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Electro-chemo-mechanical Studies of Perovskite-Structured Mixed Ionic-Electronic Conducting SrSni-xFex03-x/2+6 by Chang Sub Kim B.S. Physics California Institute of Technology, 2013 SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN MATERIALS SCIENCE AND ENGINEERING MASSACHUSETTS INSTITUTE OF TECHNOLOGY

AT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

OCT 28 2015

SEPTEMBER 2015

LIBRARIES C 2015 Massachusetts Institute of Technology. All rights reserved.

Signature redacted

Authored by

Chang Sub Kim Department of Materials Science and Engineering

July 24, 2015

A

Signature redacted

Certified by

Hari L. Tuller Professor of Ceramics an Electronic aterials Thesis Supervisor

Accepted by

Signature redacted______ Donald R. Sadoway Chair, Departmental Committee on Graduate Students

I

2

Electro-chemo-mechanical Studies of Perovskite-Structured Mixed Ionic-Electronic Conducting SrSnl-xFex03-x/2+6 by

Chang Sub Kim Submitted to the Department of Materials Science and Engineering

On July 24, 2015 in Partial Fulfillment of the Requirements for the Degree of Master of Science in Materials Science and Engineering

ABSTRACT High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive. However, when operating at reduced temperatures, there is significant loss in efficiency, of which slow surface reaction kinetics at the cathode are most responsible. Previously, the mixed ionic and electronic conducting (MIEC) perovskite-structured SrTixFex03-O2+6 (STF) materials system was identified as a promising candidate for SOFC cathodes given rapid oxygen surface exchange kinetics. The exchange kinetics were correlated with the minority electron charge density in STF, which in turn depends on its defect chemistry and band structure. In this work, an alternate B site host cation, Sn, was selected to replicate and extend the STF studies, due to its distinct band structure and higher electron mobility. Oxygen nonstoichiometry and the defect chemistry of the SrSnxFex03-/2+6 (SSF) system were examined by means of thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure while surface kinetics were examined by means of AC impedance spectroscopy. The electrochemo-mechanical properties of SSF were found not to differ significantly from the corresponding composition in STF. It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted by Sn's larger size, the defect equilibria - including the oxygen vacancy concentration - were found to also be largely dominated by Fe and thus differed only in a limited way from that in STF. Key thermodynamic parameters of SrSnosFeo.3502.825 6 SSF35 obtained include the reduction enthalpy (4.30 eV) the electronic band gap (1.72 eV) and the anion Frenkel enthalpy (0.52 eV). Key kinetic parameters include the migration enthalpy of oxygen vacancies (0.70 eV), the activation energy of area-specific-resistance (1.65 eV) and the electron (0.0002 0.00005 cm 2 /V-s) and hole (0.0037 0.0015 cm 2 /V-s) mobilities. With the surface exchange rate nearly identical to the STF35 counterpart, the main advantage of SSF35 as a SOFC electrode would be its enhanced chemical stability and slower degradation.

Thesis Supervisor: Harry L. Tuller Title: Professor of Ceramics and Electronic Materials

3

TABLE OF CONTENTS LIST OF FIGU RES......................................................................................................................................

6

LIST OF TA BLES .......................................................................................................................................

8

ACKN OW LED GEM ENTS .........................................................................................................................

9

CHA PTER 1. IN TRODUCTION ...............................................................................................................

10

1.1

M otivation ..................................................................................................................................

10

1.2

Solid oxide fuel cells (SO FCs).................................................................................................

12

1.2.1

Principles of operation and characteristics .....................................................................

12

1.2.2

Technical challenges of SO FCs......................................................................................

14

1.2.3

SOFC Cathodes ..................................................................................................................

15

1.3

M ixed ionic and electronic conducting (M IEC) oxides ..........................................................

16

1.3.1

M odel system STF..............................................................................................................

17

1.3.2

M odel system SSF ..............................................................................................................

18

Objectives of the research...........................................................................................................

19

CHA PTER 2. EX PERIM EN TS .................................................................................................................

20

2.1 Sam ple Preparation ..........................................................................................................................

20

1.4

2.1.1

Powder synthesis ................................................................................................................

20

2.1.2

Bulk sam ples ......................................................................................................................

20

2.1.3

Deposition of STF and SSF thin films by PLD ...............................................................

20

2.1.4

Deposition of Au or Pt current collectors........................................................................

21

2.2

Physical Characterization .......................................................................................................

22

2.2.1

X-ray diffraction .................................................................................................................

22

2.2.2

Density m easurem ent .....................................................................................................

22

2.3 Therm ogravim etry............................................................................................................................

22

2.4 Sim ultaneous conductivity and chem ical expansion m easurem ents .............................................

23

2.4.1

Bulk sam ple configuration...............................................................................................

23

2.4.2

M easurem ent setup .............................................................................................................

23

2.4.3

M easurem ent conditions.................................................................................................

24

2.5 Thin film EIS m easurem ents............................................................................................................

