PERFORMANCE ANALYSIS OF SINGLE STEP
CO-FIRED SOLID OXIDE FUEL CELLS (SOFCS)
KYUNG JOONG YOON, PETER A. ZINK, UDAY B. PAL, SRIKANTH GOPALAN
DEPARTMENT OF MECHANICAL ENGINEERING
DIVISION OF MATERIALS SCIENCE AND ENGINEERING
BOSTON UNIVERSITY
OUTLINE
™ Introduction
™ Single Step CoCo-firing Process Development
™ Polarization Modeling
™ Polarization Analysis
™ Fuel Utilization Test
™ Advanced
Ad
d Cathode
C h d Investigation
I
i i
™ Summary & Future Work
INTRODUCTION
™ Major Challenges for Commercialization of SOFCs
High Manufacturing Costs
Reduced Cell Performance at Low Operating
(SOFC System Costs per Unit of Power ($/kW))
Temperature (Interconnects, Sealing, etc.)
Single Step CoCo-Firing Process
Polarization Modeling:
Modeling: Systematic Analysis of
with Conventional Material System
Cell Performance and Polarization Losses
™ Research
R
hG
Goals
l
ƒ Manufacture SOFCs with the Lowest Manufacturing Costs
by Single Step Co
Co--Firing of the Entire SOFCs
ƒ Achieve the Highest Cell Performance and Lower Operating Temperature
by Optimization of Materials, Microstructures, and Process Parameters
using Polarization Modeling
Minimize $/kW
SINGLE STEP CO-FIRING PROCESS
Cathode Current Collector:
Collector: Screen Printing
Ca
CaC -doped
d
d LaMnO
L M O3 (Porous)
(P
)
Cathode Active Layer:
Layer: Screen Printing
Ca--doped LaMnO3 + YSZ (Fine & Porous)
Ca
Electrolyte:: Screen Printing
Electrolyte
YSZ ((Dense))
Co--Firing in Air
Co
(1300--1330oC)
(1300
Anode Active Layer:
Layer: Screen Printing
Ni + YSZ (Fine & Porous)
Anode Support:
Support: Tape Casting
Ni + YSZ (Porous)
PROCESS DEVELOPMENT
™ Lowered Electrolyte Sintering Temperature
ƒ Density of YSZ : Without Sintering Aid: ~ 94%,
94%, With Sintering Aid: ~ 99+% @1300oC
™ Matched Thermal Expansion Coefficients and Sintering Shrinkages
™ Developed Refractory Cathode Composition
ƒ Doped
Doped--(La,Ca
La,Ca)MnO
)MnO3
Doped-LCM
LSM
™ Optimized Thicknesses and Porosities of Electrodes
™ Optimized
p
Particle Sizes of Initial Powders
™ Evaluated Pore Former Material
ƒ Carbon Black
Carbon Black
™ Employed Cathode Current Collector
Methyl Cellulose
Cathode Current Collector
: (La,Ca)MnO
(L C )M O3
50μm thick, 50% porous
PROCESS DEVELOPMENT
2
Power Density ((W/cm )
0.6
o
Temperature: 900 C
H2+3% H2O / Air
Cathode Particle Size Optimization
Cathode Current Collector
Anode Porosity Optimization
0.5
0.4
Pore Former Material Optimization
0.3
0.2
0.1
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2
Current Density (A/cm )
Cathode Current Collector:
LCM 50μ
LCM,
50μm thick
thick, 50% porous
Cathode Active Layer:
LCM--YSZ, 30
LCM
30μ
μm thick, 31% porous
Electrolyte: YSZ, 15μ
