Ammonia fuel cells 1st edition ibrahim dincer

Ammonia Fuel Cells 1st Edition Ibrahim Dincer

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AmmoniaFuelCells

AmmoniaFuelCells

IbrahimDincer

OsamahSiddiqui

Elsevier

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Duringthepastdecade,ammoniahasreceivedincreasingattentionfromvarioussectorsandmanypeopleduetoitsexcellentpropertiesandcarbon-freenaturealthough ithasbeenessentiallyverywellknownintherefrigerationindustryforrefrigeration applications.Wehavegoneintoanerawherehydrogenisessentialformanysectors asafuelandasanenergycarrier.Duetotheconsiderablechangesrequiredforhydrogenenergysystemsandapplications,ammoniaappearstoentailavitalpositioninthe developmentofthehydrogeneconomy.Havingnumerousfavorablepropertiesand characteristics,ammoniaisconsideredtobeapromisingcandidatethatcanaddress severalchallengesfacedbyhydrogenusageasafuel.Ammoniahas,formany decades,beenservinginvarioussectors,forexample,fertilizers,workingfluids, cleaningchemicals,refrigerants,andmanymoreinadditiontorefrigerationsystems asstatedabove.However,fromtheperspectiveoffuels,thenecessityofshifting towardcarbon-freefuelsinthiserahasmadeammoniastandoutasapromisingfuel thatcanaidinreducingthedependenceonfossilfuel-basedenergyproduction.Also, ammoniaisrecognizedasacleansolutionaslongasitisproducedcleanlybyrenewables.Inthisregard,ammoniafuelcellsplayakeyroletowardtheflourishmentof ammoniaenergy.Entailinghigherefficienciesthancombustionengines,fuelcells alongwithotherrenewablesareanticipatedtobetheupcomingrevolutioninpower generation.

Inthisbook, Chapter1 firsthighlightstheimportanceofimplementingclean energyresourcesincludingtheutilizationofammoniaandhydrogenfuelsandthen discussestheminbrief.Next, Chapter2 coverstheunderlyingfundamentalsoffuel celltechnologiesaswellasbackgroundinformationessentialtounderstandthe workingmethodologyofdifferenttypesofammoniafuelcells.Thenecessarycomponentsrequiredtoconstituteafuelcelldevicearedescribedandthephysicalaswell aschemicalphenomenaoccurringateachofthesecomponentsisdiscussed.Also,the classificationoffuelcellsaccordingtodifferentclassificationcategoriesisdescribed wherethecategorizationaccordingtothetypeofelectrolytes,fuel,andoperating conditionsispresented. Chapter3 dwellsonidentifyingdifferenttypesoffuelsthat havebeeninvestigatedforfuelcellapplications.Theserangefromhydrogenasthe mostcommonlyemployedfueltodifferenttypesofalcoholsaswellasalkanes. Ammoniaasapromisingfuelisthendescribedcomprehensivelypresentingvarious advantagesandfavorablepropertiesitentails.Theseprovidesufficientinformation neededtoproceedtowardthedetailsofammoniafuelcells. Chapter4 explicitlyconcernstheammoniafuelcellswhereitcomprehensivelycoversdifferenttypesof ammoniafuelcellsthathavebeendeveloped.Theperformanceofeachtypeof ammoniafuelcellislinkedtothetypeofelectrolyte,electrochemicalcatalyst, andelectrodesused.Ammoniafueledhigh-temperaturesolidoxidefuelcellswith bothproton-conductingandanion-conductingelectrolytesisdiscussed.Furthermore,low-temperaturedirectammoniaalkalinefuelcellsarecoveredindepth.

Thedevelopment,materials,operation,systemconditions,andcatalystsofthese ammoniafuelcellsarepresented.Moreover, Chapter5 presentstheanalysisand modelingoffuelcellswithspecificfocusonelectrochemicalinteractionofammonia moleculesthatoccursindirectammoniafuelcells.Theperformanceofthesedifferenttypesofammoniafuelcellsisalsodiscussed. Chapter6 describesvariousnew integratedammoniafuelcell-basedsystemsthathavebeendevelopedintherecent pastandtheirperformancesareanalyzedthroughoverallenergyandexergyefficiencies. Chapter7 discussesnovelammoniafuelcell-basedtechnologiesascasestudies wheretheirperformancesareinvestigatedatvaryingoperatingconditionsandsystemparameters.Lastly,thebookcloseswith Chapter8 providingseveralrecommendationsforfuturedevelopmentofammoniafuelcellsanddepictingthedevelopment ofammonia-basedenergytechnologiesmovingtowardanewammonia-basedera.

Nomenclature

A area(m2)

C charge(Coloumbs),concentration(mol/m3)

cny conductivity(S/m)

D dayangle(°),diffusioncoefficient(m2/s)

ex specificexergy(kJ/kg)

E potential(V)

Ex exergyrate(kW)

F Faraday’sconstant(96,500C/mol)

g specificGibbsenergy(J/mol)

G Gibbsfreeenergy(J)

h specificmolarenthalpy(kJ/mol)

h specificenthalpy(kJ/kg)

H enthalpy(kJ) _ I solarintensity

J currentdensity(A/m2)

m mass(kg)

m massflowrate(kg/s)

M molarconcentration(mol/m3)

n numberofelectrons _ N molarflowrate(mol/s)

P pressure

P power(kW)

q heattransfer(kJ)

Q heattransferrate(kW)

R idealgasconstant(J/molK),Ohmicresistance(Ohm)

s specificentropy(kJ/kgK)

s specificmolarentropy(kJ/molK)

S entropy(kJ/K)

ST solartime(h)

T temperature(°C)

v specificvolume(m3/kg)

V voltage(V)

w specificwork(kJ/kg) _

W workrate(kW)

Greekletters

Ω resistance(Ohmcm2)

α chargetransfercoefficient

 half-cellpotential(V)

ρ density(kg/m3)

