Degradation of a PEM fuel cell stack with Nafion® membranes of different thicknesses. Part II : Ex situ diagnosis

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Journal of Power Sources, 205, pp. 324-334, 2012-01-21

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Degradation of a PEM fuel cell stack with Nafion® membranes of

different thicknesses. Part II : Ex situ diagnosis

Yuan, Xiao-Zi; Zhang, Shengsheng; Ban, Shuai; Huang, Cheng; Wang,

Haijiang; Singara, Vengatesan; Fowler, Michael; Schulze, Mathias; Haug,

Andrea; Friedrich, K. Andreas; Hiesgen, Renate

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ContentslistsavailableatSciVerseScienceDirect

Journal

of

Power

Sources

j o ur n a l ho m e p age :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r

Degradation

of

a

PEM

fuel

cell

stack

with

Nafion

®

_membranes

_of

_different

thicknesses.

Part

II:

Ex

situ

diagnosis

Xiao-Zi

Yuan

a,∗

_,

_Shengsheng

_Zhang

a

_,

_Shuai

_Ban

a

_,

_Cheng

_Huang

a

_,

_Haijiang

_Wang

a,∗∗

_,

Vengatesan

Singara

b

_,

_Michael

_Fowler

b

_,

_Mathias

_Schulze

c

_,

_Andrea

_Haug

c

_,

_K.

_Andreas

_Friedrich

c

_,

Renate

Hiesgen

d

a_Institute_for_Fuel_Cell_Innovation,_National_Research_Council_Canada,_Vancouver,_BC,_Canada_V6T_1W5

b_Department_of_Chemical_Engineering,_University_of_Waterloo,₂₀₀_University_Avenue_West,_Waterloo,_ON,_Canada_N2L_3G1

c_Institute_of_Technical_{Thermodynamics,}_German_Aerospace_Center,_{Pfaffenwaldring}_38-40,₇₀₅₆₉_Stuttgart,_Germany

d_University_of_Applied_Sciences_Esslingen,_Department_of_Basic_Science,_Kanalstrasse_33,₇₃₇₂₈_Esslingen,_Germany

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received13October2011

Receivedinrevisedform7January2012 Accepted12January2012

Available online 21 January 2012

Keywords:

Protonexchangemembranefuelcell Stack Exsitu Durability Idleconditions Degradation

a

b

s

t

r

a

c

t

PartIofthisstudycarriedoutmembraneelectrodeassemblydegradationofafour-cellstackwithNafion membranesofdifferentthicknesses,includingN117,N115,NR212,andNR211,for1000hunderidle conditions.Throughon-lineelectrochemicalmeasurementsitwasfoundthatasdegradationadvanced, cellswiththinnermembranesexperiencedmuchmorerapidperformancedegradationthanthosewith thickermembranes,especiallyafter800hofoperation,duetoadramaticincreaseinhydrogencrossover. Inthepresentworkweinvestigatethedegradationmechanismsofthisfour-cellstackusingseveralexsitu diagnostictools,includingscanningelectronmicroscopy(SEM),infrared(IR)imaging,ion chromatogra-phy(IC),gaspermeabilitymeasurement,contactanglemeasurement,andsimulation.Theresultsindicate thatthedrasticincreaseinhydrogencrossoverisduetomembranethicknesslossandpinholeformation. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Thedurabilityofprotonexchangemembrane(PEM)fuelcells isoneofthebiggestchallengesandbarriersforcommercializing this promising technology. The 2015US Department ofEnergy lifetimerequirementsfortransportationapplicationsrangefrom 5000hforcarsto20,000hforbuses,andforstationaryapplications therequirementis40,000hofcontinuousoperation.Currently,the lifetimesoffuelcellvehiclesandstationarycogenerationsystems arearound1700hand10,000h,respectively[1].Accordingly, con-tinuousR&D ontheissues relatedtoPEMfuelcelldurabilityis stillrequiredtogainafundamentalunderstandingofdegradation mechanisms,whichinturnwillhelpreducethetechnologygapand providemitigationstrategies.

PEMfuelcellsconsistofmanycomponents,includingcatalysts, catalystsupports,membranes,gasdiffusionlayers(GDLs),bipolar plates,sealings,andgaskets.Eachofthesecomponentscandegrade

∗ Correspondingauthor.Tel.:+6042213000x5576.

∗∗ Correspondingauthor.Tel.:+6042213038;fax:+6042213001.

E-mailaddresses:xiao-zi.yuan@nrc.gc.ca(X.-Z.Yuan),

haijiang.wang@nrc-cnrc.gc.ca(H.Wang).

orfailtofunction,thuscausingthefuelcellsystemtodegradeor fail.Sofar,variousfailuremodesofcomponentdegradationhave beenidentified,suchascatalystparticleagglomeration/dissolution, carbonsupportoxidation,membranedissolution,ionomer degra-dationinthecatalystlayer(CL),hydrophobicitychangesintheCL and/orGDL,andPTFEdecompositionintheCLand/orGDL. Depend-ingoninternalfactors(e.g.,materialproperties, andthedesign ofthecomponentsandthestack)andexternalfactors(e.g.,fuel celloperatingconditions,impuritiesorcontaminantsinthefeeds, andenvironmentalconditions),oneorseveralfailuremodesare predominantovertheothers[2].