15μm thick
Anode Support:
Ni--YSZ, 850
Ni
850μ
μm thick, 32% porous
BASELINE CELL: POWER DENSITY
2
Power Density (W
W/cm )
0.6
2
Max.=0.55 W/cm
™ Test Condition
ƒ Temperature: 700~800oC
0.5
ƒ Fuel: 97% H2+3% H2O
0.4
2
Max.=0.36 W/cm
ƒ Oxidant: Air
0.3
2
Max =0 23 W/cm
Max.=0.23
0.2
o
800 C
o
750 C
o
700 C
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2
Current Density (A/cm )
POLARIZATION MODEL: THEORY
E
C
= E − iR −
0
i
η
act
−
η
conc
ƒ Activation Polarization (ηact)* :
α ne η act F
i = i 0 exp(
RT
) − i0 exp(-
( 1 − α ) neη act F
2
η actt = RT ln{ 1 [( i ) + ( i ) + 4 ]}
F
2 i0
i0
RT
) (α=1/2, ne=2)
(Anode + Cathode)
o
pH 2 i
RT
i
RT
ln(1 − ) +
ƒ Concentration Polarization (ηconc)** : ηconc = − RT ln(1 − i ) −
+
ln(1 o
)
2F
ias
4F
ics
p
2F
H Oi
2
as
1 i
i
RT
i 2
RT
i
RT
RT
p Ho 2i
EC = Eo − i R i −
ln{ [( ) + ( ) + 4 ]} +
ln(1 − ) +
ln(1 − ) −
ln(1 + o
)
F
i
4
F
i
2
F
2
F
p
i
2 io
ias
o
cs
H 2 O as
Open Circuit Ohmic
Potential
Polarization
Activation
Polarization
Concentration
Polarization (Cathode)
* P.W.Li
P.W.Li,, M.K.Chyu
M.K.Chyu,, J. Heat Transfer,
Transfer, vol.127, 1344 (2005)
** J.W.Kim,
J.W.Kim, A.V.Virkar,
A.V.Virkar, KK-Z.Fung
Z.Fung,, K.Mehta,
K.Mehta, S.C.Singhal
S.C.Singhal,, J.Electrochem.Soc
J.Electrochem.Soc.146
.146 (1) (1999) 69
Concentration
Polarization (Anode)
POLARIZATION MODELING: CURVE-FITTING (EXAMPLE)
™ Test Condition
ƒ Temperature: 800oC
ƒ Fuel: 97% H2+3% H2O (300cc/min): Fixed
1.2
Fitting
Parameters
100% O2
21% O2 +
79% N2
8% O2 +
92% N2
Ri (Ω·
Ω·cm
cm2)
0.082
0.082
0.082
io (A/cm2)
0.52
0.28
0.097
ias (A/cm
( / 2)
5.77
5.77
5.77
ics (A/cm2)
-
5.42
1.77
ƒ Oxidant: 100%,
100% 21%,
21% 8% O2 + N2 (1000cc/min)
100% O2
21% O2 + 79% N2
1.0
8% O2 + 92% N2
Voltage (V)
0.8
Curve-fitted
Curve
fitted
0.6
0.4
0.2
Assumption: ics >> ias and ηconc,c<< ηconc,a, ηact with 100% O2
0.0
0
1
2
3
4
5
6
2
C
Current
tD
Density
it (A/cm
(A/
)
ƒ Anode Limiting Current
i as =
2 F p Ho 2 D Heff2 − H 2 O
2
DeffH2
H2--H2O = 0.23 cm /s
R T la
4 F`p o D
ƒ Cathode Limiting Current i cs =
O2
(
p − po
O2
p
eff
O 2− N 2
2
DeffO2
O2--N2 = 0.037 cm /s
)RTl
c
POLARIZATION MODELING: VERIFICATION OF ASSUMPTION
Various Polarization Losses @800oC with
Cathodic Limiting Current Density
Humidified Hydrogen (3% H2O) and Air
@800oC as a Function of pO2 in Oxidant
1000
2
Cathodic L
Limiting Currentt Density (A/cm )
Polarization
n (V)
o
0.6
Temperature: 800 C
Fuel: 97% H2+3% H2O
0.5
Oxidant: Air
0.4
0.3
0.2
ic
Anod
01
0.1
0.0
n
rizatio
n Pola
io
t
a
iv
t
Ac
n
zatio
olari
P
n
o
ti
entra
Conc
olarization
centration P
Cathodic Con
0
1
2
3
4
5
6
o
Temperature: 800 C
800
600
400
200
0
0.0
2
0.2
0.4
0.6
0.8
1.0
