SECA Core Technology Program - PNNL:SOFC Component Materials DevelopmentSECA Core Technology Program - PNNL:SOFC Component Materials Development

Jeff StevensonPacific Northwest National LaboratoryRichland, WA 99352

Presented at the SECA CTP Review MeetingSacramento, CA, Feb. 19, 2003

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SECA Core Technology Program - PNNL:SOFC Component Materials DevelopmentSECA Core Technology Program - PNNL:SOFC Component Materials Development

Program Scope:Intermediate Temperature Cathode Materials DevelopmentAdvanced Anode Materials DevelopmentMetallic Interconnect Materials Evaluation and DevelopmentSOFC Stack Seal Development

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Presentation OutlinePresentation OutlineIntermediate Temperature Cathode Materials DevelopmentMetallic Interconnect Materials Evaluation and Development

For each task:ObjectivePrevious StatusResultsFuture Work

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Intermediate Temperature Cathode Materials Development

Intermediate Temperature Cathode Materials Development

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Cathode Materials DevelopmentCathode Materials DevelopmentObjective: Develop and optimize high performance, stable cathode materials for intermediate temperature SOFC

Variables:Base composition, type and amount of dopantInitial particle size distribution (calcination and milling conditions), fugitive phasesSintering temperature and time

Approach:Synthesis (glycine-nitrate) and characterization of candidate cathode powders (XRD, dilatometry, SEM, PSA, TGA, electrical conductivity)Fabrication of cathodes on anode-supported membranes via screen printing and sinteringEvaluation of cathode performance by electrochemical testing andSEM

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Sr-Doped LaFeO3 Cathode Development

Advantages:•High ionic conductivity

•Rapid oxygen surface exchange kinetics

•TEC match to other components

•High electronic conductivity

•Iron is inexpensive B-site constituent

Introduction of doped ceria layer improves performance

La(Sr)FeOLa(Sr)FeO333030--5050µµmm

YSZYSZ77µµmm

AnodeAnode~550~550µµmm

Ce(Sm)OCe(Sm)O2233µµmm

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Anode-supported cell w/ LSF-20 cathode(Previous status)

Anode-supported cell w/ LSF-20 cathode(Previous status)

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2

Current Density (A/cm2)

Volta

ge (V

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Pow

er D

ensi

ty (W

/cm

2)

800ºC750ºC700ºC650ºC

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700 800

Time (h)

Pow

er D

ensi

ty (W

/cm

2)

750ºC

Cell: LSF Cathode / SDC Interlayer / YSZ Electrolyte / Ni-YSZ anode

Fuel: 97% H2 / 3% H2O (Low Fuel Utilization)

Oxidant: Air

T(ºC) Power at 0.7V (W/cm2)

650 0.36

700 0.63

750 0.85

800 1.21

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Role of ceria layer:Prevention of reaction between YSZ and LSF?

Role of ceria layer:Prevention of reaction between YSZ and LSF?

25 30 35 40 45 50 55

1400oC/2h

RT

Diffraction Angle, 2Θ

E

EE

H

H

H

H

H

H

E LSF-20H 8-YSZ

E

E

E

v

E

E

E

Mixtures of LSF and YSZ, heated to 1400ºC for 2 h, showed no evidence of zirconate formation. (Contrary to case with LSCo and YSZ).

25 30 35 40 45 50 55

1050oC/2h

RT

Diffraction Angle, 2Θ

E

EE

H

H

H

GGH

H H

F

F

F

F

SS

S

E LSCo-20H 8-YSZG SrZrO3F La2Zr2O7S Co3O4

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Role of ceria layer:Prevention of reaction between YSZ and LSF?

Role of ceria layer:Prevention of reaction between YSZ and LSF?

However, for T ≥ 1000ºC, LSF peaks were shifted, indicating expansion of lattice due to change in composition of perovskite phase.

29 29.5 30 30.5 31 31.5 32 32.5 33

YSZ 100%Peak

LSF 100%Peak

1000oC

RT

Diffraction Angle, 2Θ

1400oC1200oC

239.5240.8245.7248.0

31 31.2 31.4 31.6 31.8 32 32.2 32.4 32.6 32.8 33Diffraction Angle, 2Θ

EH LSF-20 + ZrO2 1200oC/2h

LSF-20 + 8-YSZ 1200oC/2h

E

E

EE LSF-20H ZrO2

LSF-20 + Y2O3 1200oC/2h

LSF-20 (unreacted)

Results using ZrO2 and Y2O3 indicate Zr4+ from YSZ incorporated onto B-site of perovskite lattice; conclusion is supported by EDX results

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Impact of Zr4+ on LSF ConductivityImpact of Zr4+ on LSF Conductivity

1.5

2

2.5

3

3.5

4

4.5

5

5.5

0.9 1.1 1.3 1.5 1.7 1.9 2.11000/T (K-1)

LSF-20

LSFZr-8291

LSFZr-8282

LSF-20 + 10 mol.% ZrB-site excess

Significant reduction in electrical conductivity of LSF w/ increasing Zr content:

[ ] [ ] [ ] [ ] [ ]•••• ++=+ FeOFeFeLa ZrVFeFeSr 2''

Conclusion: Ceria interlayer required for LSF cathodes sintered at T ≥ 1000ºC

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Enhancing LSF SinterabilityGoal: Modify the LSF cathode to sinter onto YSZ below 1000oC to avoid the LSF-YSZ interaction. Compositions of the type (La0.8Sr0.2)0.98Fe0.98M0.02O3 are being considered.