Generally,itisimpracticalandcostlytooperateafuelcellunder itsnormalconditionsforseveralthousandhours;acceleratedtest methodsarethuspreferredtofacilitaterapidlearningaboutkey durability issues.The USDepartment of Energyhasestablished acceleratedstresstest(AST)protocolsforcellcomponents, includ-ingelectrocatalysts,catalystsupports,membranes,andmembrane electrodeassemblies,inanattempttoprovideastandardsetoftest conditionsandoperatingprocedurestoevaluatenewcell compo-nentmaterialsandstructures[3].Differentresearchgroupsmay prefertheirowntestprotocolsordesignspecialdegradationteststo investigatefailuremodes,withaspecialtargetinmind.For exam-ple,astheidletimemayamounttoseveralthousandhoursover

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theentirelifetimeofafuelcell,itisconsideredoneofthemost crucialfactorsaffectingthecell’sdurability,especiallyfor automo-tivefuelcellsystems[4,5].Wuetal.[6]conductedasix-cellstack degradationprocedureunderclosetoopencircuitvoltage(OCV) conditions,findingconsiderablehydrogencrossoverafter800hof operation.Tofacilitatemembranedegradationstudies,Yuanetal.

[7]reportedanaccelerateddegradationtestinvolvingafour-cell stackdegradationprocedureusingNafionmembranesofdifferent thicknesses,includingN117,N115,NR212,andNR211,for1000h underidleconditions.Thecellperformanceandelectrochemical characteristicsweremonitoredapproximatelyevery200hof oper-ation.

WhilevariousASTsaredesignedandperformedonPEMfuel cells,numerousdiagnostictools,includinginsituandexsitutools, areimplementedtointerpretexperimentalresults,toinvestigate failuremodes,andtogainafundamentalunderstandingof compo-nentdegradation.Todate,awiderangeofexperimentaldiagnostic toolsfortheaccurateanalysisofPEMfuelcellsandstackshavebeen developed,asreviewedbyWuetal.[8,9].Forexample,

•_Atomic_force_microscopy_(AFM)_–_Nanoscale_Nafion_membrane andGDLdegradation[10,11]

•_Infrared_(IR)_imaging_–_Membrane_thinning_and_pinhole_detection

[12,13]

•_X-ray_diffraction_(XRD)_–_Particle_size_change_[14] _and_loss_of surfacearea[15]

•_Scanning_electron_microscopy_(SEM)_–_{Morphological} _changes

[16]anddegradationofmembrane[17],CL[18],andGDL[19]

•_Transmission_electron_microscopy_(TEM)_–_Diagnosis_of electro-catalystdegradation[20]

•_Fourier_transform_infrared_spectroscopy_(FTIR)_–_Polymer alter-ationsinducedbydegradation[21]

•_X-ray_{photoelectron}_spectroscopy _(XPS)_– _Surface _component loss[22]

•_Ion_{chromatography}_(IC)_–_Membrane_and_ionomer_degradation throughanalysisofdissolvedfluorideions[23,24].

Theseexsitutoolsplayanimportantroleininvestigatingfailure modesanddegradationmechanisms.

Previously,weperformedaninsitudiagnosisofafour-cellstack withmembranesofdifferentthicknesses,wheretraditional elec-trochemicaldiagnostictools (including OCV,polarizationcurve, electrochemicalimpedancespectroscopy(EIS),andlinearsweep voltammetry(LSV))wereusedtoobserveperformancelossesafter 1000hofoperationunderidleconditions[7].Inthepresentwork, anex situ diagnosisis carriedout toidentifythemajor failure mode.TheseexsitumeasurementsincludeSEM,IRimaging,IC, gaspermeabilitymeasurement,andcontactanglemeasurement.

2. Experimental

2.1. Fuelcellmaterialsandtestapparatus

Thefuelcelltestedwasafour-cellstackwithanactiveareaof 50cm2_._The_materials_used_were_Ion _Power_customized_catalyst coatedmembranes(CCMs)(N117,N115,NR212,andNR211)with aPtcatalystloadingof0.3mgcm−2_,_supported_on_carbon_on_both sides,andSGLGDLs.Duringthedegradationtest,thefour-cellstack wasoperatedataconstantcurrentof0.5A(10mAcm−2_)._The_fuel cellstacktemperaturewaskeptat70◦_C,_and_air_and_hydrogen_were fullyhumidifiedpriortotheirdeliveryintothefuelcell.Detailed proceduresofthisstackdegradationtest,aswellaselectrochemical measurements,havebeendescribedintheliterature[7].

2.2. Exsitudiagnosis 2.2.1. SEM

Cross-sectional SEM imageswere taken using a HITACHI S-3500N before and after degradation to identify morphological changesaswellasthicknesschangesinthemembraneandCLs.All thesamplesusedforSEMscanning,includingfreshanddegraded NR211, NR212, N115, and N117, were cut from 50cm2 _sheets into10mm×20mmpieces.Thesepieceswerethenfracturedafter immersioninliquidnitrogen.

