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Manganese oxide catalysts for secondary zinc air batteries: from electrocatalytic activity to bifunctional air electrode performance

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Manganese

oxide

catalysts

for

secondary

zinc

air

batteries:

from

electrocatalytic

activity

to

bifunctional

air

electrode

performance

Aroa

R.

Mainar

a,b,

*

,

Luis

C.

Colmenares

a

,

Olatz

Leonet

a

,

Francisco

Alcaide

a

,

Juan

J.

Iruin

b

,

Stephan

Weinberger

c

,

Viktor

Hacker

c

,

Elena

Iruin

a

,

Idoia

Urdanpilleta

a

,

J.

Alberto

Blazquez

a,

*

a

IK4-CIDETEC,PMiramón,196,20014Donostia-SanSebastián,Guipúzcoa,Spain b

DepartamentodeCienciayTecnologíadePolímeros,FacultaddeQuímica,(UPV/EHU),PManueldeLardizábal3,20018Donostia-SanSebastián, Guipúzcoa,Spain

cInstituteofChemicalEngineeringandEnvironmentalTechnology,GrazUniversityofTechnology,Inffeldgasse25C,8010Graz,Austria

ARTICLE INFO Articlehistory: Received9June2016

Receivedinrevisedform7August2016 Accepted9September2016

Availableonline13September2016 Keywords:

Bifunctionalairelectrode Secondaryzinc-airbattery

Manganeseoxidebasedcatalysts(MnxOy)

Oxygenevolutionreaction(OER) Oxygenreductionreaction(ORR)

ABSTRACT

Anefficient,durableandlowcostaircathodewithlowpolarizationbetweentheoxygenreduction reaction(ORR)andoxygenevolutionreaction(OER)isessentialforahighperformanceanddurable secondary zinc-air battery. Different valence states and morphologies of MnxOy catalysts were

synthetized via thermal treatmentof EMD (generating Mn2O3 and Mn3O4) and acid digestion of

synthetizedMn2O3(producinga-MnO2)inordertodevelopanefficientBifunctionalAirElectrode(BAE).

Change in theratio H+ toMn

2O3during the acid digestionaffectsthe sample microporosity,the

crystallographicplanedistribution,aswellasthephysicalandchemicaladsorbedwaterwhichwas relatedtodefects,i.e.cationvacancies(Mn4+)andMn3+.Thesecharacteristicswerediscussedandlinked

totheelectrocatalyticactivity.ThebestORRperformingcatalystwasthatwiththehighersurfacewater content(associatedtomaterialBETsurfacearea)anda(310)surfaceasthe2ndmorecontributingplane (after211).Ontheotherhand,thecatalystwiththehigherstructuralwaterandwith(110)and(200) crystallographicplanes beingthemostintensitycontributors(after 211)was themost OERactive material.Inthiswork,itwasabletofindarelationshipbetweencatalyststructureandair-efficiency throughavolcano-likerelationshipbetweenair-efficiencyandsurfacewatercontent.Air-efficiency(also takeasround-efficiencydischarge/chargeinbatterycontext)canbe takenasagooddescriptor of potentiallygood materialsforZn-Airsecondarybatteriestechnology. Inthisterm,wewereableto prepareaBifunctionalAirElectrodebasedontheselecteda-MnO2samplewhichdemonstrateda

round-efficiencyof53%,aDVaround1Vandaneglectedlossofthechargepotential(about2.1V)overtheentire lifecycletest(more200cyclesover30hours)withacapacityretentionsuperiorto95%.

ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Theworldwideconsumptionoffossilfuelsresultsgloballyina dramatic increase of greenhouse gases in the atmosphere and locally in pollution of the air. When in future intermittent renewablesources likewind or solarradiation substitutefossil fuels,anefficient,safeandinexpensiveenergystoragesystemswill berequiredtostoreenergyduringproductionpeaksandtosupply energyaccordingtothedemandsoftheconsumer[1].

Incomparisontocommonrechargeablebatteriessuchas lead-acid and Li-ion batteries, the metal-air batteries, especially secondaryzinc-airbatteriespresentlowcost,lowpollution,light weight,relativelyhighspecificcapacity/energydensity,andthey are generally considered as safe technology. However, the secondary zinc-air batteries, which normally are based on an aqueousalkalineelectrolyte[2],arestillunderdevelopmentdueto theshortcyclelifeoftheirelectrodes[3].

Regardingthebifunctionalairelectrode,thelargeoverpotential (

D

V)betweentheoxygenevolutionreaction(OER)andtheoxygen reduction reaction (ORR) reduces the cycle life limiting the performance of the secondary zinc-air batteries. Hence, the developmentofefficientandstablebifunctionalcatalyststowards theOERandORRiscriticaltodevelopthetechnology[4].

*Correspondingauthor.Tel.:+34943309022;fax:+34943309136. E-mailaddress:ablazquez@cidetec.es(J.A.Blazquez).

http://dx.doi.org/10.1016/j.electacta.2016.09.052

0013-4686/ã2016TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

ContentslistsavailableatScienceDirect

Electrochimica

Acta

(2)

The precious metals as platinum have been the most used catalystsforairelectrodesbuttheirhighcostandscarcitylimit their widespread applications in clean energy technology. The search for low cost catalysts with sufficient catalytic activity promotedthescientificinterestintransitionmetaloxidesasan alternative. Among them, manganese oxides (MnxOy) exhibit promisingelectrocatalyticperformancesforOERandORRunder alkaline conditions and possess many advantages such as abundanceinnaturalores,lowtoxicity,lowcostand environmen-talfriendliness[5,6].

TheperformanceofMnxOycatalystsstronglydependsontheir morphologies,Mnvalencestate,preparationmethods,crystalline phasesandstructures[7–12].MnxOyhasover30differentcrystal structures and variable valences of Mn center in different polymorphs[13].Thedifferentpolymorphicformswithdifferent propertiesincludeone-dimensional(1D)tunneledtypes(e.g.

a

-,

b

-,and

g

-MnO2),2Dlayeredcompound(

d

-MnO2),and3Dspinel structure(

l

-MnO2).

The1Dstructurediffersinthearrangedmannerandsizeofthe tunnels.Amongthem,

a

-MnO2possessesboth(22)and(11) tunnelssurroundedbydoublebindingoctahedralchains.

b

-MnO2 consistsofmerely(11)tunnelsseparatedbysinglechains,and

g

-MnO2displays(11)and(12)tunnelsenvelopedindouble chains[7].Thealphaallotropicformisconsideredtobethemost active.Thesuperiorperformanceof

a

-MnO2isattributedtothe existenceofdifferentsurfaceconcentrationsofMn3+ions,whose aresupposedlyhigherandmoreaccessibleinthisspeciesthanon theotherallotropicformsofmanganesedioxide[6].Ithasbeen reported[7,14]thatthepolymorphicformspresentORRactivities following the sequence:

b

-MnO2<

l

-MnO2<

g

-MnO2<

a

-MnO2

d

-MnO2.

