Air
Breathing
Cathodes
for
Microbial
Fuel
Cell
using
Mn-,
Fe-,
Co-
and
Ni-containing
Platinum
Group
Metal-free
Catalysts
Mounika
Kodali
a,
Carlo
Santoro
a,
Alexey
Serov
a,
Sadia
Kabir
a,
Kateryna
Artyushkova
a,
Ivana
Matanovic
a,b,
Plamen
Atanassov
a,*
aCenterMicro-EngineeredMaterials(CMEM),DepartmentofChemicalandBiologicalEngineering,UniversityofNewMexico,Albuquerque,NM,USA bTheoreticalDivision,LosAlamosNationalLaboratory,LosAlamos,NM87545,USA
ARTICLE INFO
Articlehistory:
Received7September2016
Receivedinrevisedform5February2017 Accepted6February2017
Availableonline7February2017
Keywords: MicrobialFuelCells OxygenReductionReaction PGM-freecatalysts Fe-AAPyr
HighPowerGeneration
ABSTRACT
Theoxygenreductionreaction(ORR)isoneofthemajorfactorsthatislimitingtheoverallperformance
outputofmicrobialfuelcells(MFC).Inthisstudy,PlatinumGroupMetal-free(PGM-free)ORRcatalysts
basedonFe,Co,Ni,Mnand thesameprecursor(Aminoantipyrine,AAPyr)weresynthesizedusing
identicalsacrificialsupportmethod(SSM).Thecatalystswereinvestigatedfortheirelectrochemical
performance,andthenintegratedintoanair-breathingcathodetobetestedin“clean”environmentand
inaworkingmicrobialfuelcell(MFC).Theirperformanceswerealsocomparedtoactivatedcarbon(AC)
basedcathodeundersimilarconditions.ResultsshowedthattheadditionofMn,Fe,CoandNitoAAPyr
increasedtheperformancescomparedtoAC.Fe-AAPyrshowedthehighestopencircuitpotential(OCP)
thatwas0.307!0.001V(vs.Ag/AgCl)andthehighestelectrocatalyticactivityatpH7.5.Onthecontrary,
AChadanOCPof0.203!0.002V(vs.Ag/AgCl)andhadthelowestelectrochemicalactivity.InMFC,
Fe-AAPyralsohadthehighestoutputof251!2.3mWcm"2,followedbyCo-AAPyrwith196
!1.5mWcm"2,
Ni-AAPyrwith171!3.6mWcm"2,Mn-AAPyrwith160!2.8mWcm"2andAC129!4.2mWcm"2.The
bestperformingcatalyst(Fe-AAPyr)wasthentestedinMFCwithincreasingsolutionconductivityfrom
12.4mScm"1to63.1 mScm"1.A maximum powerdensityof 482
!5mWcm"2was obtained with
increasingsolutionconductivity,whichisoneofthehighestvaluesreportedinthefield.
©2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense
(http://creativecommons.org/licenses/by/4.0/).
1.Introduction
Microbial fuel cells (MFCs) are bio-electrochemical systems that can treat wastewater while simultaneously generating electricity. This co-generative configuration can theoretically replace the existing energy-intensive treatments plants [1,2]. Unfortunately,theperformancesofMFCsarelimitedbyserveral factorsthathinder itslarge-scaleapplication[3–5].It hasbeen
establishedthatoneofthemainfactorslimitingthepoweroutput ofMFCsisthereductionreactioninthecathode[6].Themostused oxidantatthecathodeisoxygen,andthisisduetothefactthat oxygenhasahighreductionpotential,andisnaturallyabundantin
theatmosphere.Severalissuesconcerningtheoxygenreduction reaction (ORR) are as follows: i) high overpotentials; ii) low kineticsandiii)highohmicresistancesoftheexistingcathodes
[6]. It has been shown that enzymes have low activation overpotentials within 100mV, [7–9] but their utilization in a
pollutedenvironmentisprohibitiveduetotheirrapiddegradation anddeactivation[10].Fromthetheoreticalopencircuitpotential (OCP)which "at pH7"is 606mVvs. Ag/AgCl (3M KCl),the activationoverpotentialsreportedusingmetalbasedcatalystsare roughly300mV[11]thatcanbefurtherincreasedupto400mV whenactivatedcarbon(metal-free)isused[12]andevento500– 600mVwhendifferentcarbonaceousorsteelmaterialsareused
[13,14]. The activationoverpotentials enormously contribute to initiallossesconcerningtheelectroreductionofoxygeninneutral media.
While ORR pathways have been widely studied in both acidic [15–18] and alkaline [19–22] media, the kinetic * Correspondingauthor.
