MFCs using novel HER iron-based catalysts Co-generation of hydrogen and power/current pulses fromsupercapacitive Electrochimica Acta

Co-generation

of

hydrogen

and

power/current

pulses

from

supercapacitive

MFCs

using

novel

HER

iron-based

catalysts

Carlo

Santoro

a,

*

,1

,

Francesca

Soavi

b,

*

,1

,

Catia

Arbizzani

b

,

Alexey

Serov

a

,

Sadia

Kabir

a

,

Kayla

Carpenter

c

,

Orianna

Bretschger

c

,

Plamen

Atanassov

a

a_{DepartmentofChemicalandBiologicalEngineering,CenterforMicro-EngineeredMaterials(CMEM),UniversityofNewMexico,Albuquerque,NM87131,}

USA

b

DepartmentofChemistry“GiacomoCiamician”,AlmaMaterStudiorum–UniversitàdiBologna,ViaSelmi,2,40126Bologna,Italy

c

J.CraigVenterInstitute,4120CapricornLane,LaJolla,CA92037,USA

ARTICLE INFO Articlehistory:

Received3September2016

Receivedinrevisedform15October2016 Accepted22October2016

Availableonline24October2016 Keywords:

Supercapacitor microbialfuelcells hydrogenevolution bioenergy PGM-freecatalysts

ABSTRACT

Inthiswork,fourdifferentsupercapacitivemicrobialfuelcells(SC-MFCs)withcarbonbrushastheanode

andanair-breathingcathodewithFe-Aminoantipyrine(Fe-AAPyr)asthecatalysthavebeeninvestigated

usinggalvanostaticdischarges.Themaximumpower(Pmax)obtainedwasintherangefrom1.7mWto

1.9mWforeachSC-MFC.This in-seriesconnection offourSC-MFCs almostquadrupledPmaxtoan

operatingvoltageof3025mVandaPmaxof8.1mW,oneofthehighestpoweroutputsreportedinthe

literature.Anadditionalelectrode(AdHER)connectedtotheanodeofthefirstSC-MFCandplacedinthe

fourthSC-MFCevolvedhydrogen.Thehydrogenevolutionreaction(HER)takingplaceattheelectrode

wasstudiedonPtandtwonovelplatinumgroupmetal-free(PGM-free)catalysts:Fe-Aminoantipyrine

(Fe-AAPyr)andFe-Mebendazole(Fe-MBZ).TheamountofH2producedwasestimatedusingtheFaraday

lawas0.86mMd
1cm
2(0.132Lday
1)forPt,0.83mMd
1cm
2(0.127Lday
1)forFe-AAPyrand0.8

mMd
1cm
2(0.123Lday
1)forFe-MBZ.Hydrogenevolutionwasalsodetectedusinggas

chromatogra-phy.WhileHERwastakingplace,galvanostaticdischargeswerealsoperformedshowingsimultaneous

H2productionandpulsedpowergenerationwithnoneedofexternalpowersources.

ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense

(http://creativecommons.org/licenses/by/4.0/).

1.Introduction

Inthelastdecade,bioelectrochemicalsystems(BES)havebeen studiedfortheirapplicationsinbioenergy,resourcerecoveryand biomassdegradation [1,2]. The two mostinvestigated BESsare namelymicrobialfuelcells(MFCs)[3,4]andmicrobialelectrolysis cells (MECs) [5,6]. While MFCs generate electricity from the oxidationoforganicwasteasafuel[3,4],MECsproducehydrogen or other value added products (VAPs) utilizing electricity generatedfromexternalsources[5,6].

MFCs and MECs utilize electroactive bacteria that degrade organic compounds in the electrolyte, releasing the electrons eitherthroughmediatorsordirectlytothesurfaceoftheelectrode (anode) that works as an electron acceptor [7,8]. In the MFC, electronsliberated fromthedegradationof electrolyteorganics

movethroughtheexternalcircuittothecathodewhereoxygenis reduced[9,10],andnetcurrent/powerisgenerated.Ontheother hand,inanMEC,anexternalpowersupplyisconnectedinorderto achievetheoperationalpotential(
1VvsAg/AgCl)atwhichthe hydrogen evolution reaction (HER) takes place at the cathode working in neutral media. Consequently, organics are also degradedandH2isproduced,buttheenergybalanceisnegative inMECsbecauseanexternalsourceofenergyisneeded[11].

Themain differenceofMFCs and MECscomparedtoproton exchangemembranefuelcells(PEMFCs)orelectrolyzesare:i)the bioticcatalyst(electroactivebacteria)attheanode;ii)thecathode directlyexposedtothewaste(oftenbioticcathode)iii)ambient working temperatures;iv) neutral media; and v) utilization of complexbiomassasanodicfuel[12,13].MFCsandMECsstillhave severaltechnicalchallengessuchashighcostmaterialsandlow powerdensities thatmustbeaddressedtobecompetitivewith conventionalfuelcellsorelectrolyzers;however,manyofthese canbesolvedthroughbetter-suitedelectrocatalyticmaterialsand operationalchanges.

OneofthemainchallengesforMECsistheoptimizationofthe H2 evolution reaction (HER) that takes place on the cathode.

*Correspondingauthors.

E-mailaddresses:carlo.santoro830@gmail.com,santoro@unm.edu(C.Santoro),

francesca.soavi@unibo.it(F.Soavi).

1

Thetwoauthorscontributedequallytothemanuscript.

http://dx.doi.org/10.1016/j.electacta.2016.10.154

0013-4686/ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).

