Study of electrocatalysis in solid oxide fuel cells using well … · 2020. 9. 17. · Ionic or...

Study of electrocatalysis in solid oxide fuel cells using well-defined model

electrode structures

WooChul Jung

Advisor: Sossina M. Haile

Caltech

Sep. 19, 2012, TUDelft

2

Electrode inefficiency dominates in thin-electrolyte IT-SOFC

O2-

H2

O2

Cathode Electrolyte Anode

e-

H+

V

Separate oxidative and

reductive steps of combustion

Oxide ions diffuse through

solid-oxide electrolyte

Electrons forced through

external circuit

Result: efficiently converted

chemical energy into electricity

Solid Oxide Fuel Cell Basics

IT SOFC: intermediate-temperature Solid Oxide Fuel Cell

3

Gas accessibility

Ionic pathway

Electronic pathway

Electro-catalysis

O2 + 2e → O2 1 2

Gases

Electrode Reactions

Cathode :

H2 + O2 → H2O + 2e Anode :

Electronic species Ionic species

4

Challenges in SOFC Electrode Research O2

Electronic

e-

O2-

Electrolyte O2-

O2

Triple phase boundary (3PB)

Surface Pathway

Ionic or MIEC

Ionic phase

e.g., doped zirconia or ceria

perovskite ferrite or cobalite

(MIEC: Mixed Electronic & Ionic Conductor)

Electronic phase

e.g., metals or (La,Sr)MnO3

Pores

Image from J.R. Wilson, et al. Nature Materials 2006, 5, 541.

5

Challenges in SOFC Electrode Research

Electronic

e-

Electrolyte

O2

e-

e-

O2-

O2-

Double phase boundary (2PB)

Bulk pathway

Ionic or MIEC

Ionic phase

e.g., doped zirconia or ceria

perovskite ferrite or cobalite

(MIEC: Mixed Electronic & Ionic Conductor)

Electronic phase

e.g., metals or (La,Sr)MnO3

Pores

Image from J.R. Wilson, et al. Nature Materials 2006, 5, 541.

6

Challenges in SOFC Electrode Research

Electronic

Electrolyte

Ionic or MIEC

Ionic phase

e.g., doped zirconia or ceria

perovskite ferrite or cobalite

(MIEC: Mixed Electronic & Ionic Conductor)

Electronic phase

e.g., metals or (La,Sr)MnO3

Pores

Image from J.R. Wilson, et al. Nature Materials 2006, 5, 541.

+ + + + + + + - - - - - -

+ +

+

+ + + + +

+

Adsorbate layer

Surface layer

Space charge layer

Bulk

Chemical & Morphological Complexities

+

Multiple, Simultaneous Reactions

+

Difficulties in Surface Characterizations

7

• The relative activities of the various

reaction sites?

• Dominant reaction pathway?

• Characteristics of surface properties?

• Rate determining step? Factors

governing reaction rate?

Conventional Approach

Optimize materials & microstructure

A complex system is not well

understood:

Trial-and-Error

Develop model systems

Design materials &

microstructures

Decouple multi-phase

reaction-diffusion interactions

Bottom-Up Approach

Fabricate devices

Optimization of Electrode

Identify rate governing factors

+

/26

Ceria

Ceria

S.P. Yoon, et al. J. Power Source. 2002, 106,160.

S.P. Jiang, et al. Electrochem. Solid State Lett. 2003, 6, A67.

S.P. Jiang, et al. J. Mater. Sci. 2004, 39, 4405.

K. Eguchi, et al. Solid State Ion. 1992, 52, 165.

T. Tsa , et al. Solid State Ion. 1997, 98, 191.

T. Tsa , et al. J. Electrochem. Soc. 1998, 145, 1696.

K. Eguchi, et al. Solid State Ion. 1992, 52, 165.

T. Setoguchi, et al. J. Electrochem. Soc. 1992, 139, 2875.

C. Lu, J. Electrochem. Soc. 2003, 150, A1357.

Example1

Enhanced performance in the presence of Ceria

8

H2 + O2 → H2O + 2e

CeO2

9

Metal

Ceria

1 2 3

Metal

Ceria

Metal

Ceria

Reaction rate at metal | ceria | gas sites

Reaction rate at ceria | gas sites

Electronic conductivity of ceria

Metal Pathway

Limited

Ceria Surface

Limited

Lateral Electron

Diffusion Limited

Material Properties

3PB Site Density

2PB Site Density

Inter-metal distance

Microstructural Parameters

Ionic conductivity of ceria

Gas Gas Gas

Probing Coupled Surface Reaction-Diffusion

In courtesy of W.C. Chueh for this slide

10

Ceria Thin Film

Ionic Conducting Sub.