E-mailaddresses:plamen.b.atanassov@gmail.com,plamen@unm.edu (P.Atanassov).
http://dx.doi.org/10.1016/j.electacta.2017.02.033
0013-4686/©2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/). ContentslistsavailableatScienceDirect
Electrochimica
Acta
mechanisms taking place in neutral media " in which MFCs usuallyoperate"arenotfullyunderstood.Usingtherotatingring disk electrode (RRDE) technique, it was recently shown that oxygen is reducedvia a 2e" mechanism on carbon black and
activatedcarbon[23,24],whereasa4e"pathwayisdominantfor
Pt[24]andFe-basedcatalyst[25,26].InordertoenhancetheORR kinetics, usually three different pathways can be selected for integrationofcatalystsintothecathodelayer.Thefirstoneisthe utilizationofhighsurfaceareacarbonslikeactivatedcarbon(AC)
[27,28],graphene-basedmaterials[29],carbonnanotubes(CNTs)
[30], carbon nanofibers (CNFs) [31] or nitrogen doped carbon
[32].Activatedcarbonseemstobethebestcompromisebetween costandperformancesandlatelyithasbeenlargelyutilizedasa cathodeinMFCs[32].ThesecondapproachisbasedonusageofPt orPt-basedmaterialsnamedasplatinumgroupmetals(PGMs)as cathodecatalysts.These catalystswere extensivelyused inthe past,butboththehighcostsandlowdurabilityhavesignificantly loweredtheirutilizationinMFCs[33].Particularly,Ptispoisoned severelywithanions(mainlyS2"andSO
42")whicharenaturally
presentedinthewastewaterasrecentlydemonstrated[34]. Thethirdandmoreemergingcategoryistheutilizationof PGM-freecatalysts[33,35,36]basedonM-N-CmaterialsinwhichMisa transitionmetal,NisnitrogenandCiscarbon.PGM-freecatalysts havebeenheavilystudied,andseveralcatalystscontainingFe[37– 41],Mn [42–44], Co[45–47] and Ni[48,49] havebeenused in MFCs.Despiteslightlyhighercostincathodemanufacturing,the utilizationofPGM-freecatalystsguaranteeshigherperformances compare to AC, and it assures higher durability in long-terms operations[50,51].Inpreviousstudies,weshowedthatsprayed Fe-Aminoantipyrine (Fe-AAPyr) cathode outperformed AC and Pt during both linear scan voltammetry in clean media and in a workingMFC[50].Morerecently,weshowedthathigh perform-anceswereachievedbyFe-N-Ccathodesynthesizedfromdifferent organicprecursors(namely,RicobendazoleandNiclosamide),and high stability output was demonstrated during 32days of durabilitytest[51].
Inthecurrentliterature,tothebestofourknowledge,there arenoaclearanddirectcomparisonsthathavebeenmadeamong thePGM-freeORRcatalysts fabricatedwithdifferent transition metals(Mn,Fe,CoandNi).Comparisonofexistingliteratureon PGM-free catalysts in MFCs is quite complicated, and this is because the catalysts are often synthesized using different fabricationmethods and precursors and the performances are onlycomparedtoACorPt.Moreover,diverseworkingconditions lead to further differences in the output and the comparison becomesevenhardertodetermine.Lastbutnotleast,ithasnot beenwellestablishedwhichtransitionmetal(M)amongthe M-N-CPGM-freecatalysts"Fe,Co,MnandNi"hasthesuperior electrochemical performances towards the electroreduction of oxygeninneutralmedia.
In viewof that, we have "for thefirst time-made a direct comparison of Fe-, Co-, Mn- and Ni- based PGM-free cathode catalysts that were synthesized using the same method and organic precursor (Aminoantipyrine, AAPyr) under the same working conditions. Particularly, in this study, we studied the electrochemicalperformance of the above mentioned catalysts usinglinearsweepvoltammetry(LSV)inneutralmedia,whereAC was used as the benchmark. Then the performance of these catalystsincorporatedintoanair-breathingcathodewasevaluated inrunningMFCswithexactlythesameoperatingconditions.Once thebestperformingcatalystwasidentified,solutionconductivity of the working MFC was increased, and effect of solution conductivity on power curves and anode/cathode polarization curves was carried out to determine the one of the highest performingM-N-CcatalystsinMFCs
2.Experimental
2.1.Catalystspreparation
ThePGM-freecatalystsweremadeusingthesacrificialsupport method (SSM) previously described [52,53]. Particularly, the metal’ssalt(Fe(NO3)3
#
9H2O,Co(NO3)2#
6H2O,Mn(NO3)2#
4H2O,Ni(NO3)2
#
6H2O) was separatelywetimpregnated withaminoanti-pyrine (AAPyr) on the surface of fumed silica (Cab-O-Sil M5; surfacearea:$250m2g"1).AAPyr
"arichsourceofcarbonand nitrogen–wasusedasanorganicprecursor.Themixturewasthen ultrasonicatedanddriedovernightatroughly85%C.Theobtained
materialswerethengroundtofinepowderusingballmilling.The powderwasthen subjectedtoheattreatmentunderUltraHigh Purity nitrogen (flow rate 100mLmin"1) atmosphere. The
temperature was increased from room temperature to 950%C
with a rate of 25%Cmin"1. Once the desired temperature was
reached,pyrolysistookplacefor30minutes.Thesilicawasthen etchedandremovedfromtheobtainedpyrolyzedM-N-Cmaterials using hydrofluoric acid (20wt%). The catalysts were then extensively washed withde-ionized water and dried overnight at85%C.Thecatalystsobtainedwerenamedasafunctionofthe
metalused:Mn-AAPyr,Fe-AAPyr,Co-AAPyr,Ni-AAPyr.