24

2.5.1

Cell configuration ...............................................................................................................

24

2.5.2

M easurem ent conditions.................................................................................................

25

CHA PTER 3. RESULTS............................................................................................................................ 3.1 Physical/chem ical characterization of SSF powder and thin film s ...............................................

26 26

3.1.1

SSF powders.......................................................................................................................

26

3.1.2

SSF thin film s .....................................................................................................................

26

3.2 Therm ogravim etric analysis.............................................................................................................

29

4

3.3 Bulk conductivity.............................................................................................................................

31

3.4 Bulk chem ical expansion .................................................................................................................

33

3.5 EIS results of SSF thin film electrodes ............................................................................................

35

CHA PTER 4. DISCU SSION .....................................................................................................................

37

4.1 Defect chem ical model.....................................................................................................................

37

4.1.1

Defect chem istry of STF and SSF ...................................................................................

37

4.1.2

M odel defect diagram .....................................................................................................

40

4.1.3

Defect diagrams and thermodynamic parameters of STF35 and SSF35 ........................

41

4.2 Conductivity m odel..........................................................................................................................

44

4.3 Chem ical expansion .........................................................................................................................

49

4.4 Investigation of surface exchange kinetics of STF/SSF thin film s ...............................................

50

4.5 Problem s w ith SSF5 and SSF50 ..................................................................................................

50

4.5.1

Phase decom position of SSF5 .........................................................................................

50

4.5.2

Degradation of SSF50 thin film electrode ......................................................................

51

CHA PTER 5. CON CLU SION ...................................................................................................................

52

5.1 Sum m ary ..........................................................................................................................................

52

5.2 Recom m endations for Future W ork.............................................................................................

53

REFERENCES ...........................................................................................................................................

5

55

LIST OF FIGURES Figure 1. A reaction diagram of a SOFC running in fuel cell (top) and electrolysis (bottom) modes. ...... 11 Figure 2. Renewable energy cycle diagram. SOFCs running in electrolysis mode to generate fuel with excess energy and run in fuel cell mode to convert back to electrical power..........................................

14

Figure 3. Area specific resistance of a SOFC, its components - cathode, electrolyte, anode - and charge carrier diffusion. Reprinted from a reference [6]..................................................................................... 15 Figure 4. Sketches of the three reaction paths of the oxygen reduction and incorporation reaction and some possible rate-determining steps. Modifications of the paths (e.g., adsorption of a molecular rather than an atomic species or diffusion along the cathode/electrolyte interface) and a combination of electrode and electrolyte surface paths (adsorption on cathode and surface diffusion onto the electrolyte surface) are also possible. Reprinted from a reference [7]......................................................................................... 16 Figure 5. Ec-EF (left axis) and activation energy of RSTF (or k) measured by EIS (right axis) as a function of Fe composition at 600'C in air. Reprinted from a reference [12]....................................................... 18 Figure 6. Variation of the total conductivity vs. oxygen partial pressure at 750'C of SrSnxFe,03-6 (0 x 1). Conductivities at highly reducing and oxidizing conditions were measured, but not at intermediate oxygen partial pressure where an ionic regime is expected. Reprinted from a reference [13]................ 19 Figure 7. A schematic illustration of the current collector pattern. UV light passes through the white region (transparent, not coated with chrome), cross-linking the photoresist polymer. Photoresist on the black region (coated with chrome) dissolves in the developer.............................................................. 21 Figure 8. A Schematic illustration of the SSF bulk sample with four-point probe setup for simultaneous conductivity/chem ical expansion measurem ents................................................................................... 23 Figure 9. Schematic illustrations of asymmetric cells with YSZ electrolyte, SSF working electrode with (a) buried Pt current collector and (b) top Au current collector. Both cells had porous Ag counter e le ctro d e s.................................................................................................................................................... 24 Figure 10. XRD pattern of SrSn -,Fex03.6 (x

=

0, 5, 35) powders synthesized .....................................

Figure 11. HRXRD patterns of SrSnO3 thin films deposited on YSZ (100) single crystal substrates by PLD at 850'C and 950'C heater tem peratures .......................................................................................

26

27

Figure 12. HRXRD patterns of SSF50 thin films deposited on YSZ (100) single crystal substrates by PLD at 850'C heater temperature, and SSF35 thin film at 850'C and 9500 C................................................. 28 Figure 13. HRXRD patterns of STF35 thin films deposited on YSZ (100) single crystal substrates by PLD at 600'C and 850'C heater tem peratures................................................................................................. 28 Figure 14. Oxygen nonstoichiometry - 6 - as a function of p02 and temperature for SSF35. .............. 30 Figure 15. Comparison of oxygen nonstoichiometry of STF35 [19] and SSF35 at 700'C and 1000'C.... 31 Figure 16. Bulk conductivity of SSF35 as a function of p02 and temperature. ....................................

6

32

Figure 17. Comparison of bulk conductivities of STF35 [20] and SSF35 at 8001C. ..............................

