Experimental characterization of interna pdf

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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

Experimental

characterization

of

internal

currents

during

the

start-up

of

a

proton

exchange

membrane

fuel

cell

A. Lamibrac,

G. Maranzana

∗

_,

_O.

_Lottin,

_J.

_Dillet,

_J.

_Mainka,

_S.

_Didierjean,

_A.

_Thomas,

_C.

_Moyne

LEMTA-NancyUniversity-CNRS,2avenuedelaForêtdeHaye,BP160,54504VandoeuvrelesNancyCedex,France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received2May2011

Receivedinrevisedform28June2011 Accepted5July2011

Start-up/shutdownunderinappropriateconditionsareknowntodamageprotonexchangemembrane fuelcells.Severalmeasurementtechniqueswerealreadyusedtocharacterizetheireffectsandthe degra-dationsaremainlyattributedtocarbonoxidation:adecreaseofthecathodeactiveareaiscommonly observedafterstart-up/shut-downcycles.

Inordertoachieveabetterunderstandingoftheconditionsinwhichthisphenomenonappears,a segmentedcellwasdesignedandinternalcurrentswererecordedduringstart-upunderopencircuit conditions.Theresultsofthisworkallowestimatingthecarbonlossesataglobal,aswellasatalocalscale, asafunctionofthehydrogenflowrate.Localelectrochemicalimpedancespectroscopymeasurements carriedoutondamagedMEAshowclearlythatthesegmentsthatremainforthelongesttimeinthe presenceofair(whiletheothersarefedbyhydrogen)arethemostaffectedbycarbonoxidation.

1. Introduction

During the start-up of a PEMFC, the anode compartment is divided momentarily into an upstream part occupied by fresh hydrogenanda downstreampartfilledwiththegaswhichwas usedtoflushtheanode: airornitrogen.Theboundarybetween thedownstreamandtheupstreampartsmovesfromtheinletto theoutletoftheanodecompartmentwithavelocitythatis gov-ernedbythehydrogenflowrate.Start-upsoccurgenerallyunder opencircuitconditionssothatpositiveandnegativeinternal cur-rentsappearbetweentheanodeandthecathode.Theseinternal currentscanbeobservedthankstodualcellconfiguration[1–3]

butalsowithinstrumentedcells[4].Theiroriginisattributedto faradicreactions[1,5],asdepictedinFig.1:hydrogenisoxidizedin theinletpartoftheanodecompartmentwhileinthepresenceof oxygen,waterandcarbonoxidationcantakeplaceatthecathode. Evidenceofcarbonoxidationwasbroughtbymeasuringinreal timetheemissionsofCO2 intheairexitingthecathodeduring start/stop cycles[6–9]. Gu etal. [7] measuredbetween60 and 100ppm(partspermillion)forstopandstart-upsequences, respec-tively,withhydrogenandairflowratessetto140sccm(standard cubiccentimetersperminute)attheanode(1000sccmforairat thecathode).

∗_{Corresponding}_author._Tel.:₊₃₃₃₈₃₅₉₅₅_48;_fax:₊₃₃₃₈₃₅₉₅₅_51. E-mailaddress:[email protected](G.Maranzana).

Theaimsoftheworkpresentedinthispaperaretomeasure andanalyzetheinternalcurrentsoccurringduringPEMFCstart-up sequences,inordertobetterunderstandtheirorigin,theirnature, andtheirconsequencesintermofcathodeoxidation.These inves-tigationsarecarriedoutbymeansofasegmentedcell.Segmented cellsareincreasinglyusedforinsituinvestigationofPEMfuelcell operation[10,11],forinstancetodesignchannels[12]orto opti-mizetheirperformances[13].Althoughsuitableforthestudyof degradations[14,15],theywere rarelyemployed for character-izingstart-up andshutdownsequences[16].Theexperimental resultsareinterpretedbyreferencetoanelectricalequivalent cir-cuitproposedbyMaranzanaetal.[17],whoshowednumerically thatthecapacitivecontributiontonegativecurrentscouldbe sig-nificant,dependingontheoperatingconditions.Asaconsequence, itappearsthatcarbonoxidation,andthustheresultingfuelcell degradationandperformancelosses,dependhighlyonthe start-up/shut-downprotocol.