ContentslistsavailableatScienceDirect

Electrochimica

Acta

Similar to the oxygen reduction reaction (ORR), catalysts are generally used tosignificantlydecreasethe overpotentials and acceleratethereactioninwhichhydrogenisevolved.Inacidicand alkalineconditions,IrandPtareknowntobethebestcatalystsfor HER[14,15].Platinumisthemostusedandreportedcatalystfor HERinMEC[16,17],butthehighcostcomparedtothehydrogen producedandthepossibilityofcatalystpoisoningduetosulfur pollutantsinwastewaterdoesnotmakeitthebestcandidatefor HERinMECsforlargeapplications[18].Atpresent,therearea limitednumberof studies thathave focusedoncharacterizing catalysts operating in neutral conditions required for MEC

[19_–22].Additionally,fewplatinumgroupmetal-free(PGM-free) catalystshave been reportedas HERcatalysts in MEC [19–23]. Among them, molybdenum sulfide(MoS2)is the moststudied

[20].IthasbeenshownthattheperformancesofMoS2asHER catalystarequitegoodalsoinacidicandalkalinemedia[24,25]. However,althoughMoisconsideredtobeaPGM-freecatalyst,it isnotanearthabundantmetalandthecostofimplementing Mo-basedcatalysts in large-scaleMEC applications is not feasible. OtherPGM-freecatalystbasedonNi[19,21],stainlesssteel(SS)

[22]andactivatedcarbononastainlesssteelmesh[23]havebeen reportedasalternativestoPtasHERcatalystsinneutralmedia withpromisingresults;however,thecorresponding overpoten-tialswerestillhighrelativetoPt.PGM-freecatalystsbasedoniron havebeenrecentlyshowntobepromisinginacidicandalkaline media [26,27] for H2 evolution. However, to the best of our knowledge,PGM-free catalystsbasedonFe-N-Cmaterialshave notbeenyetimplementedorreportedasHERcatalystsinneutral media.

Fe-N-C catalysts have been extensively studied for oxygen reductionreaction(ORR) inacidic [28–31],neutral [32–35]and alkaline conditions [36–39]. The materials were found to be extremelyactiveinallpHconditionsandperformancewasbetter than Pt in alkaline media [40]. The influence of morphology, surface chemistry and ORR of Fe-N-Cmaterials synthesizedby adoptingtheSacrificialSupportMethod(SSM)wassystematically investigated[41–45].Itwasanalyticallyshownthatthe micropo-rousstructureofmaterialswillimproveORRactivityinneutraland highpHbecauseoftheaccessibilitytoactivesitesandremovalof productsofreaction[41–45].SincetheremovalandcaptureofH2 producedinrealapplicationsisquitechallenging,theSacrificial Support Method was selected for preparation of the Fe-N-C catalystsforenhancingH2degassing.

AnothermainchallengeconcerningMECsisthatenergyhasto besuppliedtothebioelectrochemicalsysteminordertocreatethe conditionsfortheHERandproductionofH2.Therefore,thesystem hasanegativeenergybalance.Thiscanbealleviatedorovercome by:i)utilizationoflowcostandefficientcatalyticmaterialsthat decreasetheenergyconsumed[23];ii)thehybridizationofanMFC withanMEC,whichhasbeenalsobeensuccessfullyreportedin literature[46,47].Inthelattercon_figuration,theexternalvoltage supplyforHERwasgivenbythevoltage/currentgeneratedbythe workingMFCs,yieldinganullenergybalanceandtheproduction of hydrogen [46,47]. Since then, several strategies have been adopted to overcome the configuration problem of connecting several hydraulically separated MFCs in series to increase the availablevoltage/current[48–51].Itwaspreviouslydemonstrated that theconnectionin series of MFCs allowed toboost up the voltage, necessary to powerpractical application, and to reach much higherpowerproduced [50–55]. Practicalapplicationsof MFCs smartly connected in series have been recently shown

[50–55].

Ithasbeenshownpreviouslythatcurrentandvoltagecanalso be boosted further by coupling MFCs with external super-capacitors[48–55].Althoughcommercial external supercapaci-tors are suitable for certain applications, some issues remain

becauseofthelongrechargetimerequiredfordevicesorsensors

[48–55].Recently,Santoroetal.shownthattheintegrationofthe MFCelectrodesasaninternalsupercapacitorinasupercapacitive microbial fuel cell (SC-MFC) [56–58] is a new approach to increasethecurrentpulsesanditcouldbeusedtosimplifythe combined system MFC-MEC system for energy neutral H2 production.

The presence and absence of O2 creates a double and opposite anaerobic/aerobic environment inside the single chamber MFC. In fact, bacteria on the anode consume the oxygen guaranteeing strictlyanaerobicconditionsin proximity of the electrode. These speci_fic conditions and oxidation of acetatesubstratepushtheanodetowardslowpotentialsvalues of 
0.5V (vsAg/AgClatpH7).In contrary,O2is neededand used as oxidant. Oxygen reduction reaction takes place at the cathode. Consequently, the cathode electrode is designed to provide oxidant to the catalytic sites. Therefore, air-breathing cathodesaredesignedtohaveconstantpresenceofoxygenand this pushes the potentials towards high values. Different environment conditions lead to the self-polarization of the electrodeswiththecarbonsurfaceattheanodeelectrostatically charged negativelyandthat at the cathodecharged positively. Once the electrodes are charged, the excess of charge is balancedbythecounterionsfromthebulksolutionforminga double layer at each surface electrode/electrolyte interface. In this case,the MFCelectrodes store electrostatically charges at OCV (Vmax,OC) behaving like a charged electrochemical double layer capacitor(EDLC).

