Influence of crystallinity and particle size on the electrochemical properties of spray pyrolyzed Nd2NiO4+δ powders

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DOI:10.1016/j.electacta.2012.07.134

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Mesguich, David and Bassat, Jean-Marc and Aymonier, Cyril and

Brüll, Annelise and Dessemond, Laurent and Djurado, Elisabeth

Influence of crystallinity and particle size on the electrochemical

properties of spray pyrolyzed Nd2NiO4+į powders. (2013)

Electrochimica Acta, vol. 87 . pp. 330-335. ISSN 0013-4686

Any correspondance concerning this service should be sent to the repository

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Influence

of

crystallinity

and

particle

size

on

the

electrochemical

properties

of

spray

pyrolyzed

Nd2NiO

4+ı

powders

D.

Mesguich

_,

_J.-M.

_Bassat

_,

_C.

_Aymonier

_,

_A.

_Brüll

_,

_L.

_Dessemond

_,

_E.

_Djurado

a_{CNRS,UniversitédeBordeaux,ICMCB,87avenueduDr.A.Schweitzer,33608Pessac,France}

b_{LEPMI,UMR5279,CNRS,GrenobleINP,UniversitédeSavoie,UniversitéJosephFourier,BP75,38402SaintMartind’HèresCedex,France}

Keywords:

Solidoxidefuelcell Cathode

Mixedionicandelectronicconductor Electrochemicalimpedancespectroscopy Nickelates

Ultrasonicspraypyrolysis

a

b

s

t

r

a

c

t

ThispaperisdedicatedtothestudyoftherelationshipbetweentheNd2NiO4+ıpowdermicro-structural

properties(especiallyparticlesizeandcrystallinity)andelectrochemicalpropertieswhentheoxide

isusedasSOFCcathodedepositedon8YSZelectrolytecoatedwiththindopedceria.Nano-structured

particlesofNd2NiO4+ıwithcontrolledcrystallinity,sizeandmorphologyhavebeensynthesizedusing

ultrasonicspraypyrolysis(USP).Theseriesandpolarizationresistancesmeasuredonsymmetricalhalf

cellsNd2NiO4+ı/YDC/8YSZ/YDC/Nd2NiO4+ıarebothfoundtobedependentonthecathode

microstruc-tureandpresentasimilarevolutionwithtemperature.Thebestresultsareobtainedforhighlycrystalline

cathodepowderscombinedwithasmallparticlesize.

1. Introduction

Among the essential efforts devoted toSOFCs development, one of the long-established pathways deals with the cathode materialoptimization.Thechallengeistoobtainhighactivityfor electrochemicaloxygenreductioninthe600–700◦_Ctemperature operatingrangewhileretainingchemicalcompatibilitywith elec-trolytematerials.

Rare-earthnickelateswiththeK2NiF4-typestructure,namely Ln2NiO4+ı (Ln=La,Nd,Pr)appeartobepromisingmaterialsfor applicationssuch as cathodes for low-temperature solid oxide fuelcells (LT-SOFCs)and oxygen separationmembranes. Inthe pastfifteenyears, severalgroups havefocusedtheir researches onthedevelopmentof theseoxides [1–6]. TheA2MO4+ı struc-ture,whichconsistsofMO6 octaedrasheetsinterleavedbyA2O2 layers,showsalternativetransportpropertiesincomparisonwith thoseoftheisotropicperovskitestructure.Indeed,theanisotropy ofanionicconductivityinthistwodimensional-typestructurewas demonstratedonLa2NiO4+ısinglecrystalsandthinfilms[7,8],the diffusionbeingatleastone totwo ordersof magnitudehigher inthe(a,b)planewhencomparedwiththeperpendicular direc-tion.Thenickelatestructureprovidespreferentialpathforoxygen

∗ Correspondingauthor.Tel.:+330476826684;fax:+330476826777.

E-mailaddress:[email protected](E.Djurado).

