Surface Chemistry, Electronic Structure, and Activity of SOFC … · 2013. 8. 22. · 15 Effects of...
Transcript of Surface Chemistry, Electronic Structure, and Activity of SOFC … · 2013. 8. 22. · 15 Effects of...
Surface Chemistry, Electronic Structure,
and Activity of SOFC Cathode Films
Wonyoung Lee and Bilge Yildiz
Laboratory for Electrochemical Interfaces
Nuclear Science and Engineering
Massachusetts Institute of Technology
13th SECA Workshop, Pittsburgh, PA, USA
July 25, 2012
SOFC cathode materials
: Main barrier to achieve higher power
2
Singhal and Kendall, High Temperature
Solid Oxide Fuel Cells, 2003.
CathodeCathodeLa(Mn,Co)OLa(Mn,Co)O33
ElectrolyteElectrolyteZrOZrO22
e-
CathodeCathodeLa(Mn,Co)OLa(Mn,Co)O33
ElectrolyteElectrolyteZrOZrO22
e-
Oxygen Reduction (OR) at SOFC cathode
Structure
Composition
Electronic structure
OR kinetic
Sholklapper et al., Electrochem. Solid. St. 10 (2007).
Solid Oxide Fuel Cells Cathode Materials
3
Dulli et al., Phys. Rev. Lett. 62 (2000).
Mn
La Sr
O
Sr rich in A-site [1]
Formation of (La,Sr)O [2] Formation of (La,Sr)2MnO4
[3]
SEM Sr
Co La
(La,Sr)CoO3 annealed at 650 C in air
Fister et al., Appl. Phys. Lett. 93 (2008).
Cai et al., Chem. Mater. 24 (2012).
Surface Structure and Chemistry: A-Site Rich?
4
To control cation segregation for enhanced activity & stability,
driving forces must be quantitatively understood
Kubicek et al., J. Electrochem. Soc. 158 (2011); Cai et al., Chem. Mater. 24 (2012)
Surface Chemistry Strongly Affects Surface Activity
5
Quantitatively assess the key
driving forces of cation
segregation on perovskite oxide
surfaces
Determine the chemical
composition and structure of
secondary phases on the surface
upon cation enrichment
Assess the effects on
electrochemical activity
Objective
Elastic Electrostatic
Cation Segregation
Elastic energy
minimization Electrostatic repulsion
minimization
Hypothesis for Cation Segregation in Perovskites
6
7
XPS
Surface Chemical State X-ray Photoelectron Spectroscopy (XPS)
Nanoscale Auger Electron Spectroscopy
(n-AES)
EB
STM
Surface Electronic Structure
Scanning Tunneling
Microscopy / Spectroscopy
(STM/STS), in situ
Mechanisms, Energetics and
Kinetics of Cation Segregation
Electronic structure (DFT+U)
+_
Tip
Tunneling direction
+_
Tip
Tunneling direction
Tip
Empty
Filled
Eg
Approach
LaMnO3 or SmMnO3
To systematically induce elastic energy
differences, radius of the selected dopant
cations is varied.
Host Dopant
Ca2+ Sr2+ Ba2+
La3+ Δr (Å) -0.02 0.08 0.25
Δr/r0 (%) -1.5 5.9 18.4
Sm3+ Δr (Å) 0.10 0.20 0.37
Δr/r0 (%) 8.1 16.1 29.8
Increasing dopant size relative
to the host cation
Control of Elastic Interactions
8
LaMnO3 or SmMnO3
Seven models to represent the variation of
electrostatic interactions are constructed
by controlling the distribution of charged
oxygen- and cation-vacancies.
Increasing attractive interaction
to the dopant cation to the surface
Control of Electrostatic Interactions
9
LaMnO3
1 2 3 4 5 6 7
-0.8
-0.6
-0.4
-0.2
0.0
0.2
ES
eg
r (e
V)
Types
Ca
Sr
Ba
Model 1 2 3 4 5 6 7
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
ES
eg
r (e
V)
Model
Ca
Sr
Ba
SmMnO3
Model
Both Elastic and Electrostatic Drivers are Important
10
Han and Yildiz,
in preparation.