The electrodes are then discharged through an electrostatic processwith:i)surfacechargesneutralizationandii)ionsreleased into the electrolyte. The electrostatically stored energy is then releasedthroughshortbut highgalvanostaticdischargespulses (GLV)withtheconsequentproductionofhighpoweroutput.After discharge,theMFCisleftinrestconditions(OCV)andtheelectrode equilibrium potential is self-recovered. Theelectrodes are then self-polarized again to the anodic and cathodic equilibrium potentialsand thedouble layerisself-regenerated.Theinternal EDLCisthenfullyrechargedandconditions,previousthedischarge pulse,arere-established.Inthoseconditions,theMFCelectrodes workasaself-powered,self-rechargedsupercapacitorthatisable togeneratepulsedcurrent/power[56–58].Ithasbeenshownthat intermittent load implementation enhances the output of a microbial fuel cell [59,60] and SC-MFC operation follows this directionandoperatingstrategy.

Herein,wereportforthefirsttimeasystemthatproducesH2by asmartseriescombinationofSC-MFCsandwithoutanyadditional power supply.It utilizes Fe-basedPGM-free HER catalysts. The powerpulsesof fourSC-MFCssinglecellsand seriesconnected (SC-MFC-SERIES)wereevaluated.TheSC-MFC-SERIESconnection was used todrive thepotential of anadditional HER electrode (AdHER)tothevaluesrequiredforH2production.Differentcatalyst materials weretestedfortheAdHERincludingPtandtwo novel PGM-freecatalystsbasedoniron.Fe-Aminoantipyrine(Fe-AAPyr) and Fe-Mebendazole(Fe-MBZ)areherepresented asnovel and alternative low cost catalysts compared to Pt for HER. By combiningtheimprovedoperationalsetup(theseriesconnection ofSC-MFCs)andtheutilizationoflow-costAdHERelectrodes,the simultaneous production of H2 and current/power pulses were shown.

2.ExperimentalMethodsandMaterials

Fig.1.SchematicofasupercapacitivemicrobialfuelcellwithillustrationofEDLCformedontheelectrodes(a).Schematicofelectrochemicalreactorconsistingofthefour SC-MFCs-SERIES(b)inwhichAistheanode,Cisthecathode,Risthereferenceelectrodeandthenumbersrepresentthenumberofthecells.Schematicofthefour SC-MFCs-SERIESwiththeAdHERinMFC4forhydrogenevolution(c).AnandCnelectrodepotentialsrefertoanominalAg/AgClreferenceelectrodeeventuallyplacedineachcelln(not

2.1.MaterialsDescription 2.1.1.AnodeElectrode

Theanodeelectrodeswerecommercialcarbonbrush(Millirose, USA)withdiameterof3cmandheightof3cm.Itshouldbenoted that theexperiments wereconducted withthe anodesalready well-colonizedbyelectroactive bacteriain MFCsrunning forat least6months[32].Theanodewasfullyimmersedinthesolution throughoutallexperiments.

2.1.2.CathodeElectrode

The air-breathing cathodewas prepared using a mixture of activatedcarbon(AC,SXUltra,USA),carbonblack(CB,AlfaAesar), polytetrafluoroethylene(PTFE, 60wt% emulsion, Sigma Aldrich) andiron-aminoantipyrine (Fe-AAPyr).Theiron-aminoantipyrine (Fe-AAPyr)catalystwassynthesizedusingthesacrificialsupport method(SSM)previouslypresented[61–63].Briefly,AC,CBand PTFEweremixedinagrinderwithweightpercentagesof70, 10and 20%, respectively. After5minutes of grinding, the mixturewas appliedtoastainlesssteelmesh(McMaster,USA)currentcollector usingametallicdie,andFe-AAPyrwasaddedandmixedwiththe existingpowder.Theelectrodewaspressedat2mT(metrictons) for5minuteswithahydraulicdie(Carver,USA).AC,CBandPTFE loading was 402mgcm
2 and the Fe-AAPyr loading was 1.50.1mgcm
2.Thecathodeshad ageometricareaof2.9cm2 exposedtothesolution.New cathodeswereutilized duringall experiments.

2.1.3.Additionalelectrodeforhydrogenevolutionreaction(AdHER) Threedifferentcatalystswereusedastheadditionalelectrode forhydrogenevolutionreaction(AdEHER):Ptasthecontrol,and Fe-AAPyrandFe-MBZasnewPGM-freecatalystsforHERinneutral media. AdEHER electrodes had a geometric area of 2.32.3cm (5.3cm2_{).Toraycarbonpaperwasusedascurrentcollectorforthe} HER electrodes. The catalyst inks wereprepared following the procedure described previously [63]. Briefly, Pt, Fe-AAPyr or Fe-MBZ(120mg each)were mixedwithNafion1 _(45wt.%)and isopropanolandthensonicatedforatleast1hour.Anairbrushwas usedtospraytheinkdirectlyontothecarbonpaper.Thecarbon paper was set up on a hot plate (T=60C) to evaporate the isopropanolandquicklydrytheelectrode.Fe-AAPyrandFe-MBZ loadingwas50.5mgcm
2,whilePtloadingwas 0.50.05mg cm
2.Thecathodewasconnectedtoaplastic-coveredcopperwire andthecontactwasgluedusinganepoxyresintoavoidexposure tothesolution.

2.2.Microbialfuelcellandmicrobialelectrolysiscellconfigurationand operation

Theelectrochemicalreactorwasfilledwithamixtureof50% (v/v)0.1Mpotassiumphosphatebufferedsaline(K-PBS)and0.1M ofpotassiumchloride(KCl)solution,and50%(v/v)activatedsludge from the Albuquerque water treatment plant (Albuquerque SoutheastWaterReclamationFacility,NewMexico,USA).Acetate in concentration of 3gL
1 (36.5mM) was used as the carbon sourcefor theelectroactive bacteria. Allexperiments wererun with pre-colonized anodes and new sterile cathodes. The four SC-MFCs were characterized separately (Fig. 1a) and then connected in series toincrease theoperating voltage (Fig.1b). ThefourSC-MFCsconnectedinserieswerethennamed SC-MFC-SERIES(Fig.1b).Theseriesconnectionwaspossiblebecauseeach MFCwas hydraulicallyuniqueandthey didnotshare thesame electrolyte[51,52].

