Experimental investigation of the effect of free openings of current collectors on a DMFC_图文

导读:JournalofPowerSources196(2011)717–728ContentslistsavailableatScienceDirectJournalofPowerSourcesjournalhomepage:www.elsevier.com/locate/jpowsourExperimentalinvestigationoftheeffecto

Experimental investigation of the effect of free openings of current collectors on a DMFC_图文

JournalofPowerSources196 (2011) 717–728

ContentslistsavailableatScienceDirect

JournalofPower

Sources

journalhomepage:www.elsevier.com/locate/jpowsou

r

Experimentalinvestigationoftheeffectoffreeopeningsofcurrentcollectorsonadirectmethanolfuelcell

Yean-DerKuana,?,Jing-YiChangb,Shi-MinLeec

DepartmentofRefrigeration,Air-ConditioningandEnergyEngineering,NationalChin-YiUniversityofTechnology,No.35,Lane215,Section1,Chung-ShanRoad,TaipingCity,411TaichungCounty,Taiwanb

IntegratedResearchCenterforGreenLivingTechnologies,NationalChin-YiUniversityofTechnology,411Taichung,Taiwanc

DepartmentofAerospaceEngineering,TamkangUniversity,251Tamsui,Taiwan

a

articleinfoabstract

Acurrentcollectorisoneofthekeycomponentsofadirectmethanolfuelcell(DMFC).Foraplanar-typeDMFC,thecurrentcollectorisusuallyfabricatedfromathinmetalthathasopenholes.Thegeometryofthecurrentcollectormayhaveasigni?canteffectonDMFCperformance.Therefore,thedesignofthecurrentcollectorisimportantforDMFCdesign.TheobjectiveofthisstudyistomakeasystematicexperimentalinvestigationofDMFCperformanceinthepresenceofcurrentcollectorswithdifferentfreeopenratiosandtotalperimeterlengthsofthefreeopenings.Currentcollectorswith5×5,7×7,and10×10holearrangementsunderdifferenttotalfreeopenratiosof30%,40%,50%,and60%areinvestigated.Theresultsshowthatthetotalfreeopenratiocansigni?cantlyaffectcellperformance;theyalsoshowthatdecreasingthetotalfreeopenratiodecreasescellperformance,andincreasingthetotalfreeopenratioincreasescellperformance.Ahightotalfreeopenratioaffectsthetotalcontactareabetweenthemembraneelectrolyteassembly(MEA)andcurrentcollectors.ProperconsiderationofboththetotalfreeopenratioandthetotalcontactareabetweentheMEAandcurrentcollectorsisnecessaryforthedesignofDMFCcurrentcollectors.Inaddition,alongertotalperimeterofthefreeopeningsyieldshighercellperformancewiththesamefreeopenratioofthecurrentcollectors.

? 2010 Elsevier B.V. All rights reserved.

Articlehistory:

Received11June2010

Receivedinrevisedform15July2010Accepted20July2010

Available online 30 July 2010Keywords:

Currentcollector

DirectmethanolfuelcellFreeopenratioPerimeterlength

1.Introduction

Researchonalternativeenergysourceshasincreasedrapidlyinrecentyearsbecausepetroleumreserveswillbeexhaustedinafewdecades.Afuelcellisatypeofpowergenerationdevicethatdirectlyconvertschemicalenergystoredinfuelintoelectricalenergywithhighef?ciency[1].Amongdifferenttypesoffuelcells,thedirectmethanolfuelcell(DMFC)issuitableforitscompactdesign,anditisprominentinportableapplicationsbecauseithasthemajoradvantagesofquickandconvenientrefueling,nearroomtempera-tureoperatingconditions,safety,anduseslowcostmethanol.Itiseasytostoreandcarry,anditconvertsmethanoldirectlyintoelec-tricitywithoutabulkyreformer[2].Therefore,theDMFCissuitableforcompactdesign,andthemostpromisingshort-termapplicationofDMFCsisinthe?eldofportablepowersources.DMFCsystemscouldpossiblyreplaceLi-ionbatteriesduetotherecentadvancesofminimizingDMFCstacks[3].

ThebasicelectrochemicalreactionsoftheDMFCattheanode,atthecathode,andoverallareasfollows[4]:Anode:Cathode:Overall:

CH3OH+H2O→6e?+6H++CO23

O2+6e?+6H+→3H2O2CH3OH+

3

O2→2H2O+CO22

(1)(2)(3)

?Correspondingauthor.Tel.:+886423924505x8256;fax:+886423932758.E-mailaddresses:ydkuan@ncut.edu.tw,c699611@yahoo.com(Y.-D.Kuan).0378-7753/$–seefrontmatter? 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jpowsour.2010.07.056

LiquidmethanolsolutionisusuallyadoptedastheDMFCfuel.Thereactionattheanodeisanoxidationreactionthatconvertsmethanolandwaterintoelectrons,hydrogenprotons,andcarbondioxide.Thenthehydrogenprotonsontheanodesidemovethoughtheelectrolyteandarriveatthecathodeside.Theelectronsmovetothecathodethroughanexternalcircuit.Thereactioninthecathodeisareductionreactionthatformswaterfromoxygen,protons,andelectrons.