2.2.CatalystsSurfaceChemistry
Thesynthesized materials"Mn-AAPyr, Fe-AAPyr,Co-AAPyr, Ni-AAPyr werecharacterized catalysts usingX-ray fluorescence (XRF).XRFwasconductedtoidentifythemetalsinthecatalysts using an EDAX Orbis with a Rh tube source and a titanium adsorptionedgefilter.Samplesweremeasuredinavacuumwitha filament voltage of 15kV and a current of 200
m
A. Data are reportedontheSupportingInformation.High-resolution XPS was performed using Kratos Ultra DLD spectrometer.AlK
a
monochromaticsourceat225Wwasutilized. Nochargecompensationwasnecessary.High-resolutionC1s,O1s, N 1sand corresponding metalspectra(Fe2p/Co2p/Ni2p/Mn2p) wereacquiredfromthreeareaspersample ata passenergyof 20eV. CASAxps was used to process spectra. The Linear back-groundwasusedforquantifyingC1s,O1sandN1sspectraand Shirleybackgroundformetalspectra.Threeareaspersamplewere analyzedandtheaverageispresented.Themorphologiesofthe synthesized materials were determined by scanning electron microscopy(SEM,HitachiS-5200)withanacceleratingvoltageof 20keV).2.3.Electrochemicalmeasurements
TheelectrochemicalactivityofthesynthesizedMn-AAPyr, Fe-AAPyr,Co-AAPyr,Ni-AAPyrcatalystsaswell asactivatedcarbon (AC)was investigatedusinga three-electrodecellconfiguration comprising of a catalyst coated rotating disc electrode (glassy carbondisk)astheworkingelectrode,agraphiterodasthecounter electrodeand Ag/AgCl(3MKCl) asthereferenceelectrode. The workingelectrodeswerepreparedbydepositing sonicatedinks formulatedusingamixtureofabinder(150
m
Lof0.5wt%Nafion solution),850m
Lofwater:isopropanolmixture(4:1volumetric ratio)) and 5mg of the catalyst of interest. Inks were then depositedontothe0.2475cm2glassycarbondiskthroughdropcastingtechniquewithacatalystloadingof200
m
gcm"2.Thedisk2.4.Cathodepreparation
Thecathodeswerepreparedbypressinga mixtureactivated carbon(AC,NoritSXUltra,SigmaAldrich),PTFE(60wt%solution, SigmaAldrich)andCarbonblack(CB,AlfaAesar)intheratioof 70:20:10respectivelywithM-N-Ccatalysts.CBwasaddedinorder to enhance the conductivity of the formed pellet [54]. 40!1 mgcm"2 of above mentioned base materials (AC:CB:PTFE) and
2!0.1mgcm"2 ofcatalysts (Fe-AAPyr,Ni-AAPyr,Mn-AAPyr, Co-AAPyr) weresubjected tomechanical press witha pressure of 2.5mT(metrictons)forspanof5minutesonastainlesssteelmesh (SS,McMaster),whichactedasacurrentcollector[51].Cathodes withageometricareaof2.85cm2wereexposedtothesolutionby
mountingonthelaterholeoftheglasstubeMFC.
2.5.Cathodepolarizationcurvein“clean”media
AmodifiedPyrexglassbottlewithanemptyvolumeof125mL wasused asanelectrochemicalcell.A lateralholewas builtin ordertoaccommodatethecathodethatwasscrewedonitleaving the current collector (SS) exposed to the atmosphere and the catalyticlayerexposedtotheliquidelectrolyte.Inordertoattain stableelectrodepotential,removeadsorbedoxygenandincrease materialswettabilityforenhancingthethree-phaseinterface(TPI), thecathodeswereexposedtothepotassiumphosphatebuffer (K-PB)solution(0.1M)with0.1MKClovernight (atleast15hours). Theelectrocatalyticactivityofthesecathodesincleanconditions wasdoneusinglinearsweepvoltammetry(LSV)technique.Three electrodesetupwasusedinwhichcathodeactsastheworking electrode,platinummeshasthecounterelectrodeandAg/AgCl3M KCl(+210mVvs.SHE)asareferenceelectrode.LSVswererunina rangebetweenopencircuitpotentialand"0.4Vvs.Ag/AgCl(3M KCl)atascanrateof0.2mVs"1.
2.6.Microbialfuelcellconstruction
After the electrochemical measurement, the cathode was insertedinaworkingmicrobial fuelcellandleftinopencircuit voltage(OCV)foratleast3hourstilltheoutputwasstabilized.The MFCwasasimilarlymodifiedPyrexglassbottle(volumeof125mL) singlechamberwithalateralholetoaccommodatethecathode. Thecathodewasinairbreathingconfigurationwiththemetallic mesh exposed to air and the catalytic side facing directly the solutionwith nomembrane separator utilized. Passive air was used.Carbonbrush(3cmasdiameterand3cmasheight,Millirose, USA)wasusedastheanodeoftheMFC.Theanodesusedwere alreadyfully workingfrom runningexperiments[55]. TheMFC solutionwasbasedonamixtureof50:50ratioK-PB(0.1Mand 0.1M KCl) and activated sludge (Albuquerque Southeast Water Reclamation Facility, New Mexico, USA), along with 3gL"1 of
sodiumacetate(NaOAc)asanorganicsubstrate.Allthe measure-mentsweredoneatroomtemperature(22!1%C)inAlbuquerque
(NM,USA) atroughly 1600AMSL. It iswellknownthat atthis altitude,atmosphericpressureis20%lessthanatsealevel,andthis certainlyaffects thepresencenegativelyandpartial pressureof oxygen,therefore,theORRisadverselypenalized.
