A Study of Interface Reaction of Li0.35La0.55Tio3-Li2co3 and Its

A Study of Interface Reaction of Li0.35La0.55TiO3-Li2CO3 and

Its Effect on Potentiometric CO2 Gas Sensors

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Junro Yoon, M.S.

Graduate Program in Materials Science and Engineering

The Ohio State University

2012

Dissertation Committee:

Professor Sheikh A. Akbar, Advisor

Professor Prabir K. Dutta, Co-Advisor

Professor Gerald S. Frankel

Professor Patricia A. Morris

Junro Yoon

2012

Abstract

A new potentiometric CO2 gas sensor using lithium-lanthanum-titanate

(Li0.35La0.55TiO3) electrolyte, Li2CO3 sensing electrode, and Li2TiO3+TiO2 reference electrode was investigated. The microstructure and electrical properties of the optimized solid electrolyte were examined and the measured conductivity values were found consistent with those reported in literature. The sensor was tested under dry condition in

21% O2/N2 at temperatures ranging from 250 to 550°C. As the temperature increased, the percentage of Nernstian behavior improved from 50% at 250°C to 95% at 450°C, but the performance degraded above 450°C. The proposed hypothesis for the degradation is as follows. Depending on CO2 partial pressure, Li2CO3 can decompose and react with

+ Li0.35La0.55TiO3 around 475-500°C resulting in insertion of Li into Li0.35La0.55TiO3 that causes structural distortion. When the reaction between Li2CO3 and Li0.35La0.55TiO3 occurs at elevated temperatures such as at 700°C, the distorted structure transforms to disordered LaLi1/3Ti2/3O3 and the sensor performance degrades irreversibly.

Thermodynamic calculations combined with solid-state reaction under controlled atmosphere followed by X-ray diffraction (XRD) are used to confirm the hypothesis.

Finally, for device fabrication, it is demonstrated that introduction of high concentration of CO2 (~99.99%) can avoid the reaction between Li2CO3 and Li0.35La0.55TiO3 at high temperatures, which also facilitates good bonding between the electrode and the

ii electrolyte. As for long-term device performance, it is shown that the sensor can measure changes in CO2 concentrations reproducibly as long as it is operated in conditions where there is a background of CO2, such as in ambient atmosphere or combustion systems.

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Dedicated

To my wife Minkyung Baek, daughter Soojin Yoon, and son Hojin Yoon

To my father Kisup Yoon, mother Eunsoon Joo, and brother Hyungro Yoon

Acknowledgments

Ahead of all others, I would like to give my deepest thanks to my thesis advisor,

Prof. Sheikh A. Akbar for giving me the opportunity to work in this field and guiding me throughout the years. On a similar note, I should also thank to my thesis co-advisor, Prof.

Prabir K. Dutta for his invaluable help, inspiring me with his enthusiasm. Their passion for research and education will remain in my heart for the rest of my career.

I am also indebted to Gary Hunter from NASA for many useful discussions and also financial assistance which gave me a chance to study in The Ohio State University.

I would like to extend my thanks to Dr. Inhee Lee for bringing me at Center for

Industrial Sensors and Measurements (CISM) and for guiding me while I learned experimental technique. I would also like to thank Dr. Hojun Lim for his guidance in career life.

The CISM was a wonderful place to work. Thanks to Dr. Ben Dinan, Dr. Mark

Andio, and Dr. Harris Ansari for taking time together in order to take our candidacy exams and for all their help in academic life. Thanks to Dr. William Chiu for his friendship throughout the years. A special thanks is owed to Dr. Krenar Shaqu for his early morning coffee and discussion for experiments. I should also thank Dr. Adedunni

Adeyemo for her helpful discussion on sensor works. Also, my group members, Joe

Atria, Max Mullen, and Michael Severance are unforgettable.

The environment at MSE, OSU was very enjoyable places due to my Korean friends: Jinwook Seong, Jonghan Kwon, Hyungsung Kim, Jihun Jeon, and Changkyoo

Park.

Finally, and most importantly, I would like to thank my wife, Minkyung Baek for her endless love and devotion to family. Lastly I am indebted to my parents for their unconditional support and loves.

Vita

January 1980 ...... Born – Seoul, Korea

2006...... B.S. Materials Science and Engineering

Hanyang University, Seoul, Korea

2008...... M.S. Materials Science and Engineering

Hanyang University, Seoul, Korea

2008 to present ...... Graduate Research Associate,

Materials Science and Engineering

The Ohio State University, Columbus, Ohio

Publications

1. Junro Yoon, Gary Hunter, Sheikh A. Akbar and Prabir K. Dutta

Interface reaction and its effect on the performance of a CO2 gas sensor based on

Li0.35La0.55TiO3 electrolyte and Li2CO3 sensing electrode, Manuscript submitted; Sensors and Actuators B chemical (2012.09)

2. Junro Yoon, Dongjoo Choi, and Young-Ho Kim

Fabrication of CuO anc Cu2O Nanoparticles in a Thick Polyimide Film by Post heat- treatment in a controlled-atmosphere, Journal of Nanoscience and Nanotechnology, Vol. 11 (2011), pp. 796-800

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3. Junro Yoon, Dongjoo Choi, Kyu-Hyung Lee, Jeong Yong Lee, and Young-Ho Kim Two different approaches to the fabrication of Nano-sized Particles in Thick Polyimide Films, Electronic Materials Letters, Vol. 4, No. 4 (2008), pp. 167-173

4. Junro Yoon, Dongjoo Choi, Do-Hyun Oh, T. W. Kim, and Young-Ho Kim Formation of Copper Based Nanoparticles Embedded in a Relatively Thick Polyimide Film by Thermal Curing in Reducing Atmosphere - Journal of Nanoscience and Nanotechnology, Vol. 8 (2008), pp. 5433-5438

5. Wen-Guo Dong, Gun-Hong Kim, Jae-Youn Choi, Junro Yoon, and Young-Ho Kim A study of the curing behavior of polyamic acid coated on a Cu or Zn layer, Colloids and Surfaces A: Physicochem. Eng. Aspects 324 (2008), pp. 122-125

Fields of Study

Major Field: Materials Science and Engineering

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Table of Contents

Abstract ii

Dedication iv

Acknowledgments v

Vita vii

List of Tables xiii

List of Figures xiv

Chapter 1. Introduction 1

1.1 Non-dispersive infrared CO2 sensors 2

1.2 Electrochemical CO2 sensors 3

1.2.1 Severinghaus type CO2 sensors 3

1.2.2 Solid-electrolyte-based CO2 sensors 4

1.3 The-state-of -the-art of CO2 sensors 5 1.4 Conclusion 6

Chapter 2. Fundamentals of solid-electrolyte-based potentiometric CO2 gas sensors 16 2.1 Classification of solid-electrolyte-based potentiometric gas sensors 16

2.2 The structure of type III solid-electrolyte-based potentiometric CO2 gas sensors 18 2.2.1 Solid-electrolyte 19 ix

2.2.1.1 Lithium Lanthanum Titanate 20 2.2.2 Reference electrode 23 2.2.3 Sensing electrode 25 2.2.4 Triple phase boundary 26

2.3 The fabrication of solid-electrolyte-based potentiometric CO2 gas sensors 26 2.4 Electromotive force 27 2.4.1 Derivation of EMF in terms of lithium activity 28

2.4.2 Derivation of EMF in terms of CO2 partial pressure 30 2.4.3 Validity of Nernst equation 32 2.5 How to read the sensor response 33 2.6 Important issues in type III solid-electrolyte-based potentiometric

CO2 gas sensors 33 2.6.1 Instability of solid-electrolyte in gas sensing environment 34 2.6.2 Mixed-potential and slow kinetics at low temperature 34 2.6.3 Different cation species between auxiliary phases and solid- electrolytes 36 2.6.4 EMF dependence on oxygen 37

2.6.4.1 The role of oxygen in CO2 gas at sensing electrode 37

2.6.4.2 The role of oxygen in CO2 gas at reference electrode 39

Chapter 3. Synthesis and characterization of Li0.35La0.55TiO3 solid-electrolyte 52 3.1 Experimental 53

3.1.1 Optimization of synthesis conditions for Li0.35La0.55TiO3 electrolyte 53 3.1.2 Density measurement 55 3.1.3 Phase and microstructure identification 56 3.1.4 Electrical property measurement 56 3.1.4.1 Impedance measurement 57

3.1.4.2 DC polarization -Hebb-Wagner method 57 3.1.5 Investigation of the interface between an electrolyte and an electrode 58 3.2 Results and Discussion 59

3.2.1 Density of sintered Li0.35La0.55TiO3 59

3.2.2 Phase and microstructure of sintered Li0.35La0.55TiO3 61 3.2.3 Characterization of grain and grain boundary conductivities of

sintered Li0.35La0.55TiO3 by impedance measurement 62 3.2.4 Investigation of electronic conductivity of sintered

Li0.35La0.55TiO3 by Hebb-Wagner DC polarization method 68

3.2.5 Stability of Li0.35La0.55TiO3 with the sensor components and gases 71

3.2.5.1 Li2CO3 sensing electrode material 71

3.2.5.2 Li2TiO3 and TiO2 reference electrode materials 72 3.2.5.3 Humidity and carbon dioxide 74 3.3 Conclusions 74

Chapter 4. A study of performance and limits of a potentiometric CO2 sensor with Li0.35La0.55TiO3 electrolyte, Li2CO3 sensing and Li2TiO3+TiO2 reference electrodes 89 4.1 Experimental 90

4.1.1 CO2 sensor fabrication 90 4.1.2 Gas preparation and EMF measurement 91 4.1.3 Nernstian slope calculation 92 4.2 Results 93

4.2.1 Optimization of Li2CO3 fabrication of Li0.35La0.55TiO3 based CO2 sensor 93

4.2.2 The sensor performance in 21% O2/N2 96

4.2.3 The effect of background CO2 on Li0.35La0.55TiO3-Li2CO3 reaction 97 4.3 Discussion 99

4.3.1 Principle of CO2 sensor operation 99 xi

4.3.2 The reaction between Li2CO3 and Li0.35La0.55TiO3 101

4.3.3 The effect of the formation of LaLa1/3Ti2/3O3 on sensor performance 105 4.3.4 Practical implications of sensor fabrication and performance 107 4.4 Summary 108

Chapter 5. Modification of Li2CO3 sensing electrode for improving CO2 gas sensing: Preliminary Results 132 5.1 Mixture of carbonates as a sensing electrode 135

5.1.1 Li2CO3-BaCO3 binary carbonates sensing electrode 136

5.1.2 Li2CO3-Na2CO3-K2CO3 ternary carbonates sensing electrode139

5.2 Li2CO3 formation from LiOH solution 141

5.3 Enhancement of electrical conductivity of Li2CO3 143

5.3.1 Ionic conductivity of Li2CO3 144

5.3.2 Electronic conductivity of Li2CO3 145 5.4 Summary 146

Chapter 6. Conclusions and future work 162 6.1 Summary of key results 162 6.2 Future work 166

