A review of polymer electrolyte membrane fuel cell durability test protocols

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Journal of Power Sources, 196, 22, pp. 9107-9116, 2011-08-23

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A review of polymer electrolyte membrane fuel cell durability test

protocols

Yuan, Xiao-Zi; Li, Hui; Zhang, Shengsheng; Martin, Jonathan; Wang,

Haijiang

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JournalofPowerSources196 (2011) 9107–9116

ContentslistsavailableatScienceDirect

Journal

of

Power

Sources

j ou rn a l h o m e pa g e :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

Review

A

review

of

polymer

electrolyte

membrane

fuel

cell

durability

test

protocols

Xiao-Zi

Yuan,

Hui

Li,

Shengsheng

Zhang,

Jonathan

Martin,

Haijiang

Wang

∗

InstituteforFuelCellInnovation,NationalResearchCouncilCanada,4250WesbrookMall,Vancouver,BC,CanadaV6T1W5

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received30May2011

Receivedinrevisedform26July2011 Accepted28July2011

Available online 23 August 2011

Keywords: Acceleratedtest Degradation Diagnosis Durability PEMfuelcell Protocol

a

b

s

t

r

a

c

t

Durabilityisoneofthemajorbarriers topolymerelectrolyte membranefuelcells(PEMFCs)being acceptedasacommerciallyviableproduct.Itisthereforeimportanttounderstandtheirdegradation phenomenaandanalyzedegradationmechanismsfromthecomponentleveltothecellandstacklevel sothatnovelcomponentmaterialscanbedevelopedandnoveldesignsforcells/stackscanbeachievedto mitigateinsufficientfuelcelldurability.Itisgenerallyimpracticalandcostlytooperateafuelcellunder itsnormalconditionsforseveralthousandhours,soacceleratedtestmethodsarepreferredtofacilitate rapidlearningaboutkeydurabilityissues.BasedontheUSDepartmentofEnergy(DOE)andUSFuelCell Council(USFCC)acceleratedtestprotocols,aswellasdegradationtestsperformedbyresearchersand publishedintheliterature,wereviewdegradationtestprotocolsatbothcomponentandcell/stacklevels (drivingcycles),aimingtogathertheavailableinformationonacceleratedtestmethodsanddegradation testprotocolsforPEMFCs,andtherebyprovidepractitionerswithausefultoolboxtostudydurability issues.Theseprotocolshelppreventtheprolongedtestperiodsandhighcostsassociatedwithreal life-timetests,assesstheperformanceanddurabilityofPEMFCcomponents,andensurethatthegenerated datacanbecompared.

Contents

1. Introduction... 9107

2. AbriefreviewofPEMFCdegradation... 9108

2.1. Catalystlayerdegradation... 9108

2.2. Membranedegradation... 9109

2.3. GDLdegradation... 9109

2.4. Bipolarplatedegradation... 9109

2.5. Othercomponentdegradation... 9109

2.6. Operatingconditions,environment,contamination,operationmode,anddesign-induceddegradation... 9109

3. Acceleratedstresstestprotocolsforcellcomponents... 9110

3.1. Electrocatalyst... 9110

3.2. Catalystsupport... 9110

3.3. Membrane... 9111

3.3.1. MembranemechanicalAST... 9111

3.3.2. MembranechemicalAST... 9112

3.4. GDL... 9112

3.5. IonomerintheCL ... 9114

4. Cell/stackdurabilitytestprotocols... 9114

4.1. USDOEsinglecell/stacktestingprotocolsfortransportationapplications... 9114

4.2. Durabilitytestsforstationaryapplications... 9114

5. Concludingremarks... 9115

References... 9115

∗Correspondingauthor.Tel.:+16042213038;fax:+16042213001. E-mailaddress:haijiang.wang@nrc.gc.ca(H.Wang).

1. Introduction

Owingtotheirhighenergy efficiency,convenientoperation, andenvironmentallyfriendlycharacteristics,polymerelectrolyte

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membrane fuel cells (PEMFCs) are considered one of themost promisingfuel celltechnologiesforboth stationaryand mobile applications.Significantprogresshasbeenachievedoverthepast fewdecades.However,durabilityand costhave beenidentified as thetop two issues in PEMFC technology. On the one hand, it isrecognized that onlywhen fuel cellcostsare dramatically reducedtotheUSDepartmentofEnergy(DOE)targetof$50kW−1

willfuel cells becompetitivefor virtually everytype of power application.Ontheotherhand,toreachtechnologicalreadiness, automotivefuelcellpowersystemsneedtobeasdurableand reli-ableastoday’sinternalcombustionengines,correspondingtoa 5000-hoperatinglifetime(approximately7monthsoverarange ofvehicleoperatingconditions,includingvariablerelative humid-ity(RH),shutdown/start-up,freeze/thaw,andsubfreezingdownto −40◦_C),_and_stationary_fuel_cells_must_also_meet_operating

life-timetargetsof morethan 40,000h(approximately4.5 years of continuousoperation)tocompetewithextantdistributedpower generationsystems,basedonthe2015DOEtargetsatcomponent, stack,andsystemlevels[1].TheDOEdurabilitytargetsfor station-aryandtransportationfuelcellapplicationsstandonthelifetimes ofenergyconversiondevicesthatarecompetitivewithfuelcells, suchasmicroturbinesandinternalcombustionengines, respec-tively.However,currentlythelifetimesoffuelcellvehicles and stationarycogenerationsystemsarearound1700hand10,000h, respectively[2].Clearly,intensiveR&Disstillneededtoaddress theissuesrelatedtoPEMFCdurabilityordegradationinorderto achievesustainablecommercialization.

As fuel cells must operate over a wide range of operat-ing and cycling conditions, including temperatures from well belowfreezingtowellabovetheboilingpointofwater,relative humidity(RH) fromambient tosaturated, and half-cell poten-tialsfrom 0to >1.5V,performance degradation is unavoidable, especially fortransportation applications.Althoughthelifetime targetsforautomobilesaremuchlowerthanthoseforstationary applications,operatingconditionssuchasdynamicloadcycling, start-up/shutdown,and freezing/thawing make the target very challengingforcurrentfuelcelltechnologies.Thecell/stack condi-tionscycle,sometimesquiterapidly,betweenhighandlowvoltage, temperature,humidity,andgascomposition.Thecyclingresults in physical and chemical changes to components, sometimes withcatastrophicresults[3].Althoughperformancedegradation is unavoidable,the degradation ratecan beminimized given a comprehensiveunderstandingofdegradationandfailure mecha-nisms.Normally,degradationtargetsrequirelessthan20%lossin theefficiencyofthefuelcellsystembytheendofitslife,anda degradationrateof2–10␮Vh−1 _is_commonly_accepted_for_most

applications[4].