2.7.MicrobialFuelCellElectrochemicalMeasurements
Afterthecathodewas insertedin aworkingMFCand leftto stabilizeforfewhours,overallpolarizationcurveswererunfrom open circuit voltage to 0mV with a scan rate of 0.2mVs"1.
Particularly, the anode was used as a counter electrode, the cathodeasworkingelectrodeandthereferencechannelwas short-circuitedwiththecounter.Simultaneously,anodepotential and cathodepotentialwererecordedonanotherpotentiostat(Biologic
SP-50, France) channel. Power was calculated by multiplying currentandvoltage(P=VxI)fromthepolarizationcurve.Power densityandthecurrentdensitywascalculatedwithrespecttothe cathodegeometricarea(2.85cm2).Everyelectrochemical
experi-mentwasrunintriplicates.
2.8.Longtermstabilitytests
Durabilitytestswereconductedoveraperiodofonemonthin whichvoltageoveranexternalresistanceof100ohmwasrecorded usingadatalogsystem(PersonalDAQ/56).Duplicateandseparate MFCs were run. The solution used was the same as described beforewith50%involumeof0.1MK-PBand0.1MKCland50%in volumeofactivatedsludge.Inordertohavelongercycles,4gL"1
wasaddedineachcycle.Atday15,thesolutionwascompletely refreshed. Power curves were run also at the end of the experiments toestimatethedegradation of theMFCs in terms ofperformances.
2.9.Solutionconductivityanalysis
Oncethecatalystwiththehighestperformancewasidentified, thesolutionconductivityoftheworkingMFCwasvariedbetween 12.4mScm"1 and 63.1mScm"1. Particularly,K-PBwas prepared
usingK2HPO4andKH2PO4inconcentrationof0.05M,0.1M,0.2M,
0.3M,0.5M, 0.6M, 0.8Mand 1M.The pH was 7.5 and it was adjustedusingKClorKOH.EachdifferentK-PBconcentrationalso contained 0.1M KCl, and it was blended in 50% volume with activatedsludge(AS).Thesolutionconductivityhadthevaluesare reportedinTable1.Also,inthiscase,polarizationcurveswererun fromopencircuitvoltageto0mV(0.2mVs"1scanrate)withthe
anodeasa counterelectrode,thecathodeasworking electrode (referenceshort-circuitedwiththecounter).Anodepotentialand cathodepotentialduringthepolarizationwererecorded separate-ly.
3.ResultsandDiscussion
3.1.SurfaceChemistryandMorphologyofthecatalystsinvestigated
Scanning Electron Microscopy was used to analyze the morphology of the synthesizedmaterials. Fig.1 a-bshows the SEMmicrographsoftheFe-AAPyrcatalystsynthesizedusingthe sacrificialsupportmethod(SSM).Asitcanbeseenfromthisfigure, theporousmorphologyoftheFe-AApyrcatalystisclearly visible-asitiswithallcatalyststhathavebeenpreviouslyfabricatedusing thesacrificialsupportmethod(SSM)[50–53].Theseporeswere
formedduringtheetchingoftheamorphousfumedsilicatemplate whichwasfusedintothemetalprecursor-carbon-nitrogenmatrix duringwetimpregnation(see catalystpreparation,section 2.1). EDSanalysisofthesematerialsalsoindicatedthattheactivesites containingmetalwereatomicallydispersedinthecatalystmatrix,
Table1
Mixtureutilizedaselectrolyteandcorrespondingsolutionconductivitymeasured andpowerdensityproduced.
K-PB KCl AS SC Powerdensity
[M] [M] %involume mScm"1 mWcm"2
0.05 0.1 50 12.4 151!2
0.1 0.1 50 16.4 251!2
0.2 0.1 50 22.6 262!4
0.3 0.1 50 29.3 286!6
0.5 0.1 50 40.8 336!20
0.6 0.1 50 45.1 370!9
0.8 0.1 50 57 444!8
as the% of the dispersed metalactives was <1wt% in allthe
samples(datanotshown).OnlyFe-AAPyrisherepresentedbut also Ni-AAPyr, Co-AAPyr and Mn-AAPyr had very similar morphology.
X-RayFluorescencewasusedtodeterminethepresenceofthe anticipated transition metal in the M-N-C catalysts being investigated.Itcanbenoticed(Fig.S1)thatineachcatalyst,the onlymetalusedduringthesynthesiswasidentifiedbythespecific peakinthegraphforeachcatalyst.Noothermetalswereidentified inthe graphsindicating theabsence of metalliccontamination duringthesynthesisprocess.XRFtechniqueisveryselectiveand wasusedinthepresentstudyasanonlyqualitativemethodin ordertodeterminethepresenceofmetalswithinppb concentra-tion.
Thechemicalcompositionof4electrocatalystswasstudiedin detailbyXPS.Table2showselementalcompositionandchemical speciationfor carbon, nitrogen, and metal. Fig. 2 shows high-resolutionN1sspectraformaterialswithfourdifferentmetalsfor comparisonalongwithatomicpercentageofeachtypeofmetal presentobtainedfromcorrespondinghighresolutionspectraof metal,i.e.Fe2p,Co2p,Ni2pandMn2p(Fig.S2).Itwasshown previously that N moieties have a positive effect on oxygen reductionreaction[56,57].