Althoughmanganeseoxideshavebeenstudiedextensively,key factorsthatinfluencethespecificORRandOERactivitiesarenot welldefinedbecauseitisnotstraightforwardtocompareacross differentstudies[15].Severalmethodshavebeendevelopedfor thesynthesisofmanganesedioxides[7,9,15–25].Eachtechnique resultsinconsiderablydifferentcrystallite/particlesize[26],shape

[7]and porosity[27].Notonlythesepropertiesaffectthemass activityoftheoxides,butalsotheycorrelatewiththeelectronic structureatthesurfaceandtherebygiverisetodifferentspecific activities[15].

In this work, Mn2O3, Mn3O4 and

a

-MnO2 catalysts with different morphologies have been obtained by relatively easy synthesismethods.Theprincipalprecursorusedforthesynthesis of the catalysts has been the electrolytic manganese dioxide (EMD).HeatingthismaterialreducesMntolowervalencestates, aneffectthatgenerallyinducesstructuralchanges.Temperature between625Cand725CpromotetheformationofMn2O3and between 950C and 1050C Mn2O3 is further transformed to Mn3O4[28].Different

a

-MnO2catalystshavebeensynthetizedvia acidic digestion from the as-prepared Mn2O3. The principal advantageof theacidic digestionsynthesis method is thehigh purityofthefinalmaterial.Typically,allothermethodsleadtoa combinationofdifferent(e.g.structural,crystallographic)products reducingthepurityofthetargetingcatalyst[29].

Thesynthesisofdifferent

a

-MnO2inthisworkwasbasedon changing the ratio of H+ to Mn2O3 by incorporating different amountsofas-preparedMn2O3toaconstantacidconcentration. This is done with the intention of change the synthesis rate determinatestep(disproportionationofMn(III)intoMn(IV)and Mn(II)[29])whichcouldproduce

a

-MnO2withdifferent proper-tiesinducedmostlikelybyalterationinitsmorphology(variation ofcrystalgrowingrate).EMDandthegeneratedcatalysts(Mn2O3, Mn3O4 and

a

-MnO2)wereelectrochemically evaluatedand the resultsrelatedtotheirphysicochemicalcharacteristics.Themost promisingmaterialsintermofactivitytowardsbothORRandOER

werefurtherinvestigatedasBifunctionalAirElectrodes(BAE)in half-cellconfigurationassessingthebifunctionalactivityandthe air-efficiency at a given current density. Finally, the most air-efficientmaterialwasincorporatedascathodeinalab-scale zinc-airrechargeablebatteryandthelifecycleassessed.

2.Experimentalsection

2.1.SynthesisoftheMnxOycatalysts

2.1.1.GenerationofMn2O3andMn3O4fromEMD

Bymeansofathermalmethod[30],Mn2O3andMn3O4were synthetized from a commercial-grade electrolytic manganese dioxide (EMD). The Mn2O3 has been obtained by heating the EMDat700C(heatingrateof5Cmin1)for24hinatmospheric air. On the other hand, when EMD was treated at 1000C (temperature ramp of 20Cmin1) for 2h in atmospheric air, Mn3O4wascollected.Bothresultantsamples(Mn2O3andMn3O4) werelefttocooldowntoroomtemperatureinthefurnace,and thencrushedandstoredinadesiccator.WiththeresultingMn2O3 ithasproceededtothesynthesisof

a

-MnO2viaaciddigestion. 2.1.2.Preparationof

a

-MnO2byaciddigestion

All of the reagents used in this work are without further purification.Theas-preparedMn2O3samplewasusedasprecursor forthesynthesisofdifferentsamplesof

a

-MnO2viaaciddigestion. The acid digestion of Mn2O3 (overall reaction: Mn2O3+2H+$ MnO2#+Mn2++H2O) hasbeenreportedtofollowa dissolution-precipitationmechanism[29,31]wheretherate-determining-step (rds)isthedisproportionationofMn(III)intoMn(IV)andMn(II)in electrolyte [29]. Through the solubility of the Mn(III) as intermediate and its disproportionation, theacidconcentration and reactiontemperatureofthedigestionplaya crucialrole in definingthephaseoftheMnO2(i.e.

g

,

a

or

b

and/oracombination ofthem)asitcanbeobservedinthephasediagramshowninFig.1 [32].

The kinetic transformation of Mn2O3 into MnO2 has been regarded as an autocatalytic first order reaction (

y

=k(XA)(XB); wherekistherateconstantandXAandXBarethemolefractionsof Mn2O3 and MnO2, respectively) [31]. Hence, the additions of differentquantitiesofMn2O3maychangetherdsfortheoverall process, and, in consequence, the

a

-MnO2 properties obtained couldbedifferent.Inthissense,followingthelaststatement,the

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synthesisof

a

-MnO2inthisworkwasbasedonchangingtheH+: Mn2O3 ratioby incorporatingdifferent amounts ofas-prepared Mn2O3butkeepingtheacidconcentrationconstant.Thus,3,10,13, 18and30gofas-preparedMn2O3werecorrespondinglyaddedto 1L of6MH2SO4 (Scharlau,98% purity)solutionprepared with deionizeddistilledwater(DDW),andkeptundermagneticstirring for16hat130C(seethezoomareamarkedinFig.1).Afterthis time,theblackprecipitateswerefilteredandwashedseveraltimes withethanolandDDW.Theresultingsolidsweredriedovernight undervacuumandthendried110Cfor2hour.Thesampleswere coolingtoroomtemperatureinadesiccatorandpreservedthere untiluse.BasedontheamountofMn2O3precursorusedintheacid digestion,theresulting

a

-MnO2samplesarefromnowonlabelled as

a

-MnO2-3,

a

-MnO2-10,

a

-MnO2-13,

a

-MnO2-18and

a

-MnO2 -30.

2.2.SamplesCharacterization

2.2.1.Structuralandmorphologicalcharacterization

ThephasecompositionoftheMnxOysampleswasdetermined byX-raydiffraction(BrukerXS,D8AdvanceequippedwithCu-K

a

radiation).Eachdiffractionpatternwasrecordedinthe2

u

range from10to80withastepsizeof0.022

u

andacounttimeof2s per step. The crystallitesize of the samples was calculated by Scherrer equation [33] taking the full-width-at-half-maximum (FWHM)ofthemaincrystallinepeak.

Themorphologyofeach samplewasexaminedusinga Zeiss GeminiUltraplusfieldemissionscanningelectronmicroscopy (FE-SEM)atvariousmagnifications.

Surfacewaterandstructuralwatercontentsweredetermined followingtheproceduredescribedinref.29.Inbrief,thesurface waterorphysicallyboundwaterwas determinedbytheweight lossof500mgofcatalystheatedinanovenat110Cfor2hinair. Furtherweighlossofthesamplebyheatingupto400Cfor2his attributedtothelossof chemicallybonded water orstructural water[34andrefs.citedtherein].

Specificsurface areawas determinedbyapplying Brunauer-Emmett-Tellet (BET) approach. Data was collected form N2 physisorption using ASAP 2020 analyzer (Micromeritics, USA) recordingtheadsorptionisothermsuptoarelativepressure(P/P0) of0.3.Priortoadsorptionthesampleswereoutgassedat150C (temperaturerampof10C/min.)for5minandcoolingdownfor 7hoursundervacuum.

2.2.2.ElectrodepreparationandElectrochemicalmeasurements The electrochemical characterization was performed by the well-knownthin-filmelectrodeusingrotatingdiscelectrode(RDE) andbyhalf-cellGasDiffusionElectrodes(GDE).