pO2 in oxidant (atm)
Current Density (A/cm )
ƒ ηconc,c << ηconc,a and ηact except at high
ƒ ics rapidly increases as pO2 increases and
current density near ics
approaches 100% O2
™ Consistent with assumption that ηconc,c is negligible compared to other polarization
losses when oxygen at the cathode nears 100%
POLARIZATION MODELING & IMPEDANCE SPECTROSCOPY
ƒ Activation Polarization Resistance at OCV
ƒ Anodic Concentration Polarization Resistance at OCV
ƒ Cathodic Concentration Polarization Resistance at OCV
0.4
0.2
0.0
0.2
0.8
21% O2 + 79% N2
8% O2 + 92% N2
0.6
0.8
1.0
Impedance Spectroscopy
Polarization Model
2
100% O2
0.4
Polarizzation Resistance ((Ohm-cm )
2
Z(Im) (Ohm-cm )
Impedance Spectroscopy @800oC
1.2
2
Z(Re) (Ohm-cm )
High Frequency Intercept = RΩ
Low Frequency Intercept = Rpolar + RΩ
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
pO2 in oxidant (atm)
1.0
POLARIZATION ANALYSIS #1: BASELINE CELL
1.2
Ohmic Loss (Electrolyte)
1.0
Ohmic Loss (Electrode) : Anode + Cathode + Contacts
Volltage (V)
0.8
Activation Polarization
0.6
Cathodic Concentration Polarization
Anodic Concentration Polarization
0.4
o
Temperature: 800 C
H2-3% H2O / Air
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2
Current Density (A/cm )
™ Dominant Polarization Losses
ƒ Ohmic Loss (Electrode)
ƒ Activation
A ti ti
Polarization
P l i ti
(Cathode)
(C th d )
ƒ Concentration Polarization (Anode)
POLARIZATION ANALYSIS #2: CONTACT RESISTANCE
™ Cathode Current Collection : Ag Mesh vs. Pt Mesh
1.2
1.0
o
Temperature: 800 C
H2-3% H2O / Air
2
Max.=0.9W/cm
1.0
Voltage
e (V)
0.8
2
Max.=0.55W/cm
0.6
06
0.6
0.4
0.4
0.2
0.2
0.5
1.0
1.5
Relectrode = 0.067 Ω·
Ω·cm
cm2
Tm(Pt) = 1772oC
Relectrode = 0.156 Ω·
Ω·cm
cm2
2
0.0
0.0
Ag Mesh
Pt Mesh
Tm(Ag) = 961oC
Power Density (W/cm )
0.8
2.0
2.5
0.0
3.0
2
Current Density (A/cm )
ƒ Ag is softer than Pt at operating temperature.
⇒ Higher Interfacial Contact Area ⇒ Lower Ohmic Electrode Resistance
ƒ In SOFC stacks, the effect of the contacts between the electrode and interconnects can be
substantial.
POLARIZATION ANALYSIS #3: CATHODE MICROSTRUCTURE
™ Sintering Temperature:
1.2
vs.
1330oC
Max.=1.4W/cm
1.4
1.0
2
Voltag
ge (V)
Max.=0.9W/cm
0.6
0.8
0.6
0.4
0.4
1300 C Sintering
o
1330 C Sintering
0
1
2
0.2
3
4
io = 0.27 A/cm2
1330oC Sintering
2
o
Power Den
nsity (W/cm )
0.8
0.0
Avg. Grain Size = 2.3 μm
2
1.2
0.2
1300oC Sintering
1.6
o
Temperature: 800 C
H2-3% H2O / Air
10
1.0
1300oC
Avg Grain Size = 3.2
Avg.
3 2 μm
0.0
2
Current Density (A/cm )
0.3
2
-IM(Z) (Ω cm )
Impedance Spectroscopy
0.2
o
1300 C Sintering
o
1330 C Sintering
io = 0.16 A/cm2
0.1
0.0
1.9
ƒ No Significant Change in Anode Microstructure.
