chromium reactivity and transport · cr diffusion across surface of oxide (800°c) cr deposited...
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Chromium Reactivity and Transport
Michael Krumpelt, Terry A. Cruse, Brian Ingram, Di-Jia Liu
7th Annual SECA WorkshopPhiladelphia, 9/12-14
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Chromium Transport According to Hilpert(J. Electrochem. Soc. 143 (1996) 3642)
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Flow-channel Hydrodynamics
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CathodeCathode
ElectrolyteElectrolyte
InterconnectInterconnect
OO2(g)2(g)HH22OO(g)(g)
CrOCrO22(OH)(OH)2(g)2(g)
CrCr22OO33
CathodeCathode
ElectrolyteElectrolyte
InterconnectInterconnect
OO2(g)2(g)
HH22OO(g)(g) CrOCrO22(OH)(OH)2(g)2(g)
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Three Cell Configurations Were Tested
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E-Brite E-Brite, Au Separation Au
With and without externaldosing
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Schematic of a Fixture with ribbed Flow-field
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Metallic Interconnect– ANL
• E-Brite• Au Barrier Between E-Brite
and Cathode• Au
– PNNL• Au• Au with External Cr Dosing
LSM Contact PasteLSM CathodeLSM/8YSZ Active Cathode8YZS ElectrolyteNi/8YSZ Anode
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Cell Potentials versus Time
0
0.1
0.2
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0.5
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0.9
1
0 100 200 300 400 500 600 700 800 900 1000Time (Hrs)
Cel
l Pot
entia
l (V)
Au Interconnect, Baseline (PNNL)
E-Brite Interconnect Au Foil Spacer (ANL)
E-Brite Interconnect (ANL)
Au Interconnect External Cr Dosed (PNNL)
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Au Between E-Brite and Cathode
Direct Contact
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1Zreal (Ω∗cm2)
-Zim
ag ( Ω
∗cm
2 )
Initial
120 Hrs
264 Hrs
504 Hrs
Temperature 800CE-Brite InterconnectAir ~3% H2O, 160 sccmFuel N2:H2 (~1:1) ~3% H2O, 400 sccmActive Cathode Area 4.6 cm2
-1.15 A
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1Zreal (Ω∗cm2)
-Zim
ag ( Ω
∗cm
2 )
Initial
24 Hrs
264 Hrs
504 Hrs
Temperature 800CE-Brite InterconnectAir ~3% H2O, 160 sccmFuel N2:H2 (~1:1) ~3% H2O, 400 sccmActive Cathode Area 4.6 cm2
-1.15 A
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HEWAXS at Argonne APS is a Powerful Tool for Microscopic XRD Investigation
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HEWAXS at APS
Focused X-rayBeam
2 µm x 50 µm
Investigation for Cr poisoned SOFC
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Cr2O3 and MnCr2O4 were unambiguously identified
Cr2O3
MnCr2O4
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Profiles across Cathode
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
0 20 40 60 80 100 120Depth (micron)
Am
plitu
de (a
. u.)
LSM
YSZ
Cr2O3
MnCr2O4
Upper Cathode
Mixed Cathode
Contact Paste Anode
Electrolyte
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Formation and Transport of Chromium Compounds
Cr2O3: Cr2O3 + O2 +2H2O 2CrO2(OH)2 ∆G = +84KJ2CrO2(OH) + 6ē + 3V Cr2O3 + 3O2-o&&
MnCr2O4:
1. 7.5 Cr2O3 + 5 La.8Sr.2MnO3 4 LaCrO4 + 5 MnCr2O4+SrCrO4 + ¾ O2 ∆G = -297KJ
2. 2CrO2(OH)2 + MnO MnCr2O4 + 2H2O + 1/2O2
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Incremental Chromium Transport in MACOR fixture
K2Cr2O7 was identified as a condensate in the exhaust tube
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0 100 200 300 400 500
Time (Hrs)
Cel
l Vol
tage
(V)
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Chromium Mass Balances
Oxyhydroxide formation:- theoretical: 15 mg- measured: 0.22 mg
Cr2O3 deposits:- E-Brite with gold foil: not detectable- E-Brite: 0.12 mg- E-Brite in Macor: 1.5 mg
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Summary
Hilpert’s oxyhydroxide mechanism can account for a cell potential degradation of 5% with E-Brite current collectors
Spinel forming alloys or spinel coatings will largely prevent the chromium transport
Alkali and perhaps alkaline earth oxides can dramatically increase it
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Cr diffusion across surface of oxide (800°C)
Cr deposited from vapor on LSM surface (1000°C)
2 transport mechanisms for Cr: vapor and solid state diffusion
Both mechanisms are material and temperature dependent. Cr transport into the cathode is greatly reduced by lowering the operating temperature to a range suitable for metallic components (650-750°C). However it does not stop solid state transport through the contact paste. Barrier layers or coatings solve this problem.
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Solutions for Uncoated Interconnect
Choose a contact paste that does not have significant solid state diffusion or vapor deposition (Ag), or a contact paste that reacts with the chromia scale to form an effective barrier.
Accept degradation rate or develop Cr tolerant cathodes.
Coating decreases Cr vaporization rate by ~100x.
Contact paste can be Ag, LSM, LNF, etc.
Contact paste and/or current collection layer can include Cr getters such as Co to further protect the cathode.
Solutions for Coated Interconnect