32

Figure 18. A schematic model of a material's expansion as a function of temperature, with the chemical expansion contribution noted. Reprinted from a reference [22]............................................................ 33 Figure 19. SSF bulk sample length in different oxygen partial pressures at a constant temperature of 900'C m easured by a dilatom eter...............................................................................................................

34

Figure 20. Expansion of SSF35 as a function of 6 at 750'C. ..................................................................

34

Figure 21. Typical electrochemical impedance spectroscopy (EIS) spectrum of a SSF35 thin film deposited onto a single crystal YSZ electrolyte with buried Au current collectors and porous Ag counter 35 electrode m easured in air at 468'C ............................................................................................................. Figure 22. Comparison of temperature dependence of area specific resistance (ASR) of SSF35 and STF35 36 th in film electro des..................................................................................................................................... Figure 23. Schematic defect diagram using Brouwer approximation for the model system SSF and STF.41 Figure 24. Carrier concentrations from defect equilibria at 10000 C. ....................................................

42

Figure 25. Natural log of equilibrium constants K as a function of 1/kT...............................................

43

Figure 26. Temperature dependence of p-type (p 0 2= 0.316 atm) , n-type (pO2 = 10-18 atm) , and PO2insensitive ionic conductivities of SSF35..............................................................................................

45

Figure 27. Oxygen partial pressure dependence of (a) n-type and (b) p-type conductivities of SSF35 extracted by subtracting the respective ionic conductivities at each isotherm. .......................................

45

Figure 28. Temperature dependence of electron and hole mobilities of SSF35......................................

47

Figure 29. Expected conduction and valance band positions of STF and SSF as a function of Fe content,

x . .................................................................................................................................................................

7

52

LIST OF TABLES Table 1. Summary of properties of different types of fuel cells. Reprinted from a reference [5]........... 13 Table 2. Ionic radii of Ti"* and Sn", lattice parameters and volumes of STF35 and SSF35 calculated from

X RD peak s. ................................................................................................................................................

29

Table 3. The predicted solutions to the defect solutions with Brouwer approximations.........................

40

Table 4. Thermodynamic parameters for STF35 [19] and SSF35 from fitting the nonstoichiometry 6 as a function of oxygen partial pressure at different temperatures................................................................. 43 Table 5. Thermodynamic parameters for STF35 and SSF35 from fitting nonstoichiometry - 6 - data as functions of oxygen partial pressure over a range of temperatures simultaneously............................... 44 Table 6. Electron and hole mobilities of SSF35 calculated from defect chemistry and conductivity data. 46 Table 7. Activation energies of n-type conductivity, electron mobility, electron concentration, and the reductio n enthalpy . .....................................................................................................................................

48

Table 8. Activation energies of p-type conductivity, hole mobility, hole concentration, and activation energies of hole concentration at intermediate and high pO2.................................................................

48

Table 9. Activation energies of ionic conductivity, oxygen vacancy concentration, and enthalpy of oxygen vacan cy m igratio n . ..................................................................................................................................... 49

8

ACKNOWLEDGEMENTS I would like to express my sincerest gratitude to my advisor, Professor Harry Tuller, both intellectually and personally. He is a genuine educator, an insightful researcher, a passionate lecturer, and my personal mentor. His guidance was available whenever I needed, and was always above my expectations. Members of the Tuller group must be acknowledged for their helpful discussions, comments, suggestions in our offices, group meetings, and labs. Dr. Jae Jin Kim has guided me since I first arrived at the Institute. I cannot count how many questions I have asked and how many discussions we had. Drs. Sean Bishop and Nicola Perry have generously shared their experience and insight. Dr. Stuart Cook has helped with fitting the thermogravimetric data. I would also like to acknowledge Drs. Di Chen, Kunal Muhkerjee, Nikolai Tsvetkov, and Michael Campion for their helpful discussions, and all other members, visiting students and scholars in the group, as well as Elisabeth Anderson. There are so many individuals and groups at and outside MIT I would like to thank: Kurt Broderick and other members at the Microsystems Technology Laboratories (MTL), members of KGMSE and MIT Sailing, just to name a few, have broadened my views. Lastly, I would like to thank my parents, my only brother, and my wife Kyoung-Won for their support and unconditional love. Funding for this research was provided by Skoltech Center for Electrochemical Energy Storage (CEES).