Todate,variousfailuremodeshavebeenidentified,for exam-ple,catalystparticleripening(particlecoalescence), preferential alloydissolutionin thecatalystlayer,carbon supportoxidation (corrosion),catalystpoisoning,membranethinningandpin-hole formation,lossofsulfonicacidgroupsintheionomerphaseofthe catalystlayerorinthemembrane,bipolarplatesurfacefilmgrowth, hydrophilicitychangesinthecatalystlayerand/orgasdiffusion layer(GDL),andPTFEdecompositioninthecatalystlayerand/or GDL.Componentdecayorfailureisaffectedbymanyinternaland external factors,including material properties,fuel cell operat-ingconditions(suchashumidification,temperature,cellvoltage, etc.),impuritiesorcontaminantsinthefeeds,environmental con-ditions(e.g.,subfreezingorcoldstart),operationmodes(suchas start-up,shutdown,potentialcycling,etc.),andthedesignofthe componentsandthestack.Inaddition,thedegradationprocesses ofdifferentcomponentsareofteninterrelatedinafuelcellsystem. Itisthereforeimportanttoseparate,analyze,andsystematically understandthedegradationphenomenaofeachcomponentsothat novelcomponentmaterialscanbedeveloped andnoveldesigns

forcells/stackscanbeachievedtomitigateinsufficientfuelcell durability.

In this ongoing R&D, long-term durability tests are often requiredtoevaluatethedegradationmechanismsofvarious com-ponentsand thecorrespondingfuelcellsystems.However,itis generallyimpracticalandcostlytooperateafuelcellunderits nor-malconditionsforseveralthousandhours,soacceleratedstress test(AST)methodsarepreferredtofacilitaterapidlearningabout keydurabilityissues.ThegoaloftheASTistodeterminethe dura-bilityandperformance ofcurrentfuelcell componentswithout havingtotestovermanyyears[5].Therefore,ASTprotocolsare developedtoensurethatthetypeandformatofthedata gener-atedaresufficientsothatthedatasetscanbecompared,merged, andscaled.Asnewmaterialscontinuetobedevelopedandnew designsemerge,theneedforrelevantacceleratedtesting,aswell asASTprotocols,increases.However,asstatedbyDillaretal.[6],it isdifficultatthistimetoproposeanacceleratedprotocolwithout performingadetailedexaminationoftheexistingfuelcell perfor-manceanddiagnosticsdata,suchasionicresistance,polarization curves,lifetimetests,fuelcrossoverrates,andeffluent concentra-tions.Also,regardlessoftheacceleratedprotocoldeveloped,care mustbetakentoperformacceleratedtestingunderconditionsthat donotintroducenewfailuremechanismsthatwouldbeunrealistic infuelcellenvironments.

Theintentofthisreviewistogatheravailableinformationon ASTmethodsanddegradationtestprotocolsforPEMFCs,providing practitionerswithausefultoolboxforstudyingdurabilityissues, inthehopethattheseprotocolswillhelptopreventtheprolonged testperiodsandhighcostsassociatedwithreallifetimetests,to assesstheperformanceanddurabilityofPEMFCcomponents,and toensurethatthegenerateddatacanbecompared.Itshouldbe pointedoutthatalthoughavarietyofcelldurabilityteststandards havebeendevelopedbydifferentorganizationsaroundtheworld, e.g.,theFuelCellTEstingandSTandardisationthematicNETwork (FCTESTNET)andtheJapanAutomobileResearchInstitute(JARI), theDOEASTmethodsarethemostcommonlyacceptedprotocols. Therefore,thispaperwillmainlyreviewASTprotocolsdeveloped bytheDOEprograms,aswellasASTprotocolspublishedintheopen literature.Forclarity,thesetestingprotocolsaresummarizedinto twogeneralcategories—componentlevel,and stackand system level.WithinthecomponentASTs,electrocatalyst,catalystsupport, membrane,GDL,ionomer,andinterfacialdegradationareincluded. Atthesystemlevel,drivingcycleisdiscussed.

2. AbriefreviewofPEMFCdegradation

2.1. Catalystlayerdegradation

Thecatalystlayer(CL)isamultiple-phasedmatrixcontaininga highsurfaceareacarbonsupportloadedwithnanoscaledPtor Pt-alloyparticlesandionomerrecastdispersion.ThethinCL,witha thicknessintheorderof∼15␮m,iscriticalforfacilitatingafast cat-alyticreactionthatrequiresfreeaccessforgas,electrons,protons, andwater[4,7,8].

CLdegradation relatesto thestability of both the materials andthestructure.Amongotherdegradationphenomena(suchas crackingordelaminationoftheCL,catalystwashout,electrolyte (ionomer)dissolution,carboncoarsening,andcatalystpoisoning bycontaminants),Ptparticlegrowth,Ptmigration,andcarbon cor-rosionareconsideredthemostdominantcausesofCLdegradation. Several mechanisms have been proposed to account for Pt particle growth. Small Pt particles may dissolve and redeposit onthe surface of largerparticles, leading toparticlegrowth, a phenomenoncalledOstwaldripening[9];randomcluster–cluster collisionsof Ptparticlesmayresult in Ptagglomerationonthe

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X.-Z.Yuanetal./JournalofPowerSources196 (2011) 9107–9116 9109

carbonsupportatthenanoscale[10];theminimizationofthe cata-lystclusters’GibbsfreeenergymayleadtoPtparticlegrowthatthe atomicscale[11];andthemovementandcoalescenceofPtparticles onthecarbonsupportcancausecoarseningofthecatalyst[12]. Par-ticlesizegrowthwillresultinreductionofthecatalyticallyactive surfaceareaandultimatelyleadtoadecreaseincatalystactivity andstability.

PtmigrationoccurswhenthedissolvedPtspeciesdiffuseinto theionomerphaseandsubsequentlyprecipitateinthemembrane throughthereductionofPtionsbycrossoverhydrogenfromthe anodeside[13–15].Ptmigrationintothemembranedramatically decreasesitsstabilityandconductivity.

Carboncorrosion occurs through the electrochemical oxida-tionof carbon:C+2H2O→CO2+4H++4e− (Eo=0.207VRHE)[16].

Despitecarbon’sthermodynamicinstability,carboncorrosion dur-ingnormalfuelcelloperationisnegligibleatpotentialslowerthan 1.1Vvs.RHE,duetoitsslowkinetics[17–19].However,the pres-enceofPtcancatalyzethecarbonoxidationreactionandreduce itspotentialto0.55Vvs.RHEorlower[20].Carboncorrosionis believedtobepromotedbythetransitionbetweenstart-upand shutdowncyclesand byfuelstarvation.Start-up andshutdown causenon-uniformdistributionoffuelontheanodeandhydrogen crossoverthroughthemembrane. Undercircumstancesof non-uniformfueldistributionandfuelstarvation,theanodeelectrode ispartially coveredwithhydrogen,makingtheanodepotential negativeandthusleadingtocarboncorrosion.

2.2. Membranedegradation

AsakeycomponentoftheMEA,themembranetransports pro-tonsintheformofanelectrolyteandactsasa barrierbetween anodeandcathodetopreventgaspermeation.Themostcommonly usedmembraneiscomposedofperfluorosulfonicacid(PFSA),such asNafion®_membranes.

