Alkaline Membrane Fuel Cells

22 Pages·2017·0.36 MB·English

Chapter 2
Alkaline Membrane Fuel Cells
RobertC.T.Slade,JamieP.Kizewski,SimonD.Poynton,
RongZeng,andJohnR.Varcoe
Glossary
AEM Alkaline(anion)exchangemembrane.
AFC Alkalinefuelcell
AMFC Alkalinemembranefuelcell(alsoknownasAPEMFC)
DMFC Directmethanolfuelcell
MEA Membraneelectrodeassembly
OCV Opencircuitvoltage
PEM Proton-exchangemembrane
PEMFC Proton-exchangemembranefuelcell
QA Quaternaryammonium
RG-AEM Radiation-graftedalkaline(anion)exchangemembrane
Definition
Thedisruptiveapproachofapplyingalkalineanion-exchangemembranes(AEMs)in
alkaline membrane fuel cells (AMFCs) potentially meets several of the challenges
facingotherapproachestolowtemperaturefuelcells,includingtheotherwisehigh
catalystandfuelcosts.Thus,themovetoalkalineconditionsattheelectrodesopens
thepotentialuseofarangeoflowcostnon-precious-metalcatalysts,asopposedto
This chapter was originally published as part of the Encyclopedia of Sustainability Science
andTechnologyeditedbyRobertA.Meyers.DOI:10.1007/978-1-4419-0851-3
R.C.T.Slade(*)(cid:129)J.P.Kizewski(cid:129)S.D.Poynton(cid:129)R.Zeng(cid:129)J.R.Varcoe
DepartmentofChemistry,UniversityofSurrey,GuildfordGU27XH,UK
e-mail:[email protected]
K.-D.Kreuer(ed.),FuelCells:SelectedEntriesfromtheEncyclopedia 9
ofSustainabilityScienceandTechnology,DOI10.1007/978-1-4614-5785-5_2,
#SpringerScience+BusinessMediaNewYork2013
10 R.C.T.Sladeetal.
theotherwisenecessaryuseofplatinum-group-metal(PGM)basedcatalysts.Further,
it becomes possible to consider hydrogen fuels containing substantial amounts of
impurities,whereasanacidicmembraneapproach(thatinprotonexchangemembrane
fuelcells,PEMFCs)requireshigh-puritygasesandPGMcatalysts.
Introduction
ThefirstentryintheAMFCareawaspublishedin2005[1],sincewhenactivityand
interest have continued to increase steeply internationally. Zeng and Varcoe have
recently reviewed the developing patent literature [2]. Some researchers have
recently termed these systems HEMFCs, hydroxide exchange membrane fuel
cells; that terminology is not fully appropriate in view of the complex hydroxide/
hydrogen carbonate/carbonate equilibria that are present (even after handling
membranes in air), inevitable in the use of air (containing CO (g)) as source of
2
oxidantandalsoproducedintheoxidationofmethanolindirectmethanolfuelcells
(DMFCs). Another acronym applied to these systems, APEMFCs, is also poten-
tiallyconfusing,thefirstthreewordsofthefullformthenseeminglybeing“alkaline
protonexchange”(followingtheverysimilarPEMFCacronym)butbeingreadable
alternativelyasalkalinepolymerelectrolytemembranefuelcells(possiblyAEMFC
wouldbeclearer).