Overall chemical composition varies a lot from sample to sample. It is important to mention that the same loadings of nitrogenprecursorandmetalprecursorwereutilizedinsynthesis ofelectrocatalysts.Theresultedelementalandchemical composi-tiondetectedisdifferentduetothewaymetalandnitrogenget incorporated within the carbon matrix during pyrolysis and leaching.Fe-AAPyrhasthehighestamountofNandmetal. Mn-AAPyrhasthelargestamountofoxygenandnitrogen.Thesmaller relativeamountofmetalisdetectedforthissample,however.Both Co-AAPyr and Ni-AAPyr samples have very similar elemental composition.Carbonchemistrycanbeseparatedintothreemajor constituents"graphiticandaliphaticnetwork,carbon-nitrogen defectsandsurfaceoxidesCxOy.Co-AAPyrhasthehighestamount
ofgraphiticcarbonandthesmallestamountofsurfaceoxidesdue
to a smaller amount of defects in the graphene-like matrix. Smallest amount of C-N defects is correlated with the small amountofnitrogendetectedforthissample.
High-resolution spectra have been fitted with 6 peaks [58]. Pyridinic nitrogen at 398.3eVis largestfor Fe- and Mn-AAPyr samples.A peakthat correspondstonitrogencoordinatedwith metalhasthe highestintensityfor thesame twosamples. The fittingofthispeakwasdonebasedonthedensityfunctionaltheory calculatedbindingenergiesofN1sinFe-N4,Co-N4,Ni-N4,and
Mn-N4centers,whichareshowninFig.S3,byfollowingtheprocedure
in [59,60]. N 1s bindingenergies were calculated as 399.7eV, 399.6, 399.9,and 400.0eV for Fe-N4,Co-N4,Ni-N4, and Mn-N4
centers,respectively.AlthoughthefittingoftheMetal-Npeakwas doneusingthesameMetal-N4coordinationasanrepresentative
(Fig. S3), please note that differentcoordinations of metal are expectedinthematerial[60,61].Asitwaspreviouslyshownonthe exampleofironandcobalt[60],siteswithdifferentcoordinationof metalareexpectedtoresultinrelativelynarrowdistributionofN 1sbindingenergiescontributingtothesameXPSregionbetween pyridinicnitrogenpeakat398.3eVandpyrrolicnitrogenpeakat 401.2eV.
Pyrrolic nitrogenat401.2eVand graphitic Nat 402.1eV are largestforCo-andNi-AAPyr.Twopeaksathighestbindingenergy areduetoNOxspecies,andtheyarealsolargestforCo-and
Ni-AAPyr samples. Pyridinic nitrogen and metal coordinated to nitrogenhavebeenreportedtobeimportant chemicalsitesfor electrocatalyticreactions[58].Surfaceoxideshavebeenshownto beanimportantmeasureofdefectsincarbonnetworkwhichare also veryimportant for electrocatalytic activity [62]. Fromthis standpoint,Fe-AAPyrandMn-AAPyrsupposedtohaveverysimilar highestactivityamongfoursamples.Thechemicalstateofmetal hasbeenevaluatedfromrelevanthigh-resolutionmetallicspectra, Fe2p,Co2p,Ni2pandMn2p.Fe-AAPyrandNi-AAPyrarefreefrom metallic particles which is critical observation (Fig. S2). High amounts of metal oxides are detected for Co- and Fe-AAPyr samples.Both Co-and Mn-AAPyrhave a significantamountof metallicparticlesdetectedwhichmaybedetrimentalfor electro-catalyticactivity.
Thebestperformingsamplehasauniquebalanceofchemistry as reported by XPS.Fe-AApyr hasthe highest amount of total nitrogen, total metal, high amount of pyridinic and nitrogen coordinatedtothemetal,highamountof defectsincarbonand absence of metallic particles.The next performing sample, Co-AAPyr,hasverymuchsmalleramountofnitrogenandN-pyridinic andNx-me,ahigheramountofgraphiticcarbonwithfewerdefect sitesand metallicCodetected.Ni-AAPyr issimilarin structural chemistry to Co-AAPyr with the exception of the absence of metallic particles. The smaller absolute amount of Ni-Nx in comparisontoCo-Nxresultsinslightlyworseactivitythanthatof Co-AAPyr. Finally, the worstperforming sample Mn-AAPyrhas very similartype of nitrogen-carbon networkthat should have
Fig.1.ScanningelectronmicrographsofFe-AAPyrsynthesizedusingSSM.
Table2
Elementalcompositionandchemicalspeciationofcatalysts.
C1s% O1s% N1s% Metal% Cgr C-N CxOy Fe-AAPyr 84.8 7.2 7.9 0.25 32.7 13.5 51.3
Co-AAPyr 89.5 8.1 2.3 0.10 50.6 6.4 40.5
Ni-AAPyr 86.3 11.6 2.0 0.19 29.7 17.7 43.3 Mn-AAPyr 82.3 12.5 5.1 0.13 25.0 16.1 51.6
resultedinperformanceclosetoFe-AAPyr.Thepresenceofmetal Mnparticles,however,maybethereasonforworseperformance.