Extensivestudieshavebeenconductedonmembrane degra-dation mechanisms, and it is known that typical membrane degradationinafuelcellresultsfrommechanical,thermal, and chemicaldegradation.Mechanicaldegradationincludesmembrane cracks,tears,punctures,andpinholesduetothepresenceofforeign particlesorfibersintroducedduringtheMEAfabricationprocess thatmightperforatethemembrane [21–23].Chemical degrada-tionoriginatesfromchemicalattackbyhydrogenperoxideradicals, resultinginbreakageofthemembrane’sbackboneandside-chain groupsandsubsequentlossofmechanicalstrengthandproton con-ductivity,thusleadingtoanincreaseinresistanceanddeclining cellperformance[24–26].Thermaldegradationoccurswhenthe membranebecomesdehydratedduetohigh-temperature opera-tion,lowhumidity,andothercauses,andleadstothelossofproton conductivity[27].

2.3. GDLdegradation

TheGDLisacarbon-basedporoussubstratebetweentheCLand theflowfield thatenablesgasphase transport,watertransport, electronicandthermalconduction,andmechanicalsupport.The GDLconsistsoftwolayers,amacro-porouslayermadeofcarbon fiberpaperorcarboncloththatiscoveredwithamicro-porous layer (MPL) made of carbon black powder and a hydrophobic agent(PTFE).Todate,theGDListheleaststudiedMEAcomponent intermsofdurabilityanddegradation.SeveralGDLdegradation mechanismshavebeenproposed:carbonoxidation[28,29],PTFE decomposition[30],andmechanicaldegradationasaresultof com-pression[31].Thefirsttwomechanismscausehydrophobicityloss andchangesintheGDLporestructure,resultinginanincreasein thewatercontentoftheGDLandMPLandthusimpedinggasphase masstransport[32–34].Itshouldbenotedthatthecarbonfibersof

theGDLandthecarbonblackparticlesoftheMPLaremorestable thanthecarbonblackintheCLduetotheabsenceofPtthatcan cat-alyzetheelectrochemicaloxidationofcarbon.However,chemical surfaceoxidationofcarbonbywatercannotbeexcluded[28,29].

2.4. Bipolarplatedegradation

Bipolarplates areresponsibleforseparating thefuelandair (oxygen)gases and thecoolant, uniformlydistributing reactant andproductstreams,andcollectingthecurrentgeneratedthrough the electrochemical reactions in the anode and cathode. Low ohmicresistance,lowgaspermeability,highcorrosionresistance, goodthermalandchemicalstability,andappropriatemechanical properties are requiredfor the long-termstability ofthe bipo-larplates inanoperating fuelcellenvironment[35,36].Several materialshavebeenemployedandevaluatedasbipolarplatesfor PEMFCs,includinggraphite,metals,graphite/carbon-based com-posites, and polymer-based composites [37–39]. Graphite and graphitecompositeshavefavorablepropertiessuchashigh resis-tancetocorrosionandchemicals,lowdensity,andhighelectrical andthermal conductivity,butsufferfromsurfacecarbon oxida-tion/corrosion under extreme operating conditionssuch ascell reversal[4],leadingtoincreasedcontactresistance.Metallic bipo-larplates,dependingonthenatureofthemetals,cansufferfrom corrosionandsurfaceoxidefilmgrowth[40],leadingtotherelease ofcontaminantsandincreasedcontactresistance[41,42].

2.5. Othercomponentdegradation

Incomparisontotheextensivestudiesonthedurabilityofthe key components(catalyst,membrane, GDL, andbipolar plates), durabilitystudiesonseals,endplates,andbusplateshavelongbeen ignored,andtheliteraturedataisverylimited.

Gasketscanexperiencereducedthicknessoverlong-term oper-ation,leadingtoincreasedcompressionforceontheGDLanda subsequent decreasein GDL porosity as wellas an increase in reactant transport resistance [43]; gaskets can also experience crossoverleakage,causing damagetothemembrane [44]. End-platesandbusplatesmaysufferfromsimilarcorrosionissuesbut toamuchlesserextent.

2.6. Operatingconditions,environment,contamination, operationmode,anddesign-induceddegradation

ThedurabilityofeachcomponentofaPEMFCisaffectedbymany externalfactorsinanoperatingfuelcell,includingthefuelcell oper-atingconditions(suchashumidification,temperature,cellvoltage, etc.),impuritiesorcontaminantsinthefeeds,environmental con-ditions(e.g.,subfreezingorcoldstart),operationmodes(suchas start-up,shutdown,potentialcycling,etc.),andthedesignofthe componentsandthestack[45,46].

ThechangesintemperatureandRHassociatedwithtransitions betweenlowandhighpowercanhaveadverseeffectson compo-nentpropertiesandthusontheintegrityofthefuelcellsystem. Forexample,asRHincreases,thewateruptakeinthemembrane increasesandtheionomerswells,yieldingtensileresidualstresses duringdryingandthuscontributingtomechanicalfailuresinthe membrane[47].

ImpuritiessuchasCOandH2Sarepresentinthefuelasaresultof

thereformingprocess,orintheairintake(e.g.,NOx,SOx,orvolatile

organiccompounds)duetoairpollution[48].Impuritiescanalso comefromfuelcellcomponents(suchasmetalionsfrombipolar plates)[49]orfromthesecondelementsinPtalloycatalysts(such asCo2+₎_[50,51]_._These_impurities_are_known_to_adversely_affect

fuelcellperformanceanddurabilitybyseveralmeans:thekinetic effect,causedbythepoisoningofbothanodeandcathodecatalyst

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sites;themasstransfereffect,duetochangesinthestructureand hydrophobicityoftheCLsand/orGDLs;andtheconductivityeffect, causedbyincreasedresistanceinthemembraneandionomer[52]. Theabilitytosurviveandstartupatsubfreezingtemperatures isanimportantrequirementforPEMFCs,particularlyin automo-bileapplications.SubjectingaPEMFCtosubfreezingtemperatures hasbeenreportedtocausesignificantdropsinthecathode electro-chemicalsurfacearea(ECSA),whichwasattributedtoiceformation intheCLsthatresultedinincreasedporosityandultimate delami-nationoftheCLfromthemembrane[53].Theeffectoffreeze/thaw thermalcyclesonthepropertiesofMEAcomponents(CL,GDL,and membrane)hasbeenextensivelystudiedbutmostlythroughex situinvestigations,andtheresultsaresometimesconflicting. Gen-erally,freeze–thawcyclingwillchangethewatercontent/statein theCLs [53],theairpermeabilityoftheGDL[31],and the con-ductivityofthemembrane[54,55].Detailedcharacterizationofthe durabilityofMEAcomponentsunderfuelcelloperatingconditions duringfreeze/thawcyclesapparentlyneedstobebetterevaluated

[45].

Thedesignoffuelcellcomponentssuchasflowfieldsand mani-foldscanhaveasignificantimpactonwatermanagementandfeed flows,whichcaninturnaffectthedurabilityofMEAcomponents andofthefuelcell.Forexample,animproperdesigninthechannel depthoftheflowfieldscaninducewaterblockage,andimproper manifolddesign canresultin poorcell-to-cellflowdistribution, bothofwhichwillcauselocalizedfuelstarvation[56].Thislocalized fuelstarvationcaninduce,through“reversecurrent”mechanisms, localpotentialsontheairelectrodesignificantlyhigherthan1.0V andtherebyinducecorrosionofthecarbonsupport,resultingin permanentlossofelectrochemicallyactivearea.