Symbols 168

Bibliography 169

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List of Tables

Table 1.1 Characteristic properties of commercially available CO2 sensors ...... 8

Table 3.1 Synthesis conditions for LLTO pellets ...... 54

Table 3.2 Measured weights of sintered Li0.35La0.55TiO3 in various conditions and its computed densities ...... 60

Table 3.3 Computed average grain size and standard deviation at various magnifications ...... 61

Table 3.4 Obtained resistance, capacitance, calculated grain conductivity, grain boundary conductivity, and total conductivity of sintered Li0.35La0.55TiO3 ...... 66

Table 3.5 Activation energy for grain conduction at various temperatures ...... 67

Table 3.6 Steady state current and current density at 300, 400, and 500°C from DC Hebb-Wagner polarization measurement ...... 69

Table 3.7 Electronic conductivity, total conductivity, and Lithium ion transference number calculated from Hebb-Wagner method for Li0.35La0.55TiO3 ...... 71

Table 3.8 List of diffraction angle and relative intensities for the peaks in Figure 3.12 . 73

Table 5.1 Ternary eutectic mixtures [124] ...... 140

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List of Figures

Figure 1.1 A spectroscopy unit by Shimadzu: model UV-1650 PC [31] ...... 10

Figure 1.2 A schematic showing the structural difference between (A) an absorption spectrometer and (B) a multi-spectral NDIR gas sensor for two gases [6] ...... 10

Figure 1.3 A schematic of Severinghaus type CO2 sensor and its working principle [6] . 11

Figure 1.4 A schematic of a solid-electrolyte-based sensor ...... 11

Figure 1.5 NDIR sensor with miniaturization of sample chamber and prolongation of the absorption path; Gascheck Infrared Gas sensor by Edinburgh Sensors [29] ...... 12

Figure 1.6 (a) Conventional NDIR sensor from GE (T5000 series, dimension: 83 mm W TM × 118 mm H × 28 mm T), and (b) SprintIR LED-NDIR CO2 Sensor by CO2Meter (22.6mm x 40.0mm x 25.0mm) [24, 25]...... 12

Figure 1.7 Miniaturization of a Severinghaus-type CO2 sensors [15] ...... 13

Figure 1.8 Commercialized CO2 sensor (top) and pre-calibrated module (bottom), Figaro sensor, Inc., CO2 sensor: TGS-4161, and module: CDM4161 [16] ...... 14

Figure 1.9 The effect of microstructure of auxiliary layer of potentiometric CO2 gas sensor; tested under 0% RH and 70% RH at 400°C [20] ...... 15

Figure 2.1 The illustration of solid-electrolyte-based potentiometric gas sensors (a) Type I, (b) Type II, and (c) Type II ...... 41

Figure 2.2 ABO3 perovskite structure [99] ...... 42

Figure 2.3 Crystal structure of Li3xLa2/3-xTiO3. La1 is La-rich layer and La2 is La-poor layer [68] ...... 42

Figure 2.4 Arrhenius plots of electrical conductivity of well-known solid lithium ion conductors [60] ...... 43

Figure 2.5 Activation energy (grain part) change for Li0.35La0.55TiO3 [59] ...... 43

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Figure 2.6 Ionic conductivity at room temperature as a function of Li content for the quenched La2/3-xLi3xTiO3 (●) and the furnace-cooled La2/3-xLi3xTiO3 (■) [72] ...... 44

Figure 2.7 Lithium ion conduction path via bottleneck [72] ...... 44

Figure 2.8 Schematic illustrations of (a) the closed-reference gas electrode, (b) the closed-pure element electrode, and (c) the open-reference electrode ...... 45

Figure 2.9 Response transient to CO2 of Na2CO3 electrode at 550°C. (a) dry condition and (b) wet condition [78]...... 46

Figure 2.10 Response transient to CO2 of Na2CO3-BaCO3 electrode at 550°C. (a) dry condition and (b) wet condition [78] ...... 46

Figure 2.11 Microstructure change of sensing electrode and the sensors response transient change by morphological change [100] ...... 47

Figure 2.12 The triple phase boundary (TPB) at a sensing electrode ...... 47

Figure 2.13 Theoretical Nernstian slope for the electrochemical reaction involving 2- electrons at 300°C ...... 48

Figure 2.14 An example of sensing response ...... 48

Figure 2.15 SEM images of NASICON surfaces after heating at 450°C for 3 days in humid conditions at 50°C for (a) 3 days and (b) 14 days [91] ...... 49

Figure 2.16 The sensing behavior of the sensor based on Na-β-Al2O3 and Na2CO3 at 150°C [41] ...... 49

Figure 2.17 CO2 sensing performances of (a) LaF3-based sensor and (b) MSZ-based sensor [92] ...... 50

Figure 2.18 EMF vs. CO2 partial pressure for sensors using various metal carbonate auxiliary phases [93] ...... 50

Figure 2.19 (a) EMF changes as a function of O2 in the absence of CO2 or CO2 in the absence of O2 at 285°C, and (b) EMF vs. O2 partial pressure for sensor device using Li2CO3-BaCO3 at various temperatures [97] ...... 51

Figure 2.20 EMF as a function of O2 partial pressure for open reference electrode system [101] ...... 51

Figure 3.1 The structure of a CO2 sensor in this study ...... 76 xv

Figure 3.2 Fabricated LLTO pellets under various conditions ...... 76

Figure 3.3 Evidence of reaction between LLTO pellets and alumina crucible during sintering at 1200-1350°C for 6-12 h ...... 77

Figure 3.4 Powder X-ray diffraction pattern of Li0.35La0.55TiO3 after calcination at 1100°C for 12 h. (▼: Superstructure peaks indicative of tetragonal structure) ...... 77

Figure 3.5 Microstructure images of sintered Li0.35La0.55TiO3 at a magnification of (a) × 10,000, (b) × 7,000, and (c) × 4,000 ...... 78

Figure 3.6 (a) High frequency region (32-0.1 MHz) and (b) Low frequency region (0.1 MHz-100 Hz) of the impedance plots of the sintered Li0.35La0.55TiO3 as a function of temperature (25-100°C). The impedance was measured in N2 atmosphere (Numbers in the figure correspond to exponent of 10 in frequency, Hz) ...... 79

Figure 3.7 The impedance plots of the sintered Li0.35La0.55TiO3 in various atmospheric conditions at (a) 25°C, (b) 100°C, (c) 200°C, (d) 300°C, (e) 400°C, and (f) 500°C ...... 80

Figure 3.8 Impedance data fitting by using equivalent circuit. Original data was from 55°C ...... 82

Figure 3.9 Arrhenius plots of the conductivity of sintered pellet of Li0.35La0.55TiO3, ■: grain region, ○: grain boundary region ...... 82

Figure 3.10 Time response of the current in Hebb-Wagner measurement for sintered pellet of Li0.35La0.55TiO3 at (a) 300, (b) 400, and (c) 500°C in 4000 ppm CO2 + 21% O2/N2 ...... 83

Figure 3.11 Dependence of the electron and hole currents upon the relative potentials of the reference and reversible electrodes [110] ...... 84

Figure 3.12 Hebb-Wagner polarization curves at (a) 300, (b) 400, and (c) 500°C for sintered Li0.35La0.55TiO3; current density is computed at each voltage from steady state current. The test was conducted in 21% O2/N2 in 4000 ppm CO2 atmosphere (dashed lines correspond to the n-type electronic current density) ...... 85

Figure 3.13 XRD patterns after heating (a) a mixture of Li0.35La0.55TiO3 and Li2CO3 at (b) 500, (c) 600, and (d) 700°C for 2 h in ambient air (▼: Li0.33La0.557TiO3, ●: Li2CO3, : LaLi1/3Ti2/3O3, and ?: Unknown) ...... 86

Figure 3.14 XRD patterns after heating (a) a mixture of Li0.35La0.55TiO3, TiO2 and, Li2TiO3 at (b) 650°C for 2 h, a mixture of Li0.35La0.55TiO3 and (c) TiO2, and (d) Li2TiO3

xvi at 700°C for 12 h in ambient air (▼: Li0.33La0.557TiO3, ♦: Li2TiO3, ○: TiO2 (Anatase), and ?: Unknown)...... 87

Figure 3.15 XRD patterns after heating (a) Li0.35La0.55TiO3 at (b) 500°C for 72 h in the presence of 4000 ppm dry CO2, and (c) 50°C for 36 h in the presence of 1% wet CO2 (▼: Li0.33La0.557TiO3 and ●: Li2CO3) ...... 88

Figure 4.1 CO2 sensor fabrication flow diagram ...... 110

Figure 4.2 A fabricated CO2 sensor in this study ...... 110

Figure 4.3 A photograph of gas sensor test set-up ...... 111

Figure 4.4 Installation of a gas sensor (a) sensor connection and (b) measuring leads outlets ...... 111

Figure 4.5 Two methods to calculate the Nernstian slope: (a) magnitude and (b) end point measurements ...... 112

Figure 4.6 Two methods to calculate the Nernstian slope: (a) magnitude and (b) end point measurements...... 112

Figure 4.7 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode heat treated at (a) 350, (b) 400, (c) 500, (d) 650, and (b) 700°C for 2 h in air, respectively ...... 113

Figure 4.8 Percent Nernstian behaviors at 350°C versus fabrication temperature of Li2CO3 sensing electrode on Li0.35La0.55TiO3 (reference electrode was fabricated at 650°C for 2 h prior to deposition of Li2CO3 sensing electrode) ...... 114

Figure 4.9 Morphological changes of Li0.35La0.55TiO3 coated with Li2CO3 after fabricating at 400°C for 2 h in air (a) Li2CO3 layer on Au electrode on Li0.35La0.55TiO3, (b) Li2CO3 layer on Li0.35La0.55TiO3, and (c) Li2CO3 layer and Li0.35La0.55TiO3 surface 115

Figure 4.10 Morphological changes of Li0.35La0.55TiO3 coated with Li2CO3 after fabricating at 500°C for 2 h in air (a) Li2CO3 layer on Au electrode on Li0.35La0.55TiO3, (b) Li2CO3 layer on Li0.35La0.55TiO3, and (c) Li2CO3 layer and Li0.35La0.55TiO3 surface 116

Figure 4.11 Morphological changes of Li0.35La0.55TiO3 coated with Li2CO3 after fabricating at 650°C for 2 h in air (a) Edge of Au electrode and Li0.35La0.55TiO3 surface (b) Li2CO3 layer on Li0.35La0.55TiO3, and (c) Li2CO3 layer and Li0.35La0.55TiO3 surface 117

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Figure 4.12 Morphological changes of Li0.35La0.55TiO3 coated with Li2CO3 after fabricating at 700°C for 2 h in air (a) Au electrode; Li2CO3 layer was supposed to be here (b) Edge of Au electrode, and (c) Li2CO3 layer on Li0.35La0.55TiO3 surface ...... 118

Figure 4.13 CO2 sensing time traces from 300 to 550°C of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode (fabricated at 500°C 2 h in air) in 21% O2/N2 background gas. The numbers below traces are CO2 concentrations in ppm ...... 119