2. Segmentedcellprototype

The experimental results presented in the following were obtainedwiththetest benchdepictedinFig.2.Thisbenchwas developed inorder tomonitora uniquecellwith20 segments. Gasesaresuppliedthrough6massflowcontrollers,whichallow selectingabroadrangeofflowrates.Allofthemexceptnitrogen arehumidified.Hydrogenandair areintroducedinthecathode andanodecompartmentincounter-flow.Thegaslinesalsoallow 0378-7753/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.

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POWER-14621; No.ofPages8

2 A.Lamibracetal./JournalofPowerSourcesxxx (2011) xxx–xxx

Nomenclature

S fuelcellarea(cm2₎

jseg localcurrentdensity(Acm−2)

t time(s)

R∞ infiniteresistanceordischargeresistance()

QPN₂ passivechargeforastart-upbyinjectionof hydro-geninnitrogen(C)

QPAir passivechargeforastart-upbyinjectionof hydro-geninair(C)

QBN2 balancingchargeforastart-upbyinjectionof

hydro-geninnitrogen(C)

QBAir balancingchargeforastart-upbyinjectionof hydro-geninair(C)

QgreenareaN2 activecharge+positivebalancingforastart-up

byinjectionofhydrogeninnitrogen(C)

QgreenareaAir activecharge+positivebalancingforastart-up byinjectionofhydrogeninair(C)

QAN2 activechargeforastart-upbyinjectionofhydrogen

innitrogen(C)

QAAir activechargeforastart-upbyinjectionofhydrogen inair(C)

U fuelcellvoltage(V) RH relativehumidity

HFR highfrequencyresistance(cm2₎

C capacity(Fcm−2₎

flushing theanode with nitrogen or air, and the cathodewith nitrogen.

Fig.3showspicturesoftheinstrumentedcell.Theanodeflow fieldplateisin goldplatedalumina.Exceptfortheresults pre-sentedinSection3.1,theanodeplateisnotsegmentedanditis alsousedtocontrolthetemperatureinthecellthankstoacooling circuitandathermostatedbath(LaudaRE104-E100).Thecathode compartmentismachinedinapolycarbonateplate(transparent butnon-conductive)andthecurrentcollectionismadeby20 elec-tricallyinsulatedsegments(Fig.3).Thefour15mm,1mmthick goldplatedbrassstripsusedforeachsegmentdelimitfiveparallel airchannels(thedepthandwidthofthe5straightparallel chan-nelsare0.7mm×1mm,atthecathodeandattheanode).Kapton sheets(2.3mm×1mm×80␮m)areusedtoisolateeachstripfrom

theadjacentones.Thefourstripsofeachsegmentareelectrically connectedoutsidethecell,whichamountstoaveragingthe cur-rentoverthemembrane’swidth.Thelocalcurrentsaremeasured thanksto20shuntresistorsof5mandrecordedbyadata process-ingandacquisitionsystem(NationalInstrumentSCXImultiplexer) atafrequencyof200Hz(Fig.4).

The1cm×30cm,notsegmented,JohnsonMattheyMEA con-sists of a Nafion 212 membrane, 0.4mgPtcm−2 electrodes and 190␮mthickgasdiffusionlayers(Toraypaper).150␮mTeflon

gas-ketsareinsertedbetweentheMEAandtheflowfieldplates.The platesareheldtogetherthankstoasetof40boltstightenedtoa torqueofabout2.5Nm.