η efficiency

δ declinationangle,diffusionlayerthickness

τ resistivity

μ surfacecoverage

γ orderofreaction

Subscripts

a anode

act activation

ads adsorbed

ar aerosol

b backward

c cathode

con condenser

conc concentration

conv convection

d density

dest destroyed

diff diffusion

dl declination

E electrode

elec electric

ex exchange,exit,exergy

f formation,forward

ga gas

gr grid

gen generation in incoming

L limiting lt latitude

mb beam mix mixture

nl normal Ohm Ohmic on ozone

op overpotential

or opencircuit

ox oxidant

prod products

PV photovoltaic

reac reactants

ref. reference

rev reversible

ry Rayleigh

s solar,surface

scs solarconstant

sen sensible

sn sun

T temperature

TV throttlevalve

wr water

zh zenith

Acronyms

ABS absorptioncoolingcycle

AEM anionexchangemembrane

AS ammoniasynthesis

ASR ammoniasynthesisreactor

DAFC directammoniafuelcell

DEFC directethanolfuelcell

DMFC directmethanolfuelcell

DPFC directpropanolfuelcell

COMP compressor

CON condenser

COP coefficientofperformance

Dl dayangle

EES engineeringequationsolver

FC fuelcell

GT gasturbine

HF heliostatfield

HHV higherheatingvalue

HX heatexchanger

KOH potassiumhydroxide

LHV lowerheatingvalue

MCFC moltencarbonatefuelcell

MF molarfraction

MEA membraneelectrodeassembly

MSTR multistageturbine

ORR oxygenreductionreaction

PAFC phosphoricacidfuelcell

PEM protonexchangemembrane

PSA pressureswingadsorption

PTFE polytetrafluoroethylyne

PV photovoltaic

PVA polyvinylalcohol

RC Rankinecycle

RO reverseosmosis

SEP separator

SDC samaria-dopedceria

SOFC solidoxidefuelcell

SRC secondaryRankinecycle

ST solartower

WES waterelectrolysis

YSZ yittria-stabilizedzirconia

Introduction 1

Sincetheindustrialrevolution,powergenerationhasbeenplayingavitalrolein theadvancementofanynation,whichhas,therefore,becomeacentralelementof anyeconomythatdrivestheincreaseordecreaseinthenationalproductionlevels. Acontinuousandreliablegenerationofpowerisessentialtoattainasustainableand stableeconomyaswellasindustrialsector.However,theincreaseddependenceon fossilfuelsforpowergenerationintherecentdecadeshasdeterioratedtheenvironmentconsiderably.Thechangeintheglobalprimaryenergydemandssince2010 aredepictedin Fig.1.1.Theenergydemandsincreasedbynearly1.5%inthebeginningofthepreviousdecade.Thisriseindemandshasdecreasedmarginallyinthe beginningofthisdecade.Also,towardthemiddleofthisdecade,theprimaryenergy demandshadincreasedbyacomparativelyloweramount.Nevertheless,since2016 theenergydemandshaverisensharplywheretheriseincreasedfrom0.6%in2016 to2%in2017.Thishasincreasedfurtherto2.9%in2018.Hence,everyyeardue toseveralfactors,suchasindustrialization,urbanizationandmoderndevelopment, thespecificenergydemandstendtoincreasecontinuously.Thechangeintheamount ofincreasehasvariedintherecentpast,however,therehasbeenasteadyincreasein thedemands.Moreover,astheworldswiftlymovestowardatechnology-enriched livelihood,thedemandsforenergyareexpectedtoriserigorously.Theenergy demandsareofparticularinterestduetotheirdirectrelationwithenvironmental impacts.Currently,theglobalenergyproductionheavilyreliesonfossilfuelsand theincreaseinenergydemandswilldirectlyaffecttheusageoffossilsacross theglobe.Thisisattributabletovariousfactors.First,thepowergenerationsector thatprovideselectricitytorunanyindustry,corporateoffice,ortransportation sectorisdependentonfossilfuelsacrosstheglobe.Thisimpliesthatastechnical andinfrastructuraldevelopmentcontinuestoproceed,theusageofcarbon-rich andenvironmentallydetrimentalfuelscontinuestorise.Inaddition,invarious sectorssuchastransportation,thecurrenttechnologyincludestheusageofhydrocarbonfuels,whichhasledtosignificantenvironmentaldetrimentsacrosstheglobe.

Suchalarmingfactshaveledtoseriousconcernsacrosstheglobeandinvarious countriesattentionisbeingpaidtowardreducingthedependenceonaswellasusage offossilfuels.However,inseveralpartsoftheworld,fossilfuel-basedresources comprisethemajorenergysourcesandtheirdependencecouldnotbereduced significantlyuntilrecently. Fig.1.2 depictsthetotalusageofcoal,oil,andnatural gasacrosstheglobesincetheyear2000.Ascanbeobservedfromthefigure,the usageofeachofthesecarbon-richenergyresourceshasincreasedcontinuously. AmmoniaFuelCells. https://doi.org/10.1016/B978-0-12-822825-8.00001-3 # 2020ElsevierInc.Allrightsreserved.

Percentagechangeinglobalenergydemandbasedon2009.

DatafromRef. [1]

FIG.1.2

Globalconsumptionofcoal,naturalgas,andoil.

DatafromRef. [2]

Althoughvariousglobaleffortsandagreementsweremadetoreducefossilusageas wellasassociatedenvironmentalimpacts,theusageofsuchresourceshasincreased steadilyasshowninthefigure.Thisisattributabletovariousfactorssuchas increasedenergydemands.Tomeettheenergydemands,thesereadilyavailable resourcesareutilizedextensivelyacrosstheglobeleadingtoconsiderableenvironmentaldamage.Theusageofcoalincreasedfromnearly2300Mtoein2000tonearly 3800Mtoein2018.Thisdenotesasignificantincreaseof65%,whichshowsthe heavydependenceofvariouscountriesonfossilfuelsacrosstheglobe.Similarly, theglobalusageofnaturalgasincreasedfromapproximately2000Mtoein2000 tonearly3340Mtoein2018.Thisalsosignifiesanincreaseof67%intheusage ofthecarbon-richfossilfuel.Inaddition,oilisalsousedextensivelyinvarious sectorsrangingfromtheindustrialsectortothetransportationsector.Theusage ofoilhasalsoseenasteadyincreaseintherecentyearswhereanincreaseofnearly 25%isobservedfrom2000to2018.Theseareglobalestimatesthatconstitutethe usagebyallcountries.Somecountrieswherefossilfuel-basedenergyresources areasourceofincomeandaidinthedevelopmentoftheeconomy,entailasteady increaseintheproductionaswellasusageofcarbon-richfuels.However,these nationscouldessentiallydirecteffortstowardtheutilizationofcleanandrenewable energyresourceswhereapplicable.Forinstance,somegeographicallocations receivehigh-intensitysolarradiationacrosstheyear.Suchlocationsshouldemploy differenttypesofsolar-basedpowergenerationtechniques.Similarly,somelocationshavehighwindenergypotentialacrosstheyearthatshouldbeharvested.Also, otherrenewableenergyresourcesincludingbiomass,geothermal,andhydropower shouldalsobeconsideredwheretheyaresuitablyapplicable.

Therecentbreakdownofenergyresourceutilizationacrosstheglobeisdepicted in Fig.1.3 wheretheglobalpercentageofenergyresourcesusedforprimaryenergy supplyispresented.Asdepictedinthefigure,majorityoftheenergysupplywas attainedfromcarbon-richresources.Theseincluded27%coal,22%naturalgas, and32%oilresources [3].Thesethreecarbon-richfossilfuelscomprisenearly 80%ofthetotalfossilfuelusage.Thedeploymentofcleanenergyresourcessuch asbiofuels,nuclear,solar,etc.,entailsaminorportionoftheoverallusage.Although, aftercollectiveandcollaborativeefforts,thepercentageusageofcleanfuelshasrisen intherecentyears,thepresentscenariostillentailsaheavydependenceoncarbonrichfossilfuels.Theusageofbiofuelandwasteforpowergenerationhasgained paceandhasbeenimplementedinvariouscountriesacrosstheglobe.Thisentails theproductionofbiofuelsfromwastethatcomprisesusefulfuelssuchasethanol andbiodiesel.Ethanolcanbemadefromseveralplant-basedrawmaterialsthat aregenerallyreferredtoasplantbiomass.Moreover,biogasisanothertypeofbiofuelthatisproducedfromtheanaerobicdigestionoforganicmatter.Rawmaterials includingmanure,sewage,aswellasagriculturalwastecanbedeployedtoproduce biogas.Thiscomprisesmainlymethaneandcarbondioxide.Sincemethanehasa sufficientvalueofcombustionheatthatisemittedwhenburntwithoxygen,itisused asafuelforpowergeneration.AlthoughbiogasalsoentailsCO2 emissionswithits usageasafuel,itisgenerallyconsideredasarenewableenergyresourceowingtothe

DatafromRef. [3]

carboncycle.Inthisprocess,theCO2 emissionsresultingfromthecombustionof biofuelisrecycledthroughitsusagebyplantsduringphotosynthesis.Plantsuse CO2 tosynthesizeglucosethroughthephotosynthesisreactionthatincludesthe productionofglucosefromwaterandcarbondioxideinthepresenceofsunlight. Moreover,nuclearpowerisalsoconsideredtobeenvironmentallybenignbyvarious well-knownorganizationsasitdoesnotresultincarbonemissions.However,there areotherenvironmentalaswellassafetyhazardsassociatedwiththeusageofnuclear powerthathaveraisedconcernsinvariouspartsoftheworld.Inaddition,thenuclear powerplantaccidentsandtragediesresultedindecreasedattentiontowardnuclearbasedpowergeneration.Theseresourcesentailhighcarboncontentandthusresults inconsiderableamountofgreenhousegas(GHG)emissions.

TheCO2 emissionsthatresultedgloballyfromenergy-relatedfossilfuelusagein thepast20yearsfrom1998to2018aredepictedin Fig.1.4.Asdepictedinthefigure, therehasbeenasignificantincreaseintheCO2 emissionsinthepastdecade.In1998, theglobalCO2 emissionfromenergy-relatedfossilfuelutilizationwasrecordedto be23.4Gtonne.Thisincreasedsignificantlyto33.2Gtonnein2018 [4].Thereexists adirectrelationbetweentheusageoffossilfuelsforenergy-associatedactivitiesand theincreaseintheCO2 emissions,whichhavebeenidentifiedtoincreasetheglobal warmingphenomenon.Ascanbeobservedfromthefigure,thereisatrendofexponentialriseinCO2 emissionswheretherateofincreaseinemissionswithtimeis risingcontinuously.Everyyeartherateofchangerisesbyaspecificamountand theimportanceofreducingtheseemissionsisevidentfromsuchobservations. TheexponentialriseinCO2 emissionscanbelinkedtotheincreasedusageoffossil

FIG.1.3

Percentageofresourcesutilizedforprimaryenergysupplyglobally.

FIG.1.4

Globalcarbondioxideemissionsarisingfromfossilfuelusageforenergy-relatedactivities whichisalsocorrelatedinanexponentialform.

