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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
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Influence
of
crystallinity
and
particle
size
on
the
electrochemical
properties
of
spray
pyrolyzed
Nd2NiO
4+ı
powders
D.
Mesguich
a,1,
J.-M.
Bassat
a,
C.
Aymonier
a,
A.
Brüll
a,
L.
Dessemond
b,
E.
Djurado
b,∗aCNRS,UniversitédeBordeaux,ICMCB,87avenueduDr.A.Schweitzer,33608Pessac,France
bLEPMI,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:elisabeth.djurado@lepmi.grenoble-inp.fr(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(˛=12×10−6K−1)makes this materialcompatiblewithusualelectrolytesYSZanddopedceria
[1,2].Theelectrocatalyticproperties(surfaceexchangecoefficient
k=10−6cms−1at800◦C)andoxygentransportproperties(oxygen diffusioncoefficientD*=10−7cm2s−1at800◦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 0.5cm2at700◦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 106–10−2Hz (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−1fromroomtemperatureupto1000K).X-raypowder diffraction(XRD)characterizationwasperformedonaPANalytical X‘PertMPDdiffractometerin–configurationwithaCuKa radia-tion(=1.5418)overthe2range5–80◦ina0.036◦s−1continuous 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; decreasingtheconcentrationfrom5×10−2Mto5×10−3Mleads 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 (×10−3molL−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× 10−2M,f=1.7MHz),(b)powder3(T=700◦C,C=5×10−3M,f=1.7MHz)and(c)powder6(T=1100◦C,C=5×10−2M,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.lessthan0.3cm2at 700◦Cwhichcomparesfavorablywiththe0.5cm2valuecitedin 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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