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82 燃料電池 Vol.18 No.2 2018
1.Introduction
Polymerelectrolytefuelcells(PEFCs)areconsideredapromising technology for cleanandefficientpowergenerationandarebecoming increasingly importantasalternativepowersources forstationary, transporta-tion,orportableapplications,inparticularforresidentialco-generationsystemsorelectricvehicles,dueto theirhighefficiency,rapidstart-up,and lowemissionprofile.TheoperationofPEFCsabove100℃providesnumer-ousadvantages,likereducingtheaffinityofthePtcata-lyst tocarbonmonoxide,which isaby-productof thereformingprocess, and thus increasingCO tolerance.Asaresult,theoperationofPEFCsabove100℃allowsusing impure fuel streamsandsimplifies fuelprocess-ing.High-temperatureoperationalso increasestheheattransfer ratebecauseofa lager temperaturegradientbetween the fuel cellsandexternalenvironment, sup-plyingwasteheat thatcanbeeasilyutilizedbyotherprocesses(e.g.,co-generationofheatandpoweroron-boardreforming).Therefore,theheatmanagementsys-tem is simplified, increasing theweight-andvolume-specificenergydensitiesandtheoverallenergydensity
ofPEFCsystems.Theelectrochemicalkineticsofbothelectrodereactionsarealsoenhancedby increasingthePEFCoperation temperature. Inparticular, therateoftheoxygenreductionreaction issignificantly increased,improvingtheperformanceofPEFCsasawhole.ThesefactorsresultinincreasedefficiencyandsimplificationofPEFCsystems. Theperfluorinatedsulfonicacid(PFSA)ionomerhasbeencommonlyusedasbothapolymerelectrolytemem-brane(PEM)andprotonconductor inanodeandcath-odecatalystlayers(CLs)ofstate-of-the-artPEFCsdueto itshighprotonconductivityandhighchemicalandmechanical stability.However, this ionomer isnotap-plicabletoPEFCsoperatingabove100℃,since it leadstolosesofprotonconductivityandmechanicalstrength,alsocausingconsiderablepermeation(crossover)offuel,hydrogen,andoxidants, airoroxygen,above itsglasstransitiontemperature(~110℃).Alternative ionomersbasedonnon-fluorinatedmaterials, showing stabilityabove100℃,processability,environmentaladaptability,and lowproduction cost, in addition to chemical andelectrochemicalstability,are inhighdemand.Intensiveresearcheffortshavebeendevotedtodevelopingalter-
投稿論文 Paper
Application of Sulfonated Carbon Ionomer to Catalyst Layers of Polymer Electrolyte Fuel Cells
Tomoya Oyama, Kazuki Matsumoto, Tetsuo Take Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University
Abstract:Wehavedevelopedasulfonatedcarbonionomerandappliedittotheanodeandcathodecatalystlayers(CLs)ofpolymerelectrolytefuelcells(PEFCs).Theaboveionomercanbepreparedbythethiolgroupoxidationmethod,while its ionexchangecapacity(IEC)canbecontrolledbychangingtheproportionsofcarbonblack,3-mercaptopropyltrimethoxysilane,andH2O2usedinthesynthesis.ThesulfonatedcarbonionomercanbeusedinbothanodeandcathodeCLsofPEFCs,leadingtotheirdecreasedperformance,ascomparedwiththatofPEFCsutilizing theperfluorinatedsulfonicacid(PFSA)ionomerat80℃andarelativehumidity(RH)of100%.TheperformanceofPEFCsemployingthesulfonatedcarbon ionomerhardlydependson its IECanddecreaseswithdecreasingRHat80℃.TheseperformancedecreasesarelargerthanthoseobservedforthePFSAionomer.Ontheotherhand,PEFCsusingthesulfonatedcarbonionomeraresuperiortothosebasedonthePFSAionomerattemperaturesabove100℃andRHsbelow100%.Key Words:Polymerelectrolyte fuel cell,Sulfonatedcarbon ionomer,Catalyst layer,Gasdiffusionelectrode,Operationattemperaturesabove100℃andRHsbelow100%