3.2.ElectroreductionofOxygeninNeutralMedia
Fig.3showsthelinearsweepvoltammogramsobtainedusinga RotatingDiscElectrode(RDE)at1600RPMand5mVs"1forthe
synthesizedMn-AAPyr,Fe-AAPyr,Co-AAPyr,Ni-AAPyrcatalystsas wellasACinthepH=7.5electrolytesaturatedwithO2atroom
temperature.Thecurrentdensitieshavebeennormalizedtothe
geometricareaoftheelectrodeandpotentialsinthemanuscript arereferredtotheAg/AgCl(3MKCl)electrode.Itcanbeseenthat amongallsynthesizedtransmissionmetalcatalysts,Fe-AApyrhas thehighestonset potentialof 0.31Vvs.Ag/AgCl. Thehalfwave potentials ofthetransitionmetalcatalysts alsopositivelyshifts fromNi-AApyr(-0.12Vvs.Ag/AgCl)toFe-AApyr(+0.16Vvs.Ag/ AgCl)accordingtothefollowingtrend:Ni-AApyr<Mn-AApyr<
Co-AApyr<Fe-AApyr,summarizedinTable3.Incontrast,activated
carbonnotonlyhasthelowestonsetandhalfwavepotentialsin comparison,butitalsohasthelowestlimitingcurrentdensities, indicatingthatthepresenceoftransitionmetalactivecentersare importantfortheefficientelectroreductionofoxygeninneutral media. Among all the synthesized catalysts, Fe-AApyr has the highestelectrochemicalperformance,whichalsocorroboratesthe polarizationcurvesobtainedusingMFCsoperatingwithFe-AApyr asacathodecatalyst.
3.3.CathodePolarizationcurves
Threeseparatelinearsweepvoltammetriesoftheairbreathing cathodescontainingM-AAPyrcatalystswereruninK-PB(0.1M) and0.1MKCl(Fig.4).Thisinvestigationwasdoneinordertostudy theelectrocatalyticactivityofFe-AAPyr,Co-AAPyr,Mn-AAPyrand Ni-AAPyrcatalystsinneutralmediaand“clean”conditions.After leavingthecathodeexposedtotheelectrolyteforover15hours,it can be determined the open circuit potential of the different cathodes (Fig.4.a). Particularly, Fe-AAPyrhad the highestopen circuit potential (0.307!0.001V vs Ag/AgCl) followed by Mn-AAPyr(0.252!0.004VvsAg/AgCl),Co-AAPyr(0.233!0.004Vvs Ag/AgCl),Ni-AAPyr(0.226!0.002VvsAg/AgCl)whileAChadthe lowestopencircuitpotentialthatwas0.203!0.004V(vsAg/AgCl). These results demonstrated that the utilization of PGM-free catalysts lowers theactivationlossescompared toAC. Unfortu-nately,theactivationoverpotentialwerestillhighintherangeof 0.28-0.36V compare tothe theoreticalvalue.Fe-AAPyrhad the highestelectrocatalyticactivityfollowedbyCo-AAPyr,Ni-AAPyr, andMn-AAPyr.AChadthelowestelectrocatalyticactivityamong the material investigated (Fig. 4.b). The current density of
Fig.2.High-resolutionN1sspectrafor4catalysts.AtomicpercentageofdifferenttypesofmetalsobtainedfromFe2p,Co2o,Ni2pandMn2phighresolutionspectrais shownforeachmaterial.
-6 -5 -4 -3 -2 -1 0 1
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5
Cu
rr
en
tD
ens
ity
/m
A
cm
-2
Potentialvs(Ag/AgCl) /V
Fe-AAPyr Co-AAPyr Ni-AAPyr Mn-AAPyr AC
Fig.3. RotatingDiskElectrodeCurrentmeasuredforMn-AAPyr,Fe-AAPyr, Co-AAPyr,Ni-AAPyrandAC.
Table3
Onset(Eon)andhalfwavepotentials(E1/2)ofthecatalyststowardsoxygenreduction reactioninneutralmedia.
Eon E1/2
Catalyst (VvsAg/AgCl)) (VvsAg/AgCl))
Fe-AApyr 0.31 0.16
Co-AApyr 0.27 0
Mn-AApyr 0.21 "0.1
Ni-AApyr 0.11 "0.12
1400
m
Acm"2wasreachedatapotentialof"0.142!0.004Vfor Fe-AAPyr,"0.162!0.011forCo-AAPyr,at"0.196!0.005Vfor Ni-AAPyr,at "0.216!0.009V for Mn-AAPyrand at a potential of "0.268!0.002VforAC.Fromthoseresults,itcanbedetermined thatFe-AAPyrhadthehighestopencircuitvoltageatnullcurrent and the highest potential at a measured current. Standard deviation detected was low indicating reproducibility in manufacturingthematerials.TheseresultsfollowstheRDEtrend (Fig.3)exceptNi-AAPyrandMn-AAPyrinwhichtheperformances areversedwhenintegratedintothecathode.
