ContentslistsavailableatScienceDirectPolymerjournalhomepage:www.elsevier.com/locate/polymerAlkalinesolidpolymerelectrolytemembranesbasedonstructurallymodifiedPVA/PVPwithimprovedalkalistabilityJinliQiaoa,*,JingFub,c,RuiLinc,JianxinMac,JiansheLiuaaCollegeofEnvironmentalScienceandEngineering,DonghuaUniversity,2999Ren’minNorthRoad,Shanghai201620,ChinaSchoolofResourceandEnvironmentalEngineering,EastChinaUniversityofScienceandTechnology,MelongRoad130,Shanghai200237,ChinacCleanEnergyAutomotiveEngineeringCenter,TongjiUniversity,CaoanRoad4800,Shanghai201804,ChinabarticleinfoArticlehistory:Received6March2010Receivedinrevisedform5July2010Accepted9August2010Availableonline14August2010Keywords:AlkalineelectrolytemembranePVA/PVPblendingIonicconductivityabstractNovelalkalinesolidpolymerelectrolytemembranesthatcanconductanions(OHÀ)havebeenpreparedfrompoly(vinylalcohol)/poly(vinylpyrrolidone)(PVA/PVP)byblendingandchemicalcross-linking,fol-lowedbydopinginaqueousKOHsolution.ThephysicochemicalpropertiesofthesemembraneshavebeenstudiedindetailbyFTIR,TG,andSEManalyses.TheionicconductivitywasfoundtobegreatlydependentontheconcentrationofKOHandtheinterpenetratedPVPinthePVAmatrix.Amaximumconductivityofupto0.53ScmÀ1atroomtemperaturewasachievedforPVA/PVPinamassratioof1:0.5afterdopingin8MaqueousKOHsolution.Themembraneshowedperfectalkalinestabilitywithoutlosingitsintegrityevenuponexposureto10MKOHsolutionatupto120C.Scanningelectronmicrographsrevealedahighlyorderedmicrovoidstructureuniformlydispersedonthemembranesurfacewithaporesizeofca.200nmafterheat-curing,whichimpartedthemembranewithgoodliquidelectrolyte(KOH)retentionability.FTIRspectrashowedthatthesehighionicconductivitiesmaybeattributedtothepresenceofexcessfreeKOHinthepolymermatrixinadditiontoKOHboundtothepolymer.Almostconstant,highlystable,ionicconductivitywhilemaintainingmechanicalintegritywasretainedatroomtemperatureformorethanonemonth.Ó2010ElsevierLtd.Allrightsreserved.1.IntroductionSolidpolymerelectrolytefuelcells(SPEFCs)basedonproton-conductingpolymershavebeenidentifiedaspromisingpowersourcesforstationaryandresidentialapplications[1].InSPEFCs,theion-exchangemembranesplayavitalroleinobtainingnotonlyhighionicconductivitybutalsothermalandmechanicalstabilities.NafionÒ,aperfluorinatedionomerdevelopedbyDuPont,isthecommondenominatorinthistechnology.However,thisstilllimitswidespreadapplicationduetoitshighcost,unstablepropertiesathightemperatures,highmethanolpermeability,anddifficultiesinsynthesisandprocessing[2].Ontheotherhand,alkalinesolidpolymerelectrolytes(ASPEs)findamarketnicheinanumberofapplications,suchasinbatteries[3,4],supercapacitors[5],andalkalinemembranefuelcells(AMFCs).Forexample,whentheoperationalenvironmentisalkaline,AMFCshavemanyadvantages,including:(i)fasterkineticsofoxygenreductionreactionsinanalkalinemedium,whichallowstheuseofnon-nobleandlowcost*Correspondingauthor.Tel./fax:þ862169583891.E-mailaddress:qiaojl@dhu.edu.cn(J.Qiao).0032-3861/$eseefrontmatterÓ2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.polymer.2010.08.018metalelectrocatalystssuchassilverandnickel;(ii)improvedwatermanagementsincetheelectro-osmoticdragtransportswaterawayfromthecathode,and(iii)loweredalcohol‘crossover’problemduetohydroxideiontransportfromcathodetoanode.Uptonow,somepromisingalkaline-exchangemembranes(AAEMs)havebeenevaluatedforuseinAMFCs.Here,themembraneshavebeenconstructedmainlyfromcopolymers,thequaternizedcomonomersofwhichfeatureananionasthechargecarrier,suchaspolysiloxane[6],poly(oxyethylene)methacrylates[7],polysulfone[8],polyethersulfonecardo[9],poly(phthalazinoneethersulfoneketone)[10],poly(ether-imide)[11],andradiation-graftedPVDFandFEP[12].Unfortunately,thequaternizedpolymerisunstableinalkalinemediaattemperaturesabove60CandathighKOHconcentrations[13e17].Generally,manyorganiccompoundsaremorestableunderacidicconditionsthanunderalkalineconditions.Thechemicalstabilityofcation-exchangemembranescanbeimprovedbyperfluorinatingthepolymerback-bone,asinthecaseofNafionÒmembrane.However,thequaternaryammoniumgroups,whicharefunctionalgroupsintheAAEMs,decomposeinconcentratedalkalisolutionsviatheHofmanndegradationreaction[18].Duetothecausticenvironmentinalka-linefuelcells,thealkalineelectrolytemembranemustbechemicallyJ.Qiaoetal./Polymer51(2010)4850e48594851