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nativedual-use ionomers(asbothPEMsandprotonconductors incatalyst layers)forthePEFCs.Recently,asulfonatedsilaneionomer1)andnon-fluorinatedhydro-carbon ionomers,suchassulfonatedpolyether(SPE)2),sulfonatedpolyetheretherketone(SPEEK)3)−7),sulfo-natedpolyetherketoneketone(SPEKK)8), sulfonatedpolyaryleneether(SPAE)9),10),andsulfonatedpolysulu-fone(SPS)11)havebeendeveloped forcost reduction,elevationof the operating temperature, andavoidingenvironmentalpollutionbyfluorideafterdisposal.How-ever,theapplicationofhydrocarbonionomersascatalystlayer(CL) ionomersoftendecreases theperformanceofPEFCs. Ingeneral, theprotonconductivityofhydro-carbonionomersishighunderhighhumidityconditionsbutdecreasessteeplywithdecreasinghumidity,leadingto lowcatalystutilization inthegasdiffusionelectrodes(GDEs)ofPEFCs. Inthiswork,wehavepreparedasulfonatedcarbonionomerandapplied it tobothanodeandcathodeCLsofPEFCs,supposingthatthisionomerwithhighprotonconductivityexhibitsnoharmfuleffectson theperfor-manceandstabilityofPEFCsabove100℃,becausecar-bonmaterial isusuallyusedasaPtcatalystsupport inCLsofphosphoricacidfuelcellsat200℃andpossesseshighheatdurability.Wehaveevaluatedtheeffectsofus-ingthesulfonatedcarbon ionomer in theanodeand/orcathodeCLsofPEFCs,itsionexchangecapacity(IEC),andtherelativehumidity(RH)ofthereactantgasesonPEFCperformancebyconductingthesinglePEFCtestsat80℃andRHof100%.Subsequently,wehavealsoevaluatedtheeffectsof temperaturesabove100℃andRHsbelow100%on theperformanceofPEFCsutiliz-ingthesulfonatedcarbon ionomerbyconductingsinglePEFCtests.
2.Experimental
2.1 Preparation of the sulfonated carbon ionomer
Thesulfonatedcarbon ionomerwaspreparedbyus-ing the thiol group(SH)oxidationmethodusedbyMunakataetal. for thesurfacesulfonationofaporoussilicamembrane12).The surface sulfonationof carbonblack(Aqua-Black®162 ,TokaiCarbonCo.,Ltd.,Japan)wasconductedasshown inFig.1 ,covalentlyattaching
sulfonicacidgroups(SO3H)tothecarbonblack.First,thecarbonblackwasrefluxed in3-mercaptopropyltri-methoxysilane(3-MPTMS,KBM-802 ,98 . 9%,Shin-EtsuChemicalCo.,Ltd.,Japan)solutionat110℃for24htoattachSHprecursorgroups,withacetone(99.0%,ShowaChemicalCo.,Ltd., Japan)usedassolvent.Theweightratioofacetonetocarbonblackwassetat15 : 1 .Second,theSHgroupson thecarbonblackwereconverted toSO3Hgroupsbyoxidationwith10wt%H2O2aqueoussolution(30wt%H2O2aqueoussolution,ShowaChemi-calIndustryCo.,Ltd.,Japan)at70℃for2h.Theweightratiosof3-MPTMSandH2O2tothecarbonblackwerevariedtochangetheIECofthepreparedsulfonatedcar-bon ionomer.Theweightratioof3-MPTMStocarbonblackwassetat0 . 5and1 ,andtheratioofH2O2tothecarbonblackwassetat2and3 .
2.2 Characterization of the sulfonated carbon ionomer
Fourier transform infrared(FTIR)spectroscopywasperformedusingtheFTIRSpectrumOne®spectrometerandtheUniversalATRAccessory®(PerkinElmer Inc.,USA)toverifypresenceofSO3Hmoietiesonthepre-paredsulfonatedcarbon ionomer1),with triglycinesul-fateusedasadetector.TheIECofthepreparedionomerwasalsomeasured toestimate theamountof formedSO3Hgroups.Thesulfonatedcarbon ionomersamplesweredriedat120℃for30mininair.Thesampleswereimmersedin20cm3ofasaturatedNaClaqueoussolution(NaCl, 99 . 5%,ShowaChemicalsCo.,Ltd., Japan)andshakenfor24hwithashakertoexchangetheprotonsofSO3Hunitsforsodiumions.Thesolutionwasfiltered,
Fig. 1 Outline of surface sulfonation procedure by SH oxidation method.