1 _{Permanentaddress:UniversitédeToulouse,UPS,INP,InstitutCarnotCirimat,}

118routedeNarbonne,F31062Toulousecedex9,France.

whichresultsinhighoxygenmobility.Moreover,suchoxidesare goodelectronicconductorsthankstothemixedvalencyofnickel (Ni2+_/Ni3+_{).Referredtoasmixedionicandelectronicconductors} (MIEC), these materials are highly desirable for improving the oxygenreductionkinetics.Indeedforhighionicandelectrical con-ductivitytheoxygenreductionreactionislikelytobedelocalized overthewholeelectrodesurface.

While current investigations on nickelates mainly focus on La2NiO4+ı [9], Nd2NiO4+ı was selected in this workbecause it showsalargeover-stoichiometryinthewholetemperaturerange 0–1000◦_{C [1,2] (for example ı=0.17 at 700}◦_{C), which should} leadtoimproved properties. At700◦_{C,its ionicconductivityis} about0.01Scm−1 _{and itselectronic conductivityis 100Scm}−1_. Its thermal expansioncoefficient(˛=12×10−6_K−1_{)makes this} materialcompatiblewithusualelectrolytesYSZanddopedceria

[1,2].Theelectrocatalyticproperties(surfaceexchangecoefficient

k=10−6_cms−1_at800◦_{C)andoxygentransportproperties(oxygen} diffusioncoefficientD*=10−7_cm2_s−1_at800◦_C)ofNd2NiO

4+ı sur-passthebestvaluesobtainedforreferencesperovskitematerials byaboutoneorderofmagnitude,especiallyatmoderate temper-atures[1,2].Moreover,theactivationenergyvaluefortheoxygen diffusion(0.85eV)islowerthaninYSZorperovskitematerialssuch asLSF[10].

Electrochemicalpropertiesofnickelatematerialshavebeen pre-viouslyreported, Nd2NiO4+ı exhibitslow ASR values (lessthan 0.5cm2_at700◦_{C),highcurrentdensity(at0.70V,0.54Acm}−2 at700◦_{C)combinedwithalowreactivitywithYSZ[11–14].Here,} theauthors haveexplored anoriginal route,namely ultrasonic

spraypyrolysis(USP),tosynthesizenanostructuredcathode pow-derswhichcouldleadtoimprovedelectrochemicalproperties.USP isatechniquerelyingontheultrasonicatomizationofa precur-sorsolutiontogenerateanaerosol subsequentlypyrolyzedina veryshorttime.Themainadvantageofthistechniqueisconferred bythedropletsoftheaerosolactingasindividualmicro-reactors, leadingtomicronandsubmicron powders.Thismethodallows thesynthesis of complexoxides submicron powderswithvery shortreactiontimes:1–10s;theoxidesproducedarecomposedof nanocrystallineparticles,generallyeliminatingtheneedfor post-treatments.

Thispaperisdedicatedtothestudyoftheelectrochemical prop-ertiesofNd2NiO4+ıpowderspreparedbyUSP,whentheoxideis usedascathode,depositedon8YSZelectrolytecoatedwiththin dopedceria.Theadditionofsuchthinionicconductinginterface between8YSZandthecathodelayer,alsoactingasabufferlayer, wasshowntoimprovetheelectrochemicalproperties,especially inthecaseofnickelatematerials[6].

2. Experimental

Neodymiumoxide(Nd2O3,99.9%,AlfaAesar)andnickelnitrate hexahydrate(Ni(NO3)2·6H2O,Sigma–Aldrich)wereusedas start-ingpowders.Neodymiumoxidewasfirstdissolvedintoaslight excess of nitric acid (HNO3, 68wt% in water, Prolabo). The neodymiumnitratesolutionobtainedwasdilutedindistilledwater toreachthedesiredconcentration.Nickelnitratewaseasily dis-solvedintodistilledwater,thenbothnitratessolutionsweremixed togetherinastoichiometricamount(Nd:Ni=2:1)andtheprecursor solutionwasfinallyintroducedintothehigh-frequencyultrasonic mistgeneratoratroomtemperature.Inordertovarythe atom-izationfrequency,twodifferentpiezoelectricceramictransducers havebeenused,characterizedby2.5and1.7MHzfrequencies.A syntheticair(80%N2,20%O2)flowrateof6Lmin−1wasselectedto carrytheformedaerosolthroughthe3-zoneshightemperature fur-nace.Temperatureinthelasttwozoneswasfixedat700◦_C,900◦_C or1100◦_{C(thefirstzonealwaysbeingset50}◦_{Clowertoavoidtoo} fastevaporationofthesolventandheterogeneousprecipitationof thesolute).Drypowdersarerecoveredoutsidethefurnaceusing anelectrostaticfilter.Furtherdetailsontheexperimentalsetupcan befoundinpreviouspublications[15,16].