andthermallystable[19,20].Therefore,thedevelopmentofAAEMswithimprovedalkalinestabilityisstillanurgentneed[19,20,25].AnotherkindofASPEsconsistofaneutralpolymerdopedwithaninorganicbase.Thistypeofpolymerelectrolytehasanumberofdistinctcharacteristics,suchaseasypreparation(justdippinginalkalimetalhydroxide,KOH),lowcost,abundanceofthebasiccomponents,andhighionicconductivityascomparedwithothersolidpolymerelectrolytes[21].ASPEshavealsobeenusedinstudiesconcerningapplicationtoAMFCsandshowedpromisingpowerperformance,suchasinanalkalinedirectmethanolfuelcell(ADMFC)[22e24]andanalkalinedirectethanolfuelcell(ADEFC)[13].PVAisapolyhydroxypolymerthatisverycommonlyusedinpracticalapplicationsbecauseofitseasypreparationandbiode-gradability[26].Also,PVAhasfilm-formingcapacity,hydrophilicproperties,andahighdensityofreactivechemicalfunctionsthatarefavorableforcross-linkingbyirradiation,chemical,orthermaltreatments[27].PVP,asahydrophobicizerandstabilizer,cangreatlyimprovemembraneoxidativestabilityandchemicalstability[28].Basedonthisconsideration,wereporthereinanovelalkalinesolidpolymerelectrolytemembranecomposedofKOH-dopedpoly(vinylalcohol)/poly(vinylpyrrolidone)(PVA/PVP).ThemembranestructureisfinishedbyblendingPVAandPVP,andthenchemicalcross-linkingofthehydroxylgroupsofPVAwithacetalringformationbyusingglutaraldehydeasacross-linker.Themembraneproperties,suchasionicconductivity,swellingbehavior,wateruptake,membranemicrostructure,strongalkalinetoleranceathigh-temperature,thermalstabilityandlong-termdurabilityhavebeeninvestigatedsystematicallyinordertopursueamembranethathaspotentialforelectrochemicalapplications,suchasinAMFCs.2.3.AlkalinePVA/PVPmembranepreparationFig.1schematicallyillustratesthepreparationofchemicallycross-linkedalkalinePVA/PVP.Toclarifytheconductingmecha-nismsofKOH-dopedPVA/PVP,themembraneswerepreparedinthefollowingtwoways.(i)Themembraneswererenderedcon-ductingbyimmersionofPVA/PVPinKOHsolutionatvariousconcentrationsandequilibratedforatleast24h.Atthisstage,PVA/PVPwasdopedwithKOH(PVA/PVP/KOH),andthenthemembranesweretakenoutanduseddirectlyforionicconductingmeasurementswithoutanypost-treatment.(ii)Aftercompletionofthedoping,themembranesweretakenoutandrinsedrepeatedlywithdeionized(D.I.)watertoremovetheadsorbedKOHontheirsurfaces,andthenstoredinD.I.waterforfinalionicconductivitymeasurements.Twotypesofmembraneswerethusprepared,whichwedenoteasPVA/PVP/KOH-d(d¼doped)andPVA/PVP/KOH-r(r¼rinsed),respectively.AmodeloftheinnerstructureofPVA/PVP/KOHandtypicalmembraneimagesaregiveninFig.2.2.4.MeasurementofionicconductivityandswellingbehaviorofthemembranesTheOHÀionicconductivityoftheformedmembraneswasmeasuredbyanACimpedancetechniqueusinganelectrochemicalimpedanceanalyzer(VMP2/Z,PAR),wheretheACfrequencywasscannedfrom100kHzto0.1Hzatavoltageamplitudeof100mV.FullyhydratedmembranesweresandwichedinaTeflonconduc-tivitycellequippedwithPtfoilcontacts[28].Themembranewasincontactwithwaterthroughoutthemeasurements.Ionicconduc-tivity(S/cm)wascalculatedaccordingtothefollowingequation:s¼l/(RTW),wherelisthelengthofthemembranebetweentwopotentialsensingplatinumwires,Risthemembraneresistance,andWandTarethewidthandthethicknessofthemembrane,respectively.Thetemperaturedependenceofionicconductivitywasdeterminedbycontrollingthetemperaturefrom25to80C.Thecellwassealedwithwaterthroughoutthedurationofthemeasurements.2.Experimentalsection2.1.MaterialsandmembranepreparationThemembraneswereformedbyasolution-castingmethod.AstockPVA(99%hydrolyzed,averagemolecularweightMw¼86,000e89,000;Aldrich)aqueoussolutionwaspreparedbydissolvingPVA(50g)indistilledwater(500mL)andthenheatingat90Cwithcontinuousstirringuntilatransparentsolutionwasobtained.Thepolymericadditive,PVP,waspreparedseparately(Mw¼40,000;Aldrich).Appropriateamountsofthetwosolutionswerethenmixedintheselectedblendratios,1:0,1:0.25,1:0.5,1:1,and1:2PVA/PVPbymass.Theresultingmixturetookonahomo-geneousandtransparentappearance.Afterremovaloftheairinvacuo,theresultingsolutionswerepouredintoplasticPetridishes,andwaterwasevaporatedunderambientconditions.Whenvisu-allydry,themembranewaspeeledfromtheplasticsubstrate,andhadathicknessofabout60e80mm.2.2.Chemicalcross-linkingofPVA/PVPSamplesofsquarepiecesofmembranes(ca.1.5Â2cm)weresoakedinareactionsolutioncontaining10mass%glutaraldehyde(GA)(25wt.%solutioninwater;ShanghaiGuoyao)inacetoneat30Cfor60min.Cross-linkingproceededbetweentheeOHofPVAandtheeCHOofGAinthemembraneduetoanacid-catalyzedreactionuponadditionofasmallamountofHCltothesolution.PVPwasthusinterpenetratedintothePVAnetworkduetoincreasedcross-linkingdensity.Transparent,flatmembraneswereobtainedwithathicknessoftheorderofseveraltensofmicrometers(60e80mm).Thethicknessofthemembranescouldbeeasilycontrolledbyadjustingthevolumeofthesuspension.Fig.1.SchematicdiagramforthepreparationofalkalinePVA/PVPcompositebyblendingandchemicalcross-linking.
