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Fabrication of SOFC Electrodes byImpregnation Methods
R. J. GorteChemical & Biomolecular EngineeringUniversity of Pennsylvania
Collaborators: J. M. Vohs, M. D. Gross, S.W. Jung,and W. Wang
Support: ONR, DOE-BES (H 2 program)
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Solid Oxide Fuel Cell
Solid Oxide Fuel Cell
O 2 + 4e - 2O 2-
H 2 + O 2- H 2O + 2e -
e-e-
e-e-Anode
Electrolyte
Cathode
Air
H2O
Fuel
O 2- O 2-
Porous Ni/YSZ cermet
Porous YSZ/Sr-LaMnO 3
YSZ (Y-stabilized ZrO 2)
YSZ is a conductor for O2-
but an insulator for electrons.
Operating temperature:600 to 800C
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-0.2
0
0.2
0.4
0.6
0.8
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Z', (ohm*cm 2)
Z " , ( o h
m * c m
2 )
1 kHz
4 Hz
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1Current Density (A/cm 2)
V o
l t a g e
( V )
0
50
100
150
200
250
300
350
P o w e r
D e n s i
t y ( m W / c m
2 )Performance Analysis:
Open Circuit:V = Nernst Potential
= G/nF
Power output:P = V* i
Electrolyte Losses:V = i*R electrolyte Electrode Losses:
V = overpotential= i*R electrode
R electrode is composed of diffusion, surface kinetics, etc.
Electrolyte
Electrodes
R electrolyte
(R electrolyte +R electrode )
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Research Goals at Penn:1) Anodes for direct utilization of hydrocarbons
Reforming leads to system complexity and to energy lossesat high temperatures:
2) High-performance cathodesLSM-based cathodes used today limit SOFC performance.
Consider: C 4H 10 + 2O 2 = 4CO + 5H 2 At 700C H (kJ/mol) G #electrons
C 4H 10 + 6 O 2 = 4CO 2 + 5H 2O -2,660 -2,810 264CO + 5H 2 + 4 O 2 = 4CO 2 + 5H 2O -2,370 -1,760 18
-11% -37% -31%
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Ni-based anodes cannot be exposed to hydrocarbons:
Ni catalyzes carbon formation
This can be avoided with other metals
Ni in toluenefor 3 hrs, 700C
Ni in methanefor 3 hrs, 800C
Cu in methane
for 3 hrs, 800C
Cu in toluene
for 3 hrs, 700C
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Carbon formation is in , not just on, Ni:
(M. L. Toebes, et al., Catalysis Today, 2002)
20% CO/7% H 2550 C.
Ni Particles
Note: Fe, Co, and Ru will all form catalyze carbon formation
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Traditional electrode fabrication limits materials that
can be used because of high-temperature processing
YSZ Powder
Pressed NiO/YSZ powder
NiO/YSZ compositeReduce in H 2
800C
Porous Ni/YSZ cermet
in air Dense YSZDense YSZYSZ Powder
Cathodes: (Sr-doped LaMnO 3/YSZ)
Dense YSZ Dense YSZ
Screen print LSM-YSZ and fire to 1200C
1400C
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High temperatures restrict choices of materials:
1) Cannot use materials that will melt at sintering temp.(Ex: CuO melts at 1235C)
2) High-temperatures can cause solid-state reactions(Ex: LaCoO 3 reacts rapidly with YSZ above 1000C.)
3) Must be able to form the correct phase(Ex: Alloys and layered structures are not possible.)
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YSZ+pore formers
Electrolyte
Cathode
Anode
YSZ+pore formers
YSZ70 C, 8 MPa
Firing1400 1550 C
Lamination
Porous YSZ
Impregnation &
Calcination at 450 ~ 700 C
Electrode Fabrication by Impregnation:
Both Anodes & Cathodes formed by impregnation.