DatafromRef. [4]

fuelsaspresentedearlier.TheriseinCO2 emissionsisparticularlyalarmingowingto thegreenhouseeffectthatitentails.SolarradiationenteringtheEarth’satmosphere isreflectedbyvariousobjectsandistrappedwithintheatmosphereduetothe presenceofGHGssuchasCO2.Thetrappedsolarradiationleadstothetrapping ofthermalenergywithintheatmosphereoftheEarth.Thishasbeenfoundtodisturb theglobaltemperatureswhereanincreaseintheaveragetemperatureshasbeen evidentlyprovenintherecentyears.Thiscanhavevariousdetrimentaleffectson theecosystemwheretheiceglacierscanstartmeltingatalarmingratesleadingto ariseinsealevels.Whensealevelsrise,variousdangersareposedtowardcountries situatedneartheoceansandothermajorwaterbodies.Inadditiontothis,theweather cyclesandassociatedcoldaswellashotclimatetemperaturescanalsobeaffectedby globalwarmingphenomenon.Theglobalwarmingphenomenonhasraisedmajor concernsaboutthesustainabilityandstabilityoftheecosystemaswellasthefuture generationstocome.Hence,therehavebeenglobalconcerns,efforts,andinitiatives directedtowardobtainingasolutiontothisincreasedfossilfueldependenceandthe associatedenvironmentallyharmfulemissions.Apartfromcarbon-basedemissions, combustionoffossilfuelsalsoresultsintheemissionofnitrogenoxides(NOx)and sulfuroxides(SOx).Theseemissionshavebeenproventobedetrimentaltoboth humanhealthandtheenvironment.

Owingtothesemajorconcernsassociatedwithfossilfuelusage,moreenvironmentallybenignenergyproductiontechnologies,systems,anddevicesarebeing lookedintowiththeobjectiveofreducingthecurrentenvironmentalburden.Primary effortsintherecentpastweredirectedtowardsolar,wind,andotherenvironmentally benignpowergenerationmethods.However,theintermittencyoftheseenergy resourceshashinderedtheirwidespreadusage.Bothsolar-andwind-basedpower plantsentailthedisadvantageofnothavingareliableandcontinuouslystable inputsourceofenergy.Solarpowerplantsrequiresufficientsolarradiationinput tooperateandthisdependsheavilyontheweatherconditionsinthespecificarea wherethepowerplantissituated.Specifically,forsolarthermalpowerplants(solar heliostat,parabolictrough,etc.),theavailabilityofsufficientincomingsolarradiationisnecessarytoheattheworkingfluidtothetemperaturesrequiredforoperating theplant.Inadditiontothis,aftersunsetthesolar-basedpowerplantscanonlyrely ontheexcessenergystoredduringtheday.Theconventionalenergystoragemethods includingthermalenergystorageandbatteriesarecapableofprovidingtherequired powerforonlyalimitednumberofhours.Moreover,windturbine-basedpower plantsarealsohinderedbytheintermittencyofwindflowandvelocities.Attimes, high-velocitywindsoccurthataresufficienttomeetahighamountofload.Nevertheless,considerablefluctuationsoccurinwindvelocitiesthatmakeitdifficultto predictthetotalpoweroutputsofagivenwindturbinepowerplant.Also,thewind velocitiesattimescanbemuchlowerthantheminimumvelocitiesrequiredtooperatethewindturbines.Furthermore,otherrenewablesourcesofenergysuchashydroandgeothermalpowerplantsarelimitedbygeographicallocations.Areaswithhigh headwaterreservoirsorhighflowriversaresuitableforhydropowerplants,whereas areasthataredeprivedofthesecommoditiescannotentailthispowergeneration methodologysatisfactorily.Furthermore,geothermalplantsarealsolimitedby geographicalconstraints.Locationswheregeothermalfluidisreadilyavailable andgeothermalwellscanbebuiltaresuitedforsuchpowerplants.However,such resourcesarenotreadilyavailableatalllocationsacrosstheworld.Hence,fuelcells areconsideredpromisingcandidatesthatcanplayavitalroleintherealizationof anenvironmentallybenignenergyproductioninfrastructurewheretheutilization ofhydrogenorhydrogenentailingfuelsprovideahigh-efficiencymethodfor producingenergywithoutharmfulemissions.Inaddition,consideringthetransportationsector,therehasbeenasignificantriseintheGHGemissionsarisingfromthe useoffuelsfortransportation. Fig.1.5 depictsthecomparisonoftheCO2 emissions arisingfromcoal,oil,andnaturalgasgloballyin1990and2017.Thetotalemissions wereestimatedtoincreasefrom20.5GtonnesCO2 in1990to32.6GtonnesCO2 in 2017 [5].Thissignifiesanincreaseofnearly60%duringthistimeperiod.TheCO2 emissionsarisingfromcoalusageincreasedfromnearly8.3Gtonnesin1990to 14.5Gtonnesin2017,whichalsoconstitutesanincreaseofnearly60%.TheCO2 emissionsaredirectlylinkedtotheamountofusageofcarbon-richcoalenergy resourcesandthisincreaseshowsthattheirusagehasrisenconsiderablyintherecent decadesandthecorrespondingenvironmentalimpactshavealsoincreased. Similarly,theCO2 emissionsarisingfromtheusageofnaturalgasincreasedfrom

DatafromRef. [5].

3.4Gtonnesin1990to6.7Gtonnesin2017.Thisindicatesanincreaseofnearly 100%intheCO2 emissionsresultingfromnaturalgasusage.Thiscanbeattributed totheincreaseintheimplementationofnaturalgas-basedindustrialdevelopments. Also,thetotalriseinCO2 emissionsintherecentdecadecanbeattributedtothe increasedusageoffossilfuelvehiclesalongwithindustrializationduringtheseyears. Hence,severaleffortshavebeenexertedonthedevelopmentofenvironmentally benignvehiclesthatarefueledbycleanfuelssuchashydrogenorammoniaoroperateviabatterypower.

Cleantransportationcanplayakeyroleindecreasingtheenvironmentalimpacts thatarebeingcausedduetotheusageoffossils.Insomecitiesaroundtheworld, transportationsectorhasbeenidentifiedasthemajorcontributortowardtheworseningoftheairqualityindex.Theemissionsarisingfromautomobiletailpipes includeseveraltypesofharmfulcompoundsthatharmboththeenvironmentaswell ashumanhealth.Forinstance,carbonmonoxideisknowntobedetrimentaltohuman healthwhereitcombineswiththehemoglobininthebloodtoformpermanent compoundsthatcanhaveadverseeffectsonhumanhealth.Also,particulatematter emissionsthatareemittedfromtheusageofdifferenttypesoffossilfuelsare alsoharmfulforlungsandhavebeenidentifiedasoneofthereasonsforrespiratory diseases.Insomecountries,suchdetrimentaltailpipeemissionshaveforcedtheresidentstowearpollutionmaskswhileoutside.Moreover,theformationofsmogis anotherharmfulenvironmentaldetrimentthatiscausedduetotheincreasedusage offossilfuels.Whenexcessivesmokeisreleasedintotheatmosphereinthepresence offog,aharmfulmixtureisformedintheatmospherewhichisknownassmog. Thiscomprisesseveraltypesofharmfulcompoundsthatcanalsoleadtovarious respiratorydiseases.

FIG.1.5

Comparisonofcarbondioxideemissionsarisingfromdifferentenergyresourcesin1990 and2017.

Hence,theutilizationofautomobilesoperatingwithenvironmentallybenign fuelsisessentialforenvironmentalsustainability.Fuelcellvehiclesthathavebeen developedintherecentpastprimarilyincludetheutilizationofhydrogenfuelthatis passedthroughaprotonexchangemembrane(PEM)fuelcell,whichgeneratesthe requiredpowerneededbythevehicle.Thecentraladvantageofthesevehiclesliesin thefactthatwhenhydrogenisusedasthefuel,water(H2O)istheonlyoutputemission.Thusprovidingacleanmethodforpoweringvehicleswherenocarbonentailing emissionsarise.Also,PEMfuelcellsentailhigherefficienciesthanconventional enginesasdiscussedintheproceedingsections.Thisisalsoanadvantagethatis associatedwiththeusageoffuelcellvehicles.Intherecentpast,severalnewfuel cell-basedvehicleswereintroduced.TheHyundaiNexo,HondaClarity,Toyota Mirai,F-CellMercedes-Benzareafewexamplesoffuelcellcarsdevelopedby automotivecompaniesintherecentpast.Inadditiontothis,variouslocomotives andtrainspoweredbyfuelcellshavealsobeendevelopedintherecentyears. TheAlstomCoradiaiLintisoneofthelatestcommerciallyoperatedpassengertrain poweredbyfuelcelltechnology.Severalotherprototypetrains,locomotivesaswell asbuseswereintroducedrecentlyacrosstheglobe [6].Althougheffortsarebeing directedtowardthedevelopmentofhydrogen-basedenergyproductionforvarious sectors,severalchallengeshindertheflourishmentofthistechnology.Thehighflammabilityofhydrogenassociatesitwithahighsafetyriskwhiletransportationas wellasstorage.Further,thelowvolumetricdensityofhydrogengasnecessitates compressiontohighpressurestoattainanappropriatestoragevolume.Moreover, entailinganodorlesscharacteristic,hydrogenisnoteasytodetectwithoutproper hydrogensensors.Thus,increasingthesafetyconcernsowingtothehighflammabilityriskincaseofanyleakages.Owingtosuchchallenges,otherhydrogen-based fuelshavebeeninvestigatedforfuelcellapplications.Ammoniaisapromising hydrogen-basedfuelthatentailsthesolutionstothesechallengesbyhavingalow flammabilityrange,higherdensity,higherboilingpoint,andeasilydetectablesmell. Thus,ammoniahasbeeninvestigatedasafuelforvariouspowergenerationmethods includingfuelcells,internalcombustionenginesaswellasammoniagasturbines.

