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Deconvolution of SOFC CathodePolarization

Eric D. WachsmanUF-DOE High Temperature Electrochemistry Center

Florida Institute for Sustainable EnergyUniversity of Florida

UF-DOE HiTEC

ewach@mse.ufl.edu

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Fundamental Mechanisms of SOFC Cathode Reactions

LSM Cathode

YSZ Electrolyte

1000°C -> 700°C

Reaction Limited to TPB

!

O2

+ 2VO

••" 2O

O

#+ 4h

•

Multiple Reactions SpreadThroughout Microstructure

•Each component having differentcontribution/mechanism

≤ 700°C

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What is rate limiting step?

J. Nowotny et al., “Charge Transfer at Oxygen/Zirconia Interface atElevated Temperature”, Advances in Applied Ceramics (2005)

• Multiple potential mechanisms each having PO2 dependence• However, PO2 dependence not unique

--

Fundamental Mechanisms of SOFC Cathode Reactions

UF-DOE HiTECNeed to combine multiple techniques to determine mechanism

Stuart Adler, University of Washington

Fundamental Mechanisms of SOFC Cathode Reactions

UF-DOE HiTEC

Systematic Approach to Developing Low Polarization Cathodes:

Computational Approach - With Prof’s Susan Sinnott & Simon PhilpottProvide fundamental understandingCalculate surface and bulk energetics

Surface Science and Spectroscopic Techniques - With Prof. Scott PerryDetermine surface sites, vacancies, adsorbed species and effects of surface reconstructionMeasure surface and bulk energetics

Catalysis TechniquesDetermine O-adsorption/dissociation mechanismsDetermine rate constants (ko)

Electrochemical Characterization - With Prof. Mark OrazemSeparate contributions to impedance/polarizationFrequency dependence and relation to mechanism

Quantify Microstructural Effects - With Prof. Kevin JonesFabricate and evaluate model architecturesApply advanced characterization techniques such as FIB/SEM

Integrate (all of the above) and Deconvolute MechanismsDevelop fundamental models

Fundamental Mechanisms of SOFC Cathode Reactions

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Focused Ion Beam•Enables 3-D analysis ofelectrode microstructure

- Particle-size, pore-size, &distribution- Triple-phase boundary density- Tortuosity

Quantify Microstructural Effects - FIB/SEM

Phase Fraction vs. Distance from Interface

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3

Distance (micrometer)

Phase

Fra

ctio

n

OpenPore

LSCF

YSZ

ClosedPores

(a) (b)

(c)

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Developed phasecontrast forcomposite cathodestructures

Quantify Microstructural Effects - FIB/SEM

20 µm 20 µm

20 µm 20 µm

(d) Pore microstructure

(a) YSZ microstructure (b) YSZ/LSM microstructure

(c) Transparent microstructure

Siemens SOFC

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LSM cathode microstructural features directly related to sintering:• Pore surface area decreases linearly with increasing sintering temperature• TPB length decreases linearly with increasing sintering temperature

0

1

2

3

4

5

6

1150 1200 1250 1300 1350

Su

rfa

ce

Are

a p

er

Vo

lum

e, µ

m-1

Sintering Temperature, oC

0

0.5

1

1.5

2

1150 1200 1250 1300 1350

LT

PB p

er

Are

a, µ

m-1

Sintering Temperature, oC

Quantify Microstructural Effects - FIB/SEM

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10-1

100

101

102

103

104

105

10-1

101

103

105

107

300 oC

400 oC

500 oC

600 oC

700 oC

800 oC

900 oC

-Z'' /

oh

ms

Frequency / Hz

PO2

= 2.0 X 10-5

atm

Ionic conduction in bulk electrolyte

Artifacts minimized by nulling

Ionic conduction throughelectrolyte grain boundary

O2 pore diffusion (τ ~ 5.9 s)

Dissociation and surfacediffusion of O-species onLSM to TPB (τ ~ 0.18 s)

Charge transfer at TPB (τ ~0.0001 s)

Electrochemical Impedance Spectroscopy of LSM/YSZ

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1

10

100

1000

1 10

Rp, !

µm-1

0.3 3

Charge Transfer vs. LTPB

Adsorption vs. Sv

Microstructure - Performance Relationship

For the LSM on YSZcathode reaction:

The current is:

Direct quantified relationship between cathodemicrostructure and performance

Slope = -3.5

LSM/YSZ in air at 800°C

The correspondingcharge transferpolarization (Rct)dependence on triplephase boundarylength (LTPB) is:

Rct ~ kf-1 LTPB

-3.5

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Equivalent Circuit Comparison

LSM, sintered at 1200 °C, measured at 800 °C in air

ZGas ZAds ZCT

ZHF ZHF

ZAds ZCT

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Equivalent Circuit Comparison

100

101

102

103

1 10

Rp, !

µm-1

0.3 3

Charge Transfer vs. LTPB

Adsorption vs. Sv

m = -1.8

m = -3.5

100

101

102

103

1 10µm

-1

Rp, !