The thin-film working electrode (WE) was prepared by depositingacatalystsuspensiononaglassycarbon(GC)substrate. Thecatalyst suspension was preparedmixing 9.8mg of MnxOy catalystand9.8mgofcarbonpowder(VulcanXC-72,CabotCorp.to ensureelectronicconductivity)in5mlofasolution isopropanol-water(7:3, v:v).The suspensionwas ultrasonicallyblended for 10min.andthen10

m

lofthissuspensionwascarefullypipettedon theGC(0.196cm2)andlefttodrybyrotationatroomtemperature. Thefinalthin-filmworkingelectrodeyields0.1mg/cm2ofcatalyst (MnxOy)loading.

Electrochemicalmeasurementswereperformedin0.1MKOH electrolyteinastandardthree-electrodecellatroomtemperature. TheelectrolytewaspreparefromKOH(Sigma-Aldrich,85%purity) dissolvedinDDW.Thethin-filmWEwasaRDEdescribedabove. Platinumwire and a commercialreversible hydrogen electrode (GaskatelGmbH.)wereusedascounterandreferenceelectrode, respectively.

FortheORRandOER,theRDEvoltammogramswereobtainedin syntheticair(AirLiquideALPHAGAZ1AIR;20%O2,80%N2;purity: 5.0) saturated electrolyte at 5 mVs1 with a rotation rate of 1600rpm.TheWE’swerecycledupto5timesbetweenOCVand 0.55VfortheORRandbetweenOCVand1.6VfortheOER.Thelast cathodicsweepwasusedtoevaluatetheORRactivityvia mass-transportcorrection.FortheOERthelastanodicsweepwasused. Thespecificactivity(

m

Acm2oxide)isapracticalapproximationof theactivityperactivesite,whichisoftenunknownandreflectsthe intrinsicactivity.Thisparameterisobtainedbydividingthemass activity by the corresponding BET surface area leading to a characteristicpropertyofthecatalystsamples[15,35].Thus,for both ORR and OER,Tafel plots were generated by plotting the logarithmsoftheBETsurfaceareaandmassspecificnormalized currents. Best performing catalysts were selected for further evaluationasGDE.

TheGDEconsistedofoneactivelayerofairelectrodeprepared bymixingMnxOycatalyst(40wt.%),carbonnano-fiber(50wt.%, madeatTUGraz)andPTFE(10wt.%,DyneonTF5032PTFE)using isopropanolassolvent.TheachievedMnxOycatalystloadingwas 8mgcm2.TheslurrywasthenrolledwithSigracetCarbonPaper aswaterproof diffusionlayer, andsubsequently hot-pressedfor 20minat20kNand290Ctoobtaintheas-preparedbifunctional airelectrode(BAE).TheBAEworkingareais1cm2.

Electrochemical test was carried outin 6M ofKOH (Sigma-Aldrich, 85% purity). Due to the high ionic conductivity, the commonly used KOH concentration in primary and secondary zinc-airbatteriesrangebetween6and8M[2].RHEfromGaskatel wasusedasreferenceelectrodeandaplatinumwireascounter electrodeinathree-electrodeconfigurationglasscell.Electrolyte wassaturatedwithoxygenbyflowingsyntheticairat50mlmin1 at room temperature. Pseudo steady-state-polarization-curves weregalvanostaticallyobtainedonaBaSyTecTestCellSystem.The polarizationcurve(PC)wasdone10timesinchargeand10timesin dischargemodes.Forbothmodes,BAEwasinitiallykeptatOCVfor 15minand then at 0.5mA for another15min. Afterwards, the currentwasrampedasfollow:0.5;1;10;40and200mA (withacut-offvoltageof0.8Vor1.8Vdependingonthemode)for 8; 15; 5; 3 and 1min, respectively, and then back to OCV to continue withtheanothermodeand sountillcomplete 10full charge/discharge cycles. This test was used to evaluate the bifunctional activity retention and air-efficiency of the BAE’s. Stability was assessedcomparingthe third(3rd) and thetenth (10th) polarization curve.The first two PC’s wereused as BAE conditioning.

Thebifunctionalactivityandtheair-efficiencyoftheBAEwere obtained at different current densities from the polarization curves.Bifunctionalactivityisdefinedasthevoltagegap(EOER EORR) or overvoltage (

D

V) between charge(OER) and discharge (ORR)mode,whileair-efficiencyistakenastheratioofORRtoOER ([EORR/EOER]*100). It is also recognized as round-efficiency discharge/charge[36].

2.2.3.Batteryperformancemeasurements

Finally,themostefficientBAEwasselectedandtestedunder batteryoperatingconditions.Thus,aGDEelectrodeoftheselected materialwaspreparedasexplainedaboveusingCNFasconductive media. The battery consisted of a Zn-foil (50

m

m) anode, a separatorembeddedwiththesupportingelectrolyte(8MKOH) and theGDE.Syntheticdried airwas flowedat thecathodeby meansof40mlmin1.Thebatterycomponentswereassembledin acommercialavailableECC-Aircell(activeareaof2.5cm2)from EL-CELL GmbH. Charge/discharge cycles (5min/5min) were performed at 10mAhcm2 and the capacity retention was assessedoverthelifecycletest.

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3.Resultsanddiscussion

3.1.Physicalandmorphologicalcharacterizations

The XRD pattern of the starting material (EMD) and as synthetizedMn2O3 andMn3O4arepresentedonFig.2.AllXRD patternsare consistent withthe correspondingassigned JCPDS card number, where no additional peaks could be observed, indicatingthesingle-phasecharacterofeachsample.Theheating procedure reduces Mn to lower valence states, an effect that generallygivesrisetostructuralchanges.Hence,theXRDpatterns in Fig.2 representthe phase changefrom EMDtoMn2O3 and subsequentlytoMn3O4 bythermaltreatment.Thefirst(Mn2O3) correspondstothereductionreaction2MnO2!Mn2O3+0.5O2(g) at700Candthesecondone(Mn3O4)correspondstothereduction of 3Mn2O3 ! 2Mn3O4+0.5O2(g) at 1000C. While EMD XRD patternshows broadpeaks,Mn2O3and Mn3O4samplesdisplay narrow and very sharp peaks indicating high crystallinity and largercrystallite sizes (3rd column in Table 1). The average of crystallitesizewasderivedfromScherrerequation[37andrefs. citedtherein]usingthefull-width-at-half-maximum(FWHM)of thediffractionpeak(211).Mn3O4showsalmostadoublecrystal

size in comparison tothat for Mn2O3 as expected for sintered particlesinducedbythethermaltreatmentattemperatureashigh as1000C.