4852J.Qiaoetal./Polymer51(2010)4850e4859
Fig.2.InnerstructuremodelofKOH-dopedPVA/PVP(A)andMembranepicturesfor(B)PVAconditionedin4MKOHat25C,(C)PVAconditionedin4MKOHat80C,(D)PVA/PVPconditionedin10MKOHat25C,(E)PVA/PVPconditionedin10MKOHat80Cand(F)PVA/PVPconditionedin10MKOHat120C.Conditiontime:24h,fol-lowedbycompleteremovaloffreeKOHpriortotesting.PVA/PVP¼1:1inmassratio.
Theswellingofthemembraneswasevaluatedbyboththewateruptake(WU)andthemethanoluptakefromthemasschangebeforeandaftercompletedrynessofthemembrane.Adrymembranewasswollenindeionizedwaterforaday,thenthesurfacewaterwascarefullywipedwithafilterpaper,anditwasimmediatelyweighed.Afterdryingthesampleovernightat60C,thewateruptake(WU),wascalculatedusingtheexpression:WU¼(WwetÀWdry)/Wdry,whereWwetandWdryarethemassesofthefullyhydratedmembraneandofthedrymembrane,respec-tively.Themethanoluptakewasmeasuredbythesameprocedure.2.5.CharacterizationofPVA/PVP/KOHmembranesThemembraneswereexaminedbyFourier-transforminfrared(FTIR)spectroscopyinordertocharacterizetheirmolecularstruc-turesafteralkalinedoping.IRspectrawererecordedonanFTIR-4200spectrometer(Shimadzu)withawavenumberresolutionof4cmÀ1intherange400e4000cmÀ1.Samplesintheformofthin0.6a0.50.40.30.20.10.00.00.51.01.52.0
Mass ratio of PVA/PVP
0.6b0.50.40.30.20.10.0024681012Concentration of KOH / mol L
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Fig.3.(a)IonicconductivityofPVA/PVP/KOH-dasafunctionofPVPcontent.(b)ConductivitychangeswithdopingKOHconcentrationinaqueoussolution.(a)DopingKOHconcentration:4M.(b)Polymercomposition:PVA/PVP¼1:0.5inmass.Dopingtime:24h.
filmsweresandwichedbetweentwoKBrplatesandplacedinthecelltobemeasured.Airwasemployedasabackgroundreference.ThecompositemorphologywasevaluatedusinganFEISirion200field-emissionscanningelectronmicroscope(SEM)operatingat5kV.Priortoobservations,themembranesampleswerefrac-turedinliquidnitrogenandsputteredwithgold,thenexaminedat4000Â,6000Â,and10,000Âmagnifications.Theelementdistri-butioninthecross-sectionwasdeterminedbymeansofanOxfordInstrumentsX-rayMicroanalysisINCA,operatingat20kVwithadatacollectiontimeof10min.2.6.ThermalanalysisThermalanalysis(TGA)ofthepolymermembraneswasper-formedusingaSTA449Capparatus(NETZSCH).Samplesofabout10mgwereloadedintoanaluminapan,thenheatedfrom25to600Catarateof10CminÀ1.Allmeasurementswereconductedundernitrogen.Thevacantaluminapanwasusedasareferencethroughoutthewholeexperiment.3.Resultsanddiscussion3.1.IonicconductivityandwateruptakeThealkalinePVA/PVPblendmembranesappearedtransparentandhomogeneouswithmechanicalflexibility.WhenimmersedinKOHsolution,themembranebecameorangeincolor,butitalmostJ.Qiaoetal./Polymer51(2010)4850e48594853
86420PVAPVA/PVP (1:0.25)aexcessKOHtoremainfixedintheinterspacesofthepolymermatrixtogetherwithwater,resultinginhigherionicconductivity.Similartrendswereobservedfortheconcentrationdependence,wheretheionicconductivityofPVA/PVPfirstincreasedtoamaximumof0.53ScmÀ1astheKOHconcentrationwasincreasedfrom1to8molLÀ1,andthendecreasedwhentheKOHconcen-trationexceeded10molLÀ1(Fig.3(b)).ThedecreasingtrendintheionicconductivitywithhigherKOHconcentrationisacommonphenomenonobservedinpolymerelectrolytesandcanbeexplainedintermsofweakionicmobilitysincethedegreeof2.52.01.51.00.50.02.5
PVA:PVP1:0.251:0.51:11:21Ma2.01.51.00.50.0
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-0.5
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Concentration of KOH / mol L
2M6M10Mbs / S cm X 10Fig.4.Ionicconductivityof(a)PVA/KOH-rand(b)PVA/PVP/KOH-rasafunctionofdopingKOHconcentrationinaqueoussolution.Dopingtime:24h.
3-12.01.51.00.50.0
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decoloredcompletelyuponrinsinginD.I.water.Fig.3(a)showstheionicconductivityofPVA/PVPdopedwith4MKOH,thatis,PVA/PVP/KOH-dasafunctionofPVPcontent,wherethePVA/PVPmassratiorangedfrom1:0to1:2.Itcanbeseenthattheionicconduc-tivityofthemembraneatroomtemperaturedependedonthePVPcontent.Theionicconductivityincreasedfrom0.023ScmÀ1forPVA/PVPata1:0massratiotoamaximumof0.25ScmÀ1forPVA/PVPata1:0.5massratio.ThiscanbeattributedtoanincreaseinthenumberofchargecarriersinthePVApolymermatrixduetotheinterpenetratingPVPdomain.DuetothehydrophilicnatureofPVP,thepolymerwouldbeswollenwellinwater.Hence,itallowsthe3.0
1M2.5
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/ S cm X 10-11.51.00.50.0-0.5100806040
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3Water uptake / %Temperature / C
Fig.6.IonicconductivityofPVAandPVA/PVPcompositeafterKOHconditioningatelevatedtemperatures.Themembraneswereconditionedin(a)1,4,10MKOHforPVA,(b)2,6,10MKOHforPVA/PVP(1:0.25inmass),(c)1,4,10MKOHforPVA/PVP(1:1inmass)atelevatedtemperaturesfor24h,followedbycompleteremovaloffreeKOHpriortoconductivitytesting.Theconductivitywasmeasuredat25C.
o
Mass ratio of PVA/PVP
Fig.5.IonicconductivityandwateruptakeofPVA/PVP/KOH-rasafunctionofPVPcontent.Polymercomposition:PVA:PVP¼1:1inmass.DopingKOHconcentrationinsolution:8M.Donditiontime:24h.