July 17, 2015 Chang Sub Kim

9

CHAPTER 1. INTRODUCTION 1.1 Motivation The discovery of electricity has changed the way people live. Everyday life begins and ends with electricity. The world's electrical energy consumption has been rapidly increasing over the years, and the majority of energy production finds its source from fossil fuels [1]. However, there are limited fossil fuel reserves, and the efficiency of converting fossil fuels into electricity is limited by the efficiency of the Carnot cycle: W

TC

QH

TH

W

where W is the work produced by the system,

QH

_ _1

(1)

is the heat going into the system, Tc is the temperature

of the cold reservoir, and TH is the temperature of the hot reservoir. Demand for electricity continues to rise, and the world needs more sustainable, renewable, and efficient way of generating electricity. Solar, wind, hydro, and geothermal technologies are sustainable and have become the most prominent sources of renewable energy. All sustainable technologies, however, have limitations: solar energy is limited by time (only available during the day), while wind, hydro and geothermal by location. Therefore, efficient means of energy storage and transportation are required to control the supply of sustainable energy. Electrochemical devices, such as batteries and fuel cells, can solve the problem by redistributing the uneven supply of renewable energy. Batteries store electrical energy in chemical form while charging, and it is converted back to electrical form during discharge. Batteries are widely used in portable electronics such as cell phones and laptops, but low energy density limits their scalability. Fuel cells are also electrochemical devices, but unlike batteries, fuel cells do not store energy in devices, but generate electrical power from the electrochemical reaction of fuels - usually hydrogen or hydrocarbons - with oxygen (Figure 1). Therefore, fuel cells are able to run continuously as long as the fuel and oxidant are provided. The high energy density of chemical fuels is particularly advantageous when it comes to the transportation sector and thus the use of fuel cells rather than combustion engines to provide the needed energy offers higher efficiency as well as reduced 10

difficulties and emissions. Despite these advantages, fuel cells are not widely used due to technological high cost.

2e-2 2e

2e

2e-

0

H2gas H 2 0 + 2e (- H 2 +

2e

/1202 +2e-

2

02-

,gas

--

t2e2 4-

-4 --

H

22

-

2e

> --0

2.cras

H 2 0gas H 2 0 + 2e -

H2 + 02

2

/202 + 2e

(bottom) modes. Figure 1. A reaction diagram of a SOFC running in fuel cell (top) and electrolysis

11

1.2 Solid oxide fuel cells (SOFCs) 1.2.1

Principlesof operationand characteristics

Fuel cells can be categorized according to the types of electrolyte membranes (Table 1). Different electrolytes conduct different ions, and thus exhibit different reactions at the electrodes, operate with different fuels and in different temperature regimes. SOFCs are characterized by metal oxide electrolytes which conduct oxygen ions. Solid electrolytes have several advantages over liquid or polymer electrolytes, including thermal, mechanical and chemical stability against corrosion and contamination. Operation at high temperatures (between 700'C and I 0000 C) gives SOFCs fuel flexibility: certain hydrocarbons, such as methane, can be used directly as a fuel without external reforming, because C-C bonds in hydrocarbons can be easily broken at the temperature range. Furthermore, in contrast to polymer electrolyte membrane (PEM) fuel cells, they do not require noble metal catalysts such as Pt and are less susceptible to poisoning. SOFCs can also be operated reversely as electrolysis cells by applying an electric potential, as shown in Figure 1. A number of studies were conducted using conventional SOFCs running in reverse mode, or symmetric cells with electrodes more chemically stable in both oxidizing and reducing conditions [2-4]. Excess energy from solar, wind, or hydro can be stored as hydrogen or hydrocarbon fuels by running reversible SOFCs in electrolysis mode to decompose water or C0 2, and the fuels can then be converted back to electrical power by running the SOFCs in the fuel cell mode as demand increases. This enables a carbon-free or carbon-neutral energy supply by combining renewable energy with SOFCs (Figure 2).

12

I

Table 1. Summary of properties of different types of fuel cells. Reprinted from a reference [5].

- Backup power

* Portable power

V +2e' +O2

(10)

1 K'e

2

- [V]n (PO2)f

(11)

where n is the concentration of electrons in the conduction band. As for the anion Frenkel equation above, provided the change in [V] is much less than [Ox], the latter can be treated as a constant, and the defect equations simplified as done below. Kred = Kred[Oox]

(12)

Equation (11) can be written as [Vj]n 2 (pO 2 )f = Kred = [VS]on1 2 (P02 0)f

(13)

assuming [Ox] is much greater than [V ]O and [O'], where [V]O and ni are the concentrations at the stoichiometric oxygen partial pressure P02 0 . Therefore, one can write ni =

R

(14)

from equation (9). Lastly, charge neutrality gives: 2[V ] + p = 2[0j'] + n

38

(15)

[V

Writing n as a function of other variables from equation (15) and substituting [0k"] and p as [v~lI and from equations (7) and (9), respectively, - [0']) + p = 2 =2([V-]

K4

Kaf'\ ]+--

(16)

n

[Vd]/

Rewriting equation (13), one can write oxygen vacancy concentration as: 1

1

Ka2 K-(p02)21 1 [VI] = 2 n p0g

(17)

By substituting equation (17) into equation (16) and then solving the quadratic equation for n,

[V ]

(18)

K

Kaf +y [V+] [Vd]

Substituting equation (17) into equation (18) and solving for [Vs] and n using Mathematica 10.1, analytical solutions for the defect concentrations as a function of thermodynamic parameters were found: n=Root[4KafKiP0202

