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Journal of Power Sources, 205, pp. 290-300, 2012-01-21

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Effect of open circuit voltage on degradation of a short proton

exchange membrane fuel cell stack with bilayer membrane

configurations

Zhang, Shengsheng; Yuan, Xiao-Zi; Hiesgen, Renate; Friedrich, K. Andreas;

Wang, Haijiang; Schulze, Mathias; Haug, Andrea; Li, Hui

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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

Effect

of

open

circuit

voltage

on

degradation

of

a

short

proton

exchange

membrane

fuel

cell

stack

with

bilayer

membrane

configurations

Shengsheng

Zhang

a

,

Xiao-Zi

Yuan

a,∗∗

,

Renate

Hiesgen

b

,

K.

Andreas

Friedrich

c

,

Haijiang

Wang

a,∗

,

Mathias

Schulze

c

,

Andrea

Haug

c

,

Hui

Li

a

aInstituteforFuelCellInnovation,NationalResearchCouncilCanada,Vancouver,BC,CanadaV6T1W5

bUniversityofAppliedSciencesEsslingen,DepartmentofBasicScience,Kanalstrasse33,73728Esslingen,Germany cInstituteofTechnicalThermodynamics,GermanAerospaceCenter,Pfaffenwaldring38-40,70569Stuttgart,Germany

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received18November2011 Receivedinrevisedform 22December2011 Accepted1January2012 Available online 21 January 2012 Keywords:

PEMfuelcell Twin-CCM Degradation Membrane Opencircuitvoltage

a

b

s

t

r

a

c

t

Inthepresentwork,membraneelectrodeassemblydegradationofa4-cellstackwithspeciallydesigned twincatalystcoatedmembranes(twin-CCMs)wascarriedoutfor1600hunderopencircuitvoltage(OCV) conditions.Fourtypesofmembranewithvariousthicknesseswereemployedintheexperiment.Each twin-CCMwascomposedoftwomembranesofthesametypewithacatalystlayercoatedonlyonone side.Thepurposeofthisspecialconfigurationwastofacilitatepostmortemanalysisofthemembrane afterdegradationduetothedetachablemembranestructure.Bymeansofseveralinsituelectrochemical measurements,theperformanceoftheindividualcellswasanalysedevery200hduringthe degrada-tionprocess.Postmortemanalysessuchasscanningelectronmicroscopy,atomicforcemicroscopy,and infraredimagingwerealsoconductedtoidentifythemembranedegradationmechanismsafterexposure toOCV.Theresultsindicatethatmembranedegradation,whichcorrelateswithelectrodedegradation, isthemostdirectreasonforthefailureofthewholecell/stackduringOCVoperation,especiallyforcells withthinnermembranes.

Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Durabilityandcostarerecognizedastwoofthemainbarriersto theextensiveapplicationofprotonexchangemembrane(PEM)fuel cellsasapromisingcleanenergytechnology.Todate,considerable efforthasbeenmadetostudytheirperformanceandcomponent degradationvia diverseacceleratedtest methods. Amongthese methods,OCVisoneofthemostfrequentlyemployedstressors forPEMfuelcellacceleratedtesting.Considerableresearcheffort hasbeenappliedtothistopicrecently[1–5].Generally,two fac-torsareofprimaryconsiderationintheOCVdegradationprocess. First,asweknow,highpotentialisinfluentialoncellperformance anddurability,duetodissolution/migrationofthePtcatalystand oxidationofthecarbonsupport,whichcanbeconfirmedbya sig-nificantdecreasein theelectrochemicalsurfacearea(ECSA)[6]. Second,duetozeroconsumptionofthereactantunderOCV,the relativelyhigherreactiongasconcentrationisanotherfactor caus-ingthehighdegradationrateofthemembrane.Anelevatedgas crossoverratethroughthemembraneresultsinahigherperoxide

∗Correspondingauthor.Tel.:+6042213038;fax:+6042213001. ∗∗Correspondingauthor.Tel.:+6042213000x5576;fax:+6042213001.

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

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

radicalconcentration,whichinturncausesamoreseriouschemical decompositionofthemembrane[7].Highfluorideemissionrates (FER)fromeffluentwater,aswellasseriousmembranethinning, havealsobeenconsideredevidenceofPEMdegradationunderOCV, correlatingtolossesinbothOCVandcellperformance[3,8].

GiventhecurrentresearchdevelopmentsinPEMfuelcell degra-dation under OCV, further understandingis certainly required, especially regarding thecorrelations betweenvariouscausesof degradation. Experimental investigations into the degradation mechanisms of different components and theircomplex inter-dependence with operating conditions are increasingly being correlatedwithmodelingactivitiestogainimproved understand-ing.Forexample,basedonatheoreticalmodelofmixedpotential, VilekarandDatta[9]suggestedthatbothhydrogencrossoverand the resulting oxygenreduction reaction (ORR) overpotential at thecathodeplaycritical rolesinOCV.More recently,Yoonand Huang[10]havecarriedoutanexperimentatlowrelative humid-ityandhightemperatureunderOCV.Theyexposedthefuelcell (withhomemadebilayerMEAs)toOCVundertwogasfeeding con-ditions,hydrogen/airand 4%hydrogen/oxygen.ThebilayerMEA wasseparableintotwopiecesofone-sidecoatedmembranesto facilitatethecorrespondinganalysis.In additiontoinsitu elec-trochemical analysis,theyperformed postmortemexamination, includinguniaxialmechanicaltesting,infrared(IR)spectroscopy, scanningelectronmicroscopy(SEM),andenergy-dispersiveX-ray

0378-7753/$–seefrontmatter.Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2012.01.031

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(EDX)spectroscopytodetectthedegradationprocesses.Theresults suggestedthathighlylocalizeddegradationoccurs,andaPtband isobservablewithinthemembrane.