2.2.2. IC

Thefluoridecontentofthewatersamplescollectedfromthe anodeandcathodeatthebeginningoftest(BOT)andendoftest (EOT)wasanalyzedusinganionchromatograph(DionexDX500) withanelectrochemicaldetector(DionexED40)andworkingwith agradientpump(DionexGP40).Theanalyticalcolumnusedto separatethefluorideionsfromotheranionswasaDionexAS17 plusAG17guardcolumn,and20mMNaOHwasusedastheeluent. Basedontherelativeexchangeofanionsbetweenthestationary columnandmobilephase,theanionswereelutedanddetectedat differenttimeintervals,makingthechromatogram.Theminimum detectablelimitoffluorideionsinthesamplewas0.011mgL−1_.

2.2.3. IRimaging

Todetectpinholesgeneratedinthemembranesafter degrada-tion,IRimagingwascarriedouttoobservethelocationofhigher hydrogencrossover,reflectedbythetemperaturedistributionon themembraneelectrodeassembly(MEA),whichisbasedonthe heatoutputfromthechemicalreactionbetweenhydrogen(crossed over from the anode) and oxygen in the presence of Pt cata-lyst[12].ThetemperaturedistributionobtainedfromIRimages demonstratesthelevelofreactantcrossoverthroughthePEM.For comparison,thesametestwasalsoconductedonafreshMEA.

IRimagingwasperformedusinganIRcamera(InfraTechGmbH) andaspeciallydesignedcell(50cm2₎_with_an_open_cathode,_which providedsufficientairforthereaction.Theanoderequiredadiluted hydrogensupply(e.g.,5%H2inN2,Linde).Theentire experimen-talset-upincludedahydrogencylinder,hydrogenregulator,flow meter, dummycell, IRcamera,and computer. Theentire piece offreshordegradedMEAwasinstalledintheself-designedcell, withdilutedhydrogenpassingthroughtheanodeataflowrate of30sccmmin−1_at_room_temperature_and_a_pressure_of₅_psi._The IRcamerawasplacedtowardstheopencathodetoobservethe temperaturedistribution.

2.2.4. Gaspermeability

Selectivepermeabilityofgasesthroughthemembraneisaneasy andviablemethodtoinvestigatemembranefailures,suchasrips, tears,cracks,andpinholes.Animpulsiveincreaseinpermeability maybeexpectedafteralong-termAST.Duringthisexperiment, selectivegases,suchasHe,N2,orAr,whichhavedifferent molec-ularsizes,werefedintotheanodesideofthecell,andthegasflux acrossthemembranewasmeasuredatthecathode.Thiscanbe doneforbothfreshanddegradedsamples.Byusingdifferentgases andtheirselectivepermeability,onecandistinguishwhetherthe permeationisrelatedtothesolutionortoKnudsonbehavior.The apparentpermeability(PM)ofthemembranewascalculatedusing Fick’slaw(1):

NA=PM(PI −PII)

l (1)

where PI and PII arethe partialpressures oftheanalyte gasat eithersideofthemembrane,andNAisthefluxofgasmolecules acrossthemembranewithafinitethickness(l).Itisassumedthat thegasfluxthroughthemembranewillincreaseifthemembrane

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CF

2

CF

2

CF

2

CF

OCF

2

CF

3

O

CF

2

x

y

m

CF

2

SO

3

H

Fig.1.ChemicalstructureofNafion.Inthiswork,theequivalentweightischosen tobeabout1100gofdryNafionpermoleofsulfonicacidgroups,withx=6.5and

y=m=1.

thicknesschangesduetothinningand/orthepresenceof mem-branepinholes,tears,orcracks.

2.2.5. Contactangle

ContactanglemeasurementwasconductedonaKitchener 519-748-4612fromFOLIOInstrumentsInc.,andthewaterdropimage takenandtheanglefittingprogramusedwasFTA32.Forcontact angleobservationoftheGDLs,twopieces(10mm×10mm)were cutfromeachfreshanddegradedsamplesheetsforboththeanode andcathode,andthenthreedifferentspotsweremeasuredoneach sample.Allthesampleswerestuckontoaglasssubstrateusinga double-sidedsticker.ToobservethecontactanglefortheCLs,the GDLlayerswerepeeledoffcarefullywiththehelpofascalpel.Note thatforeachsamplewechoseatleastthreelocations,andeach imagewasfittedatleastthreetimes.Therefore,thefinalresultfor thecontactangleisanaveragevalueforeachsample.

Notethatforeachoftheexsitutests,weusedthesamefreshand degradedsamples.IRimagingandgaspermeabilitymeasurements

Fig.2. Voltagedegradationtrendsforindividualcellsunderidleconditionsfor 1000h(operatingconditions:flowrateH2/air=1/2slpm,celltemperature70◦Cand

fullyhumidifiedonbothsides).

wereperformedbeforecuttingthesamplesforSEMandcontact angleobservations.The“fresh”samplesweretheonesthathave beencompressedandconditionedwithoutanyfurtheroperation asdescribedinourpreviouspublication[7].

2.3. Simulationmethods

ThekineticMonteCarlo(KMC)methodwasusedtomodelthe mainchainunzippingprocess[25,26].In thismethod,all possi-bleeventsorsimulationstepsareconsideredindependentforall

Fig.3. ComparisonofSEMimagesforNR211samplesbeforeandafterdegradation,atdifferentmagnifications:(a)beforedegradation,atamagnificationof200×,(b)after degradation,atamagnificationof200×,(c)beforedegradation,atamagnificationof1.0k×,and(d)afterdegradation,atamagnificationof1.0k×.