-

4

0

Ki P0 2 PO2 #1

2

+ (8KafKi2P0

2 PO2

0

4

+ 2Ki 3 p0PO2"0)#1

-0Root[4KaKi4p0

2

2"

-

4

0 2 Ki p02 pO2 #16 + 4Kafp0 2 2#1,]

0

0

Ki P02p02 #12 + (8KfKi2P0 2P0 2 + 2Ki 3 p02 P02 0 )#14

(19)

(20)

0

P02

KajK

IVi]J

-

2

0

- K P02P"2 #16

+ 4Kap02 2#1B]

where Root [ ] is the root of the polynomial of variable #1 inside the brackets. There are eight analytical roots to the above function, and two of the roots have physical solution. The third root corresponds to the solution for P02 0 > P02, and the fourth root for P02 0 < P0 2The oxygen nonstoichiometry

Kaf (21)

S

-

-rs

-

[SSF] can now be obtained analytically as a function of pO2, and is fitted to the thermogravimetry data using fmincon function in MATLAB, as in Figure 14.

39

4.1.2

Model defect diagram In different PO2 regimes, certain defect species dominate and approximations can be made. For

example, at low PO2, n and [V6] will be much larger than p and [O'], such that n ~ 2[V6] from the charge neutrality equation. For the model systems SSF and STF, three different pO2 regimes and corresponding approximations can be obtained, as shown in Figure 23. Along with the three key reactions - anion Frenkel disorder, intrinsic electron-hole pair generation, oxygen reduction - and electroneutrality equation, one can obtain analytical solutions for each defect species and their pO2 dependence, as calculated in Table 3. A schematic defect diagram for SSF or STF model systems from these solutions are shown in Figure 23.

Table 3. The predicted solutions to the defect solutions with Brouwer approximations.

I 1 ( 2 Kred)P

II 1

P02

S1e1 Kred2 _

\

P0

1 2

2 aJ 1

Ka P6 Kredz

2KafK 2 K 1P Kred

)

( 2 Kred)3

K6K

red K

1 Kaf 4

1 SPO26

K-

III

0

af P 2 6

[V1

KI

f

1

3

1Kre

QKred) Kaf

0

6

Kaf

P 2_

Ka

40

2

K

K

2

1

(1Kred)3

4K

2

4Kred)

1KafKi1 PO26

v =-!o']

2[V0 ] =n

p

III

II

nI

2[O1']

1/6

1/6

M U-4

[01

p

-00

~-1/6

1/6

Pi

10g P02 SSF and Figure 23. Schematic defect diagram using Brouwer approximation for the model system

STF.

4.1.3

Defect diagrams and thermodynamic parameters of STF35 and SSF35 The carrier concentrations can be obtained as a function of pO2 (Figure 24) and the equilibrium

25 are the constants as a function of reciprocal temperature (Figure 25). Slopes derived from Figure reactions (Table 4). thermal band gap and the enthalpies of the anion Frenkel and oxygen reduction

41

22.

E

0

20

STF35 [19]

-

SSF35 Vo

...------- Vo 0 a..

18 18-n

o

16-

-25

......

p

.-^--

- n-

-20

-15

-10

-5

0

5

log p02 (atm) 0 Figure 24. Carrier concentrations from defect equilibria at 1000 C.

SSF35's defect equilibria is shifted to the right (higher pO2) upon replacing Ti with the larger Sn leading to a larger lattice parameter. This implies that SSF reduces more readily than STF. This is similar to BaSrjTij-yFey03-y/21 (BSTF), where adding the larger Ba atoms for Sr shifts the defect equilibria to the "right". As expected, we find a larger band gap for SrSnO3 than for SrTiO3. However, the enthalpies of anion Frenkel and reduction are sensitive to the conditions of the fitting, and the combined values of the two enthalpies should be considered. SSF35 has a slightly larger enthalpy sum: this agrees with the thermogravimetric analysis of STF35 and SSF35 where the difference in reducibility is greater at high temperatures than lower temperatures.

42

140

m In(Kaf) *

In(Ki) In (Kred)

120-

100-

80-

9.0

10.0

9.5

10.5

11.0

I/kT (1/eV) Figure 25. Natural log of equilibrium constants K as a function of 1/kT. Table 4. Thermodynamic parameters for STF35 [191 and SSF35 from fitting the nonstoichiometry 6 as a function of oxygen partial pressure at different temperatures. STF35[191

SSF35

AHaf (eV)

0.518

0.489

Eg (eV)

1.379

1.772

AHred (eV)

3.893

4.304

Fittings of nonstoichiometry 8 were very insensitive to the equilibrium constant of anion Frenkel disorder. Therefore, further analyses on both STF35 and SSF35 were conducted by fixing He, from that of STF35 in reference [19]. This time, data points from all temperatures were fitted simultaneously by setting enthalpies and pre-exponential terms as variables, instead of fitting equilibrium constants at each temperature. The pre-exponential term for the anion Frenkel disorder from the STF35 fitting was fixed as a constant for fitting SSF35 data. The resulting enthalpies and pre-exponentials are shown in . Note that Hred for both fittings were almost identical and Eg was within 3% of each other for SSF35. This suggests that the two different fittings are consistent. 43

-

Table 5. Thermodynamic parameters for STF35 and SSF35 from fitting nonstoichiometry - 8 data as functions of oxygen partial pressure over a range of temperatures simultaneously.