ALabviewprogrammakesitpossibletocontrolthemassflow andtheoperationof thevalvesasfunctionsof the experimen-taldata:temperature,voltageandlocalcurrents.Localimpedance spectroscopywas performedin galvanostaticmode witha fre-quencyrangeof0.25–800Hzandwithanamplitudeofthe(total) currentoscillationequalto5%ofthesteadystatevalue(with a NationalInstrumentUSB6221loadcontroller).

Fig.1. Reactionsoccurringduringstart-up,theanodecompartmentbeinginitially filledwithair.

3. Resultsanddiscussion

3.1. Comparisonofinternalcurrentsbetweenanodeandcathode duringstart-up

Internalcurrentsmeasuredatoneelectrodeprovideonly par-tialinformation aboutthedirectionof theflow of theprotons. Thoseproducedintheactivepartoftheanodeshouldlogicallyflow throughthemembranetowardthecathode.However,onecannot excludeapriorithepossibilitythatsomeprotonsflowalongthe membranetoreactwithoxygenpresentinthepassivepartofthe fuelcell(Fig.5).

Thisinformationcanbeobtainedbyusingacellsegmentedon bothelectrodesandbycomparingtheinternalcurrentsmeasured onbothsides.TheresultsarepresentedinFig.6.

Thecurvesin Fig.6showonlya smalldiscrepancy between theinternalcurrentsmeasuredattheanodeandatthecathode. Thisisprobablyduetothenon-uniformityofthecontact resis-tancesbetweenthecollectingstripsandthegasdiffusionlayers, whichleadstoaredistributionoftheelectronsbetweensuccessive segments:onecancheckthatthedifferencebetweentheanode andcathodecurrentsisexactlycompensatedfromonesegment toanother.Moreoverthetotalamountofchargeproducedatthe anodeisthesameastheoneconsumedatthecathode(the differ-enceislessthan3%).Consequently,theflowofprotonsalongthe membranecanbeconsideredasnegligible:usingafuelcell seg-mentedatthecathodeonlymakesitpossibletomeasureaccurately theinternalcurrents.

3.2. Equivalentelectricalcircuit

3.2.1. Start-upwiththeanodecompartmentpreviouslyfilled withnitrogen

Theequivalentelectricalcircuitthatcanbeusedtounderstand andtomodeltheoccurrenceof internalcurrentswasproposed byMaranzanaetal.[17](Fig.7).Thepurposeofthismodelisto contributetotheunderstandingofthephenomenagoverningthe internalcurrents.Masstransportisnotaccountedfor.Forthesake ofsimplicity,thefuelcellissegmentedintoonlytwosegmentsin theexampledescribedbelow.

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Fig.2.Gaslinesanddataacquisitionandprocessingsystem.

increaseoftheelectrochemicalpotentialleadstothechargeofthe doublelayercapacityatbothelectrodes.

Thefuelcellbeinginopencircuit,theelectronsproducedinthe activepart(attheanode)flowtowardthepassivepartwhere

neg-Fig.3. Segmentedcellandclose-uponthecurrentcollectionstrips.

ativecurrents(byreferencetothenormalfuelcelloperation)are measured.Intheabsenceofoxygen,noelectrochemicalreaction canoccurinthepassivepart:thesenegativecurrentsarecapacitive currents.Thecorrespondingchargeofthedoublelayercapacities isofsignoppositetothatoccurringintheactivepart.

Thesecondstagecorrespondstoananodecompletelyfilledwith hydrogen(Fig.7b),thatistosaytotheinstantthesecondsegment becomesactivewhenreachedbyhydrogen.Atthistime,the poten-tialoftheelectrodes(ofthecathode)increasesuptoitssteady statevaluewhereasthedouble layercapacities remaincharged negatively(byreferencetotheregularoperation).