Figure 4.14 CO2 sensing time traces at 350°C before and after testing at 450 and 500°C of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode (fabricated at 500°C 2 h in air) in 21% O2/N2 background gas. The numbers below traces are CO2 concentrations in ppm ...... 119

Figure 4.15 The change of percent Nernstian behavior for the Li0.35La0.55TiO3 based CO2 sensor with Li2CO3 sensing electrode fabricated at 500°C for 2 h in air depending on test temperature; tested in 21% O2/N2 background ...... 120

Figure 4.16 (a) CO2 sensing time traces at 350°C of Li0.35La0.55TiO3 sensors with Li2CO3 sensing electrode heat-treated with/ and without CO2 during Li2CO3 layer fabrication 121

Figure 4.16 (b) CO2 sensing time traces of Li0.35La0.55TiO3 based CO2 sensor with Li2CO3 heat treated with/ and without 4000 ppm CO2 for 72 h ...... 121

Figure 4.17 (a) Calculated ∆G (free energy) for the electrochemical reactions at the sensing and reference electrodes in standard states...... 122

Figure 4.17 (b) The change of Gibbs free energy for the overall reaction (Li2TiO3 + CO2  Li2CO3 + TiO2) depending on CO2 partial pressure and temperature ...... 122

Figure 4.18 (a) The change of the composition of the reference electrode during sensor test at low temperature according to thermodynamic result shown in Figure 4.17 (b) .. 123

Figure 4.18 (b) Stability of Li2TiO3 in the presence of 10000 ppm CO2 at 100°C for 120 h (♦: Li2TiO3) ...... 123

Figure 4.19 Calculated ∆G (free energy) with varying CO2 partial pressure for Li2CO3 decomposition ...... 124

Figure 4.20 The effect of CO2 on the reaction between Li0.35La0.55TiO3 and Li2CO3 at (a) 500 and (b) 650°C ...... 125

Figure 4.21 The change of XRD patterns for the mixture of Li0.35La0.55TiO3 and Li2CO3 after heating at 500°C for 0, 2, 10, and 55h in 21% O2/N2 ...... 126

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Figure 4.22 The change of intensity ratio between (110) peak of Li0.33La0.557TiO3 and (002) peak of Li2CO3 in Figure 4.21 as a function of heating time ...... 127

Figure 4.23 The XRD patterns for (a) the mixture of Li0.35La0.55TiO3 and Li2CO3, (b) the mixture of Li0.35La0.55TiO3 and Li2CO3 after heating at 700°C for 2 h in air, (c) lithium excess LLTO (Li1.75La0.55TiO3 is initial composition), and (d) the mixture of Li1.75La0.55TiO3 and Li2CO3 after heating at 700°C for 12 h in air ...... 128

Figure 4.24 Schematics of the cross-section for Li2CO3-Li0.35La0.55TiO3 sensor (a) as fabricated, (b) formation of LaLi1/3Ti2/3O3, and (c) delaminated Li2CO3 sensing layer . 129

Figure 4.25 Delamination of Li2CO3 sensing layer after test: 500°C-2 h (4000ppm CO2+21% O2/N2)  R.T.–1 h  350°C-8h (various CO2 with background 21% O2/N2)  R.T.  500°C-20 h (various CO2 with background 21% O2/N2) R.T  500°C-50 h (various CO2 with background 21% O2/N2 and subsequently no CO2 in 21% O2/N2) ... 129

Figure 4.26 CO2 sensing time traces of Li0.35La0.55TiO3 based sensor with Li2CO3 heated at 650°C for 2 h in 99.99% CO2. The sensor was tested from 250 to 475°C in 21% O2/N2 background gas ...... 130

Figure 4.27 Percent Nernstian behaviors of the sensors with Li2CO3 fabricated at 650°C- 2h in 99.99% CO2 (□), at 500°C-2h in air (●), and at 650°C-2h in air (■); tested in 21% O2/N2 atmosphere ...... 130

Figure 4.28 Long term stability of the Li0.35La0.55TiO3 sensor at 350°C in 400 ppm +21% O2/N2; the sensor fabricated at 500°C for 2 h in air ...... 131

Figure 4.29 Long term stability of the Li0.35La0.55TiO3 sensor at 475°C in (a) 400 ppm +21% O2/N2 and (b) 21% O2/N2; sensors fabricated (a) at 500°C for 2 h in 99.99% CO2 and (b) at 500°C for 2 h in air ...... 131

Figure 5.1 CO2 sensing time traces of Sb-doped SnO2 electrode-based gas sensor at 25°C in wet condition [117] ...... 147

Figure 5.2 Nernstian slopes at 30, 50 and 70% RH for devices using (a) Li2CO3-BaCO3 and (b) NaHCO3 at 30°C [118] ...... 147

Figure 5.3 Schematic of gas paths in (a) thick and (b) thin Li2CO3 sensing layer ...... 148

Figure 5.4 Change of XRD pattern from Ba(NO3)2 (black line) to BaCO3 (red line) after heating at 630°C for 2 h in 1% CO2 with 90 sccm flow; Ba(NO3)2 was diluted in DI water ...... 148

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Figure 5.5 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Ba(NO3) 2 coating and Li2CO3 powder painted sensing electrode at (a) 400, (b) 350, (c) 300, (d) 250, (e) 200, and (f) 180°C. The sensing electrode was heated at 630°C for 2 h in 1% CO2 ...... 149

Figure 5.6 Percent Nernstian behavior of the sensors with Li2CO3 fabricated at 500°C-2h in air (○) and with Ba(NO3)2 coating-Li2CO3 fabricated at 630°C-2h in 1% CO2 (■); tested in 21% O2/N2 atmosphere ...... 150

Figure 5.7 SEM images of (a) edge of binary carbonate layer (1: binary carbonate, 2: Li0.35La0.55TiO3 surface, 3: precipitates) and (b) Li0.35La0.55TiO3 surface where originally contacted with binary carbonate layer ...... 151

Figure 5.8 The change of XRD patterns for (a) the mixture of Li0.35La0.55TiO3, Li2CO3, and BaCO3 after heating at (b) 630°C, (c) 650°C, and (d) 700°C for 2 h in 1% CO2 (▼:Li0.33La0.557TiO3, ●: Li2CO3, ○: BaCO3, and ♦: LaTiO3 or BaTiO3) ...... 152

Figure 5.9 XRD pattern of a mixture of Li0.35La0.55TiO3, Li2CO3, K2CO3, and Na2CO3 after heating at 390°C for 30 min in air (▼:Li0.33La0.557TiO3, ●: Li2CO3, ○: Na2CO3, and ■: K2CO3) ...... 153

Figure 5.10 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Li2CO3-K2CO3- Na2CO3 (43.5:25:31.5 in mole) sensing electrode heated at 390°C for 30 min in air. (a) and (b) both were obtained from different samples fabricated under identical procedures ...... 154

Figure 5.11 The change of percent Nernstian behavior for the Li0.35La0.55TiO3 based CO2 sensor with Li2CO3 sensing electrode fabricated at 500°C for 2 h in air (○), with Li2CO3- K2CO3-Na2CO3 ternary sensing electrode fabricated at 390°C for 2 h in air (■ and▲); tested in 21% O2/N2 background...... 155

Figure 5.12 Schematic of gas paths in (a) thick and (b) thin Li2CO3 sensing layer ...... 156

Figure 5.13 Schematic for Li2CO3 film morphology based on fabrication methods: (a) Li2CO3 powder paste and (b) LiOH solution coating...... 156

Figure 5.14 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 500°C for 2 h in 4000 ppm CO2...... 157

Figure 5.15 CO2 sensing time traces (a) 300°C and (b) 350°C for the Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 200°C for 12 h in 1% CO2 ...... 157

Figure 5.16 CO2 sensing time traces (a) 350°C and (b) 400°C for the Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 650°C for 2 h in 1% CO2...... 158

Figure 5.17 SEM images of the Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 200°C for 12 h in 1% CO2. (a) Plan-view and (b) Cross-section view of carbonate layer from LiOH solution...... 158

Figure 5.18 SEM images of the Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 650°C for 2 h in 1% CO2. (a) Sensor plan-view and (b) Carbonate layer from LiOH solution ...... 159

Figure 5.19 Ionic conductivity (a) parallel and (b) perpendicular to (002) of Li3PO4- doped Li2CO3 [127] ...... 159

Figure 5.20 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with 9 mol % Li3PO4 doped Li2CO3 sensing electrode heat treated at 650°C for 2 h in 99% CO2 ...... 160

Figure 5.21 Comparison of Percent Nernstian behaviors of the sensor with Li2CO3 sensing electrode (■) and with 9 mol % Li3PO4 doped Li2CO3 sensing electrode (▼) . 160

Figure 5.22 CO2 sensing time traces of Li0.35La0.55TiO3 sensors with conductive carbon+Li2CO3 sensing electrode heat treated at 500°C for 2 h in 99% CO2; (a) Li2CO3:C=6.7:4.1 and (b) Li2CO3:C=3:1 in molar ratio ...... 161

xxi

Chapter 1

Introduction

The need for reliable monitoring of carbon dioxide (CO2) gas is growing rapidly in the areas of: environmental control of greenhouse gases [1, 2], health care [3], in-door air quality control, fire detection systems [4, 5], and many industries [6]. For a gas sensor to be effective and reliable it must meet the “3S’s” requirements: Sensitivity, Selectivity, and Stability [7]. Sensitivity is the ratio of change in the sensing signal to the gas concentration. A gas sensor must also be able to distinguish the concentration of a specific gas in the presence of other gases. This is referred to as selectivity. In addition, stability refers to the integrity of sensor materials in different environments over time.

Besides “3S’s”, a wide detection range, low cost, fast response time and low power consumption are also required for gas sensors in practical applications [8, 9].

In order to fulfill the requirements, various types of CO2 sensors have been developed, and among these, optical and electrochemical sensors have been the most studied [6]. Optical sensor is mainly based on non-dispersive infrared (NDIR) type.

Severinghaus type and solid-electrolyte-based type are common in electrochemical sensors. While the detail of each CO2 sensor technology is not the main focus in chapter

1, the operation principles of each type and the-state-of-the-art in CO2 sensors are briefly introduced. The detail of each type is described elsewhere [8, 10-12].

1.1. Non-dispersive infrared CO2 sensors

NDIR type CO2 gas sensors achieved highly specific detection of CO2 by means of the optical absorption of CO2 in the infrared region [13, 14]. In general a spectrometer is used for measuring absorption wavelengths and intensities of materials but the equipment is usually expensive and bulky as shown in Figure 1.1. To reduce manufacturing cost and exterior size of devices, the NDIR sensor eliminates optical dispersion component but uses particular wavelength (4.24 µm) to get characteristic absorption of CO2 gas molecules. Figure 1.2 shows the structural difference between an absorption spectrometer and a multi channel-spectral NDIR gas sensor. In early studies, researchers expected that optical type CO2 sensor would be inferior to the electrochemical CO2 sensors for portable applications because of its bulky and expensive optical parts such as Infra-Red (IR) radiation source, beam splitter, IR filter, IR detector and sensor signal processing components. However, recent innovation of micro-manufacturing in semiconductors industry has led to the size reduction of the components and makes their integration in small area possible [6]. In addition, the development of light emitting diode (LED) can lower the power consumption of NDIR sensor [6]. Due to less energy consumption, faster response time and low maintenance, an abundance of NDIR CO2 sensors is available in the market. However it still needs to improve poor detection at high (>5%) and very

2 small CO2 concentration (parts per billion, ppb), increase detection range to high operation temperature (T>50°C), simplify complex setup, and lower the cost.

