A review on performance degradation of proton exchange membrane fuel cells during startup and shutdown processes : causes, consequences, and mitigation strategies

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Journal of Power Sources, 205, pp. 10-23, 2012-01-20

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A review on performance degradation of proton exchange membrane

fuel cells during startup and shutdown processes : causes,

consequences, and mitigation strategies

Yu, Yi; Li, Hui; Wang, Haijiang; Yuan, Xiao-Zi; Wang, Guangjin; Pan, Mu

https://publications-cnrc.canada.ca/fra/droits

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JournalofPowerSources205 (2012) 10–23

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

Review

A

review

on

performance

degradation

of

proton

exchange

membrane

fuel

cells

during

startup

and

shutdown

processes:

Causes,

consequences,

and

mitigation

strategies

Yi

Yu

a,b

_,

_Hui

_Li

a,∗

,

Haijiang

Wang

a,∗∗

,

Xiao-Zi

Yuan

a

,

Guangjin

Wang

b

,

Mu

Pan

b

a_Institute_for_Fuel_Cell_Innovation,_National_Research_Council_Canada,₄₂₅₀_Wesbrook_Mall,_Vancouver,_BC,_Canada_V6T_1W5

b_State_Key_Laboratory_of_Advanced_Technology_for_Materials_Synthesis_and_Processing,_Wuhan_University_of_Technology,_Wuhan_430070,_PR_China

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received5November2011

Receivedinrevisedform4January2012 Accepted5January2012

Available online 20 January 2012

Keywords:

Protonexchangemembranefuelcell Startupandshutdownprocess Hydrogen/airinterface Reversecurrent Systemstrategies

a

b

s

t

r

a

c

t

Performancedegradationduringstartupandshutdownisconsideredanimportantissueaffectingthe durabilityandlifetimeofprotonexchangemembranefuelcells(PEMFCs).Duetothehighpotentials experiencedbythecathodeduringstartupandshutdown,theconventionalcarbonsupportforthe cath-odecatalystispronetooxidationbyreactingwithoxygenorwater.Thispaperpresentsanoverviewof thecausesandconsequencesofperformancedegradationafterfrequentstartup–shutdowncycles. Miti-gationstrategiesarealsosummarized,includingtheuseofnovelcatalystsupportsandtheapplicationof systemstrategiestopreventperformancedegradationinPEMFCs.Itisfoundfromtheliteraturereview thatimprovementsincatalystsupportstopreventoxidationcomeattheexpenseofhighcost,andthe novelsupportsdevelopedtodatearenotsufficienttocompletelypreventcarbonoxidationinfuelcell engines.Systemstrategies,includingpotentialcontrolandreactiongascontrol,havebeendeveloped andappliedinfuelcellenginestoalleviateorevenavoidperformancedecay.Thisreviewaimsto pro-videaclearunderstandingofthemechanismsrelatedtodegradationbehaviorsduringthestartupand shutdownprocesses,therebyhelpingfuelcellmaterialorsystemdevelopersintheireffortstoprevent performancedegradationandprolongthelifetimeofPEMFCs.

Contents

1. Introduction... 11

2. Startupandshutdownprocesses... 11

2.1. Startupprocess... 11

2.2. Shutdownprocess... 12

3. DegradationofPEMFCduringstartupandshutdown... 12

3.1. Acceleratedlifetimetestsunderstartup–shutdowncycles... 12

3.2. RootcauseofPEMFCdegradationduringstartupandshutdown... 14

3.2.1. Reversecurrent... 14

3.2.2. Fuelstarvation... 16

3.2.3. Carbonoxidation... 16

3.2.4. Agglomerationand/ordissolutionofPtparticles... 17

4. Mitigationstrategies... 18

4.1. Alternativecatalystsupports... 18

4.2. Systemstrategies... 19

4.2.1. Gaspurge... 19

4.2.2. Auxiliaryloadwithpotentialcontrol... 19

4.2.3. Othersystemstrategies... 20

4.2.4. Summaryofsystemstrategies... 21

∗_{Corresponding}_author._Tel.:₊₁₆₀₄₂₂₁_3000. ∗∗_{Corresponding}_author._Tel.:₊₁₆₀₄₂₂₁_3038.

E-mailaddresses:hui.li@nrc.gc.ca(H.Li),haijiang.wang@nrc.gc.ca(H.Wang).

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5. Concludingremarks... 21 Acknowledgements... 22 References... 22

1. Introduction

Afuelcellisanelectrochemicaldevicethatcandirectly con-verthydrogenenergytoelectricity.Amongthevarioustypesoffuel cell,theprotonexchangemembranefuelcell(PEMFC)isconsidered oneofthemostpromisingcleanenergysourcesofthetwenty-first centuryfortransportationandstationaryapplications,duetoits highenergyconversionefficiencyandpowerdensity,faststartup, andlow/zeroemissionlevel[1,2].Considerableresearchoverthe pastfewdecadeshassignificantlyadvanced PEMFCtechnology. However,sometechnological“bottlenecks”havelimitedits fur-thercommercialization.Forexample,therelativelyshortlifetime ofPEMFCs,inducedbymaterialsdegradation,isstillunsatisfactory forstationaryandautomotiveapplications.TheU.S.Departmentof Energy(DOE)lifetimetargetsfor2015are5000hfor transporta-tionpowersystemsand40,000hforstationarypowersystems[3], butcurrent PEMFC technologyyieldsonly1700hand 10,000h, respectively[4].Insufficientfuelcellsystemdurabilityiscaused bydegradationofthefuelcellcomponents.Thedurabilityofeach componentinaPEMFCisaffectedbymanyinternalandexternal factors,includingmaterialproperties,fuelcelloperatingconditions (suchashumidification,temperature,cellvoltage,etc.),impurities orcontaminantsinthefeeds,environmentalconditions(e.g., sub-freezingorcoldstart),operationmodes(suchasstartup,shutdown, potentialcycling,etc.),andthedesignofthecomponentsandthe stack.

For automobile applications, PEMFCs must operate under variousconditions,suchasloadchangingcycles,highpower con-ditions,idlingconditions,andstartupandshutdowncycles.Among thesevariousdynamicconditions,startupandshutdownprocesses presentauniquechallengeforPEMFCsystems,astheycausethe cathodepotentialtobecomeabnormallyhigh,atwhichpointthe catalystsupportispronetobeoxidized,resultinginadverseeffects forfuelcelldurability.Peiet al.investigatedthedurabilityof a PEMFCandevaluateditslifetimeunderstartupandshutdown con-ditions[5].Theresultssuggestedthatperformancedecayunder frequentstartupandshutdowncyclesisveryserious,buttheeffect ofstartupandshutdowncyclesonfuelcelllifetimecanbeignored ifthestackvoltageispromptlydispelledafterthefuelcellstops operating.However,quicklyandcompletelydispellingthestack voltage is not easy, due to residual gas in the flow field after shutdown.

Therecentliteraturecontains severalreviewpapers onPEM fuel cell durability/reliability issues. Wu et al. [6] published a comprehensivereview onPEMFC degradation mechanisms and mitigationstrategies,inwhichdurabilitytestsundersteadystate

[7–17]and dynamic state [18–30]conditions were also briefly summarized.Inaddition,Wuet al.[6]alsodiscussedthemajor failuremodesand mitigationstrategies ofdifferentcomponents inPEMFCs.Borupetal.[2]publishedanimportantreviewpaper contributedby56researchersfromnationallaboratoriesand uni-versitiesintheUnitedStatesandJapanwhoparticipatedinaPEMFC DurabilityWorkshopfundedbytheUnitedStatesDepartmentof Energy(DOE).Thisreviewprovidedcomprehensivediscussionson fundamentaland scientificaspectsof PEMFCdurability,suchas operationaleffectsonfuelcelldurability,butitsmainfocuswas onthedegradationofcomponents,includingthemembrane, cata-lystlayer,andgasdiffusionlayer.Vahidietal.[31]intheirreview introducedthemain parametersinfluencing thelong-term per-formanceanddurabilityofPEMFCs.Zhangetal.[32]provideda

reviewofPt-basedcatalystlayerdegradationinPEMFCs, includ-ingaverydetaileddiscussionofthecarboncorrosionmechanisms under gross fuel starvation and the air/fuel boundary; mitiga-tionstrategieswerealsointroduced,amongthemcarbonsupport improvementandsystemstrategies.

