Transcript Structure and Electrical Conductivity of Mn

Structure and Electrical
Conductivity of Mn-based Spinels
Used as SOFC Interconnect coating
Supervisor: Dr. A. Petric
Yadi Wang
Jan. 25.2013
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OUTLINE
 Literature Review
 Objectives
 Preliminary Results
 Conclusions & Future Work
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Literature Review
-----SOFC
Reactions:
Cathode
O2 + 4e- = 2O 2Anode
H2 +O2- =H2O+2eCO +O2-= CO2 +2eCH4 + 4O2- =2H2O+CO2+8e-
Environmental friendly
High Efficiency
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Literature Review
-----Interconnect
 Interconnect is a critical part of the fuel cell
electrical connection between cells
gas separation within the cell stack
 Requirements
High electrical conductivity
Chemical stability in both fuel and air
Thermal expansion match to other cell components
High mechanical strength
Chemical stability with regard to other cell components
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Literature Review
-----Interconnect
Hard to fabricate,
high cost
Ceramic interconnect
1990s
Al---alumina
Metallic interconnect
Si---silica
Cr---chromia
Ferritic stainless
steel 16%<Cr<18%
Chromium-based
alloy Cr ~95%
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High
resistance
Literature Review
-----Metallic Interconnect
 Main disadvantages of metallic interconnects
--Growth of Cr2O3 scale causes high resistance in the cell stack
--Volatile Cr (VI) causes chromium poisoning of the cell
2Cr2O3 + 3O2 + 4H2O ⟺ 4H2CrO4 (g)
2Cr2O3 + 3O2 ⟺ 4CrO3 (g)
2Cr2O3 + 3O2 + 4H2O ⟺ 4CrO2(OH)2 (g)
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Literature Review
-----Metallic Interconnect
 A protective coating is needed on the cathode side to
isolate Cr from air :
--maintain electrical conductivity
--prevent chromium (VI) formation
 Candidates to use for coating
--Perovskite oxides (high cost, difficult to apply coating)
--Spinel oxides
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Literature Review
-----Spinel
Many famous rubies
on the crown turned
out to be spinels
“Black Prince's ruby”
A magnificent
170-carat red spinel
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Literature Review
-----Spinel
Has formula of AB2O4
Cubic closed-packed oxygen
lattice with 32 O2- in one cell
A and B cations occupy
interstitial sites
1/8 of tetrahedral and 1/2 of
octahedral sites are occupied
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Literature Review
-----Spinel
 Spinels can form solutions with many different cations
 A element: Mg, Ni, Mn, Zn, Cu, Co
 B element: Al, Cr, Mn, Fe, Co
 Multivalent elements needed for conductivity
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Literature Review
-----Spinel
 Table 1. Properties of common spinels (800oC except where noted)
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Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] 1515-1520 (2007).
Literature Review
-----Spinel Conductivity
 Conduction occurs by hopping of charge on octahedral sites
----Thus presence of different valence states among octahedral
cations is essential to conduction. (Al-based spinels have low
conductivity.)
----Spinels containing manganese have high conductivity because of
multiple valence states of Mn
 Conduction is temperature activated
σ=σ0 exp(-EF /kT)
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Literature Review
-----Mn-based Spinel
 Mn valence state
Tetrahedral: Mn2+
Octahedral : Mn2+ , Mn3+ , Mn4+ (2Mn3+  Mn4+ +Mn2+ )
 Conductivity of Mn-based spinel is attributed to
hopping between Mn3+/Mn4+ on octahedral sites.
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Literature Review
-----Mn-based Spinel
 Co-Mn spinel
MnCo2O4---has conductivity of 60 S/cm
Promising coating material for SOFC metallic interconnect
 CuMn2O4 has conductivity around 200 S/cm, volatile
 Cu doped Co-Mn spinel
Increase conductivity
Decrease cost
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Objectives
 Measure electrical conductivity of Co-Mn-Cu spinel
 Electroplate spinel coating for stainless steel 430 and
chromium alloy interconnects
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Co - Mn - O2
p(O2) = 0.21 atm
2000
Aukrust & Muan (air)
Liquid
1800
1600
Monoxide
1400
T(C)
1200
1000
Spinel
800
Mn2O3 + Sp
600
Mn2O3 + CoMnO3
400
CoMnO3 + Sp
MnO2 + CoMnO3
200
MnO2 + Sp
0
0
.2
.4
.6
.8
1
mole Co/(Co+Mn)
Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] 1515-1520 (2007).
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Co-Cu-Mn Spinel Conductivity
 Measured
conductivity
of 13
compositions
within the
cubic region
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Co-Cu-Mn Spinel Conductivity
Blue region
identifies
the region
of optimum
properties
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Coating methods
 Coating methods:
Slurry coating, screen-printing, physical vapor deposition,
electroplating, plasma spray
 Electroplating + oxidation
--cost-effective
--low processing temperature (room temperature)
--uniform films on complex substrate
--easy to control thickness
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Electroplating
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Reactions:
Cathode: Cu2++2e-= Cu
Co2++2e-=Co
Mn2++2e-=Mn
Anode: 2H2O= O2+ 4H++4eUse Faraday’s law:
m=(Q*M)/(F*Z) =
to control electroplating time
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On stainless steel 430, 24h
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On Stainless steel 430, 150h
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Preliminary Results
-----C00.5Cu0.5Mn2O4 Coating
On chromium-iron plate, 24h
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Conclusions & Future Work
 Cu doped Co-Mn spinels have ideal electrical conductivity
 Co0.5Cu0.5Mn2O4 coating successfully stops Cr migration
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Improve the quality of spinel coating.
Apply different compositions
Characterize the coating
Apply to large interconnect plate
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Acknowledgement
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Dr. A. Petric
Dr. Y. Mozharivskyj
Dr. P. Wei, Dr. S. Joshi, Xavier Michaud
Chris Butcher, Doug Culley, Xiaogang Li, Wenhe Gong
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