1.2. Electrochemical CO2 sensors

Most electrochemical sensors work in the potentiometric or amperometric mode

[15]; when the signal is an electrical potential, the electrochemical sensor is referred to as a potentiometric type. Since potentiometric method is widely used for measurements of

CO2 gas, we only discuss potentiometric type in this section. Details of other methods are available elsewhere [15].

1.2.1. Severinghaus type CO2 sensors

The Severinghaus type CO2 sensor is so-called conventional electrochemical sensor since it works with aqueous or liquid electrolytes. It measures indirectly CO2 concentration by measuring the pH value of a bicarbonate solution. A schematic of the

Severinghaus CO2 sensor is shown in Figure 1.3. A polymer membrane separates the solution from the surroundings and is only permeable to CO2 gas. Depending on CO2 concentration in the sample gas, the bicarbonate solution can be acidified or be alkalized.

Since CO2 molecules diffuse through the polymer membrane, the transport property of the membrane is a critical factor for the response time of the sensor. Though

Severinghaus CO2 sensors show negligible power consumption, its operation temperature

(T<140°C), slow response time, and difficulty in calibration limit its universal usage.

1.2.2. Solid-electrolyte-based CO2 sensors

A solid-electrolyte-based CO2 sensor has a structure of a galvanic cell, which consists of a solid electrolyte, a sensing electrode, and a reference electrode as shown in

Figure 1.4. Partial pressure of CO2 is directly measured from an electrical signal between a sensing electrode and a reference electrode. The output signal is related to the target gas partial pressure from an electrochemical equilibrium at triple phase boundary (TPB) where the ions of the electrolyte, gas (CO2, O2), and electrons in the electrodes meet.

As compared to NDIR sensors and Severinghaus sensors, solid-electrolyte-based sensors are simple in their structure and smaller in size, and can operate at high temperature (T>500°C, e.g. combustion engines) and be higher in selectivity and sensitivity, and less in cost. In spite of the advantages, solid-electrolyte-based sensors work properly only above certain temperature, Tmin=350-400°C, due to the temperature dependence of ionic conductivity of the electrolyte and CO2 electrochemical reaction on the sensing electrode. In order to maintain the minimum operation temperature, solid- electrolyte-based sensors require a heater, which consumes large power (300 mW-1.3

W)1 and takes warm-up time up to 2 hr2. The elimination of the heater can extend longer operation time with less power supply; this is required in aerospace applications or portable devices.

In this dissertation, the author focuses on the study of solid-electrolyte-based CO2 sensor and the details of this type is described in chapter 2.

1 The power consumption of NDIR with LED light source sensors is 35-150 mW. 2 The response time of NDIR sensors is typically 3-5 min and maximum 30 min. 4

1.3. The-state-of-the-art of CO2 sensors

Most commercially available CO2 sensors are intended for monitoring air quality control in specific area or spaces such as buildings, mines, caves and storages. In addition, increasing attention for personal safety favors the development of portable CO2 sensors. As mentioned in the previous section, NDIR, Severinghaus, and solid- electrolyte-based CO2 sensors get most attention in the commercial market and research fields. Table 1 summarizes the characteristics of the commercially available devices which are obtained from the manufacturer’s web sites. Comprehensive lists of solid- electrolyte-based CO2 sensors in literature can be found elsewhere [9].

Most NDIR products have single-channel structure because of less manufacturing cost, smaller size, and less energy consumption. To lower the detection limit, the further development of NDIR sensor focuses on miniaturization of sample chamber and prolongation of the absorption path as shown in Figure 1.5. The use of IR-LED brings size reduction in some parts (Figure 1.6(b)), but the performance is still inferior to incandescent IR lamps [6].

The Severinghaus type CO2 sensor is appropriate for liquid and gaseous samples, but its function in small volume sample is limited. The improvement can be achieved by the miniaturization as shown in Figure 1.7; the sensor on the far left side is a commercial standard, whereas three sensors on the far right side are fabricated by screen-printing technique [15].

Commercialized solid-electrolyte-based CO2 sensors usually require an integrated heater in order to maintain the sensors at a specific temperature; it requires larges power

5 consumption and need several hours of warm-up time. Since emf can drift, extra modules can be attached to the sensors for accuracy [16]. Figure 1.8 shows a potentiometric sensor and its support module; the module has a microprocessor for signal processing. To lower the operation temperature, new investigations are forward on developing and modifying electrode and electrolyte materials. Researchers have studied solid-electrolyte-based CO2 gas sensors without the heater, but sensing responses are sluggish and sensitivity is poor below 350°C in dry condition [17-19]. To improve the sluggish sensing response, Morio et al. fabricated porous auxiliary layer (sensing layer) and achieved enhanced response time as shown in Figure 1.9 [20]. Though their results give us an idea that a porous sensing layer can enhance sensing response, it was not enough to lower the operation temperature of the sensor. Some reported CO2 sensors operate at room temperature without the need for a heater [21]. However, all these sensors require humidity levels varying from 10 to 100 RH %.

1.4. Conclusion

The current development of CO2 sensors is characterized by low power consumption and miniaturization, and manufactured by thin- and thick-film technology.

According to our survey, solid-electrolyte-based CO2 sensors can be the most promising candidate for a reliable, accurate, portable, and low cost CO2 monitoring device compared to NDIR and Severinghaus type. However, high operation temperature of the solid-electrolyte-based sensor has limited its successful commercialization in aerospace and portable applications. Based on literature survey, it is clear that solid-electrolyte- 6 based CO2 sensors without heaters are not yet available. Therefore, this dissertation focuses on lowering the operation temperature of solid-electrolyte-based CO2 sensors by using the fastest solid electrolyte and by modifying sensing electrode for the enhancement of electrochemical reaction.

Table 1 Characteristic properties of commercially available CO2 sensors

Company & model Operating Power Response time (t90) Accuracy Detection range Ref (Type) conditions Consumption Warm up time Figaro TGS4160 -10~50°C 1.3 W approx. 2 min approx. ±20% 350-50,000 ppm [16] (solid-electrolyte-based) 5-95% RH (a heater) 2 hr at 1,000 ppm Figaro TGS4161 -10~50°C approx. 1.5 min approx. ±20% 300 mW 350-10,000 ppm [16] (solid-electrolyte-based) 5-95% RH 2 hr at 1,000 ppm Alphasense CO -D 10~35°C Voltage unknown 2-4 min 2 1 Unknown 0.2-95% [22] (solid-electrolyte-based) 15-95% RH 30 µA for 6-12 mo Unknown Futurlec MG811 -20~50°C < 60 sec 1200 mW Unknown 350-10,000 ppm [23] (solid-electrolyte-based) Unknown RH Unknown Alphasense IRC-A1 -10~40°C < 40 s 1% at 5,000 ppm (NDIR, LED) 300 mW 0-100% [22] 8 0-95% RH 30 s-30 min 4% at 100%

GE T5000 series 0~50°C 3-5 min ±75 ppm 1.75-2.75 W 0-2,000 ppm [24] (NDIR) 0-95% RH 2-10 min at 22°C CO Meter K-30 series 0~50°C 180-560 mW < 20 sec ±30 ppm 0-10,000 ppm 2 [25] (NDIR) 0-95% RH Max 4.2 W ≤ 1 min ± 3% of reading 0-10%vol CO Meter K-31 series -10~50°C 180-560 mW < 20 sec ±0.2 % 0-10,000 ppm 2 vol [25] (NDIR) 0-95% RH Max 4.2 W ≤ 1 min ± 3% of reading 0-30%vol 0-5% CO Meter SprintIR vol 2 -25~55°C < 2 sec ±70 ppm 0-20% series 35 mW vol [25] 0-95% RH ± 5% of reading 0-65% (NDIR, LED) ≤ 1 min vol 0-100%vol CO Meter S8 0~50°C 2 min ±0.02% 0.04-2% 2 150 mW vol [25] (NDIR, LED) 0-95% RH ≤ 1 min ± 3% of reading 0.04-3.2%vol CO Meter pSense series 0~50°C 24 hrs with < 30 sec ±30 ppm (high) 2 0-9,999 ppm [25] (NDIR, portable) 0-95% RH 4 “AA” batteries 30 sec ±75 ppm (ave) Continued 8

Table 1 continued.

Manufacturer Operating Power Response time (t90) Accuracy Detection range Ref (Type) conditions Consumption Warm up time Veris CWL series 0~50°C 1 min ±30 ppm 3 Wmax 0-2,000/5,000 ppm [26] (NDIR) 0-95% RH unknown ±2% of reading Veris CDL series 0~50°C 1 min ±30 ppm 3 Wmax 0-2,000/5,000 ppm [26] (NDIR) 0-95% RH unknown ±2% of reading ±50 ppm +2% 0-2,000 ppm E+E Elektronik EE891 -40~60°C 27 mW 195 sec ±50 ppm +3% 0-5,000 ppm [27] (NDIR) 5-95% RH 3.75 W max Unknown ±100 ppm +5% 0-10,000 ppm Mettler Toledo < 120 sec at 25°C 0~60°C Lower detection limit InPro 5000i Unknown > ±10% [28] (Dissolved CO ) Unknown 10 mbar (Severinghaus) 2 Edinburgh sensors 9 0~45°C 10 sec 0-5000 ppm Gascard NG 480 mW ±2% [29] 0-95% RH 0-100% (NDIR) 1-30 min Edinburgh sensors 0~45°C 30-260 sec 0-3,000 ppm Gascheck 0.9 W ±3% [29] 0-95% RH 0-3/10% (NDIR) 5-30 min Edinburgh sensors 0~50°C 30-300 sec IRgaskiT 0.9 W ±5% 0-2/5/10/20/30% [29] 0-95% RH (NDIR) 5-40 min Edinburgh sensors 30 sec 0~40°C GuardCard 13 W ±2% 0-1/3/5/10/30/100% [29] 0-99% RH 3-40 min (NDIR) ±50 ppm +2% 0-2,000 ppm Digitron HLX871 -40~60°C 27 mW < 195 sec ±50 ppm +3% 0-5,000 ppm [30] (NDIR) 0-100% RH 3.75 W max Unknown ±100 ppm +5% 0-10,000 ppm

Figure 1.1 A spectroscopy unit by Shimadzu: model UV-1650 PC [31].

Figure 1.2 A schematic showing the structural difference between (A) an absorption spectrometer and (B) a multi-spectral NDIR gas sensor for two gases [6]. 10

Figure 1.3 A schematic of Severinghaus type CO2 sensor and its working principle [6].

