Effect
of
Cerium
on
the
Electrical
Properties
of
a
Cobalt
Conversion
Coating
for
Solid
Oxide
Fuel
Cell
Interconnects
–
A
Study
Using
Impedance
Spectroscopy
Jan
Gustav
Grolig
*
,
Jan
Froitzheim,
Jan-Erik
Svensson
EnvironmentalInorganicChemistry,ChalmersUniversityofTechnology,Kemivägen10,41296Gothenburg,Sweden
A R T I C L E I N F O
Articlehistory:
Received2July2015
Receivedinrevisedform19October2015 Accepted20October2015
Availableonline26October2015
A B S T R A C T
Coatingsofmetalliccobalt,whichconvertintoacobaltmanganesespineloxideareknowntoimprovethe propertiesofinterconnectsforsolidoxidefuelcells(SOFCs).Theadditionofceriumtothecobaltcoating further improves thecorrosionproperties ofthe material.Forthisstudytraditional four-pointDC measurementsathightemperatureswerecombinedwithimpedancespectroscopyatlowtemperatures inordertoinvestigatetheeffectofceriumontheelectricalpropertiesofacobaltconversioncoating.It was found that combination-coatings of cerium and cobalt exhibit superior electrical properties comparedtopurecobaltcoatings.Ceriumslowsdownthegrowthofchromiaandpreventstheoutward diffusionofironintothecobaltspinellayer.Botheffectsarebeneficialfortheelectricalpropertiesofthe interconnect.Impedancespectroscopymeasurementsrevealedthatevenaftermorethan3000hof exposuretheoutercobaltmanganesespinellayerstillhasahigherelectricalconductivitywhencerium waspresent.
ã2015TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.Introduction
Solidoxidefuelcells(SOFCs)arerecognizedasaninteresting alternativetechnologyforelectricityandheatproduction.Several planarfuelcellelementsareusuallyconnectedinseriestoreach sufficientpowerdensities.Thisisachievedbytheutilizationof so-called interconnector plates or interconnects. Recent
develop-ments, such as improved cathode performance and thinner
electrolytes,haveledtoloweroperationaltemperaturesandthus have allowed the use of interconnects fabricated of metallic materials. Extensive research has led to the development and investigation ofseveral suitable alloysfor useasinterconnects. ExamplesoftheseareCrofer22APUandCrofer22HfromVDM ThyssenKrupp,SanergyHTfromSandvikMaterialsTechnologyor ZMG232fromHitachiMaterials[1–3].Othernon-specificSOFC applicationdevelopmentssuchasAISI430or441havealsobeen reported[4–6].The degradation of theinterconnect is a major limitingfactor forthelife-timeof SOFCstacks[7].Threemajor degradationmechanismscanbeidentifiedthatdegrade intercon-nectperformance;theevaporationofchromium,theoxidationof the metal, and an increase in electrical resistance due to the
oxidationprocess.Metalliccobaltconversioncoatingshaveshown greatpotentialinovercomingtheissuesofchromiumevaporation andincreasingelectricalresistance[8–10].Thecombinationofa cobaltconversioncoatingwithceriumhaspreviouslybeenshown to improve the corrosion resistance of interconnect materials
[4,9,11].Previousstudiesonthesecoatingshaveshownthatthe metallic cobalt coating is oxidized to Co3O4 in the very first moments of exposure [9,10]. With time, manganese, which is contained in the steel, diffuses outwards to the Co3O4 spinel convertedcobaltcoating.Canovicetal.haveshownasignificantly higheramountofironintheouterspinellayerwhenonlycobaltis used,incontrasttoanalmostiron-freeouterspinelwhenacobalt coatingisappliedontoa10nmthickceriumlayer[9].Thecobalt manganesespinelactsasacaplayeranddecreasestheevaporation ofchromiumandcan,insomecases,evendecreasetheoxidation rate oftheinterconnect[4].Furthermoreithaspreviouslybeen reportedthatceriumresidesmostlyintheouter(spinel-)oxide layerwhenitisusedforexposuretimeslongerthan168h[12,13]. Inthiscontext,theeffectofceriumontheelectricalpropertiesof thesetypesofcoatingsisinteresting.AlowerASRcanbeexpected sincetheoxidegrowthrateisdecreasedbythepresenceofcerium
[14].Besidestheeffectsofachangeinoxidechemistry,asreported byCanovicetal.,andthereducedgrowthratementionedabove,a dopingeffectoftheceriumitselfonthespinellayermightalsobe suspected.Impedancespectroscopyhasrarelybeenusedtostudy * Correspondingauthor.Tel.:+46317722828;fax:+46317722853.