Cathode inks have been prepared mixing USP Nd2NiO4+ı powders(70wt%)withaterpineol-based dispersantmedium, a commercialsolvent(alsocontainingterpineol)fromDupontand ahome-madeorganicbinderusinga Dispermatmillingsystem. Yttria-dopedceria(YDCCe0.8Y0.2O1.9)inkswereprepared simi-larlyfromcommercialpowderfromPraxair.YDCbufferlayerswere screen-printedonbothsidesof8YSZelectrolytesandannealedin airat1400◦_{Cfor1h,resultingin2–3mmthickporouslayers.} Cath-odelayers(20–25mmthick)weresubsequentlyscreen-printedon bothsidesofYDC-coated8YSZandannealedat1100◦_Cfor3h.

Each cell consists of a 100mm thick dense 8YSZ ((ZrO2)0.92(Y2O3)0.08) electrolyte coated on both sides with a 2–3mmthickYDCbufferlayeranda20–25mmthickNd2NiO4+ı cathode layer. The only difference between each cell is the cathodemicrostructureand crystallinity,everyotherparameter (electrolyte, YDC interlayer, ink composition, screen printing parameters,annealing steps)beingidentical. Comparisons have beenperformedusingcommercialNd2NiO4+ıpowdersdeposited usingthesameprocedure.Forrelevantcomparisonsallsamples have beenprocessed in conditions optimized for thereference commercialpowder.

Thesesymmetricalcellshavebeencharacterizedbyimpedance spectroscopymeasurements(AutolabPGSTAT302Ncoupledwith a frequency responseanalyzer) at open circuit potential in the

frequency range 106_–10−2_{Hz (10points per decade) underair} between500◦_Cand800◦_{C.Themagnitudeofthemeasuring} sinu-soidalvoltagewaschosenequalto50mVtoensurethelinearity of theelectrical response.Platinum grids (mesh225) mechani-callypressedagainstbothelectrodeswereusedaselectricalcurrent collectors.Platinumwireswereusedtoconnectelectrodestothe externalelectricalcircuit.

The particles morphology and cathode layer microstructure werestudiedusinghighresolutionscanningelectronmicroscopy (HR-SEM,JEOL6700)under5.0kVacceleratedvoltageand trans-missionelectronmicroscopy(TEMJEOL2000).Theparticlesizewas evaluatedfromtheSEMmicrographsbycountingover500 par-ticlesusingImageToolsoftware;themainparticlesizereported herecorrespondstod0.5(50%oftheparticlesarebelowthissize). DilatometricanalyseswereperformedwiththehelpofaNetzsch STA409C/CDapparatusunderanairflowof100mLmin−1_(heating rate2Kmin−1_{fromroomtemperatureupto1000K).X-raypowder} diffraction(XRD)characterizationwasperformedonaPANalytical X‘PertMPDdiffractometerin–configurationwithaCuKa radia-tion(=1.5418)overthe2range5–80◦_ina0.036◦_s−1_continuous scanmodeatroomtemperature.

