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Journal of Power Sources, 205, pp. 340-344, 2012-01-10
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Accelerated conditioning for a proton exchange membrane fuel cell
Yuan, Xiao-Zi; Sun, Jian Colin; Wang, Haijiang; Li, Hui
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ContentslistsavailableatSciVerseScienceDirect
Journal
of
Power
Sources
j o ur n a l ho m e p age :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r
Short
communication
Accelerated
conditioning
for
a
proton
exchange
membrane
fuel
cell
Xiao-Zi
Yuan
∗,
Jian
Colin
Sun,
Haijiang
Wang
∗∗,
Hui
Li
InstituteforFuelCellInnovation,NationalResearchCouncilCanada,4250WesbrookMall,Vancouver,BC,CanadaV6T1W5
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received26October2011 Receivedinrevisedform 21December2011 Accepted1January2012 Available online 10 January 2012
Keywords: Conditioning Pre-conditioning Activating Commissioning Break-in
a
b
s
t
r
a
c
t
Aconditioningprocessisusuallyneededforanewlyfabricatedprotonexchangemembrane(PEM)fuel celltobeactivated.Dependingonthemembraneelectrodeassemblies,thisprocesscantakehoursand evendaystocomplete.Toprovideforacceleratedconditioningtechniquesthatcancompletetheprocess inashorttime,thispapercomparesvariousreportedmethodstoconditionaPEMsinglecell.Themajor objectivesaretoidentifyacceleratedconditioningapproachesthatcansignificantlyreducethe condi-tioningdurationfortheexistingconditioningregimeinanoperationallyeasymanner,andtounderstand thefundamentalprinciplesthatgovernacceleratedconditioning.Variouseffectsinvestigatedinclude temperature,cyclingsteps,andcyclingfrequencies.Othertechniques,suchasshortcircuiting,hydrogen pumping,andhotwatercirculation,arealsodiscussed.Foreachtechnique,measurementsaretakenusing electrochemicalimpedancespectroscopy(EIS),cyclicvoltammetry(CV),andlinearsweepvoltammetry (LSV).
Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction
A conditioningprocess is usually needed for a newly fabri-catedpolymerelectrolytemembrane(PEM)fuelcelltobeactivated
[1].Throughthisprocess,alsocalledbreak-in,commissioning,or incubation,thenewcellcanreachitsbestperformance.Typically, duringthisbreak-inperiodthecellperformanceincreases gradu-ally,thenreachesaplateauwithoutfurtherincrease.Depending onthemembraneelectrodeassemblies(MEAs),thisprocesscan takehoursand even daystocomplete.Anidealscenariois not onlytohavethehighestpossiblepowerdensityafterthe break-inprocedure,butalsotominimizetheactivationcompletiontime
[2,3].
Variousapproaches havebeeninvestigatedtomaximizefuel cellperformanceandshortentheelectrodeactivationtime.On-line techniquesexploredincludecurrentcontrol[5,6],potential con-trol[7,8],temperaturecontrol[9–11],hydrogenpumping[4,12], COstripping[13],andairbraking[14,15].Off-linemethodsinclude electrochemicalconditioning ofthe MEA[16], and steamingor boilingtheelectrode[17].TheUS FuelCellCouncil(USFCC)has evenestablishedcellbreak-inprotocolstostandardizetheprocess
[18].However,nostandardmeasurementhasbeenestablishedto determinetheeffectivenessofabreak-inorconditioning proce-dure.Murthyetal.[3]recommendedmonitoringafuelcell’soutput
∗Correspondingauthor.Tel.:+16042213000x5576;fax:+16042213001. ∗∗Correspondingauthor.Tel.:+16042213038;fax:+16042213001.
E-mailaddresses:xiao-zi.yuan@nrc.gc.ca(X.-Z.Yuan),haijiang.wang@nrc.gc.ca
(H.Wang).
currentdensityat0.6Vandrecordingitasafunctionoftimeduring theapplicationofagivenconditioningprocedure.
Comparedwithfuelcelldurabilitystudies,researchonfuelcell conditioningisrelatively limited.In mostcases,proceduresare givenandresultsarepresentedwithoutdiggingfurtherintothe mechanisms.Asaresult,thereportscontainmorehypothesesthan facts.Mostmechanismsproposedarehypotheticalbecausethey lackdirectexperimentalsupportorconcreteexperimental verifica-tion.Asystematicinvestigationofconditioninganditsmechanisms isstill required.Tothis end,thepresent paperaimstoidentify acceleratedconditioningapproachesthatcansignificantlyreduce the conditioning duration for an existing conditioning regime, witheasyoperationalconditions,and tounderstandthe funda-mental principlesthat governaccelerated conditioning.Various effectsinvestigatedincludetemperature,cyclingsteps,andcycling frequencies.Othertechniques,suchasshortcircuiting,hydrogen pumping,andhotwatercirculation,arealsodiscussed.