Alkalinefuelcells(AFCs,hydrogen-fuelledcellswithanalkalineliquidelectro-
lytesuchasKOH(aq))arethebestperformingofallknownconventionalhydrogen–
oxygen fuel cells operable at temperatures below 200(cid:1)C. This is due to the facile
kinetics at the cathode and at the anode; cheaper non-noble metal catalysts can be
used(suchasnickelandsilver[3,4]),reducingcost.McLeanetal.gavecomprehen-
sivereviewofalkalinefuelcelltechnology[5].Theassociatedfuelcellreactionsboth
foratraditionalAFCandalsoforanAMFCare:
Anode: 2H þ4OH(cid:3)!4H Oþ4e(cid:3)
2 2
ðE ¼0:83Vvs:SHEat1bar;298:15KÞ
a
Cathode: O þ2H Oþ4e(cid:3) !4OH(cid:3)
2 2
ðE ¼0:40 V vs: SHE at 1 bar; 298:15 KÞ
c
Overall: 2H þO !2H O
2 2 2
ðE ¼1:23 V at 1 bar; 298:15 KÞ
cell
AmajorissuewithtraditionalAFCsisthatofelectrolyteandelectrodedegradation
caused by the formation of carbonate/bicarbonate (CO 2(cid:3)/HCO (cid:3)) on reaction of
3 3
(cid:3)
OH ionswithCO contaminationintheoxidantgasstream[5–7]:
2
2 AlkalineMembraneFuelCells 11
Carbonate formation: CO þ2OH(cid:3)
2
!CO 2(cid:3)þH O
3 2
Bicarbonate formation: CO þOH(cid:3) !HCO (cid:3)
2 3
ThemajorcauseofthedegradingperformanceofAFCsistheconsequentprecipi-
tationofmetalcarbonatecrystals(mostcommonlyNa CO orK CO ,dependingon
2 3 2 3
thealkalineelectrolyteused)intheelectrolyte-filledporesoftheelectrodes,blocking
poresandmechanicallydisruptinganddestroyingactivelayers.
The cost offuel cells still retards commercialization in most markets. AFCs are
promising on a cost basis mainly because cheap and relatively abundant non-plati-
num-groupmetals(non-PGM)areviablecatalysts,butarehinderedbydegradation
duetoformationofprecipitatesasabove.Catalystelectrokinetics(forfueloxidation
andoxygenreduction)areimprovedinalkaline,asopposedtoacidic,conditions(the
acid-stabilitycriterionprecludestheuseofmostnon-PGMcatalystsinPEMFCs).The
replacement of the KOH(aq) electrolyte with an alkaline electrolyte membrane
(AEM), to giveAMFCs,retainstheelectrocatalyticadvantagesbutintroduces CO
2
tolerance (there being no mobile cations that could give carbonate/bicarbonate
precipitates) with the additional advantage of being an all-solid-state fuel cell (as
withPEMFCs–i.e.,noseepingoutofKOH(aq)).Additionally,thin(lowelectronic
resistance)andeasilystamped(cheap)metalmono/bipolarplatescanbeused,with
reducedcorrosion-derivedproblemsathighpH(thecostofbipolarplatesforPEMFCs
canbeasmuchasonethirdofthecostofthestacksthemselves).Akeyandyettobe
convincinglymetrequirementis,however,thedevelopmentofadispersiblealkaline
ionomer (sometimes termed an anionomer) to maximize ionic contact between the
catalyst reaction sites and the ion-conductive membranes. As in the case of AFCs,
waterisproducedatanodeandconsumedatcathodeinAMFCs(whenfuelledwith
hydrogen and with four electron reduction of oxygen at the cathode), which is
fundamentally different to what occurs in PEMFCs containing acidic electrolytes;
thiscancausehighoverpotentialsatAMFCanodes,duetosuspectedflooding[8].
The use of an AEM as a solid electrolyte including no metal cations prevents
precipitationofcarbonate/bicarbonatesalts.(Theelectrolytecontainingthecationic
groupsisalreadyasolid.)ThecarbonationprocessisquickeveniftheAEMhasbeen
exposed tothe air for onlya short time[9, 10].The conductivities ofthe AEMsin
(cid:3)
OH form may have been underestimated because most studies to date have not
disclosedvigorousCO exclusion proceduresduringconductivitymeasurements. It
2
(cid:3)
has been hypothesized that OH ion conductivities in AEMs can be estimated by
(cid:3)
measuring the ionic conductivities of HCO from AEMs and multiplying by 3.8
3
[11].Thiscarbonationprocessmaynottobeaseriousproblemduetoaninsitu“self-
(cid:3)
purgingmechanism”becauseOH anionsarecontinuouslygeneratedatthecathode
inAMFCs[12].