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andthefiltratewastitratedagainst0 . 01MNaOHaque-oussolution(NaOH,93 . 0%,ShowaChemicalsCo.,Ltd.,Japan)usingapotentiometric titrator(AT-510 ,KyotoElectronics IndustryCo.,Ltd., Japan)toquantify theamountofprotons.Basedontheobtainedresultandtheweightofthesulfonatedcarbonionomersample,theIECwasestimatedusingEq.(1)13): IEC=(V× C)/Wdry (1)whereIEC[mmolg−1=meqg−1]istheionexchangecapacity,V[L]isthevolumeoftheNaOHsolutionusedfor titration,C[mmolL−1] is theconcentrationof theaboveNaOHsolution,andWdry[g]isthedryweightofthesulfonatedcarbonionomer.
2.3 Preparation of GDEs and membrane-electrode assemblies
GDEswitha three-layerstructure, i.e.,CL/micropo-rous layer(MPL)/wet-proofed carbonpaper,werepreparedforeachsinglePEFCasfollows.Carbonpaper(360µmthick,TGP-H-090 ,TorayIndustryInc.,Japan)was immersed in anaqueousdispersion ofpolytetra-fluoroethylene(PTFE,31-JR,60wt%,DuPont-MitsuiFluorochemicalsCo.,Ltd.,Japan)for2min,driedat60℃for30mininair,andthensinteredat350℃for1min.Thiswet-proofingprocedurewasrepeateduntilaPTFEloadingof 5wt%was reached.TheMPLwas formedon thewet-proofedcarbonpaperas follows.Ultrapurewater,methanol,andthePTFEdispersionweremixedwithcarbonblack(VulcanXC-72R,CabotCorporation,USA).Theweightratioofultrapurewater/methanol/carbonblack/PTFEwascontrolledat10/10/1 .0/0 . 84 .Thecomponentsweremixedunderultrasonicagitation(SU-9THultrasoniccleaner,ShibataScientificTechnol-ogyLtd.,Japan)untilauniformpastewasobtained.Theresultingpastewasrepeatedlycoatedontoone faceofthewet-proofed carbonpaper followedbydryingatambienttemperatureuntilreachingaloadingof10 − 15wt%.Thepaste-coatedwet-proofedcarbonpaperwasthenhot-pressedwithaminiTESTPRESS®-10hot-pressingapparatus(ToyoSeikiSeisaku-sho,Ltd.,Japan)at120℃under1MPafor1minandheatedat350℃inairfor1htoformtheMPLonthewet-proofedcarbonpaper.Thegasdiffusion layer(GDL)of theGDEwascomposedof theMPLandthewet-proofedcarbonpa-per.
TheanodeandcathodeCLsweredepositedovertheGDLsurface.Toprepare thecatalystpaste,500mgofPt-loadedcarbonblackcatalystpowder(Pt/Ccatalyst;46 . 7wt%Pt supportedonhigh-specific-surface-areacarbonblack,TEC10E50E,TanakaKikinzokuKogyoK.K.,Japan)weremechanicallymixedwith2451mgofultrapurewaterand549mgof1-propanol(99.5%,WakoPureChemicalIndustries,Ltd.,Japan)inaplanetaryballmillfor1husingzirconiaballs.Thepreparedsulfonatedcarbon ionomeror the commercialPFSA ionomer(5wt%Nafion®solution;D-521 ,E.I.duPontdeNemoursandCompany,USA)wereaddedintothemixture,whichwasmechanicallyhomogenizedintheplanetaryballmillfor1husingzirconiaballs.Theweightratioofthedrysulfonatedcarbon ionomeror thedryNafion® ionomertoPt in thecatalystpastewasset to1 .ThepreparedcatalystpastewasfourtimesspreadontotheGDLswithasurfaceareaof27cm2.Theobtainedcatalyst-coatedGDLsweredriedat2℃ inair for1h.TheamountofloadedPtwasadjustedto0 . 45±0 .02mgcm−2intheanodeCLand0 . 30±0 .02mgcm−2inthecathodeCL. Themembrane-electrodeassemblies(MEAs)werefabricatedbyhot-pressing thePEMsandwichedbe-tween twoGDEs(anodeandcathode)using thehot-pressingapparatusat120℃under1MPafor1min.ThecommerciallyavailablePFSAmembrane,Nafion®NRE212(50µmthick,E. I.duPontdeNemoursandCompany,USA),andapoly(styrenesulfonicacid)-graftedpoly-ether ether ketone(ssPEEK)membrane 14)(30 µmthick)wereusedasPEMs.ThessPEEKmembranewasdonatedbyIHICorporation,Japan.Thegeometricalsur-faceareasofallelectrodeswere25cm2.TheMEAwasassembledintoasinglePEFCunit,usingcarbonsepara-torswithserpentinegas-channels.
