Gas Transport and Electrochemistry in Solid Oxide Fuel ... · Gas Transport and Electrochemistry in Solid Oxide Fuel Cell Electrodes Wilson K. S. Chiu Department of Mechanical Engineering

Gas Transport and Electrochemistry in Solid Oxide Fuel Cell Electrodes

Wilson K. S. ChiuDepartment of Mechanical EngineeringUniversity of Connecticut

Chiu Research LaboratoryDepartment of Mechanical Engineering


Overview of Research ActivitiesHeat & Mass Transfer with Chemical Rxns:

P. Russell, Science 299:358, 2003.

PhotonicsCVD Nanomaterials Fuel Cells

Ni-YSZAnode

7 microns


OutlineIntroduction – Energy ConversionFuel Cell Background & MotivationImaging & Analysis at the Pore Level

Pore ImagingGas TransportElectrochemistry

Electrode Microstructure and GradingConclusions & Future Research Plans


Global Energy Usage


U.S. Energy Usage

Energy Information Association Annual Energy Outlook, DOE/EIA Report # 0383, 2007.C. Song, Catal. Today 115:2-32. 2006.


Energy Conversion Efficiency

US Power Plants, 2003, in quadrillion BTUC. Song, Catal. Today 115:2-32. 2006.


Energy Conversion TechnologiesFor Fossil Fuels:

SOA: Heat EnginesSteam CyclesGas TurbinesInternal Combustion

Fuel Cells?

High Electrical EfficiencyFuel FlexibilityScalable






Types of Fuel Cells

Small to large Power Plants

Power Plantsup to 1 MW

Power Plants50 to 200 kW

small units, up to automobiles

small units, up to automobiles

Applications

All Fuels,direct feed

Selected fuelor Converter

Hydrogen or Converter

Hydrogen or Converter

Hydrogen or NH3-Cracker

Fuel Supply

650-1000

600-1000

175-200

60-100

50-250

Operating Temperature

ceramic Zr/Y-oxides

Li/Na-carbonate melt

concentrated acid gel

immobilized, acidic

circulating liquid or matrix

Electrolyte

Solid Oxide (SOFC)

Molten Carbonate (MCFC)

Phosphoric Acid (PAFC)

Proton Exchange Membrane(PEM)

Alkaline (AFC)

Fuel Cells


Solid Oxide Fuel Cell

from http://www.thirdorbitpower.com/SOFC_mech.html

Hydrogen (& CO) Fueled Internal Reforming

COHOHCH +⇔+ 224 3

222 COHOHCO +⇔+

Steam Reforming

Shift Reaction


Atkinson et al., Nature Mat. 3:17-27, 2004. La0.3Sr0.7TiO3 current collector

Porous YSZ-active regionYSZ electrolyte

La0.3Sr0.7TiO3 current collector

Porous YSZ-active regionYSZ electrolyte

Previous Generation SOFC Current Generation SOFC

?Gorte et al., 2006

Next Generation

Electrochemistry Materials Design Fabricate TestDesignInput

How, specifically, can we design fuel cell microstructure to optimize performance and durability?

Modeling Tool needed to Determine Optimal Microstructure.






SOFC Pore Structure ImagingX-Ray Microtomography (XMT)

Non-destructive tomography capable of reconstructing 3-D pore structure.X-rays penetrate sample, which is mounted on a rotation stage, and pass through a scintillation screen. The 2-D slice is captured by a camera.The sample is rotated through 180 degrees where each slice is captured at specified angle intervals and reconstructed into a 3-D volume of the pore structure.


SOFC Pore Structure Imaging – cont’d

XMT Experiment detailsXradia (Concord, CA)8 keV copper source181 projections at 300 sec per projection22.6 µm field of view50 nm resolution3-D tomographicreconstruction


Imaging Processing and Modeling

Actual SEM image Model Image

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 00 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 00 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 1 1 1 1 1 0 0 00 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 10 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 10 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 10 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 10 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 10 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 1 11 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 11 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0

anodeDigital image file

5 µm

Actual SEM image Model Image

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 00 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 00 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 1 1 1 1 1 0 0 00 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 10 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 10 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 10 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 10 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 10 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 11 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 11 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 1 11 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 11 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0

anodeDigital image file

5 µmhttp://rsb.info.nih.gov/ij/

Geometry Input file for Model

ImageJ

A.S. Joshi, W.K.S. Chiu, et al., 9th AIAA-2006-3820, 2006.


Modeling Challenges

SOLID

• Gas diffusion occurs at high temperature and through micron size pores. Continuum theory is no longer valid.

• Gas particles can get adsorbed on the solid material.

• Complex structure.