E-mailaddress:[email protected](J.G.Grolig).
http://dx.doi.org/10.1016/j.electacta.2015.10.111
0013-4686/ã2015TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/). ContentslistsavailableatScienceDirect
Electrochimica
Acta
thecontributionof individualoxidelayersthat growonasteel substrate.Impedancespectroscopyisaversatilenon-destructive materialtestingmethodthatcanbeusedtogiveinformationabout differenttimedependentconductionmechanismsinmaterials.It canbealsousedtoinvestigatedielectricandelectricproperties,as wellaspropertiessuchasgrowth,continuityorthepresenceof oxidelayers[15,16].Songetal.usedimpedancespectroscopyto examineoxidelayersgrownonAISI304andrevealedthepresence ofaspineloxideandachromiumoxide,theelectricalpropertiesof which could be determined [16]. Velraj et al. have presented impedance data on cobalt-plated Crofer 22 APU, where they observed three individual oxide layers; chromia, chromium containingcobaltoxideandacobaltchromiumspineloxide[15]. Sincethereisalreadyextensiveliteratureavailableondifferent substratematerialswith cobaltmanganese coatings,this study aimed at investigating the effect of cerium on the electric properties of the cobalt manganese spinel oxide scale. The additionof ceriumisexpectedtoslowdownthegrowthofthe chromia,and,therefore,alowerareaspecificresistance(ASR)can beanticipated.Theeffectsofceriumontheactivationenergyofthe electrical conduction and the electrical propertiesof theouter spinellayerwereinvestigatedusingacombinationofclassicDC characterizationmethods athightemperatureswithimpedance spectroscopyatlowtemperatures.
2.Experimental 2.1.SamplePreparation
Pre-coated steel sheets of Sanergy HT with a thickness of 0.2mm werereceived fromSandvik MaterialsTechnology. The composition of the substrate material is given in Table 1. The sampleswerecoatedwithanindustriallyavailablephysicalvapor depositionmethod and consistedof either 640nm cobalt or a doublelayercoatingofa10nmlayerofcerium(innerlayer)and 640nmofcobalt.Forthedepositionofbothcoatinglayers,metallic targets were used. The steel sheets were cut into coupons of 1515mm2.Afterultrasoniccleaningwithacetoneandethanol, thesampleswereweighedusingaXP6scale(MettlerToledo). 2.2.Exposure
The sampleswere exposed at 850C in tubular furnaces in simulatedcathodeatmosphere(air,3%watervapor,flowof27cm/ s),moreinformationcanbefoundin[17].Thewatercontentwas adjustedwithacoilcondenserandwasmaintainedat thermody-namicequilibrium[17].Thesampleswereexposedisothermally fordifferentdurationsofupto3270h.
2.3.Areaspecificresistancemeasurement
ThesampleswerepreparedforASRcharacterizationasshown inFig.1:Apre-oxidizedsamplewasmaskedwithashadowmask of 1010mm2 and sputtered with platinum using a Quorum 150sputtercoater(60mAand10minduration)toensureadefined electrodeareawithanapproximateplatinumthicknessofabout 100nm.Thisprocedurewasrepeatedforbothsidesofeachsample. Theelectrodeswererepaintedwithplatinumink(Metalor6082) usingafinebrush.Thesampleswerethendriedfor1.5hat150C
andfiredat850Cfor10mintoburnoffthebinderoftheplatinum ink. The prepared sampleswere put intoa Probostat1 sample holderwithafour-pointsetupwhich wasplacedintoa tubular furnace.TheDCresistanceofthesampleswasmeasuredontwo test stands using either a Keithley 2440Current Source in combination with a Keithley 2701 digital multimeter or just a using Keithley 2400 in four-point mode. A current density of 100mA/cm2was appliedduringthemeasurement,andtheASR was measuredat 50C intervalsfrom850C downto500C to enableanactivationenergycalculation.Themeasuredresistance valuesweredividedbytwoinordertoobtaintheASRvaluefora singleoxidescale.Themeasurementswereperformedusingan ex-situmethod,sinceitwaspreviouslyshownthatin-situ measure-mentsaffecttheoxidationmechanism[18].