The X-ray diffraction pattern of various

a

-MnO2 samples derivedfromtheaciddigestionofMn2O3areshowninFig.3.All XRDpatternareverysimilar.Itcanbeseenthatallthereflections including thefive characteristic peaks at 17.86, 28.84, 37.52, 49.86,and60.27(allofthemcontributingmorethan50%ofthe intensitycompareto100%assignedto37.52)canbeindexedtothe bodycentredtetragonal

a

-MnO2(JCPDSNo.44-0141)indicating thephasepureofthesynthetized

a

-MnO2.Anyreflectionsfrom precursorMn2O3canbeapparentlyobserved(seeFig.S1ofESI). However,amagnificationoftheXRDpatternsbetween2

u

54and 58(Fig.S2ofESI)showsforsamples13and20aslightshiftof peak(431)(for

a

-MnO2)tothereflectionpeak(440)assignedto Mn2O3 (JCPDS No. 01-071-0636). Nevertheless, all

a

-MnO2 samplesare consideredasphase pure. Thecrystallitesizes(3rd columninTable1)obtainedfromthemajordiffractionpeak(211) rankfrom19.5nm(

a

-MnO2-20)to43.5nm(

a

-MnO2-18)andnot trendswiththeinitialamountofMn2O3wasobserved.

MorphologyofthesampleswasexaminedwithFE-SEM.Figs.4

(a-c)and5 (a-g) showthemicrographsofthedifferentMnxOy obtainedinthiswork.AmorphousstartingmaterialEMD(Fig.4a) presents flaky and porous morphology, while Mn2O3 powder obtained by heat treatment of EMD (Fig. 4b) has a reef-like morphology.Itismainlycomposedofmicro-sizedparticlesinthe range 4.5–70

m

m, constituteby quasi-spherical nano-sized par-ticles.Themicrographreflexesthefactofsinteredparticles(inthe rangeof81.81nmand650nm)byheating.Thismorphologyhas been also observed by Y. Park et al. [38] in the synthesis of Na0.55Mn2O41.5H2O and Na4Mn9O18 using Mn2O3 as precursor material. Denser non-porous smoothed boulder morphology is observedfortheMn3O4sample(Fig.4c).

Fig. 5(a-g) illustrates the micrographs for the as prepared

a

-MnO2.Thesamplessurfaceshapecanbecategorizedas nano-flake, needle, rodand fiber/wire like morphologies.Hence, the micro-sized

a

-MnO2surfaceisinfluencedbytheinitialamountof Mn2O3 used during the acid digestion. Synthetized

a

-MnO2-3 (Fig.5a)presentssmoothboulderparticlesintherangeof100nm to350nm.Thiscatalystdoesnotseemtoconsistofagglomerated nanoparticles,butnano-needlescanbeobservedcoveringsomeof largeparticles.Ontheotherhand,samples

a

-MnO2-10;-13;-15; 18; and

a

-MnO2-30 (Fig. 5-b; -c; -d; -e; and -g, respectively) consistofrandomlydispersednano-rod-likeshapewithdifferent degreeofagglomeration,lengths,widths,roughnessandporosity. Finally,

a

-MnO2-20(Fig.5f)presentsamorelikenanowire/fiber morphology.Inallcases,itisclearthatthematerialsobtainedare appreciablehomogeneousintheirrespectiveshapes.However,a clearrelationshipbetweeninitialamountofMn2O3and morphol-ogycannotbenoticed.

TheBETsurfacearea(SA)isaphysicalpropertywhichvariesas functionofthesynthesismethodoranysampletreatment[3,39– 41] whichcan affecte.g.theporosityor thesurfaceroughness. Hence,BETsurfaceareafortheMnxOysamplesisshowninTable1. As it can be seem, theBET SAof starting material (EMD with 34.15m2g1) decreases in almost one order of magnitude (3.33m2g1 for Mn2O3) or even closer to two orders (0.48m2g1inMn

3O4)byheattreatmentat700Cand1000C, respectively. As it was shown above, beside to induce phase changes, the thermal treatment also induces morphological changes(Fig.4a-c)includingthelossofSAbysintering.During thisprocessthe(micro)porositypracticallydisappears(seeFig.S3 inESI).Fortheassynthetizedseven

a

-MnO2samples,theBETSA ranks from13 to67m2g1. Thislarge range of BET SAcan be assigned to different micro and macro porosity amongst the sampleswhichisnotstrictlyrelatedtotheinitialamountofMn2O3

Fig.2.X-raydiffractionpatternsofprecursorEMDandthermalsynthetizedMn2O3

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addedtotheacidmediabuttotherateofdisproportionationofa solubleMn(III)intermediateandprecipitationof

a

-MnO2.Onthe otherhand,after

a

-MnO2formation,theexcessofacidmightalso leadtosomedefectsatthesurfacethroughetchingprocess.Thus, boththeBETsurfacearea(Table1)andtheassociated micropo-rosity(region0.3 p/po in theadsorption isotherm in Fig. S3) decreaseinthesequent

a

-MnO2–18>30>10>13>15>20>3.

In order to learn more about the structural defects in the synthetized samples, the surface and structural water content weredeterminedforallsamples[31].InTable1thecorresponding resultsofsurface(physicallybound),structural(chemicallybound) andtotalwaterareshownforeachMnxOysample.Surfacewater and,ingeneral,thewatercontent(surfaceplusstructuralwater) decreasesastheheattreatmenttemperatureincreases(EMDto Mn2O3toMn3O4)inlinewiththedecreaseobservedinBETSA.Itis explainedbythefactthattheamountofwaterabsorbedonthe surfaceorallocatedintheporesand/orvoidsaredeterminedby the available surface area, and hence, as expected, the water content is largely determined by the BET surface area of the material.Thisbehaviorisalso,ingeneral,observedforthe

a

-MnO2 samples. Among

a

-MnO2 samples, No. 18 shows the high physically bounded water, while sample 10 presents the high contentofstructuralwater.However,itisnoticeablethatthereis notdirectrelationshipbetweenwatercontentandstartingamount ofMn2O3intheaciddigestionprocesses.

Throughthecationvacancymodel[30,34],Ruetschiexplained therelationbetweenwatercontentandelectrochemicalactivity. Heassociatedthestructuralwaterwithbothcationvacancies(Mn4 +) and Mn(III), i.e. with defects, and hence materials with significant amountof chemically absorbed water exhibitsgood electrochemical performance. The presence of structural water

influences also other material properties such as density and electronicconductivity[34andrefs.citedtherein].Thus,onone handhigherdefectcontent(structuralwater)favorstheinterfacial chargestoragebyincreasingtheelectronicconductivity,andon theotherhand, largeporosityand higher contentof physically (surface)boundwatermayfacilitatetheintercalation(protonsor alkalinecations)process(MnO2+H+(M+)+e!MnOOH(M);M+: Na+, K+, Li+) [37], where water molecules reside in the (22) channelsofthestructure,whichisthepositionnormallyoccupied byintercalatedcations[42].In addition,takingintoaccountthe structuraldefects,Prélotetal.usingthePannetierstructuralmodel

[43 and refs. cited therein] were able to explain with it the

variationofthewaterdissociationconstantofthesurfacesitesat theinterfacesolid-electrolyte,and thuslocal topographyof the outermost oxide layers influence the properties of the first adsorbedwater layer. Selvakumar et al. [41]have also demon-strated through their comparative study between

a

-MnO2 catalystswithdifferentmorphologies(nanowire,nanotube, nano-particles, nanoflower) that desorption of physisorbed and/or chemisorbed water molecules are largely influenced by the shape/morphologyofthematerial.Thus,theauthorshaveshown thatstructuralwaterinteractsstrongerwithe.g.

a

-MnO2 nano-tubes than with

a

-MnO2 nanoflower. Thereby surface and structuralproperties (here representedbytheBET surfacearea and surfaceand structuralwater)would influencethecatalytic activityof materials towards the reduction and/orevolution of oxygen.