1.1 Historicalbackground

Thehistoryofutilizationofenergyresourcesisknowntohavebegunfromwoodasa sourceforobtainingthermalenergy.Intheearliertimes,woodwasburntthrough sparksemittedfromtherubbingofstones.Thethermalenergyobtainedfromthis processwasusedforvariouspurposes.Anestimatedtimelinedepictingtheevolution ofdominantandprevalentenergyresourcesfromwoodtofuelswithlowercarbon tohydrogenratiosisdepictedin Fig.1.6.Untilthemid-1700,theusageofwood asanenergyresourcewascommonthatdecreasedconsiderablyintheupcoming years.Throughtheexplorationofcoalasaneffectivefueltogeneratethermal energy,itsusagegainedextensivepopularity,especiallyintheindustrialrevolution. Specifically,incountrieswhereconsiderablecoalreserveswerefound,itwas

FIG.1.6

Evolutionofdominantenergyresourcesduringdifferenttimesdepictingthechangetoward lowcarbontohydrogenratiofuels.

adoptedastheprimarysourceofenergyproductionacrossthenation.Inaddition, whencoalpoweredlocomotiveswereintroduced,theusageofcoalincreasedextensivelyinmajorpartsoftheworld.Eventoday,somecountrieshavingmassivecoal reservesrelyonthecarbonintensivefuelforenergyproduction.Next,theadventof oilreservesoccurredwherecountrieswithsignificantamountofoilresourceswere observedtoflourishmassively.Infewyears,theusagehydrocarbongasessuchas naturalgas,propane,etc.,becameincreasinglycommoninvariouscountriesowing tothediscoveryofnewgasreserves.Also,theusageofgasolinepoweredcarsand automobilesbecameaglobalcommoditywheretransportationbecameheavily dependentonfossilfuels.Nearlyallmodesoftransportationincludingroad,air, andwatertransportmediumsweremanufacturedtooperatewithhydrocarbonfuels. Although,theevolutionofenergyresourceswasdependentonnumerousreasons, situations,andconditions,ageneraltrendofdecreasingcarbontohydrogenratios canbeobserved.Intherecentyears,whentheusageofhydrogenasafuelgained pace,thistrendfurthercontinuedtowardlowerandlowerusageofcarbon-based fuels.Entailingzerocarboncontent,hydrogenfuelisconsideredasapromising alternativetofossilfuelsthatcanovercomethemassiveenvironmentaldetriments, whichtheusageoffossilshavecausedintherecentpast.Theprimaryusageofhydrogenasafuelcomprisesusingelectrochemicaldevicesreferredtoasfuelcells.

Fuelcellsworkontheprincipleofthegenerationofelectricalenergyasa consequenceofaseriesofelectrochemicalreactions.Thisphenomenoncanbefound invariousaquaticanimalsincludingtheeel,torpedoray,etc.,thatareknownto generatevoltagesofnearly300V [7].Theprincipleofutilizingfuelcellsfor electricitygenerationbyprovidingthenecessaryelectrolytes,ions,andelectrodes isgenerallyattributedtoSirWilliamGrove,whohadestablishedtheworking principlesatisfactorilyin1839.

Nevertheless,theelectrochemicalprinciplesassociatedwiththeinteractionof dissimilarmetalsproducingelectricalsignalswereintroducedbySirLuigiGalvani, SirHumphreyDavy,andSirAlessandroVoltaintheearly19thcentury.Thesetypes ofelectrochemicalcellswerenamedthegalvanicandvoltaiccells.TheelectrochemicalcelldevelopedbySirHumphreyDavyutilizedtheinteractionbetweenoxygen obtainedvianitricacidandcarbonatoms.Thiscellwasparticularlyinterestinginthe sensethatitcouldproduceelectricalenergyviaconsumptionofcarbonentailing coal.However,thepracticalityrequiredforusefulapplicationswasnotachieved throughthismethodology.Nevertheless,theseembodimentsprovidedtheinitiative fordevelopingnewtypesofdirectcoalfuelcells,whichentailedtheworkingprincipleofusingcoal(readilyavailablefuelatthattime)toproduceelectricalenergy directlywithouttheusageofanintermediateheatengine.Moreover,theterm“fuel cell”isreportedtobecoinedbyMondandLanger [8].Sincethesetimes,several effortsweredirectedtowardproposing,developing,andinvestigatingdifferenttypes offuelcells.Considerableprogresswasachievedascomparedtotheinitialdevelopments.Theseprogressiveeffortswerenotcontinuousthroughouttimeandspans existedwherepeoplelosttheirinterestinfuelcelltechnologies.Thefirsttimeperiod whereincreasedinterestandeffortsexistedisgenerallystatedbetween1839and 1890.Duringthisperiod,theinitialdevelopmentoffuelcellsaswellastheintroductionofelectricitykeptthekeennessaliveinfuelcelltechnology.Specifically,owing totheuseoflow-efficiencysteamenginesatthosetimes,fuelcellswereconsidered promisingtechnologiesthatcanaidinutilizingcoaldirectlyforpowergenerationin amoreefficientway.Efficienciesofmorethan50%wereexpectedthroughthecoalbasedfuelcelltechnologies.However,afterseveralefforts,thisideawasabandoned consideringittobeimpracticalforusefulapplications.

Thenexttimespanafter1890swheretherewasconsiderableprogressinfuelcell inventionsexistedbetween1950and1960s.Significanteffortstowardresearchand theinventionofnewandbetterfuelcelltechnologiesstartedafterthesuccessful operationofthealkalinetypefuelcelltechnology.Theseweresatisfactorilyutilized inspacecraftapplications.TheApolloaswellasGeminiprogramsdevelopedbythe UnitedStatesraisedpositiveexpectationstowardthedevelopmentofmoreeconomicallyviableandtechnologicallypracticalfuelcellenergysystems.Furthermore, duringthisperiodseveralprivateaswellaspublicinitiativesandinvestmentswere madeinfuelcelldevelopment.Manytypesoffuelcellspopularlyknowntoday werestudiedandtheiradvancementwasinitiatedduringthisera.Theseinclude thepopularPEMfuelcell,solidoxidefuelcell(SOFC),moltencarbonatefuelcell (MCFC),andphosphoricacidfuelcell.Moreover,duringthiserathefuelcellswere differentiatedonthebasisoftheelectrolytemobility.Thealkalinefuelcells, forinstance,entailedelectrolytesthatcouldflowreadily,however,theSOFCwere associatedwithsolidelectrolytes.Thechoiceofelectrolytewasdependentonthe applicationandusage.Theinitiativesundertakenduringthiseracouldnotmeet theirexpectedoutcomesandobjectivesforfuelcelldevelopmentandapplication. Thisagainledtoalaybackinthedevelopmentofviablefuelcelltechnologies andtheinterestswerewithdrawnowingtotheunfavorableoutcomes.Nevertheless,