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Fig.4.ComparisonofSEMimagesforNR212samplesbeforeandafterdegradation,atdifferentmagnifications:(a)beforedegradation,atamagnificationof180×,(b)after degradation,atamagnificationof180×,(c)beforedegradation,atamagnificationof1.0k×,and(d)afterdegradation,atamagnificationof1.0k×.

Nafionatoms.Acertaineventisselectedwithaprobability pro-portionaltoitsrate.Inparticular,thecarboxylgroupsareassumed tobethemajorreactant,witharateconstantof1.TheArrhenius equation k=Aexp

_E_a RT

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isoftenusedtoderivetherateconstantsfromthereactionenthalpy calculatedbydensityfunctiontheory[27].Asthepre-exponential termAisnotdeterminedexplicitlyinthiswork,theprocessing timecalculatedbyKMChasanarbitraryunit.Thechemical struc-tureofNafionisshowninFig.1[28].Inthepresentwork,aNafion polymerconsistedof20monomers,namely20sidechainsand300 CFxgroupsinthePTFEbackbone.Intotal,1000Nafionchainswere used.TheconcentrationofperoxideradicalsOH•_was_assumed_to beconstant[27].Byneglectingtheformationprocedureoffree rad-icalsinacceleratedstresstestwemayhaveslightlyoverestimated thequantityofradicalsintheinitialstage,whereionomer disso-lutionmayhavetakenplacelessrapidlythanwemodeled[27].A typicalsimulationwasstartedfromanetworkofNafionionomers partiallyterminatedwithcarboxylendgroups,withamaximumof twoperNafionchain.Aftertheexecutionofthisevent,thelistof possibleeventswasupdatedandtheprocedurewasrepeateduntil thedesiredweightlossofNafionwasreached.Statisticaldatafor thedegradationproductswascollectedduringthewholeprocess.

3. Resultsanddiscussion 3.1. Summaryofinsituresults

Basedonthepreviousinsitudiagnosis,theOCVofthe individ-ualcellsdecreasesmuchfasterforthinnermembranes,especially after800hofoperation.Beforedegradation,thethinnerthe mem-braneis,thehigherthecellperforms. However,asdegradation progresses,thecellswiththinnermembranesdegrademuchfaster.

Fig.2showsthevoltagedegradationcurvesofeachindividualcell ofthestack upto1000hunderidleconditions.Ascanbeseen, from600to800hthecellwiththeNR211membranedegrades significantly, whereas theother cells do not experiencedrastic degradation;from800to1000h, both cellswiththinner mem-branes(NR211andNR212)degradedramatically,whilethetwo cellswiththickermembranes(N115andN117)stillretainaslight degradation rate. Fig. 2 yields an average degradation rate of approximately0.18mVh−1_for_NR212_and_0.26_mV_h−1_for_NR211, andofapproximately0.09mVh−1_for_both_N115_and_N117._Note thatthevoltagejumpswerecausedbyinterruptionsofthe degrada-tionprocessforelectrochemicalmeasurementsat200hintervals orunexpectedshutdownsoftheteststationasindicatedinthe figure.Basedonthelinearsweepvoltammetry(LSV)curves mea-suredaftereach200h,increasedhydrogencrossoverwasfound toberesponsibleforthedrasticperformancelossforcells with thinnermembranes[7].Thisisinagreementwithourprevious degradationtestunderidleconditions[6].Theresultsindicatethat

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Fig.5.ComparisonofSEMimagesforN115samplesbeforeandafterdegradation,atdifferentmagnifications:(a)beforedegradation,atamagnificationof200×,(b)after degradation,atamagnificationof200×,(c)beforedegradation,atamagnificationof500×,and(d)afterdegradation,atamagnificationof500×.

membranedegradationisthemajorsourceoftheentirecell per-formancedegradationunderidleconditions.Itwasassumedthat membranethinningorpinholeformationwasresponsibleforthe increasedhydrogencrossover[7].

3.2. Exsituresults 3.2.1. Thicknessloss

Figs.3–6showtherespectiveSEMimagesforNR211,NR212, N115,andN117samplesbeforeandafterdegradation,at differ-entmagnifications.Macroscopically,obviousthicknessdecreaseis observableafter1000hofdegradationunderidleconditions, espe-ciallyforthinmembranes.Wemeasuredthethicknesschangesfor allthemembranes;theresultsarelistedinTable1.

AsshowninTable1,thereisa3–4␮mdifferenceinmembrane thicknessbetweenthefreshsamplesandsamplesconditionedby cellcompression(100psi).Afterdegradation,membranethickness decreasessignificantly,especiallyforthinmembranes,withatotal thickness lossof 66% for NR211 and 68% for NR212. However, thisdoesnotmeanthatthinmembranesdegradefaster.Thickness

Table1

Membranethicknesschangebeforeandafterdegradation.

Fresh (␮m) After conditioning (␮m) Degradedafter 1000h(␮m) Thickness lost(␮m) Thickness decrease(%) NR211 25.4 22 7.5 14.5 66 NR212 50.8 44 14 30 68 N115 127 124 78 46 37 N117 183 179 142 37 21

losses for thin membranes arestill lowerthan for thick mem-branes,sincethereislessNafioninthethinmembranestodegrade. Webelievethatforthinmembranes,onlyasmallfractionofthe membraneneedstodegradetoobserveacrossoverincreaseand subsequentOCVdecrease.