Figure 1.4 A schematic of a solid-electrolyte-based sensor.

Figure 1.5 NDIR sensor with miniaturization of sample chamber and prolongation of the absorption path; Gascheck Infrared Gas sensor by Edinburgh Sensors [29].

(a) (b)

Figure 1.6 (a) Conventional NDIR sensor from GE (T5000 series, dimension: 83 mm TM W × 118 mm H × 28 mm T), and (b) SprintIR LED-NDIR CO2 Sensor by CO2Meter (22.6mm x 40.0mm x 25.0mm) [24, 25].

Figure 1.7 Miniaturization of a Severinghaus-type CO2 sensors [15].

Figure 1.8 Commercialized CO2 sensor (top) and pre-calibrated module (bottom), Figaro sensor, Inc., CO2 sensor: TGS-4161, and module: CDM4161 [16].

Figure 1.9 The effect of microstructure of auxiliary layer of potentiometric CO2 gas sensor; tested under 0% RH and 70% RH at 400°C [20].

Chapter 2

Fundamentals of Solid-Electrolyte-Based Potentiometric CO2 Gas Sensors

For the past four decades, various types of solid-electrolyte-based potentiometric gas sensors have been developed and proposed. It is the aim of this chapter to provide comprehensive and basic knowledge for solid-electrolyte-based potentiometric CO2 gas sensors. This chapter specifically deals with the classification, working principles, sensor components, and current issues.

2.1. Classification of solid-electrolyte-based potentiometric gas sensors

As mentioned in chapter 1, a solid-electrolyte separates a sensing electrode and a reference electrode in potentiometric gas sensors. Weppner classified the sensors into type I, II, and III in terms of the relation between gas species and mobile species in the solid-electrolyte [8, 10].

In the type I gas sensor, the target gas species is identical to the mobile species in the solid-electrolyte. For example, yttria stabilized zirconia (YSZ) based O2 gas sensor, a well-known type I gas sensor, is illustrated in Figure 2.1(a). Other examples of type I gas

2- sensors are for F2, Cl2, and H2 [32]. Solid-electrolytes conducting CO3 have not been developed and thus type I solid-electrolyte-based CO2 sensor is unknown. 16

In the type II gas sensor, the target gas species is identical to the immobile species

+ in the solid-electrolyte. In Figure 2.1(b), the mobile species is K in K2CO3 electrolyte and the following reaction makes CO2 detection possible.

1 K CO (s)  2K  (s)  CO (g)  O (g)  2e  (2.1) 2 3 2 2 2

The first type II solid-electrolyte-based potentiometric CO2 gas sensor was presented by

Gauthier and Chamberland using K2CO3 electrolyte [33]. However, K2CO3 electrolyte was not stable in harsh environments and thus was not a reliable sensor. Other carbonates, e.g. Li2CO3, were investigated as solid-electrolytes for type II as well. Zhang et al. reported type II sensor with 5 mol% Li3PO4 doped Li2CO3 that worked at 350-

400°C [34].

In terms of developing gas sensors for various gases, however, type I and II have a fundamental limitation; the number of available solid-electrolytes is limited and the list of satisfactory solid-electrolytes is very restricted with respect to stability, and operation temperature [9]. To overcome this problem, type III sensors were introduced. In the type

III gas sensor, the target gas species has no direct relation to mobile and immobile species in the solid-electrolyte. Instead, the co-called auxiliary phase contains the target species and the mobile species of the solid-electrolyte both. As shown in Figure 2.1(c), Na2CO3 is the phase which contains Na+ and carbonate ion while sodium super ion conductor

(NASICON, Na3Zr2Si2PO12) does not have any CO2 related species.

Among the three types of the solid-electrolyte-based potentiometric sensors, the type III sensor has been mostly studied because of the following advantages for sensor developments [35].

● Versatility in selection of materials

● Flexibility in device construction

● Design of auxiliary phase for gas sensing

Hence, in the following sections, only the type III CO2 gas sensors will be discussed.

2.2. The structure of type III solid-electrolyte-based potentiometric CO2 gas sensors

The basic structure of solid-electrolyte-based potentiometric gas sensors consists of a solid-electrolyte placed between a sensing and a reference electrode. Metal layers contact with the solid-electrolyte and are connected to an external circuit [36] as shown in

Figure 1.4. An electrochemical reaction involving a target gas species occurs at the sensing electrode but not at the reference electrode. This creates an electrical potential difference between the sensing and the reference electrodes. As a result, target gas concentration can be measured from the electrical potential difference between the electrodes. In the type III gas sensor, the solid-electrolyte, the sensing and the reference electrodes have their own requirements because the role of each component is different.

Therefore, the specific requirement of each component is essential to understand the type

III gas sensor. 18

2.2.1. Solid-electrolyte

Generally, solid-electrolytes are ceramic materials that exhibit dominant ionic conductivity which is accomplished by ion hopping through point defects in a crystal structure [32]. In potentiometric gas sensors, solid-electrolytes should fulfill these requirements [11, 37, 38]: i) high ionic conductivity for mobile species; ii) negligible electronic conductivity; iii) chemical stability in operation environments; iv) mechanical stability at the operating temperature; and v) coefficient of thermal expansion (C.T.E.) compatible with other materials of the sensor.

High ionic conductivity of the solid-electrolyte can provide fast kinetics to achieve a quick sensing response. The ionic conductivity of solid-electrolytes increases as the temperature rises because ion hopping is thermally activated. On the other hand, the electronic conductivity of the solid-electrolyte should be negligible because potential change by electronic conductivity can cause erroneous gas sensing. The chemical stability means that the solid-electrolyte must not react with gases and materials in contact. The mechanical stability of the solid-electrolyte can guarantee good thermal shock strength and no creep. C.T.E. mismatch of the solid-electrolyte with other materials in the sensor can cause cracks during high temperature sensing operation. This can eventually cause sensor failure by delamination.

Early research on solid-electrolyte-based potentiometric CO2 sensors have incorporated sodium ion (Na+) conducting solid-electrolytes such as Na+- β/β”-Alumina and NASICON [19, 35, 39] because of their high ionic conductivities. Na+-β/β”-Alumina was used in early studies [18, 40-42], but its 2-dimensional sodium ion conductivity was

19 a problem, because it requires conduction plane alignment [43-45]. As an alternative,

NASICON has been investigated but the electromotive force (EMF) of the sensor with

NASICON drifted under dry and/or humid atmosphere [46-48] because of instability of the electrolyte in contact with metal-carbonates or humid atmosphere [49-53].

In recent years, lithium ion (Li+) conductors have been used as solid-electrolytes because of their resistance to humidity as well as availability of new materials because of

Lithium ion battery R&D. Unlike sodium ion electrolyte-based CO2 sensor, various lithium ion electrolytes such as Li1.3Ti1.7Al0.3(PO4)3, Li3N, Li3.6V0.4Ge0.6O4, Li2CO3-

Li3PO4-Al2O3 [54], Li3PO4 [55], Li2.88PO3.73N0.14 (LIPON) Li1.3Ti1.7Al0.3(PO4)3, and

Li2.88PO3.73N0.14 have been investigated as solid-electrolyte for CO2 gas sensors.

2.2.1.1. Lithium Lanthanum Titanate

Lithium lanthanum titanate (LLTO) has nominal composition of Li3xLa((2/3)- x)□((1/3)-2x)TiO3, where □ represents a vacancy. According to Robertson et al., the

Li3xLa((2/3)-x)□((1/3)-2x)TiO3 solid solutions exist in the composition range 0.06

The structure of LLTO is ABO3 perovskite as shown in Figure 2.2 [57]; vacancies, lanthanum, and lithium ions are randomly distributed on A-sites while titanium occupies the B sites. The TiO6 octahedron constructs the unit cell and A-cages are formed by 12 oxygen ions [57, 58]. Robertson et al. have found that three polymorphs, two different tetragonal structures and a cubic phase existed depending on temperature. The differences between the polymorphs are correlated with La ordering in the structure; at high temperature La atoms are arranged in disordered manner, whereas 20 the La ordering occurs at low temperature; the order/disorder temperatures dependent on the La concentration [56]. Since the structure of LLTO is influenced by La ordering, various synthesis procedures and initial compositions of reactants yield different LLTO structures: cubic, tetragonal, hexagonal, and orthorhombic [57, 59-63]. Particularly, lithium loss during high temperature synthesis (T>1250°C) in LLTO preparation is difficult to control and gives problems with reproducible product composition.

Among the structures, cubic and tetragonal LLTOs have been extensively studied due to their electrical property. According to literature, a cubic perovskite LLTO (space group Pm3m) with the lattice parameter a=3.854Å is prepared by quenching while a tetragonal perovskite LLTO (space group P4/mmm) is synthesized by slow furnace cooling [58-60, 64, 65]. The structure of tetragonal perovskite LLTO is double stacked cubic LLTO; a=b≈3.873 Å and c≈2a. Due to the ordering of La3+, the superstructure lines, which are from cubic-double stacking, are observed in XRD study. The unequal distribution of La3+ in the tetragonal perovskite structure forms La-rich and La-poor layers, which causes the slight tilting of the TiO6 octahedra and provides lithium ion migration path as presented in Figure 2.3 [61, 66].

The lithium ion conductor LLTO has attracted much attention because of its high lithium ion conductivity as shown in Figure 2.4 [60]. Cubic Li0.35L0.55TiO3 exhibited σ

-3 -5 -5 grain=1-1.5×10 S/cm, σ grain boundary=7.5×10 S/cm, and σ total=7.5×10 S/cm at 25°C; the activation energies are reported as 0.33-0.40 eV and 0.42 eV for conduction in grain and grain boundary, respectively (T<100°C) [59, 60]. For tetragonal Li0.33La0.557TiO3, grain conductivity was 6-9×10-4 S/cm (0.35 eV for activation energy), grain boundary

21 conductivity was of the order of 10-6-10-7 S/cm (0.4 eV for activation energy) at room temperature. The grain conductivity of LLTO is comparable to the conductivities of organic and polymer electrolytes (10-2-10-3 S/cm at R.T.) [67]. However since polycrystalline materials are commonly used for applications, grain boundary conduction exhibits a rate-limiting step for ionic transport; hence total conductivity of polycrystalline

LLTO is reduced to 10-5-10-7 S/cm at room temperature [68].

As temperature increases, the change of activation energy (grain) of ionic conductivity of LLTO was observed as shown in Figure 2.5 [57, 59, 69-71]. Belous et al. observed the phenomenon but did not explain it in detail. Inaguma et al. and Lee et al. correlated the changes of activation energy and the phase transitions. On the other hand,

Bohnke et al. used Vogel-Tamman-Fulcher (VTF) law to account for non-Arrhenius behavior of the grain activation energy; they suggested that the conduction mechanism involves the tilting of TiO6 octahedra at high temperature. The origin of the temperature dependence of the grain activation energy in LLTO has not been revealed clearly.