Allthesereviewshave touchedonperformancedegradation and carbon oxidation during thestartup and shutdown cycles, butthereisnocomprehensivereviewofPEMFCdurabilitystudies andmitigationstrategiesunderstartupandshutdownconditions. Over thelast fewyears, in anefforttoenhance thelifetimeof PEMFCsystemsforautomobileapplicationsundergoingfrequent startup and shutdown, significant progress has been made in thedevelopmentofnovelcatalystsupports.Inaddition,various systemstrategiesfortacklingstartupandshutdownissueshave beendevelopedandreportedthroughaconsiderablenumberof papers and patents.There isthus a need for a detailed review ofdegradationmechanismsandallknownmitigationstrategies, includingmaterialsimprovementandsystemstrategiesforstartup andshutdownprocesses,tohelpfuelcellmaterialdevelopersor fuelcellsystemdevelopersintheireffortstopreventperformance degradationandprolongthelifetimeofPEMFCsforautomotive applications.

Thepurposeofthisreviewisthereforetosummarizethestudies conductedbyacademicandindustrialresearchersonthe durabil-ityofPEMFCsduringstartupandshutdown.First,adescriptionof startupandshutdownprocessesisprovidedforaclear understand-ingofwhathappenstofuelcellsduringtheseprocesses.Second, accelerated lifetimetests under startup–shutdown cycle condi-tions,conductedbybothacademicandindustrialresearchers,are summarizedanddiscussed.Inaddition,themajorfailuremodes androotcausesofdegradationduringstartupandshutdown pro-cessesarediscussedindetail.Thereviewconcludeswithadetailed introductionofrecentlydevelopedmitigationstrategies,including materialsimprovementandsystemstrategies,basedonan exhaus-tivesurveyofjournalpapersandpatents.

2. Startupandshutdownprocesses

Bothstartupandshutdownaredynamicprocessesthatafuel cellinevitablymustconfrontinautomobileapplications.Compared tosteady-stateprocesses,startupandshutdownprocesses experi-encedifferentprofilesunderoperatingconditions.Forexample,the celltemperature,gashumidity,andlocalgasmixturearedifferent thanundersteady-stateconditions—forexample,increasing tem-peratureandhumidityduringstartup,anddecreasingtemperature andhumidityduringshutdown.However,themajorfeature dur-ingstartupandshutdown,afeaturethatisalsothemajorcause ofperformance degradationduringthose processes,isthelocal gasmixtureattheanode,whichiscommonlycalledthe hydro-gen/airinterfaceorthefuel/airinterface.Toclearlyunderstandthe acceleratedlifetimetestsanddegradationmechanismsassociated withstartupandshutdownprocesses,abriefintroductiontothese processesisgiven belowandaschematicgraphis presentedin

Fig.2.

2.1. Startupprocess

Thestartupprocessforthefuelcellsreferredtointhispaper excludescold start belowfreezingtemperatures. In thenormal operationoffuelcellengines,airfillstheanodeflowfieldafter

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extendedshutdown,duetopermeationbyatmosphericairfrom theanodeexhaustoracrossthemembrane.Thefirststepofthe startupprocessistointroduceairoroxygenintothecathodeand hydrogenintotheanode. Becauseof thepresenceof air inthe anode,thereisahydrogen/airinterfaceintheanodeflowfield dur-inghydrogenintroduction.Ashydrogenissuppliedcontinuously duringthestartupprocess,theairisdispelledfromtheanodeinlet totheanodeoutlet,resultinginafloatinghydrogen/airinterface. Thefasterthehydrogenissupplied,themorebrieflyisthis inter-facepresent.Eventually,alltheairintheanodeisdispelledbythe hydrogen,andtheair/hydrogeninterfacedisappears,leadingtothe stateoftheopencircuitvoltage(OCV)forfuelcells.Inalaboratory test,topreventfuelcellsfromformingthishydrogen/airinterface, theanodeandcathodeflowfieldsarepurgedwithnitrogento pro-tectthefuelcellsbeforethestartupprocess.However,inthereal operationoffuelcellengines,anitrogensupplyisnotpractical, resultinginanair/hydrogeninterfaceduringstartup.

Inaddition,ifthefuelcellexperiencesbadgasflow distribu-tionduringthestartupprocess(calledlocalfuelstarvation),oxygen couldcrossthemembranefromthecathodetotheanodeunder apressureorconcentrationgradient,resultinginahydrogen/air interface.

2.2. Shutdownprocess

Thesamesituationofahydrogen/airinterfacecanalsooccurin theshutdownprocess.Whentheprimaryloadisshutoff,thefuel cell’sshutdownprocessbegins.Afterthehydrogenand air sup-pliesareshutoff,residualgaswillremaininthegaschannelsat theanodeandcathode.Becauseofthegasconcentrationdifference betweentheanodeandcathode,theoxygenatthecathodecrosses themembranetotheanode,resultinginahydrogen/airinterface attheanode.Moreover,afterextendedshutdown,atmosphericair willpermeatethefuelcellfromtheanodeoutletduetoseal fail-ure.Theair/hydrogeninterfacewillremainforamuchlongertime thanwhenitisintroduceddirectlyviathehydrogensupplyinthe startupprocess,duetoslowairpermeationduringshutdown.

Toinvestigatetheeffectthatthehydrogen/airinterfacehason theperformanceofPEMfuelcellsduringthestartupandshutdown processes,manyresearchershavefocusedonacceleratedlifetime testsunderstartup–shutdowncycleconditionsinthelaboratory. 3. DegradationofPEMFCduringstartupandshutdown 3.1. Acceleratedlifetimetestsunderstartup–shutdowncycles

Inrecentyears,significantresearcheffortshavebeenfocused ondegradationbehaviorsunderstartupandshutdownconditions, asindicated by thenumber of publications shown in Fig.1. In thesepublications, mostofthedurability testswereconducted underacceleratedconditions,duetothecomplexityanddifficulty ofmimickingandcontrollingstartupandshutdownprocedures.To conductdurabilitytestsforstartupandshutdowncycles,the hydro-gen/airinterfacemustbeintroducedattheanode,whichdemands highstandardsoftestconditioncontrolsandsafetycontrolsinthe laboratory.Asaresult,durabilitytestsunderstartupandshutdown conditions arecostly,as therequisiteexperiments are lengthy. Acceleratedtestsarethereforecommonlyused.United Technolo-gies Corporation (UTC) and a group at the Fuel Cell Research Center,KoreaInstituteofScienceandTechnology,haveconducted a considerableamountworkonacceleratedteststoinvestigate degradationmechanismsanddevelopsystemstrategiesforstartup andshutdownprocesses.

AsshowninTable1,Choetal.fromtheFuelCellResearchCenter attheKoreaInstituteofScienceandTechnologyhaveinvestigated

Fig.1.Publicationhistoryofarticlesafter2004onthestartupandshutdown pro-cessesofPEMFCs.

several operating parameters [33–39] that can have an impact onthedegradationrateofPEMFCsunderstartupandshutdown cycles,includingcathodehumidity[37,38],celltemperature[33], applicationofa dummyload[35,36],andgassupplysequences

[34]. The hydrogen/air interface during the startup and shut-downprocesseswasmimickedbypurgingtheanodeandcathode channelswithair aftercontinuousair and hydrogenwere sup-plied.Duringthispurgingprocess,thetestconditions—including celltemperature,gashumidity,andgassupplysequences—were changed toinvestigate theresulting effects onthedegradation behaviors of fuel cells. The results indicated that performance degradationwasalleviatedatlowerhumidityandlowercell tem-perature with the dummy load during startup and shutdown. Similarly,Leeetal.[39]alsoinvestigatedtheeffectofthe hydro-gen/air interface onfuelcell performance degradation. In their study,thehydrogen/airinterfacewascreatedbyreplacing hydro-genwithairattheanode,toinvestigatecarboncorrosionatthe cathode.Itwasconcludedthattopreventthedegradationof PEM-FCscausedbyresidualgases,hydrogenshouldberemovedfrom theanodegaschannelbyairpurging,whichwasfoundtobevery effective.