Patterns interconnected

Approach: Patterned Thin Film Model Electrodes

Varying metal-catalyzed

reaction site density (d3PB)

Ceria surface area (d2PB) held constant

Varying d2PB

d3PB held constant

In courtesy of W.C. Chueh for this slide

Monitoring Technique for Reaction Rate:

Electrochemical Impedance Spectroscopy (EIS)

11

YSZ (100)

SDC

SDC

Metal

Metal

YSZ (100)

Sm0.20Ce0.80O1.9-d (SDC)

SDC

Y0.16Zr0.84O1.92

(YSZ) (100)

Pulsed-Laser

Deposition Metal

Patterning

By Liftoff

Lithography

Post testing 72 hours 500 – 600 °C in H2

-150 -100 -50 0 50 100 150

SDC Thinfilm (200)

(°)

Inte

nsity (

Arb

.)

YSZ Substrate (200)

20 30 40 50 60 70 80

YS

Z (

11

0)

SD

C (

11

0)

Inte

nsity (

Arb

.)

2 (°)

20 22 24 26

Inte

nsity (

Arb

.) (°)

Fabrication Route

Nature Materials 2012, 11, 155. In courtesy of W.C. Chueh for this slide

12

Activity of Different Reaction Sites

Fixed reaction rate

regardless of d3PB

Reaction rate is linearly

dependent on d2PB

Nature Materials 2012, 11, 155.

102

103

10-3

10-2

10-1

d3PB

/ cm-1

pH2O = 0.0058 atm (Pt)

1/R

ele

ctr

od

e

/

-1 c

m-2

1

102

103

10-3

10-2

10-1

d3PB

/ cm-1

pH2O = 0.0058 atm (Pt)

0.0030 atm

0.0015 atm

0.00078 atm

1/R

ele

ctr

od

e

/

-1 c

m-2

1

102

103

10-3

10-2

10-1

0.0051 atm (Ni)

0.0026 atm

0.0013 atm

0.00065 atm

d3PB

/ cm-1

pH2O = 0.0058 atm (Pt)

0.0030 atm

0.0015 atm

0.00078 atm

1/R

ele

ctr

od

e

/

-1 c

m-2

1

0.1 1

10-3

10-2

10-1

d2PB

pH2O = 0.0057 atm

0.0015 atm

0.0029 atm

0.00076 atm

1/R

ele

ctr

od

e

/

-1 c

m-2

1

13

Lateral Electron Diffusion

Ceria

YSZ

Ceria

YSZ

seon = 0.1 -1cm-1, k2PB = 0.1 -1cm-2, L=10-4 cm, Cchem=500 Fcm-3

Increasing

inter-metal

distance

In courtesy of W.C. Chueh for simulations

14

Metal

Ceria

1 2 3

Metal

Ceria

Metal

Ceria

Metal Pathway

Limited

Ceria Surface

Limited

Lateral Electron

Diffusion Limited

Gas Gas Gas

X X O 1.3 x 102 to 2.0 x 103 cm/cm2

0.06 to 0.75 cm2/cm2

3PB Site Density

2PB Site Density

Inter-metal distance: 5 to 120 mm

3PB / 2PB (this work): ~ 4 x 103 cm-1

*Commercial SOFCs: ~ 2 x 104 cm-1

Implications of Technological Applications

*: J. R. Wilson, et al. Nature Materials 2006, 5, 541.

15

Rational Fuel Cell Electrode Design

Ceria Metal

Electrolyte

Pulsed Laser Deposition

(PLD)

16

2 mm

PLD

Energy & Environ. Sci. 2012, 5, 8682.

17

500 nm

PLD

Energy & Environ. Sci. 2012, 5, 8682.