Inourresearchactivities,wehaveperformedseveral experi-mentsunderOCVoridleconditions;bothsinglecellsandcellstacks wereinvolvedinpreviousexperiments[11–13].Theresultsofthe previousmeasurementsclearlyrevealedaccelerateddegradation ofmembranesandelectrodesunderhigherpotentialconditions. However,otherfactors,suchascelldesignandcomponent selec-tion,canalsoinfluencethedegradationrateofPEMfuelcells.

Inthisstudy,wereportdegradationtestingona4-cellstackwith Nafion®membranesofdifferentthicknesses,tobuildonthe pre-viousknowledgeobtainedinOCVstudies.Atwincatalystcoated membrane(CCM) configurationwasappliedtoeach cell ofthe stack.Eachtwin-CCMwascomposedoftwosemi-CCMs(i.e., cata-lystcoatedononesideofthemembrane),withtheuncoatedsides facingeachother.Thepurposeofthisspecialconfigurationwas tofacilitatepostmortemanalysisof themembraneafter degra-dationoccurredduetothedetachable membrane structure.An additionalbenefitofthisconfigurationwasthattheinitialposition ofthemembranecenterwasidentifiable.Usingbothinsituandex situdiagnostictools,thisstack’sdegradationunderOCVfor1600h wasinvestigated.Insitu characterizationsincludedpolarization curve(V(I)curve),electrochemicalimpedancespectroscopy(EIS), cyclicvoltammetry(CV),andlinearsweepvoltagescans(LSV).For exsitudiagnosis,SEMisgenerallyusedtoobservemorphological changesafterdegradation,butitssignificanceisstrengthenedby combiningitwithadditionalanalyticalmethods.Therefore,inthis study,theCCMswerecharacterizedafteroperationusingexsitu diagnostictoolslikeSEM,aswellasIRimagingandatomicforce microscopy(AFM). Thecombinedanalysisimprovesour under-standingofdegradation,especiallyasitoccursinthemembrane. 2. Experimental

2.1. Testsetup

Thecellused for theOCVdegradation experiment wasa 4-cellPEMfuelcellstack,eachcellhavinga50cm2activearea.The materialsusedwerecommerciallyavailableproductsasdescribed below:

Forthegasdiffusionlayers(GDLs),SIGRACET25BCfromSGL Technologieswasemployedatthecathode.TheGDL25BC con-tained5wt%PTFE.Attheanode,aGDLSIGRACET25DCwith20wt% PTFEloadingwasused.BothGDLswerecoatedwithamicroporous layer(MPL).

IonPowercustomizedsemi-CCMswithacatalystlayeronone sideofthemembranewerecombinedtoformcompletemembrane electrodeassemblies(MEAs).ThePtloadingwas0.3mgcm−2inthe catalystlayer.ThemembranesusedwereN117,N115,N212,and N211.

Theschematicdiagramofthesetwin-CCMsisshowninFig.1. TheMEAs wereformedbysandwichingtheKaptonfilm sealed twin-CCMsbetweentheanodeandcathodeGDLs.Graphite bipo-larplateswereused,containingawaterflowfieldonthebackto thermalizethecellataconstanttemperature.Thealignedcellstack wasthencompresseduniformlyunder100Psi(689,475.7Pa)using nitrogen,andconnectedwithanArbin500Wfuelcellteststation. Theanodeandcathodegasesfollowedacounterflowpatternfor allofthe4cellsinthestackduringthetest.

2.2. Experimentprocedure

Conditioning was performed in the test station. The stack wascheckedforgasleakagesbeforeconditioningbyloadcycling

Fig.1.Schematicdiagramofthetwin-CCM.

(describedbelow).The4cellswithinthestackwerenamedCell 1(withN117membrane),Cell2(withN115membrane),Cell3 (withN212membrane),andCell4(withN211membrane),from thebottomtothetop.Eachcurrentcycleoftheconditioning pro-cessconsistedofOCVfor10min,constantcurrentat20Afor30min, andconstantcurrentat10Afor30min.Thecyclewasrepeatedfor atotalof22huntiltherewasnofurtherperformanceimprovement foratleast3h.

Thefuelcelltemperaturewaskeptat70◦Cusinganexternally heatedcirculator.Airandhydrogenwerefullyhumidifiedin bub-blersat70◦Cbeforetheirentranceintothefuelcell.Bothanode andcathodewereunderambientpressureduringtheentire exper-iment.Notethatallthetemperaturesettingsforthefollowingtests werethesametoensureconstantconditions.Thestoichiometries forairandhydrogenweresetat2.5and1.5,respectively.The min-imumflowratesforairandhydrogenwereabout2slpm(standard litreperminute)and1slpm,respectively.