Takagi et al. [40] investigated the effect that the shutoff sequence ofhydrogenandair hadonthefuelcellperformance degradation rate.Thestartup and shutdownprocess was oper-atedasfollows:(1)hydrogenandairweresuppliedtotheanode andcathode,respectively,andthecellwasoperatedfor1–1.5min underaconstantloadcurrent.(2)Theloadcurrentwasturnedoff tomaintaintheOCVstate,andsimultaneouslyeithertheairorthe hydrogensupplywasshutoffatthatpoint.(3)Whenthecellvoltage droppedto0.2V,theothergassupplywasshutoff.(4)Afterleaving thecellshutdownfor5minwhilemaintainingthecelltemperature atthesamelevelashadbeenusedduringoperation,both hydro-genandairweresuppliedsimultaneouslyandtheloadcurrentwas appliedagaintooperatethecell.Itwasconcludedthatinthe inter-estofsystemsafetyandfuelefficiency,itwouldbemorebeneficial toshutoffthehydrogensupplybeforetheairsupply.Nevertheless, topreventperformancedegradationduringtheshutoffprocess,the airsupplyshouldbeclosedpriortothehydrogensupplyto pre-ventairpermeationtotheanode,whichwouldotherwiseresultin ahydrogen/airinterfaceattheanodeduringtheshutdownprocess. Inukaietal.[41]simulatedstartup/shutdowncyclesbyexchanging hydrogenandairattheanode.Duringthisgasexchange,the dis-tributionofoxygenpartialpressuresattheanodewasvisualized

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Fig.2.Descriptionofthestartup(A)andshutdown(B)processes. usingareal-time/spacevisualizationsystem,whichclearlyshowed

thelocationofH2-richandO2-richareasalongthegas-flowchannel

fromtheinlettotheoutlet.Theyobservedthatthegasexchange ratewasmuch slowerthanwhatwouldbepredictedfrom sim-plereplacement,andthatit correlatedwiththeprotontransfer derivedfromcarboncorrosionofthecathodecatalystlayer.From

thesevisualizationresultstheyfoundthattheshutdownprocess resultedinmoreseriouseffectsthanthestartupprocess.

Inadditiontothosestudyingthestartupandshutdown pro-cessesbyreplacinghydrogen/airorexchanginggasestointroduce thehydrogen/air interfaceat the anode,other research groups havestudiedtheprocesses usinga reformattedgassuppliedto Table1

SummaryofPEMFCdurabilitytestsunderstartupandshutdowncycles.

Authors Experimentalfactors Numberofcycles Degradationrate Reference Qietal. Air/fuelboundary 80 5mVdroppercycleat400mAcm−2 _[42]

Darlingetal. Localizedfuelstarvation 100h Severedamagetothecatalystlayer [44] Takagietal. Shutoffsequenceofhydrogenandairwithout

adummyload

40 Outputpowerdeclinedby17%at1.0Acm−2 _[40]

Peietal. Start-stopcyclingwithnitrogenpurge 80h Cellvoltagedecayedby0.00196%at100Apercycle with280cm2_active_area_PEMFC_stack [5]

Owejanetal. GraphitizedcarboninMPL 25h 63%lossincurrentdensityat0.6Vwithoutgraphitized carbon,25%improvementinvoltagedegradationat 1.2Acm−2

[30] Limetal. Shutdownprocess 200 0.31mVdroppercycleat80◦_C_and₄₀₀_mA_cm−2 _[35]

Sakamotoetal. 50–90␮Vdroppercycle [22]

Choetal. Airpurgingeffect 50 4.2mVdroppercyclewithair/hydrogenpurge 2.0mVpercyclewithair/airpurge

[39] Applyingthedummyload 1200 Cellvoltagedecayedby0.030%withdummyloadbut

by0.068%withoutdummyload

[36] Cathodeinletrelativehumidity(RH) 1500 0.186mAcm−2_drop_per_cycle_for_0%RH

0.240mAcm−2_drop_per_cycle_for_50%RH

0.266mAcm−2_drop_per_cycle_for_100%_RH

[37,38] Supplysequenceofhydrogenandair 1200 0.68mAcm−2_drop_per_cycle_with_concurrent_air_and

hydrogensupply;0.47mAcm−2_per_cycle_with

hydrogensuppliedpriortoair

[34] Fuelcelltemperature 1200 0.13mVdroppercycleat40◦_C

0.24mVdroppercycleat65◦_C

0.31mVdroppercycleat80◦_C_at₄₀₀_mA_cm−2

[33] Ettingshausenetal. Start/stopcycling 500 About0.66mVdroppercycleat1000mAcm−2 _[45]

Yuetal. Cathodeexhaustcondition 1500 0.024mVdroppercycleforclosedcell;0.093mVdrop percycleforopen-endedcellat1000mAcm−2 [43]

Inukaietal. Gasexchangecycling 500 About0.9mVdroppercycleat200mAcm−2 _[41]

Knightsetal. RelativehumidityonRudissolutionand

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theanode,orusinganopencathodeexhaustend.Insteadofpure hydrogenfortheanodegassupply,Qietal.[42]usedareformate gascomposedof10ppmCO,49%H2,and17%CO2,balancedby

N2.A2%airbleedwasusedattheanodesidetomimica

hydro-gen/airinterface.Theresultsindicatedadegradationrateofabout 5mVpercycleat400mAcm−2_after₈₀_cycles._However,₁₀_ppm

COand17%CO2inthereformattedgasalsoaugmentedthePEMFC

degradationrate,whichshouldnotbeignored.Yietal.[43] stud-iedtheeffectofcathodeexhaustconditionsonthedegradation behaviorsofPEMFCsduringtheshutoffprocess.Thehydrogen/air interfacewassimulatedbyusingtheopenexhaustatthecathodeto leadtoatmosphericairpermeation.Theyconcludedthattheclosed cathodeexhaustwasbeneficialforPEMFCdurability.

Theacceleratedtestingresultsdiscussedabovenotonly demon-stratetheadverseeffectsthatstartupandshutdownprocessescan haveonthedurabilityofaPEMFC,butalsoprovideinformationon howtoalleviatetheseadverseeffectsbychangingoperating condi-tions,suchashumidity,temperature,etc.However,thecondition changesmentionedinthosepapersarenotpracticalforrealfuelcell operation.Forexample,theperformancedegradationcausedby startupandshutdowncouldbemitigatedwithlowertemperatures andlowergashumidityduringthestartupandshutdowncycles. However,underrealconditionsitwouldbedifficulttocontrolthe temperatureandhumiditybeforestartinguporshuttingdownthe system.Furthermore,itwouldtakealongtimetolowerthe tem-peratureandhumidityfromtheirworkingpointstotheirshutdown points.Anotherimpracticaltacticwouldbeshuttingofftheair sup-plypriortothehydrogensupply;althoughthis wouldmitigate theadverseeffectsofthestartupandshutdownprocess,itisnot

advisableforeithersystemsafetyorfuelefficiency.Inaddition,as showninTable1,althoughmosttestsconductedunderoptimized operatingconditionsandfrequentcyclesdemonstratedsomelevel ofimprovementindegradationrates,theseimprovementswere stillnotenoughtomeettheDOElifetimetargets.

Hence,morepracticalandeffectiveproceduresshouldbe devel-opedtomitigatetheperformancedegradationcausedbystartup andshutdowncycles.InSection4.2,weexploreindepthafew sys-temstrategiesputforwardbyUTCthatcouldbepracticallyapplied inrealfuelcellsystemsandresultinmuchbetterdurability. 3.2. RootcauseofPEMFCdegradationduringstartupand shutdown

Asdiscussedearlier,thehydrogen/airinterfaceformedduring thestartupandshutdownprocessesistherootcauseofthenegative effectsthattheseprocesseshaveonPEMFCdurability.Inthispart wewilldiscusstheunderlyingdegradationmechanismscausedby thehydrogen/airinterfaceinaPEMFC.

