Effect of operating parameters and anode diffusion layer on the direct ethanol fuel cell performance

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Journal of Power Sources, 196, 24, pp. 10625-10631, 2011-09-03

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Effect of operating parameters and anode diffusion layer on the direct

ethanol fuel cell performance

Alzate, V.; Fatih, K.; Wang, H.

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ContentslistsavailableatSciVerseScienceDirect

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

Effect

of

operating

parameters

and

anode

diffusion

layer

on

the

direct

ethanol

fuel

cell

performance

V. Alzate,

K. Fatih

∗

_,

_H.

_Wang

NationalResearchCouncilCanada-InstituteforFuelCellInnovation,4250WesbrookMall,Vancouver,BritishColumbia,V6T1W5,Canada

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received24May2011

Receivedinrevisedform19August2011 Accepted20August2011

Available online 3 September 2011 Keywords:

Directethanolfuelcells Directalcoholfuelcells Ethanoloxidation

Fuelcelloperatingparameters Cellperformance

a

b

s

t

r

a

c

t

Aparametricstudywasconductedontheperformanceofdirectethanolfuelcells.Themembrane elec-trodeassembliesemployedwerecomposedofaNafion®₁₁₇_membrane,_a_Pt/C_cathode_and_a_PtRu/C

anode.Theeffectofcathodebackpressure,celltemperature,ethanolsolutionflowrate,ethanol concen-tration,andoxygenflowratewereevaluatedbymeasuringthecellvoltageasafunctionofcurrentdensity foreachsetofconditions.Theeffectoftheanodediffusionmediawasalsostudied.Itwasfoundthatthe cellperformancewasenhancedbyincreasingthecelltemperatureandthecathodebackpressure.Onthe contrary,thecellperformancewasvirtuallyindependentofoxygenandfuelsolutionflowrates. Perfor-mancevariationswereencounteredonlyatverylowflowrates.Theeffectoftheethanolconcentrationon theperformancewasasexpected,masstransportlosesobservedatlowconcentrationsandkineticloses athighethanolconcentrationduetofuelcrossover.Theopencircuitvoltageappearedtobeindependent ofmostoperatingparametersandwasonlysignificantlyaffectedbytheethanolconcentration.Itwas alsoestablishedthattheanodediffusionmediahadanimportanteffectonthecellperformance.

1. Introduction

Directliquidfuelcellsareanattractivetechnologybecauseof thefuel’shighvolumetricenergydensity,whichtranslatesin sys-temcompactnessandsimplicity.Intheliquidfueloptions,ethanol hastwomainadvantages;itslowtoxicityanditsestablished pro-ductioninfrastructure.Themainissueofdirectethanolfuelcells (DEFC)istheirlowefficiency,mainlyduetothedifficultytobreak theethanol’scarbon–carbonbondatthefuelcell’soperating tem-perature.Inaddition,thepresenceofethanolinthecathode,due tothecrossoverthroughtheNafion®_membrane,_reduces_the_open

circuitvoltageandpoisonsthecatalyst.Becauseoftheimportance oftheelectrochemicalreactionkineticsinthissystemmostofthe researchworkhasbeenfocusedontheanodecatalyst.Platinum–tin basedcatalystshaveshownthebestinitialperformanceforthe ethanolelectro-oxidationreaction(EOR)[1,2],while Pt–Sn con-tainingIrshowedbetterlongtermperformance[3].Almostallof thecatalystsappliedtoEORshowedverylowCO2yieldswithacetic

acidandacetaldehydebeingthemainoxidationproducts.Effortsto increasetheplatinum–tinelectrocatalyticactivityandCO2

selectiv-ityincludedesignofitsmicrostructure[4],addingathird[5–9],and fourthcatalystcomponent[3]andmodifyingthecatalystsupport [10–12].

∗Correspondingauthor.Tel.:+16042213071;fax:+16042213001. E-mailaddress:Khalid.Fatih@nrc.gc.ca(K.Fatih).