AsanalyzedbyCollieretal.[29],thecausesofmembrane degra-dationincludemechanicaldegradation,thermaldegradation,and chemicaland electrochemicaldegradation.Webelievethat dra-matic membrane thickness loss or membrane thinning can be attributedtoradicalattacks,amechanismproposedbyCurtinetal.

[26].AsradicalsformedattheanodewillattackH-containingend groups,Nafionmembranedegradationbeginswiththeabstraction ofhydrogen.Theformedperfluorocarbonradicalcanthenreact witha hydroxylradicaltoproduceHFandanacidfluoride.The finalstepincludeshydrolysisoftheacidfluoride:

(i)R-CF2COOH+•OH→ R-CF₂• +CO₂+H₂O (3) (ii)R-CF2• +•OH →R-CF₂OH→ R-COF+HF (4) (iii)R-COF+H₂O→R-COOH+HF (5) The water samples wereanalyzed using IC. This analysisof thefluoride concentrationinwater from thestack outletsmay beessentialinrevealingthelevelofmembrane dissolutionand ionomerlossintheCL.Theresultshows thatfluorineemission ratesincreaseddramaticallyfrom2.2mgL−1_and_2.4_mg_L−1_at_BOT to11.9mgL−1_and_10.3_mg_L−1_at_EOT_for_the_anode_and_cathode_of thestack,respectively.Therefore,thetremendousthicknesslossin themembraneswasfurtherconfirmedbythefluorinerelease mea-suredbyIC.Unfortunately,thistechniquecouldnotdetectfluorine releaseratesforindividualcells.

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Fig.6.ComparisonofSEMimagesforN117samplesbeforeandafterdegradation,atdifferentmagnifications:(a)beforedegradation,atamagnificationof70×,(b)after degradation,atamagnificationof70×,(c)beforedegradation,atamagnificationof200×,and(d)afterdegradation,atamagnificationof200×.

FromourSEMimages,wealsomeasuredthethicknesschanges intheCLs;theresultsarelistedinTable2.Wefoundthat:theCL thicknessesforbothelectrodeswerebetween9and13␮mbefore andafterdegradation;noobviouscathodeCLthinningwas iden-tifiable;andathicknesslossofabout1–2␮mwasobservablefor theanodeCL,but thechangewasnotsignificant.TheCLlossis attributabletoPtdissolution/migration/washoutandionomerloss, asobservedbyfluoriderelease.Itisstillunclearwhytheanodelost morecatalystthanthecathode.

3.2.2. Pinholeformation

Fig.7(a)–(d)comparesfourfreshMEAs withdifferent mem-branes.Ascanbeseenfromthefourimages,temperatureisevenly distributedthroughouttheMEAforallthefreshsampleswith dif-ferentmembranes.Noobvioushotspotcanbeobserved,evenfor thethinnestmembrane.However,fromthecolororaverage tem-peratureof each sample,we canconcludethat thethinnerthe membrane,thehighertheaveragetemperatureandthegreater the hydrogen crossover through the membrane, which agrees

Table2

Catalystlayerthicknesschangebeforeandafterdegradation.

Anodeafter conditioning (␮m) Cathodeafter conditioning (␮m) Anodeafter degradation (␮m) Cathodeafter degradation (␮m) NR211 11–12 11–12 10–11 11 NR212 9–10 9–10 8–9 10–11 N115 11–12 11–12 9–10 11–12 N117 13 13 11 13

perfectlywiththeresultsfromLSV inourpreviouspublication [7].Apparently,hydrogencrossoverthroughmembranes,aswell asmembranethicknesses,canbecomparedordetermined quali-tativelyusingIRimaging.

Figs.8–11compareIRimagesforMEAsampleswithdifferent membranesbeforeandafterdegradation.Forthickermembranes (N117andN115)thereisaveryslightincreaseintheaverage tem-perature,asshownbythecolorchangeintheimages.However,this temperature/crossoverchangeisnotatallsignificantbeforeorafter degradation,whichmatchestheslightperformancedecayobserved fromthepolarizationcurve.Forthinnermembranes(NR212and NR211),hydrogencrossesoversignificantlythroughaweakarea orpinholesafter1000hofoperationunderidleconditions.This resultagreesverywellwiththeLSVcurvesdescribedinour pre-viouspaper, and explains the OCVdegradation, as well asthe performancedegradationmeasuredbythepolarizationcurves.We furtherspeculate thatfor thinnermembranes,pinholes arethe majorreasonformembranedegradation,ratherthanmembrane thinning,indicatedbyaslightincreaseinthebackground temper-ature.

Theformationofpinholescanbeexplainedasacomprehensive effectoftheaforementionedmembranedegradationmechanisms. Radicalsformedfromelectrochemicalreactionsattackthe mem-brane,causinglossesofhydrogen,fluorine,sulfur,andendgroups, whichisadirectreasonformembranethinning.Athinner mem-braneleadstomorehydrogencrossover,whichfurtherworsensthe situation.Also,thecrossedhydrogenmayreactwithoxygen, caus-inglocalheatstress.Together,thecontinuousradicalattacks,heat stress,andmechanicalstressarisingfromthewettabilitychange leadtopinholeformation.