2.4.Impedancespectroscopy
For the impedance spectroscopy measurements, the same Probostat1 sample holder was used and placed into a tubular furnace. A Solartron 1260 Impedance Analyzer was used to measureinACmode.Ameasurementpotentialof100mVwithout abiaswasappliedtomeasuretheimpedanceofthesamplesina
frequency range from 10mHz to 6MHz. The impedance was
measuredat10Cintervalsinatemperaturerangefrom200Cto 60C. For the analysis and the fitting of the impedance data, ZView1softwarefromScribnerwasused.
3.Results
3.1.Gravimetricalevolution
ThemassgainofindividualsamplesisplottedinFig.2.Afteran initialmassgain of 0.21mg/cm2after only2hofexposure, the massgainfollowedanalmostparabolictrend.Thecerium/cobalt coatedsamplesshowedasignificantlylowermassgainthanthe cobaltcoatedsamples.Withexposuretimeslongerthan1000h, themassgainofthecerium/cobaltcoatedsampleswasalmosthalf asmuchasthecobaltcoatedsamples.After3300hofexposure,an average mass gain of about 1.7mg/cm2 was observed for the cerium/cobaltcoatedsamplesin contrastto2.8mg/cm2for the cobaltcoatedsamples.
3.2.DCcharacterization
TheASRofbothcobaltandcerium/cobaltcoatedsampleswas measuredatdifferenttimeintervalsfrom2–3270h.Theevolution oftheASRvaluesovertimeisplottedinFig.3.Itcanbeseenthat, over the entire exposure time, the ASR values of both sample
systems increase. The cobalt coated samples were always
significantlyhigher in ASRthan thecerium/cobaltcoated,even aftertheshortestexposuretimeof2h.TheobservedtrendinASR evolutionwascomparabletothepreviouslyshownmassgain.This canbealsoseeninFig.4,inwhichtherelationbetweenthemass gain and ASR is plotted, indicating an approximately linear relationship.However,somesamples,especiallythecobaltcoated ones,showedarelativelargespread.Theactivationenergyofeach measuredsamplewascalculatedusingtheArrheniusequation.At very low mass gains and short exposure times, the activation energy was, for both sample systems, in the range of 0.55 to
Table1
SanergyHTcompositioninwt.%givenbythesupplier.
Fe C Si Mn P S Cr Ni Mo W Cu Nb Ti N Zr
0.65eV.Whereasatlongerexposuretimesandhighmassgain,the activationenergydecreasedtoavalueofabout0.45eV,ascanbe seeninFig.5.Nosignificantdifferenceinactivationenergywas notedbetweencobaltcoatedandcerium/cobaltcoatedsamples. 3.3.Impedancespectroscopy
Besides DC characterization, selected samples were also measuredatlowertemperaturesusingimpedancespectroscopy. A representativeNyquist plotof a cobaltcoated and a cerium/
cobaltcoatedsampleexposedfor3300hat850Candmeasuredat 150CisdepictedinFig.6.Forthetemperatureintervalbetween 200 and 60C, two semi-circles for both samples could be recorded. An equivalent circuit of two R/CPE elements (R1/ CPE1andR2/CPE2)inserieswithanadditionalresistor(R3)was usedforfitting(seeinsetFig.6).TheadditionalresistorR3which wasinallcasessignificantlysmallerthanR1andR2represented theresistanceofthebulksteelandtheelectrodes.Theresulting resistancesR1andR2ofeachtemperaturewereusedtocalculate area specific conductivities, which were consequently plotted againsttheinversetemperatureseeninFig.7.Theconductivityfor the cobalt coated sample was, for both R/CPE elements, much lowerthanthecerium/cobaltcoatedsample.Bothsamplesshowed similaractivationenergiesforconductivityderivedfromthehigh frequencysemi-circlewith0.40eVforthecobaltcoatedsample and 0.45eVforthecerium/cobaltcoatedsample.Theactivation energyfortheconductivityderivedfromthelowfrequency semi-circlewas0.51eVforthecobaltcoatedand0.53eVforthecerium/ cobaltcoatedsample.Similaractivationenergieswereobserved forsamplesexposedforshorterexposuretimes.Acompilationof resistance values, capacitance values and peak frequencies is showninTables2and3.
