Hydrogen Fuel Cell Power System

83~ChapterThreeThehydrogenfuelcellpowersystem~84Thischapterdiscussestwomajorissues:(i)fuelcelltheoryandengineering,and(ii)providingfuelforthefuelcell.Thefirstsectionthoroughlydiscussesthesciencebehindfuelcellsingeneral.Ofthedifferentkindsoffuelcells,theprotonexchangemembranetypeisidentifiedastheunquestionedbestcandidateforthesmallvehicleapplication.Engineeringissueslikehowtocoolthefuelcellstackandwhethertopressurizethefuelcellarediscussedingeneraltermsandthefeasibilityofcertaindesignoptionsisdetermined.(Ontheotherhand,morecomplete,quantitativeanalysesofcoolingandpressurizationrequiremoredetailedinformationaboutsteady-stateandtransientpowerrequirementsthatarenotcalculateduntilChapter4,andarethusproperlyanalyzedwithinthatchapter).Thesizeandweightofthefuelcellarediscussedwithreferencetopreviousresearchers’results.Thesecondhalfofthischapterconcernsfuelingofhydrogenfuelcells,whichisconsiderablymorecomplexthanfillingatankwithgasoline.Itexplainswhyreformedfossilfuelscannotyetbeusedtopowerafuelcellvehicle,anddescribesvariousoptionsforstoringpurehydrogenonboardthevehicle.Theissueofsafetyisdiscussed.853.1.FuelCellScience3.1.1.FundamentalsFuelcellsareelectrochemicalenginesthatproduceelectricityfrompairedoxidation/reductionreactions.Onecanthinkofthemasbatterieswithflowsofreactantsinandproductsout.Incontrast,thebatteryhasafixedsupplyofreactantsthattransformintoproductswithoutbeingsteadilyreplaced.Thedistinctionisaniceoneaszinc-air“batteries”havereplaceablezincelectrodes,makingthemverylikefuelcells.Astandardhighschoolchemistrydemonstrationinvolvespassinganelectriccurrentfromabatterythroughajarofwater(withdissolvedsalts)bywayoftwometalelectrodessuspendedinthewater.Hydrogenevolvesatthecathodeandoxygenattheanodeasthewaterisbrokenintoitsconstituentelementsbyelectrolysis.Essentially,afuelcellusesthereverseprocess:hydrogenandoxygenarecombinedtoformwater,andelectricityisproduced.Technically,thetwochemicalsdonothavetobehydrogenandoxygen;theredoxreactionrequiresonlyareducerandanoxidizer,butsinceoxygeniseasytoobtainfromtheairandhydrogenhassuitablyfastreactionkinetics,thesearethetwomostoftenchosen.Allfurtherexampleswillusehydrogenandoxygenunlessotherwisenoted.Specifically,thehydrogenandoxygendiffuseintotheirrespectiveelectrodes,ionize,andonetypeofionmigratesthroughanelectrolyteandrecombinesattheothersidewiththeotheriontoformwater.Atanygiventemperaturethereisanequilibriumratioofionstomolecules.Coatingsof86noblemetalcatalystontheelectrodeslowertheactivationenergyoftheionization/recombinationprocess,acceleratingtherestorationofequilibriumastheionsareconsumedbythefuelcell.Theprinciplebehindfuelcellswasdiscoveredasearlyas1839byWelshphysicistandjudgeSirWilliamGrove.However,duetohighcosts,thetechnologywasnotsignificantlyuseduntiltheAmericanGeminispacemissionsofthe1960's.Forthisandsubsequentspacemissions,fuelcellswerethoughttobesaferthannuclearelectricgenerationandcheaperthansolar.Theyhavebeenthrusttotheforefrontofenergytechnologyinthe1990's,however,ashighpowerdensitieshavemadethemfeasibleforbothstationaryandportableapplications.ONSIcorporation,asubsidiaryofUnitedTechnologies,hasproducedover170ofitsPC25stationary200kWfuelcellsystemssincetheirintroductionin1992.Fuelcellshavetheadvantagesofhighefficiency,loworevenzeropollution,quietoperation,andfewermovingparts–onlypumpsandfanstocirculatecoolantandreactantgases,respectively–forgreaterreliabilitythaninternalcombustionengines(oncefuelcellsystemsarewell-developed).3.1.1.1ThermodynamicsAnelectrolytephysicallyseparatesthetworeactantsandalsopreventselectronicconduction,whileallowingionstopassthrough;theelectronstravelthroughanexternallooptosupplytheload.Electrodesareattachedtoeithersideoftheelectrolyte.Attheanode,thehydrogenisoxidized:H2(g)2H++2e-Theelectronspassthroughtheloadtoprovidethedesiredcurrentandendupatthecathode,wherethematchingreductionreactionoccurs:872H24e-4e-O24H+2O2-conductiveseparatorplateconductiveseparatorplateanodeelectrodecathodeelectrodepolymerelectrolytemembrane2H2Oelectricalload2e-+½O2(g)O2-Electrostaticbalanceisreachedasthehydrogenionsdiffusethroughtheelectrolytetogettothecathode:2H++O2-H2O(l)Figure3.1FuelcellschematicThetheoreticalenergyreleaseoftheoverallreactionisdeterminedbytheenthalpychangeHintheoverall(isothermal)reaction,88H2(g)+½O2(g)H2O(l)(H(=-285.8kJ/mol;G(=-237.2kJ/mol;)GistheGibbsfreeenergyandstandardconditions,asindicatedbythenoughtsuperscript,areT=25(C,partialpressuresof1atmforeachofthegases,andwaterintheliquidstate.Thislastdistinctionisimportant.Forhightemperaturefuelcells,wateremergesinthegaseousstate,sothe“lowerheatingvalue”wouldbeused.Inthis(non-standard)case,H=-241.8kJ/molandG=-228.6kJ/mol.Thehigherheatingvalueisusedintheremainingcalculationssincemostfuelcellsoperatebelowtheboilingpointofwater.Thenextstepistodeterminefuelcellefficiency.G(=H(-TS(sothestandardchangeinentropyis-0.163kJ•mol-1•K-1.Anenergybalanceonafuelcellshowsthatd/dt(dQ+dWelec)=d/dt(dH+dKE+dPE)Kineticenergy(KE)andpotentialenergy(PE)changesareassumedtobenegligible,andsteadystate