AEMs are solid polymer electrolyte membranes that contain positive ionic
groups (typically quaternary ammonium (QA) functional groups such as poly-
NMe +) and mobile negatively charged anions. A widely quoted concern with
3
12 R.C.T.Sladeetal.
AEMs is membrane stability, especially at elevated temperatures [13, 14]. The
generalissuesare:
1. ThediffusioncoefficientsandmobilitiesofOH(cid:3)anionsarelessthanthatofH+
inmostmedia,andQAionicgroupsarelessdissociatedthanthetypicalsulfonic
acidgroups(pK forsulfonicacidgroupsaretypically(cid:3)1butforQAgroupsthe
a
related pK values are around +4); there were concerns that AEMs would not
b
possessadequateintrinsicionicconductivitiesforapplicationinfuelcells.
(cid:3)
2. The OH anions are effective nucleophiles which potentially cause degradation
via (a) a direct nucleophilic displacement and/or (b) a Hofmann elimination
reaction when a b-hydrogen is present; methyl ((cid:3)CH ) groups may also be
3
(cid:3)
displaced by OH ions forming tertiary amines and methanol [13, 14]. If the
AEMscontaingoodleavinggroups(e.g.,QA–NMe +groups)thenthechemical
3
stabilityoftheAEMsmighthavebeeninadequateforuseinfuelcells,particularly
atelevatedtemperatures.
3. Precursor anion-exchange membranes are generally submerged in aqueous
(cid:3)
NaOH/KOHsolutionstoexchangethemtotheOH formAEM;theAEMmust
havethechemicalstabilitytowithstandthisprocess.Despitethis,overadecade
ago,thestabilitiesofvariousbenzyltrimethylammonium-basedAEMswerefound
to be stable at up 75(cid:1)C in NaOH(aq) at concentrations up to 6 mol dm(cid:3)3 for
severaldays[15].
AmajorpotentialapplicationofAMFCsis,however,aspowersourcesforator
near room temperature (as for PEMFCs), which means such degradation can be
minimal.
This entry considers the current understanding and application of AEMs in
hydrogen-fuelled AMFCs and other fuel cell types employing AEMs. Figure 2.1
enablescomparisonbetweenhydrogen-ormethanol-fuelledPEMFCsandAMFCs;
theelectrodereactionsinahydrogen-fuelledAMFCarediscussedabove.Incontrast
toPEMFCs,operationofanAMFCrequiresthepresenceofwaterasareagentatthe
(cid:3)
cathode(oxygenreductionreaction,ORR–toformOH )andtheproductwateris
formedbythehydrogenoxidationreaction(HOR)attheanode(asopposedtobeing
formedatthecathodeinPEMFCs).Theentrybeginswithconsiderationofthemain
classesofAEMs.
An Overview of Alkaline Anion-Exchange Membranes (AEMs)
AEMs and alkaline ionomers (anionomers) are key tothe successful implementa-
tion of AMFCs. Anion-exchange membranes have, for a long time, been used as
separation membranes for seawater desalination, the recovery of metal ions from
wastewaters, electrodialysis and bio-separation processes, for example [16–26].
Thesemembranesmay,however,notbestableorconductiveenoughtobeapplied
in AMFCs. AEMs used in early AMFC studies were reviewed in 2005 [1] and
included polybenzimidazole (PBI) doped with KOH, epichlorohydrin polymer
2 AlkalineMembraneFuelCells 13
a b
3H2 or 6e− Load 6e− 3H2 or 6e− Load 6e−
[CH3 OH + H2O] 3/2O2 [CH3 OH ] 3/2O2+3H2O
6H+ 6OH−
Anode Cathode Anode Cathode
6H2 or
[CO2] 3H2O [CO2 + 5H2O]
Proton-Exchange Membrane Alkaline Anion-Exchange Membrane
[PEM] [AAEM]
Fig. 2.1 A schematic presentation of (a) a proton-exchange membrane (PEMFC) and (b) an
alkalinemembranefuelcell(AMFC),bothfuelledeitherwithH gasordirectlywithmethanol
2
(DMFCmode).Thestoichiometricratiosofreactantsandproductsareshownineachcase
quaternizedwith1,4–diazabicyclo[2,2,2]octane(DABCO)orquaternizedwitha1:1
ratio of DABCO and triethylamine, and commercial membranes such as AHA
(TokuyamaCo,Japan),MorganeADP(SolveyS.A.),Tosflex®SF–17(Tosoh)and
2259–60(PallRAI).MostofthesefuelcellscontainingAEMswere,however,being
operatedinthepresenceofaqueousalkalinesolutions(containingNaOHorKOH).