Chemical reactions among gas components need to be included for the case of internal reforming and at the TPB.5 [µm]


A Comparison of Modeling Approaches

10-4 10-3 10-2 10-1 100 101 102

KNUDSEN NUMBER

SIM

ULA

TIO

N C

OS

T

Lattice Boltzmann Method

Monte Carlo

Molecular Dynamics

continuum slip transition free molecular

Dusty Gas Model

10-4 10-3 10-2 10-1 100 101 102

KNUDSEN NUMBER

SIM

ULA

TIO

N C

OS

T

Lattice Boltzmann Method

Monte Carlo

Molecular Dynamics

continuum slip transition free molecular

Dusty Gas Model


Lattice Boltzmann Method (LBM)

SOLID FLUID

• Multiple species

• Complex geometry

• Parallel algorithm

• Wall interactions

• Non-continuum regime

• Historically derived from the lattice gas cellular automata

• LBM is a numerical approximation to the Boltzmann equation

M.E. McCracken M. E. and J. Abraham, Phys. Rev. E, 71:046704. 2005.


Basic LBM Algorithm: Stream and Collide

ni

ni

ni nn

i

,,

,,

1,,

σσσrrer Ω−=+

+

species old time-level

velocity distribution function BGK collision termdirection

discrete velocity

new time level

11e

12e1

3e14e

15e

16e 1

7e 18e

10e 1

1e

12e1

3e14e

15e

16e 1

7e 18e

10e


Multi-Component Gas Transport in a SOFC Anode

0 50 100 1500

50

100

150

0

0.1

0.2

0.3

0.4

0.5

X (lattice units)

Z (l

attic

e un

its)

N2 m

ole fraction

0 50 100 1500

50

100

150

0

0.1

0.2

0.3

0.4

0.5

X (lattice units)

Z (l

attic

e un

its)

N2 m

ole fraction

0 50 100 1500

50

100

150

0.1

0.2

0.3

0.4

X (lattice units)

Z (l

attic

e un

its)

H2 m

ole fraction

0 50 100 1500

50

100

150

0.1

0.2

0.3

0.4

X (lattice units)

Z (l

attic

e un

its)

H2 m

ole fraction

0 50 100 1500

50

100

150

0

0.1

0.2

0.3

0.4

X (lattice units)

Z (l

attic

e un

its)

H2 O

mole fraction

0 50 100 1500

50

100

150

0

0.1

0.2

0.3

0.4

X (lattice units)

Z (l

attic

e un

its)

H2 O

mole fraction

Digitized Pore

Structure

φ = 0.5

GasChannel TPB Gas

Channel TPB

A.S. Joshi, W.K.S. Chiu, et al., J. Power Sources, accepted, 2006.A.S. Joshi, W.K.S. Chiu, et al., 42nd Power Sources Conference, pp. 131-134, 2006.


LBM ValidationC

once

ntra

tion

pola

rizat

ion

[ V ]

Non-dimensional current density ( i* )

0

0.1

0.2

0.3

0 0.02 0.04 0.06 0.08 0.1

Zhao & Virkar (2005)

Chan et al. (2001)

Present LB model

Con

cent

ratio

n po

lariz

atio

n [ V

]

Non-dimensional current density ( i* )

0

0.1

0.2

0.3

0 0.02 0.04 0.06 0.08 0.1

Zhao & Virkar (2005)

Chan et al. (2001)

Present LB model


LBM Validation – cont’d.

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10

Diffusivity ratio ( D* )

Mol

e fra

ctio

n ra

tio (

X*

)236 23.6 2.36 0.23

Knudsen number ( Kn )

Pe=

0.01

Pe=

0.08

Pe=

0.16

Pe=

0.32

Pe=

0.48

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10

Diffusivity ratio ( D* )

Mol

e fra

ctio

n ra

tio (

X*

)236 23.6 2.36 0.23

Knudsen number ( Kn )

Pe=

0.01

Pe=

0.08

Pe=

0.16

Pe=

0.32

Pe=

0.48

LBM: data points Dusty Gas: solid linesA.S. Joshi, W.K.S. Chiu, et al., ASME IMECE2006-13620, 2006.


3D LBM Analysis of a SOFC Anode

X

Z

Y

Anode

(Ni + YSZ)

Electrolyte

(YSZ)

Pore space

H2

H2O

Surface representing the interface between the solid region and pores.