3.2.Oxygenreductionandevolutioncharacteristics

ThecatalyticactivityoftheaspreparedMnxOysamplestowards theORRand theOERwereassessedin air-saturated0.1MKOH electrolyte solution. The 5th ORR and OER linealsweep volta-mogramms (LSV) recorded at 5 mVs1 under rotation rate of 1600rpmareshownrespectivelyinFig.S4andFig.S5.FortheORR, the mass-transfer corrected kinetic currents derived from the Koutecky-Levichequationalongwiththecorrespondingmassand catalystBETsurfaceareawereusedtoobtainthekineticcurrent densities(Jk).RepresentingtheTafelplotsfromtheJkondifferent MnxOycatalysts,thesurface(Figs.6a)andmass(Fig.7a)specific activitiesatdifferentpotentialsduringthe5thLSVwereaccessed.

Table2summarizesacomparisonoftheORRsurface(2ndcolumn)

andmass(3rdcolumn)specificactivitiesat0.8VRHE.ForOER,Tafel plotsalsoillustratetheBETSA(Fig.6b)andmass(Fig.7b)specific activities.Column4thand5thinTable2showsthecorresponding catalystsOERactivityat1.55VRHE.

TheORRproceedsonMnO2bytwomainroutes:(i)thedirect four-electronreductionpathway(O2+2H2O+4e!4OH)and(ii) via the hydrogen peroxide pathway (O2+H2O+2e ! OOH+ OH) which can be furtherreduce to complete the overall 4-electron ORR in alkaline medium. However, complete

four-Table1

PhysicochemicalcharacterizationofMnxOysamples.

Samples BET

(m2

g1)

Cristalsize(nm) SurfaceWater (%) StructuralWater (%) Watercontent (%) EMD 34.15 N.A. 1.00 3.00 4.00 Mn2O3 3.33 32.5 0.23 0.09 0.32 M3O4 0.48 62.3 0.15 0.06 0.21 a-MnO2-03 13.00 25.3 0.93 3.25 4.18 a-MnO2-10 55.58 32.2 4.70 7.28 11.98 a-MnO2-13 46.58 25.6 9.20 2.86 12.06 a-MnO2-15 40.37 27.7 3.60 3.01 6.61 a-MnO2-18 67.36 43.5 19.80 2.74 22.54 a-MnO2-20 38.40 19.5 10.50 2.46 12.96 a-MnO2-30 60.12 24.9 15.00 2.00 17.00

Fig.3.X-raydiffractionpatternsofa-MnO2synthetizedviaaciddigestionof

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electron reduction pathway is the most common pathway occurringonmostcrystallographicstructures[6].Inbothcases, thefirst step involves the conversion of manganesedioxide to manganese oxyhydroxide (MnOOH), on which the dissociative adsorptionofmolecularoxygenproceeds. Therate-determining step(rds)istheelectrontransfertotheadsorbedoxygenmolecule ineither thecomplete4-electron reductionpathwayorvia the hydrogenperoxide2-electronreductionpathway.K.Selvakumar etal.havedemonstratedthaton

a

-MnO2nanowiresandnanorods (similarshapestothoseshowninthisstudy(Fig.5))areableto reduce oxygen via the direct 4-electron pathway on a single catalyticsiteduetoitsnano-geometryeffect[41].Theyfoundby DFTcalculationthatthisdirectORRpathwayismainlyassociated tosuchof nanostructurewhich possessesactivesiteswithtwo shortenedMn-ObondsalongwithanoptimumrequiredMn-Mn distance which favors the adsorption of O2 in a bridge-type interactionthatconductstodissociationofoxygenmoleculeand hence,thefourelectronreductionpathway.P.-C.Lietal.[3]have also confirmed that the ORR on nearly pure

a

-MnO2 catalysts proceeds via 4-electrons. In his case, it was explained by a sequential (serie) two-electrode pathwayor socalled quasi-4e reductionpathway(seerefs.citedinref.3)duetotheabilityof

MnO2todisproportionationofHO2intoOHandO2.Basedon thesefacts,andsincethesamplessynthetizedinthisworkpresent mainly a nanorod shape, it hasassumed that thereduction of oxygen on as-synthetized

a

-MnO2 samples proceeds via 4-electronpathwaywhereH2O2 generationcouldbeneglectedat thepotentialofinterest.

AsitcanbeobservedinthecomparativeTable2,

a

-MnO2-18for theORRand

a

-MnO2-10fortheOERarethemostactivecatalysts amongthesynthetizedmaterialsandevenbetterperformingthan thoseactivitiesreportedintheliteratureforotherkindof

a

-MnO2 catalysts[7,10,12,44].Incaseoftheinvestigated

a

-MnO2-18,the mass activity achieves about 15Ag1ox at 0.8VRHE which is outperformingthebestpublishedcatalystbasedon

a

-MnO2[44]. Mn2O3 and Mn3O4 catalysts present very low ORR activity compared tothe

a

-MnO2 catalysts (Fig. 7a). The (22)tunnel structures(

a

-MnO2)wereproventohavehigherintrinsicactivities overlayeredmaterials,consideringtheaccessibilityofwaterand thedefectsbythesetwosystems[12].ForMnO2-basedcatalysts,it hasbeendemonstratedthattheORRactivityfollowsanorderof

a

->

b

->

g

-MnO2, whichis attributedtoacombinative effectof theirintrinsictunnelsizeandelectricalconductivity.Ontheother hand, MnO2 catalysts withthesame phase, thenanostructures apparentlyoutperform themicro-sizedparticles duetosmaller size and higher specific surface areas. Otherpossible reason is associated to the nature of preferentially exposed plane of manganeseoxides[45].

Recently J. Liu et al. [46] has shown a significantly ORR enhancement on morphological tailored Mn3O4 catalysts. By synthetizingMn3O4withsmallnanoparticles(NP-S),large nano-particles(NP-L),nanoflake(NF)andnanorod(NR)morphologies theauthorswereabletoachieve 16.4Ag1ox at0.83VonNP-S Mn3O4. In the same kind of idea, but only apply to

a

-MnO2 crystallographicform,K.Selvakumaretal.[40]developed shape-engineered(nanoparticles(NP), nanowires(NW) andnanotubes (NT))

a

-MnO2asbifunctionalORR/OERcatalyst.Theyfound NW-shape as thebestperforming towards bothORR and OER.This result was explained by the associated surface energies and adsorptionofwateronthesurface,wheretheshapeofthematerial largelyinfluencedthephysically(surface)andchemically (struc-tural) bounded water. Both groups [40,46] pointed out the remarkable shape effect on the ORR activity for MnxOy based catalyst, as wellas, withthe help of density functional theory (DFT), the correlation between enhanced ORR activity and preferentiallyexposed planes (001 for NF Mn3O4 [46] and 310 forNW

a

-MnO2[40]).