In the UK (at the University of Surrey), several kinds of QA-containing
radiation-grafted AEMs (RG-AEMs) based on poly(vinylidene fluoride) PVDF
[27–31], poly(tetrafluoroethylene–co– hexafluoropropylene) FEP [27, 28, 32] and
poly(ethylene-co-tetrafluoroethylene) ETFE [32, 33], with good ion-exchange
capacities (IEC) and ionic conductivities and with sufficient stabilities to test the
proof of concept of using AEMs in fuel cells, have been developed. ETFE base
filmsproducethebestAEMsfortestinginAMFCs.Theradiation-graftedmethod-
ology(usinggammaraysorelectronbeamsforirradiation)allowedfortheproduc-
tionofAEMsofdifferentthicknesses,ion-exchangecapacity,physical/mechanical
properties, and chemistries that facilitated fundamental investigations. A water-
insolublealkalineionomerthatusedN,N,N’,N’–tetramethylhexane–1,6–diamineas
thejointaminationandcross-linkingagentfirstenabledstudiesoftheperformance
ofmetal-cation-freeall-solid-statealkalinefuelcellswith stableperformanceinto
the medium term at 50(cid:1)C [33, 34]. Many other groups have now developed new
conductive and chemically and thermally stable AEMs (and candidate alkaline
ionomers), as discussed below. AEMs in the hydroxide form typically become
brittle if allowed to dry; hydroxide forms are typically prepared just before use,
fromthechlorideformanalogue.
14 R.C.T.Sladeetal.
Properties of AEMs
ThemostcommonclassofNaOH-/KOH-freeAEMsbeinginvestigatedforuseinfuel
cellsisbasedonQAchemistryandhasreasonablestabilityinalkalineenvironments
(especially those AEMs containing benzyltrimethyl ammonium exchange sites).
There are three main classes of chemical degradation reaction mechanisms by
(cid:3)
whichnucleophilicOH anionscanremoveQAgroups.Thepresenceofb-hydrogens
allows the Hofmann elimination reaction to occur (Scheme 2.1), often in parallel
tothereactionsdiscussedbelow,yieldingalkene(vinyl)groups;thiscangiveriseto
QA–AEMs that have low thermal and chemical (to alkali) stabilities. If, as in RG-
AEMs, no b-hydrogen atoms are present, direct nucleophilic substitution reactions
weretraditionally thoughttotakeplaceyieldingalcoholand tertiaryamine groups.
However, recent density functional theory (DFT) calculations and deuterium ex-
changeexperimentsatLosAlamosNationalLaboratory(USA)onmodelsmallQA-
containing compounds indicate that a mechanism involving an ylide intermediate
(trimethylammoniummethylide,alsoknownasa1,2-dipolarylidecompound)may
predominateandbemoreseverewhentheAEMsaredehydrated[35].
H
HO C
H
H
H
C N+Me2CH2 Me2NCH2
H
H
direct nucleophillic
displacement mechanism
+ NMe
3
nucleophile
OH-
C HO C
H H N+Me3 H H
benzyltrimethylammonium cation
H
β
Hβ N+Me3 Hβ + NMe3 + HOBβ
C
C C C Hα
Hα Hα Hofmann elimination Hα
mechanism
Scheme2.1 AlternativemechanismsfordegradationofAEMsbydisplacementofthetrimethy-
lammoniumgroupsbyhydroxideanionsatelevatedtemperatures
2 AlkalineMembraneFuelCells 15
AEMsfromTokuyamaCohavegoodthermochemicalstability.Thethin(10mm)
Tokuyama “fuel cell grade” AEMs (A010, A201 – formerly A006 and A901)
[9, 36–39] and their developmental dispersible alkaline ionomers (A3ver.2 and
AS–4) have also been tested at up to 50(cid:1)C by several research groups in direct
alcoholfuelcells[40–42].