Theelectrochemicalcharacteristicsmeasuredafterthe condi-tioningprocesseswereregardedasthebaselinedata.Thecurrent andvoltagedatawererecordedwiththeArbinteststation.For impedance testing, a Solartron SI-1260 impedance/gain-phase analyzerwasusedincombinationwithaSolartronSI-1287 electro-chemicalinterface,inthefrequencyrangeof10kHzto0.1Hzand withanexcitationamplitudeof10mV.FortheCVandLSVtests, thecathodegaswasswitchedtonitrogentorecordtheECSAand thehydrogencrossoverrate,respectively.Therefore,theanodeside remainedunderhydrogenfeedandtheflowratesfornitrogenand hydrogenwerekeptconstantat2slpmand1slpm,respectively. Inthiscase,theanodeservedasboththereferenceandcounter electrodes,andthecathodeservedastheworkingelectrode.The cyclingwasperformedbetween0.05and1.2Vvs.reversible hydro-genelectrode (RHE) ata scan rateof 50mVs−1.The ECSA was calculatedbasedonthecatalystloadingandthetotalchargearea ofthehydrogendesorption/absorptionpeakintegratedfromabout 0.1Vto0.4V(vs.RHE),aftersubtractingthedoublelayercharge ontheCVcurves.DuringLSVmeasurement,alltheoperating con-ditionsforthefuelcellwerethesameasintheCVtest,exceptfor theapplicationofalinearpotentialscanfrom0.05to0.5Vonthe cathodeatascanrateof5mVs−1.

Itisalsoworthnotingthattheapplicableelectrochemical evalu-ationtestsconductedinthisexperimentallreferredtothecathode side.Theseelectrochemicaldiagnoseswerethenperformedatthe endofeach200hofOCVholding.After1200hofdegradationand correspondingtests,thewholetestsetupwasmovedtoanother teststationwiththesamefunctionduetotheschedulingofthe teststations.Therefore,thelast400hofOCVholdingandthelast twoelectrochemicaltestswerefinishedonthesecondteststation.

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Apart from these electrochemical measurements, fluoride releasedatawasalsocollectedevery200htoanalysethe efflu-entwater(bothanodeandcathode)usingafluorideionselective electrode.

Asapostmortemmeasurement,IRimagingwasconductedon thedegradedMEAstodetectwhetheranypinholeswere gener-atedafterthedegradationtest.AnIRcamera(InfraTechGmbH)was employedinthistest.ThedegradedMEAsremovedfromthe degra-dationcellswereinstalledonebyoneinadummycellwithanopen cathode.Hydrogenpassedthroughtheanodesideataflowrateof 0.1slpmatroomtemperature.TheIRcamerawasplacedonthe cathodesidetoobservethetemperaturedistributionofeach indi-vidualcellunderthesamegasflowrateandroomtemperature.The principleofmeasuringtemperaturedistributionusingIRimaging isbasedontheheatoutputfromthechemicalreactionbetween hydrogenandoxygeninthepresenceofPtcatalyst,demonstrating thelevelofreactantcrossoverthroughthePEM.Forcomparison, thesametestwasalsoconductedonfreshMEAswiththesame twin-CCMconfiguration.

Toidentifymembranethicknesschanges,cross-sectionalSEM imagesweretakenafterdegradation,usingaHITACHIS-3500N SEM.Inaddition,thePtcontentofthemembraneswasanalysed usinghigh-resolutionSEM-EDX(LEO982GEMINI)at6different sitesofthecross-sectionsofeachMEA.

AFMwasperformedusingaMultimode8systemwithtwo dif-ferenttappingmodetechniques.Inthe“HarmoniX”mode(Bruker Corp.),aspeciallydesignedasymmetricalAFMtipisneeded,which enhancesthehigherharmonicoscillationaftercontactwiththe samplesurface.Fromtheseoscillations,theforce–distancecurve canbereconstructedateveryimagepoint.Adhesionforce,energy dissipation,phase shift, stiffness, and peak force can be evalu-atedtogetherwiththetopography,butnocurrentmeasurementis possiblebecausethespecialtipisnotconductive[14–17].Inthe “QuantitativeNanomechanicalPeakForce” mode(QNM;Bruker Corp.),thetipvibratesatafixedfrequencyof2kHztowardsthe sampleandtouchesthesurfacewithanultralowforceduringeach cycle.Duringapproachandretraction,force–distancecurvesare recorded,whichcanbedeterminedtogetherwiththetopography andthefollowingproperties:adhesionforce,energydissipation, maximumforce,hardness(DMTmodulus),deformation,stiffness, andphaseshiftofoscillationateveryimagepoint.Theinstrument anditsanalysissoftwaredeliveramappingofeachproperty.Witha conductivetipintappingmode,thecurrentduringcontactcanalso bemeasuredateachpoint;previously,conductivitywasmeasured intheso-calledcontactmode,whichwasassociatedwith signifi-cantpressureonthesample.Statisticalevaluationoftheproperties ofeachimagewasperformed,yieldingahistogramofthe occur-renceof thevaluefor a specificproperty. Thepeak occurrence valuesweretakenasmeanvaluesoftheproperty.Fromseveral images,ageneralmeanvalueforapropertywascalculated,with thecorrespondingerrorforonestandarddeviation.