theerafilledwithconcernsregardingenvironmentalsustainabilityledtotherevival ofthedevelopmentoffuelcelltechnologies.Sinceearly1980s,continuousefforts havebeenexertedtowardreducingtheenvironmentalburdencausedduetothe colossalusageoffossilfuels.Further,thehumanhealthdetrimentscausedby theharmfulemissionsarisingfromfossilfuelusageaswellasdepletingfossil fuelreservesmadetheinvestigationanddevelopmentofnewenvironmentally benignenergyresourcesinevitable.Severaltypesoffuelcellsweredevelopedfor commercialapplicationsincludingPEM,AFC,SOFC,andMCFCintheseyears. Also,thefuelcelldevelopmentinthiserafocusedprimarilyonhydrogenfuel. However,severalchallengeshinderingtheflourishmentofhydrogenfuelincluding highflammability,lowvolumetricdensity,andsafetyandeconomicdisadvantages fortransportationandstoragehaveledtoincreasedeffortsinthedevelopmentof alternativefuels.Thedetaileddiscussionaboutthechallengesfacedbyhydrogenthat hinderitscurrentwidespreadusagearediscussedinthisbook.Owingtothosereasons,ammoniahasbeenidentifiedasacarbon-freefuelthatcanalsobeutilizedfor cleanpowerproductionanditentailsseveraladvantageouspropertiesandcharacteristics.Severaltypesofammonia-basedpowergenerationtechniquesexist.Energy canbeobtainedfromammoniafuelthroughconventionalenginesincluding compressionorsparkignitionsystems.Nevertheless,thecompressionratiosrequired forammoniacombustionaredifferentfromdieselfuel.Further,duetospecific flammabilitylimitsofammonia,ithastobeoftenblendedwithdifferentcombustion promotersthatcompriseconventionalfuels.Moreover,ammoniagasturbinescan alsobeemployedforpowergenerationwherethegas-basedpowergeneration cyclecomprisingcompression,combustion,andexpansionprocessescanbeimplemented.Inammoniagasturbines,ammoniafuelisusedinsteadoftheconventional gasturbinesthatarefueledwithnaturalgasasthefuel.Moreover,asammonia isusedinthermalenginesthroughtheprocessofcombustion,itcanalsobeused inelectrochemicalenginescomposedofdifferenttypesoffuelcelltechnologies whereelectrochemicalreactionofammoniaandoxygenoccurs.Thisistheprimary focusofthisbook.In1960,Cairnsetal. [9] studiedthepossibilityofutilizingammoniaforgeneratingelectricalenergyviafuelcells.Theyutilizedapotassiumhydroxideelectrolyte,thusconstitutinganalkalinefuelcelltype.Sincethen,several researchersandscientistshaveexertedeffortsonthedevelopmentofammonia-based fuelcellsforpowergenerationthataredescribedcomprehensivelyintheproceeding chapters.

Inthisbook,first,theunderlyingfundamentalsoffuelcelltechnologiesare describedindetailastheyprovidetheb ackgroundinformationneededtounderstandtheworkingmethodologyofdifferenttypesofammoniafuelcells.The necessarycomponentsrequiredtoconstituteafuelcel ldevicearedescribedand thephysicalaswellaschemicalphenomenaoccurringateachofthesecomponents arediscussed.Further,theclassificationoffuelcellsaccordingtodifferentclassificationcategoriesisdescribedwheret hecategorizationaccordingtothetype ofelectrolytes,fuel,andoperatingconditionsispresented.Next,thebookcovers differenttypesoffuelsthathavebeeni nvestigatedforfuelcellapplications.

Theserangefromhydrogenasthemostc ommonlyemployedfueltodifferent typesofalcoholsaswellasalkanes.Ammoniaasapromisingfuelisthendescribed comprehensivelypresentingvariousadvantagesandfavorablepropertiesitentails. Further,ammoniafuelcellsarecovered comprehensivelywheredevelopmentof differenttypesofammoniafuelcellsaredescribed.Theperformanceofeachtype ofammoniafuelcellhasbeenlinkedtothetypeofelectrolyte,electrochemical catalyst,andelectrodesused.Ammoniafueledhigh-temperatureSOFCswith bothproton-conductingandanion-conductingelectrolytesisdiscussed.Also, low-temperaturedirectammoniaalkalinefuelcellsarecoveredindepth.The development,materials,operation,andsystemconditionsoftheseammoniafuel cellsarepresented.Moreover,theanalysisandmodelingoffuelcellsispresented withaspecificfocusonelectrochemicalinteractionof ammoniamolecules thatoccursindirectammoniafuelcells.Theperformanceofthesedifferent typesofammoniafuelcellsis alsodiscussed.Severalnewintegratedammoniafuel cell-basedsystemsthathavebeendevelopedintherecentpastarepresentedand theirperformancesareassessedthroughoverallenergyandexergyefficiencies. Lastly,novelammoniafuelcell-basedtechnologiesarepresentedascasestudies andtheirperformancesareinvestigatedatvaryingoperatingconditionsand systemparameters.Thebookcloseswith severalrecommendationsforfuture developmentofammoniafuelcellsidentifyingthekeychallengesfacedbythese technologies.

1.2 Closingremarks

Inthischapter,theimportanceofreducingthedependencyonfossilfuel-based energyproductionishighlighted.Therisingenergydemandsacrosstheglobeare shownthroughrecentstatisticscovering.Furthermore,thecorrespondingrisein CO2 emissionsisexplainedshowingthetrendintheexponentialriseintheseemissionsintherecentyears.Also,acomparisonofCO2 emissionsfromcarbon-richcoal, naturalgas,oil,andbiomassenergyresourcesismadecomparingrecentemission datawithrecentdecades.Thestatisticsarealarmingandshowthatimmediateattentionisneededtowardthereductionoffossilfuelusageandriseofrenewableenergy utilization.Moreover,ahistoricalperspectiveofenergyresourcesispresentedshowingthetrendtowardtheutilizationoflowcarboncontentenergyresources,wherethe initialusageofwoodasanenergyfuelwasreplacedwithcoalwhencoalminingwas introduced.Further,theadventofnaturalgasenergyreservesledtotheshifttoward naturalgas-basedpowergenerationinvariouspartsoftheworld.Further,afteroil wasintroducedalongwithitsvariousby-productsforutilizationinvariousindustries,itsusagegainedmomentum.Next,theincreasedattentiontowardlow-carbon fuelsledtotheusageofcarbon-freehydrogenfuelinrecentyears.Thehistorical backgroundoffuelcelltechnologiesisalsopresentedprovidinganoverviewof thetechnologicaldevelopmentandadvancementsthatoccurredsincethe19th century.

Fundamentals 2

Inthischapter,thefundamentalsoffuelcellsarepresentedandcomprehensively elucidated.Theconceptofhydrogeneconomyisfirstdiscussedfollowedbythe underlyingoperatingphenomenaoffuelcells.Thesimilitudeoffuelcellstoother electrochemicaldevicesispresentedandtheessentialcomponentsconstitutinga fuelcelldevicearedescribed.Furthermore,thechemicalandphysicalphenomena thatoccurwithinafuelcellduringtheelectrochemicalinteractionsareexplained. Theclassificationofdifferenttypesoffuelcellsaccordingtothetypeofelectrolyte, electrode,operatingtemperature,etc.,isalsopresented.

2.1 Hydrogeneconomy

Thehydrogeneconomyentailstheconceptofutilizinghydrogenastheprimefuelfor variousapplications.Someoftheseapplicationsincludeelectricitygeneration,transportation,metalrefining,fertilizerproduction,syntheticfuelproduction,etc.The ecosystembasedonhydrogencomprisescleanelectricitygenerationfromsolar, wind,ornuclearpower.Theelectricityproducedfromsuchenvironmentallybenign energyresourcesissuppliedtothegridandtheexcessenergyavailableintheformof solarorwindenergyisstoredintheformofhydrogen.Hydrogenissynthesizedvia waterelectrolysisthatdoesnotentailcarbonemissions,whichareassociatedwith steammethanereforming-basedhydrogenproduction.Thehydrogensynthesized fromthesecleanenergyresourcesisusedasafueltopowerautomobilesaswell asothertransportationvehicles.Further,theproducedhydrogenisalsousedtosynthesizesyntheticfuelssuchasammonia.Also,otherapplicationsincludeindustrial usagesuchasrefiningofmetals.Inadditiontothis,duringperiodsoflowenergy availability,theproducedhydrogencanbeusedasafueltogenerateelectricity throughtheusageoffuelcells.Moreover,toutilizehydrogenasafuelintransportationvehicles,fuelcelltechnologiesarerequired.Hence,fuelcellsentailhigh significanceinthehydrogeneconomy.Fuelcellsentailsomeresemblancetobatteries.Consideranon-rechargeablebatteryasdepictedin Fig.2.1.Insuchchemicalto electricalenergyconversiondevices,thereactantfuelaswellastherequiredreactant oxidantsarestoredwithinanenclosedcontainment.Owingtothisprinciple,they aresometimesknownasonboardstoragedevices.Batteriesarethusenergystorage devicessuchthattheystorechemicalenergyintheformofthefuelandoxidants.

FIG.2.1

Schematicrepresentationofabatterywithenclosedreactantfuelandoxidant.