Quaternized pyridinium- or phosphonium-based AEMs were thought to have
thermochemical stabilities that are not suitable for use in AEM fuel cells but
there are reports of recent work on polysulfone-phosphonium-based AEMs and
anionomers(seelater).
Surrey’s benzyltrimethylammonium-containing S80, S50, and S20 RG–AEMs
(thenumberdesignatingthefullyhydratedthicknessesinmicrometers)arechemically
stable up to 80(cid:1)C inaqueous KOH [32–34] (aq, 1 moldm(cid:3)3) and can exhibit ionic
conductivities>0.03Scm(cid:3)1atroomtemperaturewhenfullyhydrated;theexsituand
insituionicconductivityoffullyhydratedS80is0.06Scm(cid:3)1at60(cid:1)C(cf.Nafion®
acidicPEMsaretypically>0.1Scm(cid:3)1atthesetemperatures).AEMconductivities
are, however, considerably reduced at humidities RH/% < 100 and drop to values
between0.01and0.02Scm(cid:3)1afteronlyanhourwhenexposedtoair(especiallywith
verythinmembranes)duetothereactionofOH(cid:3)anionswithCO formingCO 2(cid:3)and
2 3
(cid:3)
HCO withinthemembrane.Thelowerdissociationconstantfor–NMe OHgroups
3 3
(requiringahighernumberofwatermoleculesforcompletedissociation),compared
to –SO H groups in PEMs, and a very low number of water molecules directly
3
associated with the ionic groups lead to the poor performance at low RHs (at high
humiditiesmuchofthewaterpresentinAEMsislocatedinaggregatesnotdirectly
associatedwiththeionicgroups)[32].
The developing published and patent literature in this area has recently been
reviewed by Zeng and Varcoe [2]. Further classes of AEM for fuel cells have
included membranes based on quaternized poly(epichlorohydrin), polysulfone,
poly(phthalalazinone ether ketone), poly(2,6–dimethyl–1,4–phenylene oxide),
and poly(vinyl alcohol) grafted with (2,3–epoxypropyl)trimethylammonium
chloride. Common quaternizing agents include alkyliodides, trialkylamines,
N,N,N’,N’–tetramethylalkyl–1,n–diamines, polyethyleneimine, 1,4–diazabicy-
clo–[2.2.2]–octane (DABCO), and 1–azabicyclo–[2.2.2]–octane. The final two
of these have been used extensively; they contain b–hydrogen but their structures
do not permit molecular conformations favored in the Hofmann elimination
mechanism. Metal-cation-containing AEMs based on doping polymer films
(e.g., polybenzimidazole (PBI), poly(vinyl alcohol) and its composites, and bio-
compatible chitosan) with NaOH/KOH(aq) are also being investigated, but the
presenceofmobilecationsmayintroduceproblemsassociatedwithprecipitation
ofcarbonatesalts.
IonicconductivitiesofAEMsaregenerallylowerthanthoseofcomparablePEMs.
(cid:3)
ThisisnotsurprisingasthesolutionmobilityofOH isonethirdtoonehalfofthatof
aH+(dependingontheenvironmentandifthere–m (cid:4)104/cm2V(cid:3)1s(cid:3)1=20.64for
o
OH(cid:3)(aq) anions and 36.23 for H+(aq) cations at 298 K) [1, 43]. One strategy for
enhanced ionic conductivities is to increase the ion-exchange capacity (IEC) via
syntheticmethodology,butthisoftenleadstoadecreaseinthemechanicalstrength
16 R.C.T.Sladeetal.
duetoexcessivewateruptakes.Anotherstrategyistosynthesizetailoredmembranes
that will exhibit hydrophilic(ionic)–hydrophobic(nonionic) phase segregation and
continuous ionic domains, which is hypothesized to increase ionic conductivities
[44,45].