2.0
2.1
2.2
2.3
2.4
2
Re(Z) (Ω cm )
2.5
2.6
ƒ Less Sintering of Cathode ⇒ Low Activation Polarization
POLARIZATION ANALYSIS #4: ANODE POROSITY
8 wt% Carbon Black
™ Pore Former in Anode: 8 wt% C vs. 5 wt% C
12
1.2
o
Temperature: 800 C
H2-3% H2O / Air
1.0
1.6
2
Max.= 1.5 W/cm
1.4
Max.= 1.4 W/cm
Voltage (V)
V
1.2
1.0
0.6
0.8
0.6
0.4
0.4
0.0
8 wt% Carbon Black
5 wt% Carbon Black
0
1
2
0.2
3
4
5
ias = 5.77 A/cm2
2 -1
DeffH2
H2--H2O = 0.23 cm s
5 wt% Carbon Black
Anode Porosity: 32%
2
0.2
Power Density (W
W/cm )
2
0.8
Anode Porosity: 37%
0.0
2
Current Density (A/cm )
ƒ Increased Anode Porosity
⇒ Low Anode Concentration Polarization
ias = 4.97 A/cm2
2 -1
DeffH2
H2--H2O = 0.19 cm s
2
Maxim
mum Pow
wer Density (W/cm
m)
CELL PERFORMANCE IMPROVEMENT CHART
1.6
1.4
1.5 W/cm2
o
Temperature: 800 C
97% H2+3% H2O - Air
Comparable to the StateState-of
of-the--Art Cells Fabricated with
the
1.2
Multiple
p Firing
g Steps
p
1.0
0.8
0.6
0.4
0.2
0.0
11/4/0511/30 12/231/30/06 2/15 4/10 4/27
5/3
Date
5/8
6/15 6/19 6/29 7/14 7/20 9/28
FUEL UTILIZATION TEST : BACKGROUND
ƒ Fuel Utilization
Ut =
Molar flow rate of reactants consumed in a cell
Molar flow rate of reactants supplied into the cell
&
Nconsumed
=
&
Nin
ƒ Fuel utilization increases along the flow path over the electrode surface.
⇐ Fuels are consumed and products are formed along the flow path.
ƒ Cell Performance Loss near Exit (High Fuel Utilization)
• Loss of Nernst Potential
• Anodic Activation Polarization
• Anodic Concentration Polarization
ƒ Simulate the effect of practical fuel utilization on single cell performance
by increasing H2O content in fuel
FUEL UTILIZATION TEST : EFFECT OF ANODE ACTIVE LAYER
With Anode Active Layer
o
Temperature: 800 C
o
Temperature: 800 C
1.2
2
1.2
Power De
ensity (W/cm )
1.4
2
Power De
ensity (W/cm )
1.4
Without Anode Active Layer
1.0
0.8
H2-3% H2O / Air
H2-30% H2O / Air
0.6
H2-40% H2O / Air
0.4
H2-50% H2O / Air
H2-60% H2O / Air
0.2
0.0
0.0
H2-70% H2O / Air
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1.0
0.8
H2-40% H2O / Air
0.4
H2-50% H2O / Air
H2-60% H2O / Air
0.2
0.0
0.0
2
H2-70% H2O / Air
0.5
1.0
1.5
2.0
2.5
3.0
3.5
2
Current Density (A/cm )
Porosity = 26%
Fuel
Compositions
p
Porosity = 37%
Avg. Grain Size = 4.3 μm
Avg. Pore Size = 2.6 μm
Max. Power Density
(W/cm2)
With
AAL
Without
AAL
H2 – 3% H2O
1.41
1.40
H2 – 30% H2O
1.27
1.25
H2 – 50% H2O
1.17
0.91
H2 – 70% H2O
0.84
0.45
Avg. Grain Size = 1.3 μm
Avg. Pore Size = 0.7 μm
Anode Support
H2-30% H2O / Air
0.6
Current Density (A/cm )
Anode Active Layer
H2-3% H2O / Air
FUEL UTILIZATION TEST : POLARIZATION MODELING
Ri, DeffO2-N2 : Independent of Fuel Composition
DeffH2-H2O : Independent of H2/H2O ratio (Kinetic Theory of Gases)*
