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Surface Modification of MCFC
Current Collectors for Improved
Lifetime
Héctor Colón-Mercado, Anand Durairajan,
Bala Haran, and Branko Popov
Department of Chemical Engineering
University of South Carolina
Columbia, SC 29208
State of the Art Current Collectors
• SS 316 is currently used as a current
collector
– Oxidation of SS occur in the cathode
atmosphere
– SS components (Cr) dissolves in the carbonate
melt
Materials Used for Surface Modification
of the Current Collector
Coating
Materials
Metals and Alloys
Ceramics
(non-metals)
Ceramics
(non-metals and
metals)
Advantages
Disadvantages
Good Conductivity Low corrosion resistance
in MC
Good corrosion
properties
Poor electronic
conductivity
Combine higher oxidation resistance with
higher electronic conductivity.
Mixed oxides from corrosion products are
suitable coating materials
Objective
• Increase the corrosion resistance of SS 304, in
cathode gas, used as current collectors and bipolar
separator plates
• Decrease dissolution of SS 304 (Fe, 10%Ni,
18%Cr) components (Cr)
• Create a more conductive corrosion scale
Approach
• Modify the surface by encapsulation of the SS304
with Ni-Co to form a layer of lithiated Ni-Co
oxides
Experimental
• The SS 304 current collector was encapsulated with
Ni-Co by in-house develop auto catalytic reduction
process
• Dissolution studies were carried out
• Oxidation behavior studies were carried out using a
three-electrode pot cell:
– Open circuit potential
– Cyclic voltammetry
• Polarization Studies were carried out using a threeelectrode 3 cm2 half cell:
– Tafel polarization
– Impedance analysis
Chromium Dissolution
(AAS Data)
0.75
a
SS304
Weight (mg/cm2 )
0.60
0.45
0.30
Co-Ni-SS304
0.15
0.00
0
100
200
300
Time (hours)
400
500
Nickel Dissolution (AAS Data)
2.0
SS304
b
Weight (mg/cm2 )
1.7
1.4
1.1
Co-Ni-SS304
0.8
0.5
0
100
200
300
Time (hours)
400
500
SEM Micrographs
Elt. Conc.
Elt. Conc.
Cr 18.193 wt.%
Fresh
SS304
Co-Ni
SS304
Ni 10.377 wt.%
Co 52.046 wt.%
Ni 28.060 wt.%
Cr 0.505 wt.%
Fe 70.722 wt.%
Fe 1.032 wt.%
P
12 mm
11.764 wt.%
9.5 mm
Elt. Conc.
Elt. Conc.
Cr 6.082 wt.%
Ni 8.704 wt.%
Fe 84.901 wt.%
12 mm
SS304
500 h
Co-Ni
500 h
Co 64.075 wt.%
Ni 29.646 wt.%
Cr 0.518 wt.%
Fe 4.785 wt%
P
0.976 wt%
12 mm
XRD Result (Posttest)
Intensity (arbitrary units)
Co-Ni-SS304
LiNiO2
and LiCoO2
LiFe5O8
SS304
LiFeO2
20
40
2 theta
60
Separator Results
SS304
Co-Ni-SS304
EDAX 6.5 wt.% Cr
EDAX 0.24 wt.% Cr
Open Circuit Potential as a Function
of Time (650º C)
0.1
Potential (V vs. Au/2CO 2 +1O2 )
2
-0.2
2
Cr  4CO3  CrO4  4CO2  6e 
2
Co  CO3  CoCO3  2e 
Co-Ni-SS304
-0.5
2
3Co  4CO3  Co3O4  4CO2  8e 
2
-0.8
Ni  CO3  NiO  CO2  2e 
2
Fe  CO3  FeO  CO2  2e 
SS 304
-1.1
0
3
6
Time (hours)
9
12
15
