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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


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


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


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


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


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


• 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


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.


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


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


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


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


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


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


Possible Surface Reaction Processes

For (101) phase

Diffusion

Diffusion

Oxygen vacancy Adsorbed Oxygen


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


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-


-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


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


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


-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


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


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


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


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.


Fabrication of Functionally Graded Electrodes

↑O2-

FuelFunctionally GradedAnode

electrolyte

Functionally GradedCathode

e-

e-

Load↑O2-


electrolyte


↑O2-↑O2-


electrolyte


Functionally GradedAnode

electrolyte


e-

e-

Load

e-

e-

Load

• Templated Synthesis

• Combustion CVD


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


Preliminary ResultsPreliminary ResultsPreliminary Results

• SEM pictures

PMMA template Porous GDC-SSC MIEC


Preliminary ResultsPreliminary ResultsPreliminary Results

Porous GDC-NiO MIEC

Walls consist of particles of about 100 nm in diameter


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


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



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


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


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


Deposition Time: Microstructures

2 min 5 min

10 min 20 min


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


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)


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)


AnodeNi +SDC

CathodeSSC+SDC

An SOFC Fabricated by CCVD

250 µm GDC


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


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 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


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


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


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


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


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


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

functionally graded cathodes for functionally graded cathodes for ... library/research/coal/energy...

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