Asaconsequence,thefuelcellvoltageexceedstheopencircuit valueforashorttime(Fig.8);simultaneouslythenon-uniformity of thechargeof thecapacitiesover thewholemembrane elec-trodeassembly (MEA)givesrisetocapacitiveinternal currents, untilthecompleteredistributionofcharges.Then,thefuelcell volt-agedecreasesslowlydowntoitssteadystateopencircuitvalue; thisprocessisnotyetwellunderstoodbutitcanbeassumedthat thereisasmallleakagecurrentbetweenanodeandcathode,which correspondstotheresistanceR∞(actuallynotinfinite,buthigh)in parallelwiththedoublelayercapacityintheequivalentcircuit.

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Fig.4. Segmentedcellelectricalconnections.

Table1

Fuelcellvoltageovershootinthepresenceofnitrogenvs.hydrogenflowrate.Airandhydrogenhumidifiedto100%RH,fuelcelltemperaturesetto50◦_C.

H2flowrate(slph) 0.1 0.2 0.5 1 2 5 10 20 30

Fuelcellovershoot(V) 1.06 1.09 1.10 1.09 1.08 1.06 1.03 1.01 1.00

thelastmeasurements(thefirstpointsinTable1)areaffectedby carbonoxidation.

Notethat avoltageovershoot of0.1V represents10%ofthe steadystateOCV,and15–20%ofthefuelcellvoltageinoperation.A 100cellsstackdesignedforworkingat0.6V/cell(60V/stack)could reachanOCVof110Vatstart-up,avalue80%higherthaninregular operation:onesimplewaytoreducethefuelcellvoltageovershoot duringstart-upconsistsinincreasingmonetarilythehydrogenflow rate.

3.2.2. Start-upwiththeanodecompartmentpreviouslyfilled withair

Thepresenceofoxygenintheanodecompartmententails oxi-dationreactionsatthecathodeinthepresenceofnegativecurrents (thecathodebecomingtransientlyananodeduringthefirststage ofthestart-upsequence).Theseoxidationreactions(eitheroxygen evolutionorcarboncorrosion)correspondtoafaradiccontribution tothenegativecurrentdensitymeasuredinthepassivepart;they areaccountedforintheequivalentcircuitwithachargetransfer resistance–setinparalleltothedoublelayercapacity–aswellas withanelectrodepotential(Fig.9).

Fig.5.Reactionsoccurringduringstart-upassumingpossibleflowofprotonsalong themembranefromtheactiveparttothepassivepartoftheanodecompartment (initiallyfilledwithair).

Notethatbecauseofthepresenceofairattheanode,the poten-tialdifferencebetweenthecathodeandtheelectrolytecanreach upto1.6V,correspondingtoveryoxidizingconditions[1,5–7,18].

3.3. Experimentalresults:localcurrentdensities

Fig.10ashowstheevolutionofthelocalcurrentdensities mea-suredinallthesegmentsduringastart-upwiththeanodeinitially filledwithnitrogen.Wefocusinthefollowingoncurrentdensity profilesmeasuredonthefirstsegmentsupplied withhydrogen (Fig.10b),onasegmentlocatedatanequaldistancefromtheanode inletandoutlet(Fig.10c)andonthelastsegment(Fig.10d).

Thefirstsegmentsuppliedwithhydrogenisalwaysactivewhile alltheothersbecomesuccessivelyactiveonceinthepresenceof hydrogen.Theredistributionofchargesbeginswhenthefaradic reactionsstop.Atthistime,thefirstsegmentreleasessomeofthe electronsithasaccumulatedatthecathode—whichcorresponds toanegativecurrentinFig.10bandcwhereastheinverse

phe-Fig.6. Localcurrentdensitiesmeasuredattheanodeandatthecathodesegments duringastart-upwiththeanodecompartmentinitiallyfilledwithair(H2flow rate=2slph,airflowrate=20slph).Airandhydrogenhumidifiedto100%RH,fuel celltemperaturefixedto50◦

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Fig.7.(a)Equivalentelectricalcircuitcorrespondingtoastart-upwiththeanode initiallyfilledwithnitrogen—firststage,anodepartiallyfilledwithhydrogen.(b) Equivalentelectricalcircuitcorrespondingtoastart-upwithanodeinitiallyfilled withnitrogen—secondstage,anodecompletelyfilledwithhydrogen.