ThesamplesfortheAFMinvestigationwereallcutfromthe sameposition,atthe4MEAsclosetothehydrogeninletofthecell. FromtheN115asecondsamplewascutforcomparisonreasons, atalargerdistancefromthehydrogeninlet.For theAFM mea-surements,thetwinMEAswerecarefullyseparatedmanuallyand attachedtothesampleholderwithadhesivetape.Forcurrent mea-surements,thesampleswerestoredinMilliporewater.Forionic conductivitymeasurements,theGDLwasremovedandsamples wereputwiththeirelectrodesonaporous,conducting,hydrophilic layer that provided a water supply to the back electrode. The schematicdiagramoftheAFMobservationoftheinterfacebetween thetwosemi-CCMsisshowninFig.2.Inadditiontoinspection oftheinnerinterfacesurfaces,MEAcross-sectionsperformedby cryo-cleavageinliquidnitrogenwereinvestigatedusingtheQNM mode.

Fig.2. SchematicrepresentationoftheAFMmeasurementperformedontheinner membraneinterfaces. 0 1 2 3 4 1600 1400 1200 1000 800 600 400 200 0 Time (h) Potential (V) OCV

Fig.3. TimeevolutionoftheOCVofthestackduring1600hofoperationatOCV.

3. Resultsanddiscussion 3.1. Insitudiagnosis 3.1.1. OCVtimedependence

Fig.3showsthetimeevolutionanddecreaseinOCVofthestack duringthe1600hoftheexperiment,associatedwithdegradation processes.Throughouttheexperiment,OCVdeclinesgreatlyfor eachtimeinterval. Thisisinaccordancewithprevious observa-tionsbySahinetal.[14].TheinitialOCVatthebeginningofeach sessionremainsalmostthesameforupto800h,althoughtheend OCVofeachsessionislowanddecreaseswithtime.Afterabout 800h,therecoveredOCVvalueatthebeginningofthetime inter-valdecreasessignificantly.Thissizeablevoltagelossindicatesthat thecomponentpropertiesaredegradingandtherebyinfluencing thecell’sfunctionality.Duringthelast200hofOCVoperation,an unexpectedgassupplyfailurecausesashutdown,leadingtosimilar timebehaviourfortheOCV.

By comparingtheOCV time recordsfor each individualcell showninFig.5,itwasfoundthatOCVdecaywasnotuniformamong thefourcells.Thethinnermembranesexperienceda more seri-ousvoltagedecreasethanthethickerones.Forexample,withthe thinnestmembrane,theOCVofCell4decreasedtobelow0.8Vafter 1600h.Themostsignificantdecreaseinthiscelloccurredduring thelast600h.However,theOCVofCells1and2,withtheirthicker membranes,showedaverystabletrendduringthewhole degra-dationperiod.Inaddition,voltagefluctuationswereobservedafter 1200h,especiallyforthefuelcellswiththinnermembranes(Cells 3and4),whichshouldbeindicativeofmembranedeterioration. 3.1.2. Evolutionofcurrentvoltagecurves

Fig.4showstheV(I)curvesofthecellstackafterpotentialOCV holdingfordifferentdurations,comparedwiththebaseline.The relativelylow performanceofthestackwasmostprobablydue tothehigherresistanceofthedouble-layeredthickmembranes.

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0 0.5 1 1.5 2 2.5 3 3.5 4 0 100 200 300 400 500

Current density (mA cm-2)

Voltage (V) baseline 200hrs 400hrs 600hrs 800hrs 1000hrs 1200hrs 1400hrs 1600hrs

Fig.4. IVcurvesofthecellstackafterpotentialholdingfordifferentdurations, comparedwiththebaseline.

Afterthefirst200h,thestackperformanceshowedasignificant improvementcomparedwiththebaselineaftertheconditioning. This indicatesthat the stack was not fully activated, although a conditioningprocessof 22hhad beenconducted. Duetothe double-sizedmembranethickness,especiallyforCell1,more con-ditioningtimemightbeneeded.Duringthesubsequent600h,the stackperformanceexperiencedaminordecrease.Then,the per-formancedropbecamemorepronouncedafter1400and1600hof operation.

Fig. 5 compares the potential of each cell at OCV and 160mAcm−2.Duringthewholeexperimentalperiod,the perfor-manceof thestack waslimitedby Cell 1,which containedthe thickest membrane, N117. As thelimiting current density was foundto decrease during operation, a rather low current den-sityof160mAcm−2(thelimitingcurrentdensityofCell1inthe lastV(I)curve)waschosenforcomparingthecells.Fig.5 demon-stratesthatatthebeginningofthetestOCVishigherforthicker membranes(Cells1and2),correspondingtothelowerhydrogen crossoverratesthroughthickermembranes.Withincreasing oper-ationtime,thecellswiththickermembranesshowedmorestable OCVthanthosewiththinnermembranes(Cells3and4)asthereare morematerialstodegradebeforepinholesformforthicker mem-branes.ArapidOCVdecreaseoccurredforCell4(withthethinnest membrane)afterabout1400h,followedbyCell3(withthe sec-ondthinnestmembrane)after1600hduetomembranethinning orpinholeformationthroughthinnermembranes.This observa-tionconfirmsthatmembranethicknessdeterminescelldurability underOCVconditions.