Fuelcellstart-upandshutdownareoperationalmodesthatcan haveaprofoundinfluenceonfuelcelldurability.Underconditions ofprolongedshutdown,allthehydrogenwilleventuallycrossover fromtheanodetothecathode,resultingintheanodeflow chan-nelsbeingfilledwithair.Inthiscase,fuelcellstart-upwillcreate atransientconditioninwhichfuelexistsattheinletbuttheoutlet isstillfuel-starved attheanodeside. Thislocalizedfuel starva-tioncaninducethelocalpotentialatthecathodetobehigherthan 1.8V,causingseriousdeterioration infuelcellperformance and durability[57].

3. Acceleratedstresstestprotocolsforcellcomponents Afuelcellisacomplicatedsystemcomprisingvarious compo-nents.AnASTshouldnotonlyactivatethetargetedfailuremodeof thespecificcomponent,butalsominimizetheconfoundingeffects fromother components.Specific ASTconditions and cyclesare thusintendedtoisolateeffectsandfailuremodes,andarebased onassumed,butwidelyaccepted,mechanisms.Forinstance,the ASTprotocolforcatalystsupportsisdifferentfromtheprotocol forelectrocatalystsbecausethecomponentsexperiencedifferent degradationmechanismsunderdifferentconditions.Similarly,the ASTformechanicaldegradationofthemembraneshouldisolate theeffectsofchemicaldegradationofthemembrane.

Tounderstanddegradation mechanisms,component interac-tions, and the effects of operating conditions, and to increase samplethroughputandreduceexperimentaltime,fuelcell devel-opershaveimplementedvariousASTstoanalyzethefailuremodes ofcurrentfuelcellcomponents.TheseASTsattempttoensurethat thetest conditionsandproceduresdo notresultindegradation mechanismsthataredifferentfromthoseencounteredduring nor-maloperation.TheDOEandtheUSFuelCellCouncil(USFCC)have establishedPEMFCdurabilitytestingprotocolswiththeintentof providingastandardsetoftestconditionsandoperating proce-duresforevaluatingnewcellcomponentmaterialsandstructures.

TheDOEASTprotocolsweredevelopedbytheFreedomCARFuel CellTechnicalTeam(FCTT),comprisedofrepresentativesfromthe DOEand US automakers,toprovide guidanceand standardiza-tionforDOEcomponentdevelopmentprojects.TheFreedomCAR targets[58]arecloselyalignedwiththeDOEsystem,stack,and componenttechnicaltargets.Inputwasalsosolicitedandreceived fromseveralfuelcelldevelopersandfromtheUSFCC,which is developingASTsforbroader,long-termapplications.Basically,the USFCCprotocolsareinagreementwiththeDOEprotocols.Other thantheDOEandUSFCCprotocols,avarietyofASTsatdifferent levelsunder variousoperating conditions havebeen published. SomeareinlinewiththeDOEprotocolswhileothersarespecially designedfordifferentpurposes.

ThefollowingsectionssummarizetheDOEcellcomponentAST protocols,includingforelectrocatalysts,catalystsupports, mem-brane/MEA chemical stability, and membrane/MEA mechanical durability,derivedfromreferences[59,60],aswellasexamplesof ASTsavailableintheliterature.

3.1. Electrocatalyst

Electrocatalyst degradation mechanisms depend on factors suchaspotential,temperature,humidity,contaminants,and car-bonsupportstability.Hence,electrocatalystdegradationcanbe accelerated by potential control, undesirable temperatures and humidities,contaminants,andloadcycling[8].Inparticular,athigh electrodepotentialsthedurabilityofcatalystscanbecompromised byPtsintering,particlegrowth,anddissolution.ASTstressorscan beanycombinationofthesefactorswhentestingisconductedina complexenvironment.

Because catalyst sintering/dissolution is accelerated under potentialcycling [61], the DOEelectrocatalyst ASTis based on potentialcycling. Table1liststheDOEtestingprotocolsaswell asthetestingmetricsforelectrocatalysts[3].Herecatalytic activ-ityischaracterizedinAmg−1_at₁₅₀_kPa_absolute_backpressure_and

at900mVwithiR-correctedonH2/O2.

TheASTforelectrocatalystspecifiesloadcyclingfrom0.7Vto 0.9Vfor30,000cyclesoruntilcatalyticactivitylossreaches60%, theECSAdecreasesby40%,orperformanceisreducedby30mV at0.8Acm−2_._Cycling_to_higher_voltage_would_accelerate

degra-dationbutmightalsoincrease corrosionofthecarbon support, convolutinginterpretationoftheresults.

TheUSFCCpresentstwo protocolsforelectrocatalysttesting. ThefirstisverysimilartotheDOEprotocoldescribedinTable1

butwithaslightlywider voltagecyclingrange(0.6–0.96V)and aironthecathode.Thesecond hasa stillwider voltagecycling range(0.6–1.2V)withN2onthecathode[62].TheDOErangeis

restrictedto0.7–0.9Vtoreducethepossibilityofcatalystsupport degradationathighervoltagesandtoisolatedegradationeffects.

Avarietyofinvestigationsintocatalystdegradationhavebeen conducted[63–70].CatalystASTsdescribedintheliterature gener-allysimulatedutycycleinducedcatalystdegradationbypotential cyclingfromalowerpotential,intherangeof0.1–0.7V,toincreased potential,suchasOCV,1.0,or1.2V.Someexamplesofthesestudies aregiveninTable2.

3.2. Catalystsupport

Durabilityofcatalystsupportsisanothertechnicalbarrierfor stationary and transportation applications of PEMFCs. Conven-tionalPEMelectrodesconsistofplatinumparticlessupportedon carbon.Carbonisanexcellentmaterialforsupporting electrocat-alysts,allowing facilemass transport of reactantsand fuelcell reactionproducts,andprovidinggoodelectricalconductivityand stabilityundernormalconditions.However,underprolonged con-ditionsof hightemperature, highwater content, low pH, high

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X.-Z.Yuanetal./JournalofPowerSources196 (2011) 9107–9116 9111 Table1

DOEASTprotocolsforelectrocatalyst[3](reproducedwithpermissionfromtheElectrochemicalSociety). Cell Singlecell25–50cm2

Operating condi-tions

Temperature:80◦_C

Relativehumidity:anode/cathode100/100% Fuel/oxidant:H2/N2

Pressure:150kPaabsolute

Cycle Stepchange:30sat0.7Vand30sat0.9V Cycletime:60s

Cyclenumber:30,000

Metrics Catalyticactivity Frequency:beginningandendoflife Target:<60%lossofinitialcatalyticactivity Polarizationcurve Frequency:after0,1k,5k,10k,and30kcycles

Target:<30mVlossat0.8Acm−2

ECSA/cyclic voltammetry

Frequency:after1,10,30,100,300,1000,3000cyclesandevery5000cyclesthereafter Target:<40%lossofinitialarea

Table2

Catalyst/CLASTprotocolsintheliterature.

