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Functionally Graded Cathodes for SOFCs

Project Manager: Dr. Lane Wilson DOE National Energy Technology Laboratory

E. Koep, Q. Wu, H. Abernathy, J. Dong, Y. Liu & M. Liu

Center for Innovative Fuel Cell and Battery Technologies School of Materials Science and Engineering Georgia Institute of Technology Collaborators: T. Orlando and H. Chen Chemistry and Biochemistry, GT May 11 ­ 13, 2004

Functionally Graded Electrodes

Outline

· Technical Issues Addressed · Objectives & Approach · Phase I Accomplishments · Recent Results (Phase II) ­ Further versification and interpretation of IR data ­ DFT Calculations and ESD ­ In-situ Raman and TERS ­ Patterned Electrodes with some sites selectively blocked ­ Measurement of Local (micro or nano-scale) properties ­ Fabrication of Graded Electrodes · Applicability to SECA · Activities for the next 6-12 Months

Functionally Graded Electrodes

Critical Factor: Interfacial Resistance

10

Interfacial Resistance

2 Resistance, cm

8

Cathode

30 µm Electrolyte

6

Anode

4

2

30 µm Electrolyte

Performance is determined by Rp at low temperatures

0 400 450 500 550 600

Temperature, °C

Functionally Graded Electrodes

Origin of RP for a Porous MIEC Electrode

Ions & e' (current flow path)

The Concept of FGE

Mass Transport

gas through pores

O2

Macro-porous structure Large pores for fast Transport High Electronic Conductivity Compatible with Interconnect

Reaction Zones: TPBs & MIEC Surfaces

Inter-Mixed Layer Produce Turbulence Flow

Electrolyte

Functionally Graded Electrodes

Nano-porous structure Highly Catalytic Active Compatible with electrolyte

Critical Issues

· 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

Functionally Graded Electrodes

Objectives

· To develop tools for in-situ characterization of electrode reactions in SOFCs; · To gain a profound understanding of the elemental processes occurring at cathodeelectrolyte interfaces; and · To rationally design and fabricate efficient cathodes for low temperature operation to make SOFC technology economically competitive.

Functionally Graded Electrodes

Technical Approach

Patterned Electrodes

· Reaction Pathway · Active sites

Fabrication of FGE Modeling

· Transport in Porous Media · Active Reaction Sites · Reaction Pathways · Mechanism · Optimal Microstructure · Graded in Composition · Cost-effective/Reproducible

SOFC Performance

In-situ Characterization

· FTIR/Raman, TERS · Micro- or nano-IS · GC/MS

Functionally Graded Electrodes

· High Performance · Long-Term Stability

Major Phase I Accomplishments

· · · · ·

Successfully fabricated patterned electrodes (SSC and LSM) with well-defined geometries for determination of TPB width Studied SOFC cathode materials using in-situ FTIR emission spectroscopy under practical conditions, including Pt, SSC, LSC, LSF, and LSCF Characterized surface structures of SOFC materials using Raman spectroscopy Fabricated electrodes with vastly different microstructures and morphologies using combustion CVD and template synthesis Demonstrated functionally graded cathodes of low polarization resistances at low temperatures.

Functionally Graded Electrodes

Recent Progress ­ Phase II · · · · · · ·

Further versification and interpretation of in-situ vibrational spectra of cathode materials (thickness and time dependence) DFT calculation of vibration frequencies of surface oxygen species (adsorbed O2, O2-, and O22-) ­ peak assignment Started study of electron-stimulated desorption (ESD) of oxygen Dramatically increased sensitivity and spatial resolution in probing surfaces using tip enhanced Raman scattering (TERS) Successfully fabricated patterned electrodes for specific site isolation Set up for local (micro or nano-scale) electrochemical measurements (IS) using SPM tips and patterned electrodes Developed processes for fabrication of electrodes graded in both composition and microstructure

Functionally Graded Electrodes

Typical IR Spectra: Bulk and Kinetic Properties

24

P e a k a ss ig n m e n ts : 1 1 2 4 c m -1 : O 2- 1 2 3 6 c m -1 : p e rtu rb e d O 2- 9 3 0 c m -1 : O 22 - S u rfa c e S p e c ie s