2.4 Fuel cell operation AllofsinglePEFCswereoperatedunderatmosphericpressure,with the setupof the experimental systemused in thisstudyshown inFig.2 .HumidifiedH2andairweresuppliedtotheanodeandcathode,respectively,as reactantgases.The flowratesofH2andairwerecontrolledat500and2000sccm(standardcubiccenti-meterperminute,20℃),respectively,usingtwomass-flowcontrollers.These flowratescorresponded toO2andH2utilizationof21 . 0and34 .8%atacurrentdensity
投稿論文 Paper
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of1Acm−2 .At80℃,eachreactantgaswashumidifiedbybubbling throughahumidifierprior tobeing fedtothesinglePEFC.TheRHsofreactantgasesweresettoequalvalues,controlledbychangingthehumidifiertem-perature.Fortemperaturesabove100℃,eachreactantgaswasmixedwithvaporizedwater(steam)usingava-porizerpriortobeingfedtothesinglePEFC.TheRHsofreactantgasesweresettoequalvalues,controlledbychangingtheamountsofwatersuppliedtothevaporizerbyapump.Bycontrollingthecathodewatersupplyat1.19gmin−1andtheanodewatersupplyat0.30gmin−1,theRHsofH2suppliedtotheanodeandairsuppliedtothecathodewereset to100 ,72 ,and53%at100 ,110 ,and120℃,respectively. Current−voltage(I −V)andIRdropcharacteristicsofsinglePEFCsweremeasuredusinga fuel-cellevalu-ation system(TFC-2000G,TsukasaSokkenCo.,Ltd.,Japan)understeady-stateoperationconditions.TheIRdropcharacteristicsweremeasuredbyapplying100-µs current-off pulses to singlePEFCsand recordingtheresultingpotentialresponses.TheoverallresistancepolarizationwascalculatedusingthemeasuredIR-dropcharacteristics.Thecurrentwasnormalizedbasedontheelectrodegeometricalsurfaceareaof25cm2.TheIRdropdatawas included inthe I −Vcharacteristicsre-portedherein. I −Vcharacteristicswereanalyzed toevaluate thesourcesofpolarization15),withtheoverallactivationpo-
larizationdefinedinEq.(2): ηa=blog(I / I 0 .90) (2)whereηa[V] is theoverallactivationpolarization,b istheTafelslope, I 0 .90[Acm−2]isthecurrentdensityat0 . 90Vand I [Acm−2]isthemeasuredcurrentdensity.TheTafel slopewasestimatedbyapplying the leastsquaresmethodto thecurrentdensitiesatIR-freecellvoltagesbetween0 . 82and0 . 92V.
3.Results and discussion
3.1 Characterization of the prepared sul-fonated carbon ionomer
TheFTIRspectrumofthepreparedsulfonatedcarbonionomer isshown inFig.3 , thecorrespondingIECbe-
Fig. 2 Experimental setup.
Fig. 3 FTIR spectrum of prepared sulfonated-carbon ionomer.