2. Experimental
2.1. Acceleratedconditioning
ThePEMfuelcellhadanactiveareaof50cm2.Allthe
experi-mentswerecarriedoutusingGore57catalystcoatedmembranes (CCMs).Toensureinstanttemperaturecontrolinthecell,allof thegasesandwatercouldbeoperatedtobypassthecellthrough three-wayvalves.
Thebaseline datawasobtainedbydrawingthecurrent ata constantvoltageof0.6V.Oncetheinitialsetpointswerereached (70◦C,with100%relativehumidity(RH)onbothsides),thecellwas
0378-7753/$–seefrontmatter.Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2012.01.039
connectedtothesystem.After6hofoperationat0.6V,EIS,CV,and LSVweremeasured. Thencelloperationwascontinuedat0.6V untiltherewasnofurtherperformance improvement,at which pointEIS,CV,andLSVmeasurementsweretakenagain.
Factorsthataffectconditioningwerechosenbasedoneaseof operationandtheeffectivenessofthemethod,andincluded tem-perature, cycling steps, and cycling frequencies. Assessment of temperatureeffectswasperformedat50◦C,70◦C(baseline),and
90◦C,ataconstantvoltageof0.6Vand100%RHonbothsidesfor
6h.Todeterminetheeffectofcyclingsteps,threeexperimentswere carriedout:(1)thecellwascycledbetween0.6Vand0.3V,with eachsetpointheldfor60s,andthecyclewasrepeatedfor6h.(2) Thecellwascycledbetween0.6V,0.3V,andOCV,witheachset pointheldfor60s,andthecyclewasrepeatedfor6h.(3)Thecell wascycledinasequenceofpotentialcyclingsteps,asfollows:
(i)Cellvoltagewasdecreasedfrom0.9Vto0.2Vandmaintained for60sateachdecreaseof0.1V.
(ii)Cellvoltagewasmaintainedat0.1Vfor60s.
(iii) Cellvoltagewasincreasedfrom0.2Vto0.9Vandmaintained for60sateachincreaseof0.1V.
(iv) Theabovethreestepswererepeatedfor6h.
Toinvestigatetheinfluenceofcyclingfrequency,twoothertests wereadded:thecellwascycledbetween0.6Vand0.3V,witheach setpointheldfor(1)5minor(2)20s.Theconditionsofallthese testswerethesameasforthebaseline.Each conditioning pro-cesswasperformedfor6h,andEIS,CV,andLSVweremeasured immediatelyafterward.
Other techniques investigated in this paper included short circuit,hot watercirculation, andhydrogenevolution/hydrogen pumping.Theprocedures forshortcircuitwereas follows.The anodeand thecathodewereconnectedtoshortthecell,witha lowflowratefor1minandahighflowratefor3min.This pro-cesswasrepeatedfor0.5h.Attheendofthisprocess,hydrogen wassuppliedataminimumrateandoxygensupplywasstopped. Whenthecellvoltagewasbelow0.1V,thehydrogensupplywas stopped.Forhotwatercirculation,hotwater(80◦C)wascirculated
forbothanodeandcathodefor0.5h,andthentheelectrochemical performancesweremeasured.Forhydrogenevolution/hydrogen pumping,airatthecathodesidewasreplacedbynitrogen,while theanodesidewasfedwithpurehydrogen.Anexternalpower supplywasusedtogenerateacurrentdensityofca.200mAcm−2
throughthecell,withhydrogenbeingoxidizedattheanode;the protonsweretransportedthroughthemembranetothecathode, wheretheywerereduced.Theprocedurewascontinuedfor30min. 2.2. Electrochemicaltechniques
Toassesstheeffectivenessofactivationandtounderstandthe underlyingprinciples,EIS,CV,andLSVweremeasuredaftereach conditioningprocess.ASolartronSI-1260Impedance/Gain-Phase analyzerandaSolartronSI-1287ElectrochemicalInterfacewere usedforEIStestingatacurrentof36Aoverthefrequencyrangeof 10kHzto0.1Hz.