20

16

E/Eo(%)

12

CO2

1236

930

1124

8

A t o v e rp o te n tia l o f -0 .7 6 V

4

B a se lin e O ffse t (b u lk p ro p e rtie s)

At OCV

0

4000

3500

3000

2500

2000

1500

1000

W a v e N u m b e r , c m -1

Functionally Graded Electrodes

FITR spectra of SSC film at different O2 partial pressure

0.5%

Gas switching from N2 to a gas containing Po2 (0.5 to 100%)

E/E, %

Both negative-going baseline shift and positive-going IR peak increase with the oxygen partial pressure.

100%

3600 2600 1600

-1

600

W ave n u m b e r, cm

Functionally Graded Electrodes

IR peak & Baseline shift ~ O2 partial pressures

0.36 0.34 0.32 0.3 0.28 0.26 0.24 0.22 0.2 0 20 40 60 80 100 120

Oxygen partial pressure (%)

Peak height E/E

The saturated oxygen partial pressure for oxygen adsorption is about 20%.

-0.8

Baseline shift E/E

-1.3 -1.8 -2.3 -2.8 -3.3 0 20 40 60 80 100 120

Oxygen partial pressure (%)

Corresponding to the IR peak height, the baseline shift at 825 cm-1 is also saturated when the O2 is about 20%.

Baseline shift and the IR peaks seem to go together!

Lower O2 pressure means higher oxygen vacancy in the bulk. Negative-going baseline shift therefore means decrease in bulk oxygen vacancy concentration

Functionally Graded Electrodes

Validity of Interpretation?

· How do we know that the peaks are corresponding to species adsorbed on surfaces, not changes in bulk properties? · If so, can we prove that the peak assignments are correct? · How do we know that the baseline shift is due really to changes in bulk properties, not in surface properties ?

Functionally Graded Electrodes

Thickness Dependence

Emitted IR

Adsorbed molecules

Active bulk < skin depth

Inactive bulk

· If the peaks are corresponding to species adsorbed on surface, the heights of the peaks should be independent of film thickness. · If the baseline shift is due really to changes in bulk properties, it should increase with thickness (until the thickness is greater than the skin depth).

Functionally Graded Electrodes

Equilibrium FITR spectra for SSC thin film electrodes

2.0µm 2.5µm 3.0µm

E/E, %

3.5µm 5.0µm 6.0µm 7.5µm 8.0µm 10 µm 15 µm

3650 2650 1650 650 W a ve num be r, cm -1

· A broad oxide adsorbent peak centered at 1124 cm-1 · A negative-going baseline shift with increase thickness.

Functionally Graded Electrodes

IR peak height and baseline shift

0.5 0.45 0.4 0.35

E/E 0.3

The O2- peak height is almost independent of film thickness.

0.25

0.2

0.15 0.1 0.05 0

1

3

5

7

9

11

13

15

Thickness (µ m)

0.5 0 -0.5 E/E -1 -1.5 -2 -2.5 -3 1 3 5 7 9 11 13 15

(c) (a) (b)

The baseline shifts at (a) 3020, (b) 2128, and (c) 825 cm-1 increase with film thickness. Larger baseline shift is observed at lower wavenumber (or longer wavelength with larger skin depth).

Thickness (µ m)

Functionally Graded Electrodes

Time Dependence of pd-FTIR ES pd-FTIR

OPO2 O2- OO O

DC Voltage

Heating Cartridge

·

If the peaks are corresponding to species adsorbed on surface and the baseline shift is due really to changes in bulk properties, the rate of peak height change (rate of surface reaction) should be different from that of baseline shift (the rate of bulk transport) unless the rates of the two processes are coincidently identical.

