedpositionsofthearyl

back-bone ringsand highDS, theyshowedvery highproton

conductivities.Inparticular,sulfonatedcopolyimide

poly-merizedwithhexane-1,6-diamine,SBTDA,andBTDAhad

excellentwaterstabilityat90◦Cwithoutsacrificingproton

conductivity.

2.2. Recenthigh-performancesulfonatedhydrocarbon PEMs

Inthemid-2000s,asknowledgeregardingthe

synthe-sisofnewsulfonatedhydrocarbonPEMsprogressedand

thewell-orderedhydrophilicchannelstructureinNafion®

hadbeendisclosed,theimportanceofnewpolymerdesigns

havinghighprotonconductivitiesevenat lowIECswas

addressed.ThisissuewasexpandedtohighIECPEMs with-outsacrificingmechanicalstability.EarlierPEMsexhibited relativelylowprotonconductivityandhighwaterswelling duetopoorwaterchannelformation(asmentionedin Sec-tion1).Particularattentionwaspaidtocontrollingthenano (ormicro)-structureofthesePEMsthroughpolymer topol-ogy.

Inthissection,recenttrendsinhigh-performance

sul-fonatedhydrocarbonPEMswillbediscussed.

2.2.1. Introductionoffunctionalgroups

Intheinitialdevelopmentstagesoffuelcellsbasedon

sulfonatedhydrocarbonPEMs,theirMEAssufferedfrom

poor adhesion and delamination between the Nafion®

ionomerbinderinthecatalystlayersandthehydrocarbon PEMsduetodifferencesinchemicalproperties.However,

extensiveresearchhasbeendevoted toimprovingMEA

fabricationmethodsforhydrocarbonPEMsandenhancing

membranepropertiessuchasthebalancebetween

pro-tonconductivityandwateruptake. Theincorporationof

functionalgroupsintosulfonatedPEMsisaneffectiveway

toimprovePEMperformance(includingthe

electrochemi-calperformance)withoutsacrificingmembraneproperties suchastheIECvalue.

TheMcGrathgroupfirstintroducedfluorinegroupsinto

hydrocarbon PEMs using 4,4′-hexafluoroisopropylidene

bisphenol(6F-BPA)monomersforSPAEScopolymers[81].

The authors studied the effect of bisphenol monomers

with various chemical structures on the properties of

theresultingSPAES copolymers,asshown in Fig.20(a).

6F-BPA copolymer systems exhibited the lowest water

sorptionduetothehydrophobicityofthefluorinegroups. Adisulfonatedpoly(aryleneetherbenzonitrile)copolymer

containing6F-BPA (6FCN,seeFig.20(b)) hadmuch less

wateruptakethanboththecopolymerwithout

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* O O C O X C O O Y O C O C O SO3H m 1-m X = , Y = ,

(a) SPAEE

KK

* O O O X O CN SO3H CN m 1-m X = ,

(b) SPAEEN

HO3S O X O O O CN CN n H3OS H3OS SO3H 1-n X =

(c)

m-SPAEEN P-S

PAEEN

D-S

PAEEN

Fig.15. Chemicalstructureofpoly(aryleneetherethernitrile)copolymerhavingdifferentsulfonicacidbondingsitefromcommercialsulfonatedbisphenol monomers[87–90].

[89]Copyright2006,ElsevierLtd.;[90]Copyright2007,AmericanChemicalSociety.

groupsatequivalentIECs;theprotonconductivitiesofthe structureswereallcomparable [102].In particular,Kim etal.reportedthatthe6FCNcopolymerdisplayedthe high-estfuelcellperformance[103].Suchperformancewasdue totwofactors:alowwateruptakethatresultedfromthe interactionbetweennitrilegroupsandsulfonicacidand thereducedinterfacialresistancefromtheimproved

adhe-sionbetweenfluorinegroups inthecopolymer and the

Nafion®-bondedelectrodes[83,103].