3. Resultsanddiscussion

3.1. Controlofcrystallinity,particlesizeandmorphology

Before addressing the electrochemical properties of USP Nd2NiO4+ıcathodes,wewillfirstsummarizetheresultspreviously reportedconcerningthehighcontrolofferedbyUSPovernickelate powderproperties;foradditionaldetailsonecanreferto[16]. Con-troloftheprimaryparticlediameterusingtheprocessparameters isoneofthemostimportantadvantagesofthissynthesismethod sinceit allowsfinecontroloftheparticle sizeandcrystallinity. Indeed,thereactionsresponsibleforproductformationare con-finedwithintheindividualmicrometer-sizeddropletswhichactas individualchemicalreactors.Thefourmainoperatingparameters controllingpropertiesofthefinalpowdersaretheconcentration oftheprecursorsolution,theatomizationfrequency,thefurnace temperatureandthecarriergasflowrate.Simplychangingoneof thesefourparameterswillchangethesizeoftheparticle,and con-sequentlytheprimaryparticlediameterandmorphology,aswell asitscrystallinity[16,17].

Adjustingtheprecursorsolutionconcentrationaswellasthe atomization frequency allows fine tuning of the particle size.

Table1presentstheexperimentalparametersforUSPsynthesisand thecorrespondingmainparticlesizeforeachsample.Theoretically, theparticlesizevarieslinearlywiththedropletsize(influencedby atomizationfrequency)andwiththeprecursorconcentrationto thepower1/3asconfirmedwithourexperiments.Fig.1provides someexamplesofmorphologiesofNd2NiO4+ıUSPpowders. Com-parisonbetweensamples1(Fig.1a)and3(Fig.1b)clearlyshows theinfluence of theprecursor concentrationover particle size; decreasingtheconcentrationfrom5×10−2_Mto5×10−3_Mleads toareductioninmainparticlesizefrom542nmto190nm. Atom-izationfrequencydoesnotonlycontroltheparticlesizebutalso theparticle sizedistribution(PSD).Highatomizationfrequency (2.5MHzinsteadof1.7MHz)generatessmalleraerosoldroplets, reducingtheprobabilityofcollisionsbetweendropletsresponsible forthebroadparticlesizedistributionusuallyobservedusingspray pyrolysis.

The powder morphology varies in regard to the precursor concentrationand pyrolysistemperature.Smooth and spherical nanoparticles are synthesized at lower concentrations (Fig. 1b) whilerougherparticleswithmoreirregularshapesareobtainedat higherconcentrations(Fig.1aandc).Surfaceprecipitationbecomes

Table1

ExperimentalparametersforthesynthesisofUSPNd2NiO4+ıpowdersandRpvaluesat600◦CforthecorrespondingNd2NiO4+ı/YDC/8YSZhalfcells.

ExperimentalparametersforUSPNd2NiO4+ıpowdersynthesis Mainparticlesize(nm) Rp600◦C(cm2)

Pyrolysistemperature (◦_C) Precursorconcentration (×10−3_molL−1₎ Atomizationfrequency (MHz)

Nd2NiO4+ı/YDC/8YSZcells

1 700 50 1.7 542 8.65

1annealed900◦_{C1h 700 – – (Sinteredgrains) 4.08}

2 700 50 2.5 463 4.27

3 700 5 1.7 190 4.53

4 900 50 1.7 521 2.59

5 1100 50 2.5 423 1.80

6 1100 50 1.7 474 1.91

significantforlargedropletsizescombinedwithfastevaporation

[20] (i.e.for high precursor concentrationcombined with high pyrolysistemperature)leadingtotheemergenceofopenporosity forthelargerparticles(Fig.1c).

Finally,powdercrystallinitydependsonthepyrolysis tempera-ture:amorphouspowdersareobtainedforamoderatetemperature (i.e.700◦_{C)whereashighlycrystallinepowdersareformedathigh} pyrolysistemperature(i.e.1100◦_{C). XRDpatternrepresentative} ofNd2NiO4+ıpowderssynthesizedat1100◦CisshowninFig.1d. Becausetheamorphouspowdersexhibithighreactivity (calcina-tionof1hat900◦_{Cunderair issufficienttoinduceNd2NiO}

4+ı crystallization),theNd2NiO4+ıcathodelayerdepositedfromUSP powdersisalwayswellcrystallizedafterashortannealingstep.It isremarkabletosynthesizehighlycrystallineNd2NiO4+ıpowders (averagecrystallitesize=35±3nm)withsuchshortreactiontimes (8s)whilelowtemperaturesyntheses[11,13,14,18,19]andsolid statereaction[2,13]systematicallyrequireasubsequentannealing step(8–12h)over1000◦_C.

