Materials for the Hydrogen Economy (2007) Episode 9 pot

Journal of Materials Science, 2001,.. char-acteristics.of.BICUVOX.1.solid.electrolyte,.Journal of Materials Science, 2006,.41,.. Journal of Solid State Electrochemistry, 2001,... a.new.o

An oxygen ion conductor can be tailored because of the geometrical and

chemi-cal flexibility of the perovskite structure This is borne out by (La,Sr)(Mg,Ga)O3

(1) Ln(Al,In,Sc,Y)O3-based materials, (2) the doped and undoped brownmillerite

Ba2In2O5,.and.(3).La2Mo2O9 The.transference.number.of.doped.La2Mo2O9.can.be

higher than 0.99 in an oxidant environment The drawbacks of La2Mo2O9-based

has been reported in the literature, however, considerable interdiffusion between.

ZrO2- and CeO2-based materials occurs at elevated temperatures (>1,400°C).58–61

Solid.solutions.between.ZrO2.and.CeO2

5.15 5.20 5.25 5.30 5.35 5.40

YSZ (111) CGO

fIgure 0.   A plot of d spacing for CGO films on YSZ substrates as a function of

anneal-ing temperature The inset illustrates the XRD of CGO and YSZ powders annealed at various

temperatures 62

by.plotting.ln(σT).versus.1/T.for.CGOxYSZ1–x

.at.high.oxygen.activity;.the.preex-ponential factor,.σ_, appears to be independent of the CGO ratio Therefore, the

observed decrease in conductivity, in particular at the intermediate-temperature

Kosacki et al.67–69,75 studied the electrical conductivity of yttria- and

scandia-doped zirconia thin films deposited onto either single-crystal alumina or

magne-sia substrates Their study showed that the electrical conductivity of YSZ can be

-2.5 -2.0 -1.5 -1.0 -0.5

25mol% CGO 50mol% CGO 75mol% CGO CGO YSZ

10.4.5 G rain b Oundary e FFeCtS

fIgure 0.   Relative expansion of GDC10 and GDC 20 (After Yasuda, I and Hishinuma,

M., Electrochem Soc Proc., 97, 178, 1997.)

500 o C

in an oxidizing and reducing environment Doped CeO2, on the other hand, is a.

of Applied Ceramic Technology, 2004,.1,.5–15.

3 Steele, B.C.H., Material science and engineering: the enabling technology for the.

commercialisation of fuel cell systems, Journal of Materials Science, 2001, 36,.

Alcaide,.F.,.Cabot,.P.L.,.and.Brillas,.E.,.Fuel.cells.for.chemicals.and.energy.cogenera-tion,.Journal of Power Sources, 2006,.153,.47–60.

13 Paydar, M.H., Hadian, A.M., and Fafilek, G., A new look at oxygen pumping

char-acteristics.of.BICUVOX.1.solid.electrolyte,.Journal of Materials Science, 2006,.41,.

1953–1957.

14 Kharton,.V.V.,.Naumovich,.E.N.,.Yaremchenko,.A.A.,.and.Marques,.F.M.B.,.Research.

on the electrochemistry of oxygen ion conductors in the former Soviet Union IV

Bismuth oxide-based ceramics, Journal of Solid State Electrochemistry, 2001, 5,.

18 Yang,.J.H.,.Wen,.Z.Y.,.Gu,.Z.H.,.and.Yan,.D.S.,.Ionic.conductivity.and.micro.structure.

of.solid.electrolyte.La 2 Mo 2 O 9.prepared.by.spark-plasma.sintering,.Journal of the

Euro-pean Ceramic Society, 2005,.25,.3315–3321.

23 Takamura,.H and.Tuller,.H.L.,.Ionic.conductivity.of.Gd2GaSbO7-Gd2Zr2

O7.solid.solu-tions.with.structural.disorder,.Solid State Ionics, 2000,.134,.67–73.

compounds,.Solid State Ionics, 1992,.52,.135–146.

29 Sansom, J.E.H., Najib, A., and Slater, P.R., Oxide ion conductivity in mixed

Si/Ge-based.apatite-type.systems,.Solid State Ionics, 2004,.175,.353–355.

30 Yaremchenko, A.A., Shaula, A.L., Kharton, V.V., Waerenborgh, J.C., Rojas, D.P.,.

Patrakeev, M.V., and Marques, F.M.B., Ionic and electronic conductivity of La9.83

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31

Arachi,.Y.,.Sakai,.H.,.Yamamoto,.O.,.Takeda,.Y.,.and.Imanishai,.N.,.Electrical.conduc-tivity.of.the.ZrO2-Ln(2)O(3).(Ln.=.lanthanides).system,.Solid State Ionics, 1999,.121,.