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Structure near electrolyte interface:
Electrode I
electrolyte
Electrode II
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Add active phase separately:
Ex: La 0.8Sr 0.2FeO 3-YSZ
40 wt% LSF-YSZ by impregnationPorous YSZ
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Advantages:
1. Separate firing temperatures for YSZ and active phase.- Avoids solid-state reactions between LSF & YSZ.- Can use low-melting solids (CuO).
2. Composite is non-random structure.
Mixed powdersImpregnated
LSM
Percolation thresholdfor random media
12.612.611.710.3CTE (10 -6/K), 300 to 1073 K
55%45%35%0%LSCo Weight Fraction in YSZb) CTE of LSCo-YSZ
a) Electrical conductivity of LSM-YSZ
CTE of LSCo is 23x10 -6/K
700 C in air, compositescalcined at 1523 K.
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YSZO2-
CeO 2
Cu Cu
C2H6
e-
CO 2H2O
Our past focus: Cu-ceria anodes
a) Cu for electronic conductivityb) Ceria for catalysis
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Cu-ceria composites perform in real fuels!
15%CeO2 25%Cu, 24um electrolyte, laminated cell, LSF, Heavy Naph 700 oC
0
0.2
0.4
0.60.8
1
1.2
0 5 10 15 20 25 30 35 40 45
Time, hour
V o l
t a g e , V
0
0.2
0.4
0.60.8
1
1.2
P o w e r
d e n s
i t y ,
W / c m
2Voltage
Power density
Operation on Heavy Naphtha, 700C; fuel was undiluted.
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0
100
200
300
400
500
0 200 400 600 800 1000 1200 1400
700 C, 30% conversion at peak power725 C, 40% conversion at peak power750 C, 44% conversion at peak power775C, 47% conversion at peak power800C, 50% conversion at peak power
P o w e r
D e n s i
t y ( m W / c m
2 )
Current Density (mA/cm2
)
700 C
725 C
750 C
775 C
800 C
0
200
400
600
800
1000
1200
0 200 400 600 800 1000 1200 1400
700 C, 1.2 ml/h decane725 C, 1.2 ml/h decane750 C, 1.2 ml/h decane775C, 1.4 ml/h decane800C, 1.7 ml/h decane
P o t e n
t i a l ( m
V )
Current(mA/cm2)
700 C725 C 750 C
775 C
800 C
Power densities can be good with optimized cells:Fuel: pure n-decane
Data from SRI InternationalCell area 6.4 cm 2
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0 50 100 150 200 250 300 3500.0
0.1
0.2
0.3
0.4
800 0C, Keep at ~0.6V
H2 with 10vol% H 2Oand
50ppm H 2S100ppm H 2S200ppm H 2S300ppm450ppm
600ppm900ppm
P o w e r
d e n s
i t y ( W / c m
2 )
Time (h) -22.0 -21.5 -21.0 -20.5 -20.0-14
-13
-12
-11
-10
-9
800 0C
Ce 2O2S
log P O 2 (P O 2 in atm)
l o g
P S 2
( P S 2
i n a t m
)
CeO 1.83
Cu-ceria-YSZ electrodes stable in sulfur:
Measure performance in H 2-H 2O-H 2S mixtures
900 ppm600 ppm450 ppm300 ppm200 ppm
100 ppm
50 ppm
Sulfur poisoning occurs when Ce 2O 2S is formed.
Note: Ni-anodes are poisoned at surface monolayer, ~ 5 ppm.
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Cu-based anodes have limitations:
Low catalytic activity (ceria is the catalyst)Limited to use at lower temperatures.