Singlefuelcelltestingisoneimportanttoolforfuelcellcatalyst designasitrevealstheperformanceofthecatalystinactual oper-atingconditions.Publishedworkhasshownthatthedirectethanol fuel cell performance is significantly affected by theemployed testingconditions,thefabricationprocessesandmaterialsofthe membraneelectrodeassembly(MEA).Theeffectoftemperature andethanol concentrationontheethanolcrossoverrateandits impactoncellperformancehasbeenstudiedbySongetal.[13]. Theauthorsfoundthatethanolcrossoverincreasedwithincreasing temperatureandethanolconcentration.Similarly,usinga refer-enceelectrodeLiandPickupfoundthattemperatureandethanol concentration have a positive effect on the ethanol oxidation reaction,buttheoppositeeffectwasfoundfortheoxygen reduc-tionreaction,demonstratingthesignificantimpactthatethanol crossoverhasontheoxygenreductionreaction[14].Praminiketal. [15],studiedtheeffectofthetemperatureoftheanodeandthe cathode,separately,aswellastheeffectoftheethanol concentra-tion.Theauthorsfoundaperformancemaximumat90◦_C_for_the

anodeand60◦_C_for_the_cathode_with_the_testing_temperature_range

of42–120◦_C_and_42–88◦_C_for_anode_and_cathode,_{respectively.}_The

authorsestablishedanoptimumethanolconcentrationof2M.In additiontothetestingparameters,theMEAfabricationprocesses alsoaffecttheDEFCperformance.Thishasbeendemonstratedby Songetal.,whocomparedtheperformanceandstabilityofagas diffusionelectrode(GDE)basedMEAwithacatalystcoated mem-brane(CCM)basedMEA[16].AlthoughtheCCMMEAhadhigher ethanolcrossoverrate,itsperformance andstabilitywere supe-riorcomparedtotheGDEMEA,havingapeakpowerdegradation

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10626 V.Alzateetal./JournalofPowerSources196 (2011) 10625–10631

Table1

Propertiesoftheanodediffusionlayer.

Anodediffusionlayer Thickness[␮m] Porosity[%] PTFEloading[wt.%] Description

CFPToray-120 370 78 0 WithoutMPL

CFP-25BCSGL 235 80 5 WithMPL(23%PTFE)

CFCE-tekB-1/B(cloth) 445 – 0 WithoutMPL

of15%against34%fortheGDEMEAina10hlifetest,whichwas attributedtolessdelaminationproblemsinthecaseoftheCCM MEA.Thestructureofthecatalystlayerhasalsobeeninvestigated forDEFC[17].Inthiswork,theauthorsobtainedhighercell per-formanceusingporeformersintheanodecatalystlayer,andby increasingPTFEcontentinthecatalystlayerfrom10to20wt%. Thesetwoparametersweresaidtoimprovetheflowsystem net-workfortheremovalofethanolelectro-oxidationproductspecies andthereforehavingmorecatalystsitesavailableforthereaction. Theobjectiveofthisworkistocreateaclearerandbroader pic-tureoftheeffectofthemainparametersusedduringdirectethanol fuelcelltesting. Specifically,theeffectofcathodebackpressure, celltemperature,ethanolsolutionflow rate,ethanol concentra-tion,andoxygenflowratewillbepresented.Furthermore,three differentanodediffusionmediaweretested.Theeffectofusing car-bonfibrecloth(CFC)andcarbonfibrepaper(CFP)isexamined,as wellasthepresenceofamicroporouslayer(MPL).Thecell perfor-mancewasevaluatedmeasuringthefuelcellvoltageasafunction ofcurrentdensityandcalculatingthepowerdensityfromthese measurements.Theresultsarepresentedaspolarizationandpower curves.