Fig.8. Fuelcellvoltageduringastart-up withtheanodeinitially filledwith nitrogen—experimentalresultsmeasuredwithahydrogenflowratesetto0.2slph. Airandhydrogenhumidifiedto100%RH,fuelcelltemperaturefixedto50◦_C.

Fig.9.Equivalentelectricalcircuitcorrespondingtoastart-upwiththeanode ini-tiallyfilledwithair–Firststage,anodepartiallyfilledwithhydrogen.

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Fig.11.Totalamountofpassiveandactivechargesexchangedduringastart-up sequencevs.thehydrogenflowrate.Anodeinitiallyfilledwithnitrogen.Airand hydrogenhumidifiedto100%RH,fuelcelltemperaturefixedto50◦

C.

nomenonoccursinthelastsegment.Thebalanceofchargeshows thatthetimeandspaceintegralofthenegativecurrentsisstrictly equaltothatofthepositivecurrents.Startingfromthese consider-ations,itispossibletodefinethreetypesofchargeexchangeduring thestart-upsequence:

(i)Thepassivechargescorrespondtothenegativecurrents mea-suredinthepassivepartofthefuelcell(orangehatchedarea inFig.10candd),

(ii)Balancing charges are exchanged during the redistribution stageonceallsegmentsbecameactive;thenegative balanc-ingchargescorrespondtothebluehatchedareainFig.10b andc,

(iii)Activechargescorrespondtothepositivecurrentmeasuredin theactivepartminusthepositivebalancingcharges(equalin absolutevaluetothenegativebalancingcharges),

Qgreen areaN₂ =S

IntermofMEAdegradation,carbonoxidationoccursinthe pres-enceofnegativecurrentsinthepassivepart,i.e.,itislinkedtothe amountofchargesflowingthroughthepassivesegments(aslong astheyremainpassive),whichdependsonthehydrogenflowrate andonthegasfillinginitiallytheanodecompartment,asshownin

Fig.11.

ThemostimportantresultinFig.11 concernsthesignificant impactofthegasfillinginitiallytheanodecompartmentonthe amountof charge produced duringthestart-up sequence: it is much lower in the presence of nitrogen, this difference being attributedtotheabsenceofafaradiccontributiontothenegative currentsflowingthroughthepassivesegments,i.e.,totheabsence ofcarboncorrosionatthecathodewhenthereisnooxygenatthe anode.

Onecanalsonoticethattheimpactofaironthefaradic contribu-tiontothepassivechargesdecreaseswiththehydrogenflowrates: thisisprobablyduetothelowkineticofcarbonoxidation(and possiblyofoxygenevolution)comparedtothecapacitivecurrent densitiesinthepassivepart.

3.4. Experimentalresults:carbonoxidationandperformance losses

Assumingthatthedoublelayercapacitiesareindependentof thegasintheanodecompartment,andthatoxygenevolutionis negligible compared tocarbon corrosion (which seems reason-ablebuthastobechecked),onecanestimatethemassofcarbon oxidizedduringastart-upsequencestartingfromthedifference betweentheamountofpassivechargesmeasuredwhentheanode compartmentisinitiallyfiledwithairandwithnitrogen.

ThislastresultisillustratedinFig.12a,whereonecanclearlysee thedecreaseofcarboncorrosionathighhydrogenflowrate.The estimatedweightofoxidizedcarbonperstart-upisconsistentwith theinitialamountusedascatalystsupportatthecathode(about 50mg)andwiththeamountofcarbondioxidemeasuredatthe cathodeoutletbyOfstadetal.[9].