The time dependence of the cell voltages at 160mAcm−2 showedpronounceddifferencescomparedtoOCVbehaviour.The lowestcellperformancewasobservedatCell1thatcontainedthe thickestmembranedue tothelargemembraneresistances.The

0 0.2 0.4 0.6 0.8 1 1.2 1600 1400 1200 1000 800 600 400 200 0 Time (h) Po te n tia l ( V )

Cell 1 OCV Cell 1 160mA cm-2 Cell 2 OCV Cell 2 160mA cm-2 Cell 3 OCV Cell 3 160mA cm-2 Cell 4 OCV Cell 4 160mA cm-2

OCV

160mAcm

Fig.5.Performancecomparisonofeachsinglecellfordifferentdurations,atOCV and160mAcm−2,respectively.

Fig.6.ComparisonofthecalculatedECSAforeachsinglecell,obtainedfromtheCV test,fordifferentdurations;theinsetisatypicalCVcurve(baselineofCell1).

voltages of thecells increasedduringthe first400h, especially forthecellswiththickermembranes(Cells1and2).The perfor-manceof Cells1 and2 showedobviousimprovement ofabout 100mV.However,theimprovementinCells3and4wereminor, evenwithintherangeofmeasurementerror.Theseresultsalso confirmedthattheevidentperformanceenhancementofthestack duringthefirst200hwasmostlycontributed bythecells with thickermembranes.Alreadyafter400hofoperation,Cell1 expe-riencedastrongerperformancelosscomparedtotheotherthree cells,whichwasoppositetotheOCVdegradationtrend.Sinceno strongincreaseingascrossoverwasobservedforCell1,themost probablereasonfor theperformancelossshouldbeeither elec-trodedegradationorgasdiffusionlayerdegradation.Asshownby theECSAdegradationcurvesinFig.6,noevidencewasshownthat catalystsurfaceareachangewasrelevanttotheperformanceloss. However,afterthedegradationtest,remarkableGDLdamagesdue tothecompressiononCell1andCell2(especiallyforCell1)were observedwithnakedeye.Theshapeoftheflowfieldwasclearly embeddedintotheGDL.Whileforcellswiththinnermembranes (Cell3 and Cell4), nosuchphenomenon wasobserved. There-fore,seriousGDLdeteriorationoftheCCMwithdoubledthicker membranesmaybethereasonfor theunexpectedperformance losstrendobservedinFig.5.Thisconfirmsthatcellperformance degradation is influenced by many factors, including cell com-ponents(membrane,catalyst,GDL), componentdesign, andcell configuration.

3.1.3. Timedependenceofcyclicvoltammetry

Commonly,theECSAattainedfromCVmeasurementisusedas anindicatorofcatalystlayerperformance.Fig.6comparesthe cal-culatedECSAforeachcellobtainedfromtheCVtest.Theinsetis atypicalCVcurveobtainedfromthebaselineofCell1.Thelast CVmeasurementforCell4wasnotobtainedduetotheexcessive hydrogencrossovercurrent.TheECSAsofallcellsdecreasedwith increasingOCVholdingtime.ThereasonfortheECSAdecreasewas Ptdissolution/redeposition,aswellaspossiblecorrosionofcarbon support[6].Ptevolutionwithinthetwin-CCMafterthe degrada-tiontestwillbediscussedinSection3.2.ECSAlosswashighest duringthefirst200h,andthenlevelledofftoalossofabout45% after1200h.Thisbehaviourwasalsofoundinourprevious degra-dationresearchunderOCVstressandidleconditions.However,the ECSAvaluesafter1400and1600hshowedanunexpectedincrease compared withprior measurements.Considering the operating recordofthetest,themostplausiblereasonforthisECSAchange isinstrumentaldifference,asthelasttwo testswereconducted withasecondteststation;considerthatamovementofthestack involvesrelease and tightening of thealignment, which might causedifferencesevenincalibratedidenticalteststations.TheECSA measurementsreflectthestrongerdecreaseinperformanceofCell

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Fig.7.Comparisonofthehydrogencrossovercurrentdensitiesofeachsinglecell at0.5VaftereachtimeintervalduringOCVoperation.

1,whichmayindicatethatotherfactors,likedeformationofthe GDL,areimportant.

3.1.4. Timedependenceofhydrogencrossover,determinedbyLSV Thehydrogencrossovercurrentdensitiesforeachcellat0.5V aftereach time intervalare compared inFig.7.For thethicker membranes (Cells 1 and 2) there were no significant changes in hydrogencrossoverduringthe 1600h, while theLSV curves showedaremarkablechangeforCells3and4,withtheirthinner membranes.Thehydrogencrossovercurrentincreasedwiththe relativedecreaseinmembranethickness(seeTable1and descrip-tionbelow).Ahugeincreaseinthehydrogencrossovercurrentof Cell4,withthethinnestmembrane,happenedafter1000hofOCV holding.Thecurrentincreasedfurther,continuously,untilthe cur-rentvalueexceededtheinstrumentlimit(2.5A).Notethat,Cell3 showedanabnormalhighincreaseinhydrogencrossovercurrent atalatertime,1600h,compared tothecontinuouslyincreasing hydrogencrossovertrend forCell 4.Thismayberelated tothe stacktransfertoanotherteststationafter1200-htest. Neverthe-less,membranethicknessplaysakeyroleinfuelcelldurability underOCVconditions.Thiscoincideswiththeresultsofour previ-ousexperiments[12,13].Therefore,althoughthecellwithathinner membrane may achieve a higher performance under thesame workingconditions,itslifespanisrelativelyshortcomparedtocells withthickermembraneswhenexposedtothesamestressors. 3.1.5. Time-dependentEISmeasurements