TheSC-MFC-SERIESreactorswerethenadaptedtotheAdHER studiestoevaluateH2production.Inthiscase,anAdHERelectrode ofappropriatematerialwasaddedtothelastSC-MFC(MFC4)ofthe

SC-MFC-SERIES and electrically connectedto theanode of first SC-MFC(A1)intheSC-MFC-SERIES(Fig.1c).Thepulsetimingfor openingandclosingthecircuitbetweentheanodeofMFC1(A1) andtheAdHERinMFC4wasvariedtoevaluatetheenergyrecovery andH2productionactivitiesunderdifferentpulse-lengths. 2.3.Electrochemicalmeasurements

A BioLogic SP-50 potentiostat was used for carrying out electrochemical measurements. Galvanostatic (GLV) discharge curveswereperformedatdifferentcurrents(ipulse)withdischarge time (tpulse) of 2s and 10ms. The four SC-MFCs were tested separatelyusingathree-electrodesetupwithaAg/AgClreference electrode(3MKCl,+210mVvsSHE)insertedintothesolution,the anodeelectrodeasthecounter,andthecathodeelectrodeasthe workingelectrode.

Each SC-MFC was kept under rest conditions (Open Circuit Voltage,OCVorVmax,OC)untilstablevoltage(1mV)wasreached andaspecificGLVdischargewasrunforagiventime(Fig.2).After thedischarge,theSC-MFCwereallowedtorecovertotheinitial Vmax,OCvalueandequilibriumconditionswererestored,meaning theSC-MFCwasself-recharged.

Following discharge pulse application, the voltage dropped verticallytoalowervalue(Vmax)–thatisdefinedasthepractical miximumvalueofthevoltageatwhichenergyandpowercanbe obtained. The ohmic losses (

D

Vohmic) were quantified as the differencebetweenVmax,OCandVmax,asshowedbyEq.(1):

D

Vohmic¼Vmax;OC
Vmax ð1Þ

Asaconsequence,theequivalentseriesresistanceofeachcell (ESR) canbe calculated as theratio between theohmic losses (

D

Vohmic)andtheconstantcurrentappliedduringthepulse(ipulse) asshowedbyEq.(2):

ESR¼

D

Vohmic

ipulse ð2Þ

RearrangingEqs.(1)andEq.(2),itispossibletoexpressVmax differently,accordinglytoEq.(3):

Vmax¼Vmax;OC

D

Vohmic¼Vmax;OC
ESRipulse



ð3Þ Theutilizationofthethree-electrodestechniqueallowedusto monitortheelectrodepotentialtrendsduringdischarge.Afterthe initialohmicdrop(

D

Vohmic),thecellvoltagelinearly decreased overtimeduringtheelectrostaticdischarge(

D

Vcapacitive)ofthe polarizedcarbon surfaces (anode and cathode). Given that the kineticsofORRaremuchslowerthantheelectrostaticprocess,at highcurrentpulsesandshorttimes,theoverallSC-MFCresponse was mainly governed by the electrostatic, capacitive GLV discharge.The ratio between thecurrent pulse (ipulse)and the voltagedischargeslope(s,dV/dt)duringtpulsecorrespondstothe capacitanceoftheSC-MFCasshowedbyEq.(4):

Ccell¼ ipulse dV dt ¼ipulse s ð4Þ

Inordertohavehighenergyandpower,

D

Vcapacitiveshouldbe minimizedandCcellmaximized.Thecellcapacitanceisrelatedto theanodecapacitance(CA)andcathodecapacitance(CC)bythe followingequation: Ccell¼ 1 CAþ 1 CC  
1 ð5Þ Power curves were constructed considering the maximum poweroutput(Pmax)andthepowerobtainedafterpulses(Ppulse) of2sand10ms.Particularly,Pmaxwascalculatedbymultiplying Vmaxandtheappliedcurrentpulse(ipulse).

Pmax¼Vmaxipulse¼ðVmax;OC
ESRipulse



Þipulse ð6Þ

Theenergyobtainedafterapulse(Epulse)wascalculatedasthe area under the discharge curve between Vmax and Vfinal,pulse accordingtoEq.(7):

Epulse¼ipulse

Z t 0

Vdt _ð7Þ

Consequently, the power under the GLV pulse (Ppulse) was calculatedastheratiobetweentheenergyproducedduringapulse (Epulse)andtpulseaccordingtoEq.(8):

Ppulse¼

Epulse

tpulse ð8Þ

PmaxandPpulsewerealsonormalizedtothecathodearea(2.9cm2₎ andtothevolumeofthereactor(125mL)toestablishsurfacearea andvolumetricpowerdensitiesforPmaxandPpulse,respectively. The four SC-MFCs were then connected in series (SC-MFC-SERIES)andgalvanostatically(GLV)dischargedafterthe SC-MFC-SERIESreachedastablevoltageunderopen-circuitconditions.The GLVdischargeswereconductedwithtpulseof2sand10msata givencurrent(ipulse).ThedatarecordedfromtheSC-MFC-SERIES system were used in the equations above to calculate the maximumenergyandpoweravailablefromthesystem.However, thethreeelectrodetechniquewasnotpossiblesincethe SC-MFC-SERIES did not share the same electrolyte and consequently, 2-electrodedischargeswereperformedusingthecathodefrom MFC4(C4)asthepositiveelectrodeandtheanodefromMFC1(A1) asthenegativeelectrode(Fig. 1b).PmaxandPpulsecurveswerethen calculatedaspreviouslydescribed.