700C 900CB
Cu
B
Cu
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-0.5
0
0.5
1
1.5
2
0 0.5 1 1.5 2 2.5 3 3.5 4
Z re cm 2
- Z
i m
c m
2
Effect of thermal treatment on cell performance:
After heating to 800C After heating to 900C
Sintering of Cu:1) Loss of anode conduction2) Increased ohmic resistance
Cu-ceria-YSZ YSZ LSM-YSZ
3% H2O-97% H
2at 700C
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Solution 1: Electroplate Co onto Cu cermets
Idea: Coat thermally stable Cowith chemically stable Cu:
Co
Cu
Cu and Co do not form alloys
Cu Anode Ni Cermet
CuSO 4 + H 2S0 4Solution
DC
Cu 2+
e
e
V
Cu Anode Ni Cermet
CuSO 4 + H 2S0 4Solution
DC
Cu 2+
e
e
Cu Anode Ni Cermet
CuSO 4 + H 2S0 4Solution
DC
Cu Anode Ni Cermet
CuSO 4 + H 2S0 4Solution
DCC
Cu 2+
e
e
V
Co
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Key is to deposit evenly
throughout the pores; Co platingis much easier than Cu.
But Cu segregates to Co surface
XPS of 250-nm Co filmon Cu with heating
Co plated Cu stable inCH 4 at 800C for 3 h
Cu-Co Co only
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Enhanced thermal stability achieved:
Ohmic Resistances of Cells at 900C
(o) Co-Cu-ceria-YSZ(5-vol% Co electroplated;13-vol% Cu)
() Cu-ceria-YSZ(18-vol% Cu)
( ) Co-ceria-YSZ(18-vol% Co)
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0
0. 2
0. 4
0. 6
0. 8
1
1. 2
0 50 100 150 200 250 300 350 400 450 500
Time, hour
V o l
t a g e , V
0
0. 1
0. 2
0. 3
0. 4
0. 5
0. 6
P o w e r
d e n s
i t y , W / c m
2voltage
power denisityDry CH 4, 800C50:50 Co-Cu
Cu-Co composites stable for long times
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Solution 2: Ceramic anodes (the Holy Grail?)
Problem: Oxides have either poor conductivityat low P(O 2) or poor catalytic activity
Concept: Separate the two required functions
YSZ electrolyte
Anode current collector
Porous ceramic, optimized for conductivity.
Anode functional layer
Optimized for catalytic performance.Thin to avoid need for high conductivity.
Key point:If = 10 m and R ohmic must be < 0.1 .cm 2, need only be 0.01 S/cm!
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Concept works:1. Functional layer can provide high performance when
Ag used for current collection.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0 0.5 1.0 1.5 2.0 2.5
Current Density, A/cm2
V o l t a g e ,
V
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P o w e r
D e n s i
t y , W
/ c m
2
650C700C
750C800C
LSF-YSZ cathode (300 m)
YSZ electrolyte (75 m)Anode, Ceria-YSZ (10 m)Current collector, Ag paint
10 m
H 2 (3% H 2O)
800C 750C 700C 650C
(Pd doped)
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2. Ag can be replaced by La 0.3Sr 0.7TiO 3:
20 mP
or o u s Y
S Z - a
c t i v e
r e gi on
La0.3 Sr 0.7 TiO 3 current collector
Y S Z
el e
c t r ol y
t e
0
0.2
0.4
0.6
0.8
1
1.2
0 0.4 0.8 1.2 1.6
Current Density / A cm -2
V o
l t a g e
/ V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
P o w e r
D e n s i
t y / W c m
- 2
650C
800C750C
H 2 (3% H 2O)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.4 0.8 1.2 1.6
Current Density / A cm -2
V o l
t a g e
/ V
0
0.1
0.2
0.3
0.4
0.5
0.6
P o w e r
D e n s i t y
/ W c m
- 2
CH 4 (3% H 2O)
800C
750C
700C
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Cathode Research: Background
Standard Material: LSM (La 0.8Sr 0.2MnO 3)-YSZ compositeLSM is used because- it has good electronic conductivity.- low reactivity with YSZ, even at 1200C.- good long-term stability.- good CTE match with YSZ.
But,- it has poor ionic conductivity, so that performance poor below 800C.- activated by cathodic polarization (implications for electrolysis.).
LSCo ((La 0.8Sr 0.2CoO 3):- good electronic and ionic conductivity- can provide outstanding electrode performance at below 700C.