3.4.Microstructuralevolution
Overtheentireexposuretime,themicrostructuralevolutionof both sample systems was in line with previously reported observations of similar coating and substrate combinations
[9,10,13].Cross-sectionsofacobaltcoatedsampleandacerium/ cobalt coatedsample after100hof exposure andthe electrical characterization of the samples are shown in Fig. 8a and b. WhereasinFig.9aand b,twocross-sectionsofa cobaltcoated sampleandacerium/cobaltcoatedsampleaftermorethan3000h ofexposure,includingelectricalcharacterizationareshown.After 100hofexposure,adoublelayeredoxidewasobservedonboth sampleswitha relativelythickouteroxideandarelativelythin inner oxide.PreviouslyreportedEDXanalysesrevealedthatthe outeroxidelayercontainedoxygen,cobalt,manganeseandironin thecobaltcoatedcase,andoxygen,cobaltandmanganeseinthe cerium/cobaltcoatedcase[9].Theverythin inneroxidemainly consisted ofchromium oxide [9]. After3300h(Fig.9), a much thickeroxidescalewas observed.Theouteroxide scale,in both cases didnot growsignificantly. However,onthecobaltcoated
sample, a much thicker inner oxide with a thickness of
approximately10
m
m, comparedtoa5m
mthickinneroxide in the cerium/cobalt coated case, was observed, examining SEM cross-sectionsofseveralsamplesatdifferentlocations.4.Discussion
4.1.Corrosionmechanism
Thecorrosionmechanismofthesetypesofcoatingshasbeen extensivelystudiedinpreviouspublications[4,9–11,13,19]. Gener-ally, themechanism canbesummarized as follows:The cobalt
Fig.1.Schemeofelectrodepreparation.
Fig.2.Massgainofindividualsamplesovertime.
coatingoxidizesintheveryfirstmomentsofexposuretoCo3O4. Whennoceriumispresent,asubstantialamountofirondiffuses intothecobaltcoatingduringthefirstheating.Inthefirst200hof exposure,manganese,presentinthesteel,diffusesoutwardsand togetherwiththecobaltoxideformsacobaltmanganesespinel,in the form of (Co,Mn,Fe)3O4, with a stoichiometry ranging from Co1.4Fe1Mn0.6O4 at the innermost part of the oxide to Co2.3Fe0.4Mn0.3O4 at the oxide surface [9]. The ratio between
cobalt and manganese depends on the amount of manganese
presentinthesteel,thedurationofexposureandthethicknessof the cobalt coating. In the present case, the ratio of cobalt to manganesewasabout2:1forbothusedcoatingsafterexposure timeslongerthan200h.Overtheentireexposuretime,anoxide layerofchromiagrewunderneaththespineloxide.Whenadouble layerofceriumandcobaltwasusedasthecoating,thediffusionof ironisinhibitedresultinginanouterspineloxideintheformof (Co,Mn)3O4 and a drastically reduced chromia growth was observed.Afteranexposuretimeof3300h, anaveragechromia thicknessofapproximately 10
m
mand 5m
mforthecobalt and cerium/cobaltcoatingsrespectivelyhadbeenobtained.4.2.ASRevolution
The ASR values of thesamples wereexpectedto bemainly dominatedbythegrowingoxidescales.Sincetheoxidescaleafter veryshortexposuretimesismainlycomprisedoftheoutercobalt manganesespinel,itcanbeassumedthatthecorrespondingASR valueandespeciallyactivationenergywillbelargelydetermined bytheelectricpropertiesofthisspinellayer.Theactivationenergy fortheelectricalconductionwasdeterminedtobe0.6eVafteronly 2h of exposure. Literature values available for the activation energy of cobalt manganese spinels are