With Anode Active Layer
97% H2-3% H2O
97% H2-3% H2O
1.2
40% H2-60% H2O
40% H2-60% H2O
---- Curve-fitted
0.6
0.4
0.2
0.8
Voltage (V)
30% H2-70% H2O
0.5
1.0
1.5
2.0
2.5
3.0
2
Current Density (A/cm )
3.5
Without
AAL
H2 – 3% H2O
0.87
0.98
H2 – 30% H2O
0.84
0.78
H2 – 40% H2O
0.82
0.70
H2 – 50% H2O
0.79
0.57
H2 – 60% H2O
0.75
0.43
H2 – 70% H2O
0.53
0.22
0.6
0.4
Temperature: 800 C
0.0
0.0
30% H2-70% H2O
---- Curve-fitted
0.2
o
50% H2-50% H2O
1.0
50% H2-50% H2O
Exchange Current
Density (A/cm2)
With
AAL
60% H2-40% H2O
60% H2-40% H2O
0.8
Fuel
Compositions
70% H2-30% H2O
1.2
70% H2-30% H2O
1.0
Volttage (V)
Without Anode Active Layer
0.0
0.0
o
Temperature: 800 C
0.5
1.0
1.5
2.0
2.5
3.0
2
Current Density (A/cm )
3.5
* R. Byron Bird, Warren E. Stewart, Edwin N. Lightfoot, Transport Phenomena, John Wiley & Sons (1960)
FUEL UTILIZATION TEST : PERFORMANCE ANALYSIS
™ Activation Polarization:
ƒ At Low Fuel Utilization (H2-3% H2O)
⇒ Dominated by Cathode
(No Difference in Cell Performance and Exchange Current Density due
to Anode Active Layer at Low Fuel Utilization)
ƒ At High Fuel Utilization
⇒ Anodic Activation Polarization Increases
⇒ Cathodic Activation Polarization: Independent of Fuel Composition
FUEL UTILIZATION TEST : PERFORMANCE ANALYSIS
™ Anodic Activation Polarization:
Without Anode Active Layer
With Anode Active Layer
30% H2-70% H2O
40% H2-60% H2O
Anodic Activation Polarization (V
V)
Anodic Activation Polarization (V
V)
30% H2-70% H2O
0.20
50% H2-50% H2O
0.15
60% H2-40% H2O
70% H2-30% H2O
---- Curve-fitted
0.10
0.05
0.00
00
0.0
02
0.2
04
0.4
06
0.6
08
0.8
2
Current Density (A/cm )
10
1.0
0.20
40% H2-60% H2O
Fuel
Compositions
Anodic Exchange
Current Density
(A/cm2)
With
AAL
Without
AAL
H2 – 30% H2O
26.73
4.18
H2 – 40% H2O
15.68
2.70
H2 – 50% H2O
9.47
1.53
H2 – 60% H2O
6.02
0.88
H2 – 70% H2O
1.55
0.34
Cathodic
Exchange
Current
Density
(A/cm2)
0.87
0.98
50% H2-50% H2O
0.15
60% H2-40% H2O
70% H2-30% H2O
---- Curve-fitted
0.10
0.05
0.00
00
0.0
02
0.2
04
0.4
06
0.6
08
0.8
2
Current Density (A/cm )
10
1.0
FUEL UTILIZATION TEST : PERFORMANCE ANALYSIS
™ Anodic Concentration Polarization:
Determined by Local H2-H2O Equilibrium:
Equilibrium:
Calculation of pi,aO2
In Steady State,
FUEL UTILIZATION TEST : PERFORMANCE ANALYSIS
Anodic C
Concentratio
on Polarizattion (V)
™ Anodic Concentration Polarization:
0.4
2
i = 4.0 A/cm
0.3
Temperature = 800o C
Deff H2-H2O = 0.27cm2//s
la = 850μm
i = 3.0 A/cm2
i = 2.0
2 0 A/cm2
0.2
i = 1.0 A/cm2
0.1
i = 0.1 A/cm2
0.0
0.0
0.2
0.4
p
H2O
0.6
0.8
(
(atm)
)
ƒ Anodic concentration polarization is low when the fuel is in the intermediate H2O
partial pressure region.
ƒ Anode active layer had no significant effect on anodic concentration polarization.