3.2.1. Reversecurrent

ThereversecurrentmechanismwasfirstproposedbyUTCin 2005[47].AsdiscussedinSection2,theypointedoutthata hydro-gen/airinterfaceintheanodewascreatedduringboththestartup andshutdownprocesses.Whenthishappens,asshowninFig.3, partoftheanodeisfilledwithhydrogenandtherestisfilledwith air,leading toa regionwhereoxygenreductionoccursonboth thecathodeandanodesides,resultinginahighpotential differ-ence(about 1.44V) atthe cathode.This highcathodepotential

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Fig.5.Intensitypotentialcurvesfortwoelectrodesofanelectrochemicalcell. ReprintedfromRef.[48]withpermission.

temporarilyreversesthecurrentwhereairispresentintheanode, causingseverecarbonoxidation(alsoshowninFig.3).They pro-posedusinghighcathodepotentialand“reversedcurrent”,based onthesignificantdecreaseinthecatalystlayerthattheyobserved inasingle-cellexperiment.However,theydidnotdirectly mea-surethevalueofthehighcathodepotential;instead,theyuseda dualcellconfigurationtoestimatethecathodevoltagetobearound 1.44V.AsshowninFig.4,intheirdualcellconfiguration,thetwo singlecellswereconnectedbyconductivewires,whichwould cer-tainlyleadtosomevoltagelossattheinterfaceofthetwosingle cells.However,undertherealconditionofasinglecell,Cell1and Cell2inFig.4wouldbeconnectedbyanelectrolyteandan elec-trode,sotheresistancebetweenthetwocellsshouldbenegligible. Therefore,theaccuracyoftheirestimated1.44Vcathodepotential isdebatable.

Basedonthesamepremiseof“reversedcurrent”andhigh cath-odepotential,othergroupsalsoconductedsimilaranalysesand tests.Yousfi-Steiner[48]explained theobserveddegradationat thecathodebyheterogeneitiesincurrentdistribution,withlocal currentclosetozerointheareafedwithnitrogenordiluted oxy-gen.AccordingtoYousfi-Steiner’sassumption,sincethetwocellsin thisconfigurationareconnectedtoneitheranelectronicloadnor apotentiostat,andsincetheyareonlyexternallyconnected,the setuppresentedinFig.4mostlikelyrepresentsagenerator(Cell1) connectedinseriestoanelectrolysiscell(Cell2),ratherthantwo singlefuelcellsconnectedinparallel[49].Analyzingthecurrent flowandcalculatingthevoltageinthetwodevicesyieldsthe con-clusionthatthepositiveelectrodeofCell2canreachhighenough potentialsforfastcarbonandPtcorrosiontooccur,asshowninthe givenformulasandFig.5.

Similarly,Owenjanetal.[30]haveconsideredthePEMFCunder thestartupand shutdowncyclesastwo shortedcells. The sec-tionofthecellwhereairwaspresentinboththeanodeandthe cathodewasconsideredanair/aircell,andthesectionwherethe cathodewasfilledwithairandtheanodewasfilledwithhydrogen wasconsideredanormalPEMFC.Theirassumptionwassimilarto Yousfi-Steiner’s,buttheydidnotpredictormeasurethevalueof thecathodicpotentialduringthestartupandshutdownprocesses. Reversecurrenthasbecomeacommonlyacceptedmechanism toexplainthedegradationinducedbythestartupandshutdown processes.Asaresult,manyresearcheffortshavebeendirected tomeasuringorpredictingthehighpotentialatthecathode.The followingparagraphssummarizetheexperimentalmeasurements andmodelpredictionsofthecathodepotentialduringstartupand shutdown.Duetotheinstantaneousnatureofthehydrogen/air interfaceat the anode,the occurrence of high potentialat the cathodeisalsoinstantaneous,whichpresentsgreatchallengesfor

correctlymeasuringthispotential.Inrecentpapers,several meth-odshavebeenreportedtotestthecathodepotential:

•Dualcellconfiguration:Twosinglecellswereconnected exter-nallybyelectricwires.Theanodeandthecathodeofthefirst singlecellweresuppliedwithhydrogenandair,respectively,as inanormalPEMFC.Intheothercell,airfilledtheanodeand cath-odetosimulateanelectrolysiscelloradrivencell[42,47,48].The cathodepotentialofthesecondcellwasthemaximumcathode potentialofaPEMFCexposedtoreversecurrent.Asdiscussed above,intherealconditionofasinglecell,Cells1and2wouldbe connectedbyanelectrolyteandanelectrode,notexternalwires, whichproducedavoltagelossattheinterfaceofthetwosingle cells.Amuchsmallervoltagelosswouldoccurinarealsituation. Consequently,someerrorsprobablyresultedfromtheexternal connection.

•Referenceelectrodemethod:Sinceitisnotpossibleto distin-guishbetweentheanodeandcathodehalf-cellreactionswith asegmentedfuelcellassembly,thereferenceelectrodemethod hasbeendevelopedandemployedbyseveralresearchgroups

[50–55].Measurementofthecathodepotentialwasachievedby introducingaconstantpotentialintothetestcell,utilizinga refer-enceelectrode,andthentheanodeandcathodepotentialscould bedeterminedrelativetothisconstantpotential.Inthismethod, asmallstripofmembraneintheMEAwassoakedinsulfuricacid withamercurysulfatereferenceelectrode[50]orahydrogen ref-erenceelectrode[56].Boththeanodesideandthecathodeside oftheMEAwereconsideredtheworkingelectrode.Thecathode potentialcouldbemeasuredasthepotentialdifferencebetween theworkingelectrodeandthereferenceelectrode.However,it shouldbepointedoutthatthereferenceelectrodemethodalso hasitsdrawbacks.Becausethereferenceelectrodeisplacedata specialpointonthemembrane,thepotentialmeasuredatthis particularpointmaynotcorrectlyreflect therealpotentialof thewholemembrane.Therefore,itwouldbemoreaccurateto developatestdevicethatcanevaluatethepotentialofthewhole membrane,ratherthanthepotentialofa specialpointonthe membrane.

•Modelprediction:Toovercomethedisadvantagesinthe experi-mentalmeasurementsmentionedabove,someresearchershave developedmathematicalmodelstoobtaintheelectrolyte poten-tialprofileforthisphenomenon[47,57,58].Theresultscanassist researchersinclearlyunderstandingdegradationmechanisms.

Table 2 summarizes the methods used to measure cathode potentialduringstartupandshutdown,asreportedinrecent pub-lications. Theresearchers atPlug Powermeasuredthe valueof thecathodicpotentialwithreformatehydrogenasthefuel[42]. Byusingadualcellconfiguration,acathodepotentialashighas 1.75Vwasmeasured,whichwasdifferentfromthecalculatedvalue reportedbyReiseretal.[47].Baumgartneretal.[50]obtaineda cathodeelectrodepotentialofupto1.5Vwithfourreference elec-trodesunderhydrogenstarvationconditions,whichalsoresultedin aH2/airboundaryintheanode.Shenetal.[56]measuredthe

cath-ode,anode,andmembranepotentialsversusahydrogenreference electrodeusingaspecialelectrodedesign,asshowninFig.6;the resultsindicatedthattheinterfacialpotentialdifferencebetween thecathodeandthemembranewasashighasabout1.6V.Allthese experimentsprovethathighpotentialdoesexistatthecathodeof aPEMFCduringthestartupandshutdownprocesses.

However,Sidik etal. [57] proposeda different viewonthe maximumpotentialaPEMFCcathodecouldexperienceduetothe formation of a hydrogen/air interface atthe anode. They clari-fiedthatthemaximumpotentialwasabouttwicethepotential observedwhenoneconnectedadrivingfuelcelltoadrivencell, basedontheButler–Volmerequationandtheexperimentaldata

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Table2

Summaryofthecathodepotentialmeasurementsorpredictionsforthestartupandshutdownprocesses,asreportedinrecentpublications.