133–139.

32

Steele,.B.C.H.,.Materials.for.IT-SOFC.stacks.35.years.R&D:.the.inevitability.of.gradu-alness?.Solid State Ionics, 2000,.134,.3–20.

33 Steele,.B.C.H.,.Appraisal.of.Ce 1–y Gd y O 2–y/2 electrolytes.for.IT-SOFC.operation.at.500.

degrees.C,.Solid State Ionics, 2000,.129,.95–110.

34 Steele,.B.C.H.,.Oxygen-transport.and.exchange.in.oxide.ceramics Journal of Power

Sources, 1994,.49,.1–14.

35 Inaba,.H and.Tagawa,.H.,.Ceria-based.solid.electrolytes:.review,.Solid State Ionics,

1996,.83,.1–16.

36 Shannon, R.D., Revised effective ionic-radii and systematic studies of interatomic.

distances.in.halides.and.chalcogenides,.Acta Crystallographica Section A, 1976,.32,.

751–767.

37 Azad,.A.M.,.Larose,.S.,.and.Akbar,.S.A.,.Bismuth.oxide-based.solid.electrolytes.for.

fuel-cells,.Journal of Materials Science, 1994,.29,.4135–4151.

38 Wachsman,.E.D.,.Effect.of.oxygen.sublattice.order.on.conductivity.in.highly.defective.

fluorite.oxides,.Journal of the European Ceramic Society, 2004,.24,.1281–1285.

39 Wachsman, E.D., Functionally gradient bilayer oxide membranes and electrolytes,.

Solid State Ionics, 2002,.152,.657–662.

45 Boyapati, S., Wachsman, E.D., and Chakoumakos, B.C., Neutron diffraction study.

of occupancy and positional order of oxygen ions in phase stabilized cubic bismuth.

oxides,.Solid State Ionics, 2001,.138,.293–304.

52 Ishihara, T., Matsuda, H., and Takita, Y., Doped LaGaO3 perovskite-type oxide as.

a.new.oxide.ionic.conductor,.Journal of the American Chemical Society, 1994,.116,.

58 Eguchi,.K.,.Akasaka,.N.,.Mitsuyasu,.H.,.and.Nonaka,.Y.,.Process.of.solid.state.reaction.

between.doped.ceria.and.zirconia,.Solid State Ionics, 2000,.135,.589–594.

59 Lee, C.H and Choi, G.M., Electrical conductivity of CeO2-doped YSZ, Solid State

Ionics, 2000,.135,.653–661.

60 Tsoga, A., Naoumidis, A., and Stover, D., Total electrical conductivity and defect.

structure of ZrO2-CeO2-Y2O3-Gd2O3 solid solutions, Solid State Ionics, 2000, 135,.

ionic-conductivity.in.cerium.dioxide,.Solid State Ionics, 1983,.8,.109–113.

64 Kilner,.J.A.,.Fast.oxygen.transport.in.acceptor.doped.oxides,.Solid State Ionics, 2000,.

Balducci,.G.,.Kaspar,.J.,.Fornasiero,.P.,.Graziani,.M.,.Islam,.M.S.,.and.Gale,.J.D.,.Com-puter.simulation.studies.of.bulk.reduction.and.oxygen.migration.in.CeO2-ZrO2.solid.

solutions,.Journal of Physical Chemistry B, 1997,.101,.1750–1753.

69 Kosacki, I., Suzuki, T., Petrovsky, V., and Anderson, H.U., Electrical

conductiv-ity of nanocrystalline ceria and zirconia thin films, Solid State Ionics, 2000, 136,.

1225–1233.

70 Zhang,.Y.W.,.Jin,.S.,.Yang,.Y.,.Li,.G.B.,.Tian,.S.J.,.Jia,.J.T.,.Liao,.C.S.,.and.Yan,.C.H.,.

Electrical conductivity enhancement in nanocrystalline (RE 2 O 3 )(0.08)(ZrO 2 )(0.92).

(RE.=.Sc,.Y).thin.films,.Applied Physics Letters, 2000,.77,.3409–3411.

71 Knoner, G., Reimann, K., Rower, R., Sodervall, U., and Schaefer, H.E., Enhanced.

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Proceed-ings of the National Academy of Sciences of the United States of America, 2003,.100,.

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72 Mondal, P and Hahn, H., Investigation of the complex conductivity of

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73 Jiang, S.S., Schulze, W.A., Amarakoon, V.R.W., and Stangle, G.C., Electrical

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74 Guo,.X.,.Vasco,.E.,.Mi,.S.B.,.Szot,.K.,.Wachsman,.E.,.and.Waser,.R.,.Ionic.conduction.

in.zirconia.films.of.nanometer.thickness,.Acta Materialia, 2005,.53,.5161–5166.