Prior tothe CV and LSV tests, the air supply was switched tonitrogenand the systemwasrun at OCVwithH2 min flow
(0.5lmin−1) and N
2 minflow (2lmin−1)until steady potential
wasachieved.InsituCVwasperformedonthefuelcellusingthe SolartronSI-1287ElectrochemicalInterfacetocomparetherelative electrochemicalsurfacearea(ECSA).Toavoidpossibledegradation fromcarbonsupportoxidationinducedbyhighpotentials,thecell wascycledbetween0.05and0.8Vonthecathodevs.reversible hydrogenelectrode(RHE)atascanrateof50mVs−1.
Thegaspermeabilityorcrossoverthroughthemembranewas assessed using LSV to measure the limiting oxidation current
10 15 20 25 30 35 40 45 50 30 25 20 15 10 5 0 Time (H) Current at 0.6V (A)
Fig.1. Timedependenceofcurrentat0.6Vforthebaseline.
densitiesofthecrossoverhydrogen.ForLSVmeasurement,allthe operatingconditionsforthefuelcellwerethesameasintheCV test,exceptforalinearpotentialscanfrom0to0.5V(vs.RHE)on thecathodeatascanrateof5mVs−1.
Foreachexperiment,theoutputcurrentdensityofthecellat 0.6Vwasmonitoredandextractedfromthedataasafunctionof time.
3. Resultsanddiscussion 3.1. Baseline
Thebaseline datawasobtainedbydrawingthecurrentat a constantvoltageof0.6Vandat70◦Cfor100h.Fig.1showsthe
timedependenceofcurrentat0.6Vduringbaselineconditioning (notethatthefigurepresentsonlythefirst28h).Ascanbeseen, thecurrentincreaseswithtimeforabout20h,withthegreatest increase occurringatthe verybeginning. After20h, nofurther performanceimprovementisobserved.After6,28,and100hof operationat 0.6V, EIS, CV, and LSV were measured(shown in
Figs.2–4,respectively).ThereisaspikeinFig.1duringconditioning at∼6h,whichwascausedbytheinterruptionofthe condition-ingoperationformeasuringEIS,CV,andLSV.Thecurvesat0time, “fresh”,werealsoaddedtothefiguresforcomparison.These“fresh” valueswereobtainedbymeasuringanewsamplethat hadnot undergoneanyconditioning.Clearly,afterconditioningthe elec-trochemicalactiveareaincreased,andthemembraneresistance (high-frequencyintercept)andpolarizationresistance(the differ-encebetweenthelow-andhigh-frequencyintercepts)decreased. Basedontheimpedancespectra,membraneresistance(Rel),charge
transferresistance(Rct),andmasstransferresistance(Rmt)canbe
extractedusingtheequivalentcircuitdepictedinFig.5,whereCPE representstheconstantphaseelement.Thesimulatedparameters arelistedinTable1.
-0.05 0 0.05 0.1 0.15 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 Z' (Ω cm2) -Z'' ( Ω cm 2 ) Baseline 6H Baseline 28H Baseline 100H Fresh
Table1
ParametersextractedfromimpedancespectrausingtheequivalentcircuitdepictedinFig.5.
Acceleratedconditioningoperation Rel(cm2) Rct(cm2) Rmt(cm2) Acceleratedconditioningoperation Rel(cm2) Rct(cm2) Rmt(cm2)
Fresh 0.17 0.225 0.045 Baseline6H 0.165 0.178 0.045 Baseline28H 0.155 0.148 0.04 Baseline100H 0.152 0.148 0.04 50◦C 0.167 0.212 0.045 0.6–0.3V,20s 0.157 0.235 0.05 70◦C 0.165 0.178 0.045 0.6–0.3V,60s 0.157 0.152 0.03 90◦C 0.16 0.135 0.024 0.6V–0.3V,5min 0.169 0.155 0.032 0.6–0.3V 0.157 0.152 0.03 Shortcircuit 0.152 0.175 0.043
0.6–0.3V–OCV 0.157 0.175 0.038 Hotwatercirculation 0.191 0.268 0.051
Sequentialcycling 0.152 0.16 0.04 Hydrogenpumping 0.17 0.19 0.041
Fig.3. ComparisonofCVcurvesatdifferenttimesduringbaselineconditioning.
Fig.4. ComparisonofLSVcurvesatdifferenttimesduringbaselineconditioning.