Charge Transfer vs. LTPB

Adsorption vs. Sv

0.3

m = -1.6

m = -1.3

3

LSM, sintered at 1200 °C, measured at 800 °C in air

ZGas ZAds ZCT

ZHF ZHF

ZAds ZCT

Need independent determination of mechanism

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• Temperature programmed desorption (TPD)– Ramp temperature in He to determine adsorbed and/or decomposition species

• Temperature programmed oxidation (TPO)– Ramp temperature in O2 gas mixture to determine reaction rates

• Isotope exchange (O16 vs. O18)– Switch gas to separate solid vs gas species contribution to mechanism

Fundamental Rate Constants - Catalysis

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TPD of LSCFBulk-O desorption

LaSrCoFeO3 -> LaSrCoFeO3-δ + δ/2O2

0

500

1000

1500

200 400 600 800

O2 C

once

ntr

atio

n (

ppm

)

Temperature (oC)

O2

kb= 2.1 x 10

-8 cm/sec

io = kf PO21/2 [VO

••] - kb [OOX] [h•]2

Fundamental Rate Constants - Catalysis

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TPO of LSCFO-Absorption to fill VO

••

depending on PO2 history

LaSrCoFeO3-δ + δ/2O2 -> LaSrCoFeO3

0

500

1000

1500

200 400 600 800

O2 C

on

cen

trat

ion

(p

pm

)

Temperature (oC)

O2

kb= 2.13 x 10

-8 cm/sec

800

1200

1600

2000

200 400 600 800

O2 C

once

ntr

atio

n (

ppm

)

Temperature (oC)

He (<1ppm O2)

! = 0.009

1000 ppm O

! = 0.0004

10 ppm O2

! = 0.008

100 ppm On

! = 0.005

1087 ppm O2

Baseline

kb = 2.1 x 10

-8 cm/sec

O2 Release

O2 Adsorption

kchem = kf [VO••]

Fundamental Rate Constants - Catalysis

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TPD of LSCF in 3000 ppm O2

Bulk-O desorption

LaSrCoFeO3 -> LaSrCoFeO3-δ + δ/2O2

0

500

1000

1500

200 400 600 800

O2 C

on

cen

trat

ion

(p

pm

)

Temperature (oC)

O2

kb= 2.13 x 10

-8 cm/sec

0

2000

4000

6000

8000

0 200 400 600

total oxygen

O2 C

on

cen

trat

ion

(p

pm

)

Temperature (°C)

Fundamental Rate Constants - Catalysis

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TPD of LSCF in 3000 ppm O2

Isotopically Labeled - 18O2

LaSrCoFeO3 -> LaSrCoFeO3-δ + δ/2O2

0

2000

4000

6000

8000

0 200 400 600

O2 C

once

ntr

atio

n (

ppm

)

Temperature (°C)

16O

18O

16O

2

18O

2

Total Oxygen

Indicates complex mechanism18O2 = gas phase oxygen16O2 = lattice oxygen16O18O = scrambled product due tosurface reaction

Fundamental Rate Constants - Catalysis

UF-DOE HiTECTPD of LSCF in 3000 ppm 18O2

0

2000

4000

6000

8000

0 200 400 600

O2 C

once

ntr

atio

n (

ppm

)

Temperature (°C)

16O

18O

16O

2

18O

2

Total Oxygen

• At low temp 18O2 is incorporatedinto lattice 18O2 + 2VO

•• = 218OOx

• At intermediate temp 18O2dissociates on surface 18O2 = 218Oadsand is then either incorporatedinto lattice 18Oads + VO

•• = 18OOx

or reacts with bulk/surface 16OOx

and desorbs 18Oads+ 16OO

x = 18O16O

• At high temp 18O2 incorporatescompletely into lattice but at arate less than 16O2 evolutionfrom the lattice kf P18O2

n [VO••] < kb [16OO

X]

Fundamental Rate Constants - Catalysis

UF-DOE HiTECTPD in 3000 ppm 18O2

0

2000

4000

6000

8000

0 200 400 600

O2 C

once

ntr

atio

n (

ppm

)

Temperature (°C)

16O

18O

16O

2

18O

2

Total Oxygen

LSCF

0

2000

4000

6000

8000

0 200 400 600

Ox

yg

en C

on

cen

trat

ion

(p

pm

)

Temperature (°C)

Total Oxygen

18O

2

16O

18O

16O

2

LSM

Fundamental Rate Constants - Catalysis

UF-DOE HiTEC

• Isothermal O18 isotope exchange of LSMand LSCF

• Determine reaction order/mechanisms fromPO2 and O18/O16 dependence r ~ k PO2

n

Future Work

• Integrate isotope results into impedance/microstructure resultsto deconvolute contributions to cathode polarization

• Compare/contrast LSM vs. LSCF results and gain fundamentalinsight into cathode materials/microstructure development

• Determine rate constant activationenergies from temperature dependence

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ACKNOWLEDGEMENTSupport:U.S. DOE High Temperature Electrochemistry CenterContract DE-AC05-76RL01830Siemens Embryonic Research Grant U05-01

FIB/SEM Characterization:Dan Gostovic, Aijie Chen and Kevin Jones

Impedance Spectra:Jeremiah Smith, Keith Duncan and Mark Orazem

TPD & Isotope Exchange:Cynthia Kan, Martin VanAssche and Eric Armstrong