Functionally Graded Electrodes

Rapid scan: Time resolved pd-FTIR on LSC pd-FTIR

0.05 0 -0.05 600 Peak height Peack height -0.1 -0.15 -0.2 -0.25 -0.3 -0.35 1 6 11 16 21 26 31 36 41 46 51 56 61 Time (Sec.) 650 Peak height 700 Peak height 600 Baseline 650 Baseline 700 Baseline 1 6

5

4

2

Baseline

3

Peak height change is much faster than baseline shift, indicating again that the peak and the baseline shift correspond to different processes. Further, it's clear that surface reaction is faster than bulk diffusion. Electrode kinetics Bulk diffusivity

0

-1

Functionally Graded Electrodes

Conclusions for FTIR Studies

· Indeed, the IR peaks correspond to species adsorbed on surfaces or a change in surface properties; · The IR baseline shift is due to a change in bulk properties of electrode material, most likely to the change in oxygen vacancy concentration in the bulk phase; · However, we are still unable to conclude which peak corresponds to which species.

- Theoretical Calculation - Isotope Exchange

Functionally Graded Electrodes

DFT Calculation of O-O vibrations O-O

· Used DFT method (B3LYP) on Q-Chem software with 6-311+G(3df) basis set · Calculated theoretical frequency of O-O bond for free O2, O2-, and O22Species Frequency, cm-1 O2 1592 O21173 O22835

Functionally Graded Electrodes

Effect of transition metal cation

· Began calculations of O-O vibrational frequency when bonded to cobalt cation in SSC or LSC · Optimized geometry first, then calculated frequency

n+ O Optimize Co O Co O O n+ n+

or

O Co

Start

Finish

Functionally Graded Electrodes

O

Most Probable Surface Configuration

For (101) phase

Oxygen vacancy

Functionally Graded Electrodes

Adsorbed Oxygen

Effect of transition metal cation

Species O-O bond, Å Co-O-O, ° O-O vibration, cm-1 CoO2+ 1.3100 141.5 1487 CoO22+ 1.2074 70.5 467 CoO23+ DNC DNC N/A CoO24+ DNC DNC N/A

*DNC denotes that geometry optimization calculation did not converge

· Calculations reveal superoxo-/peroxo- nature of oxygen species · Frequencies unreliable ­ must include more of the lattice for better results

Functionally Graded Electrodes

Electron Stimulated Desorption (ESD)

E-beam Auger Electron

eX-ay

1

2

Ionization

Auger relaxation

Functionally Graded Electrodes

3-Step ESD Model

e-

Excitation ~ 10-15 sec

Nuclear Separation ~ 10-13 sec

Desorption ~ 10-11 sec

Electron excites the target via an inelastic scattering Nuclear motion on the excited state potential surface Outgoing atom or molecule interacts with surface

Functionally Graded Electrodes

What Can ESD Offer?

· Structures of adsorbate/adsorbent system

­ Local bonding ­ Bonding geometry ­ Binding energy

· Dynamics of charge transfer · Site specific desorption

­ Defects (for example: oxygen vacancy) ­ Local disorder

Functionally Graded Electrodes

ESD Experimental Set-Up

XYZ-Rotation Stage

Sample Holder QMS

Loading Cell Sample Transfer Stage

E-Gun

Turbo

Prof. Thom Orlando Chemistry, GT

Functionally Graded Electrodes

GDC Sample for ESD Study

Compostition : Ce0.9Gd0.1O1.95 (gadolinia doped ceria, GDC ) Preparation: Combustion of metal nitrate Calcine in air at 600ºC for 2 h Cold press into pellets Fired at 1450ºC for 5 h Characterization: Relative density: 95% to 97% XRD: fluorite structure SEM: grain size about 5 µm

Functionally Graded Electrodes

Electron Energy Dependence of ESD

O+

O+ threshold energy is 22 eV relevant to the direct ionization of O2s level followed by an intra-atomic Auger cascade The feature around 50 eV is related to the Auger process that involves the excitation of 5s level of cerium and gadolinium

0

20

40

60

80

100

Ready to study cathode materials (e.g, SSC)

Electron Energy (eV)

Functionally Graded Electrodes

FTIR ­ Isotope Dosing

· Difficulties

-- Precise control of Po2 -- Fast oxygen exchange at high temperature

· Approach ­ FTIR in UHV

-- Diffuse-reflectance IR (DRIFTS) set up for low temperature FTIR measurement -- Emission IR set up for high temperature FTIR measurement

