LowLow--Cr FeCr Fe--NiNi--Co Alloys as Interconnect for Co Alloys as Interconnect for IntermediateIntermediate--Temperature SOFCTemperature SOFC

J. H. Zhu, S. J. J. H. Zhu, S. J. GengGeng, Z. H. Bi, D. A. Ballard, and J. , Z. H. Bi, D. A. Ballard, and J. BateyBateyDepartment of Mechanical EngineeringDepartment of Mechanical Engineering

Tennessee Technological University, Cookeville, TNTennessee Technological University, Cookeville, TN

M. P. Brady and K. L. MoreM. P. Brady and K. L. MoreMaterials Science and Technology DivisionMaterials Science and Technology Division

Oak Ridge National Laboratory, Oak Ridge, TNOak Ridge National Laboratory, Oak Ridge, TN

H. U. AndersonH. U. AndersonElectronic Materials Applied Research CenterElectronic Materials Applied Research CenterUniversity of MissouriUniversity of Missouri--Rolla (UMR), Rolla, MORolla (UMR), Rolla, MO

SECA Core Technology Program Peer Review MeetingSECA Core Technology Program Peer Review MeetingSan Antonio, TX, Aug. 7 San Antonio, TX, Aug. 7 -- 9, 20079, 2007

Why CrWhy Cr--free or low Cr alloys as solid oxide free or low Cr alloys as solid oxide fuel cell (SOFC) interconnect material?fuel cell (SOFC) interconnect material?

Currently, the metallic interconnects for intermediateCurrently, the metallic interconnects for intermediate--temperature SOFC are the Crtemperature SOFC are the Cr22OO33--forming alloys such as forming alloys such as Ebrite, Crofer, and Haynes 230 due to the electrically Ebrite, Crofer, and Haynes 230 due to the electrically conductive nature of Crconductive nature of Cr22OO33 compared to Alcompared to Al22OO33 and SiOand SiO22. . However, However, an inherent weakness of Cr2O3-forming alloys is the formation of volatile Cr species due to chromium evaporation, which will migrate to and thus poison the cathode, resulting in SOFC performance degradation Two approaches can be taken to address this issue:

Surface coating approachAlloy design approach

Cr-free or low Cr Fe-Ni-Co alloys tailored for SOFC interconnect application might completely resolve the Cr poisoning issue in

SOFC stacks without the need of a surface coating.

Why FeWhy Fe--NiNi--Co Alloys as SOFC Interconnect?Co Alloys as SOFC Interconnect?

Features of FeFeatures of Fe--Ni and Ni and FeFe--NiNi--Co alloys:Co alloys:–– Low CTE due to the Low CTE due to the

““InvarInvar”” effecteffect–– No Cr or low Cr content No Cr or low Cr content

for lowering Cr volatilityfor lowering Cr volatility–– Poor oxidation Poor oxidation

resistance at SOFC resistance at SOFC operating temperatureoperating temperature

Potential exists for Potential exists for further development further development for SOFC interconnect for SOFC interconnect application.application.

0

2

4

6

8

10

0 200 400 600 800

Fe-50NiCathode LSMAnode Ni+YSZEletrolyte YSZ

Ther

mal

Exp

ansi

on, Δ

L/L

10-3

Temperature (oC)

Alloy Design of New FeAlloy Design of New Fe--NiNi--Co Base Alloys Forming Co Base Alloys Forming DoubleDouble--Layer Oxide Structure During OxidationLayer Oxide Structure During Oxidation

CrCr--free Outer Layer:free Outer Layer:(Ni,Fe,Co)(Ni,Fe,Co)33OO44

Inner Layer:Inner Layer:Protective OxidesProtective Oxides

Substrate: Substrate: FeFe--NiNi--Co Alloy Co Alloy

Schematic of ThermallySchematic of Thermally--Grown Grown DoubleDouble--Layer Oxide StructureLayer Oxide Structure

Provide low CrProvide low Cr--volatility surface sealvolatility surface seal

Provide oxidation Provide oxidation diffusion barrierdiffusion barrier

Provide CTE Provide CTE match with other match with other cell componentscell components

In addition to providing In addition to providing electron conduction path, electron conduction path, these attributes are desired:these attributes are desired:

Alloy Design Philosophy for the New Alloy Design Philosophy for the New FeFe--NiNi--Co Alloy SystemCo Alloy System

DoubleDouble--layer concept for layer concept for protecting the alloyprotecting the alloy