Fig.12bshowsthatthecarbonlossismoreimportantforthe segmentslocatednearthehydrogenoutlet,becausetheyare sub-mitted for a longer time to higher oxidizing conditions. As in

Fig.12a,thelowerthehydrogenflowrate,thehighertheamount ofcarbonoxidized.

Theperformancelossesresultingfromcarbonoxidationcanbe characterizedbytheevolutionofthefuelcellperformanceandof thelocalimpedancespectra(EIS)measuredalongthegaschannels. Thiswasdoneforasuccessionofstart/stopsequencesaccordingto threedifferentprotocols:

1.anodeflushedwithnitrogen,nitrogenflowrateatshutdown andhydrogenflowrateatstart-upsetto12slph;

2.anodeflushedwithair,airflowrateatshutdownandhydrogen flowrateatstart-upsetto12slph;

3.anodeflushedwithair,airflowrateatshutdownandhydrogen flowrateatstart-upsetto2slph.

Betweentwosuccessivesequences,thefuelcellcurrentdensity wassetto0.5Acm−2_during₁_h,_with_air_and_hydrogen_humidified to60%RH,stoichiometricratiosequalto3and1.2,anda temper-aturefixedto50◦_C._The_local_impedance_spectra_were_measured duringthissteadystateoperation.

Theevolutionofthefuelcellvoltageasafunctionofthe num-berofstart-up/shut-downisdepictedbythecurvesofFig.13.They showadramaticvoltagedropwhentheanodeisflushedwithair, allthemore sowhenthehydrogenflow rateis low.Note that theseexperimentshavebeencarriedoutwithnewMEA,which explainsthesharpincreaseinperformanceobservedduringthe firstsequencesinthethreecases.Whentheanodeisflushedwith nitrogen,theimprovementof theperformance canbeobserved approximatelyuntilthe150thsequence;beyond,thefuelcell volt-ageat0.5Acm−2 _starts_to_decrease _slowly, _probably_under_the effectofpotentialcyclingassociatedwiththevariousstagesofthe start/stopprotocol.

Theaging testswerestoppedwhen thefuelcellvoltagefell under0.63Vat0.5Acm−2_._This_value_was_reached_after_about₆₀ cycleswiththethirdprotocol(airflushat2slph)and180cycles withthesecondprotocol(airflushat12slph).Startingfromthe resultsdepictedinFig.12a,onecanestimatethatthe correspond-ingcarbonlosseswereequaltoabout0.1 and0.02mg/start-up, respectively.

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Fig.12. (a)Estimationofthecarbonloss/start-upasafunctionofthehydrogenflowrate.Airandhydrogenhumidifiedto100%RH,fuelcelltemperaturefixedto50◦ C.(b) Estimationofthecarbonloss/start-upforeachsegmentasafunctionofthehydrogenflowrate.Airandhydrogenhumidifiedto100%RH,fuelcelltemperaturefixedto50◦

C.

[email protected]−2_vs._number_of_start/stop_sequences_following threedifferentstart/stopprocedures.

us(rough)estimatesofthecarbonlossesperstart/stopsequence (0.15and 0.03mg,respectively)and thus overthewholeaging tests:about10mgwiththethirdprotocol(airflushat2slph)and 6mgwiththesecondprotocol(airflushat12slph).Thesevalues represent10–20%ofthetotal massofcarbonusedtomake the electrodes,whichisintheorderofmagnitudeofthevoltagedrop (about10%,byreferencetothemaximumvalue– 0.69V– reached withnitrogen).

Notethattheseestimationsonlytendtoconfirmour interpre-tationsofthefaradicandcapacitiveoriginsofnegativecurrents. Accuratemeasurementofcarbonoxidationandofitsconsequences startingfromtheanalysisofinternalcurrentsisanobjective diffi-culttoachievebecauseofthecomplexityofthisphenomenon:for instance,theproductionofcarbonmonoxidepoisoningtheactive sites[19–22]shouldbetakenintoaccount,aswellasthePt coars-eningresultingfromcarbonloss[23,24].