But,- electrode performance is not stable.- it reacts with YSZ, even at 700C (forms La 2Zr 2O 7).- poor CTE match with YSZ.
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Performance independent of LSM precursorPerformance independent of LSM precursor
Fewest steps required by Molten SaltFewest steps required by Molten Salt
Impregnated LSM indistinguishable from screenImpregnated LSM indistinguishable from screen --printedprinted
LSM/YSZLSM/YSZ
ObservationsObservations
We need a minimum of 30 to 40 wt% of LSM.We need a minimum of 30 to 40 wt% of LSM.
a) Nano-particles ( )20 wt% in butandiol
b) Aqueous solutions ( )1.6 molar, nitrate solution
c) Molten salts ( )
ESSL, 9 (2006) A237ESSL, 9 (2006) A237 --240240
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1Current Density (A/cm 2)
V o l
t a g e
( V )
0
50
100
150
200
250
300
350
P o w e r
D e n s i
t y ( m W / c m
2 )
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1Current Density (A/cm 2)
V o l
t a g e
( V )
0
50
100
150
200
250
300
350
P o w e r
D e n s i
t y ( m W / c m
2 )
700700 CC
CoCo --ceriaceria |YSZ|LSM|YSZ|LSM --YSZYSZ
LSM by impregnation:
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-1
-0.5
0
0.5
1
1.5
2
0 0.5 1 1.5 2 2.5 3 3.5
Z Re, .cm 2
- Z I m , . c
m 2
2 kHz
0.8 Hz
1.6 Hz
3 Hz4 Hz
Understanding LSMUnderstanding LSM --YSZ cathodes: PolarizationYSZ cathodes: Polarization activationactivation
LSM-YSZ
CeO 2 Pd- C - YSZ
700C, H 2/3%H 2O, OCV after applying current
- 850 mA/cm 2- 60 mA/cm 2- 0 mA/cm 2 - 150 mA/cm 2
Note: These changes are reversible.Note: These changes are reversible. ~ 120 minutes.~ 120 minutes.
ESSL, 7 (2004) A111ESSL, 7 (2004) A111 --A114.A114.
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What we believe is happening:
YSZ
1) Dense LSM covers YSZ Gaps in LSM film caused by reduction
2) Performance limited by Gas can get to YSZ interface.oxygen diffusion.
Process driven by surface interactions between LSM & YSZ
YSZ
LSM
Electrode before activation Activated Electrode
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LSM Particles on YSZ (100): Effect of LSM Particles on YSZ (100): Effect of calcinationcalcination temperaturetemperature
850C 1050C 1150C
ESSL, 9 (2006) A237ESSL, 9 (2006) A237 --240240
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2 m x 2 mCalcination at 1150C Reduced in H 2 (10%H 2O) at 700C
Movement of particles is reversible:Movement of particles is reversible:
1)1) LSM is stable in 10%HLSM is stable in 10%H 22OO --90% H90% H 22 at 700at 700 C.C.
2)2) Reducing LSMReducing LSM --YSZ electrode activate it.YSZ electrode activate it.
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Consequences for electrolysis (anode environment is oxidizing):Consequences for electrolysis (anode environment is oxidizing):
0 50 100 150 200 250 3000.0
0.2
0.4
0.6
0.8
1.0
1.21.4
C e l
l p o t e n t
i a l ( V )
Time (min)
1.25
1.3
1.35
1.4
1.45
-20 10 40 70 100 130 160 190
Time (min)
C e l l p
o t e n
t i a l ( V )
0.5
1
1.5
2
2.5
3
3.5
E l e c t r o
d e i m
p e d a n c e
( - c m
2 )
285285 mAmA /cm/cm 22; 700; 700 C; 85%HC; 85%H 22--15%H15%H 22OO | air| air
LSMLSM --YSZ anodeYSZ anode LSFLSF --YSZ anodeYSZ anode
JECS, in press.JECS, in press.