1.0
FUEL UTILIZATION TEST : POLARIZATION ANALYSIS
™ Anodic Electrode Polarization Loss:
Without Anode Active Layer
0.40
Activation+Concentration
Activation
0.35
o
Temperature: 800 C
0.30 Current Density: 1.5A/cm2
0 25
0.25
0.20
0.15
Anodic Activation
Polarization
0.10
0.05
0.00
0.0
Anodic Concentration
Polarization
0.1
0.2
0.3
o
p
H2O
0.4
0.5
0.6
0.7
0.8
0.9
Anodic
c Electrode Polarization (V)
Anodic
c Electrode Polarization (V)
With Anode Active Layer
0.40
Activation+Concentration
Activation
0.35
o
Temperature: 800 C
0.30 Current Density: 1.5A/cm2
0 25
0.25
0.20
Anodic Activation
Polarization
0.15
0.10
0.05
0.00
0.0
in Fuel (atm)
Anodic Concentration
Polarization
0.1
0.2
0.3
o
p
H2O
0.4
0.5
in Fuel (atm)
ƒ Activation Polarization: Dominant Loss at High Fuel Utilization
⇒ Significantly Reduced by Anode Active Layer
0.6
0.7
0.8
0.9
ADVANCED CATHODE INVESTIGATION : BACKGROUND
ƒ Cathode Polarization Loss : Major Difficulty in Lowering the Operating Temperature
ƒ A sitesite-doped Lanthanum Cobaltite
• High Catalytic Activity and Mixed Electronic
Electronic--Ionic Conductivity
• High Thermal Expansion Coefficient
• Solid State Reaction with YSZ at Low Temperature
ƒ A sitesite-doped Lanthanum Ferrite
•
•
•
•
High Catalytic Activity and Mixed Electronic
Electronic--Ionic Conductivity
Adj t bl Thermal
Adjustable
Th
l Expansion
E
i
No Solid State Reaction with YSZ up to 1400oC
Diffusion of Zr4+ into Lanthanum Ferrite : Doped Ceria Interlayer
ƒ Calcium
Calcium--doped Lanthanum Ferrite
• Defect Model
• Thermogravimetry
g
y Measurements
pO2 - Weight Relationship
⇒ Equilibrium Defect Concentration
• Electrical Conductivity Measurements ⇒ Hole Mobility
ADVANCED CATHODE INVESTIGATION : POINT DEFECT MODEL
™ Point Defect Model for (La0.8Ca0.2)0.95FeO3-δ
A-site
B-site
LaxLa
FexFe
CaLa = 0.2×
0.2×0.95
VLa
= 0.05
Fe•Fe
FeFe
VFe
ƒ Oxygen Incorporation Reaction
ƒ Charge
Ch
Di
Disproportionation
ti
ti Reaction
R
ti
ƒ Schottky Equilibrium Reaction
O-site
OxO
≈0
≈0
VO••
ADVANCED CATHODE INVESTIGATION : POINT DEFECT MODEL
ƒ Charge Neutrality Condition
2 VO••
Fe•Fe
CaLa
Fe•Fe
3 VLa
CaLa
= 0.2 0.95
3 VLa
= 0.05
2 VO••
ƒ A-site Restriction
LaxLa
VLa
CaLa
1
ƒ B-site
it Restriction
R t i ti
FexFe
Fe•Fe
1
FexFe
Fe•Fe
1
ƒ O-site Restriction
OxO
VO••
3
OxO
3
VO••
ƒ Mass Action Coefficient for Oxygen Exchange Reaction
1
CaLa
= 0.2×
0.2×0.95
3 VLa
= 0.05
2 VO••
ADVANCED CATHODE INVESTIGATION : POINT DEFECT MODEL
ƒ Relationship between pO2 and Weight of (La0.8Ca0.2)0.95FeO3-δ
ADVANCED CATHODE INVESTIGATION : THERMOGRAVIMETRY
™ Thermogravimetry Measurements
o
o
106.3
o
o
o
o
o
o
900 C 850 C 800 C 750 C 700 C 650 C 600 C 550 C
o
850 C
o
800 C
o
750 C
o
700 C
o
650 C
o
600 C
o
550 C
----- Curve-Fitted
C
Fitt d
1.0
0.8
pO2 (atm)
Weigh
ht (μg)
106.2
106.1
pO2=1
pO2=0.21
106.0
0.6
0.4
0.2
pO2=0.05
pO2=0.001
0
200
400
600
800
1000
1200
0.0
0.1060
0.1061
0.1062
Weight (g)
Time (min)
n = 0.00049 mol
Temperature
850oC
800oC
750oC
700oC
650oC
600oC
550oC
Kox
186
235
317
376
504
630
813
0.1063
ADVANCED CATHODE INVESTIGATION : DEFECT EQUILIBRIUM
ƒ Equilibrium Defect Concentration
ƒ ln
ln((Kox) vs. 1/T
0.35
7.0
---- Linear Fit
0.25
o
.
550oC
650 C
o
750 C
o
850 C
65
6.5
[FeFe]
6.0
o
Slope = -ΔH ox/R
0.20
ln(K
Kox)
Defect Con
ncentration
0.30
0.15
5.0
0.10
0.05
0.00
5.5
..
o
850oC
750oC
650oC
550 C
-3.5
-3.0
4.5
[VO ]
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
4.0
log(pO2(atm))
0.9
1.0
1.1
-1
1000/T (K )
¾ High pO2 range
•
•
[Vo••]: low
Charge neutrality is maintained by hole formation.
¾ Low
L
pO2 range
• [Vo••]: high
• Hole concentration decreases.
