Zinc

JournalofIndustrialandEngineeringChemistry

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ReviewZinc–airfuelcell,apotentialcandidateforalternativeenergyPrabalSapkota,HonggonKim*EnergyandEnvironmentResearchDivision,KoreaInstituteofScienceandTechnology,Hawolgokdong39-1,Seongbukgu,Seoul136-791,RepublicofKoreaARTICLEINFOABSTRACTArticlehistory:

Received8October2008Accepted6January2009Keywords:Zinc–airfuelcellZAFCMetalairfuelcellPowergeneratorElectrochemicalcellAzinc–airfuelcell(ZAFC),whichgenerateselectricitybythereactionbetweenoxygenandzincpalletsinaliquidalkalineelectrolyte,isapotentialcandidateforanalternativeenergygenerator.Itisefficient,completelyrenewable,andcheapinfabricationbecausepreciousmetalcatalystsarenotnecessary.Inaddition,itisenvironmentallybenignbecauseofproducingsolelyrecyclablezincoxidewithoutgasemission.Itisapplicabletoportable,mobile,stationary,andmilitarypurposes.Inspiteofitshighpotentialasanalternativepowersource,itisyetinapreliminarystageofcommercializationbecauseofafewuncertaintiesremained.ThispaperreviewsthepresentstatusoftheZAFCtechnologyandtheproblemstobeovercomeforfurtheradvancementtowardthepotentialnext-generationalternativeenergy.ß2009PublishedbyElsevierB.V.onbehalfofTheKoreanSocietyofIndustrialandEngineeringChemistry.Contents1.2.Introduction............................FundamentalsofvoltagegenerationinaZAFC2.1.Thermodynamics..................2.2.Standardelectrodepotentials.........DesignofaZAFC........................MaterialsforZAFC.......................4.1.Anode...........................4.2.Cathode..........................4.3.Electrolyteandseparator............FactorsaffectingtheperformanceofZAFC....5.1.Ohmiclosses......................5.2.Activationlosses...................5.3.Dendriteformation.................5.4.Carbondioxideabsorption...........Cellperformanceevaluation...............6.1.Powerdensity.....................6.2.Polarizationcurves.................Applications............................Conclusions............................References.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................4444474474484484484484494494494494494494494494504504503.4.5.6.7.8.1.IntroductionTheissueofglobalwarmingandimmensethrustofenergythatisgrowingondaybydayhaveforcedtosearchnewsustainable*Correspondingauthor.Tel.:+8229585858;fax:+8229585859.E-mailaddress:hkim@kist.re.kr(H.Kim).energieswhichcanreplacefossilfuelsandnuclearenergy.Variousalternativeenergiessuchassolar,hydropower,wind,andgeothermalenergiesarealreadyinpractice,andsomeareinthephaseofcommercialization.Thefuelcell,aplausiblenext-generationpowergeneratingsystemwhichisknowntoefficientlyconvertfuelstoelectricitywithproducingenvironmentallybenignbyproducts,isalsounderdevelopmentandnearlyabouttobecommercialized.Varioussizesofthefuelcellcanextendits1226-086X/$–seefrontmatterß2009PublishedbyElsevierB.V.onbehalfofTheKoreanSocietyofIndustrialandEngineeringChemistry.doi:10.1016/j.jiec.2009.01.002446