STF35 (data points from [19])

SSF35

tHaf (eV)

0.518

0.518

K f

99.16

99.16

Eg (eV)

1.287

1.718

NcNv AHred (eV)

99.63 3.758

103.28 4.302

Ke

159.95

166.74

4.2 Conductivity model Four-point impedance measurements of SSF bulk samples showed a single semicircle in the frequency range of the instrument (HP 4192A, 5 Hz ~ 13 MHz). This is equivalent to a parallel RC circuit, where the resistance value can be used to calculate the electrical conductivity taking into account the geometry of the device. The PO2 independent ionic conductivity can be subtracted from the total conductivity in Figure 16 and n- and p-type conductivities can be extracted, as shown in Figure 26 and Figure 27. With subtraction of the background ionic conductivity, now the PO2 dependence of both the ptype and n-type conductivities have slopes of 0.25 0.01 (negative for n-type) in log-log plots, as shown in Figure 27.

44

0

-1-

Hole Electron Ion

A

* * -20

-3I

I

I

0.75

0.80

0.85

0.90

I

-

-

I

1.00

0.95

1.05

1000/T (K-') Figure 26. Temperature dependence of p-type (pO2= 0.316 atm), n-type (pO2 pO2-insensitive ionic conductivities of SSF35.

-0.

10-18 atm) , and

(b)0

5

,

(a)

=

-10-0.6

A__

-2.0-

L

-"S

o -2.5-

01

m800*C

-1.0

700'C

e

-

*

0

-

-3

-0.8

.

-3.5 -22

-20

a

a

-18

i

16

A V

900*C 1000*C

,

i

* A

V -1.2 -2.5

-14

-2

0

.

i i -1.5

4

i -1.0

05 -0.5

700*C 800C 900*C 1000*C

0.0

0.0

0,5

log p02 (atm)

log p02 (atm)

Figure 27. Oxygen partial pressure dependence of (a) n-type and (b) p-type conductivities of SSF35 extracted by subtracting the respective ionic conductivities at each isotherm.

The conductivity of a material is defined by f=

45

niqiqi

(22)

where i refers to a charge carrier (electron, hole, or oxygen ion), n, is the concentration, q, is the charge and p, is the mobility of the carrier. Since the charge carrier concentrations were obtained from the defect equilibrium model, the conductivity dependence on temperature and pO2 can be predicted by assuming carrier mobilities. Conversely, carrier mobilities can be calculated from the combination of the conductivity data and the defect model, as shown in Table 6 and plotted in Figure 28. The electron mobility of SSF35 (~0.0002 cm 2 /V-s) is less than a quarter that of STF35 (-0.0009 cm 2 /V-s [19]). This suggests that the conduction band of SSF35 is dominated by the Fe 3d derived band, rather than the higher mobility Sn 5s derived band. STF35 seems to have Fe 3d orbital hybridized with Ti 3d, resulting in higher mobility than the sole Fe 3d derived band in SSF35. However, SSF35 (Table 6) and STF35 (0.005 cm2 /V-s [19]) show similar hole mobilities; this can be explained by similar valence

band structure - Fe 3d and 0 2p hydbridization - in both SSF53 and STF35.

Table 6. Electron and hole mobilities of SSF35 calculated from defect chemistry and conductivity data. T (*C)

P. (cm 2/V-s)

pp (cm 2 /V-s)

700

0.000252

0.00238

800 900 1000

0.000152 0.000215 0.000204

0.00291 0.00429 0.00523

46

-2.0 p-p P-fl

-3.0

-

c

E 0) 0 -

-3.5-

U

-4,0 0.75

0.95

0.90

0.85

0.80

1.00

1.05

1OOO/T (K) Figure 28. Temperature dependence of electron and hole mobilities of SSF35.

The activation energies of each type of conductivity can be extracted from the slopes in Figure 26. These activation energies are the sum of the charge carrier concentration and mobility (migration) activation energies. Therefore we can separate the contributions to the activation energy contributed by the mobility and carrier concentration (Table 7). Let us look at the electronic part first. From equations (11) and (12),

(

1 1 Kredf(PO2 ) n= KI(23)

At intermediate pO2, [Vs] varies with temperature as -f-, therefore the activation energy related to n generation will be approximately

Hred

2

-

2

. At low pO2, n ~ 2[V ], so: 1

n = ( 2 Kred)jPO2

1

and the activation energy related to n generation will be approximately

(24) 3

. Reducing conditions at

which n-type conductivity measurements were done were near the left edge of the ionic conduction

regime, but well within the regime in which the vacancy concentration dominates. This is consistent with

47

1

the P0 2 - dependence of the n-type conductivity. Therefore the activation energy for electron generation, taking into account the effective activation energy for electron mobility of -0.040 eV, is 1.872 eV which should be given by 4.262 eV as shown in Table 7. This

Thus Hred 2 .a.