2. Experimental 2.1. Materials

Nafion®_N117_membranes_(DuPont)_were_pre-treated_at₈₀◦_C

with3wt%H2O2and0.5MH2SO4solutionsfor1hand,rinsedand

storedinde-ionisedwater.HiSPEC(JohnsonMatthey)4000(Pt/C) and5000(Pt1Ru1/C)wereusedasthecathodeandtheanode

cata-lyst,respectively.ThecathodediffusionmediawasSigracet®_GDL

25DC(SGLGroup),whichhas20%PTFEcontent.Fortheanode,three typesofdiffusionlayerswereused:(i)Sigracet® _GDL_25BC_(SGL

Group),whichisaCFPwitha5%PTFEcontentandamicro-porous layer(MPL)(23%PTFE),(ii)TGP-H-120CFP(Toray)and (iii)CFC (E-TEK).Table1summarizesthepropertiesofthethreedifferent diffusionmediaused.

2.2. ElectrodeandMEApreparation

Electrodeswerepreparedbysprayingthecatalystonthe dif-fusion media. The catalyst ink was composed of the catalyst, Nafion® _ionomer_solution₍₅_wt% _in_{alcohols/water,} _Alfa _Aesar),

andalcohol/watersolution.Thismixturewastreatedwithan ultra-sonicprocessor(Cole-Palmer)inpulsemodefor1h.Theinkwas sprayedusingan auto-spray(nozzle-XYtable) system.The cat-alystmetalloadingwas2mgcm−2 _for_both_anode_and_cathode,

whiletheNafion®_ionomer_content_in_the_catalyst_layer_was₂₀_wt%.

TheMEAs werefabricatedby hot-pressingthe Pt/Cand PtRu/C electrodes(5cm2₎_onto _each _side _of _the_Nafion® _membrane _at

90kgcm−2_and₁₄₀◦_C_for₄_min.

2.3. Fuelcellmeasurements

DEFCperformancetestswereconductedina5cm2_single_fuel

cellhardware(Fig.1).Serpentineflowfieldgraphiteplateswere used for both cathode and anode. Tests were performed with a commercial test station (Fideris). Prior to polarization curve

measurements,thebreak-inoftheMEAwasperformedbysetting thecellataconstantvoltageof0.2Vfor2–4huntilthecurrent wasstableusing1Mmethanolsolutionasfuel.Fortheactual mea-surementsethanolsolutionsandun-humidifiedoxygenwereused asreactants.Thevariablesandtestingconditionsaresummarized inTable2.Polarizationcurveswereperformedgalvanostatically, eachpointwasmeasuredfor2min,whichwasenoughtimetoget astablepotentialresponse.Aftereachpolarizationcurvedeionised water wasflown throughtheanodecompartment toavoidcell degradation.Beforemeasuringeachpolarizationcurve,thecellwas keptatopencircuitvoltage(OCV)underthetestingconditionsfor 30minforstabilization.

3. Resultsanddiscussion

3.1. Effectofanodediffusionlayer

Theeffectoftheanodediffusionlayerwasstudiedusingthree typesofsubstrates,CFPwithMPL,CFP,andCFC.InhydrogenPEM fuelcellstheMPLisknowntoimprovethecellperformance affect-ingthewatertransportproperties,thecatalystlayerstructureand electricalcontact;inthecaseofmethanolfuelcells,theMPLcan alsoaffectthefuelcrossoverandCO2transport[18].Fig.2shows

theSEMimagesoftheuncoatedandcoatedsurfacesofthe diffu-sionlayers.TheCFPwithMPLhasaveryhomogeneoussurfacewith poresizeslessin1␮minsize(Fig.2a),whileCFP(Fig.2b)andCFC (Fig.2c)havegreaterandbroaderporesizedistribution.Afterthe catalystlayerissprayedonthesesubstratesacleardifferenceinthe structureisobservedbetweentheCFC(Fig.2f)andtheCFPbased samples(Figs.2dande).ThecatalystlayerdepositedontheCFC hasamoreopenstructurethatfollowsthedirectionofthewoven fibrescomparedtoamoreflatsurfaceintheCFPsamples.Onthe otherhand,thereisnovisibledifferenceinthecatalystlayersurface betweenthesampleswithandwithoutMPL(Figs.2bandd).