AftertheSC-MFC-SERIESsystemwascharacterizedforpower and energy, the additional electrode for hydrogen evolution reaction(AdHER)wasinsertedinMFC4 andshort-circuitedtoA1 asshowninFig.1c.ThisconnectiondrovetheAdHERpotentialto thelowvaluesnecessaryforgeneratingH2.Ifredoxprocessesare

nottakingplace(e.g.inpresenceofnon-aqueous,aproticmedia) this connectionshould drive theAdHER tothe potential corre-spondingtothesumofallotheranodeelectrodes(A1+A2+A3+ A4).Practically,in anaqueousenvironment thelowest feasible potential forAdHERis setbythepotential atwhichthe electro-catalytic hydrogenevolution occurs. Thepotentialof theAdHER electrodevsAg/AgCl(R4)andthepotentialdifferencebetweenA1 andC4oftheSC-MFC-SERIES(i.e.theSC-MFC-SERIESvoltage)were monitored over time. The system reached the operational equilibrium (stable AdHER potential and stable SC-MFC-SERIES voltage)afterroughly1hour.Afterthestabilization,thefollowing electrochemicalperformancesweremonitoredsimultaneously.

GLV discharges were then carried out with simultaneous production of H2. Particularly, two tests were simultaneously performed:i)thepotentialoftheAdHERelectrodevsAg/AgCl(R4) was monitoredover time,and ii)galvanostaticdischargeswere performed using C4 as positive electrode and A1 as negative electrodewhiletheHERwastakingplace.Theseexperimentswere feasiblebyusingatwo-channelpotentiostat.

2.4.HydrogenEvolutionReactionMeasurements 2.4.1.HERmeasurements

The hydrogen evolution reaction of the Pt and PGM-free catalysts were initially run in neutral media for studying the electroactivityofthosecatalyststowardsHER.Theparametersof interestswere:i)onsetpotential;ii)overpotential(inrespectto platinum); and iii) potential at i=20mAcm
2. The HER was initiallymeasuredseparatelyinahermeticallyclosedPyrexglass chamber(volume100mL)containingpotassiumphosphatebuffer (K-PB)solution(0.1Mwith0.1MKCl)atpH7.5(Fig.S1).TheHER was measured using a three-electrode setup with the HER electrode as theworking, a platinum mesh asthe counterand Ag/AgCl(3MKCl,+210mVvsSHE)asthereference.Theworking electrodes were Pt, Fe-AAPyr, Fe-MBZ and carbon paper (CP), wherePtandCPwereusedascontrols.EachHERelectrodewas submergedin theelectrolytesolutionovernight toincrease the electrodewettabilityandavoidanyoxygenmoleculesadsorbedon thesurface.N2waspurgedvigorouslyforatleast30minutesprior to starting the experiments. Linear Sweep Voltammetry (LSV) betweenopencircuitpotential(OCP)and
2VvsAg/AgClwasrun atascanrateof1mVs
1inseparatetriplicateselectrodes.

AftertheHERtestsinneutralmedia,theelectrodeswereused asAdHERasshowedinFig.1c.H2productionfromtheAdHERduring theoperationswasestimatedusingtheFaradaylaw.Particularly, themolarofH2evolutionratewasdeterminedusingthefollowing Eq.(9):

_n¼2iHER

F ð9Þ

where_nisthemolarrate(mols
1)ofhydrogenproduction,2isthe numbermolesofelectronsnecessarytogenerate1moleofH2,iHER is the HER current (A) and F is the Faraday constant 96485 (Cmol
1).

reportedasvolumeproduced(Lday
1).H2was consideredasa perfectgasandtheequationPV=nRTwas usedtoestimatethe volumeoccupiedby1molofH2at1600AMSLwithanatmospheric pressurecalculatedas0.83atmandaroomtemperatureof22C (295K). The volume of H2 in those operating conditions is 29Lmol
1.

2.4.2.HydrogenproductionusingGasChromatography

TheFaradaylawallowedestimationoftheH2producedatthe AdEHER.ToensurethatH2wasproducedduringtheexperiments, paralleltrialsonthefourSC-MFC-SERIESsystemwiththeAdHER wereexecuted withtheFe-AAPyrAdHERcatalystand headspace sampleswereextractedforgaschromatography(GC).The SC-MFC-SERIES system was run overnight (15hours) and then H2 productionwasquantitativelymeasuredusinganAgilent6890N gaschromatographwithChemStationsoftwareRev.B.02.01-SR1 (260)andaVarianChrompackCapillarycolumn,CP-Molsieve5Å 10mx0.53mm50

m

m(PNCP7573).A0.25mLmanualinjection ofreactorheadspacewasrunona1minmethodusingasplitmode inletat120CwithN2carriergas(5.84psi,115mLmin
1),column flowofN2(5.84psi,10.2mLmin
1,115cmsec
1),oven tempera-ture of 80_{C, and a thermal conductivity detector at 120}_C

(referenceflowat10mLmin
1,makeupflowN2at5mLmin
1).A standardcalibrationcurvewaspreparedusing5%H2:95%N2(v/v) at different injection volumes, producing a sharp H2 peak at 0.35minsandabroadbackgroundpeakat0.42mins. 3.ResultsandDiscussion

3.1.Supercapacitivemicrobialfuelcellperformances

FourSC-MFCsweretestedseparatelyusingGLVdischargeswith tpulseof 2s and ipulseof 3mA. Thecell voltageand theanode/

cathodepotentialtrendswererecordedandshowninFig.3aandb, respectively.Thosespecificvaluesoftpulseandipulsewereadopted inordertocomparetopreviousreporteddataonsimilaroperating conditions [56–58,64].The behaviors of thefour separate cells were very similar and comparable to each other indicating reproducibility of the materials utilized and the process. The SC-MFCs exhibited a Vmax,OC of 74010mV (Fig. 3a) which resultedfromacontributionof1755mV(vsAg/AgCl3MKCl)of thecathodeand
57011mV(vsAg/AgCl3MKCl)oftheanode (Fig.3b).Theoveralldischargewithanipulseof3mAshowedan ohmicdropof 2306mVcorrespondingtoanESRof772

V

. This was mainly due tothe cathode(Fig.3b)ohmic drop that contributed roughly 95% to

D

Vohmic. These results are in agreement with previously presented data [56–58]. Moreover, thecapacitanceofthecell(Ccell)canbeextrapolated(Eq.(5))bythe curvesshowninFig.3a.OverallCcellwas161.1mF,Ccathodewas 719mFandCanodewas202mF.