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Fig.7.ComparisonofIRimagesforfreshMEAsampleswithdifferentmembranesusing5%H2inN2:(a)freshN117MEA(averagetemp:23.091◦C),(b)freshN115MEA

(averagetemp:23.429◦

C),(c)freshNR212MEA(averagetemp:23.724◦

C),and(d)freshNR211MEA(averagetemp:24.063◦

C).

Tofurtherconfirmthatpinholesdoexistafter1000hof mem-branedegradationtestingforthinnermembranes,wealsodesigned adifferentmeasurementusingthesameIRimagingset-upto visu-allyobservepinholes.InsteadofemployinganIRcameraforthe cathode,weputacertainamountofdeionizedwaterontopofthe opencathode,withhydrogenpassingthroughtheanodechannel ofthecellatapressureof5psiandaflowrateof30sccmmin−1_. Nowaterbubbleswereobservableforthefreshsamples,orforthe degradedN117andN115samplesafter1000hofdegradation. Bub-bleswereobservedforthedegradedNR212andNR211MEAs,as showninFig.12.Unfortunately,thebubblelocationsdonotexactly matchthelocationsofthehotspotsobservedfromIRimages.This canbeexplainedbytheporousnatureofGDLs,asthehydrogen

crossingoverthroughpinholesprefersaneasierpaththroughthe GDL(notnecessarilyrightinlinewiththepinholes).

Furtherproofofpinholeformationcanbeobtainedthroughagas permeabilitytest.Fig.13showsthepermeabilityofheliumthrough highlypermeableandlesspermeableMEAs,definedthus:

•_{non-permeable:}_no_gas_flow_across_the_MEA_until₅₀_psi •_less_permeable:_very_low_gas_flow,_indicated_by_bubble_formation

atlongerintervals

•_highly_permeable:_considerable_gas_flow_across_the_MEA, mea-suredbyrotameter

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Fig.9.ComparisonofIRimagesforN115MEAsamplesbeforeandafterdegradationunderidleconditionsusing5%H2inN2:(a)beforedegradationand(b)afterdegradation.

Fig.10. ComparisonofIRimagesforNR212MEAsamplesbeforeandafterdegradationunderidleconditionsusing5%H2inN2:(a)beforedegradationand(b)after

degradation.

Basedontheexperimentalresults,allthesamplesare catego-rizedintothesethreepermeabilitylevels,aslistedinTable3.The highpermeabilityofheliumthroughdegradedNR211andNR212 furtherconfirmstheformationofpinholesduringthe1000hof operationunderidleconditions.

3.2.3. LossofPTFE

Themostdirectmeasurementofcontactangleisachievedusing thesessiledropmethod,whereadropofliquidisplacedonasolid surfaceandtheresultingangleformedbetweentheplaneofthe solidandthetangenttothedropatthesolid–liquidcontactpoint ismeasured.Thisapproachbecomescomplicatedwhenthesolid

Table3

OverallcomparisonoffreshanddegradedMEApermeability.

Observation MEAsamples(fresh)

N117 Non-permeable N115 Lesspermeable NR212 Lesspermeable NR211 Lesspermeable MEAsamples(degraded)

N117 Lesspermeable N115 Lesspermeable NR212 Highlypermeable NR211 Highlypermeable

Fig.11. ComparisonofIRimagesforNR211MEAsamplesbeforeandafterdegradationunderidleconditionsusing5%H2inN2:(a)beforedegradationand(b)after

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Fig.12.Picturesofbubblesafter1000hofdegradationunderidleconditionsusing 5%H2inN2:(a)degradedNR211and(b)degradedNR212.

ofinterestisaporousmedium.Duetotheroughnessand poros-ityofthesurface,adropletonlycontactsafractionofthesolid, causingtheobservedcontactangletodeviatefromtheactualvalue. Despite suchdifficultiesin measuring theactualvalue, contact anglecanstillbeausefulqualitativetoolformakingdirect com-parisonsbetweenGDLs,suchascomparisonsofchangesbetween freshanddegradedsamples.

Table4showsthatstatistically,thecontactangleofthefresh anodeisabout2◦_higher_than_that_of_the_cathode,_as_we_used_SGL 25DC(20%PTFE)fortheanodeandSGL 25BC(5%PTFE)forthe cathode.Afterdegradation,thecontactangleforboththeanode andthecathodedecreasesabout2◦_,_indicating_PTFE_loss_in_the_GDL forbothsidesduringthedegradationtestasthehydrophobicityof theGDLisbasicallydependantonitsPTFEcontent,andthemore hydrophobictheGDLis,themorePTFEtheGDLcontains.

ToobtaincontactangleobservationsfortheCLs,theGDLlayers werepeeledoffcarefullywiththehelpofascalpel,thenthecontact

Table4

ComparisonofcontactanglechangeofGDLs.