Testmode Stressor Availableprotocols Authors Reference Insitu Potentialcycling Linearpotentialsweepfrom0.1Vtoanupperlimit(varying

from0.8Vto1.5V)at10mVs−1_in_increments_of₃₀₀_cycles;

80◦_C,_N

2with226%RHatthecathode,H2with100%RHatthe

anode

R.L.Borupetal. [71]

Exsitu Fixedpotential 20◦_C,_0.5_M_H₂_SO₄_,_a_fixed_potential_at_1.2_V_(vs._RHE)_for₁₉₂_h _Y.Y._Shao_et_al. _[72]

Exsitu(Ptdissolution) Constantpotential 40–80◦_C,₁_M_HClO₄_,_constant_potential_between_0.85_V_(vs.

RHE)and1.4V(vs.RHE)

V.A.T.Dametal. [73]

Insitu Potentialcycling Linearsweepfrom0.1toeither1.0or1.2Vat10mVs−1_;₆₀◦_C

or80◦_C,_50%_or_100%_RH,_N

2onthecathodeandH2onthe

anode

K.L.More [74]

Insitu Potential Cathode1.2Vrelativetotheanode;80◦_C,_100%_RH,_N 2onthe

cathodeandH2ontheanode

J.Frisketal. [75]

oxygen concentration, and/or high potential, oxidation of car-bon(carboncorrosion)ispronetoacceleration.Corrosionofhigh surfaceareacarbonsupportsathighelectrodepotentialsposes sig-nificantconcernsandistypicallyacceleratedduringfuelstarvation andstart-up/shutdowncycling.Evaluationofcarbonsupport dura-bilityisthusperformedathighvoltagewithhydrogenontheanode andnitrogenonthecathodetoisolateelectrochemicalcorrosion fromchemicalcorrosioninthepresenceofair.Table3liststheDOE ASTprotocolsaswellasthetestingmetricsforcatalystsupports[3]. TheASTforthesupportspecifiesasteady-stateholdat1.2Vfor 200horuntilcatalyticactivitylossreaches60%,ECSAdecreasesby 40%,orperformanceisreducedby30mVat1.5Acm−2_or_at_rated

power.SupportsotherthancarbonmayrequirechangestotheAST toreflectdifferentdegradationmechanisms.

TheUSFCCprovidestwotestcyclesforevaluatingcatalyst sup-ports.OnecycleusesthesameholdvoltageastheDOE(1.2V)and theotheruses1.5Vtoacceleratecorrosion.BothUSFCCprotocols uselowertemperature(80◦_C_vs._DOE’s₉₅◦_C)_and_higher_RH_(100%

vs.DOE’s80%).

OtherprotocolsusedforcatalystsupportASTsaresimilartothe DOE/USFCCones,whicharemoreorlessrelatedtohighpotential ortopotentialcyclingwithhighpotentialinvolved.Someexamples arelistedinTable4.

3.3. Membrane

Acceleratedmembranetestscanincludeundesirable tempera-tureandRH,OCV,loadcycling,Fenton’stest,freeze/thawcycling, stress cycling under tension, high-pressure testing, and high-temperatureexposure[8].

3.3.1. MembranemechanicalAST

MembranesarethekeycomponentsofthefuelcellMEAand stack;theymustbedurableandtolerateawiderangeof operat-ingconditions,includinghumidityrangingfromambientto100% RHandtemperaturerangingfrom−40to120◦_C._Improved

mem-branesareneededthatperformbetterandarelessexpensivethan thecurrentgenerationofpolymermembranes.

Table3

DOEASTprotocolsforcatalystsupport[3](reproducedwithpermissionfromtheElectrochemicalSociety). Cell Singlecell25–50cm2

Operatingconditions Temperature:95◦_C

Relativehumidity:anode/cathode80/80% Fuel/oxidant:H2/N2

Pressure:150kPaabsolute Cycle Step:holdat1.2V

Cycletime:24h Totaltime:200h

Metrics CO2

release

Frequency:on-line Target:<10%massloss Catalyticactivity Frequency:every24h

Target:<60%lossofinitialcatalyticactivity Polarizationcurve Frequency:every24h

Target:<30mVlossat1.5Acm−2_or_rated_power

ECSA/cyclic voltammetry

Frequency:every24h Target:<40%lossofinitialarea

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Table4

CatalystsupportASTprotocolsintheliterature.

Testmode Stressor Availableprotocols Authors Reference Insitu Potentialcycling Potentialcyclingfrom0.04to1.2Vvs.RHEat2mVs−1_;₅₀◦_C,

withfullyhumidified4%H2/N2andHefortheanodeand

cathode,respectively

L.M.Roenetal. [20]

Insitu IdleandOCV Idle(0.9Vvs.RHE)andOCVfor2000hat80◦_C _R._Makharia_et_al. _[76]

Insitu Potentialcycling Linearpotentialsweepfrom0.1to0.96V,1.0V,1.2V,and1.5V R.L.Borupetal. [71]

Mechanicalandchemicaldegradationofthepolymerionomer thatcomprisesthePEMbothincreasewithtemperature.Changein RHisanotherseriousconditionthatresultsinmechanical degra-dationof themembrane during practical operation. RH cycling resultsinswellingofthemembrane asit absorbswater athigh RHand shrinkageasitloseswateratlow RH.Thisswell/shrink cyclingresultsinhighmechanicalstressesinthemembraneand subsequentmechanicalfailure,leadingtogascrossoverthrough themembrane.AsdemonstratedbyCrumandLiu[77],RHcycling canbeusedtoexaminemechanicaldurabilityinisolation,orto examinecombinedmechanicalandchemicaldurability.Inaninert nitrogenatmospherewithRHcycling,themechanicaldurability associatedwithexpansionandcontractionoftheionomerunder theflowfieldchannelsisisolated.WhenthesameRHcyclingis employedwithairandhydrogenfeedgases,mechanicaland chem-icaldurabilityarecombined.Table5showstheDOEmembrane mechanicalASTprotocolsbasedonRHcycling[3].Thetest con-tinuesfor20,000cyclesoruntilcrossoveris10sccm.Membrane performanceismeasuredasthenumberofcyclesbeforeathreshold crossoverisobserved.

Gascrossoverisagoodindicatortomonitormembrane degra-dationormechanicalstability.Thiscanbedonebyeithermeasuring gasflowrateaccordingtotheproceduresdevelopedbytheUSFCC or measuringhydrogen crossoverelectrochemically (mAcm−2_),

thelatterbeingwellacceptedbyfuelcellresearchers.

3.3.2. MembranechemicalAST

The DOE AST protocol for membrane chemical durability involvesasteady-stateholdfor200hatOCVand90◦_C,_as_listed

inTable6 [3].AtOCV,thefueland oxidantarenotbeing con-sumedelectrochemically,sothereispotentiallymorecrossover, resulting in peroxide production and radicals that attack the membrane[78].Similarly,crossovercanbemeasuredbyeither gas flow rate or hydrogen crossover. The test continues until OCVdecays by 20% or crossoverreaches 20mAcm−2_. _Fluoride

ionrelease(oranequivalentfornon-fluorinatedmembranes)is alsomeasuredduringthetestformonitoringpurposes.Fluoride ion release is an indicationof membrane deterioration for flu-orinatedmembranesand hasbeencorrelatedtomembrane life

[79,80].