∆Hoox = -37.5
37 5 kJ moll-1
∆Soox = 10.6 J mol-1 K-1
1.2
ADVANCED CATHODE INVESTIGATION : ELECTRICAL CONDUCTIVITY
™ Electrical Conductivity Measurements
ƒ Conductivity vs. Temperature
ƒ Mobility vs. Temperature
140
0.6
-1 -1
Hole Mobility
y (cm V s )
100
2
Conductiv
vity (S/cm)
120
80
60
pO2=1 atm
40
pO2=0.21 atm
pO2=0.05 atm
20
0
0.5
0.4
0.3
pO2=1 atm
0.2
pO2=0.21 atm
pO2=0.05 atm
0.1
pO2=0.001 atm
pO2=0.001 atm
550
600
650
700
750
800
850
o
Temperature ( C)
ƒ pO2↑ ⇒ σe ↑ : p-type conductor
ƒ Low Temperature:
p
Thermally
y Activated Behavior
(Small Polaron Hopping)
ƒ High Temperature: Decrease in Hole Concentration
00
0.0
550
600
650
700
750
800
850
o
Temperature ( C)
ƒ pO2↓, T ↑ ⇒ [VO··] ↑
ƒ Hopping Conduction Via Fe4+-O-Fe3+ Chain
ƒ VO·· : Scattering Centers or Random Traps
for Electrons
ADVANCED CATHODE INVESTIGATION : CONDUCTION MECHANISM
ƒ Adiabatic Small Polaron Hopping
ƒ Non
Non--adiabatic Small Polaron Hopping
10.0
6.4
9.8
6.2
.
ln(μpT /(1-[FeFe]))
6.0
5.8
5.6
5.4
5.2
pO2=1 atm
pO2=0.21 atm
pO2=0.05
=0 05 atm
pO2=0.001 atm
9.2
pO2=1 atm
9.0
pO2=0.21 atm
1.0
1.1
pO2=0.05 atm
8.8
8.6
---- Linear Fit
0.9
9.4
3/2
ln(μpT/(1-[FeFe]))
.
9.6
pO2=0.001 atm
---- Linear Fit
0.9
1.2
1.0
1000/T (K )
-1
1000/T (K )
pO2 (atm
atm))
1.1
-1
Adiabatic Case
Non
Non--adiabatic Case
Activation Energy ((eV
eV))
R2
Activation Energy ((eV
eV))
R2
1
0.108
0.99353
0.149
0.99788
0.21
0.111
0.99344
0.152
0.99776
0.05
0.195
0.99423
0.235
0.99673
0.001
0.272
0.98927
0.310
0.99801
1.2
SUMMARY
ƒ
Successfully developed singlesingle-step un
un--constrained co
co--firing of the solid oxide
fuel cell @ 1300oC.
ƒ
Modeled cell performance.
ƒ
Achieved maximum power density of 1.50 W/cm2 at 800oC and 0.87 W/cm2 at
700oC with humidified hydrogen (3% H2O) and air.
ƒ
Simulated the effect of practical fuel utilization on single cell performance.
ƒ
Improved cell performance at high fuel utilization by employing anode active
layer.
ƒ
Investigated defect chemistry and electrical conduction mechanism of novel
cathode material (calcium(calcium-doped lanthanum ferrite).
FUTURE WORK
ƒ
Employ advanced cathode material in coco-firing process.
ƒ
Analyze performance at low operating temperature (600(600-700oC).
PUBLICATION LIST
ƒ Journal Papers
1. Kyung Joong Yoon, Wenhua Huang, Guosheng Ye, Srikanth Gopalan, Uday B. Pal, Donald A. Seccombe, Jr., “Electrochemical Performance
of Solid Oxide Fuel Cells (SOFCs) Manufactured by Single Step Co-firing Process,” Journal of the Electrochemical Society, 154 (4) B389 (2007).
2. Kyung Joong Yoon, Srikanth Gopalan, Uday B. Pal, “Effect of Fuel Composition on Performance of Single Step Co-fired Solid Oxide Fuel
Cells (SOFCs),” Journal of the Electrochemical Society, 154 (10) B1080 (2007).
3. Kyung Joong Yoon, Peter Zink, Srikanth Gopalan, Uday B. Pal, “Polarization Measurements on Single Step Co-fired Solid Oxide Fuel Cells
(SOFCs),” Journal of Power Sources, 172 (1) 39 (2007).