3.2.SC-MFCpowercurves

Powercurveswerethengeneratedwiththeintentofevaluating the performances of the four separate SC-MFCs. Power curves (Pmax)werebuiltfromthedischargecurvesat3mAconsidering Vmax,OC of 73710mV and ESR of 772

V

(Fig. 4a). Pmax of 1.75mW (at 5mA),1.68mW (at 4mA), 1.72mW (at 5mA) and 1.87mW(at5mA)wasdetectedforSC-MFC1,SC-MFC2,SC-MFC3 andSC-MFC4,respectively(Fig.4a).Pmaxwasalsoexpressedasa functionofthecathodeareaandthevolumeofthereactorwith SC-MFC1producing6Wm
2(14Wm
3),5.8Wm
2(13.4Wm
3) forSC-MFC2,5.9Wm
2(13.8Wm
3)forSC-MFC3and6.4Wm
2 (14.9Wm
3)SC-MFC4.Amaximumcurrentof8mAwasmeasured forSC-MFC2,whileSC-MFC1,SC-MFC3andSC-MFC4wereableto reach9mAasmaximumipulse.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 4 4.5 5 5.5 6 6.5 7

Cell V

o

lt

age

/ V

Time /

s

a

SC-MFC 1

b

SC-MFC 2 SC-MFC 3 SC-MFC 4 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 4 4.5 5 5.5 6 6.5 7 Time (s) SC-MFC 1 SC-MFC 2 SC-MFC 3 SC-MFC 4 cathode anode Potential vs. (Ag/AgCl) / V

Fig.3.Cellvoltage(a)andelectrodepotential(b)profilesoffourdifferentSC-MFCsunder5srestand2spulsesat3mA.

0 1 2 3 4 5 6 7 8 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 0 1 2 3 4 5 6 7 8 9 10 Pow e r Density / W m -2 Po wer / mW Current / mA

a

SC-MFC 1 SC-MFC 2 SC-MFC 3 SC-MFC 4 0 1 2 3 4 5 6 7 8 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 0 1 2 3 4 5 6 7 8 9 10 Po wer Dens ity / W m -2 Power / m W Current / mA

b

SC-MFC1-2sSC-MFC3-2s SC-MFC2-2s SC-MFC4-2s SC-MFC1-10ms SC-MFC2-10ms SC-MFC3-10ms SC-MFC4-10ms

Ppulseof10mswasslightlylowerthanPmaxvaryingbetween 1.6mW(5.3Wm
2,12.4Wm
3,SC-MFC2)and1.8mW(6.1Wm
2, 14.2Wm
3, SC-MFC4)(Fig.4b).Lower Ppulsewas thenachieved withatpulseof2sforallSC-MFCs.Infact,Ppulse(2s)variedbetween aminimumof 1.1mW forSC-MFC2 (3.9Wm
2,9Wm
3)and a maximum of 1.3mW for SC-MFC1 (4.6Wm
2, 10.6Wm
3) (Fig. 4b). Performance differences of only 10% were detected amongthefourSC-MFCs,indicatinggoodreproducibility. Conse-quently, the utilization of SC-MFC follows the direction of enhancingpoweroutputof a MFC system.Thoseperformances weresimilartotheonepreviouslypresentedinSC-MFCsworking withsimilar electrolyte and same electrode materials [56_–58]. Powerpulsesobtainedaremuchhigher thancontinuouspower producedinasinglechamberMFC[56–58].

3.3.SC-MFC-SERIESSystemPerformance

TheconnectioninseriesofthefourSC-MFCs(SC-MFC-SERIES) providedavoltage,i.e.thethepotentialdifferencebetweenA1and C4,thatwasapproximatelyfour-foldsthevalueofeach SC-MFC. The SC-MFC-SERIES system had an OCV of 3025mV. The connectioninserieswaspossibleduetothehydraulic disconnec-tion of the different reactors. To evaluate the system, cell dischargesat3mAwereexecutedandthedataarepresentedin

Fig.5a.TheESRofSC-MFC-SERIESwas_300

V

,equivalenttoan averageof75

V

foreachSC-MFC.ThisaverageESRwassimilarto theonesmeasuredforeachsinglereactor.Powercurveswerethen measuredforSC-MFC-SERIES(Fig.5b).SC-MFC-SERIEShadaPmax of8.1mW(6.2Wm
2,14.4Wm
3,at5mA),Ppulse(tpulse10ms)of 7.9mW (6Wm
2,14.1Wm
3, at 4mA) and Ppulse (tpulse 2s) of 5.5mW(4.3Wm
2,9.6Wm
3,at4mA).Hereforthefirsttime,itis shown the successful improvements in power/current/voltage outputofSC-MFCconnectedinseries.

3.4.HERonPt,Fe-AAPyrandFe-MBZelectrodesinneutralmedia Hydrogen evolutionreactionatFe-MBZ,Fe-AAPyr, Ptand CP electrodes was investigated by linear sweep voltammetry in neutral pH (K-PB solution, pH=7.5, T=25C) and N2-saturated electrolytebetween0and
2V(vsAg/AgCl)and1mVs
1usinga conventional three-electrode as showed in Fig. 6. Separate experimentswereruntodetermineofHERparametersofinterest suchasonsetpotential,overpotential(inrespecttoplatinum),and potentialati=20mAcm
2forthedifferentHERcatalysts(Fig.6). Aswediscussedearlier,theneutralconditionsarenotconsidered favorableforpairedreactions(ORR/OERandHOR/HER)duetothe factthat protonsand hydroxylsarethemain reagentsin those processes.ItisclearthattheconcentrationofbothH+_andOH
is thelowestatpHvaluesapproaching7.