in the range of 0.76– 0.55eV[20–23].Byincreasingexposuretime,theinneroxidescale (Cr2O3)increasedinthickness,whereastheouterspinelgrowsonly minimallyafteraboutaweekofexposure(168h)[10].Sincethe conductivityofchromiaisseveralordersofmagnitudelowerthan theconductivityofthecobaltmanganesespinel,itwasexpected thattheASRvalueswouldbedominatedbytheinnerchromialayer assoonasithadgrowntosufficientthickness[21].Additionally, theactivationenergydecreasedtoavalueofabout0.45eVoverthe investigatedtimeframe,whichmightbeseenasanindicationofa changefromacobaltmanganesespineldominatedconductionfor exposure times shorter than 100h to a chromia dominated conduction. Literature values for the activation energy of low temperatureelectronicconductionofchromiarangefrom0.18eV upto0.6eV[24–28].Ithasbeenmentionedbyotherresearchers thatthislargespreadmightbeduetothelongequilibriumtimes and the high sensitivity of chromia conductivity to impurities
[24–26,28,29].Anobviousobservationincomparingthesamples exposedforshorttimeswasthehigherASRofthecobaltcoated samplesthanthecerium/cobaltcoatedsamples.Afteronlytwo hoursofexposure,notalargedifferenceineffectivethicknessof thechromialayercanbeexpected.Ifitisassumedthattheouter spineloxidelayerofthesampledominatesitsconductivityafter theseshortexposuretimes,thechemicaldifferenceintheouter spinellayermustcausethedifferenceinASRvaluesforthetwo samplesystems.Areasonforthiscouldbethehigheramountof ironobservedintheouterspinellayer.Kieferetal.havereporteda decrease in conductivityfor MnCo(2-x)FexO4 when when x was morethan0.25,butreportedanincreasingconductivitywhenx was 0.1–0.25 [30]. Liu et al. observed a similar effect when investigatingaspinelofthesamecomposition,incontrasttothe previouspublication,onlydecreasingconductivitywasobserved when iron was added [23]. In a previous study on the same materials,anirontomanganeseratioofabout1:1wasseenforthe cobaltcoatedsamples[9].Basedonthetwopreviousmentioned papers,thiswouldleadonlytoadecreaseoftheconductivityofthe cobaltmanganesespinel.Cerium/cobaltcoatedsamplescontained only very small amounts of iron (below the quantitive EDX detectionlimit).For this reason,itis possiblethattheoutward diffusionof iron caused thedifference in conductivity.Another possibilitycouldbeaneffectofceriumdoping,whichcouldhave thepotentialtoimprovetheconductivityofthespineloxidelayer of the cerium/cobalt coated samples, even though there is no literatureavailabletosupportthattheory.
If the ASR values after longer exposure times are mainly dominatedbythegrowingchromialayer,assuspectedaboveand, whichcanbealsoseenintheplotoftheevolutionoftheactivation energy(Fig.5),thebeneficialeffectofabetterconductingouter spinelbecomesnegligible.Therefore,thetechniqueofimpedance spectroscopywasusedtoinvestigatetheinfluenceofbothoxide layersafterlongerexposuretimes.Sincethistechniqueworksbest athighresistance and capacitances,temperatures below200C werechosenforthecharacterization.Thetwosemicirclesshownin
Fig.6couldbeobservedforbothtestedsamplesinatemperature range from 200–60C, which resulted in two slopes for each sample in the derived Arrhenius plot. The difficulty with
Fig.4.MassgainofindividualsamplesplottedagainstASRvalues.