3.2. Electrodemicrostructure

The electrode microstructurehas been investigatedby SEM. Goodadhesionhasbeenobservedbetweenthecathodelayerand theYDCinterlayer(Fig.2a).Althoughlowparticlesizecanenhance

crackingproblems[6],itisworthmentioningthatthisissuehasnot beenobservedwithUSPpowdersexhibitingparticlesizeaslowas 190nm.ComparisonbetweenFig.2bandcprovesmoreadvanced sinteringforthecathodemadefromNd2NiO4+ıUSPpowderthan forthecathodemadefromacommercialpowderwithd0.5equalto about800nm.

EnhancedsinteringofthepowderssynthesizedbyUSPisalso demonstratedbydilatometrymeasurementsshowninFig.3.For instance, the sintering of powder 6 begins approximately at a temperature100◦_{Clowerthanforthereferencepowder.} Further-more,powderdensificationismuch higherfortheUSPpowder (dL/L=236×10−3 _{instead of 124×10}−3 _{at 1300}◦_{C). The signal} derivativealsoshowsmultiplesinteringstepsthatcouldbedue to theparticle size dispersion, the smaller particles being sin-teredatlower temperaturesthanthelargerones.Here particle sizedistributionandmorphologyclearlyhavearole.Anarrower particlesizedistribution(PSD)(enabledbyhigheratomization fre-quency)eliminatesthescarcelargerparticles,whichleadstomuch higher sinterability. Powders synthesized withhigh concentra-tionsandhightemperaturealreadyunderwentplasticdeformation andshrinkageduringthepyrolysisprocesswhichshouldalsobe beneficial to the sintering activity. One major benefit of using Nd2NiO4+ı powders produced by USP comes from their high reactivity(illustrated bythefastcrystallizationof Nd2NiO4+ı at

Fig.1. SEMmicrographsshowingthedifferentparticlesizesandmorphologiesofNd2NiO4+ıpowderssynthesizedbyultrasonicspraypyrolysis:(a)powder1(T=700◦C,

C=5× 10−2_{M,f=1.7MHz),(b)powder3(T=700}◦_C,C=5×10−3_{M,f=1.7MHz)and(c)powder6(T=1100}◦_C,C=5×10−2_{M,f=1.7MHz),(d)X-raydiffractionpatternof}

Fig.2.SEMmicrographsof(a)cell5,(b)cathodelayerofcell5and(c)cathodelayerofreferencecell.

moderatetemperatureswhenstartingfromamorphouspowders) inducinghighsinteringactivity.Consequently,thistechniqueoffers finercontrolofthecathodemicrostructurewhichcouldbe associ-atedwithareductionintemperatureduringtheannealingstep ofthecathode layerin anefforttominimize thechemical dif-fusion at the cathode/buffer layer and buffer layer/electrolyte interfaces.

3.3. Electrochemicalperformances

The influenceof theelectrode microstructureon the corre-spondingelectrochemicalpropertiesofNd2NiO4+ıcathodelayers depositedbyscreenprintinghasbeencarefullystudied.The elec-trochemicalperformancesofsymmetricalcellsmadewithdifferent USPNd2NiO4+ıpowdershavebeencomparedwiththoseofa refer-encecellmadewithNd2NiO4+ıcommercialpowder[21].Therefore thedifferencesobservedarediscussedinregardtotheinitial intrin-sic properties of the cathode powder (mainly crystallinity and particlesize)andthedifferencesinelectrodemicrostructurewhich canbeinducedduringthermalannealing.Hereitisworthnoting thattheparametersadoptedforthecathodelayerdepositionand subsequentannealingstepforallsamplesinthisstudyhavebeen determinedafteralengthy optimizationprocessconcerningthe referencepowder[13].In ordertoattributedifferencesin elec-trochemicalproperties solely tothecathode powderused, USP sampleshavebeenprocessedinthesameexactconditions.