Recent intensive studies have, however, been reported to lead to AEMs with
high ionic conductivities, reportedly comparable to Nafion®. These promising
AEMs [11, 45–51] are still to be evaluated in AMFCs. Most hydrocarbon AEMs
are soluble in various solvents, which is potentially useful for the formulation of
alkaline ionomers required for the preparation of high-performance membrane
electrodeassemblies(MEAs).Iftheconductivepropertiesreportedcanbetranslated
intohighpoweroutputs,thenAMFCperformancescomparabletothoseofPEMFCs
canbeexpectedinthenearfuture.
Synthetic Routes to AEMs
ThepreparationofapplicableAEMsinvolvesacompromisebetweentheproperties
ofthemembrane,suchasthechemicalandthermalstability,ion-exchangecapacity
(IEC),ionconductivity,mechanicalproperties,wateruptake,anddimensionalstabil-
ity. In general, alkaline anion-exchange polymer electrolytes can be polymerized
directly from functionalized monomers, polymerized from monomers with sub-
sequent functionalization or by functionalizing a commercially available polymer.
Thebackboneofthepolymerisusuallyselectedforitsgoodchemicalandthermal
stabilityand,therefore,typicallyincludesaromaticringsand/oradegreeoffluorina-
tion:typicalpolymerclassesincludepolysulfonesandpolyetherketones,polyimides,
poly(phenylene), poly(phthalazinone ether sulfone ketone), polyepichlorohydrin
homopolymer, polybenzimidazole (PBI), poly(phenylene oxide), radiation-grafted
copolymers,inorganic–organichybrids,andevenperfluoronatedmembranessuchas
Nafion. The active functional groups are commonly quaternary ammonium type
((cid:3)NR +) with a clear preference for trimethylammonium ((cid:3)N(CH ) +) groups
3 3 3
(pK (H O)=9.8).
a 2
AsuitableAEMwillhaveahighion-exchangecapacity,highionicconductivity,
andthermochemicalstability,butwillexhibitalowdegreeofswellingonhydration.
There are several general synthetic methodologies for the preparation of AEMs
[52, 53]. Fluorine-containing polymers generally show higher thermal stabilities
thanhydrocarbonpolymers.AEMsbasedonapoly(aryleneethersulfone)containing
fluorineatomsshowhighionicconductivities(63mScm(cid:3)1inCO 2(cid:3)format70(cid:1)C)
3
[53]; this is a key result as CO 2(cid:3) anions have dilute solution mobilities that are
3
lessthan33% ofthat ofOH(cid:3) anions. (It is rare tosee CO 2(cid:3) conductivities above
3
30mScm(cid:3)1).
Irradiationofpolymerfilms(andpowders,etc.)usingX-rays,g-rays,orelectron
beams (as at Surrey, see previous section) is a flexible way to introduce various
functional groups on the polymer backbones (Scheme 2.2). A wide range of
2 AlkalineMembraneFuelCells 17
Scheme2.2 Theradiation-
graftingofvinylbenzyl vinylbenzyl
chlorideontoETFEand chloride
subsequentaminationand
alkali-exchange,yielding
alkalineanion-exchange
membranes(RG-AEMs) CH Cl
2
ETFE ETFE
electron
beaming
(CF CF -CH CH )
2 2 2 2n
m
CH Cl
2
1) N(Me) (aq)
ETFE 3
2) KOH (aq)
m
−
OH
+
CH N (Me)
2 3
chemicallyandthermalstablepolymers,suchasETFEandFEP,canbechosenas
thebasefilmsfortheproductionofAEMs.Additionally,thereisawidechoiceof
functionalmonomersavailablethatcanbeusedtointroduceion-exchangegroups
intothegraftedpolymericchains.
A common strategy in the synthesis of AEMs is to introduce halogen alkyl
groups onto the backbone or side chains of the polymer via chloroalkylation,
fluorination, bromination, or chlorination, followed by amination/quaternization
and finally ion exchange. Highly carcinogenic chloromethylethers have tradition-
ally been used as the chloroalkylation agent but safer strategies have been
introduced, for example, the chloro methylation agent is generated in situ (e.g.,
[44, 55, 56]). An alternative strategy is the prior introduction of tertiary amine
groupsintothepolymer,followedbyquaternization.