Finally,theevolutionoftheimpedancespectraprovides com-plementaryinformationaboutthelocaleffectsofcarboncorrosion. TheresultsdepictedinFig.14correspondtothesecondaging pro-tocol(airflushat12slph):acomparisonismadebetweenlocal

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Fig.15.(a)Highfrequencyresistanceprofilesafter49and173start/stopsequencesfollowingprotocolNo.2.(b)Doublelayercapacityprofilesafter49and173start/stop sequencesfollowingprotocolNo.2.

impedancespectrameasuredafter49and173start/stopsequences, whichconfirmsthatthesegmentlocatednearthehydrogen out-let (i.e.,near theend of the anode compartment, where air is introduced)aremoreaffected,asshownbytheincreaseoftheir impedance.Nevertheless,itremainspossibletofitthese experi-mentalspectrausingtheusualRandle’sequivalentcircuitandto identifythemainimpedanceparameters[25].

Adetailedanalysisoftheirprofilewouldbeoutofthescopeof thispaper,allthemoresosincetheusualhypothesesand simpli-ficationsleadingtotheexpressionoftheWarburgimpedanceare probablynotadaptedtomodelmasstransport.However,itappears clearlythatthehighfrequencyresistanceincreasesdramatically nearthehydrogenoutlet(Fig.15a).Thishighfrequencyresistance isthesumofthecontactresistancesbetweentheMEAcomponents, oftheprotonicresistanceoftheelectrolyte,andoftheelectronic resistancesoftheelectrodes,gasdiffusionlayersandbipolarplates: inourcase,itsincreaseismostprobablyduetodamagestothe catalystlayerentailedbycarboncorrosion(i.e.,totheincreaseof thecathodeelectronicresistance).Thisisconfirmedbythe simul-taneousdecreaseofthedoublelayercapacity(Fig.15b)nearthe hydrogenoutlet,whichisinterpretedasalossofelectrochemically activearea.Notethattheimpedanceofthesegmentslocatedinthe centreofthecellorclosetotheoppositeendshowsnosignificant variation.

4. Conclusion

Amethodbasedontheanalysisoflocalcurrentdensities mea-suredinasegmentedcellisdevelopedandusedtoestimatecarbon oxidationduringfuelcellstart-up.Theresultsshowclearlythatthe rateatwhichhydrogenisintroducedintotheanodecompartment hasasignificantimpactonfuelcelldurability:itmustbeashigh aspossibleinthepresenceofairoroxygen.Similarly,ifairhasto beintroducedintotheanodecompartmentatshut-down,itsflow ratemustbeashighaspossible.

Introducinghydrogeninananodecompartmentinitially occu-piedbyaircausesprofounddamagetothecathode,andthiscan entailasteepdecreaseofthefuelcellperformance.Local electro-chemicalimpedancespectroscopymeasurementscarriedouton damagedMEAshowthatthesegmentsthatremainforthelongest timeinthepresenceofair(whiletheothersarefedbyhydrogen) arethemostaffectedbycarbonoxidation.Carbonoxidationoccurs atthecathode,inthepresenceofnegativecurrents.However,these negativecurrentshavealsoacapacitiveorigin,whichexplainsthe fuelcellvoltageovershootobservedduringstart-up.Itispossible toevaluatethefaradicandthecapacitivecontributionstothe neg-ativecurrentsbycomparingthevaluemeasuredduringstart-up,in thepresenceofairintheanodecompartment,andinthepresence

ofnitrogen.Inthislastcase,onecanreasonablyassumethatthe faradiccontributionisnegligiblesincenosignificantdegradation occursafterasuccessionofstart/stopsequences.

Theresultsofthis workshouldhelpoptimisingstart-upand shut-downprotocolsinordertoimprovefuelcellsdurability.They canalsobeusedtoimprovecarbonoxidationmodelsduring start-up[26].

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