4854J.Qiaoetal./Polymer51(2010)4850e4859
Fig.7.FTIRspectraregisteredforPVA/PVPcomposite(a)withoutKOHconditioning(b)conditioningin6MKOHat25C(c)conditioningin6MKOHat60C.Polymercomposition:PVA/PVP¼1:1inmass.
freedomofiontransportisreducedassomuchKOHentersintothepolymermatrix[29].ComparedtothePVAmembrane,theionicconductivityofthePVA/PVP(1:0.5bymass)increasedfrom0.023to0.53ScmÀ1at25C.ThesvalueforthePVA/PVP(1:0.5bymass)membranewasca.20timesthatofthePVAmembrane.ItseemsthatourPVA/PVP(1:0.5bymass)membraneisahighlyionicconductingASPEMincomparisonwithrecentoutstandingresults[22À25].Here,PVPwasalsonotedtohaveagreatinfluenceonthemembranemechanicalstability.ThemembranebecamebrittleandeasilybrokenwhenthePVA/PVPratiowashigherthan1:1.5bymass.Therefore,itcanbeconcludedthatPVPfunctionedbothasaplas-ticizerinstabilizingthepolymermembraneandasacomplexingreagentimpartingahighsvalue.InordertoclarifyanyinteractionsbetweenthedopantKOHandPVA/PVP,Fig.4showsthechangeintheionicconductivityofPVA/PVP/KOH-rwithdopantKOHconcentrationinsolution.Itcanbeseenthattheionicconductivitymeasuredat25CbyACimpedancespectroscopyreachedamaximumvalueof4.73Â10À4ScmÀ1forPVAafterKOHdopingfromanaqueoussolutionof2e4molLÀ1.Thisconductivityvaluegreatlyincreasedto7.73Â10À4ScmÀ1forPVA/PVPblendata1:0.25massratio,andfurtherincreasedto1.77Â10À3ScmÀ1ata1:1massratioafterKOHdopingfromanaqueoussolutionof6e8molLÀ1.AlthoughthesvaluesofPVA/PVP/KOH-r(Fig.4)areratherlowcomparedwiththatofPVA/PVP/KOH-d(Fig.3),theyareofthesameorderastheionicconductivityobtainedbydirectlymixingaviscousPVAsolutionwithaconcen-tratedaqueousKOHsolution,asreportedelsewhere[30].ThisclearlydemonstratedthatsomeKOHwastakenintothepolymermatrix,althoughtheadsorbedKOHonthesurfaceofthemembranewasremovedafterrepeatedrinsingwithD.I.water.Evidently,theionicconductivityofPVA/PVP/KOH-rstemsmainlyfromthe“bond”KOHthroughchemicalinteractionssuchashydrogenbondingandinductionforcesbetweenC]OgroupsonPVPandKOHtogetherwithCeOandOHgroupsofPVAandtheKOHdopant.Onthecontrary,forPVA/PVP/KOH-d,thehighionicconductivitystemsmainlyfromthecontributionofexcess“free”KOHadsorbedonthemembranesurface.Itisreportedthatalkali-dopedPVAwithvariousadditives,suchasHAP[24],SSA[23],orPAA[25],andTiO2filler[22],amongothers,givesmuchhigherionicconductivitiesinexcessof10À2ScmÀ1.SincethemembraneswerepreparedbydirectlyaddingKOHtoPVAsolution[22]orbyimmersingthemembraneinaqueousKOHsolution[23À25]withoutanypost-treatmentsuchasrinsingwithD.I.water,itisconcludedthatthehighionicconductivityvaluesintheaboveworkwereattributedtoexcess“free”KOHadsorbedonthemembranesurface,asforPVA/PVP/KOH-dinthiswork.AsinthecaseofPVA/PVP/KOH-d,anadditionaldopingconcentrationofKOH,i.e.,largerthan4MforPVAor8MforPVA/PVP,ledtoadecreaseintheionicconductivityofPVA/KOH-rorPVA/PVP/KOH-r.Thatis,anadditionalhigherdopingKOHconcentrationinsolutiondoesnotsimplymakeafurthercontri-butiontotheconductivity.ThemembranesamplesarefoundtobesupernatantathighdopingKOHconcentration(>8M)insolution.ItseemsthatmoreOHÀcouldnotbetakenintothepolymerduetotheweakionicmobility(owingtotheformationofion-pairsorincreasedviscosity),thusleadingtoadecreaseinionicconductivity[29].ThedecreaseinionicconductivityseenwhenthePVA/PVPratioexceeds1:1bymassmaystemfromthehighwateruptakeofthemembraneduetotheincreasedhydrophiliceffectofPVP.Inordertoseethistrend,theionicconductivitydatainFig.4,obtainedatatypicaldopingKOHconcentrationof8M,andthewateruptakehavebeenplottedagaininFig.5asafunctionofPVPcontentinthepolymer.ItcanclearlybeseenthatthewateruptakeinthemembraneincreasedsharplywithincreasingPVPcontentandexceeded120%atPVA/PVP¼1:2bymass.Largersorptionofwatermaybeduetogreaterflexibilityofthepolymerchainsorthemicrostructureofthepolymer,whichwouldallowmorewatertoresidebetweenthepolymerchains.This,togetherwiththehydrophiliccharacterofPVP,resultsinlargesorptionofwaterathighPVPcontents,whichdoesnotsimplygiveimprovedionicconductivitybutratheradilutionofchargecarriers[28].3.2.ChemicalstabilityThechemicalstabilityofalkalineelectrolytemembranesisrecognizedasakeyfactorthataffectstheirelectrochemicalappli-cations,especiallyinalkalinemediaattemperaturesabove60CandathighKOHconcentrations[14,15,18,19].ThechemicalstabilityofthemembraneswasassessedbyimmersingPVAandPVA/PVPindifferentconcentrationsofKOHatelevatedtemperaturesfor24h.Thisexperimentwasthusdesignedtotestthetoleranceofthemembranetobasetreatmentsatelevatedtemperatures[11].AftercompleteremovalofthefreeKOHonthemembranesurface,theionicconductivityofthemembraneswasmeasuredatroomtemperature.Fig.6showstheresultsofthetypicalrunsatthreeconcentrationseach,specifically1,4,and10MKOHforPVA,2,6,and10MKOHforPVA/PVPat1:0.25bymass,and1,4,and10MKOHforPVA/PVPat1:1bymass,respectively.ItisnotedthateachofthetestedmembranesdisplayedanincreasedconductivitywithincreasingKOHconcentrationatelevatedtreatmenttemperatures.ThePVAsoakedin10MKOHshowedarapidincreaseinionicconductivity,andexhibiteditshighestconductivityof9.77Â10À4ScmÀ1whentreatedat80C(Fig.6(a)).Byincorpo-rationofasmallamountofPVP,i.e.,atPVA/PVP¼1:0.25bymass,membranessoakedineither6.0or10MKOHshowedarapidincreaseinionicconductivity,attainingmaximumvaluesof1.61and2.09Â10À3ScmÀ1,respectively,afterconditioningat80C(Fig.6(b)).Similarly,anincreasedionicconductivitywasobservedforPVA/PVP¼1:1bymassafterconditioningthemembranein10MKOHat80C(Fig.6(b)).ItisknownthatAAEMsarefrequentlylessstablesincebasicgroupsareinherentlylessstablethanacidicgroups[15,18,19,26].Becauseofthis,purelyquaternizedpolymersareonlystableoveralimitedrangeofKOHconcentrations,typicallyupto0.5or1.0Mat80C.TheyareknowntodeteriorateataKOHconcentrationofjust2.0Mattemperaturesaslowas60C[11]oreveninpurewateratJ.Qiaoetal./Polymer51(2010)4850e48594855
Fig.8.SEMpicturesofthecutviewof(a)PVAmembraneconditionedin10MKOHatr.t.,(b)PVAmembraneconditionedin4MKOHat80C,(c)PVA/PVPcompositeconditionedin10MKOHatr.t.,(d)PVA/PVPcompositeconditionedin10MKOHat120C,(e)surfacepictureofPVAmembraneconditionedin10MKOHat80C,(f)surfacepictureofPVA/PVPconditionedin10MKOHat120C.Conditiontime:24h,followedbycompleteremovaloffreeKOHpriortotesting.PVA/PVP¼1:1inmass.
80C[10],whereupontheionicconductivityisgreatlydecreasedorcanevennolongerbemeasured.HybridPEO/SiO2membraneshavealsobeenfoundtoundergoseriousdegradationuponimmersioninastronglyalkalinesolution[31].However,nodecreaseinionicconductivitywasobservedforPVA/PVPafterimmersioninastronglyalkalinesolutionatelevatedtemperatures.ThisdemonstratestheperfecttoleranceoftheresultingPVA/PVPtobasetreatment,whichmaybeattributedtothehighlycross-linkedPVAnetworkandentrappedPVPinthePVAmatrix.Therefore,theadditionofPVPnotonlyimprovestheionicconductivitybutalsothechemicalstabilityofthemembranes.3.3.FTIRToclarifytheconductingmechanismsofalkalinePVA/PVPmembranes,FTIRspectrawererecordedinthewavenumberregionbetween4000and500cmÀ1,whichcoversthewholerangeofallofthecharacteristicIRvibrations.Fig.7presentstypicalFTIRspectrameasuredforPVA/PVPcomposite(1:1bymass),withandwithouthigh-temperatureconditioning,afterdopingin6MaqueousKOHsolution.Allofthespectrashowanintensebandbetween3100and3750cmÀ1,whichisascribedtothestretchingvibrationofOHgroupsfromtheboundwater.Thebandsat2894and2954cmÀ1arisefromthestretchingofCH3e,eCH2e,andCHegroups.Althoughthestretchingabsorptionbandat1557cmÀ1,whichisattributedtotheIIbeltn(CeN)ofPVPcouldnotbeunambiguouslyassigned,asharppeakat1672cmÀ1,whichisascribedtotheIbeltn(C]O)absorptionfromPVP,wasclearlyobserved.Thepresenceofcross-linkinginthemembranehasbeenprovenbyIRmeasurements,wheretheappearanceofasharpabsorptionbandat1718cmÀ1(C]O)wasobservedduetothe‘free’eCHOattachedtoGA.Thisindicatesreducedaccessibilitytothereactivegroupscausedbyanincreaseinthenetworkdensity[28].Interestingly,afterconditioningthecompositemembraneinKOHsolution,thepeakat1718cmÀ1dis-appearedandanewcharacteristicabsorptionpeakcenteredat1571cmÀ1appearedandintensifiedsignificantlywithtreatment4856J.Qiaoetal./Polymer51(2010)4850e4859
Fig.9.EDXmappingof(a),(b),(c)potassiumelementand(d),(e),(f)oxygenelementwithinKOH-dopedPVA/PVPcomposite,respectively.DopingKOHconcentrationinsolution:10M;(a),(d):conditionedatr.t.;(b),(e):conditionedat80C;(c),(f):conditionedat120C.Conditiontime:24h,followedbycompleteremovaloffreeKOHpriortotesting.