Elastic Interaction: Dopant Size Mismatch
11
To systematically induce elastic energy differences, radius of
the selected dopant cations is varied.
Host
cation Size mismatch
Dopant
Ca2+ Sr2+ Ba2+
La3+ Δr (= rhost – rdopant) (Å) -0.02 0.08 +0. 25
Δr/r0 (%) -1.5 +5.9 +18.4
La0.8D0.2MnO3
SrTiO3 (100)
~20nm La0.8Ba0.2MnO3 LBM
La0.8Sr0.2MnO3 LSM
La0.8Ca0.2MnO3 LCM
Pulse laser deposition (T = 815 °C in pO2 = 10 mTorr)
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Annealing Induced Changes
830 C
530 C
RT
830 C
530 C
RT
Emission angle = 0 Emission angle = 80
Chemical changes (Sr 3d)
lattice surface
500nm 500nm 500nm
RT 530 C 830 C
Structural changes (AFM)
Emission
angle = 80
Emission
angle = 0
Segregation in
perovskites
Secondary phase
formation
Secondary phase
formation
Segregation in
perovskites
Sr/Mn
Sr/La
13
Effects of Size Mismatch on Dopant Segregation
RT 530 °C 830 °C
LB
M
LS
M
LC
M
500nm
Annealing in air for 1 hr
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Localized Chemical Identification
LBM_ann_800_1h.133.sem: MIT CMSE
2012 Apr 2 20.0 kV 0 FRR
SEM
0.0
107.0
0.200 µm
0.2
00 µ
m
SEM
LBM_ann_800_1h_map.135.map: MIT CMSE
2012 Apr 2 20.0 kV 0 FRR 12.29 min
La3
0.0
63.2
0.200 µm
0.2
00 µ
m
La3
LBM_ann_800_1h_map.135.map: MIT CMSE
2012 Apr 2 20.0 kV 0 FRR 12.29 min
Ba4
0.0%
100.0%
0.200 µm
0.2
00 µ
m
AC (%)
Ba4
LBM_ann_800_1h_map.135.map: MIT CMSE
2012 Apr 2 20.0 kV 0 FRR 12.29 min
Mn1
0.0
43.7
0.200 µm
0.2
00 µ
m
Mn1
500nm
500nm
500nm
500nm
SEM La
Ba Mn
LBM after annealing at 830 C
[Ba]↓,
[La]↑, [Mn]↑
[Ba]↑,
[La]↓, [Mn]↓
On top of surface phases
Away from surface phases
On top of surface phases
Away from surface phases
agglomeration
depletion
Auger electron spectroscopy:
elemental mapping (left) and point
spectra (right)
15
Effects of Oxygen Pressure on Dopant Segregation
LBM
LBM_700_1e-6_.124.sem: MIT CMSE
2012 May 5 20.0 kV 0 FRR
SEM
0.0
122.0
0.200 µm
0.2
00 µ
m
SEM
LBM_700_.137.sem: MIT CMSE
2012 May 1 20.0 kV 0 FRR
SEM
0.0
157.0
0.200 µm
0.2
00 µ
m
SEM
1μm
1μm
500nm
500nm
760 Torr (830 C)
1x10-6 Torr (850 C)
760 Torr (830 C)
1 10-6 Torr (850 C)
LBM_500_.111.sem: MIT CMSE
2012 May 1 10.0 kV 0 FRR
SEM
0.0
149.0
10 µm
10 µ
m
SEM
1μm 500nm
1x10-9 Torr (700 C) 1 10-9 Torr (700 C)
Cation intensity ratio away from the
surface phases (background data
was used if no features exist)
16
Effects of Oxygen Pressure on Dopant Segregation LSM_700_.122.sem: MIT CMSE
2012 May 2 20.0 kV 0 FRR
SEM
0.0
183.0
0.200 µm
0.2
00 µ
m
SEM
LSM
LBM_500_.111.sem: MIT CMSE
2012 May 1 10.0 kV 0 FRR
SEM
0.0
149.0
10 µm
10 µ
m
SEM
1μm 500nm
1x10-9 Torr (700 C) 1 10-9 Torr (700 C)
1μm 500nm
1x10-6 Torr (850 C) 1 10-6 Torr (850 C)
1μm 500nm
760 Torr (830 C) 760 Torr (830 C)