TheUSFCCASTsformembranesarevirtuallyidenticaltothe DOEprotocolsdescribedinTables3and4.Thedifferencesare asso-ciatedwiththeoxygenconcentration(airvs.theUSFCC’s40%or 100%O2)onthecathodeduringthetest.TheDOEspecifiesairto

accommodatethoselaboratoriesthatrestrictoxygenusage.

Unfortunately,membranedurabilityisalwaysstudiedusinga MEA.Thus,theDOE/USFCCmembraneASTprotocolisdescribed asamembrane/MEAASTprotocol.Despitethedifficulties,several studieshaveexaminedcandidateconditionsforexsituaccelerated testingofmembranes.LaContietal.[21]describemultiplemethods ofacceleratingdegradationinmembranes.Itispossibletosimulate thedegradationofPEMsusinganacceleratedtestmediumsuchas Fenton’sreagent(smallamountsofhydrogenperoxide(∼3%)and Fe2+_ions₍₄_ppm)_in_solution)._This_Fenton’s_test_is_also_described

bytheUSFCCasanadditionalmembrane/MEAprotocol.The fer-rousioncatalyzesthedecompositionofperoxidetoperoxyand/or hydroxyl radicalsthat chemically attacktheperfluorinated sul-fonicacidpolymers.MeasurementofF−_in_the_effluent_provides

ameasureofmembranestability[80].However,thisFenton’stest onlyappliestofluorinemembranes,suchasNafion®_._Another

addi-tionalmembrane/MEAASTprotocolrecommendedbytheUSFCC isacombinedhumidity/loadcycledevelopedbyDuPont.TheRHis cycledbetween1and100%atfixedloadfor24h,followedbyaload cyclebetween10and800mAcm−2_at_fixed_RH_for₂₄_h._The_cycle

isrepeatedforthedurationofthetest.TheDOEdidnotincludethis protocolbecausealthoughitprovidesinsightintothebehaviorofa membranewiththecombinedcycle,itdoesnotallowisolationof theeffectsanddegradationmechanismsforRHandvoltagecycling. W.L.Gore&Associateshaveconductedextensiveinvestigations intodurability testing andaccelerated testing procedures.They observedan approximately10×relationship formembrane life betweentheirstandardandacceleratedresidentialtestprocedures, while H2 crossover(monitored regularlyfor both standard and

acceleratedtesting)graduallyincreasedovertime.Comparedtothe standardprotocol(Tcell=70◦C,DPa,DPc=70◦C,PH2=PAir=0psig),

they concluded that the accelerated fuel cell life test protocol (Tcell=90◦C,DPa,DPc=83◦C,PH2=5psig,andPAir=15psig)wasa

validtestingconditiontoevaluatemembranedurabilityfor resi-dentialfuelcellapplications[81].

Numerousexperiments havetackledmembrane degradation

[82–88], examining various stressors: temperature, humidity, freeze–thawcycling, OCV,and Fenton’stest. Someexamplesof membraneASTs, aswellastheirtestingprotocols, arelistedin

Table7.

3.4. GDL

Changesinthemicrostructureandsurfacecharacteristicsofthe GDLduetomateriallossandporesizedistributionshiftsmaycause changesinthewatercontentlevelandtransportpropertiesofthe

Table5

DOEASTprotocolsformembranemechanicaldegradation[3](reproducedwithpermissionfromtheElectrochemicalSociety). Cell Singlecell25–50cm2_(test_using_a_MEA)

Fuel/oxidant:air/airat2slpmonbothsides Pressure:ambientornoback-pressure

Cycle Stepchange:cycle0%RH(2min)to90◦_C_dewpoint₍₂_min)

Cycletime:24h

Totaltime:untilcrossover>10sccmor20,000cycles

Metrics Crossover Frequency:every24h Target:<10sccm

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X.-Z.Yuanetal./JournalofPowerSources196 (2011) 9107–9116 9113

Table6

DOEASTprotocolsformembranechemicalstability[3](reproducedwithpermissionfromtheElectrochemicalSociety). Cell Singlecell25–50cm2

Relativehumidity:anode/cathode30/30%

Fuel/oxidant:hydrogen/airatstoicsof10/10at0.2Acm−2

Pressure:anode250kPa(inlet),cathode200kPa(inlet) Cycle Step:steadystateOCV

Cycletime:24h Totaltime:200h

Metrics F-releaseorequivalentfornon-fluorinemembranes Frequency:atleastevery24h Hydrogencrossover(mAcm−2₎ Frequency:every24h

Target:<20mAcm−2

OCV Frequency:continuous

Target:<20%lossinOCV High-frequencyresistance Frequency:every24hat0.2Acm−2

MEA.Todate,theDOEhasnotdevelopedanystandardAST pro-tocolsondurabilityanddegradationissuesforGDLs.Eveninthe literature,onlyalimitednumberofstudieshavefocusedonGDLs aseffectivecharacterizationtechniquesisrelativelylimited.Asa result,noDOEorothercommonlyusedASTprotocolsare avail-ableforthisreview.Instead,abriefsummaryofsomeinsituand exsitustressorsthathavebeenusedtotackleGDLdegradationis provided.

In situstressorsthat havebeen usedtoassessGDL stability includefreeze/thawcyclesandfuelstarvation.Forexample,Kim etal.[92]reportedinterfacialdelaminationbetweentheCLandGDL after100freeze/thawcyclesfrom−40◦_C_to₇₀◦_C,_which_resulted

fromiceformationandinturnsignificantlyincreasedGDL deforma-tion.Mukundanetal.[93]conductedfreeze/thawcyclesdownto −80◦Cusingdryice,andtheirresultsindicatedthatmultiplecycles couldleadtointerfacialdelaminationandGDLfailureinthefuel cell.Also,prolongedorrepeatedpulsesofH2starvationandhigh

potentialcontrolcanchangethemicrostructureandcausematerial lossintheMPLandGDL.Oszcipoketal.[94]haveobservedreduced GDLsurfacehydrophobicityincold-startconditions.

Many researchstudieshave alsoexaminedGDLdegradation usingexsitu methods.Indurability tests,ex situstressorshelp avoid potential confounding effects from other adjoining com-ponents. Ex situ stressors are intended to simulatea complex environmentfortheGDLduringfuelcelloperation.Forexample, Frisketal.[95]submergedtheGDLinhydrogenperoxidesolution, amethodsimilartoFenton’stest,andinthiswaydetectedweight lossandMPLcontactangleincreaseintheGDL.Woodetal.[96]

immersedGDLsamplesinliquidwaterwithdifferentoxygen con-centrationstolookatdecreasesinhydrophobicitywithexposure timeandwatertemperature.CompressivestrainoftheGDLunder steady-stateandfreezingconditionswasalsousedasanexsitu stressorbyLeeandMerida[31].BymonitoringtheGDL’s proper-ties–e.g.,electricalresistivity,bendingstiffness,airpermeability, surfacecontactangle,porosity,andwatervapordiffusion–they wereabletoinvestigatethedegradationmechanisms.