Authors Testmethods Cathodepotentialvalue Reference

Qietal. Dualcellconfiguration 1.75V [42]

Reiseretal. Modelprediction 1.44V [47]

Baumgartneretal. Fourreferenceelectrodes 1.5V [50]

Shenetal. Hydrogenreferenceelectrode 1.6V [56]

Taketal. Dynamichydrogenelectrode(DHE) 1.4V [59]

Sidiketal. Theoreticanalysis TwicetheOCVofthedrivingcell [57]

Ohsetal. Modelingstudy >1.2V [58]

fromPEMFCs(InFig.3,thedrivingcellisCell1andthedrivencell isCell 2.).Becauseofsomesimplificationsemployedin Reiser’s model, the predicted cathode potentialwas 1.44V, which was lowerthantwicetheOCV.

Itcanbeconcludedfromthemeasuredorpredictedcathode potentialsdiscussedabovethat,inspiteofthedifferentmethods, themaximumpotentialvalueatthecathodeismuchhigherthan theOCV.Atsuchahighpotential,thecarbonsupportofaPt-based catalyst is proneto oxidation, resulting in PEMFC performance degradation.

Inadditiontomeasuringormathematicallypredictingthehigh cathodepotentialexperiencedduringthestartupandshutdown processes,researchershavealsoconductedotherrelatedtests,such asmeasuringthecurrentdistributioninsegmentedcells[60–62], monitoringthe concentrationof CO or CO2 during startup and

shutdown[59,63],evaluatingtheeffectofcelldesign(flowfield structure andGDLthickness)onstart–stopphenomena [64,65], andconducting modelingstudies[58,66–73]onwater manage-mentandcarbon-corrosionduringstartupandshutdown.Allthe testresultshaveconfirmedperformancedegradationduringthe startupandshutdownprocessesofPEMFCs.

3.2.2. Fuelstarvation

Localizedfuelstarvationcanhappenduringnormaloperation duetopoordistributionofreactantsortowaterflooding.However, duringunprotectedfuelcellstartupandshutdown,ahydrogen/air boundaryforms,whichinsomecasesleadstoasituationsimilarto fuelstarvation.Thissectiondiscussesinageneralwaythe conse-quencesthatfuelstarvationcanhaveonfuelcellperformanceand durability.Butithastobepointedoutthatfuelstarvationisnot limitedtostartupandshutdown.

InthenormaloperationofPEMFCs,allthereactantsare suffi-cientlysuppliedtotheanodeandthecathode,resultingineven distributionattheelectrode surface.However,ifheterogeneous

Fig.6. SchematicsoftestMEAwithreferencehydrogenelectrodeandthincopper wiressandwichedbetweentwomembrane.

ReprintedfromRef.[56]withpermission.

fueldistributionoccurs,airoroxygenatthecathodewillcrossthe membranetotheanode,duetothepressuredifferencebetween theanodeandcathode. In2008,Ofstad[74]reportedcreatinga transienthydrogen/air interfacein theanodelayer.InJingwei’s modeling study[73],the resultsindicated thatthe rateof car-boncorrosionatthecathodesideofthefuelstarvationregionwas stronglydependentonoxygendiffusionthroughthemembrane. Theconcentrationgradient acrossthemembrane isthedriving forceforoxygentopermeatefromthecathodetotheanode, accord-ingtoFick’sfirstlaw.Ifwesupposeafuelcellsysteminasteady state,theconcentrationofoxygenremainsconstantatallsurfaces ofthemembrane[75].Underthisassumption,theone-dimensional diffusionequationfromFick’sfirstlawisexpressedas[76]: JO2 =−DO2 dCO2 dz =DO2 Ccathode_O 2 −C anode O2 z (1)

whereJO2istheoxygenpermeationrate,DO2istheoxygendiffusion

coefficient,C_Ocathode

2 andC

anode

O2 aretherespectiveoxygen

concen-trationsonthesurfaceofthemembraneatthecathodeandanode, andzisthemembranethickness.Asaresult,ahydrogen/air inter-faceisproducedattheanode.Accordingtothe“reverse-current mechanism”[47],thisinterfaceisnotbeneficialtothedurabilityof thecarbonsupportinPEMFCs.

Someresearchershaveinvestigatedthecurrentdistributionof PEMFCsduringlocalfuelstarvation[50,77–83],andYousfi-Steiner et al.[48] providedan excellentreview ofthecausesand con-sequences of starvation issues. So in this review, we will turn fromperformancedegradationduetolocalfuelstarvationarising fromahydrogen/airinterface,andfocusinSection4on materi-alsimprovementandsystemstrategiestomitigatecatalystdecay duringthestartupandshutdownprocesses.

3.2.3. Carbonoxidation

Catalystdegradationatthecathodeisconsideredamajorfailure mode for PEMFCs when the catalysts are exposed to reverse-currentconditionsduringstartupandshutdown.

In thecatalyst layersof PEMFCs,platinum nanoparticlesare always dispersed on supports to increase the active area and therebylowermaterialcosts[84–86].AsconcludedinRef.[87], theidealcatalystsupportshouldhavethefollowingproperties: •Reasonablyhighelectricalandthermalconductivity.

•Goodcorrosionresistanceintheelectrolyte.

•Dimensionalandmechanicalstabilitywithreasonablestrength. •Highsurfacearea.

•Availabilityatlowcostandwithreasonablyhighpurity. •Easydispersionofsmallcatalystparticles.

•Easyfabricationintoelectrodes. •Absenceofadversecorrosionproducts.

Becauseofitsgoodelectronicconductivityandlowcost, car-boniswidelyusedasafuelcellcatalystsupport.Althoughcarbon hasmostoftheaboveproperties,itissusceptibletooxidationor

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Fig.7.Quantitativeanalysisofcathodeoutletgaseswithchangeincathode poten-tial.

corrosion at high temperature and high oxygenconcentration, yieldingCO2orCO,asinthefollowingequations[42,63]:

C+2H2O →CO2+4H++4e− (2)

C+H₂O→ CO+2H++2e− (3)

Shaoetal. [88]investigatedthethermodynamicpotentialof carbonoxidationintoCO2.Theirresultsindicatedthatoncethe

potentialwashigherthan+0.207Vvs.RHE,thecarbonwouldbe oxidizedtoCO2,accordingtoreaction(2).Carboncanalsobe

oxi-dizedtoCOat0.518Vunderstandardconditions[63,89,90]. Innormalfuelcelloperation,thecarbonoxidationrateisslow duetotheslowkineticsofreactions(2)and(3)atPEMFC operat-ingtemperatures.However,carbonoxidationoccursmuchfaster underdynamicprocessessuchasstartupandshutdown,because thecathodepotential is much higher than thethermodynamic potentialofcarbonoxidation.Ithasbeenreportedthatalthoughthe kineticsofthecarbonoxidationreactionissluggishatlow poten-tialandlowtemperature,thepresenceofPtparticleswillaccelerate carbonoxidation[90].Kimetal.[59]directlymeasuredand ana-lyzedexhaustgasusingFT-IRduringstart/stopoperation.Asshown inFig.7,theamountofCO2evolvedwasproportionaltothecathode

potentialatvaluesabove1.0V.Inaddition,atpotentialshigherthan 1.2V,COandSO2weregenerated,havingdetrimentaleffectson

fuelcellperformance.Similarly,S.MassmeasuredtheCO2andCO

concentrationsincathodeexhaustusingnon-dispersiveinfrared spectroscopy(NDIR)[63].Moredirectly,Inukaietal.usedscanning transmissionelectron microscopy (STEM) imagestoinvestigate catalyst-layerdegradation at thecathode during500 start/stop cycles.Theyobservedseveralholesatthecatalystlayer,withno

Fig.8.STEMimagesneartheinlet,center,andoutletofanMEAatthecathodeafter degradation.

carbonsupport,indicatingadrasticcorrosionofthecarbonsupport

[41](Fig.8).

Nano-particlessuchasPtparticlesinthecatalystlayerhavethe inherenttendencytoagglomerateintobiggerparticlestoreduce theirhighsurfaceenergy[91,92].Carbonoxidationorcorrosion weakenstheattachmentofPtparticlestothecarbonsurfaceand eventuallyleadstostructuralcollapseanddetachmentfromthe carbonsupport,resultingintheagglomerationand/ordissolution ofPtparticlesintotheelectrolytewithoutredeposition.Moreover, Ptmaybetransportedthroughtheelectrolyteand/orthroughthe ionomerafterbeingdetached,causingareductioninelectrolyte conductivity[93].Therefore,carbonoxidationpromotedbyhigh cathodepotentialduringthestartupandshutdownprocessesis themaincauseofcatalystdegradationinthecathodeofthefuel cell[43].