Fig.6showsthatPthasthelowestoverpotentialrelatedtothe materialstested.OnsetpotentialforPtwas
0.75V(vsAg/AgCl) andati=20mAcm
2thepotentialwas
1.35V(vsAg/AgCl). PGM-freeelectrocatalysts,asexpected,haveahigheronsetintherange of
1/-1.1V(vsAg/AgCl)withapotentialof1.52V(vsAg/AgCl)at i=20mAcm
2.AccordingtotheFaradaylaw,theH2producedat i=20mAcm
2correspondsto18mMday
1cm
2or5.5Lday
1.In general, the overpotential of PGM-free catalysts was 250mV higher than platinum, indicating that Fe-based materials are promising catalysts for hydrogen production in MECs. Over-potentials are much lower compared to Ni and SS previously presentedassubstituteofPtinneutralmedia[65,66].

3.5.SC-MFC-SERIESvoltageandAdHERpotential

AfterthestudyofHERelectrodesseparatelyinneutralmedia electrolytes that was carried out to investigate their activity (section3.4), theelectrodes wereused as AdHER electrode and connectedasinFig.1c.TheAdHERelectrodewasaddedintothe anodicchamberofMFC4andshortcircuitedwithA1(Fig.1c).The AdHERpotentialwasdriventolowvalues,reachingastablevalue afterapproximately60minutes.ThestableAdHERpotentialoutput forPt,Fe-AAPyrandFe-MBZisshowninFig.7aover30minutes operation.Ptstabilizedat
0.88V(vsAg/AgCl)whileFe-AAPyrand Fe-MBZ stabilized at lower values of
_1.17V (vs Ag/AgCl) and
1.15V (vs Ag/AgCl) respectively (Fig. 7a). SC-MFC-SERIES decreased their voltage to lower values recorded in 1.27V for SC-MFC-SERIESwithFe-AAPyrasAdHER,1.23VforSC-MFC-SERIES withFe-MBZasAdHERand1.12VforSC-MFC-SERIES withPtas AdHER(Fig.7b). 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4 4.5 5 5.5 6 6.5 7

C

e

ll V

o

lta

g

e

/

V

Time / s

a

SC-MFC-SERIES 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 10 0 2 4 6 8 10 12 14

Power Densi

ty / W m

-2

Power / mW

Curre

nt / mA

b

Aaaaa Baaaa Caaa Pmax Ppulse 2s Ppulse 10ms

Fig.5. Cellvoltage(a)profilesofSC-MFC-SERIESunder5srestand2spulseat3mA(a).PmaxvsIandPpulsevsipulseplots(tpulseof10msand2s)forSC-MFC-SERIES(b).

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 -2 -1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 0 Current De n s ity / m A cm -2 Potential vs. (Ag/AgCl) / V Fe-MBZ Fe-AAPyr Pt CP

3.6.HydrogenProductionfromAdHER

Chronoamperometry was used to simulate and investigate AdHERprocessesatthepotentialsdrivenbySC-MFC-SERIESand, specifically, at those reported in from Fig.7a. For Fe-MBZ, Fe-AAPyr, and Pt the potentials
1.15V,
1.17V and
0.88V vs Ag/AgCl were applied, respectively. The average current values recordedduringchronoamperometryafterstabilization(roughly 1hour) were very similar and equal to
0.47mAcm
2 for Pt,
0.46mAcm
2 for Fe-AAPyr and
0.45mAcm
2 for Fe-MBZ (Fig. 8). This current generated was used to estimate the H2 production(supposing 100%yield)byEq.(9) whichresultedof 0.86,0.83,and0.80mMday
1cm
2withPt,Fe-AAPyr,andFe-MBZ catalysts,respectively.IntermsofvolumetricH2production,the values above correspond to 0.132L day
1 (Pt), 0.127L day
1 (Fe-AAPyr), 0.123L day
1.To confirm theproduction ofH2, the systemshowninFig.1cwithAdHERfeaturingFe-AAPyrwasrunfor 15hours and then the headspace of MFC4 was sampled and analyzedusingaGC.Themeasurementconfirmedthathydrogen occupied44%ofthegasheadspace.

TheutilizationoftheFaradaylawmaygiveanoverestimationof thehydrogenproduced.LikelypartoftheH2producedislostdueto bacteriauptakeandutilization.Infact,differentlythantheHER processeshappeninginabioticconditions,thechambercontains different kind of bacteria which utilize hydrogen directly or throughintermediateprocessinwhichthelatterisinvolvedand consumed. Consequently, we can postulate that not all the H2 producedduringtheHERissuccessfullyrecoveredandfuturework willbedevotedtoimprovecellconfigurationatominimizeH2loss duringtheHER.

3.7.SimultaneousPulsedPowerandHER

After 30min stabilizationof theAdHER potential (Fig.7),10 dischargesoftheSC-MFC-SERIESwerecarriedoutwithtpulseof2s andresttimeof100s.The10cycleswererepeatedfordischarge currents(ipulse)of1mA(Fig.9).ThepotentialoftheAdHERwas recordedcontinuouslyvsanAg/AgClreferenceelectrodeplacedin the same chamber n.4 (Fig.1c). Simultaneously, the SC-MFC-SERIES voltage under the GLV discharges/self-recharges was monitored. The SC-MFC-SERIES voltage corresponds to the differencebetweentheC4positiveelectrodeandtheA1negative electrode(Fig.1c).

