Functionally Graded ElectrodesFunctionally Graded Electrodes
Functionally Graded Cathodes for SOFCsFunctionally Graded Cathodes for Functionally Graded Cathodes for SOFCsSOFCs
Project Manager: Dr. Lane WilsonDOE National Energy Technology Laboratory
Meilin LiuCenter for Innovative Fuel Cell and Battery Technologies
School of Materials Science and EngineeringGeorgia Institute of Technology
September 30 - October 1, 2003
Functionally Graded ElectrodesFunctionally Graded Electrodes
The Research TeamThe Research TeamThe Research Team
• Rupak Das and Robert Williams (NSF Fellow)
– Modeling/simulation of FGE
• Erik Koep and Chuck Compson (NASA Fellow)
– Patterned Electrodes
• Qihui Wu and Harry Abernathy (NSF Fellow)
– In-situ Characterization: FTIR/Raman, IS, GC/MS
• Ying Liu and Yuelan Zhang
– Fabrication of FGE and performance testing
Functionally Graded ElectrodesFunctionally Graded Electrodes
OutlineOutlineOutline
• Technical Issues Addressed
• R&D Objectives & Approach
• Results to Date– Modeling of Functionally Graded Electrodes– Patterned Electrodes– In-situ Characterization Techniques– Fabrication of Graded Electrodes
• Applicability to SOFC Commercialization
• Activities for the next 6-12 Months
Functionally Graded ElectrodesFunctionally Graded Electrodes
400 450 500 550 6000
2
4
6
8
10
Res
ista
nce,
Ω
cm2
Temperature, °C
30 µm Electrolyte
Interfacial Resistance
Performance is determined by Rpat low temperatures
Critical Factor: Interfacial ResistanceCritical Factor: Interfacial ResistanceCritical Factor: Interfacial Resistance
Cathode
30 µm Electrolyte
Anode
Functionally Graded ElectrodesFunctionally Graded Electrodes
Origin of RP for a Porous MIEC ElectrodeOrigin of ROrigin of RPP for a Porous MIEC Electrodefor a Porous MIEC Electrode
Ions & e’ (current flow path)
Mass Transport
gas through pores
O2
Electrolyte
Inter-Mixed LayerProduce Turbulence Flow
Nano-porous structureHighly Catalytic ActiveCompatible with electrolyte
Macro-porous structureLarge pores for fast TransportHigh Electronic ConductivityCompatible with Interconnect
ReactionZones:
TPBs & MIEC Surfaces
The Concept of FGE
Functionally Graded ElectrodesFunctionally Graded Electrodes
Atomic/Molecular Level Steps Involving O2Atomic/Molecular Level Steps Involving O2
← →
←→
←→
←→
←→
←→
←→
←→
′+−
′++
−
+=
′−
′+
−
′−
′+−
′−
′+
−
′−
′+
XO
eV
eV
XO
V
V
ade
e
ade
e
ade
e
ad
ade
e
adcomb
diss
ad
des
ad
gas
O
OO
OOO
OOO
O
O
O
O
O
)(
)(
,2,2
,2
,2
..
..
..
..
221
21
21
A probable model of O2 reduction on MO
M
OO
→n V.. M
OO
n+1 V..
-
→
≈1.26Å
MO
On+1
-
→- MO
On+2
-
Diffusion
Functionally Graded ElectrodesFunctionally Graded Electrodes
• Intrinsic Properties of MIEC Cathodes– Fundamental processes at the surfaces?– Effect of surface defects/Nano-struture?– Effect of ionic and electronic transport?– In-situ characterization tools and predictive models?
• Effect of Microstructure/Architecture– Surface area/reaction sites– Rapid gas transport through pores– Predictive models for design of better electrodes
• Fabrication of FGE with desired microstructure and composition
Critical IssuesCritical IssuesCritical Issues
Functionally Graded ElectrodesFunctionally Graded Electrodes
ObjectivesObjectivesObjectives
• To develop novel tools for in-situ characterization of surface reactions;
• To gain a profound understanding of the processes occurring at cathode-electrolyte interfaces; and
• To rationally design and fabricate efficient cathodes for low temperature operation to make SOFC technology economically competitive.