Theobservedphenomenacan beexplained bythefollowing possibletheories[2]:
(1)Increaseintheelectrochemicalactiveareaofthecatalyst:this isachievedbytheremovalofimpuritiesintroducedduringthe processofmanufacturingtheMEAandthefuelcellstack,and theactivationofacatalystthatdoesnotparticipateinthe reac-tion.
Fig.5. EquivalentcircuitofaresistorandaVoigt’sstructureinserieswithcapacitors replacedbyCPE.
(2)Membrane“break-in”:onesuchtheoryisthatthemembranes mayincludecatalystresiduethathinderstheirperformance. Another is that the membranes are initially dry, hindering thecellperformanceuntilthemembraneshydrateduringthe incubation period (indicated by thedecrease in membrane resistance).
(3)Decreaseinchargetransferresistance:theinitialperformance ofanewMEAwithNafion-bondedelectrodesusuallyimproves withtime,astheelectrolytecontainedintheelectrodesneeds hydrationtoensurethepassageofhydrogenions.
(4)Decreaseinmasstransferresistance:conditioningcreatesmore transferpassagesforreactantstothecatalyst.
Asseen inFig.4,hydrogencrossoverincreasesslightly over time comparedwiththefreshsample.However,theincreaseis insignificant.
3.2. Effectoftemperature
Thetemperatureeffectisobtainedbycomparingthe perfor-manceafter6hofconditioningat50,70,and90◦C.Fig.6shows
thetimedependenceofcurrentat0.6Vduring6hof condition-ingat differenttemperatures. Ascanbeseen, cellperformance increaseswithincreasingtemperature.Forallthetemperatures, asimilarincreasingtrendinperformanceisobservedwithin6h, butthistimeisapparentlynotenoughforthecellstocompletely reachtheirbestperformance.For example,in thebaselinecase, 20hofoperationisneeded.Nevertheless,thegreatestperformance increaseoccursattheverybeginning(i.e.,1–2h).QiandKaufman
[9]alsofoundthatunderelevatedtemperature,thecurrent den-sityatcertaincellvoltagescouldbedoubledafteractivationand theactivationcouldbecompletedextremelyquickly,withmostof itachievedinthefirstfewminutes.
After6hofoperationat0.6Vatdifferenttemperatures,EIS,CV, andLSVweremeasured.TheresultsofCVandEISarecomparedin
Figs.7and8,respectively.Evidently,afterconditioningatdifferent temperaturestheelectrochemicalactiveareaincreases(relative tothefreshsample,theelectrochemicalactiveareaincreasesby
0 10 20 30 40 50 7 6 5 4 3 2 1 0 Time (H) Current at 0.6V (A) 50°C 0.6V 70°C 0.6V 90°C 0.6V
Fig.6.Comparisonofcurrent–timecurvesat0.6Vconditionedatdifferent temper-atures.
0.5 1 1.5
Fast conditioning operation
Relative active area
90°C 70°C
50°C Fresh
Fig.7.ComparisonofrelativeactiveareaderivedfromtheCVcurvesafter condi-tioningat0.6Vfor6hatdifferenttemperatures.Therelativeactiveareaofthefresh sampleis1.
1.4,1.2,and1.3timesfor50◦C,70◦C,and90◦C,respectively).Due
toionomerhydrationandcatalystactivation,themembraneand polarizationresistancesdecrease,asdemonstratedinFig.8. Fur-ther,accordingtothesimulatedimpedanceparameterslistedin
Table1,Rel,Rct,andRmtalldecreasewithincreasingtemperature.
Ithasbeenproposedthattheactivationprocessincreasescatalyst utilizationbyopeningmany“dead”regionsinthecatalystlayer. EvenwhenaprotonconductorsuchasNafionismixedintoa cat-alystlayertomakeitconductprotonsinthreedimensions,many ofthecatalystsitesarenotavailableforreaction,forvarious rea-sons:(1)thereactantscannotreachthecatalystsitesbecausethe latterareblocked,(2)Nafionnearthesecatalystsitescannotbe eas-ilyhydrated,or(3)ionicorelectroniccontinuityisnotestablished withthesesites.Whenafuelcellisoperatedatelevated tempera-tureandpressure,manyofthese“dead”regionsare“opened”and thenbecomeactive[9].
With increasing temperature, hydrogen crossover slightly increasesduetofasterdiffusionofhydrogenathigher tempera-tures. 0.15 0.16 0.17 0.18
a
b
Fast Conditioning Operation
Membrane resistance ( Ω cm 2 ) 90°C Fresh 50°C 70°C 0 0.1 0.2 0.3 0.4 0.5
Fast Conditioning Operation
Polarization resistance Rp ( Ω cm 2) 90°C Fresh 50°C 70°C
Fig.8. Comparisonof(a)high-frequency resistance(b)polarizationresistance derivedfromEIScurvesafterconditioningat0.6Vfor6hatdifferenttemperatures.