3.4.PerformancesinMFC
ThecathodeshavethenbeenincorporatedintorunningMFCs and polarization curves (Fig. 5.a), powercurves (Fig. 5.b), and electrodepolarizationshavebeendetermined(Fig.5.c). Polariza-tioncurves(V–I)showeddifferenttrendsthatfollowedtheresults previouslypresentedforthecathodelinearsweepvoltammetries
(Fig.4.b).TheMFCpolarizationcurvesshowedhighestopencircuit voltageandoverallperformancesforFe-AAPyr.Fe-AAPyrhadopen circuit voltage of 0.691!0.011V followed by Co-AAPyr (0.637!0.002V), Mn-AAPyr (0.628!0.008V) and by Ni-AAPyr (0.627!0.015V)(Fig.4.a).MFCwithACcathodehadthelowest open circuit voltage that was 0.616!0.003V (Fig. 5.a). Power curvesshowedthatFe-AAPyrhadthehighestpoweroutputthat was 251!2.3
m
Wcm"2. The other PGM-free had a maximumpower density of 196!1.5
m
Wcm"2 (Co-AAPyr), 171!3.6
m
W cm"2 (Ni-AAPyr),161!2.8
m
Wcm"2 (Mn-AAPyr) and 130!4.2
m
Wcm"2(AC)thatwasactuallythelowestpowerachieved(Fig.5.b).Fe-AAPyrhadapowerdensitythatwas28%highercomparedto Co-AAPyr,48%betterthanNi-AAPyr,54%betterthanMn-AAPyr and 102%bettercompared toAC.Singleelectrode polarizations taken during the overall polarization curves showed that the differenceinperformanceswascausedexclusivelybythecathode behavior(Fig.5.c).Theanodepolarizationsperformedsimilarlyas
Fig.4. CathodepotentialsmeasuredbeforeLSVs(a)andLSVinpotassiumphosphatebuffer(0.1M)(b).
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 400 800 1200 1600 2000
Ce
llP
ot
en
tia
l/
V
Current Density
/
µA
cm
-2a
Fe-AAPyrCo-AAPyr Ni-AAPyr Mn-AAPyr AC
0 50 100 150 200 250 300
0 400 800 1200 1600 2000
Po
w
er
De
ns
ity
/µ
W
cm
-2
Current Density
/
µA
cm
-2b
Fe-AAPyrCo-AAPyr Ni-AAPyr Mn-AAPyr AC
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3
0 400 800 1200 1600 2000
Po
te
nt
ia
lv
s.
(A
g/
A
gC
l)
/V
Current Density
/
µA
cm
-2c
Fe-AAPyrCo-AAPyr Ni-AAPyr Mn-AAPyr AC
expected (Fig. 5.c). Also, in this case, low standard deviation indicatedgoodreproducibility.
3.5.DurabilityTestsinMicrobialFuelCells
DuplicateMFCswererunforoveronemonthandthevoltage was recordedcontinuously. Theaverage valuesof the recorded voltageareherepresented(Fig.6)whiletheduplicatevoltagefor MFC having different cathode catalyst is presented in the SupportingInformation(Fig.S4).Itcanbenoticedthatgenerally Fe-AAPyr outperformed the other cathode catalysts along the entireexperimentation(Fig.6).Similarly,afterFe-AAPyr,Co-AAPyr had higher voltage compared to Ni-AAPyr and Mn-AAPyr. The latterwastheworstperformingmetal-basedcatalystduringthe durability test. All the M-AAPyr had higher performances comparedtoACcathodesMFCs(Fig.6).Interestingly,thecycles ofM-AAPyrcathodecatalystsMFCswereshorterthanthecyclesof ACcathodeMFCprobablyindicatingafasterconsumptionoffuel duetohighervoltage/currentgenerated(Fig.6).
Powercurveswerealsomeasuredattheendofthedurability testandpresentedintotheSupportingInformation(Fig.S5).The differencesinpowerpeaksbetweenthebeginningandtheendof thetests are herereported (Fig. 7). Generally, the powerpeak decreasesovertimeduetothebiologicalgrowthonthecathode
[63],theprecipitationofcarbonatesthatenhancetheprotonmass transferresistance[64]andthenegativeeffectoftheinteraction betweenthecatalystandthepollutantscontainedintowastewater
[50,51]. In our case, clear biofilm/fouling was formed on the cathodesurfaceasshowedbyFig.S6.Thepowerpeakdecreasedby 30%inthecaseofFe-AAPyr(from251!2.3
m
Wcm"2to175!12
m
Wcm"2),26%forCo-AAPyr(from196!1.5
m
Wcm"2to144!8.7
m
Wcm"2),21%forNi-AAPyr(from171!3.6
m
Wcm"2to135!0.6
m
Wcm"2), 22% for Mn-AAPyr (from 161!2.8
m
Wcm"2 to125!4.1
m
Wcm"2)and 19%for AC (from130!4.2
m
Wcm"2to107!6.7
m
Wcm"2).Thosedataareinagreementwithpreviouslyreported data in which the decrease in M-N-C catalyst varied between10and30%duringonemonthinvestigation[50,51,63].
