台湾大学机械工程系能源环境实验室

NTUMEH.K.MaDepartmentofMechanicalEngineeringNationalTaiwanUniversity,Taipei,TaiwanNovember,2009台灣大學機械工程系能源環境實驗室Maximumelectricwork(Wel)atconstanttemp.andpressureisgivenbyGibbsfreeenergy:•nisno.ofelectrons•FisFarady’sconstant•EisidealpotentialofthecellBesides,Gibbs’freeenergycanbewrittenas:•∆Hisenthaplychange•∆Sisentropychange•T∆SrepresentstheunavailableenergyresultingfromtheentropychangeAtconstantpressure,thespecificentropyattemperatureTisgivenbyItthanfollowsthat,TheGibbs’freeenergycanbeexpressedbytheequation•Forthegenerationreaction,•Substitutingtheequationintothen,ThisisgeneralformoftheNernestEquationNTUMENTUMENTUMETheidealefficiencyofafuelcell,operatingreversibly,isthen,•Atstandardcondition(298K,1atm),thermalenergy(∆H)inthehydrogen/oxygenreactionis285.8KJ/mole,andthefreeenergyforusefulworkis237KJ/mole,therefore,Thethermalefficiencyofahydrogen/oxygencanbewrittenintermoftheactualcellvoltageActivationlossesarecausedbysluggishelectrodekinetics.Itispossibletoapproximatethevoltagedropbyasemi-empiricalequation,calledtheTafelequation.•αistheelectrontransfercoefficientofthereactionattheelectronbeingaddressed•i0istheexchangecurrentdensityOhmiclossesoccurbecauseofresistancetotheflowintheelectrolyteandresistancetotheflowelectronsthroughtheelectrode.BecausetheelectrolyteandfuelcellelectrodesobeyOhm’slaw,theohmiclossescanbeexpressedbytheequation,Thetotalresistance,R,whichincludeselectronic,ionic,andcontactresistanceThecombinedeffectofthelossesforagivencellandgivenoperatingconditionscanbeexpressedaspolarizations.Thetotalpolarizationattheelectrodeisthesumofanodeandcathode.Thecellvoltageincludesthecontributionoftheanodeandcathodepotentialsandohmicpolarization•Whenandaresubstitutedinthen,Fuelcelltypes19AFCPEMFCDMFCPAFCMCFCSOFCIonOH-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~10M2019/8/11Idealpotentialofhydrogenfuelcells20WaterproductT(K)E(V)Liquid2981.23Liquid3531.18Gas3731.17Gas4731.14Gas6731.09Gas8731.04Gas12730.92nFEGWele.OHOH22221222)()()(OHOHGGGGsThGEmolVJmolKJnFEG/964722/2.237VE23.12019/8/11H2,O2,andAirdemandfor1Amp.21VolumeflowrateH26.964cc/min.O23.483cc/min.Air16.586cc/min..min1/.sec60/2/144.96472//1.sec1/12electronmolemolegCelectronmoleCVHeHH222molgLmolegmoleg1/4.22min/10109.3.]min/[10109.344.min/..964.6.]min/[10964.64ccL.min/10268.61/1016.2.]min/[10109.3442gmoleggmolgmH.min/10555.12/1.min/10109.3442222molegmolegmolegmolegnHOHO.]min/..[483.3.min/10483.3)1/(4.22.min/10555.1342ccLmolegLmolegVO]/[10567.3min]/[021.01/9.2821.0/1.min/10555.14422sggmoleggmolegmolegmolegmairairOairOair2019/8/11Fuelcellefficiencylimitunderthethreeformsofwater22WaterproductT(K)Efficiencylimit(%)Liquid29883Liquid35380Gas37379Gas127362121TTTEfficiencyoffuelcell=electricenergyproducedpermoleoffuel/enthalpyofformationLimitoffuelcellefficiency=)(HHVffhgCarnotLimit=Thermodynamicefficiency2019/8/11EnergybarrierReactantProductReactantProductEnergyBarrierReactionRate2019/8/1123I-VcurveActivationlosses:0logiiAVactOhmiclosses:iRVohmicConcentrationlosses:11ln2iiFRTVtrans2019/8/1124