AssessmentoftheInternalResistance

AnUpflowMicrobialFuelCellwithanInteriorCathode:AssessmentoftheInternalResistancebyImpedanceSpectroscopy†ZHENHE,‡NORBERTWAGNER,§SHELLEYD.MINTEER,|ANDLARGUST.ANGENENT*,‡DepartmentofChemicalEngineeringandEnvironmentalEngineeringScienceProgram,WashingtonUniversityinSt.Louis,St.Louis,Missouri63130,InstituteforTechnicalThermodynamics,GermanAerospaceCenter,D-70569Stuttgart,Germany,andDepartmentofChemistry,SaintLouisUniversity,St.Louis,Missouri63103Anupflowmicrobialfuelcell(UMFC)systemwithaU-shapedcathodeinsidetheanodechamberwasdevelopedandproducedamaximumvolumetricpowerof29.2W/m3atavolumetricloadingrateof3.40kgCOD/(m3day)andanoperatingtemperatureof35°Cwhilefeedingsucrosecontinuously.TheCoulombicefficiencydecreasedfrom51.0%to10.6%withtheincreaseinthevolumetricloadingratefrom0.57to4.29kgCOD/(m3day).Inaddition,thelab-scaleUMFCmaintainedsolublechemicaloxygendemand(COD)removalefficienciesexceeding90%andvolatilefattyacidconcentrationsof40mg/L,indicatingefficientwastewatertreatment.Theanalysisofimpedancespectroscopy,generatedbyfittingexperimentaldataintoanequivalentcircuit,revealedthatatavolumetricloadingrateof3.40kgCOD/(m3day)theoverallinternalresistancewas17.13¿.Thisinternalresistancewascomposedofelectrolyteresistance(8.62¿),charge-transferresistance(7.05¿),anddiffusionresistance(1.46¿).Electrolyteresistancedominatedthroughouttheentirerangeofloadingrates.Inaddition,impedancespectroscopydemonstratedthatboththeanodicandthecathodiccharge-transferresistanceswereimportantlimitingfactors.TofurtherimprovethepoweroutputoftheUMFC,wemustreducetheelectrolyteresistancebyoptimizingreactorconfiguration,reducetheanodecharge-transferresistancesbyselectingsuperioranodicmicrobiota,andreducethecathodiccharge-transferresistancebyexploringsustainableandefficientcatalysts.IntroductionTheelectrochemicallimitationsontheperformanceofmicrobialfuelcells(MFCs)areduetotheinternalresistance(Ri),whichresultsfromohmic,kinetic,andtransportlimitations(1,2).ForMFCs,ohmiclimitationismostlyduetotheresistanceofelectrolytes,suchasanolyte,catholyte,andprotonexchangemembrane(PEM),whilekineticlimitationisacharge-transferresistanceduetoslowactiva-tionreactionratesonanodeandcathodeelectrodes,andtransportlimitationisaresistancecausedbyretardeddiffusion.AreversecorrelationexistsbetweenRiandpoweroutput,andresearchers,therefore,haveattemptedtoreduceRibyoptimizingreactorconfigurationsandcathodecatalysts/mediatorswithoutlosingpracticality(3-5).CorrectmeasurementandassessmentofRiforaMFCwillhelpresearchersunderstandthelimitationsoftheirsystemsandboostpoweroutput.Impedancespectroscopy(IS)isatechniquethatmeasurestheresistanceofanelectriccircuittocurrentflow,whichisexpressedasRi(6).ThistechniquecanprovidedetailedinformationontherelativecontributionofdifferentcomponentstoRiinanelectro-chemicalcell(7).ResearchershaveutilizedIStocharacterizetheelectrochemicalkineticsofanodicandcathodicprocessesinenzyme-drivenbiofuelcellsandfoundthatthecathodicbiocatalyticprocesswasthemaincontributortotheoverallelectron-transferresistance(8).Noinformationontheelectrolyteresistancewasavailablefromthisstudy,probablyduetoitsnegligiblequantityinsuchbiofuelcells.However,theelectrolyteresistanceplaysanimportantroleinMFCs,becauseiontransportinelectrolyteswasaffectedbyreactorconfigurationandfuelcomposition(4).Toourknowledge,IShasnotbeenperformedindetailwithMFCs.Microbialfuelcellsachieveacomparableorganicremovalefficiencytocurrentbiologicalwastewatertreatmentpro-cesses,suchasanaerobicdigesters(1,9-13).Tocompetewithanaerobicdigestersforelectricityproductionafterconversionofmethanegastoelectricityviaagenerator,however,MFCsmustproduceavolumetricpowerupto160W/m3(basedonthewetvolumeoftheanodechamber)(1).Todate,amaximumvolumetricpowerof216W/m3wasproducedfromaMFCwithananodewetvolumeof40mL(9).Asensor-typeMFCwithavolumeof20mLalsoobtainedahighpoweroutputof102W/m3(13).Thus,MFCshaveconsiderablepotentialaswastewatertreatmentsystemswhilesimultaneouslygeneratingelectricity(14,15).Besidessat-isfyingorganicremovalefficiencyandpoweroutput,apracticalreactorconfigurationmustbedevelopedthatcanbeeconomicallyscaledup.Microbialfuelcellswithaflowpatternthroughaporousanodeelectrodeareadvantageous,becausecontactbetweensubstrateandbiomassismadebytheinfluentflowratherthanmechanicalmixing(1,2).Wehaveusedsuchaflowpatterninanupflowmicrobialfuelcell(UMFC)(1).Inadditiontoapracticalconfiguration,UMFCsachievedpromisingpoweroutputs.Recently,anUMFC(ortubularMFC)withawetanodevolumeof210mLgeneratedamaximumvolumetricpowerof90W/m3duringcontinuoustreatmentofacetate(2).ThisUMFCwasconfiguredwithanexteriorcathodeoverwhichhexacyanoferrate(catholyte)wasrecirculated.Theanodeconsistedofgraphitegranulesthroughwhichacetateflowed.ArelativelylowRiof4¿wasachievedbysustainingashortdistancebetweentheanodeandcathodeelectrodesandalargePEMsurfacearea(2).However,scaleupofthisUMFCwithanexteriorcathodewouldconsiderablyincreasethevolumetosurfacearearatioandthusincreasetheelectrodedistanceanddecreasetherelativePEMsurfacearea,resultinginahigherRiandalowerpoweroutput