The easiest synthetic route is to use inert polymers (as above) doped with
concentrated KOH(aq) (as above): polybenzimidazole (PBI) [57–61], poly(vinyl
alcohol)(PVA)[44],compositepolymerssuchasPVA/hydroxyapatite(PVA/HAP)
[62], quaternized–PVA/alumina (QPVA/Al O ) [63], PVA/titanium oxide (PVA/
2 3
TiO )[64,65],chitosanandcross-linkedchitosan[66–68],copolymersofepichlo-
2
rohydrinandethyleneoxide[69],andcross-linkedPVA/sulfosuccinicacid(10wt.
%SSA)[70]haveallbeendopedwithKOHandusedasAEMs.PatentUS5569559
[71]describestheuseofpolarpolymers(mostpreferredbeingpolyethyleneoxide)
doped with alkaline metal hydroxides (such as KOH), alkaline earth metal
18 R.C.T.Sladeetal.
hydroxidesorammoniumhydroxidessuchastetrabutylammoniumhydroxide;PBI
dopedwithKOHshowedthehighestionicconductivity,comparabletoNafion®(a
standard acidic proton-exchange membrane, PEM). All of these materials could,
however,leadtocarbonateprecipitates.
Anion-exchange polymers that contain methacrylate, ester, amide, or other
carbonyl (C = O double bond) functional groups show low stabilities in alkali as
(cid:3)
thesefunctionalgroupsarehighlyreactivetonucleophilessuchasOH .
Development of Alkaline Ionomers
Alkaline analogues to the oligomeric perfluorosulfonic acid dispersions used to
produce optimized ionic contact between the catalyst reaction sites and the PEM
inPEMFCshaveyettobeconvincinglydevelopedandthishasresultedintheuse
of nonideal strategies for the fabrication of alkaline MEAs: the use of Nafion
dispersions (acidic, cation exchanging);concepts involving adsorbed potassium
hydroxide, and quaternized copolymers made from 4-vinylpyridine monomer at
the electrode–AEM interface. Polysulfone-based alkaline ionomers that are com-
patible with polysulfone AEMs have been developed by various teams. On the
commercial front, it has been reported that Fumatech have developed a ionomer
concept for use with their commercial Fumasep®FAA AEMs, that uses two
precursors that react to form a cross-linked polymer when mixed together, and
Tokuyama’s alkaline ionomers can be deposited from solution (designated A3 or
AS-4)andarechemicallycompatiblewithTokuyama’sfuelcellAEMs.
Surrey developed an alkaline ionomer (SION1, Scheme 2.3) for MEA fabrica-
tion, with the objective of developing a system that would allow the testing of
differentAEMsandelectrodesinAMFCs[72];withoutthisalkalineionomer,the
performances were too low for satisfactory testing. That alkaline ionomer also
allowedfortheoperationoffuelcellsinmetal-cation-freemode,incontrasttothe
possible use of doped polymers containing alkali metal hydroxides. An alkaline
MEAthatwasdeliberatelyconvertedtotheCO 2(cid:3)formoperatedaswellasanOH(cid:3)
3
form MEA; even more interestingly, the CO 2(cid:3) content of the carbonated MEA
3
decreasedduringfuelcelloperationwithair(containingCO )atthecathode.The
2
SION1 ionomer contains b–hydrogen atoms and, therefore, allows the Hofmann
elimination degradation mechanism to operate, limiting the thermal stability to
belowca.60(cid:1)C.Surreyiscurrentlydevelopinganext-generationb-hydrogen-free
alkalineionomersdepositedfromaqueoussolutions(PatentGB0814652.4).
Many researchers have reported that alkaline polymers with hydrocarbon
backbones can be dissolved in solvents such as DMF, DMAc, and DMSO
[47–49, 54, 73, 74], and this allows their use as the ionomer for the preparation
ofMEAs.Zhuangetal.[73,75]usedaquaternaryammoniumpolysulfone(QAPS)
which can be dissolved in DMF and used as the ionomer; the QAPS polymer (in
OH(cid:3) form: IEC = 1.08 mmol g(cid:3)1) was used to fabricate the membrane, while
aQAPSfilm(OH(cid:3)form:IEC=1.18meqg(cid:3)1)wasusedastheionomer.