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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
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
OutlineIntroduction – Energy ConversionFuel Cell Background & MotivationImaging & Analysis at the Pore Level
Pore ImagingGas TransportElectrochemistry
Electrode Microstructure and GradingConclusions & Future Research Plans
Chiu Research LaboratoryDepartment of Mechanical Engineering
Global Energy Usage
Chiu Research LaboratoryDepartment of Mechanical Engineering
U.S. Energy Usage
Energy Information Association Annual Energy Outlook, DOE/EIA Report # 0383, 2007.C. Song, Catal. Today 115:2-32. 2006.
Chiu Research LaboratoryDepartment of Mechanical Engineering
Energy Conversion Efficiency
US Power Plants, 2003, in quadrillion BTUC. Song, Catal. Today 115:2-32. 2006.
Chiu Research LaboratoryDepartment of Mechanical Engineering
Energy Conversion TechnologiesFor Fossil Fuels:
SOA: Heat EnginesSteam CyclesGas TurbinesInternal Combustion
Fuel Cells?
High Electrical EfficiencyFuel FlexibilityScalable
Chiu Research LaboratoryDepartment of Mechanical Engineering
OutlineIntroduction – Energy ConversionFuel Cell Background & MotivationImaging & Analysis at the Pore Level
Pore ImagingGas TransportElectrochemistry
Electrode Microstructure and GradingConclusions & Future Research Plans
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
OutlineIntroduction – Energy ConversionFuel Cell Background & MotivationImaging & Analysis at the Pore Level
Pore ImagingGas TransportElectrochemistry
Electrode Microstructure and GradingConclusions & Future Research Plans
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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]
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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-
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
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
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
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
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
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
OutlineIntroduction – Energy ConversionFuel Cell Background & MotivationImaging & Analysis at the Pore Level
Pore ImagingGas TransportElectrochemistry
Electrode Microstructure and GradingConclusions & Future Research Plans
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
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.
Chiu Research LaboratoryDepartment of Mechanical Engineering
AcknowledgementsSponsors
Office of Naval ResearchArmy Research OfficeUniversity of Connecticut - Institute of Materials Science
CollaboratorsNaval Undersea Warfare CenterAdaptive MaterialsNexTech MaterialsXradiaAdvanced Photon Source, Argonne National Lab
Chiu Research LaboratoryDepartment of Mechanical Engineering
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
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