Freshanode(◦ ) Freshcathode (◦ ) Degraded anode(◦ ) Degraded cathode(◦ ) NR211 143.3 138.4 140.1 138.1 NR212 141.4 141.2 140.6 139.0 N115 143.1 140.9 139.6 136.0 N117 143.4 140.2 141.7 138.8 Average 142.8 140.2 140.5 138.0 0 0.2 0.4 0.6 0.8 1 1.2 60 50 40 30 20 10 0 Pressure (psi) Flux (cm 3 · cm -2 · s -1 ) Degraded NR211 Degraded NR212

(a)

0 10 20 30 40 50 60 70 60 50 40 30 20 10 0 Pressure (psi) Time (s) Degraded N117 Fresh N115 Degraded N115 Fresh NR211 Fresh NR212

(b)

Fig.13. Permeationofhelium(a)throughhighlypermeableMEAsand(b)through lesspermeableMEAs.

anglesfortheanodeandcathodeCLsweremeasured. However, fromthedatathusobtained(Table5)wedonotseeanyregular patternofincreaseordecreaseafterdegradationfortheanodeand cathode.ThisisprobablyduetounsuccessfulpeelingoftheGDL fromtheMEA.

.

3.3. Modeling

The mechanism of membrane thinning can be understood throughsimulating Nafionweightlossduring degradation test-ing.Asthemajorreactantsinvolvedinthemainchainunzipping processarethecarboxylendgroups,theirrateofweightlosswas calculatedinawiderangeofconcentrations,asshowninFig.14.A linearcorrelationwasfoundbetweenthelossrateandthe car-boxylpopulation, indicatingthat theinitialamountofcarboxyl groupspresentinthemembranedeterminedthedegradationrate ofNafionduringthetest.Moreover,suchalinearcorrelationcan alsobe noted fromtherelease of the degradationproducts, as showninFig.15.Fluorideionshadthehighestemissionrate,about twotimeshigherthanthatofCO2.Incomparison,theamountof

Table5

ComparisonofcontactanglechangeofCLs.

Fresh anode(◦ ) Freshcathode (◦ ) Degraded anode(◦ ) Degraded cathode(◦ ) NR211 142.7 137.0 142.3 138.6 NR212 132.7 135.4 138.7 144.7 N115 141.3 134.5 134.3 133.6 N117 140.3 131.9 133.3 129.3 Average 139.2 134.7 137.1 136.5

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0.4 0.3 0.2 0.1 0 0.5 0.6 0.7 0.8 -COOH fraction 0 0.1 0.2 0.3 0.4 0.5

Rate of weight loss [a.u.]

Fig.14.WeightlossofdegradedNafionwithvariousfractionsofcarboxylend groups. 0.4 0.3 0.2 0.1 0 0.5 0.6 0.7 0.8

-COOH fraction

0 1 2 3 4

Product emission rate

F -CO₂

SO₄

2-Fig.15.ProductemissionratesofdegradedNafionwithvariousfractionsofcarboxyl endgroups.

reducedsulfatefromNafionsidechainswasnegligible.Onecan expectthatthelossofsulfonicgroups,eveninasmallamount,may considerablydecreaseaNafionmembrane’sprotonconductivity. Theevolutionof–SO3Hand–COOHendgroupsisthuscomputed

120 80 40 0 160 200 Time [a.u.] 0 4 8 12 16 20 Number of -SO 3

H per Nafion chain

0 0.2 0.4 0.6 0.8 1

Number of -COOH per Nafion Chain

Fig.16.Theevolutionof–COOH/–SO3HgroupsindegradedNafionwith10%

car-boxylendgroups.

Fig.17.SnapshotofdegradedNafionwith50%carboxylendgroups.Thefragments leftaretrifluoroaceticacid.

inFig.16.Itisinterestingtonotethatthenumberofcarboxylgroups remainsconstant,whileagradualdeclineinsulfonicgroupsoccurs alongwiththetestingtime.Thelossofsulfonicgroupsisattributed tothelossofNafionsidechainsinthemainchainunzipping pro-cess.Thereasonfortheineffectivedissolutionofcarboxylgroups canbeanalyzedfromthesimulationsnapshotshowninFig.17.As carboxylgroupsareusuallyterminatedattheendsofNafionchains, theshrinkageofNafionpolymertakesplacefromtheendstothe center.Duringthisprocedure,noadditionalcarboxylgroupsare createdandtheunzippingrateisbasicallyproportionaltothe con-centrationofcarboxylendgroups.Basedonthepresentmodeling resultsofNafiondegradation,itisknownthatthecarboxyldefects, initiallypresentinthemembrane,aresusceptibletoradicalattack. 4. Conclusions

Using a four-cell stack with Nafion membranes of different thicknesses,an acceleratedstresstest under idleconditions for 1000hwascarriedout.Theresultsindicatethatunderthese con-ditions,membranedegradationisthemajorsourceoftheoverall cell performance degradation. The predominant reason for the drasticperformancedecay thatoccurredafter800hfor thinner membranesisthedramaticincreaseinhydrogencrossover,caused by significantmembrane thickness loss and pinholeformation. Although the performance of thin membranes degrades much fasterthanthatofthickmembranes,thethicknesslossforthe for-merislower,asthinmembraneshavelessmaterialtodegrade.The mechanismofmembranethinningcanbeunderstoodthroughthe simulationofNafionweightlossviamainchainunzipping.Apart frommembranedegradation,whichispredominantinthis accel-erateddegradationtestunderidleconditions,otherdegradation mechanismsalsocontributetothetotalperformancelossofthe stack–forexample,catalystdegradation(thicknesslossintheCLs) andhydrophobicitylossintheGDL(contactangledecreasesfor bothGDLs).