K.Selvakumarandco-workers[40]havecalculatedbyDFTthat (310)and(200)surfacesin

a

-MnO2arestronglyinteractedwith watermolecule,whilethe(110)surfaceisweaklybondedwiththe adsorbateduetoitshighstructuralstability.Inthecaseof(211) plane, it is expected to have high catalytic activity. This crystallographicplaneisthedominatingsurfaceforall

a

-MnO2 catalysts as it is shown above in Fig. 3 and in TableS1 (ESI). However,for

a

-MnO2-18and

a

-MnO2-10theseconddominating planeisthe(200)withthedifferentthatinsample10the(110) surfaceintensitycontributealmostinthesameproportionthan (200)andevenmorethan(310),whileinsample18thisplane(110) intensitycontributesabout18%and12%lessthanthatfor(200) and (310), respectively. For the other

a

-MnO2 materials,(310) planeisthesecondmorepredominatingintensity,beingthatmore remarkable on sample 13 and 20 (almost 20% over the (200) surfaceandabout30%respecttoplane(110)forbothcatalysts).In

a

-MnO2-3thatdifferentbetween310and200isjust 7%,while (110)surfaceintensityisabout20%lessintensethan(310)inits XRDpattern.Interestinglytonoticeisthealmostsimilarintensities ofplanes(110),(200)and(310)in

a

-MnO2-15.Thissample,spiteto showarelativelargeBETsurfacearea(40.37m2g1)whichisvery

Fig.4.FE-SEMmicrographsillustratingthemorphologyof(a)EMD,(b)Mn2O3and

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closetoe.g.samples13(46.58m2g1)and20(38.40m2g1),this materialhasresultedinthelessactive

a

-MnO2catalyststowards theORR(withexceptionofsample10)andOER,whilst

a

-MnO2-13 and20haveshownasimilarandpromisingbifunctionalactivity. Basedonthesefacts,andinaddition,asTable1shows,

a

-MnO2-18 isthecatalyst withmostlargeamountof surfacewater (about 20%),aswellasthematerialwiththelargerBETsurfacearea.The directcorrelationwatersurfaceand ORRperformanceislargely linkedtothemassspecifickineticORRactivityinsteadBETsurface area.AsitcanbeseeminTables1and2,thesurfacewaterandthe mass specific kinetic ORR activity varies almost in the same sequentwhileBETsurfaceareanormalizedkineticcurrentmight be influenced by the corresponding crystallographic plane distribution.

On theotherhand,

a

-MnO2-10,themostOERactivecatalyst (Figs.6band7b)amongstthesynthetizedmaterials,presentsthe higher structuralwater content(Table 1)with(110) and (200) crystallographicplanesbeingthemostintensitycontributorsafter the common dominating (211) plane (Table S1). In order to compare the OER catalytic activity, other

a

-MnO2 catalysts reportedinliteraturearealsoincludedinTable2.Y.Mengetal.

[12]reportedthespecificandmassOERactivitiesat1.679VRHE.In thisworkthecatalystswerenotexposedtohigherpotentialsthan 1.6VRHE because the binder free catalysts layer may detach at highervoltagesduetogasevolution.Inthiscontext,thecurrent densitiesat1.60VRHEareprovided.

Asfallows,thecrystallographicplanedistribution(intermof intensities)observedinTableS1andrealizedabove,aswellasthe sample water content (surface and structural) could be both

associated to the observed trends for ORR and OER activities amongthesynthetized

a

-MnO2samples.Inthissense,thecatalyst activitycouldberelatedtothestructural/surfacedefectscreated during theaciddigestion of Mn2O3 [32] for the morphological (nanorod/tube/wire as observed in Fig. 5) similar synthetized

a

-MnO2samples.Thus,higherdefects(mainlycationvacancies related to structural water) can be produced when crystallite growing/precipitationrateisquicker.Thelatterisacceleratedby increasing acid concentration (H2SO4) which leads to larger crystallites and the already mentioned defects within MnO2 structure[32][32andrefs.citedtherein].

TheresultsshowedinTable2andFigs.6and7,correspondingto the5thcycle(stableLSV),indicatethat thereisnotoneunique good

a

-MnO2catalyst forboth oxygenreduction andevolution reactions.Itshouldbementionedhere,thatORRandOERactivity assessment was done on the stable LSV. The slight difference betweenthefirstandthefifthcyclesarereportedinFig.S6.Such kindofdifferencecanbeassociatedtoconditioningofthesamples by removing residual chemicals from the ink and/or partial electrochemical oxidation of the carbon used as conductive medium(50wt.%ofthecomposite)whichmaycontributetothe overallFaradaiccurrent.Itcannotberuleoutpartialdeactivationof thesamplesbyformationoflessORRactivesurfacespeciessuchas MnOOH[3]orMn3O4[47][47andrefs.citedtherein].

Itshouldpointoutthatthemainobjectiveofthisstudyisto investigatetheapplicabilityof

a

-MnO2ascathodecatalystin air-electrodes under simulated Zn-air secondary battery test con-ditionsratherthaninvestigateindepththeintrinsicpropertiesof the

a

-MnO2samples.Inthisterm,themainfocusinthefollowing

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sections is the search of an effective electrode with good bifunctionalactivitytowardsORRandOERandgoodair-efficiency. The bifunctionality is measured in term of voltage gap or overpotential (

D

V=EOER  EORR) between charge(OER) and discharge(ORR)mode,andin termofair-efficiencytakenasthe ratioofORRtoOER([EORR/EOER]*100)atagivencurrentdensityin both cases. In order to do that, Gas Diffusion Electrodes were manufactured with the selected (10, 13, 18, and 20)

a

-MnO2 catalystsandcomparedtothestartingsampleMn2O3aswellas againstMn3O4material.

3.3.ElectrochemicalassessmentoftheBifunctionalAirElectrode(BAE) BAEforautomobileapplication,forexample,mustsupportfast potential changes induced by the battery discharge during acceleration (high power demand) followed by charge phase duringregenerativebraking. In ideal conditionsall the compo-nentsoftheelectrodeshouldwithstandsuchconditions.Inthis sense,a bifunctionalcatalyticmonolayerconfigurationcouldbe the most efficient and cost effective design [36]. Taking into considerationefficiencyandcost,singlecatalyticlayerofselected

a

-MnO2 catalysts were manufactured into a BAE. As-prepared Mn2O3andMn3O4werealsotestedforcomparison.Sincethemetal oxidebasedcatalystspresentingenerallowbulkconductivity,the MnxOy-based catalystinks werepreparedusing 50wt.% carbon

nanofibers(CNF)asconductivemedia.TheuseofcarboninBAEhas beencontroversialduetoitselectrochemicaloxidationatoxygen evolution potentials. However, carbons with higher graphite character[48] and/ornanostructured (such asnanofiber, nano-tube,etc.)haveshownbetterresistancetowardsthecorrosion,and havebeenusedaspartofcathodeelectrodes.P.-C.Lietal.[3]have foundaratiocarbon(XC72)to

a

-MnO2of1astheoptimalinterm of ORR performance. Furthermore, addition of carbon in the catalyst composite provides better control of the electrolyte permeability within the hydrophilic catalyst layer due to the change in permeability [49,50] helping to achieve optimal electrolyte and oxygen permeability and hence, provide an extendedreactionzoneatthethree-phaseboundary[36].