Forpolymerelectrolytemembrane(PEM)fuelcells,thebipolarplatesrepresentasigni?cantpartoftheoverallcost.Areductioninunitcostofmaterialsorareductionintheamountofmaterialusedcouldconsiderablyreducethecostofbipolarplates.Therefore,muchefforthasbeeninvestedinthedevelopmentofcost-effectivematerialsforbipolarplates,suchasgraphitematerials,polymers

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196 (2011) 717–728

Fig.1.Atraditionalsingle-cellDMFCstructure.

mixedwithgraphite,andmetallicmaterials.Inaddition,toreducethethicknessofthebipolarplates,itisnecessarytoreducetheamountofmaterialused,whichcanalsolowerthecostofthebipo-larplates[3–6].Thebipolarplatescollectandcarrycurrentfromthecells,distributefuelandoxidantinsidethecells,separatetheanodeandcathodesidesfromcelltocellinthestack,andremovewaterforthermalmanagement.Generally,thebipolarplatesofthefuelcellsshouldhavecertaincharacteristicsthatallowthemtoachievehighperformanceandtomaintainstableoperation,suchassuperiorcapabilitiesofthermalandelectricconductivities,goodcorrosionresistance,goodgasimpermeability,lowdensity,andsuf?cientcompressivestrength.Inaddition,thecostofthematerialandthefabricationofthebipolarplatesshouldbelow[7–10].

Asingle-cellDMFCstructureisshowninFig.1;theMEAissandwichedbetweentheanodeandcathodepolarplates,whichareusuallymadeofgraphiteormetal,andthe?owchannelsaregroovedonthesurfaces.Thepolarplatesdistributebothfuelandoxidantswhilesimultaneouslycollectingelectrons.ThistypeofDMFCisseriallyinterconnectedviabipolarplates,andthisisreferredtoasverticalorbipolarplatestacking.Eachbipolarplatesimultaneouslyactsastheanodeofonecellandthecathodeoftheneighboringcell.Thecellsareconnectedinaback-to-backorienta-tiontoformthestack[4].ToshrinkthesizeoftheDMFCsandmakethemmore?exibleintermsofdesign,especiallyforlow-powerportableapplications,aside-by-sidecon?gurationthatconnectsthecellsinalateraldirectionandformsaplanar-typeDMFChasbeenintensivelyexploredasanalternativetoverticalstackinginrecentyears.Themainside-by-sidecon?gurationsarethebandedmembraneconceptandthe?ip-?opplanarseriesinterconnection.Inthebandedcon?guration,theelectrodesoverlapsuchthatthecathodeofonecellisstrappedacrossthemembranetotheanodeofthenextadjacentcell,andinterconnectionscanbemadebyedgetabs.The?ip-?opcon?gurationadoptsone-sidedbipolar?lms,therebyeliminatingtheneedtocrossthemembraneplane.Suchcon?gurationscouldallowDMFCstobeusedinplanararrayfuelcells[11,12].Inaddition,theprintedcircuitboard(PCB)-typefuelcell(PCB-FC)isalsoatypicalplanar-typefuelcell.PCBtechnologyisamatureandwell-establishedmassproductionprocess.There-fore,theapplicationofthePCBmanufacturingprocesstofuelcellscouldincreasedesign?exibility,shrinkcellsize,potentiallyleadtohigherpowerdensities,easedeviceintegration,andimprovepackagingformfactors[13–15].

Fig.2isaplanar-typePCB-DMFCmodule;itisdifferentfromtheconvectionalDMFCinthatthe?owchannelandcurrentcollectorarefabricatedseparately.Themethodusedtomakethecorrect

free

Fig.2.Aplanar-typePCB-DMFCmodule.