3.2.4. Agglomerationand/ordissolutionofPtparticles

ThedegradationmechanismsforPtcatalystsincludethe follow-ing[32,48]:

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•Ptparticleagglomerationandparticlegrowth. •Ptlossandredistribution.

•Poisonouseffectsarousedbycontaminants.

Once oxidation of the catalyst support happens during the startupandshutdownprocesses,thedegradationof Ptparticles followsthefirsttwomechanisms.However,thereisno consen-susonthedominanceofthesetwomechanisms.Someresearchers haveinvestigatedthedegradationprocessofaPtcatalystduring extendedoperationunderOCV[92,93].Theresultsindicatedthat Ptparticlesdissolvedintheionomerandthengrewtolarger par-ticles.OtherresearcherssuggestedthatPtparticleswoulddetach fromtheoxidizedcarbonsupportanddissolveintheelectrolyte

[94,95].

Cross-sectionalscanningelectronmicroscopy(SEM)and trans-mission electron microscopy (TEM) have often been used to characterizePt/Ccatalystssubjectedtofrequentstartupand shut-downcycles.Yireported[43]thatthethicknessesofthecatalyst layers in theanode and cathode of fresh MEAs were9.81 and 9.60␮m,whichdecreasedto7.02and2.47␮m,respectively,after 1500frequentstartupandshutdowncyclesforanopen-endedfuel cell(seeFig.9).Ettingshausenetal.[45]observedthegrowthofPt particlesbyusingTEMandhighresolutionTEMtoanalyzeaPt/C catalystthathadexperiencedstart/stopcyclesandhighcathode potential.Afterthestart/stopcycles,therelativenumberof parti-cleswithadiameter≤2.5nmdeclinedfrom57.20%inafreshMEA to1.92%.Choetal.combinedtheresultsofcross-sectionalSEMand TEMtoevaluatePt/Cdegradation[33,34,36,38,39,45,59].

Apparently,asupportmaterialthatismorestablethanthe cur-rentcarbonsupportforPEMFCcatalystsisdesirabletoalleviatethe degradationcausedbythestartupandshutdownprocesses.Butif thecurrentcatalyst-supporttechnologyhastobeemployed,there isanurgentneedtoapplyeffectivesystemstrategiesforstartup andshutdowntomitigateoxidationofthecarbonsupportunder highcathodepotential.

4. Mitigationstrategies

Many journal publications and patents have focused onthe development of strategies to mitigate the performance decay causedbystartupandshutdown.Thestrategiescanbeclassified intotwomajorcategories:

•Materialimprovementformorestablecatalystsupports. •Systemmitigationstrategiesforconventionalcarbon-black

sup-ports.

4.1. Alternativecatalystsupports

Replacing conventional carbon supports with corrosion-resistantmaterialshasbeenone important mitigationstrategy. Yuetal.[66]reportedthatusinggraphitizedcarbonasasupport yieldeda5timeslowerdegradationratethanaconventionalcarbon supportafter1000startup/shutdowncycles.Owenjanetal.[30]

alsoreporteda25%reductioninthestartup/shutdowndegradation rateat1.2Acm−2_with_the_{implementation}_of_a_graphitized_carbon

inthemicroporouslayers(MPL).Theyexplainedthattheuseof graphitizedcarbonresultedinareducedmasstransportlimitation of the gasdiffusion layer,in comparison withthe mass trans-portlimitationenhancedbyconventionalcarbonoxidationatthe MPL/electrodeinterface.Inadditiontographitizedcarbon,other carbonmaterials,suchascarbonnanotubesorcarbonnanofibers

[96–99],andcarbonaerogelandxerogel[100–102],havealsobeen consideredascatalystsupportsbecauseoftheirmorestable elec-trochemicalbehaviors[32].However,thehighcostofsynthesizing

Fig.9. Cross-sectionalSEM images:(A)freshMEA withoutstartup–shutdown cycles;(B)MEAinopen-endedcellafter1500cycles;(C)MEAinclosedcellafter 1500cycles.

thesecarbonmaterialsincreasedtheburdenonthecommercial developmentofPEMFCs.Inadditiontocarbon-typesupports,many non-carbonsupports[63,103–108],suchassubstoichiometric tita-niumoxide[107],tungstencarbide [104],andindium tinoxide

[105], have been investigated toreplace the conventional car-bonsupportsandachievegreaterstabilityduringlong-termtests.

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Ioroietal.[107]testedasinglecellusinga Pt/Ti4O7 cathodeat

300mAcm−2 _for _more _than₃₅₀_h_with_fully _humidified_H 2/O2.

Stablevoltagewasobservedthroughoutoperation,demonstrating thatPt/Ti4O7wasapossibleoxidation-resistantcatalystmaterial

foraPEMFCcathode.However,noreportshavebeenpublishedon durabilitytestsofPEMFCsunderstartupandshutdowncyclesusing non-carbonsupports.

Asaresult,systemstrategiesseemmorepracticalatthepresent time,whenconventionalcarbonisbeingusedasacatalystsupport. 4.2. Systemstrategies

Asdiscussedinprevioussections, thehydrogen/airinterface causingreversed currentand highcathodepotentialistheroot causeforPEMFCdegradationduringstartupandshutdown.Allthe reportedorpatentedsystemstrategiesweredevelopedtoprevent ahydrogen/airinterfaceattheanodeandeliminatehighpotential atthecathodeduringstartupandshutdown.

UTC,GeneralMotorsCorporation(GM),andotherautomotive companiessuchasFord,Toyota,Nissan,andDaimlerChryslerhave beenresearchingsystemstrategiesforthestartupandshutdown processes.Table 3lists importantpatents onsystemstrategies, including:

•Gaspurgetoanodebeforestartupandaftershutdown.

•Auxiliaryloadappliedtoconsumeresidualoxygenatthecathode withpotentialcontrol.

•Exhaustgasrecycleaspurginggasorreactiongas. •Electronicshorttoeliminatehighpotentialatthecathode.

4.2.1. Gaspurge

Gaspurgingisaneffectivewaytopreventahydrogen/air inter-faceattheanode.Itcanalsominimizethetimethataninterface exists.

Fortheshutdownprocessoffuelcells,Margiott[115]andReiser

[116]atUTChaveproposedthefollowingprocedurewithair purg-ing:supplyingtheairthroughtheblowerintotheanodetodispel theresidualhydrogenattheanode,afterdisconnectingtheprimary loadandstoppingthehydrogensupply.Reiser’slong-term durabil-itytest[120]revealedanaveragevoltagelossof0.2Vwithoutair purgingafterabout250startupandshutdowncycles;however, theperformancedegradationwasnotsosevere,withonlyabout 0.055Vvoltagelossafternearly500cycleswithairpurging(see

Table4).YuandWagneratGeneralMotorsCompany[128–130]

reportedthat air purgingduringthe shutdownprocess wasan effectivemethodtopreventperformancedegradationinfuelcells. However,theyproposedthatitwasmoreeffectivetoemploythe airpurgeuntilthetemperatureofthestackwasreducedbelowa predeterminedtemperature.

Withregardtothestartupprocess,Balliet[119]andReiser[120]

inventedasafetystrategythatconsistedofastartupsystemand methodforafuelcellpowerplantusingapurgingofthecathode flowfieldwithahydrogen-rich,reducingfluidfueltominimize corrosionofthecathodeelectrode.Theirstrategyincludedthe fol-lowingsteps:(1)purgingthecathodeflowfieldwiththereducing fluidfuel;(2)directingthereducingfluidfueltoflowthroughan anodeflowfield;(3)terminatingtheflowofthefluidfuelthrough thecathodeflowfieldanddirectinganoxygen-containingoxidant toflowthroughthecathodeflowfield;and(4)connectingaprimary loadtothefuelcell.Inaddition,GeneralMotorsalsopatentedtheir inventionofastartupprocesswithgaspurging,whichincluded introducinghydrogengasintotheanodeandthecathodeto con-sume/purgeoxygeninboth[127].