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canbecontrolledbychangingtheweightratioofcarbonblack/3-MPTMS/H2O2. 3.2 Application of the sulfonated carbon
ionomer in CLs I −VandIRdropcharacteristicsofsinglePEFCsweremeasuredat80℃andanRHof100%.Nafion®NRE212wasusedasthePEMandthepreparedsulfonatedcar-bonandNafion®ionomerswereusedintheCLs.TheIECof theused thesulfonatedcarbon ionommerwas0 . 98meqg−1.Figure4(a)showsthe I −VcharacteristicsofasinglePEFCwith(i)thesulfonatedcarbonionomerinbothanodeandcathodeCLs;(ii)sulfonatedcarboniono-merinthecathodeCLandtheNafion®ionomerinthean-odeCL;and(iii)theNafion®ionomerinthecathodeCLandthesulfonatedcarbonionomerintheanodeCL.Forcomparison, the I −VcharacteristicsofasinglePEFCwiththeNafion®ionomerinbothanodeandcathodeCLsarealsoshown inFig.4(a).ThePEFCwiththesulfo-natedcarbon ionomer intheanodeand/orcathodeCLsshoweddecreasedperformance,ascomparedwith thatwiththeNafion®ionomerinbothanodeandcathodeCLs.Figure4(b)showstheactivationpolarizationcharacter-isticsofthesinglePEFCsderivedfromthe I −Vcharac-teristicsshown inFig.4(a).Theactivationpolarizationof thesinglePEFCwiththesulfonatedcarbon ionomerintheanodeand/orcathodeCLsincreasedincomparisonwiththevaluedisplayedbythePEFCwiththeNafion®
ing0 . 98meqg−1.Thepresenceof sulfonicgroupsonthesulfonatedcarbon ionomerwasconfirmedbypeaksat767 ,1012 ,and1257cm−1correspondingtotheSO3Hbond.However, these peakswere not observed forAqua-Black®162 .Thisqualitative studysuggests thatthesulfonatedcarbon ionomercanbesuccessfullypre-paredbytheSHoxidationmethod. Table1showsthesyntheticconditionandtheIECofthesulfonatedcarbonionomer.TheweightratioofAqua-black®162/3-MPTMS/H2O2wascontrolled.The IECequaledto0.98meqg−1ataweightratioof1/0.5/2,2.67meqg−1ataweightratioof1/0 .5/3 ,and4 .37meqg−1ataweightratioof1/1/3 ,respectively. IncreasingtheweightratioofH2O2from2to3promotedtheoxidationofSHgroupstoSO3Hmoieties,increasingtheIECfrom0 .98to2 .67meqg−1,respectively.Increasingtheweightratioof3-MPTMSfrom0 .5to1promotedtheformationofSHgroupsonAqua-black®162 ,andtheIECincreasedfrom2 . 67 to4 . 37meqg−1,respectively.Theseresultsindicate that the IECof thesulfonatedcarbon ionomer
投稿論文 Paper
Table 1 Systhetic condition and IEC of sulufonated carbon ionomer.
SampleWeight ratio IEC
(meq g-1)Aqua-Black Ⓡ 162 3-MPTMS H2O2
A 1 0.5 2 0.98B 1 0.5 3 2.67C 1 1 3 4.37
Fig. 4 Effect of application of sulfonated carbon ionomer (SC) to anode and/or cathode CLs on I−V and activation polarization characteristics of single PEFC at 80 ℃ and RH of 100%: (a) I−V characteristics, (b) activation polarization characteristics.
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ionomer inbothanodeandcathodeCLs.Theresistancepolarizationof thesinglePEFCwiththesulfonatedcar-bon ionomer in theanodeand/orcathodeCLsslightlyincreasedcomparedwiththatofthePEFCwiththeNa-fion® ionomer inbothanodeandcathodeCLs. InsinglePEFCwiththesulfonatedcarbon ionomer intheanodeand/orcathodeCLs,no large increasesofresistancepo-larizationdueto the increased interfacialresistanceoc-curred,despitetheionomerdifferentfromthePEM. These results indicate that the sulfonated carbonionomercanbeusedforbothanodeandcathodeCLsofPEFCsandthattheperformanceofPEFCswiththesul-fonatedcarbonionomerintheanodeand/orcathodeCLsdecreasesduetothe increaseofactivationpolarization,as comparedwith thatof thePEFCwith theNafion®ionomerinbothcathodeandanodeCLsat80℃andanRHof100%.Thedecreaseoftheactivationpolarizationis required to improve theperformanceof thePEFCswiththesulfonatedcarbonionomer.