Fig.3showsthepolarizationandpowercurvesforthethree typesofanodediffusionlayers.Thetypeofdiffusionlayerdoesnot affecttheOCV,aswellastheperformanceinthekineticcontrolled region;thethreecurvesshowthesamebehaviouruptoacurrent densityof0.020Acm−2_._At_higher_current_density,_the_anode_with

theCFPandMPLshowedalowerperformance,reachinga maxi-mumpowerdensityof0.017Wcm−2 _compared_to_0.028_W_cm−2

obtainedwithbothCFPandCFCbasedanodes.Furthermore,the

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Table2

Experimentalconditionsusedforeachsetoftest.

Exp.series Variables

Anodediffusionlayer O2flowrate(mLmin−1₎ _Ethanol_flow_rate_(mL_min−1₎ _[Ethanol]_(M) _Temp.₍◦_C) _Cathode_{back-pressure}_(psig)

1 CFP CFP–MPL CFC 300 2 1 90 30 2 CFP 300 2 1 90 0–10–20–30 3 CFP 300 2 1 60–70–80–90 0–30 4 CFP 300 0.6–1–2–5 1 90 30 5 CFP 300 2 0.1–0.5–1–2–5 90 30 6 CFP 50–100–300–500 2 1 90 0–30

cellwiththeCFCbasedanodeachievedthehighestcurrent den-sity.TherearetwodifferencesbetweentheCFPwithMPLandthe othertwosamples.ThefirstoneisthePTFEcontentinCFP(5wt.%). ThehydrophobicityoftheCFPcanreducethetransportofwater butinalesserdegreethetransportofanethanol/watersolution, whichhasalowersurfaceenergy.Inaddition,thePTFEcanreduce theelectricalconductivityoftheCFP.Theseconddifferenceisthe presenceoftheMPL,whichmightactsimilartoabarrierpreventing theflowofliquidtoandfromthecatalystlayerduetothehighPTFE content(23wt.%)aswellastheverysmallporesandlower poros-ity.ConsideringthelowcontentofPTFEintheCFP,theporesizeof

theMPLanditshydrophobicityseemtobecomplementaryfactors inthereductionofthecellperformance.Thispostulategoesinthe samedirectionasthemodellingworkdonebyAndreadisetal.[19] inwhichwasreportedthattheporosityofthediffusionlayerhasan importanteffectontheperformance.Forexamplea22%increasein performancewascalculatedwhenthediffusionlayer’sporosityis changedfrom0.4to0.8.Infacttheirsimulatedpolarizationcurves haveaverysimilarbehaviourastoFig.3,i.e.,thekineticregion isnotaffectedbychangingthediffusionlayerporositywhileat highercurrentstheslopeofthecurveincreaseswithdecreasing porosity.Furthermore,Biswasetal.[17]foundbetterperformance

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Fig.3.Polarizationandpowercurveswithdifferentanodediffusionmediafor1M EtOHfeed(2mLmin−1_and₀_psig)_and_{non-humidified}_oxygen₍₃₀₀_mL_min−1_and

30psig)at90◦_C.

byincreasingboththeporosityandthePTFEcontent(20%)inthe catalystlayer.However,intheircasethecatalystlayerwas fabri-catedusingporeformers,thereforethePTFEcontentcouldhavea verydifferenteffectcomparedtothepresentwork.

Forthisexperimentalsystemthemajorityoftheproductsare inliquid phase,for theidealDEFCsystemthereactionproduct isCO2,thereforetheflowdynamicswouldbeverysimilartothe

directmethanolfuelcell.Gasmanagementhasbeenacritical fac-torfor directmethanolfuelcells designgiventhat CO2 bubbles

canremaininthediffusionlayer.Thesebubblesgenerallyblock theporesusedfor themethanoldiffusiontothecatalystlayer, leadingtofuelstarvationandtherefore decreasingthecell per-formance.A comprehensivereview waspublishedonthemass transportphenomenainaDMFCsystem[20].Forexample,inan experimentperformedbyLuandWang[21],theeffectoftheanode diffusionmediaofaDMFCwasevaluated.Untreated(hydrophilic) CFC wascompared witha 20% PTFE treated CFP, which had a homemadeMPL.Theauthorsconcludedthattheadditionofthis layerdecreased therateof methanolcrossoverdue tothe low-permeabilitythattheMPLprovides.Ontheotherhandtheremoval ofCO2bubblesfromthebackinglayerwaseasierintheCFCfrom

visualexperiments,although it wasnotclear whetherthis dif-ferencewasduetotheporedistributiondifferencesorthePTFE contentinthematerials.Thereforetheoptimumdiffusionmediafor DEFCanodeswoulddependonthereactantflowregime,including theamountofgasproducedintheanode.