0 10 20 30 40 50 60 7 6 5 4 3 2 1 0 Time (H) Current at 0.6V (A) Baseline 0.6V - 0.3V 0.6V - 0.3V - OCV Sequential cycling
Fig.9.Comparisonofcurrent–timecurvesat0.6Vfordifferentcyclingsteps.
3.3. Effectofcyclingsteps
Theeffectofcyclingstepswasevaluatedbycomparingthe per-formanceafter6hof conditioningusing2,3, andmore cycling steps.Fig.9showsthetimedependenceofcurrentat0.6Vduring 6hofconditioningfordifferentcyclingsteps.Ascanbeseen,any potentialcyclingoperationhelpedtoactivatethecell,andcycling between0.6Vand0.3Vachievedthebestperformancewithin6h. Inotherwords,neitherOCVrelaxationnorcyclingwithmoresteps wasmuchhelpinconditioningthecell.Similartoresultsfrom pre-vioustests,thegreatestperformanceincreasedoccurredatthevery beginningofconditioning.
After6hofconditioningunderdifferentcyclingsteps,EIS,CV, andLSVweremeasured.ResultsfromCVarecomparedinFig.10, andresultsfromEISarelistedinTable1.Fig.10showsthatafter conditioningunderdifferentpotentialcyclingsteps,the electro-chemicalactiveareaincreased,especiallyfor sequentialcycling. Comparedwiththeresultsforafreshsample,Rel,Rct,andRmtall
decreasedinthethreecases.Itisgenerallybelievedthatthe mem-branehydrationlevel,thenumberofprotonconductionchannels, andthecatalystlayerporositycontinuedtoincrease duringthe conditioningperiod.AccordingtoFig.10,asthenumberofcycling stepsincreases,moreactiveareacanbeobtained.However,this trenddoesnotexactlymatchthetrendinresistancechangesas observedfromEIS;theseactivationsareeffectivefor improving onefactorbutmaynotbeeffectiveforimprovingotherfactorsthat affectcellperformance.Theultimateperformanceimprovement willbedeterminedbythecombinedeffectsofallfactors.ForLSV, therewasnosignificantchangeinhydrogencrossover.
3.4. Effectofcyclingfrequency
Theeffectofcyclingfrequencywasdeterminedbycomparing theperformanceafter6hofconditioningunderdifferentcycling
1 1.5 2 2.5 4 3 2 1 0
Accelerated conditioning operation
Relative surface area
Cycling steps Cycling frequency Other techniques
Fig.10.ComparisonoftherelativeactiveareaderivedfromtheCVcurvesafter con-ditioningwithdifferentconditioningoperations()Cyclingsteps(1:0.6V–0.3V;2: 0.6V–0.3V–OCV;3:Sequentialcycling).()Cyclingfrequency(1:0.6V–0.3V20s;2: 0.6V–0.3V60s;3:0.6V–0.3V5min).()Othertechniques(1:hotwatercirculation; 2:shortcircuit;3:hydrogenpumping).
0 10 20 30 40 50 60 7 6 5 4 3 2 1 0 Time (H) Current at 0.6V (A) Baseline 0.6V - 0.3V 1min 0.6V - 0.3V 5min 0.6V - 0.3V 20sec
Fig.11.Comparisonofcurrent–timecurvesat0.6Vfordifferentcyclingfrequencies.
frequencies.Fig.11showsthetimedependenceofcurrentat0.6V during6hofconditioningwithdifferentcyclingfrequencies.Ascan beseen,within2h,thehigherthecyclingfrequency,themore effi-cientlythecellwasactivated;atahighfrequencyof20s,thecell reacheditsbestperformancewithin2h,followedbyperformance degradation,whereasforfrequenciesof1minand5min,atleast 6hofactivationprocesswereneeded.Atthismoment,itis diffi-culttogiveanexplanationfortheeffectoffrequency.Asallthe measurementsweretakenafter6hofoperation,weareunableto trackthechangesduringthese6h.Whileothercellsarestillinthe processofconditioning,thecellatafrequencyof20shasstarted todegradewithin6h.Wemayassumethathighfrequencyworks moreeffectivelytoimprovetheelectrochemicalactiveareaofthe catalystorcreatethepathwaysforprotons.Furthermore,itis pre-dictedthatthecellwithpotentialcyclingatahighfrequencywill haveafastdegradationrate.
After6hofoperationat0.6Vunderdifferentcycling frequen-cies,EIS,CV,andLSVweremeasured.TheCVresultsarecompared inFig.10andtheEISresultsarelistedinTable1.Forallcases,after conditioningunderdifferentcyclingfrequencies,the electrochem-icalactiveareaincreased.FromTable1wecanseethatRel,Rct,and
Rmtalldecreased,exceptat20s.Inthiscase,theincreasesinRct
andRmtareattributabletodegradationafter2h.Comparedwiththe
situationatafrequencyof1minalthoughconditioningatalow fre-quencyfor5minyieldedahighECSA,itresultedinhighresistance forthemembraneandpolarization.Thelowercellperformancefor alowfrequencyof5minwasaresultofthecombinedfactors.For LSV,therewasnosignificantchangeinhydrogencrossoverwithin suchashorttimeofoperation.
3.5. Othermethods
Othermethods investigated in this report include short cir-cuit,hotwatercirculation,andhydrogenpumping.Theprinciple ofhydrogenpumpingistoimprovePEMfuelcellperformanceby movinghydrogenfromonesideofthemembranetotheother.Asa resultofthisprocess,electrodecatalystutilizationisincreasedand MEAperformanceisimproved[4].Thisisachievedbyreducingthe overpotentialofoxygenreduction.Thereductioninoverpotential isthoughttobeduetochangesintheporosityandtortuosityofthe catalystlayerswhenH2evolvesfromthem,leadingtoanincrease
inthenumberofreactant–catalyst–electrolyte3-phasesites.Hot watercirculationwaschosen,asitwasbelievedthattheNafionin theMEAcouldachievecompletehydration(includingtheNafion membraneand theNafionintheCL),leadingtoenhancedMEA performance.Hotwatercirculationmightalsohelptoopensome “dead”regionsintheCL[17].
Thesemethodswerecomparedfor0.5hofoperation.Compared withthebaselinefigures,thesemethodscouldactivatethecelltoa certainextentwithin0.5h.Cellperformancewasimprovedinthis order:shortcircuit,hydrogenpumping,andhotwatercirculation.
After0.5hofoperationunderdifferentconditions,EIS,CV,and LSVweremeasured.TheCVresultsarecomparedinFig.10,and theEISresultsarelistedinTable1.Itcanbeseenthatafter condi-tioningunderdifferentcircumstances,theelectrochemicalactive areaincreasesgreatly,especiallyfortheshortcircuitand hydro-genpumpingsituations.Theshortcircuitnotonly achievesthe largestactivearea,butalsoachievesthesmallestmembraneand chargetransferresistances(whilenotcontributingtoincreasesin themasstransferpassages),asseeninTable1.Hydrogen pump-ingcangreatlyreducethechargetransferresistance,butdoesnot contributetoimprovingthemembraneconductivityand increas-ingthemasstransferpaths.Hotwatercirculationachieved(i)the leastactiveareaincreaseand(ii)anincreasein membraneand polarizationresistances,probablyduetofloodingissues.
4. Concludingremarks
Thisreportinvestigatedvariousmethodsandfactorsto condi-tionaPEMfuelcell.Theconditioningtimesandelectrochemical propertiesafterconditioningarecompared.Itisworthnotingthat thesecomparisonsarebasedontheGoreCCM.Dependingonthe typeofMEAcomponents,theactualconditioningtimeand prop-ertiesmayvary.Accordingtoourresults:highertemperatureis more effectivetoobtainhighperformance;OCVrelaxationand morecyclingstepsarenotveryhelpfulinacceleratingtheprocess; highercycling frequencymayreducetheactivationtime, butis soonfollowedbydegradation;andtheshortcircuitmethodseems veryeffectivetoconditionthecellinaveryshorttime.However, applyingallthesestressorstoconditionthecellstronglyaffectsthe microstructuresoftheconditionedMEA,whichinturnwillstrongly affectthelong-termbehaviouranddurabilityofthecell,asMEA nanomaterialdegradationisheavilyhistory-dependent.For exam-ple,althoughshortcircuitscanconditionthecellinaveryshort time,wedonotknowhowthiswillaffectthecell’sdurability with-outalong-termdurabilitytest.Theeffectofconditioningoncell durabilitymaybestudiedinourfuturework.
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