Functionally Graded Electrodes

High-Temp Emission IR Spectroscopy High-Temp

FT IR

3" 2F

6"

1P 6 .02 "

1F

Sequence of mirrors

Type of of mirrors F: flat mirror P: parabolic mirror

Functionally Graded Electrodes

Low-Temp (LN) DRIFTS Low-Temp

B B

Vac. Chamber

FTIR

C A

DETECTOR

A - plain mirror B - parabolic mirror F=69, 100, 153, 180, 220, 250, 300, 400, 418 mm C - parabolic mirror F= 43 mm

Detector

3P 2P 6 .02 "

6.02"

3"

0 1 .7

"

FT IR

4P

3" 1F

6"

Functionally Graded Electrodes

Other Tools for in-Situ Studies in-Situ

· Probing and Mapping Surface Reactions using Raman Spectro-microscopy · Tip Enhanced Raman Spectroscopy (TERS) ­ Combination of Raman and SPM · Patterned Electrodes for Isolation of Reaction Sites · Local (micro or nano-scale) measurement using SPM tips and patterned electrodes; TERS, Raman mapping

Functionally Graded Electrodes

Raman Spectra of LSC in a Controlled Atmosphere

O2 RT-H2 H2 RT-H2 H2 RT

Intensity (a.u.)

O2 RT Ar RT

200

700

1200

-1

1700

Wavenumber (cm )

Functionally Graded Electrodes

Raman Spectra of Cobalt Compounds in Air

Intensity (a.u.)

1545 855 1140

· 3 types of oxygen species are observed

LSCF LSC SSC

· But the peaks are week

SC

200

700

1200 Wavenum ber (cm -1)

1700

Functionally Graded Electrodes

Tip-Enhanced Raman Scattering (TERS)

SERS Mechanism Objective lens

Nano-sized Au, Ag or Cu tip

Illumination

SERS

e- eSample

K. SHIBAMOTO, et al., ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT Functionally Graded Electrodes

Our Integrated Raman and SPM

Objective Scanners

Functionally Graded Electrodes

Schematic Arrangement for TERS

45o

·Tip with Ag coating and small diameter (50 nm) for TERS ·Tuning fork technology removing the interruptions of other laser source

Unique Capabilities: Spatial Resolution: up to 50 nm Very Sensitive to the Chemical Nature of Surfaces

Functionally Graded Electrodes

Capabilities of Our System

TERS

Raman Mapping

AFM Image

To be Obtained

Functionally Graded Electrodes

TERS: Single Crystal Silicon

3500 3000 2500

with tip enhancement without tip enhancement (X10)

Intensity

2000 1500 1000 500 0 0 200 400 600 800 1000

Raman shift (cm )

-1

Functionally Graded Electrodes

TERS: Oxygen species Adsorbed on LSM

3500 3000 2500

852

Intensity

2000

With enhancement

1500 1000 500 0 0 200 400 600 800 1000 1200 1400

-1

Without enhancement

1600

1800

2000

2200

Raman shift (cm )

Peroxide species (O22-) on LSM surface was not observable with ordinary Raman or FTIR, but observable with TERS. Mechanism on LSM is different from that on SSC or LSC

Functionally Graded Electrodes

TERS: Carbon deposited on YSZ

5000 4500 4000 3500

D

G

Intensity

3000 2500 2000 1500 1000 500 0

YSZ

With enhancement Without enhancement

1000 1200 1400 1600

-1

1800

2000

Raman shift (cm )

Preliminary results: The Raman intensities of the G and D mode are dramatically enhanced by the AFM tip.