3.2. Effectofcathodebackpressureandcelltemperature

Fig.4showstheeffectofcathodebackpressureonthecell per-formance.Ingeneraltheperformanceimproveswithincreasing thebackpressure,mainlybecauseofthereductionoftheactivation overpotential.Intheohmicregion,thefourcurveshavesimilar behaviour.However,inthemass transport region,the effectof increasingthebackpressureseemstodiminishasthecurrent den-sityincreases.Ethanoltransport in theNafionmembrane takes placethroughthreedifferentphenomena:electro-osmosis; diffu-sionandhydraulicpermeation.Whenthepressureatthecathodeis higherthanattheanode,thereisbackconvectionofethanolfrom thecathodetotheanodeduetothehydraulicpermeation.This resultsinanoverallreductionofethanolcrossover.Thereduction ofethanolconcentrationinthecathodewillreducetheparasitic currentsandpoisoningcreatedbythereactionofethanolonthe cathodeactivesites.Thisenhancestheoxygenreductionreaction kineticsandreducestheactivationoverpotentialasseeninFig.4. Thecathodebackpressurealsoincreasestheoxygensolubilityin

theNafionionomerpresent inthecatalyst layer.Thiswill pro-duceahigheroxygenconcentrationinthetriplephaseboundary, enhancingtheoxygenreductionreactionrate.

Asmentionedpreviously,themaximumcurrentdensityvalues areverysimilaratthefourtestedbackpressures.Athigher cur-rentdensitiestheconcentrationofethanolintheanodecatalyst layerislower,whichaffectsthefuelcellperformanceintwoways. First,theconcentrationoverpotentialattheanodeincreasesand second,thecrossoveroftheethanolduetodiffusiondecreasesas thedifferenceinconcentrationbetweentheanodeandthecathode isreduced.Therefore,thefactthatthemaximumcurrentdensity isnearlyindependentofthecathodebackpressuresuggeststhatin thisregionofthepolarizationethanolmasstransferisgoverning thecellperformance.Thereductionofthecrossoverrateand there-foretheparasiticcurrentwithincreasingcellcurrenthasbeenalso reportedbyAndreadisetal.throughamodellingstudy[22].Inthe samemodellingwork,itwasreportedthattheethanolcrossover ismaximalatOCV.Inthepresent study,Fig.4bshowsthatthe OCVisalmostunaffectedbybackpressurewithastablevalueat 0.64V(variationsareintheorderofmV).Itisimportanttonote thatastabilizationperiodof30minwasusedbeforemeasuring eachpolarizationcurve.TheOCVwasfoundtobeverysensitiveto thechangeofconditionsbutonlyatthemomentwhenthe pertur-bationwasmade.Forexample,anOCVof0.8Vwasrecordedwith increasingbackpressurebutonlyforafewseconds,anditwould slowlydecreaseuntilreachingastablelowerpoint.Itwouldhave beenexpectedthatahigherbackpressurewouldresultinahigher OCVduetothereductionoftheethanolcrossover.Thefactthat thesteadystateOCVisalmostunchangedwithincreasing back-pressurecanbeduetoasteadypoisoningofthecathodecatalyst layer.Thismaybeduetoanaccumulationofethanolatthe cath-odesideofthemembrane,whichlowerstheinitiallyrecordedhigh OCVafterapressure increase.Asforthemaximumpower den-sity,itincreasesalmostlinearlywithincreasingbackpressureupto 20psig(Fig.4b).Byincreasingthepressurefrom20to30psigthe performancedoesnotimprove,thereforeasaturationpointmight havebeenreachedat20psig.Itisimportanttonotethatthereis alsothecrossoverofspeciesfromthecathodetotheanodesuchas oxygen,ethanoland/orreactionproductsofethanoloxidationthat mayaccumulateatthecathodesideofthemembrane,whichwould increasebyincreasingthecathodebackpressure.JamesandPickup havediscussedtheissueofethanoloxidationproducts(aceticacid andacetaldehyde)crossingtotheanodeside[23].WhileJablonski etal.reportedtheeffectofoxygencrossingfromthecathodetothe anodeandreactingchemically[24]withethanoltoproduceacetic acidandacetaldehyde.Ineithercasethepresenceof electrochem-icalreactionproductsintheanodewouldlowerthecell’sOCVand performanceandcounteractthepositiveeffectofbackpressureon thecathodekinetics.