AtypicalimpedancespectrumobtainedisshowninFig.4.The experimentaldiagramsweredecomposedbymeansofanonlinear leastsquarefitusingtheZview2®_{software(ScribnerAssociates),} intotwoorthreeelementarycontributions(parallelcombinations of resistance and constant phase element connected in series), depending on the measuring temperature; the corresponding equivalentcircuitisshowninFig.4.Hereafterthediscussionwill

Fig.3. Dilatometrycurves(solidlines)andsignalderivative(dottedlines)forUSP Nd2NiO4+ıpowder6(greencurves)andreferenceNd2NiO4+ıpowder(redcurves).

(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderis referredtothewebversionofthearticle.)

befocusedontheresultsrelatedtotheseriesresistanceRsand thepolarizationresistanceRp.Forthesakeofcomparison,both resistances were normalized by the electrode area. The series resistancewasdeterminedbytheinterceptofthehighfrequency partoftheelectrodecharacteristicontherealaxisintheNyquist plane.Thepolarizationresistancewasdeducedfromthedifference betweenthelowfrequencyrealpartoftheelectrodeimpedance and the series resistance, divided by 2 since symmetrical cells werecharacterized.Table1summarizes themainexperimental parametersusedforsynthesizingthedifferentUSPpowdersused ascathodeslayersalong withtheparticlesizeand Rpvalues at 600◦_{Cforthecorrespondingcells.Fig.5presentsRpvaluesplotted} versus1000/T.Averageactivationenergyis1.16±0.04eVwhich iscomparabletotheoneofthereferencecell(1.13eV).

From the results of Fig. 5, the first peculiar feature is that cathodes madefrom highly crystallinepowders (cells5 and 6, pyrolysistemperatureof1100◦_{C)exhibitthelowestpolarization} resistances,whichareclosedtovaluesrecordedforthereference cell withinthe experimental accuracy.In situ crystallizationof thecathodematerialontopoftheYDCinterlayer(startingfrom amorphouspowdersasforcells1–3withapyrolysistemperature of700◦_{C)leadstopoorerresults.Thisislikelytooriginatefrom} smallamountsofimpurities(1.0wt%nitrogenand0.6wt% hydro-genasmeasuredbyInductive-CoupledPlasmaandCHNSanalyzer combinedwiththermogravimetricanalyses[16])whichhinder sin-teringofamorphouspowdersandyieldlowerintimatecontacts withYDC.Preliminary heattreatment (900◦_{C, 1hunderair)of} theamorphouscathodepowderstoinduceNd2NiO4+ı crystalliza-tionbeforetheirscreen-printingdepositionresultsindecreased polarizationresistancebyafactorof2,whichunderlinesthe impor-tanceof theinitial powdercrystallinity. IntermediateRpvalues recordedforcell4(pyrolysistemperatureof900◦_{C)alsosupport} thisassumption.

Fig.4.TypicalexperimentalimpedancediagramforNd2NiO4+ı/YDC/YSZcells

(mea-surementat600◦_{Cforcell6).Thecorrespondingequivalentcircuitusedforfitting}

isalsodisplayed,thefittingcurveisoverlaidonthedatapointsandsignalfrequency isindicated.

Fig.5.Arrheniusplotofpolarizationresistance(Rp)forNd2NiO4+ı/YDC/YSZcellswithdifferentNd2NiO4+ıcathodelayers.ExperimentalparametersforUSPNd2NiO4+ı

powdersusedincells1–6areindicated(pyrolysistemperature,precursorconcentrationandatomizationfrequency,respectively).