Acknowledgments

Theauthorsacknowledgethe NRC-HelmholtzJointResearch Program,NRC-MOSTJointResearchProgram,andNRC-IFCI’s Inter-nalProgramforfinancialsupport.

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References

[1]T.Payne,FuelCellsDurability&Performance,USBrookline:TheKnowledge PressInc.,2009.

[2]H.Wang,H.Li,X.Z.Yuan,PEMFuelCellFailureModeAnalysis,CRCPress,2011. [3]DOECellComponentAcceleratedStressTestProtocolsforPEMFuelCells,

http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/components.html

(March2007).

[4]S.Sugawara,T.Maruyama,Y.Nagahara,S.S.Kocha,K.Shinohra,K.Tsujita,S. Mitsushima,K.Ota,J.PowerSources187(2009)324–331.

[5] M.F.Mathias,R.Makharia,H.A.Gasteiger,J.J.Conley,T.J.Fuller,G.J.Gittleman, S.S.Kocha,D.P.Miller,C.K.Mittelsteadt,T.Xie,S.G.Yan,P.T.Yu,Electrochem. Soc.Interface14(2005)24–35.

[6]J.Wu,X.Z.Yuan,J.Martin,H.Wang,D.Yang,J.Qiao,J.Ma,J.PowerSources195 (2010)1171–1176.

[7]X.Z.Yuan,S.Zhang,H.Wang,J.Wu,C.Sun,K.A.Friedrich,M.Schulze,A.Haug, J.PowerSources195(2010)7594–7599.

[8]J.Wu,X.Z.Yuan,H.Wang,M.Blanco,J.J.Martin,J.Zhang,Int.J.HydrogenEnergy 33(2008)1735–1746.

[9] J.Wu,X.Z.Yuan,H.Wang,M.Blanco,J.J.Martin,J.Zhang,Int.J.HydrogenEnergy 33(2008)1747–1757.

[10] K.A.Friedrich,M.Schulze,A.Bauder,R.Hiesgen,I.Wehl,X.Z.Yuan,H.Wang, ECSTrans.25(2009)395–403.

[11]R.Hiesgen,I.Wehl,K.A.Friedrich,M.Schulze,A.Haug,A.Bauder,A.Carreras, X.Z.Yuan,H.Wang,ECSTrans.28(2010)79–84.

[12] S.Zhang,X.Z.Yuan,J.NgChengHin,J.Wu,H.Wang,K.A.Friedrich,M.Schulze, J.PowerSources195(2010)1142–1148.

[13]X.Z.Yuan,S.Zhang,J.Wu,H.Wang,in:H.Wang,H.Li,X.Z.Yuan(Eds.),PEM FuelCellDiagnosticTools,CRCPress,2011.

[14]V.Uvarov,I.Popov,Mater.Charact.58(2007)883–891.

[15]P.J.Ferreira,G.J.laO’,Y.Shao-Horn,D.Morgan,R.Makharia,S.Kocha,H.A. Gasteiger,J.Electrochem.Soc.152(2005)A2256–A2271.

[16]R.Lin,E.Gülzow,M.Schulze,K.A.Friedrich,J.Electrochem.Soc.158(2011) B11–B17.

[17] A.C.Fernandes,E.A.Ticianelli,J.PowerSources193(2009)547–554. [18]A.P.Young,J.Stumper,E.Gyenge,J.Electrochem.Soc.156(2009)B913–B922. [19] D.L.Wood,J.R.Davey,P.Atanassov,R.L.Borup,ECSTrans.3(2006)753–763. [20]R.Lin,B.Li,Y.P.Hou,J.X.Ma,Int.J.HydrogenEnergy34(2009)2369–2376. [21] A.Haug,R.Hiesgen,M.Schulze,G.Schiller,in:H.Wang,H.Li,X.Z.Yuan(Eds.),

PEMFuelCellDiagnosticTools,CRCPress,2011.

[22] M.Schulze,A.Haug,in:H.Wang,H.Li,X.Z.Yuan(Eds.),PEMFuelCellDiagnostic Tools,CRCPress,2011.

[23] S.Kundu,M.W.Fowler,L.C.Simon,R.Abouatallah,N.Beydokhti,J.Power Sources183(2008)619–628.

[24] S.Kundu,K.Karan,M.Fowler,L.C.Simon,B.Peppley,E.Halliop,J.PowerSources 179(2008)693–699.

[25] A.Chatterjee,D.Vlachos,J.Comput.AidedMater.Des.14(2007)253–308. [26] D.E.Curtin,R.D.Lousenberg,T.J.Henry,P.C.Tangeman,M.E.Tisack,J.Power

Sources131(2004)41–48.

[27]F.D.Coms,ECSTrans.16(2008)235–255.

[28]M.Takasaki,K.Kimura,K.Kawaguchi,A.Abe,G.Katagiri,Macromolecules38 (2005)6031–6037.

[29]A.Collier,H.Wang,X.Z.Yuan,J.Zhang,D.P.Wilkinson,Int.J.HydrogenEnergy 31(2006)1838–1854.