Fig.5showstheeffectoftemperatureontheDEFCperformance withcathodebackpressure(30psig)and withoutcathode back-pressure(0psig).ThepolarizationcurvesinFigs.5a andbshow asimilarbehaviourwithincreasingthetemperature.Thekinetic regionisenhancedandtheslopeoftheohmicregiondecreases. Electrodekinetics,membraneconductivityandmasstransfer prop-erties are thermally activated, therefore it is expected that an incrementin fuelcelltemperaturewillresult ina performance enhancement. Onthe otherhand,fuelcrossoveris alsoa ther-mallyactivatedprocess.WithincreasingtemperaturetheNafion polymerbackbonerelaxesandexpandsallowinghighertransport rates,inadditiontheethanoldiffusivityisalsoenhanced[13,25]. Therefore the cathodekineticshastwo competing effects with increasingtemperature.LiandPickupreportedthattheeffectof ethanolcrossoverissosignificantthatthecathodeperformance decreaseswithincreasingtemperaturealthoughtheoxygen reduc-tionreactionrateincreaseswithtemperature.Theyconcludedthat

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Fig.4.Effectofcathodebackpressureonfuelcellperformancefor1MEtOHfeed(2mLmin−1_and₀_psig)_and_{non-humidified}_oxygen₍₃₀₀_mL_min−1₎_at₉₀◦_C,_(a)_polarization

andpowercurvesand(b)opencircuitvoltageandmaximumpowerdensity.

theoverallcathodebehaviourmaybetheresultofanopposing dependencewithtemperatureduetotheparasiticcurrentand poi-soningofthecatalyst[14].Butoverall,thedependenceofothercell processeswithtemperatureseemstobegreaterthanthecrossover sincetheperformanceisenhanceddespitetheincreaseinethanol crossover.

The maximum power density increases with temperature almostlinearly forboth conditions as seenin Fig. 5c,withthe exemptionofthe60◦_C_point_for_the_{unpressurized}_condition._The

differenceinpowerdensityforthesystemsatthesame temper-aturebutwithandwithoutbackpressureisinthe6–7mWcm−2

rangeforallthepointsexceptforthe60◦_C_point_where_the

differ-enceis3mWcm−2_._The_smaller_difference_at_a_lowest_temperature

suggeststhatat60◦_C_the_crossover_effect_is_less_important.

There-foretheeffectofthebackpressureisreduced.TheOCVshoweda verysmallpositivedependencewithtemperatureinboth circum-stances(Fig.5c)andwasslightlyhigherwithcathodebackpressure. WhileSongetal.[13]showedthattheOCVwasgreatlyaffectedby temperature,althoughtheirworkwasdoneatarangelowerthan 75◦_C._For_example,_when_the_temperature_was_reduced_from₇₅◦_C

to55◦_C_the_OCV_decreased_from_0.62_to_0.53_V._This_could_indicate

that,atlowertemperatures,theanodekineticsaremoreimportant thantheeffectofethanolcrossover.Thereforeanincreaseinthe temperaturewouldimprovetheOCV.Inthiswork,athigher tem-peraturesasinFig.5aandc,competingeffectbetweenenhanced anodekineticsandhighercrossoverwithincreasingtemperature, resultedinanalmostinvariableOCVwithtemperature.