Pore phase

H2 mole fraction

Pore Structure (Ψ,<r>,<r2>),Properties,ηconc, ηohmic


Electrochemical Reaction Kinetics in a SOFC Anode

H O H O egelectrolyte

gNi2

22 2+ → +− −

k+ −/ #:Global Reaction:

The Gauckler Reaction Mechanism:

A. Bieberle and L.J. Gauckler, Solid State Ionics. 146: 23-41, 2002.

Reaction rate

Vo..:

X S• :S:

Oox:

Surface vacancySurface speciesOxygen vacanciesOxygen ion

SHSgH kk •⎯⎯ →←+ − 22)( 1,12

SOHSgOH kk •⎯⎯⎯ →←+ −2

2,22 )(

SSOHSHSO kk +•⎯⎯ →←•+• −3,3

SOHSOSOH kk •⎯⎯⎯ →←•+• − 24,42

SHSOHSSOH kk •+•⎯⎯ →←+• −5,52

−− ++•⎯⎯ →←+ NiOkkx

O eVSOSO 2..6,6

Electrolyte

Anode

2e-

H2(g)H2O(g)

(1)

H2O·S H·S

Triple Phase Boundary (TPB)

e-

O2-

(3)

(5)

(2) (2)

OH·S

O·S

O·S

O2-

(4)

(2)

(1)Dissociation/Adsorption/ Desorption

(2)Surface Diffusion(3)Triple Phase Boundary(4)O2- Diffusion(5) Ion Transfer into Anode

e-

Electrolyte

Anode

2e-

H2(g)H2O(g)

(1)

H2O·S H·S

Triple Phase Boundary (TPB)

e-

O2-

(3)

(5)

(2) (2)

OH·S

O·S

O·S

O2-

(4)

(2)

(1)Dissociation/Adsorption/ Desorption

(2)Surface Diffusion(3)Triple Phase Boundary(4)O2- Diffusion(5) Ion Transfer into Anode

e-


Electrochemical Model Rate EquationsTraditional analysis requires selection of single rate limiting step and equilibration

Langmuir-Hinshelwood type rate equationApproach breaks down when reaction mechanisms contain steps thatoccur at similar ratesFor proposed mechanism requires solution to set of (4) coupled non-linear, stiff differential-equations

( )( )

( )

∂θ∂

θ θ θ θ θ θ θ θ θ θ

∂θ∂

θ θ θ θ θ θ θ

∂θ∂

θ θ θ θ θ θ θ θ θ θ θ

∂θ∂

HV H H O OH V H O V OH H

H OV H O OH H O V OH H

OHH O OH V H O O OH H O V OH H

O

tk k k k k k

tk k k k k

tk k k k k k

t

= − − + + −

= − + − −

= − + − + −

− − −

− − −

− − −

2 2

2 2

12

12

3 3 5 2 5

22 2 2 4

25 2 5

3 3 4 2 42

5 2 5

( )= − + + − + + +

= + + + +

− − −k k k k k kH O OH V H O O OH V O

V H OH H O O

3 3 4 2 42

6 6

21

θ θ θ θ θ θ θ θ θ

θ θ θ θ θ

θi: Species Surface Coverage

( )

k kF

RT

k kF

RT

o

o

6 6

6 6

2

12

= ⋅⎛⎝⎜

⎞⎠⎟

= ⋅ −⎛⎝⎜

⎞⎠⎟− −

exp

exp

β η

β η

( )i FNN

k k

J niF

o

AV O= −

• =

−2

2

6 6θ θ

r r


Validation of Reaction Kinetics Function

0.0

0.2

0.4

0.6

0.8

1.0

0 0.1 0.2 0.3 0.4 0.5Overpotential [V]

Surf

ace

Cov

erag

e

HOHVacancyBessler (4)

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

0 0.1 0.2 0.3 0.4 0.5Overpotential [V]

Surf

ace

Cov

erag

e

H2OOBessler (4)

H S H S

H O S H O S

O S OH S S

g k k

g k k

k k

21 1

22 2

2

3 3

2 2+ ← →⎯⎯⎯ •

+ ← →⎯⎯⎯ •

• ← →⎯⎯⎯ • +

−

−

−

,

,

,

H O S O S OH S

H O S S OH S H S

O S O S V e

k k

k k

ox k k

o NI

24 4

25 5

6 6

2

2

• + • ← →⎯⎯⎯ •

• + ← →⎯⎯⎯ • + •

+ ← →⎯⎯⎯ • + +

−

−

− −

,

,

, ..

W.G. Bessler, Solid State Ionics 176:997-1011, 2005. K. N. Grew, W. K. S. Chiu, et al., ASME IMECE2006-13621, 2006.