The mechanism of lithium ion conduction in LLTO is mainly attributed to large concentration of A-site vacancies. Figure 2.6 shows the variation of the ionic conductivities at room temperature; the ionic conductivity of LLTO is strongly influenced by the ratio of lithium to the A-site vacancy concentration as well as by the synthesis conditions [72, 73]. The migration pathway for lithium ions is illustrated in

Figure 2.7; lithium ion in the A-site, where is surrounded by 12 oxygen ions, jumps to the adjacent A-site via the bottleneck formed by 4 oxygen ions. This indicates that activation energy for lithium ion conduction depends on the size of bottlenecks and therefore tilting

22 of TiO6 octahedra or change of lattice parameter by doping can affect ionic conductivity of LLTO.

In terms of chemical stability, LLTO is unstable in contact with elemental lithium; lithium reduces Ti4+ to Ti3+ and eventually the reduction can induce electronic conduction, which should be avoided for solid-electrolytes [59, 62]. In the presence of

CO2 and water, formation of Li2CO3 on LLTO surface was reported [74]. The instability of LLTO may limit its applications such as in lithium air battery [59, 60].

2.2.2. Reference electrode

The essential requirement for the reference electrode is to establish a stable reference potential to produce reliable sensing response during operations. If the reference electrode reacts with the target gas, it is detrimental for the accuracy of gas sensing measurement because reference potential will be changed by the reaction. To prevent the error, many researchers have used two different types of reference electrodes: a closed type and an open type.

The closed-reference electrode incorporates a reference gas (commonly, air) or a pure element to achieve a constant electrical potential. The reference gas (e.g. ambient air) flows inside quartz or an alumina tube which is tightly bound to an electrolyte by using inorganic adhesive as shown in Figure 2.8(a). In the case of the pure element, an epoxy resin separates the element from the sensing environment as can be seen in Figure

2.8(b). In both cases, the gas-tight seal is important for a stable reference potential not to be affected by sensing environment; a broken seal will cause a deviation of a reference 23 potential, therefore accurate measurements of the target gas would be impossible [54].

The main concern in such reference electrodes is that the separation of the reference electrode from the target gas complicates the sensor design, so the open reference electrode3 idea has been developed.

The open reference electrode is exposed to gas sensing environments as illustrated in Figure 2.8(c). This requires that reference materials do not react with the target gas. To fix the electrical potential of the reference electrode, a mixture of metal-oxides is used as the reference electrode to achieve a gas-solid equilibrium. For instance, a mixture of

Li2TiO3 and TiO2 in the reference electrode participates in reaction (2.2).

1 2Li (s) O (g)  TiO (s)  2e  Li TiO (s) (2.2) 2 2 2 2 3

At constant temperature and fixed O2 partial pressure, equilibrium will result in fixing the electrical potential at the reference electrode. Due to the difficulty of attaching a pure element, the closed-reference gas electrode or the open-reference electrode are common in solid-electrolyte-based potentiometric CO2 gas sensors.

3 Some authors use “solid-state reference electrode” instead of “open reference electrode”, J.W. Fergus, Sensors and Actuators B, 134 (2008) 1034-1041 24

2.2.3. Sensing electrode

As mentioned earlier, the type III CO2 sensors achieve the equilibrium between

4 mobile species in solid-electrolytes and CO2 by using auxiliary electrodes . For instance, the equilibrium between lithium ions and CO2 can be written as:

1 Li CO (s) 2Li (s)  CO (g)  O (g)  2e (2.3) 2 3 22 2

Depending on the mobile ions in solid-electrolytes, other metal-carbonates, such as

Na2CO3 or SrCO3, have been applied as auxiliary electrodes [41, 75, 76]. Since sensing electrodes react with target gas, numerous studies have been made in terms of humidity interference, response and recovery time, and low temperature gas sensing.

In early studies, single carbonate sensing electrodes were used [18, 40, 42, 77].

Sodium carbonate (Na2CO3) was widely used, but it’s sluggish sensing behavior in wet and dry conditions, as seen from Figure 2.9, led researchers to switch to binary carbonate

(e.g. BaCO3-Na2CO3) [17]; binary carbonates sensing electrode was very effective to prevent the deterioration of EMF in wet environment as shown in Figure 2.10 [17, 78].

The addition of metal oxides also showed the same effect as binary carbonate did [21, 79-

83]. However, it has not been elucidated how the binary carbonates and the metal oxides prevent the humidity interference.

4 In literature, sensing electrodes, sensing materials, and auxiliary electrodes are used interchangeably with each other. 25

In order to enhance kinetics for CO2 sensing reaction, the efforts have been made to modify the microstructure of the sensing electrode; porous electrode did not significantly improve sensitivity but reduced response and recovery times at 400°C in dry condition as seen in Figure 2.11 [20].

Low temperature CO2 sensing has met hurdles around 300°C in dry condition since electrochemical reaction becomes slow at low temperature. Some researchers reported room temperature CO2 sensing in the presence of water; CO2 reacts with H2O

- and forms HCO3 . However stability of the sensors in that condition has not been studied

[84].

2.2.4. Triple phase boundary

To accomplish the electrochemical reaction (2.3), Li+ in the electrolyte, gas species, and electrons (from a metal layer) have to meet at a particular site as illustrated in Figure 2.12. This site is called the triple phase boundary (TPB) [85]. Technically, the amount of TPBs can increase reaction sites for electrochemical reactions. Therefore, understanding and optimizing TPBs in electrochemical devices can provide excellent opportunities for enhancing performance.

2.3. The fabrication of solid-electrolyte-based potentiometric CO2 gas sensors

Typically, the solid electrolyte is fabricated by using solid-state reactions. Raw materials are mixed in the optimal ratio and ball-milled to reduce the size of particles.

The ball-milled powder is heat-treated to achieve solid-state reactions. Sometimes, the powder is repeatedly heat-treated and ground to complete solid-state reactions. The reacted powder is pressed as a green pellet; sintering transforms the green pellet into a solid electrolyte. Metal layers are placed on both sides of the electrolyte. Gold (Au) or platinum (Pt) have been commonly used. Metal wires are attached by using the same metal pastes to the electrolyte and then are heat-treated. For the open reference electrode, reference materials are painted from a paste on the metal layer and are heat-treated. In the case of the closed-reference gas electrode, the electrolyte is fixed on end of a tube using an inorganic adhesive and reference gases flow inside the tube. The sensing electrode is normally formed on the opposite side of the reference electrode; sensing materials are pasted using organic binder on the sensing electrode and heat-treated to evaporate the organic binder. The selection of the heat-treatment temperature is important because unintended chemical reactions can occur and by-products may affect the gas sensing behavior [86].

2.4. Electromotive force

The maximum useful work obtained from chemical reaction in a galvanic cell can be measured under conditions of no current flow in the cell by the following relation:

Grxn   nF E0 (2.4)

Until any current flows the electrochemical equilibrium will not be disturbed. In order to obtain accurate results, the measurement must be carried out with a high impedance electronic voltmeter (>1012Ω) [87]. The measured electrical potential difference is referred to as the electromotive force (EMF). The EMF can be written in terms of lithium activity and CO2 partial pressure as described in the next section.

2.4.1. Derivation of EMF in terms of lithium activity

When CO2 gas is introduced, potentiometric gas sensors measure the electrical potential difference between the sensing and the reference electrodes; the reaction (2.3) occurs at the sensing electrode to achieve new equilibrium while reference potential is stable under fixed oxygen partial pressure by the reaction (2.2). Depending on the direction of the reaction (2.3), electrons are consumed or produced, and eventually new electrical potential will be established at the sensing electrode.

1 EMF      ~ Reference  ~Sensing  (2.5) Sensingelectrode Reference electrode F electrons electrons

The equation (2.5) can be written in terms of the chemical potential or activity of neutral mobile species instead of the electrochemical potential of electrons. From the equilibrium condition for the ionization, the chemical potential of neutral mobile species can be written as (in the case of lithium):

(2.6)

Combination of equations (2.5) and (2.6) gives:

(2.7)

Since the electrochemical potential of lithium ion remains constant through the electrolyte [88], equation (2.7) can be rewritten as:

(2.8)

According to the definition of chemical potential, the chemical potential is related to the activity by the following relation.

0 Li LiRTa ln Li (2.9)

Hence equation (2.9) can be modified to:

Reference electrode RT aLi EMF  ln Sensing electrode (2.10) F aLi

Equation (2.10) is termed the Nernst equation and it shows the dependence of the equilibrium potential (EMF) on the concentration of ions [87]. Since the activity of lithium at the reference electrode is constant, EMF can be a function of the activity of lithium at the sensing electrode. However Equation (2.10) does not show the relationship between EMF and CO2 partial pressure directly; thus equation (2.10) needs to be rewritten in terms of CO2 partial pressure.

2.4.2. Derivation of EMF in terms of CO2 partial pressure

Equation (2.3) can be rewritten as:

1 2Li O2  CO 2  Li 2 CO 3 (2.11) 2

The equilibrium constant of (2.11) is related to the standard Gibbs free energy change of reaction (2.11) as:

1 a2 pO2 pCO GG0011 Li(Sensing) 2 2 Li2 CO 342 Li 2 CO 3 exp(  ) aLi(Sensing)  pO 2 pCO 2 exp(  ) (2.12) aLi CO ( 1) R T 2R T 23 

For the reference electrode, equation (2.2) can be modified to:

1 2Li O  TiO  Li TiO (2.13) 2 2 2 2 3

From the relation between equilibrium constant and Gibbs free energy change of (2.13), the following equation is valid:

1 a2 pO 2 GG001 Li(Ref) 2 Li2 TiO 34 Li 2 TiO 3 exp(  ) aLi(Ref)  pO 2 exp(  ) (2.14) aLi TiO ( 1) R T 2R T 23 

Insertion of equations (2.12) and (2.14) to (2.10) gives:

1 G0 4 Li23 TiO Reference electrode pO exp( ) RRTTa 2 EMF  lnLi   ln 2RT (2.15) FFaSensing electrode 11 G0 Li pO42 pCO exp( Li23 CO ) 22 2RT

Since O2 partial pressure at the sensing and the reference electrode are the same for the open reference electrode structure, equation (2.15) can be simplified as:

2.303RT EMFE0 log p ( CO ) (2.16) 2F 2

2.4.3. Validity of Nernst equation

General form of Nernst equation for CO2 sensor used in literature:

2.303RT EMF E0 log p (CO ) (2.17) zF 2

The “z” in the denominator is the number of participating electrons in the electrochemical reaction for CO2 sensing. When the sensing temperature and the “z” are known, a theoretical slope can be calculated. For example, when “z” equals 2, the plot of equation

(2.17) at 300°C is the same as Figure 2.13. The slope of the graph shows the theoretical correlation between EMF and CO2 partial pressure at 300°C. Nernst sensing behavior is obtained when slope from an experiment is similar to a theoretical one.

Nernst equation is only valid when the electrochemical reaction at the sensing electrode achieves equilibrium and the electrolyte is a pure ionic conductor. If the reaction is not in equilibrium, Nernst equation will not describe the sensing behavior. In addition, when more than one electrochemical reaction occurs at the same electrode, the sensing behavior results in non-Nernstian behavior [89] and the sensor is called a mixed- potential sensor. Another possible reaction of deviation from the Nernstian behavior is electronic conduction in the solid-electrolyte, which effectively leaks the established equilibrium potential.

