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Development of Cathode-Interconnect Contact Materials for SOFC J.W. Stevenson, G.G. Xia, Z. Lu, X. Li, Z. Nie, Y.S. Chou, and R.C. Scott Pacific Northwest National Laboratory Richland, WA 99352 July 26-28, 2011 12 th Annual SECA Workshop Pittsburgh, PA
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Development of Cathode-Interconnect Contact Materials for SOFC

J.W. Stevenson, G.G. Xia, Z. Lu, X. Li, Z. Nie, Y.S. Chou, and R.C. Scott

Pacific Northwest National Laboratory

Richland, WA 99352

July 26-28, 2011

12th Annual SECA Workshop

Pittsburgh, PA

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Cathode-Interconnect Contact Materials

Chromia-forming

alloy interconnect

Protective Coating

Contact layer

Cathode

2

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Cathode/Interconnect Contact Materials

Requirements:High electrical conductivity to reduce interfacial electrical resistance between cathode and interconnect

Chemical and structural stability in air at SOFC operating temperature

Chemical compatibility with adjacent materials (perovskite cathode, interconnect coating)

Adequate mechanical strength and bonding to adjacent components

Low cost materials and fabrication

Challenges:Low processing temperature during stack fabrication (800-1000ºC)

Low density results in low intrinsic strength and bond strength, reduced conductance

Brittle nature of ceramics; Cost/volatility of noble metals

Goal:Develop cathode/interconnect contacts with low electrical resistivity and increased mechanical strength

Modeling results suggest strengthening of contacts can relieve stresses on seals

3

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Key Characterization Methods

4

ASR Tensile Bond Strength

Dilatometer

TGA/DSC

XRD

SEM

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coatingserialcontactmatscaleASR erconnectcathode ,,int

Simulated cathode with

dense body and porous

surface layers

Interconnect (coated)

I

Current Density:

0.5A.cm-2

“Cathode”

VI

V

Conta

ct Layers

Area Specific Resistance (ASR) Measurements

~12psi

5

I

V1 V2 V3

ASR Stack (3 sets)

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Conventional Contact Pastes: LSM, LSCM, LNF and LSCF

6

Relatively low ASR (with

suitable coating on steel)

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Conventional Contact Materials

7

LN

F

LS

M

LS

CF

Conventional contact layers exhibit low

ASR but also low intrinsic strength and

bond strength

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Approaches to Improve Contact Strength

Sintering AidsGoal: Reduce the sintering temperature of contact materials (LSM, LNF,

LSCF) to obtain increased density/conductance/strength

Reaction-SinteringSimilar to process used to prepare MnCo spinel coatings for steel

interconnects

Contact material precursor powder contains multiple phases, which react

during stack assembly to form a conductive single phase

Enthalpy of reaction provides additional driving force (besides surface energy reduction) for densification

8

Pacific Northwest National Laboratory

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Approaches to Improve Contact Strength

Sintering AidsGoal: Reduce the sintering temperature of contact materials (LSM, LNF,

LSCF) to obtain increased density/conductance/strength

Reaction-SinteringSimilar to process used to prepare MnCo spinel coatings for steel

interconnects

Contact material precursor powder contains multiple phases, which react

during stack assembly to form a conductive single phase

Enthalpy of reaction provides additional driving force (besides surface energy reduction) for densification

9

Pacific Northwest National Laboratory

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Sintering Curves of LSCF with Various Additives

LSCF with CuO exhibited the highest sintering activity.

LSCF+3mol%CuO

Pacific Northwest National Laboratory

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Sintering Curves of LSCF with Various Amounts of CuO Additions

LSCF with infiltrated CuO did not show significant difference of sintering activity with powders prepared by mixing LSCF with CuO

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Contact ASR of LSCF with CuO Sintering Aid(441-Ce0.02MC|LSCF-CuO|LSCF)

12

Low ASR, but tensile stress measurements showed very weak bonding

strength between contact layer and cathode or interconnect coating

Pacific Northwest National Laboratory

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SEM Images of LSCF-x%CuO Contact Materials after ASR Measurements

13

LSCF+3mol% CuO LSCF+5mol% CuOLSCF only

Pacific Northwest National Laboratory

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Approaches to Improve Contact Strength

Sintering AidsGoal: Reduce the sintering temperature of contact materials (LSM, LNF,

LSCF) to obtain increased density/conductance/strength

Reaction-SinteringSimilar to process used to prepare MnCo spinel coatings for steel

interconnects

Contact material precursor powder contains multiple phases, which react

during stack assembly to form a conductive single phase

Enthalpy of reaction provides additional driving force (besides surface energy reduction) for densification

14

Pacific Northwest National Laboratory

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Reaction Sintering of Contact Materials

Contact material precursor powder contains multiple phases, which react during stack assembly to form a conductive single phase

Similar to process used in fabricating MnCo spinel coatings

Driving forces for densification:

Reduction of surface energy (~75 J/mol)

Enthalpy of formation (~500 kJ/mol)

Systems of primary interest

(Ni,Co)Ox

(Mn,Co,Cu)3O4

Numerous compositions evaluated (CTE, conductivity, microstructure,

strength); down-selected to Ni0.33Co0.67Ox and Mn2.7-xCoxCu0.3O4

Potential to include fillers and fugitive phases(tailor CTE and porosity,

reduce cost)15

45.05.02 O)CoMn(2O5.3CoMnO

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Ni0.33Co0.67O as cathode-interconnect contact material