Cyclic Voltammetric Results (650 ºC)
Fe  Li  2CO3
2
 LiFeO2  2CO2  3e 
2
2
Cr  4CO3  CrO4  4CO2  6e 
2
Current (A/cm2 )
0.48
Fe  CO3  FeO  CO2  2e 
2
Ni  CO3  NiO  CO2  2e 
2
FeO  Li  CO3  LiFeO2  CO2  e 
0.26
0.04
Co-Ni-SS304
-0.18
SS304
-0.40
-1.8
-1.3
-0.8
-0.3
P otential (V vs. Au/(0.67CO2 +0.33O2 )
CV done after 2hrs in CO3 melt with Cathode gas
Scan rate: 10mV/s
Potential: -1.6V to 0V
0.2
Tafel Polarization Results
SS304
Co-Ni-SS304
750C
0.2
in 30%CO2 + 70% air
-0.0
-0.2
No Gas (after 12 h)
750C
-0.4
650C
0.2
650C
Potential (V vs. Au/2CO 2 +1O2 )
Potential (V vs. Au/2CO2 +1O2 )
650C
700C
700C
750C
in 30%CO2 + 70% air
-0.0
-0.2
No Gas (after 12 h)
750C
-0.4
650C
700C
10-5
10-4
10-3
10-2
10-1
10-5
10-4
10-2
Current density (A/cm2 )
Current density (A/cm2 )
±250 mV OCP
10-3
Scan rate: 25mV/s
700C
10-1
Corrosion Currents from Tafel
Slopes
With Oxidant Gas
( 30% CO2 + 70% O2)
(A/cm2)
No Oxidant Gas
(A/cm2)
650º C 700º C 750º C 650º C 700º C 750º C
SS 304
0.010
0.030
0.060
0.042
0.083
0.10
Co-NiSS 304
0.050
0.090
0.17
0.085
0.12
0.18
Impedance Analysis (650 ºC)
30.0
12
650C SS304
650C Co-Ni-SS304
9
-Imaginary Z ()
-Imaginary Z ()
22.5
without gas
15.0
4h
without gas
without gas
with gas
2h
12 h
4h
6
12 h
2h
without gas
0h
7.5
with gas
3
without gas
0h
0.0
0
10
20
30
40
0
0
Real Z ()
Frequency: 10 kHz-10 mHz
4
8
Real Z ()
±5 mV OCP
12
16
Impedance Analysis (700 ºC)
16
6.0
700C Co-Ni-SS304
700C SS304
4.5
without gas
8
4h
without gas
12 h
without gas
with gas
2h
-Imaginary Z ()
-Imaginary Z ()
12
4h
3.0
12 h
2h
without gas
0h
4
with gas
1.5
without gas
0h
0
0.0
0
5
10
15
20
0
Real Z ()
Frequency: 10 kHz-10 mHz
2
4
Real Z ()
±5 mV OCP
6
8
Electrical Equivalent Circuit
Representation
C1
C2
DPE1
DPE2
R1
R2
R
R – Solution resistance
R1 – Porous electrode ohmic resistance
C1 – Coating capacitance
R2 – Polarization resistance
C2 – Double layer capacitance
DPE1, DPE2 – Distributed Elements, Zarc-Cole type
Equivalent Circuit Fit
16
Experimental data
__ Equivalent circuit fit
-Imaginary Z ()
12
without gas
8
4h
without gas
12 h
without gas
with gas
2h
4
without gas
0h
0
0
5
10
Real Z ()
15
20
Resistance
EIS Equivalent Circuit Fit
Current
Collector
Porous electrode
Ohmic
Resistance
()
Polarization
Resistance
()
Linear
Polarization
Polarization
Resistance
()
SS304
~5.3
120.68
98.75
Co-NiSS304
~1.3
22.86
22
Conclusions
• Immersion test indicate a decrease in the
chromium dissolution in the case of Co-Ni-SS304
• Surface composition of Co-Ni-SS304 consist
mainly of Co and Ni oxides
• Conductivity of the corrosion scale was higher in
the case of Co-Ni-SS304
• Polarization resistance for oxygen reduction was
significantly lower in the case of Co-Ni-SS304
Acknowledgements
• Financial sponsors - Dept of Energy, National
Energy Technology Laboratory