3.6.PerformancesofFe-AAPyrcatalystsinworkingMFCswith electrolytehavingdifferentsolutionconductivity
Asitwasdiscussedabove,Fe-AAPyrwasidentifiedasthemost activeelectro-catalystamongthematerialsinvestigatedforORR incorporatedintoanair-breathingcathodeoperatinginMFC.In this paragraph, theperformances ofSCMFCs usingFe-AAPyr as cathode catalyst have been investigated varying the solution conductivity of the electrolyte simulating real wastewaters. Particularly, the solutionconductivity variedbetween12.38mS cm"1 and 63.1mScm"1. Results showedthat the performances
increasedsignificantlywiththeincreaseinsolutionconductivity (Fig. 8). Polarization curves showed practically the same open circuit voltage with starting point of 0.742!0.021V (Fig. 8.a). Differentslopesinthepolarizationcurveswereidentifiedwiththe increase in solution conductivity (Fig. 8.a).Separate electrodes polarization curves showed that both anode and cathode polarization curves are positively affected by the electrolyte conductivity (Fig.8.band 8.c).In fact,both anodeand cathode polarizationcurveschangedtheirlinearslopespositivelywiththe increaseinsolutionconductivityindicatingadecreaseinohmic losseswithelectrolyteconductivity(Fig.8.band8.c).Interestingly, both anode and cathode do not reach diffusion limitation indicating theohmic losses due tothe electrolyteas the main causeoftheoveralllosses.Powerdensityoutputwasthenaffected positivelybytheincreaseinsolutionconductivity(Fig.8.d).Results aboutthepowerdensitiesobtainedaresummarizedinTable1.The maximum power recorded was 482!5
m
Wcm"2 for solutionconductivityof63.1mScm"1(Fig.8.d).Aquasi-linearincreaseof
the power peak with the solution conductivity in the range investigatedcanbenoticed(Fig.9).Theseresults underlinethe importantaspectthattheperformancesoftheMFCsareelectrolyte limited. In fact, the increase in solution conductivity of the electrolyteledtoanincreaseinoutput.
4.Conclusion
For the first time, this study showed a comparison among severalORRPGM-freecatalystshavingadifferentmetalcenterand synthesizedusingconsistentpreparationmethod,identicalinitial ingredientsandorganicprecursors,samecathodestructure and tested in repeatable and equal operating conditions. Fe-AAPyr catalystsshowedthehighestperformancecomparedtoCo-AAPyr, Ni-AAPyr,andMn-AAPyr.Theresultsalsoindicateda straightfor-ward hierarchy among the PGM-free metal based catalysts in whichthemetalsusedduringthesynthesisaffectedthe perform-ancesandobeyedtoacertainorder:Fe>Co>Ni>Mn.Therefore,Fe shouldbethemetaltoutilizeduringthesynthesisofPGM-free catalystsforMFCapplication.Theseresultsindicatedclearlyalso that the metal-based PGM-free catalysts investigated outper-formedAC-based(metal-free)cathodeby31-102%.Durabilitytests resultsindicatedarelativelygoodstabilityofMetal-AAPyrwitha lossesin32daysquantifiedin21-30%.Theadditionofnonprecious metals catalysts undoubtedly increased the capital cost of the cathode,butthissmallcostrise(7centscm"2[65])wouldcertainly
benefitinenhancingtheverylowpowergenerated.
Afairandscientificallycorrectcomparisonofourcurrentdata withexistingliteraturecannotbedonesinceworkingtemperature, configuration utilized, utilization of different electrolytes and differentatmosphericconditions(Albuquerque,NewMexico,isat 1500 AMSLwithatmosphericpressure reducedbyroughly 20%
0 0.05 0.1 0.15 0.2 0.25
0 2 4 6 8 10 1214 16 1820 2224 2628 3032
Cel
lP
ot
en
tial
/V
Time/day
Fe-AAPyr Co-AAPyr Ni-AAPyr Mn-AAPyr AC
Fig.6.Voltagerecordedduringthedurabilitytests.
0 25 50 75 100 125 150 175 200 225 250 275
AC Mn-AAPyr
Ni-AAPyr Co-AAPyr Fe-AAPyr
Power
Density
/
µW cm
-20 days 32 days
comparedtotheoneonsealevel)arequitedifferentandcanaffect significantly the performances. In other cases, the power was referredtotheanode areaand not thecathodeareawithhigh poweroutputreported[66].
We showed that the increase of the electrolyte solution conductivityincreasedtheperformanceoutputconsiderablywith amaximumof482!5
m
Wcm"2achievedatthehighestsolutionconductivityinvestigated.Thoseresultsindicatedthat theMFCs outputis severelyelectrolytesolutionlimited andconsequently the utilization of conductivity waste (e.g. urine [27,28]) is suggestedtoincreasetheoutput.Itmustbenoticedthatalsoat thehighestsolutionconductivityinvestigatedtheoverall polari-zationcurvesandtheanode/cathodepolarizationcurveshavea linear trend indicating that they are ohmic-dependent and no diffusionlimitationoccursintheseconditions.Therefore,afurther
increase in solution conductivity can be investigated with a probableadditionalincreaseinpowergeneratedtilltheplateauis reached.Furtherdirectionsshouldcertainlypursuetheutilization of PGM-free catalysts for MFCs withan additional decrease in catalystsloadingtolowerthecathodecosts.
Acknowledgements
The authors would like to thank the Bill & Melinda Gates Foundation grant: “Efficient Microbial Bio-electrochemical Sys-tems” (OPP1139954).Computationalwork was performedusing thecomputational resourcesofEMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific NorthwestNationalLaboratory.Thispaperhasbeendesignated LA-UR-16-26818.
AppendixA.Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.02.033.
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