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)

12

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

14

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

25

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)