impedancespectroscopyisassigningtheindividualsemi-circlesto the individual oxide scales. In the investigatedcase, two ways seemedtobeappropriatetoresolvethisproblem.Theactivation energies and/orthepeak frequenciescouldbe usedtolinkthe conductivitiestothetwooxidelayers.Sincethepeakfrequencyof anRCelement(orinthiscaseR/CPEelement)isproportionaltothe inverse oftheproductofcapacitanceand resistance andasthe cobaltmanganesespinellayerismuchthinnerandthushaslikelya highercapacitance,butbothoxidelayershavesimilarresistances at these low temperatures, it can be assumed that the low frequencyarcwascausedbythecobaltmanganesespinellayer. Another argument for this conclusion is the higher activation energyobservedforthelowfrequencyarc.Thesampleswithvery shortexposuretimeshadanactivationenergyof0.6–0.55eV,and, as discussed earlier, this was caused by the cobalt manganese spineloxide layer. Thelow frequency arc in both cases(cobalt coatedandcerium/cobaltcoated)alsohadanactivationenergyin the range of 0.50-0.55eV,and, based on this argument, would representthecobaltmanganesespinellayer.Thehigheramountof ironintheouterspinelofthecobaltcoatedsampleisnotexpected todrasticallychangetheactivationenergy,whichisinagreement toLiuetal.[23].Thusbothwaysofassigningthesemi-circlestothe individualoxidescalesresultinthefactthatthelowfrequencyarc iscausedbythecobaltmanganesespinellayer.However,itcannot beexcludedthatothereffects,suchasgrainboundaryandgrain conductivity of for example the chromia layer influenced the measurement. Remarkable for the plot in Fig. 6 is the higher resistivity of the cobalt coated sample, observed for the low frequency arc, when compared to the cerium/cobalt coated sample. This supports the observation that after very short exposure timestheresistanceof thespinellayerfor thecobalt coatedsampleswasfoundtobehigher.Thiswouldmeanthatthe conductivity of the spinel layer is improved over the entire exposuretimebytheadditionofcerium.
Since this investigation was performed at a temperature of 850C,thebeneficialeffectofceriumontheconductivityofthe outer spinel layer becomes rather small. The actual operation temperatureofasolidoxidefuelcellisoftenlower(600–800C) whichmightleadtoahigherimpactontotheoverallASRwhenthe samematerialswouldbeusedatlowertemperatures.Thechromia layeratlowertemperaturesgrowssignificantlylessatthesame lengthofexposureandis consideredtohavea loweractivation energy,thus,ahigherimpactoftheouterspinellayerontheASR can be expected [31]. Additionally, the use of a thicker cobalt manganese layer might be considered, since the investigated coatingthicknesswas640nm,whichisratherthincomparedto thecoatingsreportedinsimilarpublications[32].
Fig.6.Nyquistplot150Coftwosamplesexposedfor3300h.
Fig.7.Arrhenius plotoftwosamples exposedfor 3300hderivedfrom fitted Nyquistplotsincludingactivationenergies.
Table2
Impedancedataforcobaltcoatedsample. Temperature[C] R1[Vcm2
] C1[F] vpeak1[Hz] R2[Vcm2] C2[F] vpeak2[Hz] R3[V]
200 96 1,9E-09 5,5E+06 34 2,1E-07 1,4E+05 10
190 121 1,7E-09 4,9E+06 53 1,6E-07 1,2E+05 10
180 190 1,4E-09 3,8E+06 53 2,7E-07 7,0E+04 10
170 242 1,9E-09 2,2E+06 80 3,7E-07 3,4E+04 10
160 313 1,8E-09 1,8E+06 116 2,9E-07 3,0E+04 10
150 408 1,7E-09 1,5E+06 174 2,3E-07 2,5E+04 10
140 537 1,6E-09 1,2E+06 257 1,9E-07 2,0E+04 10