Functionally Graded ElectrodesFunctionally Graded Electrodes
Technical ApproachTechnical ApproachTechnical Approach
Modeling• Transport in Porous Media
• Active Reaction Sites• Reaction Pathways
• Mechanism
Patterned Electrodes• Reaction Pathway
• Active sites
In-situ Characterization• FTIR, Raman, IS, GC/MS
• Reaction Mechanism• Catalytic Properties
Fabrication of FGE• Optimal Microstructure• Graded in Composition
• Cost-effective/Reproducible
SOFC Performance • High Performance
• Long-Term Stability
Functionally Graded ElectrodesFunctionally Graded Electrodes
Modeling of Functionally Graded ElectrodeModeling of Functionally Graded ElectrodeModeling of Functionally Graded Electrode
PorosityPore Size and Size DistributionGrain Size and Size Distribution
Diffusivity/TortuosityKnudsen Diffusion
Effective Ionic ConductivityEffective Electronic ConductivityAmbipolar Conductivity
Exchange Current DensityCathodic Transference Numbers
Key Input Parameters:1st Order Approximation
Ionic Transport Limited
Dense Electrolyte
1.0 µm
Functionally Graded ElectrodesFunctionally Graded Electrodes
Tape Cast Substrates for Patterned ElectrodesTape Cast Substrates for Patterned ElectrodesTape Cast Substrates for Patterned Electrodes
Low cost, reproducible, and easy scale-upGreat Flexibility: Co-casting of multi-layers of different materials
GDC ~120µm
YSZ ~140µm
YSZ
Ni-YSZ
Interconnect
Functionally Graded ElectrodesFunctionally Graded Electrodes
Microstructures of Patterned Electrodes
Pt Line (2 µm wide)
Gap between Pt lines(YSZ)TPB
2 µm Pt Lines
SSC
Pt CurrentCollector
YSZTPB
10 µm SSC Lines50 µm Pt Current Collector
Functionally Graded ElectrodesFunctionally Graded Electrodes
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
200 400 600 800 1000 1200
Raman shift, cm-1
Inte
nsity
, a.u
.
Co3O4 pellet
Thin film SSC, 1000ºC in air
Thin film SSC, 600º in argon
SSC Pellet, 1100ºC
Thin film SSC, room temp.
*
*
*
* Denotes peak from YSZ substrate
While initial thin film resembles SSC standard, the surface structure changes upon heating.
Raman Spectra of Thin Film SSC Electrodes
Functionally Graded ElectrodesFunctionally Graded Electrodes
Probable surface reaction models
M
OO
→n V.. M
OO
n+1 V..
-
→ →
M
OOn+1 V..
-
OSr (Sm)
→
M
OOn+2 V..