Itshouldbenotedthatinimplementingtheabovementioned systemstrategies,theblowersandotherdevicesusedtomovethe gasesthroughthesystemshouldbeproperlyselectedtoachieve

thedesiredspeedforgasdisplacementtooccurinlessthanabout 1.0s,andpreferablylessthan0.2s,tominimizethetimethatthe hydrogen/airinterfaceexists.

Nitrogenisasafeandinertgasthathasbeenwidelyusedasa purgativeforfuelcells,butitisnoteasilymadeavailableinreal fuelcelloperations.Tohavenitrogenasapurginggasforstartup andshutdown,FordMotorCompanyhasinventedapurgingsystem withaseparatorthatremovesoxygenfromtheexhaustgasatthe cathode.Afterseveralcyclesofcathodeexhaustgas,areformate gaswithahighnitrogenconcentrationandlowoxygen concen-tration canbe usedas thepurge gas [139]. Thampan invented astrategy thatusedamembrane-basedhumidifiertoprovidea N2-rich exhaustasthepurginggas[151].Themembrane-based

humidifierwasusedtotransfermoisturefromamoisture-laden exhauststreamtothedryairfeed,andwasalsousedinashutdown modeinwhichthemembrane-basedhumidifierwasimplemented topermeatemoistureandoxygenfromthemoisture-ladenexhaust stream,therebyprovidingnitrogen-richgas.Thesamemethodwas alsopatentedbyScotto[154,155].

4.2.2. Auxiliaryloadwithpotentialcontrol

Gaspurgingcanpreventa hydrogen/air interfaceduringthe startupandshutdownprocesses.However,itcannotcompletely dispelresidualgaspresentintheflowfieldoffuelcells,suchasin thegasdiffusionlayerorthecatalystlayer.Anothereffectiveway todispelresidualgasistointroduceanauxiliaryload(alsocalled adummyload).Kimetal.[36]haveinvestigatedhowapplyinga dummyloadaffectsfuelcelldegradation.Theirresultsindicated thatapplicationofadummyloadduringthestartupprocedure sig-nificantlyreducedperformancedecayandlossofelectrochemically activesurfacearea,andincreasedcharge-transferresistance,which resultedinadramaticimprovementindurability.InCondit’s[112]

andBalliet’s[113]invention,adummyloadwasappliedto pre-ventcellreversalbyconsumingtheairatthecathodeduringthe shutdownprocess.Yu[129]appliedanauxiliaryloadtoconsume theoxygenatthecathode,leavingnitrogenandhydrogeninthe cathodeandanode,respectively,duringtheshutdownprocess.

Anauxiliaryloadhasalwaysbeenusedinthecompanyofan anodeexhaustrecycleloop.Yangproposedastartupprocedurefor afuelcellsystemhavingananodeexhaustrecycleloop[121].First, theauxiliaryloadwasconnectedacrossthecelltoreducethecell voltage.Thenthelimitedhydrogenfuelfromtheanodeexhaust, througharecycleloop,wasprovidedtotheanode.Theaddedfuel reactedwiththeoxygenthatremainedintherecirculationgases untilvirtuallynooxygenremainedwithintherecycleloop.Finally, fuelandaircouldbesuppliedatthenormaloperatingflowrates intotheanodeandcathode,respectively,aftertheauxiliaryload wasshutoff.Fig.10showstheapplicationofadummyloadduring fuelintroductionatstartup[87].Thelosseswithoutadummyload weresevere,andtheperformancedecreasedby∼100␮Vcycle−1. Withadummyloadtocontrolthecellvoltage,performancedecay wasreducedtoapproximately4␮Vcycle−1.Yangetal.patented asimilarstrategythat appliedanauxiliaryloadwiththeanode recycleloop,andcouldbeusedintheshutdownprocesstoreduce thecellpotential[110,121].

Chanetal.atHyundaiMotor[149]havedesignedanapparatus foreffectivelypreventingcarboncorrosionfromoccurringatthe cathodeofafuelcell.Inthisstrategy,tocompletelyexhaustthe residualoxygenatthecathodeafterthefuelcellwasshutdown,a pressuresensorandanairdischargesolenoidvalvewereinstalled intheairdischargepipetodetecttheairpressureinthefuelcell andcontroltheairsupply.Underthefunctionofthepressure sen-sorandthesolenoidvalve,astheresidualoxygenatthecathode wascompletelyconsumed,thesolenoidvalvereceivedtheclosing signalandthedummyloadwasdisconnectedtoavoidanegative voltageinthefuelcell.

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Table3

Summaryofpatentsonsystemstrategiesforstartupandshutdownprocesses.

Inventors Year Systemstrategy Reference

UTC 2000 Makingthefuelcellsysteminertbycoolantfloodingduringshutdown [109] 2002 Shuttingdownthefuelcellsystemusingananodeexhaustrecycleloop [110]

2003 [111]

2003 Shuttingdownfuelcellswithanauxiliaryloadtoconsumetheoxygen [112] 2004 Shuttingdownfuelcellswithanauxiliaryloadtoconsumetheoxygen [113]

2004 Fuelpurgeofcascadedfuelcellstacks [114]

2004 Shuttingdownfuelcellswithanairpurgeintotheanode [115]

2005 [116]

2005 Cascadedanodeinletmanifolddesign [117]

2005 ReducingcathodepotentialfortheMEAwithanelectronicshort [118] 2005 Startingupfuelcellswithfuelpurgeintothecathode [119,120] 2006 Startingupafuelcellsystemusingananodeexhaustrecycleloop [121] 2006 Usingahydrogenreservoirtoreceiveandstorehydrogenduringfuelcelloperation,andto

releasehydrogenwheneverthefuelcellisshutdown

[122]

2011 Preventingairintrusionintohydrogenduringshutdown [123]

GMMotorsCorporation 2005 Shuttingdownandstartingupwithastoichiometricstagedcombustor [124]

2005 Shutdownandstartupwithacathoderecycleloop [125,126]

2006 Shutdownandstartupwithahydrogenpurge [127]

2006 Shuttingdownthefuelcellsystemusinganairpurgeinlowtemperature [128] 2008 Shuttingdownwithanauxiliaryloadtomakenitrogenandhydrogeninthecathodeandanode [129]

2008 Shuttingdownthefuelcellsystemusinganairpurge [130]

2008 Hydrogenpurgeintothecathodeforoperatingafuelcellstack [131] 2008 Specialelectrodedesignforreducingelectrodedegradation [132] 2009 Specialelectrodedesigncontainingoxygenevolutionreactioncatalysts [133] 2011 Fuelcelloperatingmethodsforoxygendepletionatshutdown [134]

PlugPower 2007 Gaspurgeforstartupandshutdown [135]

BallardPowerSystems 2005 Newcatalystdesigntoimprovevoltagereversaltolerance [136,137] 2006

2007 Recirculatingtheoxidantwithananodepurgepath [138]

FordMotorCompany 2009 Purgingsystemwithaseparatorthatremovesoxygenfromtheexhaustgasatthecathode [139] NissanMotor 2009 Fuelcellsystemwithvoltagesensorandaccurategas-supplycontrol [140–142]

2006 2005

DaimlerChrysler 2011 Aselectivelyconductingcomponentisincorporatedinelectricalserieswiththeanode componentsinthefuelcell

[143,144] 2009

HondaMotor 2010 Afuelcellsystemthatincludesanoxidantgassupplyapparatusandafuelgassupplier [145] ToyotaMotor 2009 Therestrictionontheoutputofthefuelcellislifted,andtheoutputofthefuelcellis

controlledaccordingtotherequestedoutput

[146–148] 2008

HyundaiMotor 2008 Apparatusforpreventingcarboncorrosionatthecathodeinthefuelcell [149] Anodesidehydrogen/oxygeninterfaceformationinhibitionstructure [150] Others 2010 Membrane-basedhumidifiertoprovideaN2-richexhaustforpurginggas [151]