temperature.Sinceadisproportionationreactionofthe‘free’eCHOoccursunderalkalineconditions,whichproducesthealcoholandcarboxylicacid[C]O(eOeH)],the[C]O(eOeH)]willfurtherreactwithKOHtoproducecarboxylate[C]O(eOeK)].Hence,thebandat1571cmÀ1bandisassignedastheasymmetricalstretchingvibrationabsorptionof[C]O(eOeK)].Therefore,itisconcludedthattheabsorbentcapacityofthePVA/PVPcompositeforKOHishighlyimprovedafterhigh-temperaturetreatment,andthatthehigherionicconductivityisduetoincreasedchargecarriersinPVA/PVP.3.4.MembranemorphologiesFig.8showsSEMphotographsofthecross-sectionalviewsofPVA/PVP/KOH-r.ThesewereobtainedtoassesswhetherthechangeininitialliquidelectrolyteabsorbentcapacityisrelatedtotheionicconductivityincrementwiththeadditionofPVP,inparticular,afterbasetreatmentatelevatedtemperatures.Forcomparison,thecorrespondingSEMphotographsofPVA/KOH-rarealsoprovided.ItcanbeseenthatSEMobservationofthesectionsofamembranerevealsahomogeneousanddensematerial.ItwasfoundthatbyincreasingthedopantKOHconcentrationinsolutionand/orbyconditioningatelevatedtemperatures,anorangecolorwasinducedinthemembranes,asshowninFig.2(b).However,nosurfacedegradationormembranedamage,suchastheappearanceofholesorphase-separationphenomena,couldbedetected.ThestructureisseentobeverycompactintheSEMimages(Fig.8(a)and(b))forPVA/KOH-rconditionedeitherathighKOHconcentration(10MKOH)orat80C.Moreover,nomorphologychangeswereobservedforPVA/PVP/KOH-rmembraneafterconditioninginKOHsolutionsofdifferentconcentrationsat25C(Figs.2(b)and8(c)).ItshouldbementionedthatPVPhasbeenreportedtobeusedasapore-formingagentsinceitleachesoutduringmembranecasting,leavingporestypicallyrangingfrom1to10mm[32À34].Whenthemembraneisimmersedinanelectrolytesolution,thesolutionpenetratesintotheporousnetwork,whichmakesthemembraneionicallyconductive.However,inthepresentwork,therewasnolossofPVPduringcasting.Therewereneithergapsnorcracksinthepolymermatrix;rather,thePVA/PVP/KOH-rmembraneexhibitedaglassyanddensecross-sectionalmorphology,evenafterconditioningathighKOHconcen-tration(10M)inaqueoussolution(Fig.8(c)).Inaddition,theflexibilityofthemembraneswasremarkablyimprovedafterKOHdoping.AnintriguingresultisthatacompactstructureofthePVA/KOH-rmembranepersistedafterhigh-temperatureconditioning,whileconditioningofPVA/PVP/KOH-rat120CledtoamicrovoidJ.Qiaoetal./Polymer51(2010)4850e48594857
100aWeight loss / wt.%806040200isingoodagreementwiththeconductivityobtainedfromFig.6andtheFTIRresults,whichfurtherdemonstratesthegoodinitialliquidelectrolyteretentionabilityofthemembraneaftertemperatureconditioning.3.5.Thermalstability(TGAanalysis)Fig.10showsthethermogravimetricanalysiscurvesofPVA/PVP/KOH-rmembraneswithdifferentPVPcontentsandheat-conditionedatelevatedtemperatures,alongwiththoseofPVA/KOH-rforcomparison.Themembranesamplesweredriedat110CpriortoTGanalysis.Allofthesamplesdisplayedthreemajorweightlossstagesataround80e200,230e380,and400e450C,followedbythefinaldecompositionofthepolymermatrix.ForthesamplesexposedtoairpriortoTGanalysis,theweightlossesinthefirst,second,andthirdstagescanberespectivelyattributedtotheexpulsionofwatermoleculesfromthepolymermatrixorthemoistureabsorbedfromtheair,thedecompositionofhydroxidegroups,andthesplittingofthemainchainofPVAfollowedbydecompositionofthepolymerbackboneabove450C.Comparedwithorganicanion-exchangemembranes,forwhichthedegradationofthequaternaryammoniumgroupsoccursatamuchlowertemperature(150e190C)[10],ourmembranesshowgreatlyenhancedthermalstability.FurtheranalysisbyD-TGArevealedthatwithincreasingPVPcontentinthepolymer,theonsetdegradationtemperaturewasincreasedbothforthemembraneswithoutKOHdoping(from235to280C)(Fig.10(a))andforthosedopedwithKOH(from200to238C)(Fig.10(b)).TheweightlossofthePVA/KOH-rmembraneconditionedin4MKOHat80C(47e48%)(Fig.10(b)andTable1)isalmostasthesameasthatofthemembraneconditionedatr.t.,whichillustratesthehighthermalstabilityofthePVAnetworkstructure.Inaddition,themembranethermalstabilityisgreatlyimprovedbytheadditionofPVP,suchthattheweightloss(31e32%)ismuchreducedincomparisonwiththatfromPVA/KOH-r(47e48%)atthemaximumthermaldecom-positiontemperatureintherange230e380C(Fig.10(b)andTable1),evenwhenaPVA/PVP(1:1bymass)membrane,forexample,wasconditionedin10MKOHat80C.ThisconfirmsthattheincorporationofPVPiscriticaltotheimprovementofthemembrane’sthermalstability. PVA PVA/PVP(1:0.25) PVA/PVP(1:0.5) PVA/PVP(1:1)100bWeight loss / wt.%806040200 PVA/KOH(4M)o PVA/KOH(4M,80C) PVA/PVP/KOH(10M)o PVA/PVP/KOH(10M,80C)0100200300400o
500600Temperature / C
Fig.10.TGprofilesofPVA/PVPcomposite(a)withoutKOHconditioning(b)condi-tionedindifferentKOHconcentrationat80C.Polymercomposition:PVA/PVP¼1:1inmass.
