SeparatorCharacteristicsforIncreasingPerformanceof

SeparatorCharacteristicsforIncreasingPerformanceofMicrobialFuelCellsXIAOYUANZHANG,†SHAOANCHENG,‡XINWANG,†XIAHUANG,†ANDBRUCEE.LOGAN*,‡StateKeyJointLaboratoryofEnvironmentSimulationandPollutionControl,DepartmentofEnvironmentalScience&Engineering,TsinghuaUniversity,Beijing100084,P.R.China,DepartmentofCivil&EnvironmentalEngineering,231QSackettBuilding,PennsylvaniaStateUniversity,UniversityPark,Pennsylvania16802,andStateKeyLaboratoryofUrbanWaterResourceandEnvironment,No.73HuangheRoad,NangangDistrict,Harbin150090,ChinaReceivedJune3,2009.RevisedmanuscriptreceivedAugust25,2009.AcceptedAugust25,2009.Twochallengesforimprovingtheperformanceofaircathode,single-chambermicrobialfuelcells(MFCs)includeincreasingCoulombicefficiency(CE)anddecreasinginternalresistance.NonbiodegradableglassfiberseparatorsbetweenthetwoelectrodeswereshowntoincreasepowerandCE,comparedtoclothseparators(J-cloth)thatweredegradedovertime.MFCtestswereconductedusingglassfibermatswiththicknessesof1.0mm(GF1)or0.4mm(GF0.4),acationexchangemembrane(CEM),andaJ-cloth(JC),usingreactorswithdifferentconfigurations.HigherpowerdensitieswereobtainedwitheitherGF1(46(4W/m3)orJC(46(1W/m3)inMFCswitha2cmelectrodespacing,whentheseparatorwasplacedagainstthecathode(S-configuration),ratherthanMFCswithGF0.4(36(1W/m3)orCEM(14(1W/m3).Powerwasincreasedto70(2W/m3byplacingtheelectrodesoneithersideoftheGF1separator(singleseparatorelectrodeassembly,SSEA)andfurtherto150(6W/m3usingtwosetsofelectrodesspaced2cmapart(doubleseparatorelectrodeassembly,DSEA).ReducingtheDSEAelectrodespacingto0.3cmincreasedpowerto696(26W/m3asaresultofadecreaseintheohmicresistancefrom5.9to2.2Ω.ThemainadvantagesofaGF1separatorcomparedtoJCwereanimprovementintheCEfrom40%to81%(S-configuration),comparedtoonly20-40%forJCundersimilarconditions,andthefactthatGF1wasnotbiodegradable.ThehighCEfortheGF1separatorwasattributedtoalowoxygenmasstransfercoefficient(kO)5.0×10-5cm/s).TheGF1andJCmaterialsdifferedintheamountofbiomassthataccumulatedontheseparatoranditsbiodegrada-bility,whichaffectedlong-termpowerproductionandoxygentransport.Theseresultsshowthatmaterialsandmasstransferpropertiesofseparatorsareimportantfactorsforimprovingpowerdensities,CE,andlong-termperformanceofMFCs.IntroductionMicrobialfuelcells(MFCs),whicharedevicesthatcangenerateelectricityfrombiomassusingbacteria(1),havedrawnincreasingattentionasapromisingtechnologyforwastewatertreatment(2-4).Single-chamber,aircathodeMFCshavethegreatestpotentialforpracticalapplicationsbecauseoftheirsimpledesignandthefactthatnochemicalregenerationisneededwhenusingair(5).ThemainchallengesforconstructingpracticalMFCsincludeanincreaseinpowerandrecoveryofelectronsfromthesubstrate(Coulombicefficiency,CE),andreducingthecostofmaterials.Onemethodtoincreasepoweristoreducetheelectrodespacingasthisdecreasesinternalresistance(6).Whentheelectrodesgettooclose,however,poweroutputcanbereducedasaresultofoxygenutilizationbybacteriaontheanode(7).Toovercomethisadverseeffectofoxygen,Fanetal.(5)placedaclothseparator(J-cloth,JC)betweentheelectrodesandavoidedsubstantialdecreasesinpowerpersurfaceareaduetooxygenintrusion.PlacingaJCseparatornexttothecathodeinitiallyreducedthevolumetricpowerdensityfrom80W/m3to71W/m3comparedtoanMFClackingaJCseparator,butpowerwasincreasedbyplacingtheelectrodesoneithersideoftheseparator(singleseparatorelectrodeassembly,SSEA)andbyusingtwocloselyplacedelectrodesinareactorwithasmallvolume(doubleseparatorelectrodeassembly,DSEA).Powerdensitiesweresubstantiallyimprovedusingaclothseparator;however,theCEremainedlow(24%under1kΩ)withasingleclothseparator(5).CEwasincreasedfurtherbyusingmultiplelayersofcloth,presumablyduetoreductioninoxygentransfertotheanode.AmainlimitationoftheJCmaterial,however,isthatitisbiodegradable,soovertimetheclothiscompletelydegradedinthereactor,effectivelyremovingtheseparator(Figure1S,SupportingInformation).Thus,nonbiodegradableseparatorsareneededtoreplaceJCinMFCs.StudiesbyFanetal.(5)andothershaveshownthatseparatorcharacteristicsareveryimportanttothedesignofanMFC,butsofartherehasbeenlittlesystematicexamina-tionoftheeffectsofseparatorcharacteristicsonpowerproductionorCE.AnidealseparatormaterialhasahighprotontransfercoefficienttoensurethatthematerialdoesnotinhibitprotonsfromreachingthecathodeandalowoxygentransfercoefficienttoimproveCE,anditmustberelativelynonbiodegradable.Cationexchangemembranes(CEMs)suchasNafion(8),anionexchangemembranes(AEM)(9),andultrafiltrationmembranes(9)havebeenusedinseveraldifferenttypesofMFCs.WhilethesemembranesimprovetheCEbyinhibitingoxygentransfer,CEMandAEMmembranessubstantiallyreducepowerproductionasaresultofpHgradientsthatdevelopacrossthemembrane(10)andbyanincreaseininternalresistance,andUFmembranessubstantiallyincreaseinternalresistancebyinhibitingprotontransfer.Severalotherseparatormaterialshavebeenused,includingnylon,cellulose,andpolycarbonatefilters(11),buttheireffectsonpowergenerationandoxygentransferhavenotbeenwell-examined.Anotherfactorthatcanaffectpowergenerationisthegrowthofbacteriaonthecathode.Asthebiofilmdevelopsonthecathode,powerproducti