2011 Shuttingdownwithcathodegasrecycle [152]

2010 Detectingtheanodegasamount [153]

2011 Generatingagasthatmaybeusedforfuelcellstartupandshutdown [154,155] 2008 Startupandshutdownwithanelectricalshortingdeviceforindividualcellshorting [156] 2006 Electricallyconnectingoutputterminalscoupledtothefuelcellwithabatteryaftershutting

downtoconsumetheoxygenandfuelgas [157]

2005 Startingupwithafuelpurge [158]

2010 Usingchemicalshortingduringstartupandshutdown [159]

2008 Detectingairpressuretopreventahydrogen/airinterface [160]

2002 Safetyprocessanddevicewiththeanodegascycle [161]

2007 Shuttingdownwithstoringfuelintheanodeandcathodechambers [162]

2009 Systemandmethodforstartingupandshuttingdown [163]

Anotherwayofapplyinganauxiliaryloadistouseanelectronic shortofthefuelcell.Bekkedahl[118]inventedaspeciallydesigned fuelcellstackwithapermanentshunt,adiode,andastripof con-ductivecarbonclothorblack.Theremovableshuntcouldberotated intoandoutofcontactwiththefuelcellanodesandcathodesto make aninternal auxiliaryload.Ramani [159]and Miller [156]

alsoappliedchemicalshortingduringthestartupandshutdown processes.

4.2.3. Othersystemstrategies

Inadditiontogaspurgingandauxiliaryloadstrategies,other systemstrategieshavebeendevelopedtoenhancethedurability

ofPEMFCsduringstartupandshutdown.Theyincludenovel cata-lystdesigntoimprovethecatalyst’stolerancetovoltagereversal

[136],unique electrode design to reduceelectrode degradation under startup and shutdown [132], use of an electrode that contains oxygenevolution reactioncatalyststopreventcurrent reversal[133], and applicationof a cascadefuel inlet manifold

[114,117] to achieve better hydrogen distribution in the flow filed.

SomeJapanese companies,suchasToyotaMotor [146–148], Seiko Instruments [164], Nissan Motor [140–142], Fuji Electric

[165],andDaihatsuMotor[166],haveproposedtheirownPEMFC systems with voltage sensors and accurate gas-supply control

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Table4

Summaryofsystemstrategies,withdurabilitytestresultsforstartupandshutdownprocesses.

Inventors Systemstrategies Durabilitytestresult Reference Reiseretal. Uncontrolledstartupandshutdown Averagevoltageloss:0.195Vafter250cycles [116,120]

StartupandshutdownwithH2purgeintothe

cathode,withanauxiliaryload

Averagevoltageloss:0.055Vafter300cycles N2purgeduringthestartupandshutdown

cycles

Averagevoltageloss:0.04Vafter1550cycles Conditetal. ShutdownwithauxiliaryloadandH2recycle

loop;startupwithauxiliaryloadandN2purge

Averagevoltagedecreasedfrom0.760Vto0.695Vin576cyclesat

400mAcm−2 [112]

ShutdownwithauxiliaryloadandH2recycle

loop;startupwithH2andN2storedatthe

anodeandcathode

Averagevoltagerecoveredfrom0.695Vto0.755Vafter2315cyclesat 400mAcm−2

Bekkedahletal. Reducingfuelcellcathodepotentialwith electronicshort

Cellvoltagedecayedmuchlessafter230–256startupandshutdown

cyclesat108mAcm−2_and₃₂₅_mA_cm−2 [118]

Yuetal. Shuttingdownfuelcellsystembyusingair purgeatlowcelltemperature(after40cycles)

30◦_C _Voltage_loss:_0.0142_V_at₂₀₀_mA_cm−2 _[128,130] Voltageloss:0.0322Vat800mAcm−2 50◦_C Voltageloss:0.0327Vat200mAcm−2 Voltageloss:0.1486Vat800mAcm−2 80◦_C _Voltage_loss:_0.1937_V_at₂₀₀_mA_cm−2 Voltageloss:0.4092Vat800mAcm−2

Yuetal. ShutdownandstartupwithH2purge Cellvoltagedecreasedfrom0.79Vto0.78Vafter200cycles [127]

Tangetal. H2purgeforstartupandshutdown PerformancewithH2-purgeprotectionisbetterthanwithN2-purge

protectionafterstartupandshutdowncycles

[135] Thampanetal. Membrane-basedhumidifiertoprovidea

N2-richexhaustforpurginggas

NoN2purge Cellvoltagedecreasedfrom0.65Vto

0.47Vat600mAcm−2 [151]

WithN2purge Cellvoltagedecreasedfrom0.65Vto

0.55Vat600mAcm−2

Ramanietal. ChemicallyshortedMEAwithH2andairbleed

purge

Averagevoltagedecreasedfrom0.69Vto0.50Vat600mAcm−2_after

350startupandshutdowncycles

[159] Paiketal. Purgesystemwithaseparatortoremove

oxygenfromtheexhaustgasatthecathode

Air/H2cyclingatanode Voltagedecreasedfrom0.78Vto

0.25Vat600mAcm−2_after₅₀₀_cycles [139]

N2purgeatanode Voltagedecreasedfrom0.39Vto

0.25Vat600mAcm−2_after₆₃₉_cycles

5%O2innitrogenpurgeatanode Voltagedecreasedfrom0.6Vto0.44V

at1000mAcm−2_after₅₀₀_cycles

10%O2innitrogenpurgeatanode Voltagedecreasedfrom0.7Vto0.4Vat

500mAcm−2_after₄₀_cycles

systemstoachieveadesirabledurabilityduringthestartupand shutdownprocesses.

4.2.4. Summaryofsystemstrategies

As shown in Table 4, recent patents have reported several durabilitytestsconductedwithdifferentsystemstrategiesunder startupandshutdowncycles.Theperformancedecaywasgreatly mitigated by various strategies, among which the most effec-tivewasthecombinationofhydrogenpurging,applicationofan

auxiliaryload,anduseofavoltagecontroldevicetoprevent volt-agereversal.AllthesystemstrategieslistedinTable4arebasedon thefollowingtwoprinciples:

•Minimizingthetimethatthehydrogen/airinterfaceexists,such asbyusingfuelpurgingduringthestartupandshutdown pro-cesses,andproducingN2-richgaswithaspecialsystemdesign.

•Reducingthepotentialsduringthestartupandshutdown pro-cesses,suchasbyapplying anexternal auxiliaryloadand by creatinganinternalshortinthefuelcells.

Incomparison withmaterialsimprovementbyusing graphi-tized carbon or non-carbon supports, system strategies are relativelysimpleandcheaptoimplementinrealfuelcellengines. Itisurgentlynecessarytosetuprelevantproceduresanddevices tomeetthedurabilityrequirementsofPEMFCs.

5. Concludingremarks

Performancedegradationduringstartupand shutdownisan importantissuethataffectsthedurabilityandlifetimeofPEMFCs. Thisreviewpaperhassurveyedandanalyzedthedurabilitytests, degradationmechanisms,andsystemstrategiesforthestartupand shutdownprocesses.

Thehighpotentialat thecathode, introducedbythe hydro-gen/airinterfaceattheanode,isthemajorcauseofperformance degradationof PEMFCs.A greatdeal ofwork hasbeen doneto developalternativenovelcatalystsupportsandsystemstrategies tomitigatecatalystdegradation.Currently,severalstrategieshave greatlyenhancedthedurabilityofPEMFCsduringthestartupand

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shutdowncycles,suchasapplyingahydrogenpurgeandan auxil-iaryload.

Furtherworkis neededtoapplytheseeffectivestrategies in morefuelcellsystems.Bycombiningsystemstrategieswithnovel catalystsupportsthathavebettercorrosionresistance,the perfor-mancedegradationcausedbythestartupandshutdownprocesses canbeavoidedtoachievelonglifetimesthatcanmeetthedurability requirementsforPEMFCs.

Acknowledgements

YiYuthankstheChinaScholarshipCounciland theNational ResearchCouncilCanada(NRC)forfinancialsupportthroughthe NRC-MOEResearchandPost-doctoralFellowshipProgram. References

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