Post on 22-Jun-2020
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
Cathode-Interconnect Contact Materials
Chromia-forming
alloy interconnect
Protective Coating
Contact layer
Cathode
2
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
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Key Characterization Methods
4
ASR Tensile Bond Strength
Dilatometer
TGA/DSC
XRD
SEM
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)
Conventional Contact Pastes: LSM, LSCM, LNF and LSCF
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Relatively low ASR (with
suitable coating on steel)
Conventional Contact Materials
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LN
F
LS
M
LS
CF
Conventional contact layers exhibit low
ASR but also low intrinsic strength and
bond strength
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
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Pacific Northwest National Laboratory
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
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Pacific Northwest National Laboratory
Sintering Curves of LSCF with Various Additives
LSCF with CuO exhibited the highest sintering activity.
LSCF+3mol%CuO
Pacific Northwest National Laboratory
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
Contact ASR of LSCF with CuO Sintering Aid(441-Ce0.02MC|LSCF-CuO|LSCF)
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Low ASR, but tensile stress measurements showed very weak bonding
strength between contact layer and cathode or interconnect coating
Pacific Northwest National Laboratory
SEM Images of LSCF-x%CuO Contact Materials after ASR Measurements
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LSCF+3mol% CuO LSCF+5mol% CuOLSCF only
Pacific Northwest National Laboratory
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
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Pacific Northwest National Laboratory
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
Ni0.33Co0.67O as cathode-interconnect contact material
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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
ASR test results for Ni0.33Co0.67O as cathode-
interconnect contact material: 800ºC
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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
Mn1.5Co1.2Cu0.3O4 as cathode-interconnect contact material
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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
Isothermal and cyclic ASR test results for
Mn1.5Co1.2Cu0.3 as cathode-interconnect contact
material: 800ºC
Thermal cycling every 24h, ~60-800oC
Mechanical Bond Strength Measurement
Cathode or Coated 441
Contact Layer
Cathode or Coated 441
Contact Bonding Strength (950oC Treated)
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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
Contact Bonding Strengths (850oC Treated)
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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
SEM Images of Specimens after Mechanical Bond Strength Measurement (950oC treated)
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MnC
oC
u0.3
MnC
oC
u0.3
LSCF
Ce-M
C441
Tensile strength results for Mn-Co-Cu contact materials with varying Co content
ASR of Mn2.7-xCoxCu0.3O4 Contact Materials
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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
Alternative Fabrication Approach: Tape-casting
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Application of cathode -
interconnect contact material
in tape form instead of paste
form
ASR of Mn2.7-xCoxCu0.3O4 Contact Materials
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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
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).
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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