-

should be equivalent to

value is within 0.040 eV of the value given for

Hred

in Table 5.

Table 7. Activation energies of n-type conductivity, electron mobility, electron concentration, and the reduction enthalpy. Eaayn

Eapn

Ea,n

Hred

1.833 eV

-0.040 eV

1.872 eV

4.262 eV

For holes, we have K i Ki [V"](p0 2 )f - 1

(25)

Kred2

At intermediate pO2, the activation energy of p will be given by Eg +

p

2

-af

Hred 2

At high pO2,

2[0'], so:

p= (Kr

iP02

(26)

KrafEgHe and the activation energy of p will be approximately the p-type conductivity, Ea, is given by Eg +

Again, given the P 0 2$ dependence of

Ha3+Eg-Hred

2 . Taking into account the derived value for

H

Hred= 4.262 eV, Haf = 0.518 eV, and Eg = 1.718 eV, one calculates a value for Ea,p = -0.154 eV. This value is 0.091 eV smaller than that shown in Table 8 derived from the electrical data and assuming an activated hole mobility with activation energy Ea,jp = 0.292 eV. Table 8. Activation energies of p-type conductivity, hole mobility, hole concentration, and activation energies of hole concentration at intermediate and high pO2.

Eagr

Eap,

Ea,p

0.047 eV

0.292 eV

-0.245 eV

48

An activation energy for a nonstoichiometric oxide semiconductor of close to zero, i.e. Ea,ap = 0.047 eV, is highly unusual. However, a similar observation was already made previously for the related STF35 system. There it was shown that with the Fermi energy close to the valence band, Ea,p took on a small positive value, while presumed phonon scattering resulted in a small effective negative value for Ea,Up which served to nearly compensate Ea,Mp to result in a near zero value for Ea,,p[2 0]. This is likely the case here as well. In subsequent work, I will re-examine the fitting routines to see where small errors may have introduced themselves to result in a reversal of signs of the two terms contributing to EaapFor the ionic component, the activation energy for oxygen vacancy formation was calculated from the defect model of SSF, i.e.

= Ea,[ve, while the migration enthalpy Hmig was obtained by

subtracting that activation energy from the total ionic conductivity activation energy, as shown in Table 9.

Table 9. Activation energies of ionic conductivity, oxygen vacancy concentration, and enthalpy of oxygen vacancy migration. Eaaion

Ea,[v, I

0.947 eV

0.245 eV

Hmig 0.702 eV

The calculated migration enthalpy of SSF35 is in the same range as other MIEC perovskite

oxides (0.5~0.9 eV) [24].

4.3 Chemical expansion When a Fe cation changes its states between Fe2+, Fe'+, and Fe 4 +, its ionic radius changes significantly. As SSF is reduced, extra electrons are created and localized on the Fe ion, decreasing its oxidation state (i.e. from Fe4 to Fe' to Fe2+). SSF35 has very similat CCE to that of STF35. This is expected as electrons are localized on the'Fe cations, and the mechanism is essentially identical for both STF35 and SSF35. Both STF35 and SSF35 have a CCE nearly an order of magnitude lower than that of

49

the fluorite oxides, and therefore will induce much less stress on the electrolyte under reducing conditions [21].

4.4 Investigation of surface exchange kinetics of STF/SSF thin films Although some bulk properties differ from that of the thin film due to strain from lattice mismatch against a substrate, bulk trends are good indicators in predicting the corresponding properties in thin films. The surface exchange coefficient k is inversely proportional to the area specific resistance (ASR) of STF/SSF thin films as [12,25]: k =kBT 4e 2 Rsco

(27)

where kB is the Boltzmann constant, Rs is the area specific resistance, and co is the total concentration of lattice oxygen. A strong correlation between k and electron carrier density, n, was found in a previous study (reviewed in section 1.3.1). SSF35 has slightly larger n derived from defect equilibria, but a lower electron mobility. Interestingly, SSF35 showed slightly better surface exchange kinetics, and nearly the identical activation energy of the ASR (or k) (Figure 22). This is not inconsistent with the hypothesis that the electrons in the conduction band are rate limiting in the oxygen surface exchange reaction.

4.5 Problems with SSF5 and SSF50 4.5.1

Phase decomposition of SSF5 Thermogravimetric measurement of SSF5 in reducing conditions (mixture of wet hydrogen and

nitrogen) resulted in phase decomposition of SSF5. Therefore the instability of a lower valent Sn seems to be the source of this instability. The addition of Fe appears to stabilize the stannate to lower PO2.