3.3 Effect of the IEC of the sulfonated car-bon ionomer
I −VandIRdropcharacteristicsofsinglePEFCsweremeasuredat80℃andanRHof100%.Nafion®NRE212wasusedas thePEM,andthesulfonatedcarbon iono-merswereusedinbothanodeandcathodeCLs.TheIECsof theemployedsulfonatedcarbon ionomerswere0 . 98 ,2 .67 ,and4 .37meqg− 1.Figure5(a)showsthe I −V
characteristics of singlePEFCemploying sulfonatedcarbon ionomerswithdifferentIECs.ThePEFCperfor-manceobtained for thesulfonatedcarbon ionomerde-creasedincomparisonwiththatobtainedfortheNafion®ionomerandtheformerslightlyincreasedwithincreasingtheIECof thesulfonatedcarbon ionomer.Figure5(b)shows theactivationpolarizationcharacteristicsof thesinglePEFCusingsulfonatecarbonionomerswithdiffer-entIECs,derivedfromthe I −VcharacteristicsshowninFig.5(a).TheactivationpolarizationofthesinglePEFCwiththesulfonatedcarbon ionomer increasedcomparedwiththatofthePEFCwiththeNafion®ionomerandtheformerslightlydecreasedwith increasing the ionomerIEC.TheresistancepolarizationofthesinglePEFCwiththesulfonatedcarbon ionomerhardlydependedontheionomerIECandwassimilartothevaluesobtained fortheNafion®ionomer. Theseresultssuggest that theprotonconductionbe-tweentheSO3Hgroupsof thesulfonatedcarbon iono-merandthePtparticlesofthePt/Ccatalyst is inferiortothatbetweentheSO3HgroupsoftheNafion®ionomerandthePtparticles.Wepresumethatthisdeferenceoftheprotonconductioniscausedbythecontactbetweenthe ionomerandPtparticles.TheNafion® ionomer iscomposedof thePFSApolymeranduniformlycontactsthePtparticles.Ontheotherhand, thesulfonatedcar-bon ionomer iscomposedof thecarbonblackparticlesanddoesnotuniformlycontact thePtparticles.Thus,
Fig. 5 Effect of IEC of sulfonated carbon ionomer (SC) in both anode and cathode CLs on I−V and activation polarization characteristics of single PEFC at 80 ℃ and RH of 100%: (a) I−V characteristics, (b) activation polarization characteristics.
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75 ,and80℃ ,respectively.Figure6(a)showsthe I −VcharacteristicsofsinglePEFCswiththesulfonatedcar-bonandNafion®ionomers.TheperformanceofthePEF-CswithbothsulfonatedcarbonandNafion® ionomersdecreasedwithdecreasingRH,however, thedecreasesfortheformerwaslargerthanthatforthelatter.Figure6(b)showstheactivationpolarizationcharacteristicsofthesinglePEFCsderivedfromthe I −Vcharacteristicsshown inFig.6(a).Theactivationpolarizationof thesinglePEFCwith the sulfonatedcarbon ionomerwaslargerthanthatofthePEFCwiththeNafion®ionomer.Thevalues forbothPEFCs increasedwithdecreasingRH,theincreasesoftheformerbeinglargerthanthatofthe latter.This indicates that theprotonconductionofthesulfonatedcarbonionomerhighlydependsontheRH
theprotonconductionbetweentheSO3Hgroupsofthesulfonatedcarbon ionomerandthePtparticles ismorepreventablethanthatbetweentheSO3HgroupsoftheNafion®ionomerandthePtparticles.TheseresultsalsosuggestthatincreasingtheIECofthesulfonatedcarbonionomerhardlyimprovesthePEFCperformance. 3.4 Effect of the RH of reactant gases I −VandIR drop characteristics of singlePEFCsweremeasuredat80℃.Nafion®NRE212wasusedasthePEMand thesulfonatedcarbonandNafion® ionomerswereused inbothcathodeandanodeCLs.TheIECoftheusedsulfonatedcarbon ionomerwas0 . 98meqg−1.TheRHofthereactantgaseswassetto40 ,60 ,80 ,and100%byadjustingthehumidifiertemperatureto59 ,68 ,
投稿論文 Paper
Fig. 6 Effect of RH of reactant gases on I−V and polarization characteristics of single PEFC with sulfonated carbon ionomer (SC) in both anode and cathode CLs at 80 ℃ : (a) I−V characteristics, (b) activation polarization characteristics, (c) resistance polarization characteristics.