18

2 – 3 % H2O + 97 – 98 % H2

High Electrode Activity

0.8 1.0 1.2 1.410

-2

10-1

100

101

102 1000 900 800 700 600 500

1

/ R

ele

ctr

od

e

/

c

m-2

1000 T -1

/ K -1

Pure SDC

Temperature / oC

Energy & Environ. Sci. 2012, 5, 8682.

19

O2

Electronic

e-

O2-

Electrolyte O2-

O2 O2

e-

e-

O2-

O2-

Ionic or MIEC

Example2-1

Identifications of rate determining steps

Important role of electron transfer to oxygen species!

O2 + 2e → O2 1

2 Complicated Nature of ORR

/27

– Perovskite solid solution, Mixed Ionic Electronic Conductor (MIEC)

– Stable over wide pO2 range

– Well studied electronic structure, defect and transport properties

– Controllable magnitudes and ratios of se and sion

Rothschild, A, Tuller, H.L., et al., Chem. Mater, 18, 3651 (2006)

SrTi1-xFexO3-d (STF)

1 atm 10-20 atm

Stable & Controllable Model Materials

20

21

Reaction Rate vs. Bulk Conductivity

– Well-defined electrode geometry 2PB limited Pathway

– Moderate sel and sion are sufficient (i.e., ~ 10-3 S/cm for STF5)

– Other factors likely control the surface exchange kinetics.

Adv. Energy Mater. 2011, 1, 1184.

YSZ

STF

Area

TPBL

STF

LSCF

330 S/cm

LSCF

22

Reaction Rate vs. Fermi Level Position

Important role of availability of electronic species!

Adv. Energy Mater. 2011, 1, 1184.

23

Remaining Questions:

Investigation of Surface Properties

+ + + + + + + - - - - - -

+ +

+

+ + + + +

+

Adsorbate layer

Surface layer

Space charge layer

Bulk

Bulk Materials Properties Surface Reaction Rate

24

Example2-2

Cation Surface Segregation

Energy & Environ. Sci. 2012, 5, 5370.

XPS surface analysis

Ratio of Sr to (Ti + Fe) is always higher than unity.

Fe to Ti ratio stays the same between the surface

and bulk.

Sr excess increases with increasing Fe content.

/27

25

Dynamic Nature of Sr Segregation

Energy & Environ. Sci. 2012, 5, 5370.

– Amount of Sr excess can be controlled by chemical etching

– Etched STF surface provides enhanced surface exchange kinetics

– Sr re-segregates upon high temperature annealing

26

In-situ characterizations

In collaboration with Prof. Bilge Yildiz (MIT)

27

Surface SrO Segregation

Cation segregation

Electronic structure Chemistry

Oxygen exchange

rate

Energy & Environ. Sci.

2012, 5, 7979.

28

Other research efforts toward

In-situ characterizations

Scattering techniques

(x-ray, photoelectron, neutron)

Spectroscopy techniques

(electron, infra-red, Raman)

Surface adsorption Surface oxygen and electronic defects.

Q.-H. Wu, et al., Surface Review and Letters, 2007 14, 587. W.C. Chueh, et al., Chem. Mater., 2012 24,1876.

29

Neutron Scattering in Fuel Cell Research

• Crystal (local) structure

(Neutron Diffraction)

• Reaction products (Neutron Diffraction)

• H+ or H2O dynamics & distribution (Quasi-Elastic Neutron Scattering, Small Angle Neutron Scattering, Neutron Radiography)

• Morphologies of Nano/Microstructure (Neutron Radiography, Neutron Activation Analysis, SANS)

Surface Sensitive, In-situ Characterizations

• Grazing incidence (or Near Surface) SANS, Neutron Reflectometry

• Nanostructures with high surface-to-volume ratio

Target: Surface adsorbates (concentration & dynamics), Structural

& Chemical evolution near surface, etc.

Global Climate and Energy Program

(Stanford University)

Acknowledgements

Supervisor: Prof. Sossina Haile (Caltech)

Collaborators: Prof. Harry Tuller (thesis advisor, MIT)

Prof. Bilge Yildiz (MIT)

Prof. William Chueh (Stanford)

Dr. Yong Hao (IET, China)