-
OSr (Sm)
-
≈1.26Å
≈1.50Å
MO
On+1
-
→- MO
On+2
-
→ M O
On+2
-
OSr (Sm)
Diffusion
Diffusion
O2 Reduction On a Metal Oxide
Functionally Graded ElectrodesFunctionally Graded Electrodes
Possible Surface Reaction Processes
For (101) phase
Diffusion
Diffusion
Oxygen vacancy Adsorbed Oxygen
Functionally Graded ElectrodesFunctionally Graded Electrodes
Air/O2
Fuel Gas
Process Control System
Mass Flow Controllers
Drier
Gas Chromatograph
Mass Spectrometer
To Vent
Impedance Spectroscopy (IS) Electroanalytical measurements
FTIR
Raman
MS
GC
• pd-FTIR ES• Raman Specroscopy• Impedance Spectroscopy• Mass Spectrometry/GC
In-Situ Characterization TechniquesInIn--Situ Characterization TechniquesSitu Characterization Techniques
Functionally Graded ElectrodesFunctionally Graded Electrodes
Gas Switching EffectGas Switching EffectGas Switching Effect
4000 3500 3000 2500 2000 1500 1000 500-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
1124
1124
44 s
36 s
32 s
SSC/SDC/SSC, at 550oC
From 1% O2 to N2, take background in 1% O2
40 spectra (4s/spectrum)
From N2 to 1% O2, take background in N2
40 spectra (4s/spectrum)
∆E
/E0(%
)
Wavenumber (cm-1)
OO-
OO-
Functionally Graded ElectrodesFunctionally Graded Electrodes
-0.002
0.008
0.018
0.028
80095011001250
Wavenumber, cm-1
LSM
LSC
LSF
SSC
11241236930
Maximum O2- signals for cathode materials at 600ºC
in 1% O2 atmosphere
O2- species
Different catalytic properties
Catalytic Properties of Cathode MaterialsCatalytic Properties of Cathode MaterialsCatalytic Properties of Cathode Materials
PEAK Height: Fast Screening Tool for Materials Development
Functionally Graded ElectrodesFunctionally Graded Electrodes
Height of 1124 cm-1 peak during gas witching experiment for different materials at 600ºC.
-0.10
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 500 1000 1500
Time, sec
Pea
k H
eigh
t
SSC
LSCLSF
First derivative of 1124 cm-1 peak height vs. time curve
-0.002
0.000
0.002
0.004
0.006
0 500 1000 1500
Time, sec
∆H/
∆t
SSCLSFLSC
Reactivity for oxygen adsorption and desorption : SSC ≥ LSF > LSC
Rates of Adsorption/DesorptionRatesRates of Adsorption/of Adsorption/DesorptionDesorption
Functionally Graded ElectrodesFunctionally Graded Electrodes
Height of 1124 cm-1 peak during gas switching experiment for SSC at different temperatures.
-0.10
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0 500 1000 1500
Time, sec
Peak
Hei
ght
SSC
650ºC
600ºC
700ºC
First derivative of 1124 cm-1 peak height vs. time curve for SSC at different temperatures
-0.002
0.000
0.002
0.004
0.006
0 500 1000 1500
Time, sec
∆H/
∆t
600
650
700
Reaction rate: 700 ≥ 650 ≥ 600
Temperature is not a significant parameter for oxygen adsorption but is for oxygen desorption
Gas Switching EffectGas Switching EffectGas Switching Effect
Functionally Graded ElectrodesFunctionally Graded Electrodes
-0.10
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0 500 1000 1500
Time, sec
Peak
Hei
ght
LSF
600ºC
650ºC
700ºC
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 500 1000 1500
Time, secPe
ak H
eigh
t
LSC
600ºC
650ºC
700ºC
Height of 1124 cm-1 peak during gas witching experiment for LSF and LSC at different temperatures.
LSF and LSC show different temperature dependence for oxygen adsorption and desorption
1% Oxygen
Temperature EffectTemperature EffectTemperature Effect
Functionally Graded ElectrodesFunctionally Graded Electrodes
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.2 0.4 0.6 0.8 1
Oxygen Partial Pressure, atm
Peak
Hei
ght
550ºC
600ºC
650ºC
700ºC
The intensity of 1124 cm-1 peak at different temperatures and in different atmospheres for LSF electrode
The saturated CO2: 20% (Air)
Effect of Oxygen Partial PressureEffect of Oxygen Partial PressureEffect of Oxygen Partial Pressure
Functionally Graded ElectrodesFunctionally Graded Electrodes
The FTIR spectra of an SSC pellet at different temperatures in oxygen
8009001000110012001300
1124 cm-1
873 cm-1
550°C
700°C
650°C
600°C
High O2 concentration → O22-: 873cm-1
Peroxide Peak at High Po2Peroxide Peak at High PoPeroxide Peak at High Po22
Functionally Graded ElectrodesFunctionally Graded Electrodes
Normalized height of 1124 cm-1 and 875 cm-1 peaks during gas switching experiment from Ar to O2 and back to Ar.