Table1Varioustypesoffuelcells.FuelcelltypeAlkaline(AFC)Protonexchangemembrane(PEMFC)Directmethanol(DMFC)Phosphoricacid(PAFC)Moltencarbonate(MCFC)Solidoxide(SOFC)Metal–air,zinc–air(ZAFC)P.Sapkota,H.Kim/JournalofIndustrialandEngineeringChemistry15(2009)445–450Operatingtemperature80–908C30–1008C20–908C$2008C6508C500–10008C0–608CFuelH2H2MethanolH2H2,CO2H2Metal,ZnCatalystPt/Pd,Pt/AuPtPt,Pt/RuPtNiPerovskitesMnO2/PtApplicationsSpaceapplicationVehicles,mobile,lowpowereddomesticuseLowpoweredportablesystemDomesticapplicationPowerplant(MW)Powerplant(MW)Mobileandstationaryapplicationsapplicationfrommicropowerdevicestoindustrypurposesincludingmobileandstationaryapplications.Theversatilityofitsapplicationinducesthemostimmenseinterestamongthealternativeenergiespresentlybeingdeveloped.Fuelcellsaretheelectrochemicaldeviceswhichdirectlyconvertchemicalenergyintoelectricalenergy.Abasicfuelcellstructureconsistsofanelectrolytelayersandwichedbetweenaporousanodeandacathode.Eventhoughthefuelcellresemblesquitesimilartoabatteryincomponents,theformerisanenergyconvertingdevicewhereasthelatterisanenergystoragedevice.Inotherwords,thefuelcellcancontinuouslyproduceenergyaslongasthefuel,hydrogeninmostcases,issupplied,whilethebatterycannot.Variousfuelcellsarebeingdevelopedaccordingtotheoperationtemperaturerange,andthemostrepresentativetypesareexampledinTable1[1–4].Therecentresearchonlow-temperaturefuelcellssuchasPEMFC(protonexchangemembranefuelcell)andDMFC(directmethanolfuelcell)formobileandstationaryapplicationsisatapeak.Butissuesofhydrogencompressionandstorageandthemethanolcrossoverhavehinderedtheircommercialization.Ametal–airfuelcell,whichusesthephenomenonthatelectronsaredischargedwhenthemetalisoxidized,canbeacandidateforanenvironmentallybenignalternativeenergygenerator.Itisefficient,environmen-tallysafe,andcompletelyrenewable,butdoesnotrequirepreciousmetalssuchasPtasitscatalyst.Inadditiontoitslowfabricationcostbecauseofcheapfuels,ametalandtheair,itcanalsobewidelyusedforportable,mobileandstationaryapplications.VariousmetalssuchasLi,Ca,Mg,Al,ZnandFecanbeadoptedformetal–airfuelcells.Asfarastheenergydensity,theenergyperunitmass,isconcerned,Alexceedsothermetals.However,itiseasilycorrodedinthealkalineelectrolyte.ThecorrosionrateofZnisslowerthanAlinanaqueousoralkalinesolution.Inaddition,Znishighlyelectro-positiveandabundantlyavailableatlowcost,andacellwithZncanproducehighspecificenergyandhighpowerdensity.Therefore,Znissofarconsideredasthemostreasonablematerialfortheanodeinametal–airfuelcell[5,6].Thezinc–airfuelcell(ZAFC)worksonthebasisofareactionbetweentheatmosphericoxygenandzincpalletsinaliquidalkalineelectrolyte.SincetheZAFCproducesonlyzincoxide,whichisentirelyrecyclable,withoutgasemissionwhilegenerat-ingelectricity,itisconsideredenvironmentallyfriendly.Fig.1showsageneralschematicdiagramofaZAFC.Inanoverallchemistry,zincisconvertedtozincoxidewhenzincpelletsarefedintothechamber.Firstly,whenairoranyoxygensourcesuchasperoxideissuppliedatthecathodeside,oxygeniselectrochemicallyreducedtohydroxideionsbyreactingwithwateronthesurfaceofthegas-permeablecathode.Meanwhile,theoxidationofZntakesplaceatthesurfaceoftheanodecurrentcollector,whereZnisconvertedtozincoxidebyreactingwithhydroxideions.DuringtheconversionofZntoZnO,electronsaredischargedandtransferredtotheanodiccurrentcollector.Theelectronsontheanodepassacrosstheexternalloadandcomebacktothecathodecurrentcollector,wherethereductionofoxygentakesplace.Therefore,theoverallchemicalreactioninaZAFCcanFig.1.SchematicofZAFC.bewrittenas2ZnþO2¼2ZnO(1)Electricity,theoretically1.65V,canbegeneratedfromtheaboveelectrochemicalreactionbetweenzincandoxygen.Thezinc–airsystemintheformofaprimarybatteryhasalreadybeeninpractice.However,inspiteofhavingenormouspotentialasanalternativeenergygenerator,theZAFCsystemisyetinapreliminarystageofcommercializationbecausetherearesomeuncertaintiesforwhichfurtherstudiesanddevelopmentneedtobeaddressed.ThepurposeofthispaperistoreviewthepresentstatusoftheZAFCtechnologyandtheproblemstobeovercomeinordertomaketheZAFCapotentialcandidateforthenext-generationalternativeenergy.2.FundamentalsofvoltagegenerationinaZAFC2.1.ThermodynamicsForgeneratingelectricityfromaZAFC,theoverallreaction(1)shouldbethermodynamicallyfavorable.Itisconvenienttoexpresstheoverallreactionintermsoftheoverallcellelectromotiveforce,Eemf(V),whichisdefinedasthepotentialdifferencebetweenacathodeandananode.TheEemfisrelatedtotheelectricalworkdoneinacell.ElectricalworkdoneðWÞ¼ÀchargeðQÞÂvoltageðEemfÞ(2)whereQ=nF,nisthenumberofelectronswhichtakepartinthechemicalreaction,andFistheFaradayconstant.W¼ÀnFEemf(3)Ifthereisnolossinthesystem,theelectricalworkdoneisequaltotheGibbsfreeenergychange(DGr).EDGremf¼ÀnF(4)P.Sapkota,H.Kim/JournalofIndustrialandEngineeringChemistry15(2009)445–450447

InthecaseofaZAFC,n=2.SotheequationbecomesEGremf¼ÀD2F(5)2.2.StandardelectrodepotentialsTheoverallelectrochemicalreactiontakingplaceintheZAFCcanbeanalyzedbyconsideringtheanodeandthecathodereactionsinseparate.Anode:Znþ4OHÀ¼ZnðOHÞ2À4þ2eÀðEo¼À0:625VÞZnðOHÞ2À4¼ZnOþ2OHÀþH2O(6)Cathode:O2þ2H2Oþ4eÀ¼4OHÀðEo¼þ0:40VÞ(7)Theoverallreactioncanbeexpressesas2ZnþO2¼2ZnOðEo¼1:65VÞ(8)Intheaboveequations,Eorepresentsthestandardelectrodepotentialofeachreactionwithrespecttothestandardhydrogenelectrodeatthestandardtemperatureandpressure.TheoverallcellelectromotiveforceiscalculatedasEemf¼EoðcathodeÞÀEoðanodeÞ(9)sothattheoverallcellelectromotiveforceofaZAFCshouldbetheoretically1.65VasshowninEq.(8).TheEemfisathermodynamicvaluewhichdoesnotincludeinternallosses.Theopencircuitvoltage(OCV)isobtainedatnoloadconditionandshouldtheoreticallybeequaltotheEemf.However,theactualopencircuitvoltageofapracticalZAFCisalwayslessthanthetheoreticalvalueduetovariouspotentiallossesandfoundtobearound1.45V[1].PossiblereasonsforthepotentiallosswillbediscussedinSection5.3.DesignofaZAFCVariousconfigurationswereproposedforZAFCs.Inthebeginningstage,azinc–airsystemwithmechanicallyrefuelableanodesshowninFig.2wasproposed[7].Thissystemcanbeconsideredasazinc–airrefuelablebatteryratherthanaZAFC.AZnplatewhichisreplaceableisplacedinthemiddleasananode,andthesurfacesoftwosidesfacetheoxygen-reductioncathodes,whichareoftencalledair-cathodes.AqueousKOHisusedasanelectrolytefortransferringions.Thetwoelectricallyoppositesides,theanodeandthecathode,areseparatedbyamembraneFig.2.Zinc–airsystemwithmechanicallyrefuelableanode.whichisionicallyconductivebutelectricallynon-conductive.Forbeingafuelcell,asystemshouldcontinuouslyproducetheelectricalenergyaslongasthefuelissupplied.Soelectricallydischargingandmechanicallyrefuelingoftheabovesystemdoesnotsupporttheexactdefinitionofafuelcell.Howevertheworkingprincipleandtheconceptaresameasthoseofafuelcell.SeveralapproachesforZAFCshavebeenproposedbydifferentresearchgroups.SincethesolidZnandzincoxideparticlesarehandledinaZAFC,thecloggingproblemofunreactedzinc,solidproducts,andbyproductsintheelectrolyteissevere.Someoftheresearchworkstosolvethisproblemwereconductedinafluidizedbedsystemorastaticbedsystem.TheMetallicPowerdevelopeda36Vfuelcelldeliveringapproximately6kWhbyadoptingtheconceptofthefluidizedbedsystem[8].However,thecloggingproblemstillremainedinthiscase.Thecloggingproblembyunreactedzincpelletscouldbesolvedbyatapered-endstructure[9].AsanexampleshowninFig.3,twonon-parallelsurfacesareplacedwithasmallverticalanglewhichproducesadifferencebetweentwoends,thelowerandupperends.Inthisapproach,thepreferableverticalangleisca.0.1–38.Acontinuousflowofelectrolyteismaintainedfromthetoptothebottomofthecell.Theupperportionofacelliselectrochemicallyactiveandcalledahopperwhichactsasareservoirforthezincpelletsorparticles.Oncethecellstartstodischargetheelectricity,thepelletsreducetheirsizesbythechemicalreactionandflowdownwardsbynaturalmovement.Solidproductsandbyproductsaswellassmall-sizeunreactedzincparticlesescapeoutfromthecellalongwiththeeffluentofelectrolyte.Themajordisadvantagesofthistapered-endapproacharethepresenceofinactivevolumeinthehopperwhichunnecessarilyincreasestheweightofacellandmakesitbulky,arelativelylongfillingtimeofzincpelletsinthehopper,andthehighshuntcurrentflowingbetweenthecellsduringrefuelingviaafeedtube.Plutoetal.suggestedanadvanceddesignofZAFCtoovercometheproblemsencounteredinthetapered-endstructure[10].Intheirdesign,aslightlyleanedanodecurrentcollectorandaverticalpermeablecathodewhichaircanpassthroughareplacedonthetwooppositesidesofthecellwhichcovertheentirecellarea.Andthefuelispumpedfromtheadjacenttanksothatalmostallspaceinthecellframeactivelyworkselectrochemically.Thisarrange-Fig.3.ZAFCwithatapered-endstructure.448P.Sapkota,H.Kim/JournalofIndustrialandEngineeringChemistry15(2009)445–450Fig.4.ZAFCcelldesignedbyPlutoetal.mentseemsusefulinreducingtheshuntcurrentbetweenunitcells.Furthermore,itmaintainsaconstantlevelofzincparticlesinsidethecell.InFig.4,theliquidelectrolytecontainingzincparticlesispumpedandsuppliedtothecell.Theverticalbaffleshelptodirectthepelletmovementinthecell.Oncetheactivespaceofthecellisfull,theelectrolytecontainingzincpelletsflowsalongthepathsandreturnsbacktothefueltankviaanoverflowexitline.Asthecellelectricallydischarges,zincpelletsgetsmallerandmovedownwards.Theelectrolytecontainingzincoxideparticles,theproduct,andsmallamountofunreactedzincpelletspassesthroughagridmeshplacedatthebottomofthecellandispumpedtotherecycletank.Thevacantspaceintheupperpartisoccupiedbynewzincpelletswhicharefedfromtheinletstream.SmedleyandZhangusedthisconceptintheirresearchworkwitha12cell-stackproviding1.8kW[11].4.MaterialsforZAFC4.1.AnodeInaZAFC,theanodematerialiszinc.Theshapeandsizeofzincdependonthecelldesign.Inthemechanicallyrefuelablezinc–airsystem,asinglezincplatewithacurrentcollectingelementembeddedinitisusedasananode.Duringrefueling,thewholeanodeassemblyisreplacedbyanewsetwithacurrentcollectingelement.AccordingtotheElectricFuelLtd.,refuelingofZnforZAFC-poweredvehiclescanbedoneinashorttimebyanautomaticrefuelingmachineinarefuelingstationjustlikethatinagasolinerefuelingstation[12].Inthecaseofthecellswithtaperedends,propersizesofzincpelletsarerequiredinordertocontinuouslychargethesolidfuel.Propershapeandsizeofzincpelletsdependontheshapeandsizeofthecellcomponents.Thecurrentcollectingelementontheanodesidecanbemadeofanymaterialhavinghighelectricalconductivityandhighanti-corrosiontotheelectrolytematerial.Nickel,copper,silver,andstainlesssteelarecommonlyusedfortheanode.Relativelyfewresearchworkshavebeendoneforimprovingtheanode.Furthermore,mostresearchworksontheanodefocusedontheelectricallyrechargeablezinc–airbatteriesandthemechanicallyrefuelablezinc–airsystemsratherthanZAFCs.Inthecaseofanelectricallyrechargeablesystem,irreversibilityofthesystemandcorrosionoftheelementsarethemajorproblems.Thesecouldbealleviatedbyusingazincoxideelectrodeoramodifiedelectrodecontainingzincoxideandleadoxide[13].Buttheharmlessnessofleadoxideontheenvironmentisnotyetproven.TheperformanceofzincanodecanbefurtherimprovedbyadoptinganalloyofNi(7.5%),Zn(90%)andIr(2.5%)[14].Althoughtheproblemassociatedwiththezincanodeseemstoplayasignificantroleinthecaseofelectricallyrechargeablebatteries,itismollifiedinthecasesofmechanicallyrefuelablezinc–airsystemsandcontinuousZAFCsystems.4.2.CathodeOneofthemajorfactorsdeterminingtheperformanceofaZAFCisanaircathodewheretheoxygenreductiontakesplace.AtypicalZAFCcathodeconsistsofthreelayers:acurrentcollectinglayer,adiffusionlayer,andacatalyticallyactivelayer.Thecurrentcollectinglayerissimplythemetalmeshsandwichedbetweenthediffusionlayerandthecatalyticallyactivelayer.Thereisnospecificmetal,butitshouldbenon-corrosiveandhighlyelectricallyconductive.Metalsmostcommonlyusedarenickel,gold,silver,copper,silver-platednickel,silver-platedornickel-platediron.Thediffusionlayercomposedofcarbonparticlesandhydro-phobicparticlesneedstobeair-permeablebutwater-impermeable.Graphiteforcarbonparticlesandpolytetrafluoroethylene(PTFE)forhydrophobicparticlesarebondedtogetherwithorwithoutepoxyresin[15].Aproperratiocanbechoseninarangeof30–70wt.%PTFEandthebalancedamountofconductivecarbon[16].ThecatalyticallyactivelayeristhemostimportantpartintheaircathodeforaZAFC.Thisistheplacewherethecatalyticreductionofoxygentakesplacesothatalargeractiveareaisdesirablefromthereactionpointofview.Theactiveareaconsistsofcatalystsmixedincarbonparticles.OneadvantageoftheZAFCisthatitdoesnotrequirepreciousandcostlymetalsforcatalysts.Inexpensivenon-noblemetaloxidessuchasMnO2canbeusedasaZAFCcatalyst[17].Aperovskitetype(La0.6Ca0.4CoO3)dopedwithmetaloxidescanbeusedforanoxygenreducingcatalyst[18].Carbonparticlessupportingthecatalystprovidenotonlyahighsurfaceareaforthereactionbutalsoelectricalpathstoelectrons.Thecellperformancedependsonthetypeandtheporesizeofcarbon.Activatedcarbonwithalargenumberofmacropores(>50nm)ormesopores(2–50nm)giveshighperformance[19].Chaoetal.suggestedtheadditionofclayintheaircathodeinordertoenhancethedispersionofcarbonparticlesandcatalyst[20].ShunandLousuggestedanadvancedcatalyticlayerconsistingofcarbonparticles,highsurfaceareaparticulatessuchasmolecularsieveorzeolite,hydrophobicparticles,andmetalhydroxideascatalystsuchasnickelhydroxide,cobalthydroxide,ironhydroxide,ceriumhydro-xide,manganesehydroxide,lanthanumhydroxideandchromiumhydroxide[15].Theactivelayercanalsobepreparedbysprayingacatalystmaterialonthecommerciallyavailablecarbonpapersorcarbonclothswhichareusedasgasdiffusionlayers.Zhuetal.proposedathinaircathodeof0.13–0.50mmthicknesswhichispreparedbyputtingcarbonparticlesinasinter-lockedmetalfibernetwork[21].Thiscouldreducethecathodethicknessby30–75%ofcommercialaircathodes.WangChendevelopedanaircathodewithmulti-layersinordertominimizethemoisturetransferfromtheinnerelectrolytetotheoutsideairandtopreservetheconstantwatercontentofzincanode[22].ThiscathodewascomposedofametallicmeshasasubstratewhichissandwichedbytwodiffusionlayersofcarbonandPTFEbinder.Differenttypesofcatalystsarecoatedonadiffusionlayeronebyonetoformamulti-layeredcatalyticsurface.Forexample,athree-layeredactivesurfacecanbepreparedbycoating1mg/cm2ofMnO2,CoOandMnO2inthefirst,thesecondandthethirdlayers,respectively.Desirablethicknessoftheaircathodeischosenintherangeof0.01and20mmaccordingtotheconfigurationandthenumberoflayers.ThenoblePtcanbeusedasacatalystmaterial,butitsuseincreasesthecostofacell.4.3.ElectrolyteandseparatorTheelectrolyteplaysanimportantroleinthetransportationofions.LiquidalkalinesareusedforZAFCelectrolytes.ThemostP.Sapkota,H.Kim/JournalofIndustrialandEngineeringChemistry15(2009)445–450449

commonlyusedonesarepotassiumhydroxide(KOH),sodiumhydroxide(NaOH)andlithiumhydroxide(LiOH).Somearticlessuggestedtousethehydroponicsgelasanelectrolytegellingagentduetoitsenormouscapabilityofstoringsolution[23,24].Aseparatororamembraneneedstobehighlyionicallyconductivebutelectricallynon-conductive.Organicpolymerssuchasacopolymerofpolyvinylalcoholandpolyvinylacetatehavebeenusedfortheseparator[25].CommerciallyavailableNafion(DuPontCo.),asulfonatedperfluoropolymer,hasbeenwidelyusedasionexchangemembranebecauseofitshighionexchangecapabilityandchemicalstability[26,27].However,becauseNafionsarefarmoreexpensivethanhydrocarbonseparators,Dewietal.suggestedtheuseofpolysulfoniummembrane,whichcanpreventthepermeationofzinccationsandonlyallowhydroxideanionstopassthrough[28].Ahydrophilicpolyestersulfonefilmwithmicroporesof0.45mmporesizewasalsoproposed[29].Wuetal.suggestedthepolypropylenemembrane(Celgard)treatedwithsulfuricacid,whichisusuallyusedinlithiumbatteries,asaseparatorforzinc–airelectrochemicalcells[30].5.FactorsaffectingtheperformanceofZAFCEventhoughthetheoreticallyavailablevoltageofaZAFCis1.65VanditsOCVis1.45V,thepracticallyattainablevalueisalwayslessthanthem.Thereareseveralfactorsresponsibleforthelowvoltagesduringoperation.5.1.OhmiclossesOhmiclossesoccurduetotheelectricalresistancesofelectrodesandinterconnections,andtheresistancetotheflowofionsintheelectrolyte.Theamountofvoltagedrop(V)dependsonthecurrent(i)andtheresistancesofcomponents(R).ThiscanbeexpressedasV¼iRTheohmiclossescanbeminimizedbyusingtheelectrodeswithhighelectricalconductivityandtheelectrolytewithhighionicconductivity.AqueousNaOH,KOHorLiOHispreferredasagoodelectrolyte[31].5.2.ActivationlossesActivationlossesresultfromtheslownessofreactionstakingplaceonthesurfaceoftheelectrodes.Inlow-temperaturefuelcells,theaircathodeisprimarilyresponsiblefortheactivationloss[2].Theactivationlossincreasesasthecurrentdensityincreases.Anditcanbereducedbyincreasingtheactivesurfaceareaofcathodes,thecatalyticactivity,ortheroughnessoftheelectrodes.IncreasingtheoxygenconcentrationbyusingpureO2insteadofaircanalsoreducetheactivationloss,butthisisnotfavorablebecauseofthehighcostofO2andthedifficultyinoxygencompressionandstorageforsmallportabledevices.5.3.DendriteformationInthecellstackswheretheelectrolyteisshared,ashuntcurrentappears.Thisshuntcurrentisresponsiblefortheformationofzincdendrite.Eventhoughitisslowlyformed,iteventuallyblockstheflowofelectrolyteandzincparticlesintothecell.Colbonsuggestedtheconstructionofdendriteeliminationzonetopreventtheaccumulationofzincparticlesinthiszone[32].Useofanacidicelectrolyteinsteadofanalkalinecouldminimizethedendriteformation[33].Additionofasmallamountofcellulose(1–10%tozincelectrode)showedapositiveeffecttoreducetheformationofzincdendriteinthecaseofrechargeablezinc–airbatteries[34,35].5.4.CarbondioxideabsorptionAsthesystemisoperatedwithairinsteadofpureoxygen,CO2presentintheairdissolvesintheelectrolyteformingcarbonate.Theformationofcarbonateincreasestheviscosityoftheelectrolyteanddecreasesitsionicconductivity.InaZAFC,thiscanbeminimizedbycontinuouslysupplyingafreshelectrolytesolution.Useofpureoxygeninsteadofaircandecreasetheformationofcarbonatebutincreasetheoperationcost.Theproblemofcarbonateformationcanpossiblybereducedifanacidicelectrolyteisadoptedinsteadofanalkaline[33].6.CellperformanceevaluationInmostcasesofelectrochemicaldevices,theperformanceisusuallyexpressedintermsofpowerdensityandapolarizationcurveeventhoughtherearediversetypesofmethodsandbases.Theevaluationofacellnormallystartsfromitsopencircuitvoltage(OCV).TheOCVofaZAFCissofarknowntobeca.1.45V.6.1.PowerdensityThepowerdensityisaspecificpowernormalizedtotheprojectedactiveareaofanelectrodeortothevolumeofasystem.P¼PAorPPdd¼VwherePdisthepowerdensity,Pisthetotalpowerexpressedinwattormilliwatt,Aistheareaofanelectrodeincm2,andVisthevolumeofasystemincm3.ForaZAFCsystem,theaircathodeareaisusuallytakenastheactiveareawheretheelectrochemicalreactionseemstotakeplace.6.2.PolarizationcurvesPolarizationoccurswhentheelectricalresistancearoundtheelectrodesabruptlyincreases.Inatypicalpolarizationcurvewhichcanrepresenttheperformanceofthecellorcanindicatecertainchangeinthecell,thevoltagechangeisexpressedintermsofthecurrentchangeorthecurrentdensitychange.ThepolarizationcurveexampledinFig.5canbedividedintothreezonesasfollows:(i)ActivationlosszonerangingfromtheOCVatzerocurrenttotheinitialsteepdecreaseofvoltage.(ii)Ohmiclosszonewherethevoltageslowlydrops.(iii)Concentrationlosszonewherethemasstransporteffectisdominantandthevoltagerapidlyfallsathighcurrentdensities.Fig.5.TypicalpolarizationcurveofZAFC.450P.Sapkota,H.Kim/JournalofIndustrialandEngineeringChemistry15(2009)445–450InaZAFC,theactivationlossdominantlyoccursintheinitialstage,andtheohmiclossoccursmoderately.Themostsignificantlossoccurswhenahighcurrentdensityisapplied.IftheZAFCisoperatedinamiddlerangeofcurrentdensity,itsteadilydischargesaconstantvoltage.InaZAFCwellmodifiedsofar,ahighcurrentdensityof360mA/cm2couldbesustainedfor1000h[11].