Whenelectricalenergygenerationisrequired,thisstoredchemicalenergyis convertedintoelectricalenergyviaelectrochemicalinteractionsbetweenthestored reactantfuelandtheoxidants.Hence,eachbatteryhasaspecificamountofoutput electricalenergythatitcansupply.Thisamountisdirectlydependentontheamount ofreactantfuelandoxidantentailedintheenclosedbatterycontainment.Inthecase ofanon-rechargeablebattery,themaximumoutputelectricalenergyisthuslimited bytheenclosedreactantamount.Oncetheycompletelyreactwitheachother,no morechemicalenergytoelectricalenergyconversionispossible.Thelifespanof abatterypowerreserveisthuslimited,accordingtothechemicalreactantssatisfactorilyenclosed.Inadditiontothis,asallthereactantsrequiredfortheelectrochemicalconversionareenclosedtogether,theelectrochemicalinteractionsbegineven beforepowerisextractedfromthebatteryforusefulpurposes.Althoughthese interactionsoccurataslowrate,ifbatteriesareunusedforlongperiodsoftime,these interactionsmaycausealargelossofusefuloutputpotential.Thisisanotherlimitationofstoringenergyintheformofbatteries.Furthermore,theelectrodeemployed inabatterydeviceisalsoconsumedduringtheelectrochemicalconversion.Itis thussometimesstatedthatthebatterylifetimeisafunctionoftheelectrodelifetime. Thehighertheelectrodelifetime,thelongerwillbethelifetimeofthebatteryunit.

However,afuelcell(so-called:anelectrochemicaldevice)differsfromabattery primarilywiththeprinciplethatthereactantfuelaswellasoxidantarenotstored withinthecellandarecontinuouslysuppliedfromanexternalsourceorfuelreservoir.Asimplifiedschematicrepresentationofthisisdepictedin Fig.2.2.Afuel reservoirthatexistsoutsidethefuelcellstoresthefuelandsuppliesaccordingto therequiredpoweroutputneededfromthecell.Also,theoxidantneededfortheelectrochemicalreactionisobtainedexternallyusuallyfromambientairoranoxygen reservoirifnecessary.Moreover,astheelectrochemicalinteractionstakeplace betweenthefuelandtheoxidant,thereactionproductisformedonanelectrode thatneedstobeextractedfromthecellcontinuously.Also,theexothermicreactions produceheat,whichalsoneedstobeextractedfromthefuelcelltopreventoverheatingofthecellthatcandeterioratetheperformancessignificantly.Therefore,unlike

batterysystems,fuelcellscanoperatecontinuouslyaslongastheyreceivethefuel andoxidantinputs.Incomparison,batterysystemscanonlyoperateinconjunction withthereactantamountenclosedwithinthecontainment.Duetothis,fuelcellsare consideredindependentoflifetimespansasfarasthefuelisconcerned.Theycan operatecontinuouslyaslongasafuelinputfeedisprovided.However,other fuelcellcomponentsincludingthecatalystlayers,membranes,electrodes,etc.,have lifetimes,whicharesufficientlylongascomparedtoabatterysystem.Moreover,as thefuelcelldoesnotstorethereactants,thereisnolossofenergyinthecaseof non-utilizationofthedeviceforlongperiodsasforthecaseofbatteries.Infuelcell systems,thefuelissafelystoredincylindersorcontainersthatdonotallowany chemicalinteractionbetweenthefuelandanyotherexternalchemicalcompound. Thefuelisdirectlypassedfromthestoragetankstothefuelcelldevice.Also,the electrodesinfuelcelldevicesdonotgenerallyparticipateintheelectrochemical reactionsandarethusnotconsumedduringtheprocess.Thisisnotthecasefor batterysystems,whereelectrodesplayakeyroleintheelectrochemicalreactions occurringwithinthecell.

Nevertheless,thereareseveralsimilaritiesbetweenfuelcellsandbatteries.First, bothentailtheprincipleofconvertingchemicalenergydirectlyintoelectrical energy.Furthermore,bothutilizeelectrochemicalinteractionsforproducingelectricalenergy.Hence,bothareelectrochemicalenergyconversiondevices.Also,both

FIG.2.2

Simplifiedschematicrepresentationofafuelcelloperatingwithexternalfuelandoxidant inputs.

entailtheusageofelectrolytesandelectrodes(anodesandcathodes).Theterm“battery,”however,canbeassociatedtoSirBenjaminFranklin,whohasutilizedthis termtodescribeacollectionofcapacitorsthatwerealsoknownasLeydenjars.

2.2 Fuelcellsandheatengines

Asdiscussedearlier,fuelcellsandheatengine-basedelectricalgeneratorsalso entailsimilaritiesintheir overallenergyconversionprinciples.Bothutilizethe inherentchemicalenergyoffueltogenerateelectricalenergy.However,inaheat engine-basedelectricitygenerationsyste m,thechemicalenergyoftheinputfuelis firstlyconvertedintothermalenergy.Th einputfueliscombustedinthepresence ofanoxidantandtheresultingcombustionchemicalreactiongeneratesheatowing toitsexothermicnature.Theheatorthermalenergythatisgeneratedisthen utilizedfurthertoproducemechanicalenergy.Theresultingmechanicalenergy isfurtherutilizedtogenerateelectricalene rgyviaanelectricgeneratorthatentails currentcarryingcoilsandthusmagneticflux,whichgenerateselectricalenergyin thepresenceofrotationalkineticormechanicalenergyoftheshaft.Thus,aheat engine-basedelectricityge nerationmethodentailsaser iesofenergyconversion stepsthatstartwiththeconversionofchemicalenergyentailedinafuelinto thermalenergy,whichisfurtherconvertedintomechanicalenergythatis convertedfinallyintoelectricalenergy. Asimpleschematicrepresentationofheat engineisdepictedin Fig.2.3.Mostimportantly,everyenergyconversionstep involveslossesaseachtypeofenergyconversionincludesirreversibilitiesand exergydestructions.Notallthechemicalenergyentailedinthereactantscanbe convertedintothermalenergyandnotallthethermalenergyproducedbyfuel combustioncanbeconvertedintomechanicalenergy.Furthermore,themechanical energyoftherotatingshaftcannotbecompletelyconvertedintoelectricalenergy. Ontheotherhand,afuelcellentailsdirectconversionofchemicalenergyinto electricalenergy,whichallowstheavoidingofseveralenergyconversionsteps andthusresultinginhigherenergyefficiencies.Inaddition,heatengine-based electricalenergygenerationincludesthelimitsposedbytheCarnotefficiency. SincenopracticalheatenginecanhaveanefficiencyofmorethantheCarnot engine,theCarnotefficiencythatdependsonthehighandlowtemperaturesin thesystemdefineanupperlimitforthe efficiencyforanygivenheatengine. Thehightemperatureoftheheatenginesy stemthatdirectlyaffectstheCarnot efficiencyisafunctionofthefuelcombustionreactionandthusisalsolimited. Theselimitationscanbeduetoseveralfactorssuchastheheatlosses,heatabsorptionsbysideprocessesandreactionsaswellasconversionratios.Also,heat enginesneedtobeequippedwithseveralmovingpartsandcomponents.These partsalwaysinvolvefrictionallosses aswellasperformancedegradationwith timeduetowear.Thesefactorsalsocontributetodecreaseintheefficiencies. Inadditiontoefficiencylimitations,heatenginesarealsoassociatedwithenvironmentaldetrimentsthathaveraisedmajorconcernsacrosstheglobeintherecent

past.Conventionalelectricitygenerationpowerplants involvetheusageofheat engines,whicharemainlypoweredbyfossilfuelresourcessuchascoalornatural gasatpresent.Whensuchfossilfuelresourcesareutilized,theyresultinlarge amountsofGHGemissionsaswellasotherharmfulemissionsthataredetrimental toboththeenvironmentandthehumanhealth.

Consideringtheabove-discussedissues,fuelcellsareseemedtobepromising electricitygenerationdevicesascomparedtoheatengines.Thisisduetothe possibilityofgeneratingelectricalener gyatahigherefficiencythanaheatengine. Also,theenergyconversionisasinglestepprocessratherthanseveralconversion stepsasinthecaseofaheatengineandthusinvolvedloweramountofirreversibilitiesandlosses.Further,unlikeheatengines,fuelcellsdonotincludeseveral movingpartsthatinvolveregulardeteri orationandthusrequiremaintenance, resultinginlowermaintenancecosts.Nevertheless,fuelcellsarealsolimitedby severalfactors.Theseincludedeteriorationofcatalystlayers,thecatalystlayers thataredevelopedtoaidintheelectrochemicalreactionalsodegradewithtime. Thetimefordegradationdependsonthetypeofcatalysts,membranes,andfuels utilized.Also,othercomponentsincluding electrodesandgasdi ffusionlayerscan alsogetdamagedovertime.

FIG.2.3

Simplifiedschematicrepresentationofaheatengine-basedelectricitygenerationsystem.

