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2011SpringFuelCellandElectrochemistry Lecture6 MCFC MoltenCarbonateFuelCell MoltenCarbonatefuelcell Advantagesnoblemetalcatalystsarenotrequired beingdevelopedfornaturalgasandcoal basedpowerplants Highefficieny Fuelflexibility forindustrial electricalutility andmilitaryapplications NeedforCO2atcathode transfertheCO2fromtheanodeexitgastothecathodeinletgas CO2transferdevice produceCO2bycombustingtheanodeexhaustgas whichismixeddirectlywiththecathodeinletgas supplyCO2fromanalternatesource HightemperatureEffect Advantagesachievehigheroverallsystemefficiencies greaterfuelflexibilityDisadvantagesseveredemandsonthecorrosionstabilityandlifeofcellcomponents particularlyintheaggressiveenvironmentofthemoltencarbonateelectrolyte Needspecialstructuretoholdmoltenelectrolyte capillarypressurestoestablishtheelectrolyteinterfacialboundaries InPAFC PTFEservesasabinderandwet proofingagenttomaintaintheintegrityoftheelectrodestructureandtoestablishastableelectrolyte gasinterfaceintheporouselectrode ThephosphoricacidisretainedinamatrixofPTFEandSiCbetweentheanodeandcathode Largestfloodedporesintheporouscomponents Overthepast28years theperformanceofsinglecellshasimprovedfromabout10mW cm2to 150mW cm2 Electrolytefabrication Electrolytemanagementcontrolovertheoptimumdistributionofmoltencarbonateelectrolyteinthedifferentcellcomponents1980 hotpressing about5 000psi mixturesofLiAlO2andalkalicarbonates 1 voidspaces 5porosity 2 pooruniformityofmicrostructure 3 generallypoormechanicalstrength and4 highiRdropNow TapecastingorelectrophreticdepositionDispersingtheceramicpowderinasolvent23thatcontainsdissolvedbinders usuallyanorganiccompound plasticizers andadditivestoyieldthepropersliprheology Theslipiscastoveramovingsmoothsubstrate andthedesiredthicknessisestablishedwithadoctorbladedevice Tapecaster Li richelectrolyte Higherionicconductivities andhencelowerohmicpolarization betterthanNa2CO3andK2CO3 Gassolubilityanddiffusivityarelower andcorrosionismorerapidinLi2CO3 Electrolytestructurescontaining45wt LiAlO2and55wt moltencarbonate 62mol Li2CO3 38mol K2CO3 haveaspecificconductivityat650 Cofabout1 3thatofthepurecarbonatephase Electrodeproblems ThemajorconsiderationswithNi basedanodesandNiOcathodesarestructuralstabilityandNiOdissolution CathodeDespitethelowsolubilityofNiOincarbonateelectrolytes 10ppm Niionsdiffuseintheelectrolytetowardstheanode andmetallicNicanprecipitateinregionswhereaH2reducingenvironmentisencountered TheprecipitationofNiprovidesasinkforNiions andthuspromotesthediffusionofdissolvedNifromthecathode ThisphenomenonbecomesworseathighCO2partialpressures Electrodeproblems ThedissolutionofNiOhasbeencorrelatedtotheacid basepropertiesofthemoltencarbonate Inrelativelyacidicmelts Bipolar Corrosionislargelyovercomebyapplyingacoating about50 mthickness atthevulnerablelocationsonthebipolarplate ThebipolarplatesusedinMCFCstacksareusuallyfabricatedfromthin 15mil sheetsofanalloy e g Incoloy825 310Sor316Lstainlesssteel thatarecoatedononeside i e thesideexposedtofuelgasesintheanodecompartment withaNilayer TheNilayerisstableinthereducinggasenvironmentoftheanodecompartment anditprovidesaconductivesurfacecoatingwithlowcontactresistance Athinaluminumcoatinginthewet sealareaofabipolarplateprovidescorrosionprotectionbyformingaprotectivelayerofLiAlO2afterreactionofAlwithLi2CO3 DevelopmentComponents Researchisaddressingalternatecathodematerialsandelectrolytes performanceimprovement lifeextensionbeyondthecommercializationgoaloffiveyears andcostreduction 1 CathodeHigherperformanceimprovementsareexpectedwithCo dopedLiFeO2 Italsohasbeenshownthat5mollithium dopedNiOwithathicknessof0 02cmprovideda43mVoverpotential higherperformance at160mA cm2comparedtothestate of the artNiOcathode Anotherideaforresolvingthecathodedissolutionproblemistoformulateamildercellenvironment DevelopmentComponents 2 ElectrolyteMatrixThepresentelectrolytestructurematerialsaretightlypacked fine or LiAlO2withfiberorparticulatereinforcement The phaseisstableat 700 C whereasthe phaseisstableat600to650 C Theindustrytrendistoswitchfrom LiAlO2to LiAlO2forbetterlong termphaseandparticle sizestabilities FCEisdevelopingalow costLiAlO2 aqueous basemanufacturingsystem butmustresolveslowdryingrateofLiAlO2anditsinstabilityinwater DevelopmentComponents 3 ElectrolytePresentelectrolyteshavethefollowingchemistry lithiumpotassiumcarbonate Li2CO3 K2CO3 62 38molpercent foratmosphericpressureoperationandlithiumsodiumcarbonate LiCO3 NaCO3 52 48o60 40molpercent thatisbetterforimprovedcathodestabilityunderpressurizedoperationandlifeextension Evaporationoftheelectrolyteisalife limitingissueforthemoltencarbonatefuelcell Li NaelectrolyteisbetterforhigherpressureoperationthanLi Kbecauseitgiveshigherperformance DevelopmentComponents 4 CoalGasTraceSpecies Performances Factorsaffectingtheselectionofoperatingconditionsstacksize heattransferrate voltagelevel loadrequirement andcost TypicalMCFCswillgenerallyoperateintherangeof100to200mA cm2at750to900mV cell Performances 1 EffectofPressure Performances Athigherpressure Performances 2 EffectofTemperatureThewatergasshiftreactionachievesrapidequilibriumattheanodeinMCFCs andconsequentlyCOservesasanindirectsourceofH2 WaterGasShift CO H2O CO2 H2 Performances 2 EffectofTemperatureinthetemperaturerangeof575to650 C 1 3ofthetotalchangeincellvoltagewithdecreasingtemperatureisduetoanincreaseinohmicpolarization Beyond650 C however therearediminishinggainswithincreasedtemperature Performances 3 EffectofReactantGasCompositionThehalf reactionatcathode O2 CO2 2e CO32 Performances 4 EffectofUtilization Performances 5 EffectofImpuritiesGasifiedcoalisexpectedtobethemajorsourceoffuelgasforMCFCs butbecausecoalcontainsmanycontaminantsinawiderangeofconcentrations fuelderivedfromthissourcealsocontainsaconsiderablenumberofcontaminants Performances 5 EffectofImpurities Sulfur Performances TheadverseeffectsofH2Soccurbecauseof ChemisorptiononNisurfacestoblockactiveelectrochemicalsites Poisoningofcatalyticreactionsitesforthewatergasshiftreaction OxidationtoSO2inacombustionreaction andsubsequentreactionwithcarbonateions
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