Cation intensity ratio away from the
surface phases (background if no
features exist)
17
Effects of Oxygen Pressure on Dopant Segregation
Annealing in high pO2
x
Annealing in low pO2
x
18
Electronic Properties of Surface Phases
500nm
LBM annealed at 830 C in air
AFM
200nm
STM
Annealing-induced surface phases are insulating (Eg,LBM, Eg,LSM ~ 2 eV, Eg,BaO ~4.5 eV, Eg,SrO ~ 5.7 eV)
500nm
LSM annealed at 830 C in air
AFM
200nm
STM
AFM
h
STM
iT
19
Summary
Agglomeration
dopant
Surface segregation
A-site
cations
surface
bulk
Ca Sr Ba
RT
C
MnDopant
MnDopanto
/
/830
Segregation = Elastic + Electrostatic
760 Torr
1×10-6 Torr
1×10-9 Torr
insulating
20
Future Work
Surface electronic structures using scanning tunneling
microscopy / spectroscopy.
Electrochemical properties using impedance
spectroscopy.
Acknowledgements
US-DOE, Office of Fossil Energy for financial support.
(Grant No. DE-NT0004117)
Prof. C. Ross and Prof. H. L. Tuller at MIT for the use of
their PLD system.
21
Supplementary
22
Angle-Resolved X-ray Photoelectron Spectroscopy
23
Information depth varies with an emission angle
Photoelectron
emission
sample
Surface
structures
Surface
segregation
Probe the chemical composition
with a depth information (Escape depth of Sr 3d is ~6nm with θ=0 ,
and ~1nm with θ=80°)
24
HR XPS for dopant peaks
500_vac
500_air
800_air
500_vac
500_air
800_air
500_vac
500_air
800_air
500_vac
500_air
800_air
500_vac
500_air
800_air
500_vac
500_air
800_air
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HR XPS
RT (0) 500 (0) 800 (0)
RT (80) 500 (80) 800 (80)
To extract more information
from HR XPS, especially to
identify the chemical
composition of surface layer or
secondary phase.
This can be useful to describe
segregation before forming
secondary phases on the
surface
Still working on this analysis
O1s
26
Effects of Vacancy Distribution on Dopant Segregation
Other thoughts on this difference (high pO2 vs. low pO2)
If there is strong interaction between the oxygen vacancy (+) and
cation vacancy (-), more oxygen vacancies on the surface in UHV
would attract more cation vacancies on the surface. Then, it will be
difficult for cations to diffuse towards the surface, and/or the cation
vacancies will provide rooms for segregated cations?
But, we need to know the diffusivity of cations at 500-800C to see they
are mobile enough
27
Effects of Oxygen Pressure on Dopant Segregation
1 10-6 Torr
1 10-9 Torr
Cation intensity ratio using XPS
(left) and AES (right)
28
Effects of Vacancy Distribution on Dopant Segregation
Annealing in high pO2
x
Annealing in low pO2
x
1 2 3 4 5 6 7
-0.8
-0.6
-0.4
-0.2
0.0
0.2
ES
eg
r (e
V)
Types
Ca
Sr
Ba
Elastic (dopant size mismatch)
+ Electrostatic (charged defects
distribution)