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ofthereactantgases,andthatthewaterreservecharac-teristicsoftheNafion®ionomeraresuperiortothoseofthesulfonatedcarbonionomer. Figure6(c)showstheresistancepolarizationcharac-teristicsofthesinglePEFCs.Theresistancepolarizationof thesinglePEFCwiththesulfonatedcarbon ionomerwassimilar tothatof thePEFCwiththeNafion® iono-meratanRHof100%.Thevalues forbothPEFCs in-creasedwithdecreasingRH,theincreasesoftheformerbeinglargerthanthatofthelatter.Thelargerincreaseof thePEFCswiththesulfonatedcarbon ionomer isat-tributed to thecharacteristicsofwater transportationfromthecathodeCLto thePEM.InthesinglePEFCswiththeNafion®ionomer,anextensivewatertransporta-tionnetworkispresentinthePEMandtheaboveiono-mer,sincetheyconsistofthesamematerial.Therefore,waterproduced in thecathodeCLduring thecathodereaction iseasily transferred to thePEMthrough theNafion® ionomer,and thePEMdryingat lowRHscanbesuppressed.Conversely, inthesinglePEFCwiththesulfonated carbon ionomer, thewater transportationnetwork in thePEMandsulfonatedcarbon ionomer isnotextensive,becausethePEMandionomerintheCLsconsistofdifferentmaterials.ThewaterproducedinthecathodeCL, therefore, isnoteasily transferred to thePEMthrough the sulfonatedcarbon ionomer, and thePEMdryingatlowRHscannotbesuppressed.
3.5 Effect of PEFC operation above 100 ℃ I −VandIR drop characteristics of singlePEFCsweremeasured at 100 , 110 and 120℃.The ssPEEKmembranewasusedasthePEM,andthesulfonatedcar-bonandNafion®ionomerswereusedinbothanodeandcathodeCLs.TheIECofthesulfonatedcarbonionomerwas0 . 98meqg−1.Figure7(a)showsthe I −Vcharac-teristicsof thesinglePEFCwiththesulfonatedcarbonionomerattemperaturesof100 ,110 ,and120℃andRHsof100 ,73and52%, respectively.TheRHsat110and120℃weresettokeepthesteamamountsconstantatthesaturatedsteamamountat100℃,supposingPEFCoperationunderatmosphericpressure.ThePEFCwiththe sulfonatedcarbon ionomerperformedbetter thanthatwith theNafion® ionomerat temperaturesof100and110℃andRHsof100and73%,respectively.TheseresultsindicatethattheperformanceofPEFCswiththe
sulfonatedcarbon ionomer issuperiortothatofPEFCswiththeNafion® ionomerabove100℃.BoththesinglePEFCwiththesulfonatedcarbonionomerandthatwiththeNafion®ionomercouldnotgeneratepowerat120℃and anRHof 52%,whichwas attributed to the lowwater-reservecharacteristicsofthessPEEKmembraneusedunderthiscondition. Figure7(b)showstheactivationpolarization char-acteristicsof thesinglePEFCsderived fromthe I −Vcharacteristics shown inFig.7(a).Theactivationpo-larizationofthesinglePEFCwiththesulfonatedcarbonionomerdecreasedat100and110℃comparedwiththatof thePEFCwith theNafion® ionomer.Thus, theap-plicationofthesulfonatedcarbonionomertobothanodeandcathodeCLscanpresumablydecreasetheactivationpolarizationabove100℃.ThisindicatesthattheprotonconductivityofthesulfonatedcarbonionomerissuperiortothatoftheNafion®ionomerabove100℃. Figure7(c)showstheresistancepolarizationcharac-teristicsofthesinglePEFCs.Theresistancepolarizationof thesinglePEFCwiththesulfonatedcarbon ionomerwasnearlyequal tothatof thePEFCwiththeNafion®ionomerat100℃andanRHof100%.Ontheotherhand,theresistancepolarizationof thePEFCwith thesulfo-natedcarbon ionomerdecreasedcomparedwiththatofthePEFCwiththeNafion®ionomerat110℃andanRHof73%.Thus, theapplicationof