Guiver and co-workersexpanded theabove concept

tosulfonatedpoly(arylether ketone)s(SPAEKs) contain-inghexafluoroisopropylidenediphenylmoieties,asshown inFig.21[104,105].Varioussulfonatedpoly(aryleneether nitrile)s(SPAENs)weresynthesizedasexplainedin Sec-tion 2.1.2 and shown in Fig. 15(b) and (c) [88–90].

The introduction of nitrile groups into the PEMs was

expectedtopromoteadhesionontoeitherthecatalystor carbonparticles inthecatalyst layers[88].SPAENs

(18)

con-Fig.16. Syntheticrouteofsulfonatedpoly(imide)sbasedonBDSA[92].

taining a naphthalene structure with the sulfonic acid

groups showninFig.15(c) exhibitedmuchlower water

uptakes and swellingratiosthan previousSPAENs with

hydroxyquinonemonomers.Suchbehaviorwasduetothe

synergisticeffectofinter-chainattractionbetween poly-merchainsviathestrongpolarinteractionbetweennitrile

groups, and to the hydrophobicity of the naphthalene

structure[89,90].Furthermore,SPAEENwithsulfonicacid

groupsmetatotheetherlinkage(m-SPAEEN)andthose

withsulfonic acid groups located pendanton a phenyl

ring(P-SPAEEN) exhibited much higher proton

conduc-tivities becausethemeta linkageand the long-distance

connectivityoftheirsulfonicacidgroups,couldeliminate deactivationofthesulfonicacidgroups,respectively[90].

Asaresult,m-SPAEENmembranesshowedhigh

perfor-mance in both PEMFC and DMFC fuel cell applications

N H2 O NH2 H2SO4/SO3 N H2 O NH2 SO3H HO3S NH2 N H2 SO3H HO3S NH2 N H2 H2SO4/SO3 80oC, 2h ODADS 80oC, 2h BAPFDS

Fig.17.Sulfonationof4,4′-diaminodiphenylether-2,2-disulfonicacid(ODADS)[94]and9,9-bis(4-aminophenyl)fluorene-2,7-disulfonicacid(BAPFDS)

[95].

(19)

O O O O O O N H2 NH2 O O SO3H SO3H N H2 NH2 N (CH2)y N N O O N O O O O O O O O n SO3H SO3H TCND BSPA (x=3) BSBA (x=4) HMDA (y=6) DMDA (y=10) TEA benzoic acid m-cresol 1. r.t. 24 h 2. 175oC 15 h 3. 195oC 3 h 1-n (CH2)x (CH2)x (CH2)x (CH2)x (CH2)y

Fig.18.Syntheticrouteofsulfonatedaliphatic/aromaticpoly(imide)s[98].

[98]Copyright2006,AmericanChemicalSociety.

[106,107]. The authors also explained the effect of the angledstructure,whichincreasedtheinterchainspacing andcreatedporeswithsulfonicacidgroupswithin.Asthe

poreseffectivelyconfinedwatermoleculesviahydrogen

bonds,theprotonconductivitycouldbeimprovedandbe

lesssensitivetotemperature.Thisisaveryimportant

con-ceptforsulfonatedPEMswithahighfreevolumeandit

willbefurtherdiscussedinthenextsection.In another paper,Guiverandco-workerssynthesizedSPAEwithahigh

fluorinecontentand SPAENwithahighnitrilecontent;

eachcontainedpendantphenylsulfonicacids,asshownin

Fig.22[108].Thesulfonicacidgroupscouldbeintroduced exclusivelyonthepara-positionofthependantphenylring withchlorosulfuricacidunderrelativelymildconditions.

TheSPAEandSPAENwithDSvaluesof1.0exhibitedhigh

IECsof1.75and2.71mequiv.g−1,highproton

conductiv-itiesof0.135and0.140Scm−1at80Cinwater,andlow

wateruptakesof32and39.6%,respectively.Onatradeoff plotofprotonconductivityversuswateruptake,thePEMs

exhibitedhighprotonconductivityandlowwateruptake

incomparisonwithNafion®.

