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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,RepublicofKoreabMacromoleculesandInterfacesInstitute,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
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
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 120◦C, 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◦Corunderreducedhumidityconditionsover100◦C.
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
O(CF
2)
2CF
CF
3OCF
2SO
3H
CF
2CF
2*
XCF
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%RHand80◦C).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
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)
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].
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
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
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.
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
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].
[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
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)
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
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
* 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
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].
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−1at80◦Cinwater,andlow
wateruptakesof32and39.6%,respectively.Onatradeoff plotofprotonconductivityversuswateruptake,thePEMs
exhibitedhighprotonconductivityandlowwateruptake
incomparisonwithNafion®.
2.2.2. SulfonatedhydrocarbonPEMswithhighfree volume
OneofthemostimportantissuesforsulfonatedPEMs
infuelcellsiswatermanagementwithinthestructures.If PEMsswellexcessivelywithwaterunderhumidified oper-atingconditions,theycanbedelaminatedfromthecatalyst layerwhichhasarelativelylowerdegreeofwaterswelling,
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].
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 120◦C 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
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
(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
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
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®at80◦Cand50–95%RH.
Leeetal.reportedongraftedSPImembranesthatwere
prepared through thermal-solution imidization and the
subsequentincorporationofsulfoalkylatedgraftingagents withdifferentalkylchainlengths,asshowninFig.31[130]. Theadditionalsulfonicacid(–SO3H)groupsattheendof