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LSFLSF --YSZ (this cannot be made by screen printing)YSZ (this cannot be made by screen printing)
2)2) Formation of Formation of ZrZr --doped LaFeOdoped LaFeO 33only above 1200only above 1200 CC
3)3) ZrZr doping is not a deactivationdoping is not a deactivation
process.process.
-0.15
-0.1
-0.05
00 0.05 0.1 0.15 0.2 0.25
Z' (ohm.cm 2)
Z ' ' ( o h m . c
m 2 )
10%Zr-LSF+YSZ
LSF+YSZ
700700 C, symmetric cellC, symmetric cell
Observations:Observations:1)1) See no additional phases withSee no additional phases withcalcinationcalcination temperature (Verytemperature (Verydifferent from LaCoOdifferent from LaCoO 33))
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Time &Time & calcinationcalcination temperature have similar effect:temperature have similar effect:
Symmetric CellsSymmetric CellsCalcineCalcine @ 850@ 850 CC
t = 0 h @700t = 0 h @700 CC
CalcineCalcine @ 850@ 850 CC
t = 2500 h @700t = 2500 h @700 CC
CalcineCalcine @ 1100@ 1100 CC
t = 0 ht = 0 h
R R = expected value for= expected value forYSZ electrolyteYSZ electrolyte
R R PP = 0.12= 0.12 cmcm 22 @ 700@ 700 CCindependent of independent of ii
R R = expected value for= expected value forYSZ electrolyteYSZ electrolyte
R R PP = 0.6 to 0.1= 0.6 to 0.1 cmcm 22 ,, depends strongly depends strongly onon ii
NoNo hysteresishysteresis ! Not like LSM.! Not like LSM.
R R = expected value for= expected value for
YSZ electrolyteYSZ electrolyte
R R PP = 2.5 to 0.1= 2.5 to 0.1 cmcm 22 ,, depends strongly depends strongly onon ii
NoNo hysteresishysteresis ..
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Effect of Effect of CalcinationCalcination Temperature, Performance @ 700Temperature, Performance @ 700 CC
CalcineCalcine
@ 850@ 850 CC
t = 0 ht = 0 h
CalcineCalcine
@ 1100@ 1100 CC
0
0.1
0.2
0.3
0.3 0.4 0.5 0.6 0.7 0.8Z Re, ohm*cm2
- Z I m
, o h m
* c m
2
450 mA/cm2OCV- 450 mA/cm2
0
0.2
0.4
0.6
0.8
1
0.3 0.5 0.7 0.9 1.1 1.3 1.5
Z Re, ohm*cm2
- Z I m
, o h m * c m
2
450 mA/cm2100 mA/cm2OCV- 100 mA/cm2- 450 mA/cm2
Anode: metalAnode: metal --doped ceriadoped ceria --YSZYSZ Ag, 50Ag, 50 m YSZ electrolytem YSZ electrolyte
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Proposed deactivation mechanism:
Implications:Implications:1.1. Deactivation is structural, not associated with interfacial reacDeactivation is structural, not associated with interfacial reac tionstions2.2. InterlayersInterlayers will probably not be effectivewill probably not be effective3.3. With LSM, activation of very good electrodes less importantWith LSM, activation of very good electrodes less important use sameuse same
concepts for stabilizing LSF?concepts for stabilizing LSF?
YSZYSZLSF
850C calcination 1100C
11 mm 11 mm
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SummaryElectrode fabrication by impregnation allows great
flexibility in preparing anodes.
Direct utilization of hydrocarbon fuels is possible inCu-ceria based anodes.