0
0.5
1
0 100 200 300 400 500 600 700Time, sec
Pea
k H
eigh
t
O22-
O2-
O2- and O2
2-: fast adsorption
O22-: slowly reach the max
Faster desorption
Kinetics for Superoxide and Peroxide IonsKinetics forKinetics for SuperoxideSuperoxide and Peroxide Ionsand Peroxide Ions
Functionally Graded ElectrodesFunctionally Graded Electrodes
Conclusions – Time-Dependent FTIR-ESConclusions Conclusions –– TimeTime--Dependent FTIRDependent FTIR--ESES
• The active sites for the oxygen reduction (oxygen adsorption) is not limited to the triple boundaries, but extended to surfaces of the MIEC electrodes.
• As expected, different cathode materials have different catalytic activity for the oxygen adsorption and desorption. In particular, SSC appears to have the highest activity for oxygen adsorption while LSF has the fastest kinetics for the oxygen desorption.
• The saturation partial pressure of oxygen is about 20% for the FTIR measurements.
• The intensity of the peroxide peaks are much weaker than those of the superoxide peak. The formation rate of peroxide species appears to be as fast as that of superoxide; however, there is some delay for peroxide to reach the maximum point. The desorption of peroxide is much faster than that of superoxide.
Functionally Graded ElectrodesFunctionally Graded Electrodes
Fabrication of Functionally Graded Electrodes
↑O2-
FuelFunctionally GradedAnode
electrolyte
Functionally GradedCathode
e-
e-
Load↑O2-
FuelFunctionally GradedAnode
electrolyte
Functionally GradedCathode
↑O2-↑O2-
FuelFunctionally GradedAnode
electrolyte
Functionally GradedCathode
Functionally GradedAnode
electrolyte
Functionally GradedCathode
e-
e-
Load
e-
e-
Load
• Templated Synthesis
• Combustion CVD
Functionally Graded ElectrodesFunctionally Graded Electrodes
Schematics – Templated SynthesisSchematics Schematics –– Templated Templated SynthesisSynthesis
PMMA spheres
PMMA aggregate by the attraction of the binder
After impregnate with slurry and template removal
Periodic interconnected porous structure
Wall is composed of smaller pores
Assembly with binder
Functionally Graded ElectrodesFunctionally Graded Electrodes
Preliminary ResultsPreliminary ResultsPreliminary Results
• SEM pictures
PMMA template Porous GDC-SSC MIEC
Functionally Graded ElectrodesFunctionally Graded Electrodes
Preliminary ResultsPreliminary ResultsPreliminary Results
Porous GDC-NiO MIEC
Walls consist of particles of about 100 nm in diameter
Functionally Graded ElectrodesFunctionally Graded Electrodes
Solutions P um p
F ilter
F low C ontroller
C ontrol & D ata A cquisition
CCVD
N ozzles
Support
P um pSolutions
F ilter
F low C ontroller
F uel andO xidant G ases
F uel andO xidant G ases
Solutions P um p
F ilter
F low C ontroller
C ontrol & D ata A cquisition
CCVDCCVD
N ozzles
Support
P um pSolutions
F ilter
F low C ontroller
F uel andO xidant G ases
F uel andO xidant G ases
Ni GDCHigh fuel-to-gas ratio
Reducing atmosphereModerate fuel-to-gas ratio
Oxidizing atmosphere
Low cost• Open-air, flame-assisted deposition process• No furnace or reaction chamber required• Inexpensive precursors (e.g., metal nitrates)
Great Flexibility• Multi-element and/or multi-layer coating capability• Capable of producing vastly different
microstructure and morphologies;
Combustion CVDCombustion CVDCombustion CVD