Chronopotentiometric measurements at different current densitieswerecarriedoutontheas-fabricatedBAEtoassesstheir bifunctional activities (in term of overpotential,

D

V) and air-efficiencyoverashort-termtest(10galvanostaticmeasuresatboth ORR and OER,meaningof 20polarization curves(PC)in total).

Fig.8representsthethirdORR/OERpolarizationcurves(firsttwo cyclesweretakenasconditioningoftheelectrodes),whilstFig.9

summarizestheretentionofbifunctionalactivity(Fig.9a)and air-efficiency(Fig.9b)at20mAcm2(pointedbythesegmentedred lineinFig.8)overtheshort-termstabilitytest.

AsitcanbeobservedinFig.8thereisnotapparentdifferencein theORR/OERpolarizationcurvesamongtestedcatalysts.However, as Fig. 9a and Table S2 show,

a

-MnO2-13,18 and 20 have outstanding bifunctional activitieswith

D

V as loweras almost 0.7V for low current densities (atthe initial state,628mV for

a

-MnO2-13at 5mAcm2)or around0.8V

D

V for intermediate current densities. P.H. Benhangi and co-workers have recently reportedanORR-OERpotentialwindow(

D

V)of662mVforMnO2 catalystat2mAcm2being563mVthelowest

D

Vreportedfor activatedMnO2-LaCoO3[47].Theyhavealsocited650mV

D

Vfor Pt/IrO2 (a nobel metal based material)and 866mV for Mn2O3, whereas here reportedMn2O3 showed a improved performing with689mVat5mAcm2and856mVat20mAcm2.

Ontheotherhand,asitobservedinFig.9a,

a

-MnO2-13catalyst showsthebestbifunctionalretention(97.7%)amongthe

a

-MnO2 samples.Thisbifunctional activityretentionwas similartothat shows byMn2O3 butslightly lowerthat the98.8% obtainedby Mn3O4catalyst.ThishighretentionofMn2O3andMn3O4couldbe relatedtothestabilityoftheirceramic-likemorphology(lowBET surfaceareaandporosity)forbothcatalysts,inducedbytheEMD thermal treatment and hence, not significant morphological/ structuralchangesareexpectedforthesematerialswiththefast potentialvariationsduringcharge/dischargecycles.This assump-tioncouldbecorroboratedbypostmortemanalysis(notperformed inthisworkbutintendedforongoingactivities).

In order to go deeper into the functionality of the tested materials,Fig.9billustratesthecatalystsair-efficiency(ORR/OER ratioat20mAcm2)anditsvariationaftertheshort-termstability test (TableS3summarizestheairefficiencies valuesatthe3rd (initial state) and after 10 PC's for comparison and retention assessment).Similartowhathasoccurredwith

D

V,

a

-MnO2-13, 18and20haveshownthebestair-efficiency.Attheinitialstate (3rd cycle) theypresent (Table S3) a respectively efficiency of 59.2%;58.0%and59.0%at5mAcm2andcorrespondingly46.9%; 46.8% and 47.3% at highcurrent density (40 mAcm2). Among them,

a

-MnO2-13wastheonlyoneabletokeepareasonable air-efficiency (46.1%)at 40mAcm2after cycling.This catalysthas demonstrated superior (about 98.8%) air-efficiency retention (Fig.9bat20mAcm2andTableS3forallcurrentdensities)in comparisontotheother

a

-MnO2samples(98.3%forsample18and 96.8%forsample20,bothat20mAcm2).Therefore,itwaschoose ascathodeforthebatterytests.

Fig.6.BETsurfaceareanormalizedORRkineticcurrents(a)andOERcurrents(b)of synthetizeda-MnO2 catalystsinair-saturated0.1MKOH.Scanrate:5mVs1;

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Ingeneral,thelossofbifunctionalactivityandair-efficiencycan be associated to the formation of Mn3O4 which has been recognizedasthereasonofthelowerrechargeabilityof manga-neseoxidesinceitcannotbeelectrochemicallyoxidizedtoactive

MnO2 duringcycling [47and refs. citedtherein].This willalso partiallyexplainthehighretentionof

D

Vandthelowerlossof air-efficiencyforMn3O4sample,althoughitsperformanceisalways lowercomparetomoreactive

a

-MnO2materials.

Fig.10illustratestherelationshipbetweencatalyststructure andair-efficiency.Thesegmentedlinesrepresentthetrendatthe initialstate(3rdcycle)andthesolidlinesafter10PC's.Asitcanbe observedthisrelationshipgoesthroughamaximum(around10%) describing a Volcano-like plot at each current density. The maximum is achievedbysamples

a

-MnO2-13 and

a

-MnO2-20, havingbothcatalystsacomparablesurfacewatercontent(Table1) andair-efficiency(TableS3).

InSection3.3,watercontentwasassociatedtotheORR/OER activity through the surface and structural defects. As it was pointed out, K. Selvakumar et al. [40] demonstrated with the supportofexperimentaldataandDFTcalculationsthatthegood performing of

a

-MnO2 towards both ORR and OER can be correlatedwithsurfaceenergiesandadsorptionofwateronthe surface. Furthermore, the shape of the material will largely influencethesurfaceandstructuralboundedwater.Theauthors havecalculatedbyDFTthat(310)and(200)surfacesin

a

-MnO2 are strongly interacted with water molecule, while the (110) surface is weakly bonded with the adsorbate due to its high structural stability. This property indicates thatthe nature of preferentially exposed facets of manganese oxides would be another influential factor [45] that may address the material reactivity.Asitcanbeassumedthisrelationshipamongexposed facet-watercontent-activitywouldalsoinduceavolcanotrend. ThistrendisreflectedinFig.10wheresample13and20havethe bestbalanceandhence, theoptimalrelationship.Bothsamples have (310) surface as the dominating plane after the main common(211) surface.Theyalsoshoweda similarsurfaceand structural water content. Although sample 10 also have the similartotal watercontent(about12%)than them,thissample doesnotshownadominatingplaneamong(110),(200)and(310), thus itsinteractionwater-surface-activity isdifferent thanthat for

a

-MnO2-13 and

a

-MnO2-20, being its relationship air-efficiency-surface water at the left side (weak interaction) of the volcano-like plot (Fig.10). In the case of

a

-MnO2-18, the dominating surface after (211) is theplane (200). The sample showedhugesurfacewatercontentandinoverallthelargertotal water content (22%) among

a

-MnO2 samples. With this structural/surface property its relationship air-efficiency and surface water is at the right side (i.e. strong interaction with water)ofthevolcano-likeplot.

Fig.7.MassspecificnormalizedORRkineticcurrents(a)andOERcurrents(b)of synthetizeda-MnO2;Mn2O3andMn3O4catalystsinair-saturated0.1MKOH.Scan

rate:5mVs1;1600rpm.

Table2

ComparisonoftheORR(0.8VRHE)andOER(1.55VRHE)surfacespecificandmassspecificnormalizedcurrents.