thesulfonatedcarbonionomertobothanodeandcathodeCLspresumablysup-presseswaterdischargefromthePEMattemperaturesabove100℃andRHsbelow100%,whichcontraststheexperimentalresultsdescribedabove.At80℃,theresis-tancepolarizationofthePEFCwiththeNafion®NRE212and thesulfonatedcarbon ionomer increasedwithde-creasingRH.This indicates that thesulfonatedcarbonionomersuppresseswaterdischarge fromthePEMattemperaturesabove100℃andRHsbelow100%.Wepresumethata largeramountof formedwater ispre-servedinthesulfonatedcarbonionomercomparedtotheNafion®ionomer,preventingthePEMfromdryingundertheseconditionsandsuppressingtheincreaseofthere-sistancepolarization. Weused theSHoxidationmethod for theprepara-tionofthesulfonatedcarbonionomers.Thismethodwasused for thesulfonationof theporoussilicamembraneandahydrothermal treatmentwasconductedat170℃
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inordertoincreasesilanolgroupsonthesilicasurface12).TheFT-IR spectra of the porous silicamembranesbeforeandafter thehydrothermal treatmentat170℃showedthatthepeakintensityofsilanolgroupswasin-creasedwithincreasingdurationforhydrothermaltreat-ment.We,therefore,presumethatthesulfonatedcarbonionomerisstableat170℃. Wehaveshowed that sulfonatedcarbon ionomer isavailable toPEFCsasprotonconductorof anodeandcathodeCLs in thispaper.Thesulfonatedcarbon iono-merincreasesPEFCperformanceover100℃comparedwiththePFSAionomer.TheperformanceofthePEFCwith the sulfonatedcarbon ionomerover100℃,how-ever,islowerthanthatwiththePESAionomerat80℃.
We suppose that the performance of the sulfonatedcarbon ionomermustbe improvedfurtherbystudyingsurfacetreatmentofcarbon,alternativecarbonmaterial,andsoon.
4.Conclusion
Thesulfonatedcarbon ionomercanbepreparedbyusingtheSHoxidationmethod,anditsIECcanbecon-trolledbychangingtheweightratioofcarbon-black/3-MPTMS/H2O2.Thesulfonatedcarbon ionomercanbeused forbothanodeandcathodeCLsofPEFCs.TheperformanceofPEFCswiththesulfonatedcarbon iono-merdecreasesincomparisontothatofPEFCswiththe
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Fig. 7 Effect of operation above 100 ℃ on I−V and polarization characteristics of single PEFC with sulfonated carbon ionomer (SC) in both cathode and anode CLs: (a) I−V characteristics, (b) activation polarization characteristics, (c) resistance polarization characteristics.
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PFSAionomerat80℃andanRHof100%,andhardlydependson the IECof thesulfonatedcarbon ionomer.Theperformanceof thePEFCswithboth sulfonatedcarbonandPFSAionomersat80℃decreasedwithde-creasingRH,however,thedecreasesfortheformerwaslarger than that for the latter.On theotherhand, theperformanceofPEFCswiththesulfonatedcarbon iono-merissuperiortothatofPEFCswiththePFSAionomerattemperaturesabove100℃andRHsbelow100%.WeconcludethatPEFCswiththesulfonatedcarbonionomercangeneratepowerandachievehigherperformancecomparedwith thatof thePFSA ionomerat tempera-turesabove100℃andRHsbelow100%.
Acknowledgement TheauthorsthankIHICorporation,Japan,forthedo-nationofthessPEEKmembraneandalsothankMs.EmiSindouofNanotechnologyResearchCenter,TokyoCityUniversityforherhelpinFTIRmeasurements.
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