2.3 Fuelcellworkingprinciples

Agivenfuelcelldevicecomprisesthreeessentialbuildingblocksthatarenecessary fortheoperationofelectrochemicalinteractions.First,theanodeorthefuelelectrode wherethefuelisinputtothecellandtheanodicelectrochemicalreactionsoccur inthepresenceofcatalysts.Second,thecathodeortheoxidantelectrodewhere theoxidantreactantsareinputtothecellandtheyreactelectrochemicallyaccording tothecathodicelectrochemicalreactionsofthecell.Third,theelectrolytethatisgenerallysituatedbetweentheanodeandcathode.Theelectrolyteisalsoakeycomponentnecessaryforthetransferofionswithinthecell.Aschematicrepresentation ofthesefuelcellcomponentsandthecorrespondingfunctionsforaprotonexchange membrane(PEM)hydrogenfuelcellaredepictedin Fig.2.4.Asdepictedinthe figure,thefuel,thatis,hydrogeninthiscaseentersattheanodesideofthecell. Theanodeorthefuelelectrodeallowsthepassageofhydrogengastoreachthe catalystlayerswheretheyreactelectrochemicallyandemitelectrons.

Theanodicelectrochemicalreactioninthiscasecanberepresentedas

Theelectronsemittedasaresultofthishalf-cellelectrochemicalreactiongeneratethe currentinthecircuitthatcanincludeanexternalloadthroughwhichelectronflowis generated.Astheanodicelectrochemicalreactionkeepsemittingelectrons,thecorrespondingcathodicreactionassociatedwithacceptingelectronsisdenotedby

wherethepositivelychargedH+ ionsaretransferredtothecathodicsideofthecell throughthemembraneelectrolyte.InPEMfuelcells,theelectrolytecomprisesa membranethatallowsthepassageofpositivelychargedH+ ions,whichareoften referredtoasprotons.Theoxygenneededatthecathodetocompletethecathodic electrochemicalreactionissuppliedfromanexternalsource.Ifambientairisutilized astheoxygensource,italsoentailsnitrogenthatexitsthecathodiccompartment unreacted.Ascanbeobservedfromthecathodicreaction,thefinalproductof theoverallreactioniswater(H2O).Thus,theseriesofion,electron,andmasstransfer stepskeepproducingelectricalenergythatcanbeutilizedforusefulpurposes. Theoverallreactionforahydrogenfuelcellisexpressedas

H2 + 1 2 O2 ! H2 O (2.3)

Inhydrogenfuelcells,aswaterisformedcontinuouslyatthecathode,itneedstobe removedfromthecellsastheaccumulationofwatercausesfloodinginthecell.High accumulationofwatermoleculeshindersthemasstransportandthusaffectsthe overallelectrochemicalactivity.Inaddition,theoverallreactionbeingexothermic resultsinthegenerationofheat.Hence,continuousheatremovalfromthefuelcell isnecessarytopreventdamageofcellcomponents.

Theworkingprinciplesofammoniafuelcellsaresimilartohydrogenfuelcells entailingelectrodereactionsaswellasmembraneelectrolytes.However,alkaline electrolyte-basedammoniafuelcellsentailseveraldifferencesascomparedto hydrogenfueledPEMfuelcells.Aschematicrepresentationofanalkaline electrolyte-baseddirectammoniafuelcells(DAFCs)isshownin Fig.2.5 illustrating thereactantsandproductsatboththeanodeandthecathode. FIG.2.5

Schematicrepresentationofadirectammoniafuelcell.

ThefuelinputfeedinaDAFCcomprisesadirectinletofammonia(NH3).In alkaline-basedDAFC,thisinputfuelreactselectrochemicallyattheanodeinthe presenceofcatalystwithnegativelychargedhydroxylions.Thisanodicreaction isrepresentedby

Theaboveanodicelectrochemicalreactionemitselectronsthatgenerateaflowof electronsthroughanyexternalloadandallowusefulpowerutilization.Thecorrespondingcathodicelectrochemicalreactionthatacceptselectronsandcompletes theoverallreactionisrepresentedas

Thenecessarycathodicreactantsofoxygen(O2)andwater(H2O)moleculesneedto beprovidedatthecathode,whichalsoreactinthepresenceofanelectrochemical catalysttoformhydroxyl(OH )ions.Thesehydroxylionsformedatthecathode migratetotheanodicsideofthefuelcellthroughanalkalineelectrolytethatallows thepassageofonlynegativelychargedanions.Hence,inthisway,theelectrochemicalreactionskeepoccurringateitherelectrodeswithfuelandoxidantinputsand usefulpowercanbecontinuouslyextractedfromthecell.Theoverallfuelcellreactionforanalkalineelectrolyte-basedDAFCcanberepresentedas

ThereareothertypesofDAFCthathavebeendevelopedintherecentpast,which willbediscussedindetailintheupcomingsectionsofthisbook.AlthoughtheworkingprinciplesofDAFCaresatisfactorilywellestablished,theirperformanceis considerablylowascomparedtohydrogenfuelcells.Thisisprimarilyattributed totheinsufficientelectrooxidationofammoniamolecules.Thetypeofcatalyst activityobservedinthecaseofhydrogenoxidationforfuelcellshasnotbeen achievedyetforthecaseofammoniaoxidation.Thisremainsoneofthekeyfactors thatprohibitthedevelopmentofhigh-performanceDAFC.

2.4 Chemicalandphysicalphenomenainfuelcells

Theworkingprinciplesdescribedforhydrogenorammoniafuelcellsintheprecedingsectionrepresenttheoverallprocesses.However,thereareseveralotherphenomenathatoccurduringtheoperationoffuelcellsandaffecttheelectrochemical

behaviorandthustheperformance.Thesecanbemainlyclassifiedintochemicaland physicalphenomena.Theunderlyingchemicalphenomenonincludesprocessesof dissociationofhydrogenmoleculestohydrogenatomsandthesubsequentadsorptionprocessthatisfollowedfinallybytheoxidationprocess.Thephysicalphenomenonincludesmassandspeciestransportprocessesofthereactantsaswellas productsthattraveltoandfromthereactionsitestotheexitandbulkconcentration sides.Thetransportphenomenonoccurringwithinthecellaffectsvariousparameters includingvoltagelossesthatoccurwithinthecellaswellasthetransferofheat. Forinstance,considerafuelcellcomprisingaliquidelectrolyte.Asthefuelenters thecell,itcomprisesamixtureofthefuelaswellassomeminoramountsofwater vapor,hydrocarbons,orothercarbon-basedcompoundsdependingonthesourceof thefuel.Thefuelmixtureortheoxidantmixturereachesthesurfaceoftheelectrode bymeansofconvection.Theelectrodeisgenerallymadeofporousmaterialtoallow thepassageofreactantoroxidantmoleculesandbymeansofdiffusion,thesegases arriveattheadjacentinterfacecomprisingtheelectrolyteandreactantgascontact. Next,inthecaseofaliquidelectrolyte,dissolvingofreactantmoleculesoccursat thisinterface.Thisinterfacecomprisesboththeliquidelectrolyteandthegaseous reactantandisthusoftenreferredtoastwo-phaseinterface.Furthermore,afterdissolvingintheelectrolyte,diffusionofthesedissolvedspeciesoccursfromthetwophaseinterfacetothesurfaceoftheelectrode.Duringtheseprocesses,someundesiredreactionsmayalsooccur.Theseincludethereactionsbetweentheelectrolyte andanyimpuritiesthatwereentailedinthereactantmixture.Also,corrosionof theelectrodemayoccurduetocontactwithanyunwantedoxidizingspecies.However,thenextmajorprocesscomprisesadsorption.Thosespeciesthatreactelectrochemicallyarefirstadsorbedonthesurfaceoftheelectrode.Themigrationof adsorbedspeciesonthesurfaceoftheelectrodeoccursbydiffusion.Next,atthe surfaceoftheelectrodethatisincontactwiththeliquidelectrolyte,theelectrochemicalreactionstakeplaceduetowhichpositiveornegativeionsaswellaselectrons aregenerated.Thisinterfaceisgenerallyreferredtoasthethree-phaseboundary. Astheionsaccumulateatthesurfaceoftheelectrode,diffusionofthesecharged speciesoccursacrossthesurface.Further,theproductsoftheelectrochemical reactionaredesorbedfromthesurfaceoftheelectrode.Intheelectrochemicalcell, duetotheformationofoppositechargesacrossthetwoelectrodes,anelectricfield iscreatedthataidsinthetransferofelectronsaswellions.Theelectrochemical reactionproductsthataredesorbed,diffusethroughtheelectrolytetoreachthe two-phaseinterfacefromwheretheyfinallyexitthecell.