2.2.2. SulfonatedhydrocarbonPEMswithhighfree volume

OneofthemostimportantissuesforsulfonatedPEMs

infuelcellsiswatermanagementwithinthestructures.If PEMsswellexcessivelywithwaterunderhumidified oper-atingconditions,theycanbedelaminatedfromthecatalyst layerwhichhasarelativelylowerdegreeofwaterswelling,

(20)

Ar = S O Ar O Ar' O O O Ar O H3OS SO3H x C CH3 CH3 C CF3 CF3 Ar' = S O O 1-x Ar = C CF3 CF3 Ar' = S O O CN

Fig.20.Sulfonatedpoly(aryleneethersulfone)copolymershavingvariousbisphenolandfunctionalgroups[81,102].

duetodifferencesindimensionalswellingratiobetween

thehydrocarbonPEMandthePFSAionomer-catalyst.This

dimensionalmiss-matchphenomenonhasbeenreported

asoneofthemajorreasonsforlowdurabilityinfuelcell systems[109].However,sincefuelcellsystemsare

oper-atedunderreducedhumiditywithincreasingtemperature

(asmentionedinSection1),PEMsshouldbedesignedto

retainwaterevenunderalowRH,toensurehighproton

conductivity.Consequently,PEMsthatareableto

main-tainwatercontentandhavehighprotonconductivitywhen

operatedinarangeofRH(40–100%)mustbedeveloped.

Apossiblewaytosolvethisistoutilizepolymer hav-ingahighfreevolumeimpartedthroughtheintroduction

of bulky monomers. In general, the sorption of small

molecules,suchasgasmoleculesinapolymerhost mate-rial,isaffectedbytheamountoffreevolumeofthepolymer

F C F HO3S SO3H O O H C CF3 CF3 OH F X F C O O C CF3 CF3 O C HO3S SO3H O O C CF3 CF3 X O n X = C O C O 1-n + +

Fig.21.Sulfonatedpoly(aryletherketone)scontaininghexafluoroisopropylidenediphenylmoiety[104,105].

(21)

Fig.22.Copoly(aryleneether)scontainingpendantsulfonicacidgroupsandotherfunctionalgroups[108].

whichtheabsorbedmoleculeshaveaccessto.This behav-iorcanbelikenedtosulfonatedPEMswhichabsorbwater molecules.Usingthisconcept,PEMswithahighfree

vol-umehavebeeninvestigatedtoabsorbalargeamountof

watermoleculesandtoretainthemevenunderlow humid-ifiedconditions. Ofcourse,water swellingin sulfonated PEMsisdifferentfromandmorecomplexthangassorption behaviorduetothemuchhighersolvationeffectofwater moleculesandtheresultingstructuralchangeofsulfonated PEMs.Asaresult,mostresearcheshavedemonstratedthe

effectofhighfreevolumeinsulfonatedPEMswith

phe-nomenologicalexperimentalresults suchas highwater

uptake.Also,thedistributionofsulfonicacidgroupsinthe vicinityofthefreevolumeaffectstheabsorptionofwaterin sulfonatedpolymers.Inparticular,littleattentionhasbeen paidtothelatterpoint,despitethemanytrialstoprepare

PEMswithhighfreevolume.

IntheearlystagesofPEMresearchin1999and2000,

Littandco-workersused theaboveconcept foraseries

ofhighlyprotonconductiveSPIswithhighchemicaland

mechanicalstabilities[110,111].Theincorporated angu-larandrigidrod-likebulkycomonomersandtheresulting polymerstructuresfromthisworkareshowninFig.23.

Sulfonatedpolyimideswithcomonomersincategories(b)

and(c)exhibitedahigherd-spacingthanthosewitha

lin-earcomonomerfrom(a)andahomopolymerbecausethe

bulkyorangularcomonomerscouldpreventpolymerchain

packing.Thestructuresalsodisplayedhigherwateruptake andprotonconductivitiesathighandlowhumidities.The authorsexplained that, with largerinterchain spacings,

morefreevolumecouldbeavailableforwatermolecules

tooccupy(i.e.,highwateruptake)andthus,ahigh conduc-tivitycouldbemaintainedevenatlowhumidity.However, theimportanceofconnectivityoffreevolumewasnot con-sideredinthisstudy.