Eitherapotentiostatoravariableresistorboxcanbeusedtosetvariableexternalloads.Thevoltagechangeismeasuredwhentheloaddecreases(orincreases)periodically.ThecurrentorthecurrentdensityiscalculatedusingOhmslaw.Whentheexternalresistanceisvaried,thechangesofcurrentandvoltageshouldbecountedafteratemporarypseudo-steadystateconditionisestablishedinafewminutes.7.ApplicationsDuetoitshighspecificenergy,highpowerdensity,cheapandabundantlyavailablefuel,nouseofpreciousmetalsascatalysts,andnoissueofdifficultyinfuelstorageandtransportation,theZAFCisdefinitelyonepromisingoptionforbothstationaryandmobileapplications.Zinc–airbatteriesarealreadyinpracticeasaprimarybatteryinsmalldeviceslikehearingaids.Itcanreplacealkalineormercurybatteriesbecauseitsenergydensityisuptofivetimesofthesebatteries.Asthezinc–airbatteries,theZAFCissuitablyappliedfortheareaswhereahighenergydensity,theenergyperweight,iscontinuouslyrequired.Recently,severalcompaniesareinvolvedindevelopmentandcommercializationofZAFCsforelectricvehicles,indoorpowergenerators,industrialfacilities,andmilitarypurposes.However,fewresearchgroupsandcompaniesaresofarworkingonthedevelopmentofZn–airsystemsthroughouttheworld.TheElectricFuel,Ltd.hasworkedonazinc–airbatterysystemforelectricvehicles,demonstratedthesystemsforvans,andbeendevelopingthesystemsforbuses.Thecompanyisalsoworkingontheprimaryandsecondarybatteriesformilitaryuses[12,36–41].ThePowerZinchascommercializedzinc–airbatteriesformilitaryusesandelectricvehicles,anddevelopedfirst-stagemodelsofZAFCproducts[42].ThePowerAirCo.hasproducedvariousZAFCsystemsformobiledevicepowerpacks,backuppowerapplicationsandindoorgenerators,andhascurrentlydemonstrateditsninthgenerationZAFCsystemin2007[43].TheMetallicPowerInc.producesZAFCsasalternativebackuporemergencyelectricalsourcesforthegeneratorswhichuseinternalcombustionengines[6].8.ConclusionsEventhoughthezinc–airfuelcellsystemisoneofthepotentialcandidatestofulfilltheworld’senergyrequirements,severalproblemsincludingfullexploitationandmodificationofzincasasourcefortheenvironmentallybenignenergyproductionneedtobesolvedforitscommercialization.ThemainproblemassociatedwiththeZAFCsofaristheperformanceofanaircathode.Amulti-layeredcatalystelectrodecanbeaplausibleoption.ImprovementofthecellperformancecanalsobeachievedbychangingtheconfigurationofacellassemblyinsuchamannerthatOHÀionscantravelashortdistance.Theadditionofionicallyconductivematerialsintotheelectrolytecanhopefullyhelptosolvetheproblemsofdendriteformation,carbonateformationandcorro-sionofelectrodes.Inordertoimprovethelife-timeofacell,severalfactorsneedtobesolvedsimultaneously.Afterall,thedevelop-mentandcommercializationofaZAFCisoneoftheinterestingresearchfieldsforthescientistsandengineerswhowishtodevotetotheexcavationoffutureenergies.References

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