75 Kosacki, I., Petrovsky, V., and Anderson, H.U., Band gap energy in nanocrystalline.

ZrO2:.16%Y.thin.films,.Applied Physics Letters, 1999,.74,.341–343.

76

Mogensen,.M.,.Sammes,.N.M.,.and.Tompsett,.G.A.,.Physical,.chemical.and.electro-chemical.properties.of.pure.and.doped.ceria,.Solid State Ionics, 2000,.129,.63–94.

77 Yasuda, I and Hishinuma, M., Electrical conductivity, dimensional instability and.

internal.stresses.of.CeO2-Gd2O3.solid.solutions,.Electrochemical Society Proceedings,

Protection of Metallic Interconnects in Solid-Oxide Fuel Cells

Zhenguo Yang, Jeffry W Stevenson,

and Prabhakar Singh

ConTenTs

11.1 Introduction 229

11.2 Corrosion.of.Oxidation-Resistant.Alloys.under.SOFC.Interconnect Exposure.Conditions 232

11.2.1.Oxidation.and.Corrosion.at.Metal–Gas.Interfaces 232

11.2.1.1 Oxidation.in.Air,.Cathode-Side.Environment 233

11.2.1.2.Oxidation.and.Corrosion.in.Fuel,.Anode-Side Environment 233

11.2.1.3.Oxidation/Corrosion.under.Air–Fuel.Dual-Exposure Conditions 235

11.2.2.Corrosion.at.Interfaces.with.Adjacent.Components 239

11.3 Surface.Modification.for.Improved.Stability 241

11.4 Summary 245

References 245

Energy.security.and.increased.concern.over.environmental.protection.have.spurred

a.dramatic.worldwide.growth.in.research.and.development.of.fuel.cells,.which.elec-trochemically.convert.incoming.fuel.into.electricity.with.no.or.low.pollution Fuel

cell.technology.has.become.increasingly.attractive.to.a.number.of.sectors,.including

utility,.automotive,.and.defense.industries Among.the.various.types.of.fuel.cells,

solid-oxide.fuel.cells.(SOFCs).operate.at.high.temperature.(typically.650.to.1,000°C)

and have advantages in terms of high conversion efficiency and the flexibility of

using hydrocarbon fuels, in addition to hydrogen.1–5 The high-temperature

opera-tion, however, can lead to increased mass transport and interactions between the

surrounding environment and components that are required to be stable during a

lifetime.of.thousands.of.hours.and.up.to.hundreds.of.thermal.cycles For.stacks.with

relatively.low.operating.temperatures.(<800ºC),.the.interconnects.that.are.used.to

} PEN

Ai r or O2, H2O

Seals

Electricalcontacts

H2, CO, H2O, CO2

Repeating Unit-PEN

chromite parts at reasonable sintering temperatures,1,6–9 and the tendency of the.

chromite interconnect to partially reduce at the fuel gas–interconnect interface,

causing.the.component.to.warp.and.the.peripheral.seal.to.break.1,10.The.recent.trend

in developing lower-temperature (650 to 800°C), more cost-effective cells that

utilize anode-supported, thin electrolytes11,12 or new electrolytes with improved

and.silica.(SiO2).are.electrically.insulating,21,22.alloys.that.form.a.semiconductive

chromia.scale.(with.a.conductivity.of.~1.0.×.10–2.S-cm–1at.800°C.in.air21,23–25).are

NiB SA

NiB SA

CrBA

CrBA: base alloys

Cr-FSS: Ferritic

ASS: Austenitic stainless steels stainless steels

FeBSA: Fe-Ni base superalloys

NiBSA: Ni-Fe base superalloys

NiB SA

NiB SA

CrBA

fIgure .   Schematic of alloy options for SOFC applications.

. CorrosIon of oxIdaTIon-resIsTanT alloys

under sofC InTerConneCT exPosure CondITIons

TeC (0–·k–

oxidation resistance

19.0(RT.

800°C)

studies have also been performed to determine the oxidation/corrosion behavior.

found in air, although their morphology and minor components can be different

found that in carbon-bearing gas environments, alloys, including Fe(-Ni)-Cr- and

Ni-Fe-Cr-base alloys, are susceptible to metal dusting at temperatures in the 400

Recently, several publications reported and discussed the danger of encountering

carbon-induced corrosion for oxidation-resistant alloys under SOFC interconnect

exposure conditions at the anode side.59,60 Overall, it appears that metal dusting

is likely to occur in a hydrocarbon fuel with a carbon activity of ≥1 Toh et al.60

reported.metal.dusting.of.some.selected.oxidation-resistant.alloys.tested.in.CO–26%