Watanabeandco-workersstudiedvarioustypesof

sul-fonatedPEMs withbulky fluorenyl groups suchasSPIs

[112],SPAES[113–116],andsulfonatedpoly(aryleneether sulfoneketone)s(SPAESKs)[117,118].Basedonthe find-ingsfromtheLittgroup,SPIswith4,4′-(9-fluorenylidene)

dianiline (FDA) were synthesized with increasing FDA

compositionsupto60mol%.Theincorporation ofbulky

fluorenylgroupsover30mol%causedtheconfinementof

watermolecules.Asaresult,theprotonconductivitydid

notdecreaseevenabove100◦Candhighproton

conduc-tivity (1.67Scm−1)wasattainedat 120C and 100%RH

[112].Inthefollowingstudies,Watanabeandco-workers introducedsulfonicacidgroupstobulkyfluorenylgroups

so as to obtain the synergistic effect of a high water

affinity viatheformation ofhighfree volume andhigh

hydrolysisstability viatheintroductionofacidic groups

onthependantphenylgroups [113,114]. Thisideawas

later expanded to the concept of PEMs withhigh IECs

(detailed in Section 2.2.4) and PEMs with highly

sul-fonatable monomers (discussed in Section 2.2.5). Both

homopolymersandcopolymersofsulfonatedpoly(arylene

ether sulfone)s(SPAESs)werepreparedviathepolymer

sulfonationmethod(Fig.24),whereregioselective substi-tutionofthe2,7-positionsofthependantfluorenylgroups (notonthemainchain)withsulfonicacidgroupswas per-formedusingacarefulsulfonationreaction.Thesulfonation reactionwassuccessfullycarriedoutwithalow concentra-tionofchlorosulfuricacid.TheresultingPEMs,especially

thehomopolymer PEMwith anIEC of 1.80mequiv.g−1,

exhibited very high oxidative stability in hot Fenton’s

reagentandhighhydrolyticstabilitiesat140◦Cand100%

RHduetothelocationofthesulfonicacidgroupsonthe

pendantgroups[114].Oneofthehomopolymer-typePEMs

withanIECof1.14mequiv.g−1exhibitedaproton

conduc-tivitythatwascomparabletoNafion®112anddisplayed

thehighestmaximumprotonconductivityat140◦C.

How-ever,incontrasttotheresultsattainedbytheLittgroup,

mostofthemembranesshowedamuchhigherdependence

onhumiditythanNafion®112.

2.2.3. Hydrophilic–hydrophobicmultiblockcopolymers

An attractive way to obtain distinct phase

separa-tion is to fabricate multiblock copolymers composed

of hydrophilic and hydrophobic blocks. Highly

phase-separated hydrophilic blocks can form well-defined

nano-sizedwaterchannelsforprotonconduction,which

resultinlessdependenceofhumidityandtemperatureon protonconductivity,aswellasanoverallreductioninboth

thewateruptakeanddimensionalchangescomparedwith

randomcopolymerofthesameIEC.

McGrathandco-workersfirstadaptedtheconceptof

distinctphaseseparationinblockcopolymerstosulfonated

(22)

Fig.23. Chemicalstructureofaseriesofrigid-rodsulfonatedpolyimides[110].

[110]Copyright1999,AmericanChemicalSociety.