Thehighfrequencyresistance(HFR)valuesoftheEIS forthe cellsarecomparedinFig.8.Theequivalentelectricalelementscan bedeterminedfromtheEIS datausingafit toatypical equiva-lentcircuit,shownintheinsertedgraph,whereR1representsthe HFR,R2representsthechargetransferresistance,andCprepresents thechargetransferrelateddouble-layercapacitance.Generally,the HFRisusedtocharacterizetheohmicresistanceofaPEMfuelcell, whichiscontributedbymembraneresistance,byelectronic resis-tanceintheflowplates,currentcollectors,andgasdiffusionmedia, andbycontactresistances.Apartfrommembraneresistance,the otherfactorscanbeassumedtoremainconstantinthisexperiment. Accordingtothedata,all4cellsshowedamoderateincreaseinHFR. ThebaselinedeterminationofHFRs(at0h)showedtheexpected

Fig.8.HFRchangeofeachsinglecellbeforeandaftereachOCVdegradationtime interval;theinsetisatypicalequivalentcircuitforaPEMfuelcell.

Fig.9. FluorideconcentrationofeffluentwaterduringOCVholdingtest,forboth anodeandcathodesides.

risewithmembranethickness,whichconfirmsthattheHFRrelates stronglytothemembraneresistance.Afterthefirst200hofOCV operation,theHFRforallofthe4cellsdecreasedslightly,which mayresultfromabettermembranehumidificationasmentionedin Section3.1.TheHFRchangeafter1000hisbelievedtohaveresulted fromthecombinedeffectsofmembranethinningandmembrane structuredeterioration.ThesetwoeffectswereconfirmedbySEM and AFMobservations,respectively, which willbepresentedin detailinSection3.2.Membranethinningandmembranestructure deterioration mightfunctioninreverseways:theformermight decreasetheresistanceduetochangesinthemembrane’s phys-icalfeatures, whilethelattermight increasetheresistancedue tostructuralchangesintheNafion®ionomer.Besidesthis, trans-ferbetweenthetwoteststationsmightalsohaveinfluencedthe impedancetest.Therefore,thetrendsinHFRforthe4individual cellsarenotclearforthisperiod.However,eachendoftest(EOT) resistanceishigherthanthatatthebeginningofthetest(BOT). 3.2. Exsitudiagnoses

3.2.1. Timeevolutionoffluorideloss

Thefluorideconcentrationwasdeterminedbymeasuringthe effluentwater duringtheOCVholdingtest for both anodeand

Table1

Membranethicknesscomparisonbetweenfreshanddegradedsinglecells.

Unused(␮m) Anode Cathode

Degraded(␮m) Loss(␮m) Decrease(%) Degraded(␮m) Loss(␮m) Decrease(%)

Cell1N117 183 171 12 6.5 160 23 12.6

Cell2N115 127 115 12 9.5 88 39 33

Cell3N212 50.8 43 7.8 15.4 23.6 27.2 53.5

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Fig.10.IRimagesoffreshanddegradedsinglecells:(a)unusedCell1;(b)degradedCell1;(c)unusedCell2;(d)degradedCell2;(e)unusedCell3;(f)degradedCell3;(g) unusedCell4;(h)degradedCell4.

cathodesides.Duetothelongexperimentaltime,itwasdifficultto collectalltheaccumulatedexhaustwater.Therefore,water sam-pleswereperiodicallycollected,andonlyrelativefluoridelossis presentedinsteadofaccumulativefluorideloss.AsshowninFig.9,

thecathodesideexhaustwatercontainedmorefluoridethanthe anodesideexhaustwater,whichcanbeinterpretedasindicatinga strongerionomerdegradationonthecathodeside.Thisisfurther confirmedbytheSEMresultsbelow.Althoughdegradationofthe

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Fig.11. TemperaturedifferencesfortheunusedanddegradedsamplesintheIR images(averageandmaximumtemperatures).

catalystlayerionomermightalsocontributetofluorideloss,the mainsourceofthefluorideisconsideredtobemembraneionomer degradation,basedonitspredominantmassandmembrane thin-ning,asobservedbySEM(seeTable1).