Aschematicrepresentationoftheseprocessesisdepictedin Fig.2.6.Thegaseous fuel(consideredH2 inthiscase)entersthecellfromthebottomandreachesthe electrolyteandfuelinterface.Further,themoleculesdissolvedatthisinterface diffusethroughtheelectrolyteandreachthesurfaceoftheelectrodeandtheadsorptionphenomenonproceeds.Aftertheprocessofadsorption,theelectrochemical interactionoccursresultingintheformationofionsandelectrons.

2.5 Electrodesandelectrolytesinfuelcells

Ascanbeobservedfromthefuelcellprocessesandphenomenadiscussedearlier, electrodesaswellaselectrolytearecentr alfuelcellcomponentsthatareneededfor theelectrochemicalinteractionstooccurandproduceusefulelectricalenergy. Therearethreewaysinwhichtheutilizationofelectrodesinfuelcellscanbe described.First,electrodesprovidethe requiredreactionsitesnecessaryforthe dissociation,adsorption,andthustheelectrochemicalreactiontooccur.Second, electrodesalsoprovideappropriateflowchannelsforthefuelcellreactantsaswell asproducts.Especially,inthecaseoffuelcellstacks,electrodeswithsuitableflow channelsaredesignedandusedthatallowsmoothpassageofreactantsovereach cell.Third,electrodesalsoactaselectro ncollectors.Whenelectronsareemitted duringanelectrochemicalreaction,theyfirstaccumulateattheelectrodethat createsanelectricpotentialbetweentwoelectrodes.Thisactsasthedrivingforce tomovetheelectronsthroughanexternalcircuitwheretheloadissituated.One importantfeatureofelectrodesistheirporousstructure.Asasolidstructureis neededforprovidingadsorptionandreactionsites,andanemptypassagespace isneededfortheflowofreactantsandproducts,aporousstructureisdesired

FIG.2.6

Differentchemicalandphysicalphenomenaoccurringinaliquidelectrolytefuelcell.

forelectrodes.Furthermore,astheelectrodesaresubjectedtovarioustypesofreactantsaswellasoxidants,theirchemicals tabilityisessentialforappropriatefuel celloperation.Incaseofanyundesiredsidereactionsbetweentheelectrodeand reactantsorproducts,theperformanceofthefuelcelldeterioratesconsiderably. Inaddition,goodmechanical strength,especiallyhighcompressivestrength,is desiredforfuelcellelectrodes.Theelectrodeswithporousstructuresaregenerally developedinlayeredconfigurations.Thefirstlayercomprisestheporousstructure thatallowsthepassageofreactantsandcollectselectrons(inconventionalfuel cellstheseareknownasgasdiffusionlayers).Thesecondlayercomprisesacatalystlayeralongwiththeinterfaceofthee lectrolyte.Conventionally,platinumis themostcommonlyemployedmetalthatisutilizedforfuelcellelectrodes.However,owingtoitshighcost,severalresearchstudiesarebeingconductedtodevelop catalystscomprisingnonpreciousmetalssuchasnickeloriron.Inthecaseof ammoniafuelcells,platinumcatalystsa repoisonedduringthedissociationand adsorptionprocesses.ThisaffectstheperformanceofDAFCsadverselyresulting inlowopen-circuitvoltages,poweroutput s,andcurrentdensities.Duetothisreason,severalstudiesintherecentpasthavefocusedondevelopingnewtypesof electro-catalyststhatarecompatiblewith electrochemicalinteractionofammonia molecules.Thedifferenttypesofcatalys tsdevelopedforammoniafuelcellsand theirrespectiveperformancesaredescribedinChapter4.

Theelectrolyteisalsoakeyfuelcellcomponentthatprovidesthemedium requiredfortheionstotransferfromone electrodetotheotherelectrode.The functionsoftheelectrolytecanalsobes ummarizedinthreeways.First,electrolyteistheionconductingmediumthatallowstheionicspeciesformedduringthe electrochemicalreactionstotravelfromtheelectrodewheretheyareformedto thecounterelectrodetocompletethehalf- cellelectrochemicalreactions.Second, theelectrolytealsoactsasanelectricinsulatorthatpreventsthecellfromshort circuiting.Ashortcircuitoccursinafuelcelliftheanodeandcathodeelectrodes comeintocontact.Inthiscase,theelectr onsareprohibitedfromtravelingthrough anyexternalloadandtheanode – cathodecontactshortcircuitsthecell.Thus,the electrolytethatisgenerallysandwiched betweenthetwoelectrodespreventsany electricalcontactandthusanychancesofcellshortcircuit.Third,theelectrolyte alsoseparatesthetwocompartmentsofthefuelcellthatcomprisethefuelinput sideandtheoxidantinputside.Theelectrolytesaremadefrommaterialsthatdo notallowthecrossflowofthefueloroxidantfromeitherside.Crossflowof reactants,alsoknownascrossover,canadverselyaffectthefuelcellperformance. Also,ifthefuelutilizediscombustibleandcrossesovertotheoxidantside,there canbesafetyhazards.Hence,asuitablefuelcellelectrolyteisrequiredtoentail severalpropertiesthatmakeitcompatibleforusage.Insolubleandnonporous propertiestowardreactantsarekeycharact eristics.Further,hi ghionicconductivityisdesirableforefficientflowofionsthroughtheelectrolyte,whichisessential toachievehighfuelcellperformances.

2.6

Performanceoffuelcells

Acompletefuelcellreactioncomprisestwohalf-cellelectrochemicalreactionsthat occurateitherelectrode.Atoneelectrode(anode),theelectrochemicalreaction emitselectrons(anodicreaction)andattheotherelectrode(cathode),theelectrochemicalreactionacceptselectrons,thus,completingafullcircuit.Sinceeachelectrodeisaccompaniedbyanelectrochemicalreaction,itentailsanelectricpotential thatiscommonlyreferredtoasthehalf-cellpotential.Theanodicelectrochemical reactiongivesrisetoananodicelectricpotentialandthecathodicelectrochemical reactionresultsinacathodicelectricpotential.Theoverallfuelcellpotentialisthus determinedfromthedifferencebetweenthesetwohalf-cellpotentials.Theelectrons aregeneratedattheanodeandflowtowardthecathodethroughanexternalcircuit, hencetheelectronflowisfromtheanodetothecathode.However,accordingtothe conventionalcurrentdirectionterminology,thecurrentwouldbestatedtobeflowing intheoppositedirectionofelectronflow(cathodetoanode).Theoveralltheoretical fuelcellpotentialforahydrogenfuelcellconsideringEq. (2.1) astheanodicreaction andEq. (2.2) asthecathodicreactionis1.229V.Foranammoniafuelcell,thetheoreticalpotentialis1.17VconsideringEqs.(2.4)and(2.5)astheanodicandcathodic electrochemicalreactions,respectively.Thesepotentialsaretheoreticalandare obtainedifthefuelisconsideredcompletelypureandatambientconditionsof 1atmpressureand25°C.Furthermore,thesepotentialsaregenerallyknownas reversiblefuelcellpotentialsastheyareevaluatedforthermodynamicallyreversible operation.ThisisdiscussedindetailinChapter5.

Inactualfuelcelloperation,severalirreversibilitiesandlossesoccurinevitably. Theselossesoccurprimarilyintheformofpotentiallossesandowingtothese irreversibilities,theworkingcellpotentialreducessubstantially.Forinstance,for ahydrogenfuelcell,theworkingcellpotentialmightdroptonearly0.7Vifthe optimalpowerdensityrangesaretargetedinpracticalapplicationsofpoweroutputs. Also,inthecaseoftheammoniafuelcells,severalundesiredlossescandropthe open-circuitvoltagetonearly0.3V.Furthermore,awell-establishedphenomenon forfuelcellsisthedropincellvoltagewithanincreaseinthecurrentdensities. Asthecurrentdrawnfromthecellisincreased,thecellvoltageisobservedto decreaseduetoseveralvoltagelosses.Infuelcells,thelossesinvoltagedueto anincreaseincurrentdensitiesisgenerallyknownasoverpotentialorovervoltage. Also,thephysicalorchemicalphenomenoncausingthisoverpotentialisknownas polarization.Generally,thecathodicelectrochemicalreactionentailingthereduction ofoxygenmoleculesisassociatedwiththehighestlossinvoltage.Thiscanbeattributedtothecomparativelyslowerrateoftheelectrochemicaloxygenreduction reaction(ORR).Whenahighercurrentisdrawnfromthecell,thenumberelectrochemicalreactionsoccurringwithinthecellalsoincreasesimultaneouslytomeet thehigherelectronflowrequired.However,duetorate-limitingfactorssuchas hindrancesinthemasstransfer,theactualcelloperatingvoltagereduces.Thus, oneofthecurrentchallengesinfuelcelltechnologiesistoovercometheslow