Electrochemistry & Gas Transport in a SOFC Anode(a) (b)

(c) (d)

Reifsnider et. al. 2005 TPB at Ni-YSZ interfaces

Impermeable Obstacle

Solid Wall

Solid Wall

Traditional TPB Flux Interface (See Fig. 1d):

H2,

H2O

, & N

2C

once

ntra

tions

Spe

cifie

d

XL

Y

H H2 Flux

H2O Flux

New method allows electrochemically active boundaries to be placed on obstacles in an ad hock manner as TPB dictates in SEM (See Fig. 1c)

Solid Wall

Open Domain


Solid Wall

Solid Wall


H2,

H2O

, & N

2C

once

ntra

tions

Spe

cifie

d

XL

Y

H H2 Flux

H2O Flux


Solid Wall

Open Domain

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

(a) (b)

(c) (d)

Reifsnider et. al. 2005 TPB at Ni-YSZ interfaces


Solid Wall

Solid Wall


H2,

H2O

, & N

2C

once

ntra

tions

Spe

cifie

d

XL

Y

H H2 Flux

H2O Flux


Solid Wall

Open Domain


Solid Wall

Solid Wall


H2,

H2O

, & N

2C

once

ntra

tions

Spe

cifie

d

XL

Y

H H2 Flux

H2O Flux


Solid Wall

Open Domain

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

x / L

y / H

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

0.02

0.04

0.06

0.08

0.1

0.12

0.14

H2 M

ole Fraction

K. N. Grew, W. K. S. Chiu, et al., ASME IMECE2006-13621, 2006.

ηactk+/-#

θi






Structure can be described by three parameters:

0 d

anode electrolytecathode

model domain

Ψ,<r>, <r2>

TPB

channel

x

• Ψ – Structural Parameter (ε/τ)• Porosity: 0 ≤ ε ≤ 1

• Tortuosity: τ ≥ 1• ~10X as important upon performance as other parameters in conditions studied.

• <r> – Average pore radius• <r2> – Pore radius distribution

Electrode Microstructure

Pore-Level Analysis ⇒ Properties, ηact, ηohmic, ηconc


Effect of Grading Electrode MicrostructurePerformance evaluated with alternative anode designs

Two fuel streamsDiluted hydrogen - Conditions near cell outlet

10% H2 – 3% H2O – balance inert gasPartially reformed methanewith internal reforming- Approx. cell inlet

10% H2 – 49% CH4 –30% H2O – 6% CO – 5% CO2

Four microstructure cases1 - Ψ is a constant high value of 0.252 - Ψ is a constant low value of 0.053 - Ψ is high at the TPB and low at the gas supply channel 0.25 -> 0.054 - Ψ is low at the TPB and high at the gas supply channel 0.05 -> 0.25

Elec

trol

yte

Cat

hode

Ψ

0.25

0.05

43

2

1


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 100 200 300 400 500 600Current Density (mA/cm2)

Cel

l Pot

entia

l (V)

0.0

0.1

0.2

0.3

0.4

0.5

Pow

er D

ensi

ty (W

/cm

2 )

Cell PotentialPower Density

1173 K1123 K1073 K

Cell operating temperature

Model Validation by Experiments

E. S. Greene, W. K. S. Chiu, A. A. Burke and M. G. Medeiros, 42nd Power Sources Conference, pp. 451-454, 2006.

Electric Leads

Concentric Alumina Tubes

SOFC

Furnace


Effect of Electrode Grading on Performance

E. S. Greene, W. K. S. Chiu and M. G. Medeiros, J. Power Sources 161:225-231, 2006.

Diluted Hydrogen Feed With Internal Reforming


SOFC Performance Correlations

Curve FitsDa ~ d1.79

R2 = 0.999

Da ~ Ψ0.357

R2 = 0.998• Displays prevalently kinetically limited characteristics (Da<1)

• Exponent of power law fits display larger sensitivity of Da to d than Ψ

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.5 1.0 1.5 2.0 2.5 3.0

d [mm]

Da

0.10.20.30.40.5

INCREASING Ψ Ψ

22

2

HH

EC

reaction

diffusion

CDdRDa ==

ττ

E.S. Greene, M.G. Medeiros and W.K.S. Chiu, J. Fuel Cell Sci. Tech. 2:136-140, 2005.


Conclusions

SOFC is promising energy conversion device.Performance strongly dependent on electrode microstructure.Presented modeling/experimental approach to analyze and design SOFC electrodes.Optimized fuel cell electrodes can provide a durable high efficiency energy conversion technology for our society.


AcknowledgementsSponsors

Office of Naval ResearchArmy Research OfficeUniversity of Connecticut - Institute of Materials Science

CollaboratorsNaval Undersea Warfare CenterAdaptive MaterialsNexTech MaterialsXradiaAdvanced Photon Source, Argonne National Lab


Acknowledgements – Fuel Cell GroupAldo Peracchio, Visiting ScholarAbhijit Joshi, Post-Doctoral ResearcherGraduate Students

Srinath S. Chakravarthy Eric S. GreeneKyle N. GrewJohn R. IzzoAndrew C. Lysaght

Undergraduate & High School Students

Documents

Gas Transport and Electrochemistry in Solid Oxide Fuel ... · Gas Transport and Electrochemistry in Solid Oxide Fuel Cell Electrodes Wilson K. S. Chiu Department of Mechanical Engineering