Sample Jk(0.8V) (mAcm2BET) Jk(0.8V) (Ag1ox) J1.55V (mAcm2BET) J1.55V (Ag1ox) Reference Mn2O3 — — — 2.41 Thiswork Mn3O4 — — — 1.33 Thiswork a-MnO2-03 16.07 2.09 20.17(51.9* ) 2.62(6.8* ) Thiswork a-MnO2-10 — — 30.19(60.1* ) 16.18(32.2* ) Thiswork a-MnO2-13 10.49 4.89 5.55(22* ) 2.58(10.6* ) Thiswork

a-MnO2-15 8.36 3.37 0.96 0.38 Thiswork

a-MnO2-18 21.94 14.78 1.95 1.31 Thiswork

a-MnO2-20 12.48 4.79 3.43 1.32 Thiswork

a-MnO2-30 13.40 8.06 1.80 1.08 Thiswork

a-MnO2(nanorod_SF) 6(147m2g1) 8.8 — — [44] a-MnO2 6.4(112m2g1) 7.2 20.89** 23.40** [12] a-MnO2-HT —(67m2g1) — 26.41** 17.7** [12] a-MnO2/C 10(8m2g1) 0.8 — — [7] a-MnO2(nanorod) 2(19.5m2g1) 0.39 — — [10] a-MnO2(nanotube) 1(26m2g1) 0.26 — — [10] * At1.6VRHE. **

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3.4.BatteryperformanceofBAEunderrelevantZn-Airoperating conditions

Fig.11 shows the reversibility of a zinc-air full cell battery containingthecatalyst

a

-MnO2-13ascathode.Thistestvalidates theuseof as-synthetized

a

-MnO2 catalystand carbon (CNFas conductivemedia)asasuitableBAEunderrelevantMe-airbattery conditions. A zinc foil with 50 microns thickness has been employed as anode and standard 8M KOH embedded in the separatorasaqueousalkalineelectrolyte.Thetestconsistedina continuous charge/discharge cycle at 10mAhcm2 at room temperature.

AsitisrepresentedinFig.11thefullbatterycellsystemshows anOCVof1.42V(1.44Vinref.3),morethan30hoursofoperation, and a good reversibility even after 200 charge/discharge (C/D) cycles,maintainingalongthetestmorethanthe95%oftheinitial capacity(red dottedlinein Fig.11).Asthemagnificationinside

Fig.11ashows,thesystempresentstheclassicalcharge/discharge

profileswith an overpotential (

D

V) around 1.0V and a round-efficiencyC/Dabout53%.Fromabout175cyclesisobservedaslight capacityloss(reddottedlineinFig.11),however,the

D

V(different betweenbothsolidgreenlines)staysunaffected(Fig.11b).This lossofcapacity butstable

D

Vmight beinterpretedasa lossof accessiblezincactivematerialattheanodeand/ordriedconditions in the cell provoked by water consumption (included water evaporation)whichlimitedthelifecycleofthebattery.Inaddition, stable potential observed during the charging phase over the lifecycle(morethan200cycles)isalsoagoodindicationofthe excellentBAEstability.

With these results it has demonstrated the synthesis of a promisingandcheapmanganeseoxidebasedcatalystwhichcould be implemented as an alternative cathode material to further improvethecurrentstateofzinc-airbatterytechnology.Aspartof anongoingR&Dworkthathasasmainobjectivethedevelopment of high reversible zinc air battery based on aqueous alkaline electrolyte, furtherwork willinclude theanalysisof other cell componentsandtheireffectontheoverallbatteryperformance. ThiswillincludeanoptimalBAEengineeringprocesswhereother non-carbonaceousconductivemedia(suchasNiorAg)wouldbe considered.Atthesametime,and inordertoreducethewater consumptionorthelossoftheactivezincanodeduetopassivation and/ormigration tothecathode, differentseparatorsaswell as electrolyte formulations to protect the anode would be also assessed.

4.Conclusion

DifferentvalencestatesandmorphologiesofMnxOycatalysts weresynthetizedinordertodevelopa bifunctionalaircathode catalystforsecondary zinc-airbatteryapplication.Ithasshown

a

-MnO2material(producedbyaciddigestionofMn2O3)asavery promising catalystincontrasttoMn2O3and Mn3O4 (bothfrom thermaltreatmentofEMDprecursor).

Thesynthesisofdifferent

a

-MnO2wasbasedonchangingthe ratio ofH+ toMn

2O3 byincorporating differentamountsof as-prepared Mn2O3 to a constant acid concentration with the intentionofchangethesynthesisratedeterminatestep (dispro-portionationofMn(III))andhence,influencethecrystalgrowing rate.Thatwasdonewiththeideaofinducingchangesoneitherthe morphologyorthestructureofthe

a

-MnO2.

In general, it hasnotfound anysignificantinfluenceonthe morphologyoftheas-synthetized

a

-MnO2,although,formationof nanowireincomparisontothemostusualnanorodwasidentified. ChangesintheratioH+toMn

2O3duringtheaciddigestionaffected thesamplemicroporosityandthecrystallographicplane distribu-tion (othersthan 211plane), aswellas affectthephysical and chemicaladsorbedwaterwhichwasrelatedtodefects,i.e.cation vacancies(Mn4+)andMn3+.Thelatterwaslinkedtothe electro-catalyticactivityofthesynthetized

a

-MnO2showingthatthebest ORRperformingcatalystwasthatwiththehighersurfacewater content (associated to material BET surface area) and a (310) surface as the2nd morecontributing plane(after211). On the otherhand,thecatalystwiththehigherstructuralwaterandwith (110)and(200)crystallographicplanesbeingthemostintensity contributors(after211)wasthemostOERactivematerial.Inthis way,thecrystallographicplanedistribution(intermofintensities), aswellasthesamplewatercontent(surfaceandstructural)could be both associated to the observed trends for ORR and OER activitiesamongthesynthetized

a

-MnO2samples.These materi-al'scharacteristicscouldberelativelyeasycontrolledbythe cost-effective acid digestion method via variation of the acid concentration.

Therefore, in this work, it was able to find a relationship betweencatalyststructureandair-efficiencythrougha

volcano-Fig.8.Resultinggalvanostaticpolarizationcurves(3rdcurve)forBifunctionalAir ElectrodespreparedwithselectedMnxOycatalystsinair-saturated6MKOH.Air

flow:50mlmin1atroomtemperature.

Fig.9.Evaluationofbifunctionalactivityandair-efficiencyretentionat20mAcm2 ofselectedMnxOycatalysts.Darkcolorsrepresentthevaluesattheinitialstate(3rd

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likerelationshipbetweenair-efficiencyandsurfacewatercontent. Air-efficiency(alsotakenasround-efficiencydischarge/chargein batterycontext)canbeusedasagooddescriptorofpotentially goodmaterialsforZn-Airsecondarybatteriestechnology.Inthis term,wewereabletoprepareaBifunctionalAirElectrodebasedon theselected

a

-MnO2samplewhichsupportaZinc-Airbatterytest

showing a round-efficiency of 53%, a

D

V around 1V and a neglectedlossofthechargepotential(about2.1V)overtheentire lifecycle test including more than 200 charge/discharge cycles (over30hours)withacapacityretentionsuperiorto95%. Acknowledgments

ThisworkwassupportedbytheBasqueCountry University (UPV/EHU) under the program ZABALDUZ2012, the European Commission through the project ZAS: “Zinc Air Secondary innovativenanotechbasedbatteriesforefficientenergystorage” (GrantAgreement 646186)andby theBasqueCountry Govern-ment(ELKARTEK2016).

AppendixA.Supplementarydata

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.09.052.

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References

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