The alloy composition of FeThe alloy composition of Fe--25Ni25Ni--30Co30Co-- 6Cr6Cr--5Nb5Nb--1.5Si1.5Si--0.1Y was 0.1Y was designed to give an overall optimal designed to give an overall optimal property:property:

Co: further modify the alloy CTE Co: further modify the alloy CTE and the conductivity of the and the conductivity of the spinelspinelCr: improve the oxidation Cr: improve the oxidation resistance (increases the alloy resistance (increases the alloy CTE)CTE)NbNb: reduce the CTE of the alloy : reduce the CTE of the alloy and form and form NbNb--SiSi compoundcompoundSi: enhance oxidation resistance Si: enhance oxidation resistance and strengthen the alloy via the and strengthen the alloy via the formation of the formation of the NbNb--SiSi compoundcompound

Cr-free Outer Layer:(Ni,Fe,Co)3O4

Inner Layer:Cr2O3

Substrate: Fe-Ni-Co Alloy

Effect of Si (wt.%) on the Oxidation Behavior of Effect of Si (wt.%) on the Oxidation Behavior of Cast LowCast Low--Cr FeCr Fe--NiNi--Co Alloys in Air at 800Co Alloys in Air at 800°°CC

The mass gain of the alloys decreased with the increase The mass gain of the alloys decreased with the increase in Si content in the Fein Si content in the Fe--NiNi--Co alloysCo alloys

Oxidation kinetics of Oxidation kinetics of lowlow--Cr FeCr Fe--NiNi--Co alloys Co alloys with different Si levels with different Si levels

at 800at 800°°C in airC in air

240

0

5

10

15

20

25

0 200 400 600Time (week)

0.00 Si0.75 Si1.50 Si3.50 Si

Mas

s ga

in (m

g/cm

2 )

Time (hour)

Effect of Si on the Oxide Structure of Low-Cr Fe-Ni Alloys After Oxidation for 3 Weeks in Air at 800°C

100 μm 50 μm

Co3O4

(Fe,Ni,Co)3O4

Fe2O3

Co3O4

Fe2O3

Cr-rich oxide

0% Si 0.75% Si

A surface Co3O4 spinel layer over a (Fe,Ni,Co)3O4 layer was observed for these two alloys.A significant amount of internal oxidation was observed.

(Fe,Ni,Co)3O4

10 μm 5 μm

Cr-free (Fe,Co,Ni)3O4

Cr2O3 Cr2O3SiO2

Pt layer

Effect of Si on the Oxide Structure of FeEffect of Si on the Oxide Structure of Fe--NiNi--Co Co Alloys After Oxidation for 3 Weeks in Air at 800Alloys After Oxidation for 3 Weeks in Air at 800°°CC

1.5% Si 3.5% SiOnly (Fe,Ni,Co)Only (Fe,Ni,Co)33OO44 spinelspinel was observed for the alloy with was observed for the alloy with 1.5%Si, with a Cr1.5%Si, with a Cr22OO33 inner layerinner layerA continuous CrA continuous Cr22OO33 surface layer was formed for the alloy surface layer was formed for the alloy with 3.5%Si, with some SiOwith 3.5%Si, with some SiO22 at the at the SiSi--rich precipitates.rich precipitates.

Pt layer

Effect of Si on Scale ASR of LowEffect of Si on Scale ASR of Low--Cr FeCr Fe--NiNi--Co AlloysCo Alloys

-6

-5

-4

-3

-2

0.9 1.0 1.1 1.2 1.31000/T, K-1

Log

ASR

/T,O

hm·c

m2 K

-1 0.75Si1.5Si3.5SiCrofer22 APU

Scale ASR after 3 weeks @ 800Scale ASR after 3 weeks @ 800ººC, airC, air

The scale ASR increased The scale ASR increased with the Si content with the Si content

No continuous SiONo continuous SiO22formation formation -- even with even with 3.5%Si addition, the 3.5%Si addition, the scale ASR was still scale ASR was still relatively low.relatively low.

The alloy with 1.5%Si The alloy with 1.5%Si exhibited lower scale exhibited lower scale ASR than was ASR than was CroferCrofer 22 22 APU.APU.

The alloy with 1.5%Si is selected as for further evaluation.The alloy with 1.5%Si is selected as for further evaluation.