3.3. Effectofethanolflowrateandethanolconcentration

Fig.6showstheeffectofflowrateofethanolsolutiononthefuel cellperformanceintherangeof0.6–5mLmin−1_._The_performance

isalmostinvariableinthe1–5mLmin−1_range_but_for_0.6_mL_min−1

themaximumpowerdensityisarounda10mWcm−2_lower_than

at thehigher flow rates (Fig. 6b).The differencein the perfor-mancebetween0.6mLmin−1_and_other_flow_rates_starts_to_appear

athighercurrentdensitiesofthekineticregionofthepolarization curve(Fig.6a).Withincreasingcurrent,thedifferencebetweenthe 0.6mLmin−1_curve_and_the_other_curves_remains_constant,_i.e.,_the

curvesarealmostparallel.Thereforetheohmicandmasstransfer

Fig.5.Effectofcelltemperatureonfuelcellperformancefor1MEtOHfeed(2mLmin−1_and₀_psig)_and_{non-humidified}_oxygen₍₃₀₀_mL_min−1₎_at₉₀◦_C,_(a)_polarization_and

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Fig.6. Effectofethanolsolutionflowrateonfuelcellperformancefor1MEtOHfeed(0psig)andnon-humidifiedoxygen(300mLmin−1_,₃₀_psig)_at₉₀◦_C,_(a)_polarization

andpowercurvesand(b)opencircuitvoltageandmaximumpowerdensity.

Fig.7.EffectofethanolconcentrationfuelcellperformanceforaqueousEtOHfeed(2mLmin−1_and₀_psig)_and_{non-humidified}_oxygen₍₃₀₀_mL_min−1_,₃₀_psig)_at₉₀◦_C,_(a)

polarizationandpowercurvesand(b)opencircuitvoltageandmaximumpowerdensity.

overpotentialsarenotaffectedbytheethanolflowrate.This sug-gestsasurfacephenomenoneffectatlowethanolflowrates,such aslowerremovalofreactionproductsthatblockactivesitesorthe decreaseofin-planetransportofreactantandthereforealower utilizationoftheavailablecatalystsites.

Theeffect of ethanol concentrationon thecell performance is illustrated in Fig. 7. It is seen that the different regions in thepolarizationcurvehavedifferentdependenceontheethanol concentration(Fig.7a).Both,theOCV(Fig.7b)andthefuelcell per-formanceinthekineticregionincreasewithdecreasingethanol concentration.Whileinthemasstransferregiontheperformance increaseswithincreasingethanolconcentrationupto2M.Atvery lowethanolconcentrationtheperformancesuddenlydropsdue tomasstransferlimitations. With0.1Mtheperformancedrops atacurrentdensityofapproximately0.015Acm−2_,_whereas_this

happensat0.06Acm−2 _for_a_{concentration}_of_0.5_M._In_the_case

of1Methanol solution,thereis a slightinflection inthecurve at0.14Acm−2_._At_these_three_points_the_ethanol_{stoichiometries}

are8.04,10.72and9.18,respectively(assumingthepartial oxida-tionreactionofethanoltoaceticacid,i.e.,4electronspermolecule ofethanol).Therefore,onecanconcludethatunderthese condi-tionsaminimumstoichiometryofapproximately10isnecessary toavoidmasstransferlimitations.Byincreasingtheconcentration from1Mto2Mtheperformanceisalmostinvariable,differences areonlyseenatthemaximumcurrentdensity.Whileincreasing ethanolconcentrationfrom2Mto5Mproducesadropinthe per-formanceinallregionsofthepolarizationcurve.Intermsofthe maximumpowerdensitythereisamaximumbetween1Mand 2M(Fig.7b).Asseenfromtheseresults,byvaryingtheethanol concentrationthereisaclearcontributionoftwoeffectstothecell performance.Bettermasstransferathighethanolconcentrations andlowerethanolcrossoveratlowethanolconcentrations.Atlow currentsthereisalowconsumptionofethanol;thereforea vari-ationofethanolconcentrationdoesnotaffecttheanodekinetics inthestudiedconcentrationrange.Onthecontrary,thecathode kineticseemstobeenhancedbythelowerethanolconcentration