structurewithporesofsizeca.200nmuniformlydispersedonthemembranesurface(Fig.8(f)).Thecorrespondingcross-sectionalviewofFig.8(d)alsoshowedsomemicrovoids,andasimilarphenomenonwasobservedforPVA/PVP/KOH-rmembranecondi-tionedat80C.Itisknownthataliquidelectrolytesolutionislikelytoevaporateandleakfromapolymerstructurewithlargepores(above3mm)withtime,whichleadstoadetrimentalincreaseinelectrolyteresistance[35].Therefore,adensemembraneshowedbetterliquidretentionwithtime,despiteinitiallowliquidsolutionuptake.Comparedwiththeroomtemperatureconductivityofaround4.73Â10À4ScmÀ1forthePVA/KOH-rmembrane,theconductivityofthePVA/PVP/KOH-rmembranewasashighas1.77Â10À3ScmÀ1atambienttemperatureandcloseto6.2Â10À3ScmÀ1aftertreatmentat120C.Obviously,theincreaseinionicconductivityforPVP/PVA/KOH-risrelatedtogoodinitialliquidsolutionretention(KOH)duetoPVPmodifyingthemicro-phasestructure.Inotherwords,theliquidelectrolyteretentionisapparentlyimprovedforPVA/PVP.Thus,ifthecompositeratioisoptimized(forexample:PVA/PVP¼1:0.25to1:1andthemembraneisconditionedinaqueousKOHsolutionfrom4to10Mat60À100C),itispossibletoobtainthebestcompromisebetweeninitialliquidelectrolyteabsorbentcapacityandretentionability,evenathightemperatures.IntheEDXmappingimageshowninFig.9,thehighlightedbrightdotsrevealhighelementconcentration.Itcanbeseenthatthedistributionsofpotassiumandoxygenwerehomogeneous,indi-catingthatKOHwaswelldispersedthroughoutthePVA/PVPmembrane.Inaddition,EDXmappingofpotassiumandoxygenindicatedhigherconcentrationsofbothafterhigh-temperatureconditioningthaninthecaseofroomtemperatureconditioning.This3.6.Long-termstabilityofalkalinePVA/PVPblendmembraneInordertoinvestigatethelong-termstabilityofalkalinePVA/PVP,membraneswereconditionedin10MKOHat80Cand120C,respectively,for24h,thenrinsedandstoredinD.I.water.Here,PVA/PVPwitha1:1massratioisdiscussedasarepresentativeexample.Wedetermined:(i)dimensionalchangesofthemembrane;i.e.thechangesinthelength(Lx),width(Ly),andthickness(Lz)ofthemembraneafterdifferentstoragetimeswereevaluatedasmeasuresofmembranestabilityusingthedimensionsTable1ThermalpropertiesofPVA/PVPsemi-IPNsafterKOHdoping.PVA:PVP(inmass)1:0e4M1:0-4M-T801:1e10M1:1e10M-T80T1-onsetdecomposition(C)203200200238T1-maxdecomposition(C)367364363368Weightloss(%)47e4847e4832e3331e3210%GAasacross-linker.Cross-linkingtime:1h.1:1-4M-T80indicatesthatthemembranesamplewasconditionedin4MKOHat80Cfor24h,thentheadsorbedKOHonthesurfaceofthemembranewascompletelyremovedbyrinsingthemembraneinD.I.waternumeroustimesandsoon.4858J.Qiaoetal./Polymer51(2010)4850e4859
)-40sn oa 80oCoitt cee 120oCuridd-20 nhzotL igtdcnnaelar fny0 ie L meg,xunla LohnVci eg20hnti(llews4010o8b 80C 120oC3016 X 1-m4c S 2/ 0-2-40510152025303540Time / day
Fig.11.Timecoursesof(a)dimensionalchange(f)and(b)theionicconductivityofPVA/PVPcompositeconditionedin10MKOHat80Cand120C,respectively,thenstoredinD.I.water.Conditiontime:24h,followedbycompleteremovaloffreeKOHpriortotesting.Polymercomposition:PVA/PVP¼1:1inmass.
