稿 論文 Paper - FCDIC · 2018. 10. 31. · 2．2 Characterization of the sulfonated ... Fig.1...

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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

投稿論文　Paper

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.

91燃料電池　Vol.18　No.2　2018

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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