Materials for Solid Oxide Fuel__ Cells

pubs.acs.org/cmPublishedonWeb11/20/2009r2009AmericanChemicalSociety660Chem.Mater.2010,22,660–674DOI:10.1021/cm902640jMaterialsforSolidOxideFuelCells†AllanJ.Jacobson*DepartmentofChemistry,UniversityofHouston,Houston,Texas,77204-5003ReceivedAugust26,2009.RevisedManuscriptReceivedOctober9,2009Solidoxidefuelcells(SOFCs)havethepromisetoimproveenergyefficiencyandtoprovidesocietywithacleanenergyproducingtechnology.Thehightemperatureofoperation(500-1000
C)enablesthesolidoxidefuelcelltooperatewithexistingfossilfuelsandtobeefficientlycoupledwithturbinestogiveveryhighefficiencyconversionoffuelstoelectricity.Solidoxidefuelcellsarecomplexelectrochemicaldevicesthatcontainthreebasiccomponents,aporousanode,anelectrolytemembrane,andaporouscathode.Inthisshortreview,asurveyofthetypesandpropertiesofmaterialsthathavebeenconsideredforeachofthesecomponentsispresentedwithanemphasisontherequirementsforoperationatintermediatetemperature(500-800
C).Somedirectionsforfutureresearcharediscussed.1.IntroductionSolidoxidefuelcells(SOFCs)havethepromisetoimproveenergyefficiencyandtoprovidesocietywithacleanenergyproducingtechnology.Thehightempera-tureofoperation(500-1000
C)enablesthesolidoxidefuelcelltooperatewithexistingfossilfuelstogiveveryhighefficiencyconversionoffuelstoelectricityandtobeusedincombinedheatandpowerapplicationsorefficientlycoupledwithgasturbines.SOFCsarequietandnonpollutingandtheirinherenthighefficiencyleadstolowergreenhousegasemissions.1-4Solidoxidefuelcellsarecomplexelectrochemicaldevicesthatcontainthreebasiccomponents,aporousanode,anelectrolytemembrane,andaporouscathode.AschematicrepresentationofanelectrolytesupportedsolidoxidefuelcellisshowninFigure1a.Inthisparticulardesign,thedenseelectrolytemembranesupportstheporouselectrodes.Thecathodeistypicallyanoxidethatcatalyzestheoxygenreductionreaction:1=2O2ðgÞþ2e-fO¼ðsÞTheanodecatalyzestheoxidationoffuel,eitherhydrogenorreformedhydrocarbons:H2ðgÞþO¼fH2Oþ2e-andCOðgÞþO¼fCO2þ2e-Thedenseelectrolytemembraneseparatestheairandfuelcompartmentsandisapureoxygenionconductor.Oxygenfromairissuppliedononesideofthecellandhydrogenandcarbonmonoxidefromreformedhydro-carbonfuelontheotherside.Theelectrodesprovidetheinterfacebetweenchemicalandelectricenergyandcata-lyzethechemicalreactions.Ideally,thesereactionsshouldbeveryfast(atequilibrium)tominimizevoltagelosses;highcurrentrequiresfastreactionratesandhighfluxesofmolecules,ions,andelectrons.Asshownsche-maticallyinFigure1b,theoxygenchemicalpotentialdifferencebetweenairandfuelisdistributedacrossthecellandtheoutputvoltagedependsonthemagnitudeoftheoverpotentialsattheelectrodes.Forcompoundelec-trodes,theirproperties(forexample,electronicconduc-tivityandoxygenvacancyconcentration)dependontheeffectivechemicalpotentialthey“see”andthechemicalpotentialofthecathodeiscoupledtothatoftheanode.Anindividualcellproduces∼1.0Vonopencircuitand∼0.6-0.7Vunderload.Multiplecellsareconnectedtogetherwithbipolarplatestoformafuelcellstackthatprovidestheoutputvoltageappropriateforaparticularapplication.Thelargegradientintheoxygenpartialpressureacrossthecellfrom0.2atmontheairsideto110-15to110-20atmonthefuelsideplacessevereconstraintsonthechoiceofmaterialsintermsofstability,chemicalreactionsattheinterfaces,andcompatiblethermalexpansioncoefficients.Inthe1990s,solidoxidefuelcellsoperatingat1000
Cusinganyttriastabilizedzirconiaelectrolyte(YSZ),alanthanumstrontiummanganitecathode,andanick-el-YSZcermetanodeweredeveloped.Cellswerecon-nectedbyalanthanumstrontiumchromitebipolarplate.VariousgeometriesforthecelldesignwereinvestigatedbutthemostdevelopedistheSiemens-WestinghousetubularconfigurationinwhichtheYSZelectrolytefilm(30-40μm)issupportedona1.5mlongtubeofporouslanthanumstrontiummanganite.TheSiemens-Westing-housedesignhasbeendemonstratedsuccessfullyatthe100kWscale.5Inrecentyears,thefocusofSOFCdevelopmenthasbeenonloweringtheoperatingtemperaturesofSOFCsto†Acceptedaspartofthe2010“MaterialsChemistryofEnergyConversionSpecialIssue”.*[e-mail]:[phone].Fax:[phone].ReviewChem.Mater.,Vol.22,No.3,2010661500-800
Cinordertoreducecosts.Loweringtheoperatingtemperaturehasasignificantimpactoncostbyallowingtheuseoflessexpensivematerialsinintercon-nectsandheatexchangers.LowertemperaturesalsoleadtoanincreaseinthedurabilityofSOFCsystemsbyreducingproblemsassociatedwiththermalcyclingandperformancedegradationbecauseofinterdiffusionorreactionoftheindividualcomponents.Operationatlowertemperature,however,createsanumberofmateri-alsproblemsthatareassociatedwiththeincreaseintheelectrolyteresistanceanddecreaseintheratesoftheelectrocatalyticreactions(electrodepolarization).Bothfactorsresultinareductionofthecellvoltageandoutputpower.Findingnewcombinationsofelectrolyteandelectrodematerialsthatreducetheselossesandprovidebothrapidiontransportacrosstheelectrolyteandelec-trode-electrolyteinterfacesandefficientelectrocatalysisoftheoxygenreductionandfueloxidationreactionsremainsasignificantmaterialschallenge.Afundamentalatomisticlevelunderstandingofthefactorsthatcontrolbulk