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

Advantages Disadvantages

Metals and Alloys Good Conductivity Low corrosion resistance in MC

Ceramics

(non-metals)

Good corrosion properties

Poor electronic conductivity

Ceramics

(non-metals and metals)

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

• Modify the surface by encapsulation of the SS304 with Ni-Co to form a layer of lithiated Ni-Co oxides

Approach

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 three-electrode 3 cm2 half cell: – Tafel polarization – Impedance analysis

Chromium Dissolution (AAS Data)

0 100 200 300 400 500

Time (hours)

0.00

0.15

0.30

0.45

0.60

0.75

Wei

ght (

mg/

cm2)

Co-Ni-SS304

SS304a

Nickel Dissolution (AAS Data)

0 100 200 300 400 500

Time (hours)

0.5

0.8

1.1

1.4

1.7

2.0

Wei

ght (

mg/

cm2)

Co-Ni-SS304

SS304b

SEM Micrographs

Fresh SS304

Co-Ni SS304

SS304 500 h

Co-Ni 500 h

Elt. Conc.

Cr 18.193 wt.%

Ni 10.377 wt.%

Fe 70.722 wt.%

Elt. Conc.

Co 52.046 wt.%

Ni 28.060 wt.%

Cr 0.505 wt.%

Fe 1.032 wt.%

P 11.764 wt.%

Elt. Conc.

Co 64.075 wt.%

Ni 29.646 wt.%

Cr 0.518 wt.%

Fe 4.785 wt%

P 0.976 wt%

Elt. Conc.

Cr 6.082 wt.%

Ni 8.704 wt.%

Fe 84.901 wt.%

12 m

9.5 m12 m

12 m

XRD Result (Posttest)

20 40 60

2 theta

Inte

nsit

y (a

rbit

rary

uni

ts)

SS304

Co-Ni-SS304 LiNiO2

and LiCoO2

LiFeO2

LiFe5O8

Separator Results

SS304

EDAX 6.5 wt.% Cr

Co-Ni-SS304

EDAX 0.24 wt.% Cr

0 3 6 9 12 15

Time (hours)

-1.1

-0.8

-0.5

-0.2

0.1

Po

ten

tial

(V

vs.

Au

/2C

O2+

1O 2

)

Co-Ni-SS304

SS 304

Open Circuit Potential as a Function of Time (650º C)

eCOFeOCOFe 222

3

eCONiOCONi 222

3

eCoCOCOCo 232

3

eCOOCoCOCo 8443 2432

3

eCOCrOCOCr 644 22

42

3

-1.8 -1.3 -0.8 -0.3 0.2

Potential (V vs. Au/(0.67CO2+0.33O2)

-0.40

-0.18

0.04

0.26

0.48

Cu

rren

t (A

/cm

2)

Co-Ni-SS304

SS304

Cyclic Voltammetric Results (650 ºC)

CV done after 2hrs in CO3 melt with Cathode gas Scan rate: 10mV/s Potential: -1.6V to 0V

eCOCrOCOCr 644 22

42

3

eCONiOCONi 222

3

eCOFeOCOFe 222

3

eCOLiFeOCOLiFe 322 222

3

eCOLiFeOCOLiFeO 222

3

Tafel Polarization Results

10-5 10-4 10-3 10-2 10-1

Current density (A/cm2)

-0.4

-0.2

-0.0

0.2

Pote

ntia

l (V

vs

. A

u/2C

O2

+1O

2)

650C 700 C

750 C

750 C

700C

650 C

in 30%CO2+ 70% air

No Gas (after 12 h)

10-5 10-4 10-3 10-2 10-1

Current density (A/cm2)

-0.4

-0.2

-0.0

0.2

Pote

ntia

l (V

vs

. A

u/2C

O2

+1O

2)

650 C700 C

750 C

750 C

700 C650 C

in 30%CO2+ 70% air

No Gas (after 12 h)

SS304 Co-Ni-SS304

±250 mV OCP Scan rate: 25mV/s

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-Ni-SS 304

0.050 0.090 0.17 0.085 0.12 0.18

Impedance Analysis (650 ºC)

0 10 20 30 40

Real Z ()

0.0

7.5

15.0

22.5

30.0

-Imag

inar

y Z

(

)

with gas

without gas

0 h

without gas

2 h

without gas

4 hwithout gas

12 h

650C SS304

0 4 8 12 16

Real Z ()

0

3

6

9

12

-Imag

inar

y Z

(

)

with gas

without gas

0 h

2 h

4 h

12 h

650C Co-Ni-SS304

Frequency: 10 kHz-10 mHz ±5 mV OCP

Impedance Analysis (700 ºC)

0 5 10 15 20

Real Z ()

0

4

8

12

16

-Imag

inar

y Z

(

)

with gas

without gas

0 h

without gas

2 h

without gas

4 hwithout gas

12 h

700C SS304

0 2 4 6 8

Real Z ()

0.0

1.5

3.0

4.5

6.0

-Imag

inar

y Z

(

)

with gas

without gas

0 h

2 h

4 h

12 h

700C Co-Ni-SS304

Frequency: 10 kHz-10 mHz ±5 mV OCP

Electrical Equivalent Circuit Representation

R

R R2

C2C

DPE1 DPE2

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

0 5 10 15 20

Real Z ()

0

4

8

12

16

-Im

agin

ary

Z (

)

with gas

without gas

0 h

without gas

2 h

without gas

4 hwithout gas

12 h

Experimental data__ Equivalent circuit fit

Resistance

EIS Equivalent Circuit Fit Linear Polarization

Current Collector

Porous electrode

Ohmic Resistance

()

Polarization Resistance

()

Polarization Resistance

()

SS304 ~5.3 120.68 98.75

Co-Ni-SS304 ~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