Functionally Graded ElectrodesFunctionally Graded Electrodes
Nano Box-Beams of Semiconductor SnO2Nano Nano BoxBox--Beams of Semiconductor SnOBeams of Semiconductor SnO22
A B
quartz
C D
50 nm
end cap
3.5 nm
E
a b
c F
Functionally Graded ElectrodesFunctionally Graded Electrodes
Effect of Deposition Temperature
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60.01
0.1
1
10
100
1000800 700 600 500 400
1400oC
1200oC
1000oC
800oC
Temperature (oC)
Inte
rfaci
al re
sist
ance
(Ωcm
2 )
1000/T (K-1)
20 25 30 35 40 45 50 55 60 65 70 75
1 000 oC
80 0 oC
S S CS D C
ig
ii
i
i
i
g
gg
ggg
Inte
nsity
2θ o
1 20 0 oC
1 400 oC
2 µm
(a) 800°C (b) 1000°C
2 µm
(c) 1200°C
2 µm 2 µm
(d) 1400°C
Functionally Graded ElectrodesFunctionally Graded Electrodes
Deposition Time: Microstructures
2 min 5 min
10 min 20 min
Functionally Graded ElectrodesFunctionally Graded Electrodes
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60.01
0.1
1
10
100
800 700 600 500 400
20min
10min5min
2 min
Inte
rfa
cia
l re
sist
an
ce(Ω
cm2 )
1000/T (K-1)
Temperature (oC)
0 4 8 12 16 20 240
20
40
60
80
100
120
Film
th
ickn
ess
(µm
)
Deposition time (min)
Deposition Time: Thickness and RP
Functionally Graded ElectrodesFunctionally Graded Electrodes
0.005 M 10 µm
(a)
0.05 M 15 µm
(b)
0.25 M 10 µm
(c)
Effect of Concentration
1.0 1.1 1.2 1.3 1.4 1.5 1.60.1
1
10
800 750 700 650 600 550 500 450 400
0.005M0.05M0.25M
Interf
acial
resis
tance
(Ωcm
2 )
1000/T (K-1)
Temperature (oC)
Functionally Graded ElectrodesFunctionally Graded Electrodes
10 µm
(b)
YSZGDC
Effect of Substrate (Electrolyte)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60.1
1
10
100
1000
10000
on GDC
on YSZ
Inte
rfaci
al re
sist
ance
(Ωcm
2 )
1000/T (K-1)
800 750 700 650 600 550 500 450 400
Temperature (oC)
Functionally Graded ElectrodesFunctionally Graded Electrodes
AnodeNi +SDC
CathodeSSC+SDC
An SOFC Fabricated by CCVD
250 µm GDC
Functionally Graded ElectrodesFunctionally Graded Electrodes
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.60.01
0.1
1
10
100800 700 600 500 400
Inte
rfaci
al re
sist
ance
(Ωcm
2 )
1000/T (K-1)
LSM-GDC50, Murry[7]
LSCF-GDC35, Dusastre[6]
SSC-GDC10, Xia[8]
SSC-SDC30, CCVD
Temperature (°C)
00.10.20.30.40.50.60.70.80.9
1
0 200 400 600 800 1000 1200
Current density (mA/cm2)V
olta
ge (V
)
0
50
100
150
200
250
Pow
er d
ensi
ty (m
W/c
m2 )
700°C650°C600°C550°C500°C
Interfacial Resistances and Performanceof an SOFC supported by 250 µm GDC
Functionally Graded ElectrodesFunctionally Graded Electrodes
0 400 800 1200 1600 20000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0V
olta
ge, V
Current density, mA/cm2
0
100
200
300
400
500
P
ower
den
sity
, mW
/cm
2
650oC 600oC 550oC 500oC
Performance of an Anode-Supported Cellwith Cathode by CCVD
30 µm Electrolyte
Functionally Graded ElectrodesFunctionally Graded Electrodes
Functionally Graded Cathode (fabricated on 250µm YSZ) by CCVD, along with the EDS dot mapping of Mn and Co element distributions
5.0µm
Mn Co
(b)
Nano-structured Electrodes by Combustion CVDNanoNano--structured Electrodes by Combustion CVDstructured Electrodes by Combustion CVD
Functionally Graded ElectrodesFunctionally Graded Electrodes
550 600 650 700 750 800 850 9000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Ra+Rc
Rb
Res
ista
nce,
Ω c
m2
Temperature, oC
(b)
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.20.0