4. Kyung Joong Yoon, Srikanth Gopalan, Uday B. Pal, “Effect of Anode Active Layer on Performance of Single Step Co-fired Solid Oxide Fuel
Cells (SOFCs) at High Fuel Utilizations,” Journal of the Electrochemical Society, 155(6) B610 (2008).
5. Kyung Joong Yoon, Srikanth Gopalan, Uday B. Pal, “Analysis of Electrochemical Performance of Solid Oxide Fuel Cells (SOFCs) Using
Polarization Modeling and Impedance Measurements,” Journal of the Electrochemical Society, 156 (3) B311 (2008).
6. Kyung Joong Yoon, Guosheng Ye, Srikanth Gopalan, Uday B. Pal, “Cost-effective Single Step Co-firing Technique for Manufacturing Solid
Oxide Fuel Cells (SOFCs) using High Shear Compaction (HSC) Anode,” Journal of Fuel Cell Science and Technology, Accepted (2008).
7. Soobhankar Pati, Kyung Joong Yoon, Uday B. Pal, “Solid Oxide Electrolyte Electrolyzer with Liquid Metal Anode for Production of Hydrogen
y
from Waste and Steam,”
, submitted ((2009).
)
and Syn-Gas
8. Kyung Joong Yoon, Peter Zink, Larry Pederson, Srikanth Gopalan, Uday B. Pal, “Defect Chemistry and Electrical Properties of
(La0.8Ca0.2)0.95FeO3-δ,” in preparation (2009).
ƒ Conference Proceedings
1. Kyung Joong Yoon, Peter Zink, Srikanth Gopalan, Uday B. Pal, “Polarization Analysis in Single Step Co-fired Solid Oxide Fuel Cells
(SOFCs),” Materials Research Society Symposium Proceedings of the Fall 2006 Meeting, Vol. 972, AA 10-02
(SOFCs),
10 02 (2007).
2. Peter A. Zink, Kyung Joong Yoon, Wenhua Huang, Srikanth Gopalan, Uday B. Pal, Donald A. Seccombe, Jr., “Refractory Cathode
Investigation for Single Step Co-fired Solid Oxide Fuel Cells (SOFCs),” Materials Research Society Symposium Proceedings of the Fall 2006
Meeting, Vol. 972, AA 03-12 (2007).
3. Kyung Joong Yoon, Peter Zink, Uday B. Pal, Srikanth Gopalan, “High Performance Low Cost Co-fired Solid Oxide Fuel Cells (SOFCs),” ECS
Transactions, Vol. 7 (1) 579 (2007).
4 Kyung Joong Yoon,
4.
Yoon Srikanth Gopalan,
Gopalan Uday B.
B Pal,
Pal “Anode
Anode Polarization Effects in Single Step Co-fired Solid Oxide Fuel Cells (SOFCs)
(SOFCs),” ECS
Transactions, Vol. 7 (1) 565 (2007).
5. Peter Zink, Kyung Joong Yoon, Wenhua Huang, Uday B. Pal, Srikanth Gopalan, “Refractory Cathode Investigation for Single Step Co-fired
Solid Oxide Fuel Cells (SOFCs),” ECS Transactions, Vol. 7 (1) 399 (2007).
6. Kyung Joong Yoon, Srikanth Gopalan, Uday B. Pal, “Effect of Anode Active Layer on Performance of Single Step Co-fired Solid Oxide Fuel
Cells (SOFCs),” ECS Transactions, Vol. 13 (26) 249 (2008).
7 Kyung Joong Yoon,
7.
Yoon Srikanth Gopalan,
Gopalan Uday B.
B Pal,
Pal “Electrochemical Performance of Single Step Co-Fired
Co Fired Solid Oxide Fuel Cells (SOFCs)
Analyzed Using Polarization Modeling and Impedance Spectroscopy,” Materials Research Society Symposium Proceedings of the Fall 2008
Meeting, Vol. 1126, S10-02 (2008).
8. Peter A. Zink, Kyung Joong Yoon, Uday B. Pal, Srikanth Gopalan, “Electrical Performance of Calcium-doped Lanthanum Ferrite for Use in
Single Step Co-Fired Solid Oxide Fuel Cells (SOFCs),” Materials Research Society Symposium Proceedings of the Fall 2008 Meeting, Vol. 1126,
S11-02 (2008).
Thank you!