Isothermal Oxidation Kinetics of the LowIsothermal Oxidation Kinetics of the Low--Cr Cr FeFe--NiNi--Co Based AlloyCo Based Alloy in Air at 800in Air at 800°°CC

After the 1After the 1stst--week exposure, the oxidation rate of the Feweek exposure, the oxidation rate of the Fe--NiNi--Co Co alloy was similar to that of alloy was similar to that of CroferCrofer 22 APU. 22 APU. Large weight gain in the 1Large weight gain in the 1stst week for the Feweek for the Fe--NiNi--Co alloy was Co alloy was due to the due to the inin--situsitu formation of a Crformation of a Cr--free spinel layer.free spinel layer.

0

2

4

6

8

10

0 2 4 6 8 10 12 14Time (weeks)

Mas

s ga

in (m

g/cm

2 )

2016 hrs

Crofer 22 APU

Fe-60Ni

Fe-Ni-Co Alloy

CrossCross--Sections of the LowSections of the Low--Cr FeCr Fe--NiNi--Co Based Co Based Alloy after OxidationAlloy after Oxidation for 3 Weeks for 3 Weeks at 800at 800°°C in AirC in Air

10 μm

Pt layerCr-free Spinel layer

Cr2O3

The oxide scale formed on this alloy consisted of a CrThe oxide scale formed on this alloy consisted of a Cr--free free spinel outer layer and a finespinel outer layer and a fine--grained Crgrained Cr22OO33 inner layer inner layer An internal oxidation zone with mainly CrAn internal oxidation zone with mainly Cr22OO33, NbCrO, NbCrO44, , NbNb--NiNi--CoCo--SiSi--OO, etc. , etc. No continuous SiONo continuous SiO22 layer was detected.layer was detected.

STEMSTEM

(Fe,Ni,Co)3O4 Layer

Interface

Fine-Grained Cr2O3

SEMSEM

CoCo0.840.84NiNi0.270.27FeFe1.891.89OO44 possessed similar electrical conductivity as Ni0.9Fe2.1O4

NiNi0.90.9FeFe2.12.1OO44 similar similar to the spinel to the spinel formed on Feformed on Fe--Ni Ni alloysalloysMnMn1.51.5CrCr1.51.5OO4 4 similar similar to the spinel to the spinel formed on Crofer formed on Crofer (Fe(Fe--CrCr--MnMn))CoCo0.840.84NiNi0.270.27FeFe1.891.89OO44similar to the spinel similar to the spinel formed on the new formed on the new FeFe--NiNi--Co alloyCo alloy-2

0

2

4

6

8

10

7 8 9 10 11 12 13

104/T, K-1

Single crystal Cr2O

3

Mn1.5

Cr1.5

O4

Ni0.9

Fe2.1

O4

Ln(σ

T),

S.cm

-1.K

-2

0

2

4

6

8

10

7 8 9 10 11 12 13

104/T, K-1

Single crystal Cr2O

3

Mn1.5

Cr1.5

O4

Ni0.9

Fe2.1

O4

Ln(σ

T),

S.cm

-1.K

104/T, K-1

ln(σ

T), S

.cm

-1.K

Cr2O3

Ni0.9Fe2.1O4

Mn1.5Cr1.5O4

(Co,Ni,Fe)3O4

Scale ASR of the Low Cr FeScale ASR of the Low Cr Fe--NiNi--Co Based Co Based Alloy after Oxidation at 800Alloy after Oxidation at 800°°C in AirC in Air

-6-5.5

-5-4.5

-4-3.5

-3-2.5

-2

0.8 1 1.2 1.41000/T, K-1

log

ASR

/T, o

hmcm

2 K-1

ASR vs. 1/T after 3ASR vs. 1/T after 3--week Exposureweek Exposure ASR at 800ASR at 800°°C vs. Oxidation TimeC vs. Oxidation Time

The scale ASR for the FeThe scale ASR for the Fe--NiNi--Co alloy after 3Co alloy after 3--week week oxidation was lower than that for oxidation was lower than that for CroferCrofer 22 APU22 APUThe scale ASR was quite stable upon further oxidationThe scale ASR was quite stable upon further oxidation

Crofer 22 APU

Fe-Ni-Co Alloy

0

5

10

15

20

25

30

0 10 20 3Time, week

ASR

, mΩ

•cm

2

Crofer 22 APU

Fe-Ni-Co Alloy

Cyclic Oxidation of the LowCyclic Oxidation of the Low--Cr FeCr Fe--NiNi--Co Alloy Co Alloy