openingsofcurrentcollectorsisimportantforplanar-typeDMFCs;however,thereareveryfewrelatedstudiesofthismethod.Huangetal.[16]adoptedacomputational?uiddynamics(CFD)simulationtodiscusstheeffectofbreathing-holesizeontheelectrochemicalperformanceofafree-breathingcathodeonaDMFC.Theirsimu-lationresultsshowedthatalargerbreathingholeincreasedtheohmicresistancepenalty,andtheincreasedareaincontactwiththefreshairreducedconcentrationpolarization.However,theydidnotincludetheanodereaction,andthenumericalresultswerenotcomparedwiththeexperiments.Changetal.[17]performedsys-tematicexperimentalstudiesonthefreeopeningeffectsofcurrentcollectorsonDMFCperformance.Theirinvestigationwasaccom-plishedusingasingle-cellDMFCinwhichthecurrentcollectorshadSierpinskicarpetfractalholes.Theyfoundthatthetotalfreeopenratioandthetotalperimeterlengthoftheholesaretwoimportantfactors,butthetotalperimeterlengthoftheholeshasagreatereffectonDMFCperformancethanthefreeopenratiooftheholes.Inaddition,alongertotalfreeopeningperimeterledtobettercellper-formanceunderthesametotalfreeopenratioofcurrentcollectors.Thetotalfreeopenratioofthecurrentcollectorwasonly30%,fol-lowingtheSierpinskicarpetrule.Later,Kuanetal.[18]adoptedtheHilbertcurvecontinuous-typefractalgeometryinthedesignofthecurrentcollectorsfortheDMFC,whichincreasedthetotalfreeopenratioofthecurrentcollectorupto50%;thetotalperimeterlengthoftheopeningswasalsosigni?cantlyincreased.Theexperimentalresultsshowedthatcurrentcollectorswithmoreuniformopeningdistributionsandhighertotalfreeopeningperimeterlengthscouldreducetheeffectofanode?owrateoncellperformance.In

addition,

Fig.3.Asingle-cellDMFCtest?xture.

Y.-D.Kuanetal./JournalofPowerSources196 (2011) 717–728

719

Fig.4.Thegeometryanddimensionsofthe?owboard.

thehighertotalfreeopenratioandtotalfreeopeningperimeterlengthofthecurrentcollectorscouldincreasecellperformance.Therefore,aninvestigationoftheeffectofcurrentcollectorswithhighertotalfreeopenratiosandlongertotalperimeterlengthsofopeningsisnecessary.ThepurposeofthisstudywastoconductfurtherstudyonhowtheopeningsontheelectriccollectoraffectDMFCperformance.Thecurrentcollectorsweredrilledasdifferentarraysofrectangularopeningswithdifferentsizes.Thetotalfreeopenratiovariedfrom30%to60%,andthetotalperimeterlengthsofopeningswerechangedviathenumberandsizeoftheopenings.

2.Experimentalsetup

Thisstudyevaluatestheeffectofopeningparametersinthecurrentcollectorsontheperformanceofafuelcell.Tosimplifytheexperimentsandensurecellstabilitythroughouttheresearch,aDMFCwithstainlesssteel316L(SS316L)currentcollectorswasadoptedtoalternatethemetal?lmsonthecurrentcollectorsofthePCB-DMFC.AlthoughtheSS316Lhaslowerelectricconductiv-itythangraphiteandsomemetalmaterials,suchasgoldorcopper,theSS316Lwaschosenasthematerialforthecurrentcollectorsinthisresearchbecauseithastheadvantagesoflowcost,wideavail-ability,lightweight,highmechanicalstrength,andeasymachining.ThecorrosionresistanceofSS316Lisnotasgoodasgraphite,butthiscanbeavoidedbysurfacemodi?cationmethods.BecausethelengthoftheexperimentsconductedinthisresearchwasnotlongenoughtocreatesurfacecorrosiononthesurfacesoftheSS316Lcurrentcollectorsandaffectperformance,itwasnotnecessaryto

usesurfacetreatmentsonthecurrentcollectorsusedinthisstudy[17–22].Asingle-cellDMFCtest?xturewasadoptedinthispaper,asshowninFig.3.Themembraneelectrolyteassembly(MEA)issandwichedbetweentheanodeandcathodecurrentcollectors.

Bothanodeandcathode?owboardsweremadeofpolymethyl-methacrylate(PMMA)andattachedtotwoendsidesoftheDMFCtest?xture.Eachanodeandcathode?owboardhadonitssur-faceasingle,grooved,serpentine?owchannel;thegeometryanddetaileddimensionsofthischannelareshowninFig.4.Theconstructionofthesingle-celltest?xtureisshowninFig.5.Topre-ventleakage,apolytetra?uoroethylene(PTFE)gasketwasplacedbetweenthetwolayers.TheMEAwascomposedofNa?on?117

as

Fig.5.Constructionofthesingle-cellDMFCtest?xture:(a)anode?owboard,(b)gasket,(c)anodecurrentcollector,(d)gasket,(e)MEA,(f)gasket,(g)cathodecurrentcollector,(h)gasket,and(i)cathodecurrentcollector.

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Fig.6.Schematicillustrationoftheexperimentalsetup.

theelectrolyteandcarbonclothforthediffusionlayers.Thecata-lystloadattheanodewas4mgcm?2Pt–Ru,andthecatalystloadingatthecathodewas4mgcm?2Pt.TheactiveareaoftheMEAwas35mm×35mm.