130 712 1,6E-09 8,9E+05 390 1,6E-07 1,6E+04 10
120 889 1,6E-09 7,2E+05 486 1,5E-07 1,3E+04 10
110 1206 1,5E-09 5,5E+05 833 1,1E-07 1,1E+04 10
100 1555 1,5E-09 4,3E+05 1175 9,6E-08 8,9E+03 10
90 2047 1,5E-09 3,3E+05 1859 7,4E-08 7,3E+03 10
80 3123 1,6E-09 2,0E+05 2823 8,2E-08 4,3E+03 10
70 5016 1,7E-09 1,2E+05 4062 1,1E-07 2,3E+03 10
5.Conclusion
Theelectricalperformanceof640nmcobaltand640nmcobalt incombinationwith10nmceriumonthestainlesssteelSanergy HT in an air environment at 850C was investigated. It was observed that cerium improved the corrosion properties of SanergyHTsubstantiallybyslowingdownthegrowthofchromia. Duetothethinnerchromialayer,theelectricalpropertiesofthe samplesweresignificantlybetterthanthosesamples,whichwere onlycobaltcoated.Moreover,theadditionofceriumpreventedthe outwarddiffusionofiron,whichprovedtobebeneficialforthe conductivityof theouterspineloxide. Impedancespectroscopy wassuccessfullyusedtorevealindividualelectricalpropertiesof
thetwooxidelayerspresentafterlongexposuretimes.TheASR valuesoftheinvestigatedsamplesystemsweredominatedbythe cobaltmanganesespinellayerforshortexposuretimes(lessthan 100h)and,afterlongertimes,bythegrowingandlessconductive chromialayer.Itwasfoundthatthecobaltmanganesespinellayer had anactivation energyfor theelectrical conduction of about 0.55eV,whereastheinnerchromialayerexhibitedanactivation energyofabout0.45eV.Attheinvestigatedtemperatureof850C, the effect of a betterconducting spinel layer onto the overall conductivitywasrathersmallforlongexposuretimes.Yet,amuch higherpositiveimpactwasexpected,whenthesekindsofcoatings areusedatlowertemperature,whereathinnerchromiascaleis expected.
Table3
Impedancedataforcerium/cobaltcoatedsample.. Temperature[C] R1[Vcm2
] C1[F] vpeak1[Hz] R2[Vcm2] C2[F] vpeak2[Hz] R3[V]
200 27 9,7E-09 3,9E+06 6 2,8E-06 6,1E+04 10
190 33 6,8E-09 4,5E+06 10 1,6E-06 6,1E+04 10
180 37 5,7E-09 4,8E+06 24 4,8E-07 8,5E+04 10
170 54 4,4E-09 4,2E+06 28 6,5E-07 5,6E+04 10
160 77 3,8E-09 3,4E+06 42 5,6E-07 4,3E+04 10
150 97 3,5E-09 2,9E+06 59 4,5E-07 3,8E+04 10
140 142 3,3E-09 2,2E+06 84 4,7E-07 2,5E+04 10
130 212 3,2E-09 1,5E+06 120 5,1E-07 1,6E+04 10
120 300 3,1E-09 1,1E+06 166 5,5E-07 1,1E+04 10
110 430 3,2E-09 7,3E+05 232 5,8E-07 7,4E+03 10
100 562 3,2E-09 5,5E+05 293 6,1E-07 5,6E+03 10
90 761 3,0E-09 4,4E+05 582 3,2E-07 5,4E+03 10
80 1163 3,0E-09 2,9E+05 903 3,1E-07 3,5E+03 10
70 1607 3,0E-09 2,1E+05 1427 2,7E-07 2,6E+03 10
60 2189 3,0E-09 1,5E+05 1869 2,4E-07 2,2E+03 10
Fig.8.SEMcross-sectionsofacobaltcoatedsample(left)andcobaltceriumcoatedsampleafter100hofexposureandASRcharacterization.
Acknowledgements
P.Alnegren,H.Falk-WindischandR.Sachitanandaregratefully acknowledged for proof-reading and valuable input during the discussionofthepaper.SandvikMaterialsTechnologyis acknowl-edgedforprovidingthesamples.Thefinancial supportreceived fromTheSwedishResearchCouncilandSwedishEnergyAgency (GrantAgreementNo 34140-1),TheSwedish HighTemperature Corrosion Centre as well as the Nordic NaCoSOFC project is gratefullyacknowledged.Furthermore,thefundingreceivedfrom theEuropeanUnion’sSeventhFrameworkProgramme (FP7/2007-2013)fortheFuelCellsandHydrogenJointTechnologyInitiative undergrantagreementn[278257]isthankfullyacknowledged. References
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