The good scale The good scale spallationspallation resistance resulted from the resistance resulted from the excellent match in CTE of the oxide scale and the substrate excellent match in CTE of the oxide scale and the substrate

0

5

10

15

20

25

30

35

0 20 40 60 80 100Cycles

Wei

ght c

hang

e (m

g/cm

2)

Fe-Ni-Co

Fe-50Ni

80 cycles

Oxidation kinetics of two alloys after 80 cycles Oxidation kinetics of two alloys after 80 cycles (25(25--h cycle, 800h cycle, 800°°C, air cooling)C, air cooling)

Excellent Match in Thermal Expansion Between the Excellent Match in Thermal Expansion Between the FeFe--NiNi--Co Alloy and (Fe,Ni,Co)Co Alloy and (Fe,Ni,Co)33OO44 SpinelSpinel

The (The (Fe,Ni,Co)Fe,Ni,Co)33OO44 spinelspinel layer formed on the Felayer formed on the Fe--NiNi--Co alloys Co alloys is expected to resist cracking during thermal cycling is expected to resist cracking during thermal cycling

0

2

4

6

8

10

0 200 400 600 800

Anode Ni+YSZFe-Ni-Co Alloy(Fe,Ni,Co)3O4 Mn1.5Cr1.5O4

Ther

mal

Exp

ansi

on Δ

L/L,

10-3

Temperature, oC

Cr volatility of the FeCr volatility of the Fe--NiNi--Co alloy was much Co alloy was much lower than that of lower than that of CroferCrofer 22 APU22 APU

FeFe--NiNi--Co AlloyCo Alloy

CroferCrofer

800°C x 500h, moist air, velocity of 1.1 cm/s

•• The CrThe Cr--free free spinelspinel outer layer formed on the Feouter layer formed on the Fe--NiNi--Co alloy acts Co alloy acts as a surface seal to effectively block the Cr evaporationas a surface seal to effectively block the Cr evaporation

(PNNL)

InIn--Cell Testing Results with the FeCell Testing Results with the Fe--NiNi--Co Co Alloy InterconnectAlloy Interconnect

Cell performance degradation rate with the FeCell performance degradation rate with the Fe--NiNi--Co alloy Co alloy interconnect was similar to interconnect was similar to CroferCrofer 22 APU at 80022 APU at 800°°C initially.C initially.PreoxidationPreoxidation at 800at 800°°C in air for 120 hours significantly reduced C in air for 120 hours significantly reduced the cell degradation due to the formation of the the cell degradation due to the formation of the spinelspinel layer.layer.

Cell Configuration:Alloy/Pt/LSM/LSM+YSZ/YSZ/Ni+YSZ

Test Condition: 800ºC at 0.7V

50 sccm Moist H2750 sccm AirInterconnect:1.2mmx1.2mm

2.5cm2 active area0

0.1

0.2

0.3

0.4

0 50 100 150 200

Bare Fe-Ni-Co AlloyOxidized Fe-Ni-Co AlloyCrofer 22 APU

Pow

er D

ensi

ty (W

/cm

2 )

Time (hour)

20

40

60

80

100

120

0 200 400 600 800 1000

Elongation, New AlloyElongation, Crofer

Duc

tility

(% E

long

atio

n)

Temperature (oC)

Tensile Properties of the New Fe-Ni-Co Alloy (Hot- + Cold-Rolled, 1000°Cx1h, 800°Cx10h)

The ductility of the new alloy was similar to Crofer 22 APU up to 600°C and slightly lower in the range of 600-900°C.The new alloy exhibited a much higher yield strength and tensilestrength over the entire test temperature range.

0

200

400

600

800

1000

0 200 400 600 800 1000

Yield Strength, New AlloyYield Strength, CroferTensile Strength, New AlloyTensile Strength, Crofer

Stre

ngth

(MPa

)Temperature (oC)

+

+Data for Crofer 22 APU was from R. Hojda & L. Paul, NACE Paper No. 6479, Corrosion 2006.

Current/Future Research DirectionsCurrent/Future Research Directions

Alloy Development Alloy Development Further Optimization of Alloy CompositionsFurther Optimization of Alloy CompositionsEvaluation of Other Properties (Forming, Creep, etc.)Evaluation of Other Properties (Forming, Creep, etc.)