2.5. How to read the sensor response

Figure 2.14 shows an example of the sensing response. When CO2 is introduced to the sensor, EMF starts to change from the baseline. The time to reach 90% of the equilibrium EMF is referred to as 90% response time (t90). This time is considered as the yardstick for measuring how fast a sensor respond to the target gas. The equilibrium EMF is a saturated EMF value with a fixed CO2 concentration. A magnitude of the equilibrium

EMF is used to calculate Nernstian behavior of a sensor. As CO2 flow is stopped, EMF should recover to the baseline. The time required to reach 90% of baseline is recovery time. Fast response and recovery times are essential for gas sensors.

2.6. Important issues in type III solid-electrolyte-based potentiometric CO2 gas

sensors

To date, many efforts have been made to clearly understand the CO2 gas sensing mechanism in the type III sensor. Dealing with type III potentiometric CO2 gas sensors, the most important issues are the following:

1. Instability of solid-electrolyte in gas sensing environment

2. Mixed-potential and slow kinetics at low temperature

3. Different cation species between auxiliary phases and solid-electrolytes

4. EMF dependence on oxygen

2.6.1. Instability of solid-electrolytes in gas sensing environment

The humidity interference with solid-electrolyte-based potentiometric CO2 sensors is a serious problem. In early studies, most type III CO2 sensors used NASICON as a solid electrolyte and its high water solubility caused severe humidity interference

[90].

Kida et al. studied the instability of NASICON electrolyte in humid conditions and confirmed that the segregation of Na3PO4 occurred on the surface of the NASICON in the presence of humidity [86, 91]. Scanning Electron Microscope (SEM) observation revealed that needle-like segregation of Na3PO4 became severe as the storage time in humid atmospheres increased as shown in Figures 2.15(a) and (b). They confirmed the presence of Na3PO4 using electron-probe micro-analyzer (EPMA) and claimed that the segregation of Na3PO4 could be the reason for the humidity interference of NASICON based type III CO2 sensors because the phase segregation could change the composition of interface between NASICON and auxiliary phase.

2.6.2. Mixed-potential and slow kinetics at low temperature

As mentioned earlier, Nernst equation is only valid in equilibrium reaction and when only one electrochemical reaction occurs at the same electrode. If both conditions are not met, the sensor is called a mixed-potential sensor. At high temperatures (over

400°C) and in dry conditions, type III CO2 sensors exhibit the Nernstian behavior because electrochemical reaction achieves equilibrium. As the temperature decreases,

34 however, efficiency of electrochemical reaction is lowered and the sensor shows mixed- potential.

Liu and Weppner examined the type III CO2 sensor with Na-β-Al2O3 electrolyte and Na2CO3 auxiliary phase at 150°C and dry conditions [41]. As shown in Figure 2.16, the data they obtained were far from the theoretical Nerstian value and the response time was very slow. To achieve equilibrated EMF value, it took 14 days. They supposed that more than one electrochemical reaction occurred simultaneously at the sensing electrode at 150°C. Based on thermodynamic study, they claimed that thermodynamically Na2O2 or

Na2O with CO2 could form Na2CO3 at 150°C, but the formation progressed gradually because the kinetics was slow at low temperature, whereas Na2O or/and Na2O2 formed

Na2CO3 quickly in the presence of CO2 at high temperature (over 400°C). They suggested that 77% of the observed EMF could be from Na2CO3 formation while 23% of the EMF was the result of Na2O2 formation.

The mixed-potential is difficult to analyze because sensing environments are too complex to postulate possible electrochemical reactions. As a result, the mixed-potential can be a critical problem for solid-electrolyte-based potentiometric CO2 gas sensors at low temperature operations. The slow kinetics in an electrochemical reaction also causes a sluggish response and recovery which are not adequate for practical devices.

2.6.3. Different cation species between auxiliary phases and solid-electrolytes

According to Weppner’s definition for the type III sensor, a cation in the auxiliary phase should be the same as the mobile ion in the electrolyte. So, the following examples were claimed to detect CO2.

+ + O2 | Na conductor | Na2CO3, O2, CO2 or O2 | Li conductor | Li2CO3, O2, CO2 (2.18)

However, the above criterion could not explain many reported potentiometric CO2 gas sensors. Miura et al. reported sensors which the structure of the sensors could not be described by the above criterion [92]. They demonstrated potentiometric CO2 sensors using Li2CO3 auxiliary phase and the fluoride-ion conductor (LaF3) or the oxygen-ion conductor (Magnesia-stabilized zirconia, MSZ) as the electrolytes as shown in Figure

2.17. Their sensors showed Nernstian CO2 sensing behavior at 400-650°C with t90 of 1 minute. These sensing behaviors could not be understood based on the Weppner’s classification because the mobile species of the electrolyte did not match the transporting cation of the auxiliary phases.

Ogata et al. also demonstrated the sensors which did not follow the Weppner’s classification. At 500°C, the sensors consisted of the Na-β-Al2O3 electrolyte and Na2CO3 worked well and showed theoretical Nernstian slope but also the sensor with other metal carbonates, such as Li2CO3, Cs2CO3, K2CO3, and CaCO3, exhibited Nernstian behavior as well (Figure 2.18) [93]. Except Na2CO3, none of the auxiliary phases satisfied the

Weppner’s type III CO2 sensor criterion.

Although the different cations between solid-electrolytes and auxiliary phase

(sensing electrode) does not follow Weppner’s classification, the sensors constructed by such structures have showed Nernstian behavior and therefore have become common in type III CO2 gas sensors [17, 44, 86, 94-96].

2.6.4. EMF dependence on Oxygen

2.6.4.1.The role of oxygen in CO2 gas at sensing electrode

Yao et al. investigated the role of O2 for CO2 reaction (2.3) using NASICON electrolyte, Li2CO3-BaCO3 auxiliary electrode, and the closed-reference gas electrode

[97]. At 285°C, the sensor didn’t respond at all to CO2 partial pressure changes in the absence of O2 while it exhibited O2-dependent EMF for the following electrochemical reaction as shown in Figure 2.19(a):

 2 O2 (g)  4e  2O (adsorbed) (2.19)

This indicated that the electrode could activate O2 at 285°C but not CO2 [97]; activated oxygen was likely a critical factor to detect CO2.

Furthermore EMF dependence on O2 partial pressure of the CO2 gas sensor in the presence of fixed CO2 partial pressure has been studied because the participation of O2 is essential in the electrochemical CO2 sensing reaction. Figure 2.19(b) shows the correlation between EMF and O2 partial pressure under a fixed CO2 pressure at various

37 temperatures. As the O2 partial pressure increased from 0.001 and 1 atm, the EMF of the

CO2 sensor showed less O2 dependence at 450-500°C compared to that at 350°C. This result implies that the EMF dependence on O2 can be a problem at low temperature than at high temperature [97].

There are similar reports about the EMF dependence trend on O2 as the temperature was lowered [18]. The investigations by Yao et al. revealed that oxygen activation is essential for CO2 gas sensing and indicated that the CO2 gas sensing reaction can be different depending on temperature. The issue is how the activated oxygen reacts with CO2. In most literature, it is believed the activated oxygen reacts with conducting ion and forms oxides as follows:

+2 2Li (from a solid-electrolyte) O (adsorbed) Li2 O (2.20)

CO2 (g) Li 2 O Li 2 CO 3 (2.21)

There are other possible reactions:

2 2 CO 2 (g)  O (adsorbed)  CO 3 (adsorbed) (2.22)

2+ CO3 (adsorbed) 2Li (from a solid-electrolyte) Li 2 CO 3 (2.23)

It has not been elucidated what subsequent reactions occurs during CO2 sensing. This knowledge will be useful to lower the sensor operation temperature, since the reaction step is a critical factor to achieve the equilibrium reaction.

2.6.4.2.The role of oxygen in CO2 gas at reference electrode

Early type III CO2 gas sensors had the closed reference gas electrode. Due to the structure of the closed reference electrode, the EMF dependence on O2 may be possible because O2 partial pressure at the reference electrode is constant while the sensing electrode is exposed to variable O2 atmospheres [44].

Maier et al. proposed that the closed reference electrode CO2 sensors could not avoid the EMF dependence on O2 because of its structural characteristic [42]; until reference gas exists, sensing electrode has different oxygen partial pressure compared to the reference one. In order to achieve same oxygen partial pressure at the sensing and reference electrodes, they proposed the open reference electrode for type III CO2 sensors that was expected to eliminate the EMF dependence on O2 because both electrodes are exposed to the same environment. From Figure 2.20, the open reference electrode seemed to eliminate the EMF dependence on O2 above 500°C; EMF was stable in O2 partial pressure range between 0.1-1000 mbar. However, below 500°C, the EMF deviated from the equilibrium value depending on O2; as discussed earlier, same phenomenon, O2 dependent EMF, was observed in the sensing electrode. Holzinger et al. pointed out that the failure of the open reference electrode below 500°C could be caused by slow electrode kinetics [18]. Some researchers proposed anion (such as O2-) conductors as electrolytes to eliminate the EMF dependence on O2, but such dependence was still observed below 500°C [92, 98].

Whether the type III CO2 sensors had the open reference electrode or the closed reference gas electrode, the EMF dependence on O2 was frequently observed below

500°C. This indicates that temperature can be critical in the EMF dependence on O2; different kinetics at sensing and reference electrode or the slow kinetics at electrodes can be a reason for the existing problem.

Figure 2.1 The illustration of solid-electrolyte-based potentiometric gas sensors (a) Type I, (b) Type II, and (c) Type II.

Figure 2.2 ABO3 perovskite structure [99].

Figure 2.3 Crystal structure of Li3xLa2/3-xTiO3. La1 is La-rich layer and La2 is La- poor layer [68].

Figure 2.4 Arrhenius plots of electrical conductivity of well-known solid lithium ion conductors[60].

Figure 2.5 Activation energy (grain part) change for Li0.35La0.55TiO3 [59].

Figure 2.6 Ionic conductivity at room temperature as a function of Li content for the quenched La2/3-xLi3xTiO3 (●) and the furnace-cooled La2/3-xLi3xTiO3 (■) [72].

Figure 2.7 Lithium ion conduction path via bottleneck [72].

Figure 2.8 Schematic illustrations of (a) the closed-reference gas electrode, (b) the closed-pure element electrode, and (c) the open-reference electrode.

Figure 2.9 Response transient to CO2 of Na2CO3 electrode at 550°C. (a) dry condition and (b) wet condition [78].

Figure 2.10 Response transient to CO2 of Na2CO3-BaCO3 electrode at 550°C. (a) dry condition and (b) wet condition [78].

Figure 2.11 Microstructure change of sensing electrode and the sensors response transient change by morphological change [100].

Figure 2.12 The triple phase boundary (TPB) at a sensing electrode.