ResearchersattheInstituteforFuelCellInnovation(IFCI)ofthe NationalResearchCouncilCanada(NRC)[97–99]haveestablished theirownGDLtestingprotocols.Prioritizationofthevarious stres-sorswasaccomplishedbyexposingsamplesofGDLtoelevated conditionsfor200h.Eachstressorwasappliedindividuallyatan elevatedleveltoisolatetheparticulareffectofthestressorunder examination.Thelistofstressorswastrimmedtofourinorderto accommodatethe200hoftestingrequiredforeachduringthefirst quarter.Thefourstressorswere:

•Elevatedflowrate

•Elevatedtemperature(120◦_C)

•Constantelectricalpotential(1.8V) •Dynamicelectricalpotential

Earlyon,itwasproposedthatGDLdamageduringhot-pressing orstackassemblyatoverlyhighpressurescouldhaveaneffecton degradation.Theconditionsselectedtorepresentthetwotypesof damagedGDLwere:

•Hot-pressinginahotpressat135◦_C_and₃₀_bar_for₅_min

•Stackassemblyinacellwithabladderpressureof200psigfor 30min

ToaccommodatethedamagedGDLsamplesintothestressor testing,a3-cellstackwasrequiredratherthanthesinglecell origi-nallyplannedfor.A3-cellstackallowedforsimultaneoustestingof anundamagedGDLsampleandbothdamagedGDLsamplesunder identicalconditions.

Uponcompletionofthetestingatelevatedconditions,thetest plancalledforcharacterizationoftheinduceddegradation. Charac-terizationwasaccomplishedthroughmultiplesetsofpolarization curvesacquiredusingacatalyst-coatedmembrane(CCM)in sin-glecellhardware.Characterizationwasalsoaccomplishedthrough theuseofvariousanalyticaltechniques,suchasmasslossandSEM. Additionalrecommendedtoolsare:

Table7

Membrane/MEAASTprotocolsintheliterature.

Testmode Stressor Availableprotocols Authors Reference Insitu RHcycling 65◦_C,_RH_cycling_from₃₀_to_80%_or_from₈₀_to_120%_with

30min/step,withairsuppliedtotheanodeandthecathode

X.Huangetal. [89]

Insitu RHcycling 80◦_C,_RH_cycling_from₀_to_150%_with₂_min/step,_with_air

suppliedtotheanodeandthecathode

M.F.Mathiasetal. [59]

Insitu OCV OCVatcelltemperatureof80◦_C_and_both_gases_humidified_at

60◦_C,_with_hydrogen_and_air_supplied_to_the_anode_and_the

cathode

M.Inabaetal. [90]

Exsitu Fenton’stest Solutionmethod:16mgL−1_Fe2+_in_a_solution_of_FeCl₂_/4H₂_O

and30%H2O2;72◦_C;_fresh_solution_every₁₂_h._Exchange

method:Nafion®_samples_were_treated_in_saturated

FeCl2·4H2Osolutionfor24h,then40mLofperoxidewas

added.72◦_C;_fresh_peroxide_every₂₄_h

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•Contactangleanalysis •X-raydiffraction

•X-rayphotoelectronspectroscopy •Mercuryintrusionporosimetry

•Surfaceroughness(Wyko3-DSurfaceProfiler,modelNT-2000) •TGA

3.5. IonomerintheCL

Manyresearchgroupshavepostulatedthationomer degrada-tion/lossmightbeoneofthecritical factorsleadingtoreduced CLperformance afterlong-termoperationorAST.Astheactual Nafion® _ionomer_network _within_the_CL_is_not _as_easily

distin-guished/imagedasPtandcarbonsupports,it isusuallydifficult toidentifyionomerdegradationintheCLusingtraditional mor-phologycharacterizationmethods.However,someresearchgroups haverecentlymaderemarkableprogressin confirmingNafion®

ionomerdegradationinCLsbyusinguniquediagnostictools.Xie etal.[63]usedcellimpedancetrendstorevealdegradationofthe recastionomernetworkintheCL.Zhangetal.[100]usedXPSto detectCLionomerdegradation(ordecreaseinconcentration).With EIS,Houetal.[101]measuredtheCL’sionicresistanceatdifferent currentstomonitorchangesintheionicresistanceprofile.With thehelpof statisticalinformation fromexperimentalimagesof CLs,Rongetal.[102]simulatedtheCLmicrostructure,revealing microstructuralchangesonthemicroscaleduringPEMFCageing.

Anadditionalconsiderationisthatasionomerdegradationin theCLisalwayscoupledwithmembranedegradation,itisdifficult todistinguishthemfromeach other,forexamplebymeasuring fluoridereleaserate.ResearchersattheUniversityof Waterloo, workingwithresearchersattheNRC,haveconsideredusing hydro-carbon membranes to investigate ionomer degradation in the catalystlayer.Theuseofhydrocarbonmembraneshelpseliminate theeffectsofmembranedegradation.Apreliminarytesting proto-colforthispurposecombinesvariousstressors,asshowninTable8.

4. Cell/stackdurabilitytestprotocols

Componentdegradation protocols are important in evaluat-ingPEMFCdurabilityat thematerialand componentlevelwith the intent to understand and compare the degradation mech-anismsof differentmaterials/components,and thus toimprove thecomponent’sperformanceanddurability,particularlyforthe developmentofnewmaterialsandnewdesigns.Inasinglecell orstack,manycomponentshavestrongandcomplexinteractions, complicatingthedegradation mechanismsofparticular compo-nentsatthecellorstacklevel.Therefore,eachcomponentmust ultimatelybeevaluatedinanintegratedfuelcellorstacksystem sothatthedurabilitydataofeachcomponentcanmoreaccurately reflectthecomponent’slifetimeinanactualfuelcellofareal sta-tionarypowergeneratororvehicle.Thissectionsummarizesthe degradation/durabilitytestingprotocolsthathavebeendeveloped orusedforsinglecellsorstacks.Ithastobenotedthattheexisting singlecell/stackdegradationtestingprotocolsmaynotbe compre-hensiveenoughtoevaluatetheeffectsofalloperatingconditions thatwillbeencounteredduringrealfuelcelloperation,particularly forautomobile applications,whichoften involvea combination ofvariouscomplicatedsituations.Morecomprehensiveprotocols needtobedevelopedtoaddressadditionalissues.

4.1. USDOEsinglecell/stacktestingprotocolsfortransportation applications

TheUSDOE,throughitsmultiple-yearHydrogenProgram,has developeddurabilitytestingprotocolsforfuelcellstacks.