16

Pacific Northwest National Laboratory

LSCF Ni0.33Co0.67O

MC

Spinel 441

Cross-section SEM image

of (Ni0.33Co0.67)Ox contact

heat treated at 950°C for

30 min in air

•Prepared from mixture of Ni and Co powder•Paste prepared using binder vehicle and 3 roll mill

•Primary phase is (Ni,Co)O; secondary phase is NiCo2O4

•CTE ~14.6 ppm/K•Possible inclusion of filler to reduce cost and CTE

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ASR test results for Ni0.33Co0.67O as cathode-

interconnect contact material: 800ºC

17

ASR increased from ~4 to ~9 during

initial1000 h of isothermal testing

(with 1 unscheduled thermal cycle at

~500 h)

Thermal cycling every 24h,

~60-800oC

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Mn1.5Co1.2Cu0.3O4 as cathode-interconnect contact material

18

LSCF

Porous

LSCF MCC

MC

Spinel 441

Cross-section SEM image of

Mn1.5Co1.2Cu0.3O4 heat

treated at 950ºC for 30 min

in air, 800ºC for 100 h

•Prepared from mixture of Mn, Co, and Cu powder•Paste prepared using binder vehicle and 3 roll mill

•Single phase MCC spinel

•CTE ~12.8 ppm/K

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Isothermal and cyclic ASR test results for

Mn1.5Co1.2Cu0.3 as cathode-interconnect contact

material: 800ºC

Thermal cycling every 24h, ~60-800oC

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Mechanical Bond Strength Measurement

Cathode or Coated 441

Contact Layer

Cathode or Coated 441

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Contact Bonding Strength (950oC Treated)

21

Strong bonds can be obtained for both coated 441 and cathode using Mn1.5Co1.2Cu0.3

and NiCo2 as contact pastes; bonds on cathode are stronger

MnCoCu0.3MnCoCu0.3+10%Y

SZNiCo2 NiCo2+10%YSZ

Ce-MC|441 5.27 6.47 4.26 3.16

LSCF 8.48 7.09 6.82 5.17

LSM 8.03 6.15

0

2

4

6

8

10A

vera

ge B

on

d S

tren

gth

, M

Pa

Contact Materials

950oC /0.5h-800oC/100hCe-MC|441

LSCF

LSM

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Contact Bonding Strengths (850oC Treated)

22

Stronger bonds can be obtained for both coated 441 and cathodes using

Mn1.5Co1.2Cu0.3 and NiCo2 as contact paste; bonds on cathode are stronger

MnCoCu0.3MnCoCu0.3+10

%YSZNiCo2 NiCo2+10%YSZ

Red-MnCoCu0.3

Ce-MC|441 3.37 7.28 3.38 2.19 4.23

LSCF 9.35 8.66 6.46 3.1 4.6

0

2

4

6

8

10

12

Avera

ge B

on

d S

tren

gth

, M

Pa

Contact Materials

850oC/0.5h-800oC/100h

Ce-MC|441

LSCF

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SEM Images of Specimens after Mechanical Bond Strength Measurement (950oC treated)

23

MnC

oC

u0.3

MnC

oC

u0.3

LSCF

Ce-M

C441

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Tensile strength results for Mn-Co-Cu contact materials with varying Co content

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ASR of Mn2.7-xCoxCu0.3O4 Contact Materials

25

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 500 1000 1500 2000 2500 3000 3500 4000

AS

R (

mO

hm

s-c

m2)

Time (hours)

Mn1.5Co1.2Cu0.3

Mn1.9Co0.8Cu0.3

Mn2.3Co0.4Cu0.3

Mn2.7Cu0.3

800ºC

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Alternative Fabrication Approach: Tape-casting

26

Application of cathode -

interconnect contact material

in tape form instead of paste

form

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ASR of Mn2.7-xCoxCu0.3O4 Contact Materials

27

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 1000 2000 3000 4000

AS

R (

mO

hm

-cm

2)

Time (hours)

Mn1.5Co1.2Cu0.3 paste

MnCoCu0.3 Metal Tape ~30um

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Summary and Future Work

Reactive sintering has been demonstrated as a means of preparing cathode/interconnect Ni-Co oxide and Mn-Co-Cu oxide contact materials from precursor mixtures of metallic powders.

The reactive sintering approach resulted in very low cathode-to-interconnect ASR, and high bond strength (compared to conventional contact materials).

Clearly, the high density of reaction-sintered contacts will restrict gas phase transport through the contact material.

Possible solutions:

Application of contact material to selected regions only (e.g., lands of ribs of interconnects)

Inclusion of controlled porosity through use of fugitive phases

Reactive sintering may result in stronger bulk and interfacial bonding compared to conventional contacts prepared from complex oxides with minimal sintering activity at stack fabrication temperatures

More information: Z. Lu, G. Xia, J.D. Templeton, X. Li, Z. Nie, Z. Yang, J.W. Stevenson, “Development of Ni1−xCoxO as the cathode/interconnect contact for solid oxide fuel cells,” Electrochemistry Communications, 13, 642 (2011).

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Acknowledgements

The work summarized in this paper was funded under the U.S. Department of Energy’s Solid-State Energy Conversion Alliance (SECA) Core Technology Program

NETL: Shailesh Vora, Briggs White, Rin Burke, Travis Shultz, and Joe Stoffa

PNNL: Jim Coleman, Shelley Carlson, Nat Saenz

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