studiednovelsulfonatedmultiblockcopolymersusing

var-ious sulfonated monomers [73,74,119–123]. Images of

well-defined nano-phase separation were obtained for

sulfonated multiblock SPAEScopolymers, and thePEMs

had high proton conductivity even under low

humid-ityconditions below40%RH.Thefirstreportpertaining

to multiblock copolymers from this group detailed the

use of multiblock copolymers with sulfonated poly(4′

-phenyl-2,5-benzophenone) asthehydrophilicblock and

poly(arylene ether sulfone) (PAES) as the hydrophobic

block [119]. The multiblockcopolymers didnot exhibit

characteristic nano-phase separation described above,

probably due to theirlow IECs. In subsequentresearch

usinghighlyactivatedfluorine-terminatedtelechelicsand

hydroxyl-terminated telechelics, sulfonated-fluorinated

PAEmultiblocksweresuccessfullysynthesizedwithhigh

IECsupto2.2mequiv.g−1(Fig.25).Themultiblock

copoly-mers exhibited proton conductivities that were higher

than that of Nafion®, particularly under low humidity

conditions[73].Theauthorsconcludedthatwell-defined

phase separation intheirmultiblock copolymers,which

wasconfirmed byAFM images, resultedin highproton

conductivity. The McGrathgroup used AFM imaging to

shownano-phaseseparationintheirmembranesbecause,

unlikeTEMimages, AFMimagescanbeobtainedunder

hydrated conditions. The same group also studied the

effect of block lengthon morphology and PEM

perfor-mance. Fig. 26 shows the chemical structure and the

AFMimagesofasegmentedsulfonatedpoly(aryleneether

sulfone)-b-polyimide copolymer [74]. Inrelated studies,

well-defined nano-phase separated morphologies were

attained for multiblock copolymers as theblock length

was increased. This in turn improved the proton

con-ductivity and water uptake of the structures [74,121].

Inparticular,multiblockcopolymersbasedonsulfonated andnon-sulfonatedpoly(aryleneethersulfone)oligomers exhibitedanisotropicswelling behavior(i.e.,alower in-planeswelling,buthigherthrough-planeswellingthana

randomcopolymer and Nafion® withisotropicswelling

behaviors).Thiscouldbeadvantageousinactualfuelcell

systems operated under low humidified conditions by

reducing theeffect ofdimensionalmiss-matchbetween

hydrocarbonPEMandPFSAionomer-catalyst[121].

Multi-blockcopolymersexhibit proton conductivitiesthatare

lessRH-dependentandhigherthanthatofNafion®[124].

Ueda and co-workers suggested new approaches to

synthesizesulfonated multiblock copoly(ethersulfone)s

via the nucleophilic aromatic substitution of

hydroxyl-terminated oligomerswitha chainextender for PEMFC

(seeFig.27(a))[125,126].Decafluorobiphenyl(DFB)was usedasachainextender,duetoitshighreactivity,which

couldpreventtheether–etherinterchange reactionand

theresulting randomizedpolymer architecture.

Accord-ingly,theirmultiblockcopolymerswereobtainedwithhigh

molecular weights, and the hydrophilic and

hydropho-bicblocklengthscouldbecontrolled.TheresultingPEMs hadhighoxidativestabilityandlowdimensionalswelling ratio.Inparticular,theyshowedhigherprotonconductivity

thanrandomcopolymers,and couldmaintain

compara-tivelyhighprotonconductivityevenunder50%RH[125].

These properties were further investigated with

(23)

(b) (a)

Fig.24.Chemicalstructureofsulfonatedpoly(aryleneethersulfone)shavingsulfofluorenylgroups:(a)homopolymertypeand(b)copolymertype

[113,114].

[114]Copyright2005,AmericanChemicalSociety.

resultingmembranesalsohadhighoxidativestabilityand

maintainedhighwateruptake(7.3–18.7wt%)evenunder

50%RH.Theirprotonconductivitieswiththeoptimized

oligomerlengthswerehigherthanthatofNafion®117at

80◦Cand95%RH,andshowedahighvalueof0.007Scm−1

evenunder50%RH.Fig.27(b) showsthecross-sectional morphologyofmultiblockcopolymerwithblocklengthsof 14,000/14,000(Mnofhydrophilic/hydrophobicoligomers),

whichshowedaclearhydrophilic/hydrophobic-separated

structureandcontributedtoeffectiveprotonconduction. 2.2.4. Graftedorbranchedsulfonatedhydrocarbon copolymers

In 2002, Ding et al. performed a pioneering model

study where a sulfonated graft polymer composed of

a polystyrene backbone and a poly(sodium styrene

(24)

Fig.26.Chemicalstructureofsulfonatedpoly(imide)sbasedon3,3′-disulfonicacid-bis[4-(3-aminophenoxy)phenyl]sulfone(SA-DADPS)andtheirAFM

images[74].