Steady-statedurabilitytests[103]areconductedunder steady-state conditions (constant voltage and constant current) for a periodoftime,sometimesaslongasafewthousandhours.These testsaimtoexaminethedegradationrateof asinglecell/stack and thedegradationmechanismsof componentsunder steady-stateconditions.Measurementsofpolarizationcurves,membrane resistance,hydrogencrossover,andelectrochemicalsurfacearea (throughCVs)aremadeinsituperiodicallyduringthedurability testtocharacterizechangesinthosefundamentalpropertiesasa functionoftime.Effluentwateranalysis(elementalanalysis,ionic content,and pH)isconductedtomonitorfordegradation prod-ucts.ChangesintheCL(catalystparticlesize,catalystsintering, carboncorrosion,etc.)andthemembranearecharacterizedafter thedurabilitytestsbyscanningelectronmicroscopy/energy dis-persiveX-rayspectroscopy(SEM/EDS),X-rayfluorescence(XRF), X-raydiffraction(XRD),transmissionelectronmicroscopy(TEM), andneutronscattering,etc.

Potentialcyclingdurabilitytestsareappliedtosinglecellsasan acceleratedtestingtechniquetomimicthedynamicloadofan auto-motiveapplication,whichrequiresmanyrapidchangesinloadover thecourseofthefuelcell’slifetime[103].Duringpotentialcycling (sweeping),theanodeisexposedtonitrogen.Thecathodepotential issweptlinearlyfromaninitialvoltageofusually0.1Vtoanupper limitvoltagerangingfrom0.96Vto1.2V.Thescanningrateranges from5to50mVs−1_._After_cycling_intervals,_the_polarization_curve

oftheMEAandthecatalystsurfaceareaaremeasured.Afterthe cyclingiscompleted,post-characterizationmeasurementsare per-formedbyXRD,SEM,TEM,andothermethods,asdescribedearlier. Thesepostmortemanalysesareusedtodifferentiatethe contribu-tionsoffuelcellcomponentstothedegradationoftheoverallcell performance.

Start-up/shutdown cycling durability tests [104] have been developedbyLosAlamosNationalLab(LANL)toexaminethe degra-dationoffuelcellcomponentswhenthefuelcellissubjectedto shutdown/start-upcycles.Inthisprotocol,forshutdownsthefuel cellisfirstrunatOCVwithcontinuousdryairflowatthe cath-odeside,thenpurgedattheanodesidewithdryairfor5min;for start-ups,theanodeissubjectedtodryhydrogenflowatfullpower for5min.Inadditiontothepostmortemanalysesdiscussed ear-lier,anotherimportantanalyticaltooltodeterminetheeffectof start-up/shutdownismeasurementofCO2(andCO)evolutionat

thecathodebynon-dispersiveinfrared(NDIR).

Drivecycletests[45,105]areprotocolsdevelopedand standard-izedbytheDOEtoassessthelong-termdurabilityoffuelcellsfor vehicularapplicationsandtocomparestackperformancewithDOE targets.Thedrivecycleprotocolwasconstructedbyconverting fed-eralinternalcombustionenginedrivecycle“US06”toanequivalent PEMFCenginedrivecycle.Thisprotocolinvolvessteppingthrough aseriesofdifferentcurrentdrawstypicalofautomotiveloads.To establishthecurrentdensityprofile,aninitialpolarizationcurveis plottedandthenusedtodeterminethecurrentdensitiesatwhich averagecellvoltagesof0.88,0.80,0.75,0.65,and0.6Vareobtained. Thestackisthensubjectedtothecurrentdensityprofileshownin

Table9[45].Thestackloadcyclebasedontheinitialstack polar-izationcurveisshown inFig.1[45].Asa quantitativemeasure, thestackdurabilityisdefinedasthetimeittakesfortheaverage cellvoltagetodecayby10%fromtheinitialvoltagewhentested accordingtotheaboveprocedures.

4.2. Durabilitytestsforstationaryapplications

The DOEdurability target for PEMFCs in stationary applica-tionsis muchlongerthanforautomotiveapplications(20,000h vs.5000hby2015).However,theoperationalcomplexityof sta-tionaryapplicationsisfarless. Asaresult,there havenotbeen asmanyefforts todeveloptesting protocolsfor PEMFCsin

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sta-X.-Z.Yuanetal./JournalofPowerSources196 (2011) 9107–9116 9115 Table8

ASTprotocolsforionomerdegradationintheCLusinghydrocarbonmembranes. Testcondition(1) Idlecondition(10mAcm−2_),_single_cell_25–50_cm2

Steptime 48h

Testcondition(2) Operationalcondition(300mAcm−2₎

Steptime 48h

Testcondition(3) Loadcyclingat50,400,and800mAcm−2_,₁₀_h_for₂_cycles

Steptime 60h

Testcondition(4) Thermalcyclingat35,55,70,and90◦_C_for₁₀_h_each

Steptime 40h

Metric F-releaseorequivalentfornon-fluorinemembranes Frequency:attheendofeachcycleoratleastevery24h Hydrogencrossover Frequency:attheendofeachcycleoratleastevery24h

Target:<20mAcm−2

Polarizationcurve Frequency:attheendofeachcycleoratleastevery24h High-frequencyresistance Frequency:attheendofeachcycleoratleastevery24h

Table9

Currentdensityvs.timeforthedrivecycleprofile[45](reproducedwithpermission fromBorupetal.[45].Copyright{2007}AmericanChemicalSociety).

Step Duration(s) Cxx Step Duration(s) Cxx

1 15 OCV 9 20 C75 2 25 C80 10 15 C88 3 20 C75 11 35 C80 4 15 C88 12 20 C60 5 24 C80 13 35 C65 6 20 C75 14 8 C88 7 15 C88 15 35 C75 8 25 C80 16 40 C88

Fig.1.Loadprofileforthestackdrivecyclestresstest[45](reproducedwith per-missionfromBorupetal.[45].Copyright{2007}AmericanChemicalSociety).

tionaryapplications.Mostlong-termlifetimetestsforstationary applicationsarecarriedoutata givensetpointorgroupofset pointswiththevoltageor,morecommonly,thecurrentdensity heldconstantforhundredsorthousandsofhourspertest[45].The jointresearchcouncil(JRC)hasusedfourdifferentsub-tests (nor-malefficiencytest,thermalloadcycling,electricalloadcycling,and start-up/shutdown)insequenceforthestationaryfuelcellsystem testmodule[106]asopposedtothesinglecelltestmoduleTM PEFCSC5-4[107],inwhichtheyutilizedon/offcyclingtoaccelerate ageing.

5. Concludingremarks

The US DOEhydrogen programshave established durability targetsforPEMFCsintransportationandstationaryapplications. These targets are based on the characteristics of current and future competitive technologies. AST/durability protocols have been developed by the DOE and the FreedomCARFCTT at the componentlevelinanattempttogainameasureofcomponent durabilityandperformanceagainstDOEtechnicaltargets.These protocolshavenotbeenconclusivelycorrelatedtoactualservice, andarenotintendedtobecomprehensivebecausemanyissues criticaltoatransportationfuelcell(e.g.,freeze/thawcycles)are notaddressedduetothedesign-specificnatureofoperating pro-cedures.Asaresult,thedrivecycleprotocolwasconstructedby convertingthefederalinternalcombustionenginedrivecycleto anequivalentPEMFCenginedrivecycle,whichstillrequires

cor-relationwithdatafromstacksandsystemsoperatingunderactual drivecycles.Therefore,additionalteststocorrelatetheseresults withrealworldlifetimesmaybeneeded,includingactualdriving, start/stop,andfreeze/thawcycles.

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