3.2.2. IRimagingofpinholeformation

AccordingtotheimagesfromtheIRcamera,showninFig.10, theaveragetemperaturesoftheagedMEAsfromCells1and2were similartothoseoftheunusedcells.ForCells3and4,hotspots weredetectedonthedegradedsamples,incontrasttotheiroriginal uniformtemperaturedistribution.Hotspotsarecertainareaswith ahighertemperatureduetoincreasedoxygen/hydrogenreaction

basedontheircrossoveramountthroughtheweakerand thin-nerareasofthemembrane.Whenhotspotsoccur,theyindicate fataldegradationduetomembranethinningorpinholeformation. Therefore,thehotspotsdetectedinCells3and4confirmedthat seriousmembranedamagewaspresentinthecellswiththinner membranesafterageing.Thetemperaturedifferencesbetweenthe unusedandagedsamplesaredisplayedinFig.11.Theaverage tem-peraturedifferencesreflectthesametrendfordifferentcells,but themaximumtemperaturedifferencesarehugeforCells3and4. Thisindicatesthatthemajorityofthemembraneisstillinnormal conditionwhileoccasionallyweakenedareasresultinhotspots. Notethatboththeaverageandthemaximumtemperaturesforthe unusedsamplesarehigherforthinnermembranes.The temper-aturedifferencebetweenthethickestandthethinnestiswithin 0.5◦C,whichprovesthatIRimagingisausefuldiagnostictoolto detectPEMintegrity.

3.2.3. StructuralinvestigationsusingSEM

SEMimagesofthecross-sectionedMEAafterthedegradation test areshownin Fig.12.Aslabelled,thecathodessidesareat thetopoftheseimages.ForeachMEA,theboundaryofthetwo semi-layersisclearlyidentifiedintheSEMimages,asmarkedby arrows.Beforeoperation,thethicknessesofthetwomembrane layersareequal.AftertheOCVholdingtest,thethicknessesofthe twomembranesarenolongeridentical.Themembranethinningof thesemi-CCMnearthecathodesideisgreaterthanneartheanode side,whichcoincideswiththehigherfluoridelossmentioned pre-viously.Thismightbetheresultofthedifferentcrossoverrateof hydrogenandairthroughthePEM.Ogumietal.[18]foundthat underthesameconditions,thecrossoverrateofoxygenisabout halftherateofhydrogenthroughNafion® 125membrane(with

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Fig.13.PtcontentofthecrosssectionofCell3determinedbyEDX.

athicknessof125␮m)at25◦C.Highmembranedegradationdue toradicalformationofoxygenatedspeciesisexpectedatareasof highoxygenpartialpressureinthepresenceofhydrogenandPt catalyst(highperoxideratesarefoundinthehydrogenadsorption regionofPt).Therefore,strongercathode-sidemembranethinning canbeexpected.Thethicknesslossesforeach cellarelistedin Table1,togetherwiththicknesscomparisonsforthedegradedand theunusedmembranes.Thethicknessesoftheindividual single-layermembranesoftheunusedsamplesarealsogiven.Fortheaged membranes,averagevaluesobtainedfrom5randomspotsforeach

Fig.15.Statisticalevaluationofthestiffnessofthemembranesurfacesinrelation tothecorrespondingunusedmembrane.

samplearelisted,becausethethinningisnotevenalongthewhole cross-sectiondirection.Comparisonofthe4cellsshowsthatthe absolutethicknesslossforCells1and2islargerthanforCells3 and4.However,therelativethicknesschangeislargerforthe thin-nermembranes,leadingtoarelativemembranelossorderofCell 1<Cell2<Cell3<Cell4.Thisfurtherconfirmstheresultsobtained usingIRimaging—pinholesaremorepronetodevelopinCells3 and4,withtheirhighpercentageofthicknessloss.

BesidesthegeneralSEMinvestigations,high-resolution SEM-EDXwasusedtotest thePtcontentwithinthemembranes.As

Fig.14.TypicalAFMimagesoftopography,togetherwithmappingofpeakforce,stiffness,adhesion,deformation,anddissipation,obtainedusingQNMtappingmodefrom theforce-distancecurvesonCell1membranesurfacesafter1600hOCVoperationonthecathodeside.

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Fig.16.AFMimagesbyHarmoniXmodeonCell4membranesatthecathodeside.

showninFig.13,Ptwasdetectedindifferentquantitiesallover themembranecross-sections.However,thereisnocleartrendin thedistributionofthePtthroughoutthemembranes,eventhough thechosenpositionswereallsameforthefoursamples.SimilarPt redepositionbehaviour(Ptbandinmembrane)hasbeenobserved, andthereasonsforsuchPtmigrationhavealsobeeninvestigated byotherresearchers[19,20].

3.2.4. AFM

All8innerinterfacesofthe4cellswereinvestigatedusingthe HarmoniXandQNMmodes.InFig.14,thesurfacestructureofthe Cell 1membrane(N117) measuredby QNMis shown,together withthemappingofpeakforce,stiffness,adhesionforce, defor-mation,andenergydissipationofthesurface,inasimilarwayas wasfoundfor allmembranesurfacesofthestack. Inthe adhe-sionforce image—whichis extremelysurface sensitive—surface materialheterogeneitiesarevisible.Thesurfaceareaswithahigh adhesionforcearethetopmostlayers(inthetopographyimage), exhibithigherenergydissipation,aremore deformable,andare morebrittle(lowerstiffness).Inthepeakforceimage,no differ-enceinhardnessisvisible.Theseagedsurfacesappeartohaveflakes peelingoffthesurfaceandexposingafreshersurfacebeneath.In addition,particleswerefrequentlydetected,mainlyattheinner cathode-sideinterfacesofthethinnermembranes.