Functionally Graded Electrodes

TERS ­ A Sensitive Surface Probe

· Dramatically enhanced sensitivity to surface species (up to 108) · Increased spatial resolution (up to 20 nm) for mapping of active sites for electrode reactions · Unlike IR, TERS is not influenced by gas phase H2O and CO2 and especially sensitive to carbon and sulfur, making it a powerful tool for investigation of fuel reforming and anodes of SOFCs running on hydrocarbon fuels (e.g., gasified coal)

Functionally Graded Electrodes

Patterned Electrodes

· Patterned electrodes with the length of TPB varying in 4 orders of magnitudes · Patterned electrodes with some reaction sites selectively blocked · Combination of patterned electrodes and SPM tip for performing local (micro- to nano-scale) electrochemical measurements

Functionally Graded Electrodes

Possible Reaction Sites

Path A.

O2

e-

(a)

Metallic or MIEC Electrode Electrolyt e

Path B.

Os

e-

Metallic or MIEC Electrode Electrolyte

(b)

O2 e-

(c)

Metallic or MIEC Electrode Electrolyte

O2

Path D.

e(d)

Functionally Graded Electrodes

MIEC

Electrode Electrolyte

A Photolithographic Process for Isolation of Reaction Sites

Photoresist Undercut Photoresist

PMGI

50µm YSZ electrolyte

PMGI LSM patterned electrode YSZ electrolyte

Ti-coating

LSM electrode Ti-coating Ti coating

0.26 µm LSM

Photoresist PMGI

PMGI LSM patterned electrode YSZ electrolyte

LSM patterned electrode YSZ electrolyte LSM patterned electrode YSZ electrolyte 0.26µm

Functionally Graded Electrodes

Typical Micrographs

0.26 µm x 50 µm

TiO2 LSM Electrode YSZ electrolyte

YSZ Electrolyte

3.0 µm

TPB Edge of TiO2 YSZ electrolyte TiO2 coating LSM microelectrode

µm

Functionally Graded Electrodes

Proof-of-Concept: Effect of Site Blocking Proof-of-Concept:

- 1 0 0 - 8 0 - 6 0 - 4 0 - 2 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0

With TiO2 Without TiO2

0.03

·

0.025

0.02 Without Ti coating

1/Rp

0.015

0.01 With Ti Coating

The surface of the patterned (0.26x50µm) LSM electrode contributes significantly to oxygen reduction. The micro-fabrication technique is effective in isolation of different reaction sites.

0.005

·

800

0 500 600 Temperature (ºC) 700

Functionally Graded Electrodes

Local Electrochemical Measurements

Impedance Analyzer

Patterned Electrode

SPM Controller

To probe local properties using impedance spectroscopy performed on an SPM tip and patterned electrodes with micro- or nano-scale spatial resolution

Functionally Graded Electrodes

Impedance of an Individual Grain/GB/TPB

Impedance Analyzer

1

Patterned Electrode Patterned Electrode

2

Functionally Graded Electrodes

Graded Electrodes Prepared by Combustion CVD

LSC LSC-powder spray 70 wt.% LSC+30 wt.% GDC GDC YSZ

50wt%LSC-50wt%GDC-CCVD GDC-CCVD YSZ-substrate

Functionally Graded Electrodes

Applicability to SOFC Commercialization

· Generated some basic understanding of electrode reaction mechanisms · Developed new tools for in-situ determination of electrode properties under practical conditions · Rational design of efficient electrodes

Functionally Graded Electrodes

Activities for the Next 12 Months

1) Oxygen isotrope exchange experiments in UHV chamber for study of detailed reaction mechanisms using vibrational spectroscopy · · FTRI, pd-FTIR, Rapid Scan FTIR TERS and Raman Mapping

2) Refined calculations of vibrational spectra for different cathode materials to assist interpretation of FTIR and Raman data for different electrodes · · Oxygen reaction mechanism and kinetic parameters Bulk properties such as vacancy concentration and transport properties

3) Local measurements using SPM tips and patterned electrodes to probe local properties under in-situ conditions 4) Optimize Templated Synthesis and Combustion CVD for Fabrication of FGEs

Functionally Graded Electrodes

Acknowledgement

Lane Wilson, NETL/DoE

SECA Core Technology Program Dept of Energy/National Energy Tech Laboratory Equipment Partially funded by DURIP/ARO Center for Innovative Fuel Cell and Battery Technologies, Georgia Tech

Functionally Graded Electrodes

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