The secondfeature tounderline isthat smaller particle size and narrower PSD (forinstance, by increasing theatomization frequency)arebeneficialfortheelectrochemicalpropertiesof cath-ode, particularly for the ones made from amorphous powders sincetherareverylargemicron-sizedparticlesaremostly respon-siblefor theorganic impurityrelease duringthesinteringstep. Indeedimpedancespectroscopy measurementfor cells 1and 3 showthat lowerparticlesize (542nmand190nmrespectively, withprecursor concentrationdivided by10) leadsto a signifi-cantdecreaseofRp(8.65cm2and4.53cm2respectively).The changeinducedbyanarrowerPSDforhighlycrystallinepowders is less noticeable since the microstructure is still more regu-lar.Theclosedpolarizationresistancesofcells 5and6(particle sizelowerthan500nm)andthereferenceone(particle sizeof

800nm)couldberegardedasconflicting.But,thesintering activ-ityofUSPNd2NiO4+ıishigher(Fig.3)andalowermeanporosity aftersintering can thusimpede theaccess of thegas phase to the reaction sites. At this stage, it is worth noting that, with-outanyoptimizationoftheprocessingparameters,theRpvalues reportedherearealreadyremarkablylow,i.e.lessthan0.3cm2_at 700◦_{Cwhichcomparesfavorablywiththe0.5cm}2_valuecitedin introduction.

Similartrendscanbeextractedfromthevariationoftheseries resistanceasafunctionofthepowdercrystallinityofthecathode layer(Fig.6).Thehigheristhecrystallinity,thelowertheseries resistance.ThisconfirmsthatanenhancedsinteringofNd2NiO4+ı yieldsbetter contactswiththeYDC interlayer. Moreover,for a givencrystallinity,theinfluenceoftheparticlesizeontheseries

resistanceisfarlesspronouncedthanforthepolarization resis-tances.Thisindicatesthattherespectivevolumesrelatedtoeach resistivecontributionaredifferent,ascouldbeexpectedforaMIEC electrode.ItisworthnotingthattheexperimentalvaluesofRsare higherthanthosewhichcanbecalculatedfromtheconductivities ofYSZ[22]andYDC[23]andtherelatedvolumes.Thissuggests thattheseriesresistanceincludesthecontributionofconstriction ofcurrentlinesrelatedtoaninsufficientcurrentcollectionwhich can strongly alter theperformance of a SOFC cathode [24,25]. Indeed,thesheetresistance arisingfromtheelectrontransport withinthecathode layeris expected tobenegligible since the electronicconductivityofNd2NiO4+ıishigherthan10Scm−1[2]. Inthisstudy,onecannotdistinguishtherelativecontributionsof theNd2NiO4+ı/YDCandNd2NiO4+ı/currentcollectorinterfacesto thecurrentconstrictioneffect.

4. Conclusions

ThisstudyisthefirstassessmentofthesuitabilityoftheUSP technique,offeringgreatopportunitiestotunethepowder proper-tiesinparticularpowdercrystallinityandparticlesize,foranSOFC application.Theaimofthispaperwastoshowtherelationships betweenthecathodepowderpropertiesandthecathoderesistance toidentifyexperimentalparametersleadingtobestresultsinterms ofelectrochemicalperformance.Electrochemicalimpedance spec-troscopymeasurementsshowthattheseriesresistanceRsandthe polarizationresistanceRparebothdependentonthe microstruc-tureoftheelectrodeandexhibitthesamevariationtrends.Thebest results(Rpvaluesbelow0.5cm2at700◦Cand2cm2at600◦C) areobtainedfromhighlycrystallineUSPpowderswithlow par-ticlesize(about400nm).Finally,highlyreactivepowdersexhibit asignificantlyimprovedsinteringactivitywhichleadstoa differ-entelectrodemicrostructurethanthereferencecell,showagood adhesionofthecathode layerandcontributetotheloweringof thecathoderesistance.Whilethisstudydemonstratesthe poten-tialofUSPNd2NiO4+ı powders,electrochemicalperformancesof USPpowderscouldbeagainenhancedafteranoptimizationofthe processingparameters(especiallythesinteringstepowingtotheir reactivitymuchhigherthantheoneofthereferencepowder). Fol-lowingtheseresultswhichdemonstratedtheinterestedofspray pyrolysissynthesisforSOFCsmaterials,anotherprojectcurrently underway focuses ontheUSP synthesis of Pr2NiO4+ı powders

whichhaveshownhighlypromisingelectrochemicalperformances at600◦_C[6].

Acknowledgment

Financial supportfrom theEC within theintegrated project SOFC600(ContractNo.020089)isgratefullyacknowledged. References

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