During the GLV discharges, the HER electrodes slightly increased their potentials. Indeed, the AdHER follow the A1 electrodes to which are connected and that increase their potential, following the electrostatic depolarization, during the GLVdischargepulses.Inanycase,theAdHERpotentialsalwayskeep wellwithintherangewhereH2isproduced(Fig.9).Dischargesof 1mAwereshowedwhenFe-AAPyr,Fe-MBZandPtwereusedas theAdHERcatalysts(Fig.9).

Inthisconfiguration,forthefirsttimeweshowthepossibility ofproducingH2throughasmartconnectionofMFCsutilizingan additionalHER.Moreover,forthefirsttime,Fe-AAPyrandFe-MBZ wereusedascatalystsforHERinneutralmediaandhadsimilar HER productionratestoPt,themostcommonlyusedcatalysts. However,Fe-AAPyrandFe-MBZarebetteroptionsthanPtdueto theirlowercosts,highertolerancetopollution,anddurabilityin harsherormorerealisticconditions.

4.Outlook

Theutilizationofhighsurfaceareaandconductiveelectrode materials allowed touse theMFC electrodesas supercapacitor electrodesunderpulsedoperationalmode.Pulsedpower generat-edwassubstantiallyhigherthancontinuouspowergenerationin agreement with previously presented literature [56,58]. It was previouslyshownthattheintermittentoperationalmodeisquite moreeffectivethatcontinuousmodeoperation[59,60].Ifthe SC-MFCs are not hydraulically connected, a series connection is possibleinordertoenhancetheoperatingvoltageofthesystem

[49–51]. High voltage is necessary for practical applications

[49–51] andin thisspecificexperimental study,voltageof over 3Vwasachieved.Inthiscurrentwork,weshowedthepossibility of integrating an in-series connection of MFCs working in supercapacitivemode(SC-MFC-SERIES)whichareabletogenerate H2throughanadditionalelectrodedevotedtotheHER.Integration oftheHERelectrodealongwiththesupercapacitorsallowedusto designaco-generativesystemwheretheelectricitygeneratedcan befurtherutilizedforthesimultaneousproductionofH2withno

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0 5 10 15 20 25 30 P o tential vs. (Ag/AgCl) / V Time / min

a

Fe-MBZ Fe-AAPyr Pt 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 5 10 15 20 25 30 Cell V o lt age / V Time / min

b

Fe-MBZ Fe-AAPyr Pt

Fig.7.PotentialofthedifferentHERelectrodes(AdHER)inMFC-4after5000secondstabilization(a).SC-MFC-SERIESvoltageduringdischargeswhilesimultaneously

producinghydrogen(b). -0.50 -0.49 -0.48 -0.47 -0.46 -0.45 -0.44 -0.43 -0.42 -0.41 -0.40 30 25 20 15 10 5 0 Fe-MBZ Fe-AAPyr Pt Time / min Cu rrent Dens ity / m A cm –2

Fig.8.ChronoamperometryplotsforAdHERelectrodeswithFe-MBZ,Fe-AAPyr,and

additionalexternalsourcesoperated.Thissynergisticintegrationis verypromisingsincethehighercombinedanodicactivitycreates currentpulseslargeenoughtobedeliveredtoexternalsources.

Meanwhile, H2 produced at the additional electrode can be harnessed for various applications. The hydrogen evolution reactiontookplace withinMFC4 withnoutilizationofexternal powersources aside fromthe MFCs themselves,which have a positiveelectricityproduction.Thiswayofthinkingaboutenergy and waste is quite revolutionaryand innovative asit creates a systemthat is self-sustainablewithpositiveenergy outputand marketablecompounds(H2).Tothebestof ourknowledge,the existing literature does not show simultaneous production of current/powerhighqualitypulsesandH2.Ptisoftenutilizedasa

catalyst for the HER, hindering the possibility of large-scale applications due to elevated material costs and the easy inactivation when working in polluted conditions. Here, we introducednovelPGM-freecatalystsforHERthatworkeffectively in neutral media and in wastewater conditions with many pollutants.It isthefirsttime thatFe-basedcatalysts havebeen used for HER in neutral media, and since these catalysts use commonly available and cheap metals, they are much more suitable for large-scale applications requiring durability. These materialsshowedhigheroverpotentialscomparedtoPtbutthey arestillpromisingasasubstituteandmorelong-termevaluations ofthesematerialsunderdifferentoperationalconditionsshouldbe executedtofurthervalidatethesefindings.

5.Conclusions

SC-MFCswerestudiedseparatelyandthenwereconnectedin seriestoincreasecurrent/voltageforuseinpracticalapplications. MaximumpowerofthesingleSC-MFCwas1.80.1mWthatis equivalentto6.10.3Wm
2and14.00.7Wm
3.Thosepower output aresignificantlyhigher than any presented MFC power output.WhenfourSC-MFCswereoperatedinseries,thevoltage increasedtoover3Vforthesystemandthemaximumpowerwas 8.1mW(6.2Wm
2,14.4Wm
3).Thissystemwithanadditional electrode (AdHER) in one of the SC-MFCs was utilized for the simultaneousproductionofhydrogenandpower.Currentpulses and H2 production were shown to take place simultaneously. HydrogenevolutionreactioncatalystsbasedonFe(insteadofPt) withrelativelylowoverpotentialsweresuccessfullyimplemented forthefirsttime.

Acknowledgement

This work was supported by the Bill & Melinda Gates Foundation: “Efficient Microbial Bio-electrochemical Systems” (OPP1139954).FSandCAwouldliketoacknowledgeAlmaMater Studiorum– Università di Bologna (RFO, Ricerca Fondamentale Orientata).

AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.electacta.2016. 10.154.

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