sulfonate)sidechainwassynthesizedviafree-radical poly-merization[53].Phaseseparationofionicaggregateswas

shown tobecontrolledby thechain lengthofthegraft

chains,whichhad adirecteffectontheproton

conduc-tivity. Asexpected,thecopolymer withthelongergraft chainexhibitedhigherprotonconductivity.However,the authorsstatedthatthecommercialapplicabilityofthe

fab-ricatedgraftedcopolymerswaslimited,probablydueto

thepoorstabilityofthestyrene-basedbackboneandside chain.Dingandco-workersexpandedtheaboveconceptto

amuchmorestablehighlyfluorinatedbackboneand

suc-cessfully fabricatednovelcomb-shapedcopolymers(see

Fig.28).Inparticular,the␣-methylpolystyrenehydrophilic

sidechainofthestructuresinducedstrongphase separa-tioninthehydrophobicfluorinatedmain-chain,whichwas confirmedbyTEMandSAXS[54].However,whilethe ini-tialperformanceofthestructureswassuperiortothatof

Nafion®andBPSH-35inaDMFC,thecurrentdensitywas

reducedduetotheinstabilityofthesidechain[127].

Guiverandco-workersdesigned comb-shapedSPAES

with a new sulfonated side-chain grafting unit

con-taining two or four sulfonic acid groups, as shown in

Fig.29[128].PAEScontainingamethoxygroupwasfirst synthesizedviaaconventionalaromaticnucleophilic

sub-stitution. The methoxy groups were then converted to

(25)

Fig.27. Syntheticrouteofsulfonatedmultiblockcopoly(ethersulfone)sandtheTEMimagesshowingthecross-sectionalmorphologyofmultiblock copolymerwithbocklengthsof14,000/14,000(Mnofhydrophilic/hydrophobicoligomers)with(a)lowresolutionand(b)highresolution[125,126].

the functional copolymer was grafted with previously

synthesizedsulfonated sidechains.Theresulting

comb-shapedcopolymerswithtwoorfoursulfonicacidgroups

exhibited high proton conductivities (0.034–0.147 and

0.063–0.125Scm−1,respectively)andlowwateruptakes

(18–60and27–53%,respectively).

TheUeda research group expanded theabove

strat-egy to star-shaped block copolymers (Fig. 30) [129].

Star-shapedsulfonated block copoly(etherketone) with

hydrophilicandhydrophobicblocksasarmswas

synthe-sizedviaaFriedel–Craftsreactionofatri-functionalcore.

Thestructurewassubsequentlysulfonatedwith

concen-tratedsulfuricacid.Thehydrophilicblockwithsulfonated siteswasattacheddirectlytothecoreandthe

hydropho-bicblockwasplacedattheendofeacharm.Theresulting

sulfonatedpolymersweresolubleincommonpolar

apro-ticsolventsandthesolvent-castmembranesweretough

and flexible. Due to theirstructural characteristics, the

membranesexhibitedrelativelygooddimensional

stabil-itydespitetheirhighwateruptake.Inparticular,theproton

conductivitiesofthemembraneswerecomparableand/or

higherthanthoseofNafion®at80Cand50–95%RH.

Leeetal.reportedongraftedSPImembranesthatwere

prepared through thermal-solution imidization and the

subsequentincorporationofsulfoalkylatedgraftingagents withdifferentalkylchainlengths,asshowninFig.31[130]. Theadditionalsulfonicacid(–SO3H)groupsattheendof

Figure

Fig. 2. Comparison of simulated water molecules distribution in (a) Nafion ® and (b) sulfonated block copolyimides (DS = 80) having the similar  values.
Fig. 8. Process for sulfophenylated poly(sulfone)s via lithiation [66].
Fig. 9. Synthesis of sulfophenoxybenzoyl polysulfone and sulfonaphthoxybenzoyl polysulfone [67].
Fig. 14. Synthetic route of 3,3 ′ -disulfonated 4,4 ′ -dichlorodiphenyl sulfone monomer and sulfonated poly(arylene ether sulfone) [79,80].
+7

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