H2–6%.H2

O.(vol%),.corresponding.to.a.carbon.activity.of.2.9,.at.650°C.under.ther-mal cycling The resistance to metal dusting depended on alloy composition For

fIgure .   SEM cross-sections of AISI 430 coupons after 300 h of oxidation at 800°C

in air under different exposure conditions: (a) both sides exposed to air and (b) on the air side

of the air–(H 2 + 3% H 2 O) exposure 64

and increased water vapor partial pressure on the air side E-brite, with 27% Cr,.

appeared.to.be.more.resistant.to.formation.of.hematite.nodules.at.800ºC.in.the.scale

grown.on.the.air.side.of.the.air–hydrogen.sample,.though.the.surface.microstruc-ture of the scale was different from the air-only sample At higher temperatures

(900°C),.Meier.et.al.96.observed.iron.oxide.formation.in.the.scale.grown.on.the.air

side of E-brite during air–hydrogen dual exposures Similar anomalous oxidation

behavior.was.also.observed.by.Ziomek-Moroz.et.al.68.and.Holcomb.et.al.69.not.only

exposure But unlike the ferritic chromia-forming alloys, nickel and Ni-Cr-base

alloys formed a uniform, well-adherent scale on the air side of the air–hydrogen

fIgure . Microstructures of cross-sections of silver tube walls after testing at 700°C

for 100 h: (a) with flow of (H2  + 3% H2 O) and (b) with flow of air 66

ponents.involves.rigid.glass–ceramic.seals,.including.those.made.from.barium–cal-cium–aluminosilicate (BCAS) base glasses.102–106 Previous work107–109 found that.

ferritic stainless steel interconnect candidates reacted extensively with the BCAS

been reported to react with interconnect alloys Reaction between manganites.

and chromia-forming alloys led to formation of a manganese-containing

spi-nel interlayer that appeared to help minimize the contact ASR.115–117 Sr in the

BaCrO 4

C-C

446

Glass ceramics

BaCrO 4

C-C

fIgure .   Interfacial reactions between G18 sealing glass and 446 stainless steel (a) A

schematic of the joined couple (446/G18/446) and SEM images of the interfacial cross-section

(b) at the edge area A, (c) at the interior region, and (d) from the region marked C in (b) The

446 coupons (12.7 × 12.7 × 0.5 mm) were joined to the G18 through heat treatment at 850°C

for 1 h, followed by 750°C for 4 h in air 107

Chromia forming alloy

O

M O

Conductive coat Sub-scale

Chromia forming alloy

O

M O

fIgure .   Schematic of mass transport in a conductive oxide protection layer on a

chro-mia-forming alloy.

conductive This conductivity requirement differentiates the interconnect

protec-tion layer from many traditional surface modifications as well as nonactive areas

Early reported examples of protection layers include overlay coatings of the

conductive perovskite compositions that are often used as cathode and

intercon-nect.materials.in.SOFCs For.example,.Linderoth122.and.Sakai.et.al.123.reported.the

effectiveness.of.a.(La,Sr)CrO3.protection.layer.on.Ducralloy.Cr5FeY2O3

.in.improv-ing.its.electrical.performance.and.surface.stability Kadowaki.et.al.124.found.that

(La,Sr)CoO3.protection.layers.fabricated.via.low-pressure.plasma.spray.on.Ni-Cr

base.alloys.effectively.improved.the.alloy.interconnect.electrical.conductivity In

contrast,.Batawi.et.al.125.evaluated.the.performance.of.(La,Sr)CrO3,.(La,Sr)CoO3,

and.(La,Sr)MnO3.protection.layers.thermally.sprayed.onto.both.Cr5FeY2O3

.and.Ni-Cr.base.alloys,.indicating.that.all.coatings.increased.the.alloy.oxidation.rate As

pointed.out.by.the.authors,.the.(La,Sr)CoO3.coatings.were.ineffective.because.of

rapid.diffusion.of.chromium.through.the.coatings.and.formation.of.thick.interfacial

reaction.layers,.while.the.(La,Sr)MnO3.protection.layers.on.Cr5FeY2O3.exhibited

the best performance due to the sluggish kinetics of interlayer growth and slow

diffusion of chromium through the coatings Quadakkers et al.117 also observed

significant transport of chromium into plasma-sprayed (La,Sr)CoO3 coatings on

Cr5FeY2O3 alloy Recent work by Fujita et al.126 found that (La,Sr)CoO3

protec-tion.layers.spin.coated.onto.ferritic.stainless.steels.AISI.430.and.ZMG.232.helped

improve.the.alloy.interconnect.surface.stability.and.cell.performance.by.reducing

chromium.poisoning Overall,.it.appears.that.the.chromites,.which.exhibit.a.lower