The thermal stability of Cu-based anodes can beimproved by:
1. Co electrodeposition2. Using ceramic anodes.
High-performance LSF-YSZ cathodes may be possible.
http://www.franklinfuelcells.com/http://www.seas.upenn.edu/cbe/Fuel_Cell_index.htm
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-0.1
0
0.1
0.2
0.3
0 0.1 0.2 0.3 0.4 0.5
Zre .cm 2
- Z i m
. c m
2500 Hz
LSM(850C)
-1
0
1
2
3
4
5
0 1 2 3 4 5 6 7Zre .cm 2
- Z i m
. c m
20.2 Hz
0.2 Hz
LSM(1250C)
( ) Initial spectrum.() After applying 250 mA/cm 2 for 10 minutes.
( ) 5 h after applying current.
4040 --wt% impregnated LSMwt% impregnated LSM --YSZ, fired to 850YSZ, fired to 850 C or 1250C or 1250 CCMeasurement at 700Measurement at 700 C in air at OCV.C in air at OCV. LSM/YSZ
LSM/YSZ
CalcinationCalcination temperature affects activation:temperature affects activation:
R = 0.5 cm 2 R = 6.5 cm 2
Not activated Activated by polarization
JECS, 152 (2005) A1347JECS, 152 (2005) A1347 --53.53.
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BET Surface Areas (40 wt% LSM in YSZ)BET Surface Areas (40 wt% LSM in YSZ)
0.380.38 0.020.02LSM(1250LSM(1250 C)C) --YSZYSZ
2.532.53LSM(850LSM(850 C)C) --YSZYSZ
0.770.77 0.020.02Porous YSZ without LSMPorous YSZ without LSM
Surface Area (mSurface Area (m 2 2 /g) /g)
4 [ (1-) ]
=
= porosity= surface area= YSZ density
= 0.77 m 2/g = 1.6 microns = 0.38 m 2/g = 0.58 microns
dense LSM film ondense LSM film onYSZ poresYSZ pores
0.780.78 0.030.03LSM(1250LSM(1250 ooC)C) --YSZ w/ 700YSZ w/ 700 C reductionC reduction
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Gas-phase pyrolysis.
Tony Dean, CO School of Mines
Control requires consideration of flow geometries and coating of surfaces.
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10 15 20 25 30 35 40 45 50 55
2500e3
5000e3
7500e3
10.0e6
12.5e6
15.0e6
17.5e6
20.0e6
TIC
1
2
3
4
56
7
8
9
10
11
12 13
10 15 20 25 30 35 40 45 50 55
Time (min)
0
1
2
3
4
5
6
7
8
925 950 975 1000 1025 1050
Temperature (K)
W e i g h
t G a i n P e r c e n t a g e
( % )
Wt change of YSZ slab after 4 h in butane:
0.3 ml/s
1.0 m/s
S:C=1.5
Tar formation on a YSZ Slab:
1) Rate depends on flow rate2) Steam does not affect rate3) For CH 4, negligible tar
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Time (hr)
0 5 10 15 20 25
P o w e r
d e n s
i t y ( m W / c m
2 )
0
100
200
300
400
Perform decreases as anode pores fill with tar:
Performance on un-diluted light naphtha, Saudi-Aramco:
800C
750C
700C
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Most hydrocarbons behave similarly:
Time (hr)
0 5 10 15 20 25
P o w e r
d e n s
i t y
( m W / c m
2 )
0
100
200
300
400
500
Time (hr)
0 5 10 15 20 25
P o w e r
d e n s i
t y ( m W / c m
2 )
0
50
100
150
200
250
300
350
Heavy naphtha n-Decane with 20% toluene
800C
700C
800C
700C
Butane and low-sulfur diesel are also similar
P ibl S l i
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Possible Solutions:1) Prevent gas-phase reactions (radical termination)2) Remove tar faster than it forms
TPO using 10% H 2O in He; tar on YSZ
YSZ
Temperature (K)
800 900 1000 1100 1200 1300
I n t e n s
i t y
( a . u . )
CeO 2 /YSZ
H 2
H 2
CO 2
CO
CO 2
CO
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45/45
Support Layers can be added for strength:
Laminated cellDense YSZ
Porous YSZ
Dense YSZ
Porous YSZ
Final Product
FFC, Inc.
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