atambientconditions(10MKOH,25C)asstandards;and(ii)changesintheionicconductivityofthemembranes.Thechangeinlengthduetoswellingwasfoundtobealmostthesameinalldirections(i.e.,intheLx,Ly,andLzdirections),indicatingaratherisotropicswellingofPVA/PVP/KOH-rmembranes.Thevolumefractions,j,thuscalculatedfromtheEqn.[36]:j¼(LxLyLzÀLxoLyoLzo)/LxLyLzarepresentedinFig.11.ItcanbeseenthattherewasalmostnochangeinjvalueforPVA/PVP/KOH-rwithstoragetimeinD.I.waterformorethanonemonth,neitherforthemembraneconditionedat80Cnorforthatconditionedat120Cin10MKOH(Fig.11(a)).Theionicconductivityofthesameinvestigatedmembraneremainednearlyconstant,exceptforaninitialdecreasewithinthefirst2e3days(Fig.11(b)).Aninterestingphenomenonwasnotedinthatthemembranecolorchangedtobrownafterconditioningat80Candtoblackafterconditioningat120C,butgraduallyrevertedtotheoriginalcolorafterrinsinginD.I.water,inaccordancewiththeconstantionicconduction.Thissuggeststhatthemembranemorphologyremainedintactovertheinvestigatedstoragetime.Theinitialdecreaseinionicconductivitymaybeduetoincompleterinsingofthemembraneorextractionofsurface-attachedPVPduringinitialhigh-temperatureconditioning.Thesolutionwasseentobecomesomewhatopaqueaftercondi-tioningthemembranein10MKOHat120C,butremainedwhollytransparentafterconditioningatroomtemperatureorat100C.Inshort,theresidualPVPmoleculescannotescapefromthemembranematrixbecauseofstrongentanglementofPVPentrap-pedinPVAnetworks,althoughnocovalentbondsareformedbetweenthem.Asaresult,themembraneconductivitydidnotchangeduringthewholestoragetime.4.ConclusionsWehavedevelopedanovelalkalinesolidpolymerelectrolytemembraneonthebasisofPVA/PVPblendingandchemicalcross-linking,withdopingbyKOH,inwhichPVPservesasbothaplasti-cizerandstabilizer.TheionicconductivitywasfoundtobegreatlydependentontheconcentrationofKOHandthePVPcontent.Highionicconductivityupto0.53ScmÀ1wasobtainedatroomtemperatureforPVA/PVPinamassratioof1:0.5afterdopingwith8MaqueousKOHsolution.PerfecttolerancetobasetreatmentoftheresultingmembranehasbeendemonstratedbytheincorporationofPVP,withneitherdimensionalchangesnoradecreaseinionicconductivitybeingobservedafterconditioningin10MKOHat120CandthenstoringinD.I.waterformorethanamonth.Thisisthehighestchemicalstabilityforalkalinesolidpolymerelectrolytesystemsreportedsofar.TheincorporationofPVPhasbeenshowntobecriticalfortheimprovementofthemembrane’sthermalstability.TheaboveresultsmaybeduetotheformationofinterpenetratingpolymerchainsofPVPinthehighlycross-linkedPVAnetwork,whichimprovesnotonlytheionicconductivitybutalsothestabilityofthemembranes.AcknowledgementsThisworkisfinanciallysupportedbyProjectPujiangFoundation(grantno.08PJ14096)andNaturalScienceFundation(grantno.09ZR1433300)ofShanghaiScienceandTechnologyCommittee,China.ThisworkisalsosponsoredbytheScientificResearchFoundationfortheReturnedOverseasChineseScholars,StateEducationMinistryofChina(grantno.2009(1001)).References[1]CropperMAJ,GeigerS,JollieDM.Fuelcells:asurveyofcurrentdevelopments.JPowerSources2004;131:57e61.[2]KopitzkeRW,LinkoursCA,AndersonHR,NelsonGL.Conductivityandwateruptakeofaromatic-basedprotonexchangemembraneelectrolytes.JElec-trochemSoc2000;147:1677e81.[3]MohamadAA,MohamedNS,YahyaMZA,OthmanR,RameshS,AliasY,etal.Ionicconductivitystudiesofpoly(vinylalcohol)alkalinesolidpolymerelec-trolyteanditsuseinnickelezinccells.SolidStateIonics2003;156:171e7.[4]VassalN,SalmonE,FauvarqueJF.ElectrochemicalpropertiesofanalkalinesolidpolymerelectrolytebasedonP(ECH-co-EO).ElectrochimActa2000;45:1527e32.[5]LewandowskiA,ZajdeerM,FrackowiakE,BeguinF.SupercapacitorbasedonactivatedcarbonandpolyethyleneoxideeKOHeH2Opolymerelectrolyte.ElectrochimActa2001;46:2777e80.[6]KangJJ,LiWY,LinY,LiXP,XiaoXR,FangSB.Synthesisandionicconductivityofapolysiloxanecontainingquaternaryammoniumgroups.PolymAdvTechnol2004;15:61e4.[7]YiF,YangX,LiY,FangSB.Synthesisandionconductivityofpoly(oxyethylene)methacrylatescontainingaquaternaryammoniumgroup.PolymAdvTechnol1999;10:473e5.[8]LuS,PanJ,HuangA,ZhuangL,LuJ.Alkalinepolymerelectrolytefuelcellscompletelyfreefromnoblemetalcatalysts.FuelCell2009;2:6.[9]LiL,WangYX.Quaternizedpolyethersulfonecardoanionexchangemembranesfordirectmethanolalkalinefuelcells.JMembrSci2005;262:1e4.[10]FangJ,ShenPK.Quaternizedpoly(phthalazinonethersulfoneketone)membraneforanionexchangemembranefuelcells.JMembrSci2006;285:317e22.[11]WangG,WengY,ChuD,XieD,ChenR.Preparationofalkalineanionexchangemembranesbasedonfunctionalpoly(ether-imide)polymersforpotentialfuelcellapplications.JMembrSci2009;326:4e8.[12]VarcoeJR,SladeRCT.Anelectron-beam-graftedETFEalkalineanion-exchangemembraneinmetal-cation-freesolid-statealkalinefuelcells.ElectrochemCommun2006;8:839e43.[13]WangY,LiL,HuL,ZhuangL,LuJT,XuBQ.Afeasibilityanalysisforalkalinemembranedirectmethanolfuelcell:thermodynamicdisadvantagesversuskineticadvantages.ElectrochemCommun2003;5:662e6.[14]HouH,SunG,HeR,SunB,JinW,LiuH,etal.Alkalidopedpolybenzimidazolemembraneforalkalinedirectmethanolfuelcell.IntJHydrogenEnergy2008;33:7172e6.J.Qiaoetal./Polymer51(2010)4850e4859
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