Fig.15presentsacomparisonofthemeanstiffnessofthe cath-odeside relative toan unusedmembrane of thesametype, at

theinnerinterfaceofthe4membranes,togetherwitherrorbars. ThestiffnessimagesweremeasuredusingtheQNMmode.Aclear exponentialincreaseinstiffnessfromthethickertothethinner membranesisvisible.Enhancedstiffnessisexpectedforadegraded polymer,andtheAFMresultsunderlinetheaccentuatedionomer degradation ofthe thinnermembranes,in accordance withthe resultsfor H2 permeability. In Fig.16, theparticlesdetectedin theinnerinterfaces,hereattheCell4cathodeside,arefurther investigated.Themostlikelyinterpretationofthenatureofmost oftheseparticlesisthattheyareredepositedPt,asshownbythe HarmoniXmodemeasurements.IncomparisontoNafion®,Ptis veryhard(peakforceimage)andhasasmallerenergydissipation duetoitshigherelasticity.Ptparticleswerealsodetectedon cross-sectionsofthemembrane.Fig.17showsafewlargePtparticleson thecross-sectionoftheCell3membraneinthetopographyimage. Overlayingtheadhesionforcemappingontothetopographyclearly showsverythinextendedPtlayerscoveringalargefractionofthe surface,identifiablebytheirsmalladhesionforce.

InFig.18,currentmeasurementperformedincontactmodefor Cell3showshighconductivity,butstillexposesnon-conductive areasinthe1.5␮mimageontheleftside.Theconductivityimages areinprinciplesimilartothenewmembranesamplesreportedin variouspreviousstudies[21–24].However,theconductivityareas arelargehereandtheconductivityishigher.Inaddition, interpene-tratingregionswithdifferentcurrentlevels(>3)arepresent,which havenotbeendetectedbefore.Ahistogramofthecurrentlevelsis

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Fig.18.ConductiveAFMcurrentimagesincontactmodeofCell3attheanodeside,showingconductivitydistributiononthemembranesurfacefor1.5␮m×1.5␮m(left) and115nm×115nm(right).Thehistogramontopshowstheoccurrenceinpixelsofcurrentvaluesforthewholeimage.

givenintheupperleftofFig.18.Itiswellknownthattheionic net-workofNafion®consistsofbranchedconductivechannels.Areas ofsimilarcurrentvaluesaremostlikelyinterconnectedthrougha branchedionicsubnet.Regionswiththesameconductivitybelong tothesameionicsubnetandmighthaveonlyahighresistive con-nectionwithahighresistancebeneaththesurface.Thesubnets areanindicationofanon-uniformconductivenetwork,probably causedbytheactivationprocessofthemembraneatthestartof fuelcelloperation.Also,theratherhightotalconductivityisdueto theextendedactivationoftheagedmembranesincomparisonto theunusedmembranes.Ontherightside,ahighresolutionimage isshownwithsmallconductiveareasofabout10nm,and even somesmallerspotsofonlyafewnanometers.Inprevious stud-ies,thesmallestconductivestructureswereassignedtoindividual invertedmicellesaspartofaconductivenetworkofNafion®.These micellesclusterintolargestructuresof10–30nm,alsoseeninthe agedsamples.

4. Conclusions

Using in situ and ex situ diagnostictools, theeffectof OCV operationasanacceleratedmodeofPEMdegradationina4-cell stackwithtwin-CCMswasinvestigated.Differentmembrane thick-nesses,namelyN117,N115,N212,andN211,wereusedinthe4-cell stack.TheresultsshowedperformancelossandOCVlossduring OCVdegradationforupto1600h,differingwidelyforthe vari-ouscells.DisproportionatelyhighOCVlossandhydrogencrossover wereobservedforthetwothinnestmembranes,confirmedbyIR measurementsandthinningdetectedbySEManalysis.The func-tionality lossin the cells withthin membraneswas associated withstrongthinningand evenpinholeformation. Ontheother hand,performancelossat160mAcm−2wasmorepronouncedfor

cellswiththickermembranes.TheactivePtsurfaceareadecreased stronglyatthebeginning oftheOCV,inaccordancewith previ-ousstudies.Forstackfunctioning,membranedegradationismore seriousthanelectrodedegradationunderOCVconditions. Investi-gationofcross-sectionsusingSEMindicatedstrongerthinningat thecathodeside.FurtherinvestigationswithnovelAFMmethods revealedthattheagedinnermembranesurfacesweresignificantly stiffer,inparticularforthethinnermembranes,andhadadegraded surface structure withpolymer flakes. These results supported theelectrochemicalresults.Thelocalionicconductivity,however, wasfoundtobehighercomparedwithpreviousinvestigationsof unusedsamples.Aninterpenetratingionicnetworkwithdifferent currentlevelswasfoundattheinnerinterfaces.

References

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[5]B.Sompalli,B.A.Litteer,W.B.Gu,H.A.Gasteiger,J.Electrochem.Soc.154(2007) B1349–B1357.

[6] S.Zhang,X.-Z.Yuan,J.N.C.Hin,H.Wang,K.A.Friedich,M.Schulze,J.Power Sources194(2009)588–600.

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

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