0.1
0.2
800oC
700oC
-Im
Z, Ω
cm
2
Re Z, Ω cm2
(a)
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20
950 900 850 800 750 700 650 600
0.01
0.1
1
10
GDC-impregnated LSM, Jiang [10]
Temperature, oC
Inte
rfaci
al re
sist
ance
, Ω c
m2
1000/T, K-1
Graded LSM-LSC-GDC, CCVD
LSM-GDC50, Murray [9]
LSM, Murray [9] Graded LSM-LSC-GDC, Hart [17]
Impedance Spectra/Resistance – Combustion CVDImpedance Spectra/Resistance Impedance Spectra/Resistance –– Combustion CVDCombustion CVD
Functionally Graded ElectrodesFunctionally Graded Electrodes
0 400 800 1200 1600 2000 24000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0
100
200
300
400
500
600
V
olta
ge, V
C urrent density, m A/cm 2
Pow
er d
ensi
ty, m
W/c
m2
850oC 800oC 750oC 700oC 650oC 600oC
Fuel Cell Performance – Combustion CVDFuel Cell Performance Fuel Cell Performance –– Combustion CVDCombustion CVD
250 µm Electrolyte
Functionally Graded ElectrodesFunctionally Graded Electrodes
Summary of Accomplishments• Started 3-D Modeling of graded multi-layer cathodes
• Started Microscopic modeling of surface reaction processes
• Developed micro-fabrication techniques capable of producing MIEC electrodes (SSC and LSM) with well-defined geometries
• Understanding of reduction mechanisms on different cathode materials using in-situ characterization techniques
• Used Raman spectroscopy to better characterize surface structures of electrodes under practical operating conditions
• Used combustion CVD and templated synthesis to produce vastly different microstructure and morphologies of porous mixed-conducting electrodes
• Demonstrated cathodes of lowest polarization resistances for lowtemperature SOFCs
Functionally Graded ElectrodesFunctionally Graded Electrodes
Applicability to SOFC CommercializationApplicability to SOFC CommercializationApplicability to SOFC Commercialization
• Generated some basic understanding of electrode reaction mechanisms in an effort to better design of efficient electrodes
• Developed new tools for in-situ determination of electrode properties under practical conditions
• Developed new architectures/microstructures of porous MIEC electrodes using combustion CVD and templated synthesis
Functionally Graded ElectrodesFunctionally Graded Electrodes
Activities for the Next 6-12 MonthsActivities for the Next 6Activities for the Next 6--12 Months12 Months
• Fabrication and evaluation of patterned MIEC electrodes with active phase and finer features
→ Reaction sites, pathway, and mechanism
• Refine Macroscopic and Microscopic Models
→ Optimum Microstructure/Architecture
• Optimization of templated synthesis and combustion CVD for fabrication of FGEs
• Development of new in-situ characterization tools for investigation of SOFC reactions
→ AFM/STM integrated with Raman spectro-microscope to achieve chemical mapping at nano-scale
→ AFM/STM integrated impedance spectroscopy to acquire impedance spectra of individual grains and individual grain boundaries between dissimilar materials
Functionally Graded ElectrodesFunctionally Graded Electrodes
AcknowledgementAcknowledgementAcknowledgement
SECA Core Technology ProgramDept of Energy/National Energy Tech Laboratory
DARPA/DSO-Palm Power ProgramArmy Research Office/DURIP
Center for Innovative Fuel Cell and Battery Technologies, Georgia Tech
Lane Wilson, NETL/DoE