Fig.8.Effectofoxygenflowrateonfuelcellperformancefor1MEtOHfeed(1mLmin−1_,₀_psig)_and_{non-humidified}_oxygen₍₃₀₀_mL_min−1_,₀_psig)_at₉₀◦_C,_(a)_polarization

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duetothelowerethanolcrossover.Withincreasingcurrentdensity, theethanolconcentrationstartstoplayaroleintheanoderesponse andtheextremecaseisseenwhenverylowethanolconcentrations weretestedwiththepresenceofalimitingcurrent.

3.4. Effectofoxygenflowrate

Fig.8showstheeffectofoxygenflowrateonthefuelcell per-formance.Thesetestswereperformedatatmosphericpressure.No effectontheperformancewasobservedinthe50–500mLmin−1

rangewith30psigbackpressure(resultsarenotshown).InFig.8a itisseenthattheperformanceincreaseswithincreasingoxygen flowrateupto300mLmin−1_,_increasing_the_flow_rate_further_does

notproduceanychangeontheperformance.Fig.8bshowsthatthe OCVandthemaximumpowerdensityincreasewithincreasingthe flowrate,whichisduetoahigherconcentrationofoxygenatthe cathodewithrespecttothecrossoverproductsandtoanincrease intherateofremovalofcrossoversubstancesthatcanpoisonthe cathodecatalyst.

4. Conclusions

Theeffectof operatingparameters ontheDEFCpolarization curve,OCVandmaximumpowerdensitywastestedandanalyzed. TheOCVwashighlydependantontheconcentrationofethanolin thefuelstreamandincreasesbydecreasingtheethanolcontent. Incontrast,theeffectoftheotherstudiedparametersontheOCV wasverysmall.Takingintoaccountthesesmallvariationsitcan besummarizedthattheOCVincreasedwiththeoxygenflowrate, cathodebackpressureandtemperature,andwasindependentof fuelflowrateandanodebackinglayer.

Ingeneral,thekineticallycontrolledregionofthepolarization curveswasenhancedbyincreasingthetemperature,the backpres-sureandtheoxygenflowrate,whileitdecreasedbyincreasingthe ethanolconcentration.Theseparametersaffectedeitherorboththe electrodeintrinsickineticsandtheethanolcrossover.Furthermore, thekineticresponseofthefuelcellwasfoundtobeindependent ofethanolsolutionflowrateandanodediffusionlayer.Thecell’s masstransferpropertieswereparticularlyaffectedbytheethanol concentrationandinlesserextentbytheanodebackinglayerand reactantflowrates.

Increasingthetemperatureimprovedthemaximumpower den-sityalmostlinearly.Withbackpressuretheperformanceincreased up to 20psig, further increments in the backpressure did not producehigherperformance.Similarbehaviourwasseenforthe oxygenflowrateatvalueshigherthan300mLmin−1_._Higher_power

densitieswerefoundforethanolconcentrationof1Mandethanol flowratehigherthan1mLmin−1_.

ThegasdiffusionlayersthatdidnothaveanMPLshowed supe-riorperformance.Intermsofthediffusionlayer,theDEFCsystem isuniquesincereactantsandproductsareallliquids.Thisisnot thecaseforthemoreextensivelystudiedsystemsusingmethanol (liquidfuel–gasproduct)andhydrogen(gasfuel–liquidproduct) asfuels.Thereforemoreworkisneededintheoptimizationofthe DEFCelectrodeproperties,includingdiffusionlayers.

Acknowledgments

TheNational ResearchCouncil Canada-Institute forFuel Cell Innovation(NRC-IFCI) andAgricultureAgri-FoodCanada(AAFC) through the Agricultural Bioproducts Innovation Network Pro-gram(ABIP)supportedthiswork.TheauthorsthankMariusDinu, AndrewMattie,KevinBereraandTomVanderhoekfortheir contri-butioninthefabricationoftheinkauto-spraysystemandfuelcell hardware.

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