50

4.5.2

Degradationof SSF50 thinfilm electrode Degradation of the SSF50 thin films occurred noticeably faster than that of SSF35. Degradation

of thin films during EIS measurements made it impossible to obtain meaningful impedance spectra (significantly distorted).

51

..........

CHAPTER 5. CONCLUSION 5.1 Summary The electro-chemo-mechanical properties of a new SOFC model cathode material SrSnl.xFe03. xI2+n

have been explored in this study. Defect chemistry and conductivity models of SSF35 were

established with key thermodynamic and kinetic parameters derived, namely the enthalpy of reduction (4.302 eV), electronic band gap (1.718 eV), migration enthalpy of oxygen vacancies (0.702 eV), and 2 electron (0.0002 0.00005 cm 2 /V.s) and hole (0.0037 0.0015 cm /N-s) mobilities.

The SSF defect equilibria was distinctly different from that of STF largely under oxidizing conditions at the lower temperatures of this study for which the regime of oxygen excess was shifted to higher pO2. This could be attributed to its higher band gap energy. The n-type electrical conductivity of SSF35 followed the same trend as its STF counterpart, but with lower magnitude, suggesting that the Fe 3d band is largely responsible for electron conduction in the conduction band rather than the Sn 5s band

(Figure 29). STF35 showed higher electron mobility, possibly due to the Ti 3d derived band hybridizing with the Fe 3d derived band, while only Fe 3d derived band is conducting electrons for SSF35. The hole mobility of SSF35 was almost identical to that of STF35, as expected, since Fe 3d and 0 2p orbitals constitute the valence bands of both SSF35 and STF35.

E--E-

Conduction band

EC EF

EC EE

F

--- -- --

EV

Ev

Valence band x in

Tij Fex)03-x/2+

x in S (SnFe )O 3-x/,

6

Figure 29. Expected conduction and valance band positions of STF and SSF as a function of Fe content, x.

52

5% Fe doped SrSnO3 (SSF5) showed low chemical stability under reducing conditions. In order to be used as an electrode for a reversible SOFC, stability in both oxidizing and reducing conditions must be fulfilled, and thus a sufficient Fe concentration is required for SSF to serve as a potential electrode. However, too much Fe content in SSF50 resulted in fast degradation, possibly from Sr segregation to the surface. Sr, due to its large size, is believed to segregate to the surface as a result of compressive stress [26]. Because Fe is smaller than Sn, higher Fe content decreases the lattice constant, increasing the compressive stress on Sr and the degree of its surface segregation. Therefore, the SSF35 composition seems to be the most suitable in Fe-doped SrSnO3 perovskite-structured oxide as a SOFC electrode. The chemical expansion of SSF35 was found to be similar to STF35 and other perovskite oxides. The surface exchange rate and its activation energy for SSF35 were similar to those of STF35. SSF35's higher electron density in the conduction band seems to be compensated by its lower electron mobility in terms of the magnitude of the n-type conductivity.

5.2 Recommendations for Future Work The electro-chemo-mechanical properties of the perovskite-structured SSF have been explored; however, this study has also raised a number of questions. Instead of the expected large increase in electron mobility associated with the Sn conduction band, an electron mobility even lower than in STF35 was obtained. A more detailed examination of the energy band structure of SSF35 could lead to an improved insight into this question. The activation energy and magnitude of the surface exchange coefficient of SSF were found to be very similar to those for STF. This is likely due to the fact that Sr segregates to the surface of both while both also depend on Fe for charge transfer. It would thus be interesting to study surface segregation in SSF and see how it compares with that in STF [26]. Surface exchange kinetics were found to be strongly dependent on surface termination in metal oxides, such as Sn02 [27]. Surface segregation has been

53

considered as one of the major issues for SOFC cathodes at high temperatures and in long term operations [26,28-30]. Systematic variations in surface by surface decoration or etching and surface analysis such as X-ray photoelectron spectroscopy can be employed to further study these phenomena. Surface exchange kinetics can be further studied with the aid of optical transmission spectra. A case study of Pr-doped ceria was demonstrated by characterizing changes in optical absorption of different valence states of Pr [31]. Fe cations also have different optical absorption spectra for different valence states, and so a similar technique can be employed. Unlike AC impedance spectroscopy with their need for metallic current collectors, optical measurements do not affect or interfere with the surface exchange kinetics, and therefore a direct, operando measurement is possible. Relaxation in both bulk conductivity and chemical expansion were examined following changes in PO2. However, analysis of the data was complicated by the porosity of the bulk samples. This resulted in higher than expected diffusion coefficients, given much reduced diffusion lengths than the outside dimensions of the specimens would suggest. The use of thin films is recommended for in-plane conductivity relaxation using interdigitated electrodes, as well as chemical expansion relaxation using high-temperature XRD (HTXRD), which can be done simultaneously to systematically compare the two methods - electrical and mechanical - on extracting surface exchange and diffusion coefficients. In addition, by direct measurement of the kinetics, optical relaxations studies can help complete the broader picture for this model perovskite oxide.

54

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