燃料電池效率(III)

H.K.Ma

DepartmentofMechanicalEngineering

NationalTaiwanUniversity,Taipei,Taiwan

November,2009

臺灣大學機械工程系能源環(huán)境實驗室Maximumelectricwork(Wel)atconstanttemp.andpressureisgivenbyGibbsfreeenergy:TheRoleofGibbsFreeEnergy&

NernstPotentialEquationnisno.ofelectronsFisFarady’sconstantEisidealpotentialofthecellBesides,Gibbs’freeenergycanbewrittenas:?Hisenthaplychange?SisentropychangeT?SrepresentstheunavailableenergyresultingfromtheentropychangeAtconstantpressure,thespecificentropyattemperatureTisgivenbyItthanfollowsthat,TheGibbs’freeenergycanbeexpressedbytheequationForthegenerationreaction,Substitutingtheequationintothen,ThisisgeneralformoftheNernestEquationThefuelcellreaction&correspondingNernstequationTheidealefficiencyofafuelcell,operatingreversibly,isthen,CellefficiencyAtstandardcondition(298K,1atm),thermalenergy(?H)inthehydrogen/oxygenreactionis285.8KJ/mole,andthefreeenergyforusefulworkis237KJ/mole,therefore,Thethermalefficiencyofahydrogen/oxygencanbewrittenintermoftheactualcellvoltageActualPerformanceActivationlossesarecausedbysluggishelectrodekinetics.Itispossibletoapproximatethevoltagedropbyasemi-empiricalequation,calledtheTafelequation.ActivationLossα

istheelectrontransfercoefficientofthereactionattheelectronbeingaddressedi0istheexchangecurrentdensityTafelPlotOhmiclossesoccurbecauseofresistancetotheflowintheelectrolyteandresistancetotheflowelectronsthroughtheelectrode.BecausetheelectrolyteandfuelcellelectrodesobeyOhm’slaw,theohmiclossescanbeexpressedbytheequation,OhmicLoss Thetotalresistance,R,whichincludeselectronic,ionic,andcontactresistanceThetotalresistanceThecombinedeffectofthelossesforagivencellandgivenoperatingconditionscanbeexpressedaspolarizations.Thetotalpolarizationattheelectrodeisthesumofanodeandcathode.CumulativeeffectofthelossesThecellvoltageincludesthecontributionoftheanodeandcathodepotentialsandohmicpolarizationSummingofCellVoltageWhenandaresubstitutedinthen,19AFCPEMFCDMFCPAFCMCFCSOFCIonOH-H+H+H+CO-23O-2Temperature50~200℃30~120℃20~90℃~220℃~650℃500~1000℃FuelH2H2CH3OHH2NG,H2…NG,H2…OxidesO2O2O2O2O2O2Output(W)1KW~10K1~100K1~10010K~1M1M~10M1K~10M2025/5/3120WaterproductT(K)E(V)Liquid2981.23Liquid3531.18Gas3731.17Gas4731.14Gas6731.09Gas8731.04Gas12730.922025/5/3121VolumeflowrateH26.964cc/min.O23.483cc/min.Air16.586cc/min.2025/5/3122WaterproductT(K)Efficiencylimit(%)Liquid29883Liquid35380Gas37379Gas127362

Efficiencyoffuelcell=electricenergyproducedpermoleoffuel/enthalpyofformationLimitoffuelcellefficiency=

CarnotLimit=Thermodynamicefficiency2025/5/31

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