Coating Development for Cost ReductionCoating Development for Cost ReductionElectroplating of the FeElectroplating of the Fe--NiNi--Co Alloy on Ferritic Steels Co Alloy on Ferritic Steels as a Precursor for Synthesizing the (Fe,Ni,Co)as a Precursor for Synthesizing the (Fe,Ni,Co)33OO44SpinelSpinel CoatingCoatingOther Coating Processes such as Cladding, Screen Other Coating Processes such as Cladding, Screen Printing, etc.Printing, etc.

Electroplating of the FeElectroplating of the Fe--NiNi--Co Alloy on Ferritic Co Alloy on Ferritic Steels to Achieve the Steels to Achieve the SpinelSpinel CoatingsCoatings

(Ni,Fe,Co)(Ni,Fe,Co)33OO44

Fe-Cr Steel

CrCr22OO33

Ag Paste

NiFe2O4

Cr2O3

•• CodepositionCodeposition of a Niof a Ni--FeFe--Co Co alloy layer on the ferritic steels alloy layer on the ferritic steels via electroplatingvia electroplating

•• Oxidation in air can be used to Oxidation in air can be used to convert the alloy into the spinel convert the alloy into the spinel layer with the simultaneous layer with the simultaneous formation of a Crformation of a Cr22OO33 subsub--layerlayer 5 μm

Plated FePlated Fe--NiNi--Co alloyCo alloy

Fe-Cr Steel

Thermal Oxidation

Ebrite

TTUTTU’’ss Electroplating Process: High Stability (>30 days, Electroplating Process: High Stability (>30 days, > 30 runs); Accurate Control of Coating Composition. > 30 runs); Accurate Control of Coating Composition.

Electroplating: 20mA/cmElectroplating: 20mA/cm22, 40min, 40minThermal oxidation: 800Thermal oxidation: 800°°Cx1weekCx1week

InIn--Cell Testing Results with Electroplated Cell Testing Results with Electroplated CroferCrofer 22 APU Interconnect22 APU Interconnect

•• The electroplated FeThe electroplated Fe--NiNi--Co layer on Co layer on CroferCrofer 22 APU significantly 22 APU significantly improved the cell stability, indicating its effectiveness in improved the cell stability, indicating its effectiveness in blocking the Cr migration.blocking the Cr migration.

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200

Pow

er D

ensi

ty (W

/cm

2 )

Time (hour)

Electroplated Crofer

PreoxidizedFe-Ni-Co Alloy

Bare Crofer

Bare Fe-Ni-Co Alloy

SummarySummary

A series of lowA series of low--Cr FeCr Fe--NiNi--Co alloys have been developed Co alloys have been developed that form a conductive, Crthat form a conductive, Cr--free spinel outer layer atop free spinel outer layer atop

an Cran Cr22OO33 inner layer during thermal oxidation.inner layer during thermal oxidation.

CTE (CTE (√√))The CTE of these alloys matches other cell components.The CTE of these alloys matches other cell components.Oxidation Resistance (Oxidation Resistance (√√))Effective alloying elements have been identified that Effective alloying elements have been identified that significantly improve oxidation resistance of these alloys.significantly improve oxidation resistance of these alloys.ASR (ASR (√√))The ASR of the oxide scales formed on the new alloys is The ASR of the oxide scales formed on the new alloys is comparable to that of current interconnect alloys.comparable to that of current interconnect alloys.Cr Volatility (Cr Volatility (√√))The Cr transport rate for the new alloys is much lower than The Cr transport rate for the new alloys is much lower than that of that of CroferCrofer 22 APU.22 APU.

AcknowledgementsAcknowledgements

This research was sponsored by the DOE SECA This research was sponsored by the DOE SECA Core Technology Program (DOE Award #DECore Technology Program (DOE Award #DE--FC26FC26--04NT42223). 04NT42223). Thanks are due to Drs. Lane Thanks are due to Drs. Lane Wilson and Wilson and AyyakkannuAyyakkannu ManivannanManivannan at NETL for at NETL for helpful discussions regarding this work. helpful discussions regarding this work.

Additional support was provided by the Center Additional support was provided by the Center for Manufacturing Research, Tennessee for Manufacturing Research, Tennessee Technological University.Technological University.

Gary Yang, X.D. Zhou, Jeff Stevenson, and Gary Gary Yang, X.D. Zhou, Jeff Stevenson, and Gary Maupin, PNNL, for Cr transport rate Maupin, PNNL, for Cr transport rate measurement.measurement.