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0 100 200 300 400 500 600 700 800 900 1000

La0.8Sr0.2FeO3

(La0.8Sr0.2)0.98FeO3

(La0.8Sr0.2)0.98Fe0.98Cu0.02O3

Time (min)

950oC (600 min. hold)25o- 950oC(3oC/min)

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(La0.8Sr0.2)0.98Fe0.98Cu0.02O3 Performance Data

Initial performance data indicates significantly improved power density (1.4-1.8 W/cm2

at 750oC and 0.7V) for the new cathode material sintered on YSZ at 950oC.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200 250 300 350Time (h)

LSF-20 w/ SDC Interlayer

New Cathode w/o SDC Interlayer

Data at 750oC and 0.7V

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Metallic Interconnect Materials Evaluation and Development

Metallic Interconnect Materials Evaluation and Development

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Metallic Interconnects for SOFCMetallic Interconnects for SOFC

Objectives:Identify and quantify degradation processes in candidate alloysDevelop a cost-effective optimized material for intermediate temperature interconnect applications

Approach:Screen testing of candidate alloys (chemical, electrical, mechanical properties)Materials development

Surface modification (surface doping, protective coatings)Bulk modification

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Screen testing of candidate alloysScreen testing of candidate alloys

Emphasis on “Chromia-forming” Ferritic Stainless Steels:

CTE match, conductive oxide scale, low cost, ease of fabrication

Screening Studies

Electrical Screen

Chemical Screen

Mechanical Screen

ASR measurements under SOFC exposure conditions

Oxidation in air, fuel, and dual atmosphere environments (scale thickness, composition, and microstructure)Chemical compatibility with alkaline earth-aluminosilicate glass sealsOxide scale thermodynamic stability

Investigation of thermal expansionInterfacial bonding strength with glass seals

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Thermal expansion

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 100 200 300 400 500 600 700 800

Temperature (C)

∆L/L

AL453Crofer 22EbriteReduced anode

Selected alloys offer good CTE match to SOFC components

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Electrical Resistance of Scales on Selected FSS

0

0.005

0.01

0.015

0.02

0.025

0.03

0 50 100 150 200 250 300 350 400 450 500

Time (hours)

ASR

(ohm

.cm

2 )

AL453 Crofer22 APU E-brite

Crofer22 APU

E-brite

Al453

Coupon samples were pre-oxidized in air at 800oC for 100h before carrying out tests in air at 800oC.

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Scale Resistance for Crofer22APU at 700, 800ºC (in air)

0

0.005

0.01

0.015

0.02

0.025

0 50 100 150 200 250 300 350 400 450 500

Time (hours)

ASR

(ohm

.cm

2 )

800oC

700oC

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Microstructures of cross-sections of samples from conductivity tests (in air)

800oC, 500h 700oC, 500h

Pt contactScaleCrofer22 APU

(Cr,Mn)3O4

Cr3O2

(Al,Ti,La)xOyMetal matrix

Note: Reduced Cr volatility due to (Cr,Mn)3O4 outer scale

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ASR of Crofer22 APU and Effects of Conductive Oxide Coatings

0

0.005

0.01

0.015

0.02

0.025

0 50 100 150 200 250 300

Time(hours)

ASR

(Ohm

.cm

2 )

ITO coating LSF coatingLSCr coatings Crofer22 APU

Both bare and coated samples were pre-oxidized in air at 800oC for 100h before carrying out tests in air at 800oC.

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Oxidation Behavior of Crofer22APU:Dual Atmosphere vs. Air

Air exposure at both sides Air-side of dual exposure

Fe

Cr

Mn

Fe

Cr

Mn

Repeated EDX analyses on Crofer22APU tested under dual exposure indicate the presence of Fe in the scale

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Dual Atmosphere w/ Thermal Cycling

Thermal cycling tests:

5ºC/min to 800oC, 100h dwell

3 cycles

XRD patterns from the airside of dual test vs. air exposure only

M = Fe-Cr SubstrateC = Cr2O3S = MnCr2O4O = Fe2O3 Iron Oxide

Crofer22 APU in Air Only

Crofer22 APU Dual Atmosphere Test (Air Side)

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Airside under dual exposure w/ cycling

A

A-AB

C

C-CB-B

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Elemental mapping of scale formed at the air side

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Future Work (Short-term)

Continue to study role of dual atmosphere exposure on corrosion of alloys

Extend screening studies to include ZMG232, a new ferritic stainless composition developed by Hitachi Steel for SOFC applications

Study the evaporation of scale on metallic interconnect

Investigate the feasibility of doping interconnect surface to minimize scale growth / electrical resistance

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AcknowledgementsAcknowledgements

Cathode Development: Steve Simner, Mike AndersonInterconnect Development: Gary Yang, Prabhakar Singh, Matt Walker

Financial Support:Solid-State Energy Conversion Alliance Core Technology Program (SECA)

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Compositions of FSS

0.002

0.03

0.02

0.03

0.03S

0.016

0.02

0.02

0.04

0.03P

+ other elements 22.0Bal.ZMG232

0.06La--0.08----0.50.00522.0Bal.Crofer22 APU

0.10.60.02--0.30.30.0322.0Bal.AL453

--------0.0250.010.00126.0Bal.E-brite

--------1.01.50.226.0Bal.AISI446

--------1.01.00.116.0Bal.AISI430La+CeAlTiMoSiMnCCrFeFSS