Fig.6showsaschematicillustrationoftheexperimentalsetupformeasuringtheDMFCpolarizationcurves.TheDMFCwasplacedinanenvironment-controllablechamber.Abeakerheldthemethanolsolutionthatwaspreheatedbyatemperature-controllablewaterbath.ThemethanolsolutionwaspumpedintotheDMFCanodebyasquirepump.Theair?owwasdrivenandpumpedintotheDMFCcathodebyanairpumpwithanair?owregulator.TheDMFCwasloadedbyaDCelectricloader.

ToevaluateDMFCperformance,thepolarizationcurvesfortheDMFCwithdifferentfreeopeningsonthecurrentcollectorsweredetermined.Thepolarizationcurveshowsdifferentlimitingmech-anismsoccurringduringtheoperationofafuelcellatzerocurrent,lowcurrentdensity,andhighcurrentdensityranges[4].3.CurrentcollectorswithdifferentholearrangementsInthisresearch,aseriesofSS316Lcurrentcollectorswithrectan-gularholesindifferentarrangementsweredesignedandmachined.Thefreeopenratioandthetotalperimeterofthefreeopeningsaretwoimportantfactorsforthedesignofthecurrentcollectors.Toinvestigatetheeffectofthesetwoimportantfactors,rectangularholesweredesignedas5×5,7×7and10×10arrangementswith30%,40%,50%,and60%totalfreeopenratio,whichwasachievedbyusingdifferentholesizes.Therefore,thecurrentcollectorscouldbedesignedtopossessdifferentfreeopenratioswithdifferenttotalperimeterlengthsoftheholes.Asummaryofthegeometricinfor-mationregardingholearrangement,totalfreeopenratio,andtotalperimeterlengthforthecurrentcollectorsisshowninTable1.Thesizeofeachcurrentcollectoris95mm×95mm,anditsthicknessis2mm.ThereactiveareaoftheMEAis35mm×35mm.Thefreeopenratioisreferredtoasthereactivearea(35mm×35mm),inotherwords,thetotalfreeopenareadividedbythereactivearea,asshowninFig.7.Fig.8a–dshowsthecurrentcollectorswitha5×5holearrangementwith30–60%totalfreeopenratios.Thetotalperimeterlengthoftheopeningsofthecurrentcollectorswith30%,40%,50%,and60%totalfreeopenratioswere383,443,494,and541mm,respectively.Fig.9a–dshowsthecurrentcollectorswitha7×7holearrangementwith30–60%totalfreeopenratios.Thetotalperimeterlengthoftheopeningsofthecurrentswith30%,40%,50%,and60%totalfreeopenratioswere537,619.4,693.8,and758.5mm,respectively.Fig.10a–dshows

Table1

Geometricinformationfordifferentcircular-typebipolarplates.Reactivearea

Hole

TotalperimeterTotalfreeTotalfreearrangementlengthof

openareaopenratio

openings(mm)(mm2)5×5383.0366.735mm×35mm7×7537.0367.930%

10×10768.0368.65×5443.0490.635mm×35mm7×7619.4489.340%

10×10884.0488.45×5494.0610.135mm×35mm7×7693.8614.050%

10×10988.0610.15×5541.0731.735mm×35mm7×7758.5733.960%

10×10

1084.0

734.4

currentcollectorswitha5×5holearrangementand30–60%freeopenratios.DetaileddimensionsarealsoshowninFigs.8–10.Thetotalperimeterlengthofopeningsofthecurrentcollectorswith30%,40%,50%,and60%totalfreeopenratioswere768,884,988,and1084mm,respectively.4.Resultsanddiscussion

Inalloftheexperiments,theDMFCwasplacedintoaenvironment-controllablechamber,andtheenvironmentalcondi-tionswereasfollows.Environmentaltemperaturewas55?C,andtherelativityhumidity

was60%.Theanodewassuppliedwitha2Mmethanol/DIwatersolution,andthe?owratewasmaintainedat15cm?3min?1.Thecathodewassuppliedwithair,andtheair?owratewasmaintainedat1000cm?3min?1.

Inthedesignofthecurrentcollectorsthroughouttheresearch,thetotalfreeopenratioofthecurrentcollectorsdeterminesthesizeofthespaceforthereactantsto?owintothediffusionlay-ersonboththeanodeandcathodesides,anditalsodeterminesthespaceusedtoremovetheproductsproducedfromtheelec-

Fig.7.Illustrationofthefreeopenareaofthecurrentcollectors.

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Fig.8.Currentcollectorswitha5×5holearrangement:(a)30%totalfreeopenratio,(b)40%totalfreeopenratio,(c)50%totalfreeopenratio,and(d)60%totalfreeopenratio.

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