Figure 2.13 Theoretical Nernstian slope for the electrochemical reaction involving 2- electrons at 300°C.

Figure 2.14 An example of sensing response.

Figure 2.15 SEM images of NASICON surfaces after heating at 450°C for 3 days in humid conditions at 50°C for (a) 3 days and (b) 14 days [91].

Figure 2.16 The sensing behavior of the sensor based on Na-β-Al2O3 and Na2CO3 at 150°C [41].

Figure 2.17 CO2 sensing performances of (a) LaF3-based sensor and (b) MSZ-based sensor [92].

Figure 2.18 EMF vs. CO2 partial pressure for sensors using various metal carbonate auxiliary phases [93].

Figure 2.20 EMF as a function of O2 partial pressure for open reference electrode system [101].

Chapter 3

Synthesis and Characterization of Li0.35La0.55TiO3 Solid Electrolyte

Over the past several years, our group at CISM (Center for Industrial Sensors and

Measurements) has developed CO2 sensors that can be used as a respiratory monitor, fire detection, and environmental applications. Especially, the applications in aerospace require small-size and low power consumption as well as high reliability and accuracy because of limited storage space. To achieve this goal, the research has focused on the development of solid-electrolyte-based potentiometric CO2 gas sensors operating at low temperature that would not require addition of a heater.

In earlier studies, a solid-electrolyte-based potentiometric CO2 sensor was developed with Li3PO4 electrolyte, Li2CO3 sensing electrode, and a bi-phase mixture of

Li2TiO3 and TiO2 reference electrode [55, 102]. The sensor showed good sensing performance at high temperature (500-600°C), whereas poor ionic conductivity (5.58 ×

-6 10 S/cm at 500°C) of Li3PO4 electrolyte limited the sensor’s low temperature operation.

Thus our research interest moved to explore appropriate lithium ion conductors that have high ionic conductivity, particularly at lower temperature (T<300°C) [103].

Inhee Lee used lithium-lanthanum-titanate (Li3xLa((2/3)-x)□((1/3)-2x)TiO3, LLTO,

52 conductivity at room temperature as aforementioned [59, 60]. However, the sensor with

Li2CO3 sensing electrode did not work at 300°C and Lee added K2CO3 and a catalyst Au nano-particle loaded CeO2 in the Li2CO3 sensing electrode. The modified sensor showed

20% of theoretical Nernstian value at 200°C, but suffered from irreproducibility and

EMF drift.

To understand the origin of the problems in previous study, the properties of

LLTO (particularly for the composition of Li0.35La0.55TiO3) have to be systematically investigated because there are no information about transference number and LLTO phase stability in contact with electrode materials at sensing environments. In order to understand LLTO-electrode systems better, the sensor structure was simplified as shown in Figure 3.1; Li2CO3 electrode was used instead of Li2CO3+Au nano-particle loaded

CeO2+K2CO3 electrode in previous study.

In chapter 3, emphasis is placed upon the synthesis of Li0.35La0.55TiO3 solid electrolyte and its characterization for solid-electrolyte-based potentiometric CO2 gas sensor. Note that some contents of chapter 3 and 4 are to be published in Sensors and

Actuators B-Chemical [104].

3.1. Experimental

3.1.1. Optimization of synthesis conditions for Li0.35La0.55TiO3 electrolyte

The composition of lithium-lanthanum-titanate was chosen as Li0.35La0.55TiO3 because Li3xLa((2/3)-x)□((1/3)-2x)TiO3 showed the highest bulk (grain) ionic conductivity for x ≈ 0.11 at 25°C. The electrolyte powder was prepared by a conventional solid-state

53 reaction method. Stoichiometric amounts of Li2CO3 (99.999%, Alfa Aesar), La2O3

(99.99%, Alfa Aesar), and TiO2 (99.6%, Anatase, Alfa Aesar) were mixed in ethanol as starting materials. The mixture was bi-directionally ball-milled for 20 h5 using a milling machine (PQ-N04) and then was dried to evaporate ethanol at 100°C in a convection oven followed by grinding in a mortar and pestle.

It was necessary to optimize the conditions for subsequent calcinations and sintering for synthesis of Li0.35La0.55TiO3 [58, 59, 63, 64, 70, 105-107]. Four different conditions were examined and the process parameters are summarized in Table 3.1. All processes were conducted in a box furnace with ambient air. Note that condition 4 has a different starting composition because there is a report with respect to lithium evaporation at high temperatures [59].

Table 3.1 Synthesis conditions for LLTO pellets

Condition 1 Condition 2 Condition 3 Condition 4 Initial Li0.35La0.55TiO3 Li0.35La0.55TiO3 Li0.35La0.55TiO3 Li0.55La0.55TiO3 composition 800°C-4 h 800°C-4 h 800°C-8 h 800°C-4 h Calcination 1100°C-12 h 1150°C-12 h 1100°C-12 h 1000°C-6 h Sintering 1200°C-12 h 1250°C-12 h 1300°C-12 h 1350°C-12 h

The results after final sintering are presented in Figure 3.2. Sintering at 1200°C

(condition 1) formed a clean surface and a shape as intended without distortion, whereas

5 This number is based on 20 times of ball-milling sequence. Each milling sequence took 50 minutes milling and 10 minutes rest. 54 phase segregation (condition 2 and 3) and pellet distortion (condition 2, 3, and 4) were observed from other conditions. With condition 4, we noted pellet melting, consistent with what is reported in literature [107].

Based on the results from the tested conditions, the calcination and sintering conditions of Li0.35La0.55TiO3 were optimized as follows: i) The calcination of the ground powder was conducted at 1100°C for 12 h with 5°C/min heating and cooling ramp rate in ambient atmosphere; ii) The calcined powder was ball-milled again for 20 h; iii) By using

CARVER hydraulic press machine, the milled powders were cold-pressed to make green pellets of 6 mm (3500 LBS for 5 min), 12 mm (5000 LBS for 10 min) in diameter and 1 mm thickness; iv) The green pellets were sintered in a platinum crucible6 and were heated to 1200°C for 12 h with 3°C/min heating and cooling ramp rates. Final dimensions of the sintered pellet were about 5.4 mm in diameter and a thickness of about 0.9 mm, which was about 70% of the volume of the green pellet.

3.1.2. Density measurement

The density of the sintered sample was determined by density kit (MS-DNY-54,

Mettler Toledo) based on the Archimedes method using de-ionized water as the immersion fluid. Assuming that a sample has surface-connected porosity, the procedure was as follows [108]: i) measure dry weight (D) of a sample; ii) boil a sample in water for

5 h; iii) cool a sample in water for 24 h; iv) measure suspended weight (S) of a sample; v) measure wet weight in air (W). The density calculations are shown later in this chapter.

6 The green pellets were stick to alumina crucible with 1200°C sintering. See Figure 3.3 55

3.1.3. Phase and microstructure identification

The phase identification was conducted using powder X-ray diffraction (XRD).

The data was collected by a Rigaku X-ray diffractometer using Cu Kα radiation at 40 kV accelerating voltage and 25 mA current with a step size of 0.02° 2ɵ and counting time of

1 sec. The microstructure of the sintered sample and the fabrication sensor were observed by using a scanning electron microscope (SEM, JEOL JSM-5500). For cross-section images, a sample was embedded by using PELCO 24-hour epoxy mount kit (product No.

813-501 for resin and 813-514 for hardener). The resin was mixed with the hardener with

3:1 by volume and stirred gently the mixed epoxy for 3-5 minutes. To remove bubbles in the mixed epoxy, it was necessary to wait about 5 minutes in ambient air. Then specimen was placed into a mold and was covered by the mixed epoxy. Another 5-15 minutes was required for the trapped bubbles to escape. After the majority of the bubbles were evacuated, the mold with the specimen embedded in the mixed epoxy was cured at 70°C for 24 hrs to accelerate the curing process. More detailed information for the mounting is described in manufacturer’s manual7.

3.1.4. Electrical property measurement

The characterization of electrical properties of sintered Li0.35La0.55TiO3 electrolyte mainly focused on identifying ionic transference number as well as grain and grain boundary conductivities: To identify ionic and electronic conductivities, Li0.35La0.55TiO3 pellets were examined by impedance spectroscopy and DC polarization measurement

7 http://www.tedpella.com/technote_html/813-510_TN.pdf 56

(Hebb-Wagner method). The effects of temperature and atmosphere upon the electrical properties were examined as well.

3.1.4.1. Impedance measurement

The impedance of the sample was measured using a Solartron 1260A impedance analyzer. For the measurement, sputtered gold layers were formed symmetrically on both sides of a sintered Li0.35La0.55TiO3 electrolyte as ion-blocking electrodes. The frequency was swept from 1 Hz to 32 MHz with 100 mV ac applied voltage and the measurement was conducted from 25°C to 550°C in 21% O2/N2, 21%

O2/N2 with 4000 ppm CO2, and N2 atmosphere, respectively. In order to avoid magnetic effect from heating coils in a furnace, aluminum foil covered the quartz tube where the sample was placed and the entire system was electrically grounded.

3.1.4.2. DC polarization – Hebb-Wagner method

The electronic conductivity of the sample was examined by using the Hebb-

Wagner method using a Gamry PC4/300. Unlike the sample used for the impedance measurement, a sample for DC polarization had asymmetric structure and was connected as below:

(-) a mixture of Li2CO3 + Au particles | Li0.35La0.55TiO3 | Au (+) (3.1)

The vertical line, |, indicates phase boundaries, such as those between two contact solids or between a solid and a gas. Positive and negative signs mean polarity of applied potential. A mixture of Li2CO3+Au particles and Au were used as a reversible electrode and ion-blocking electrode, respectively. The samples were examined at 300, 400, and

500°C in 21% O2/N2 with 4000 ppm CO2. To polarize the sample, the D.C. voltage was applied from 0.05 to 1.4-1.7 V and the steady-state current values were obtained after 30 minutes at each voltage.

3.1.5. Investigation of the interface between an electrolyte and an electrode

In order to investigate the stability of Li0.35La0.55TiO3 solid-electrolyte in contact with Li2CO3 sensing electrode, lithium carbonate powders were mixed with

Li0.35La0.55TiO3 as 1:3 in weight ratio (Li2CO3:Li0.35La0.55TiO3=1:3) and the mixture was heated at 500, 600, and 700°C for 2 h in ambient air. To examine the stability between the electrolyte and the reference electrode, a mixture of Li0.35La0.55TiO3, Li2TiO3, and

TiO2 was heated for 2 h at 650 and 700°C in ambient air.

3.2. Results and Discussion

3.2.1. Density of sintered Li0.35La0.55TiO3

According to the Archimedes principle, the density of an unknown material can be measured as shown below:

MM12 12, VV12

FromVV12 , MMM 1 2  2  2 (3.2) 1  2  1M 1 Set variables. 3 1 :the density of water (1 g / cm )

2 :the density of Unknown

M1 : the mass of displaced water

M2 : the mass of unknown (3.2) becomes

M 2 D 2  (3.3) MWS1  *():W Wet weight weight of water inside solid S(): Suspended weight weight of water inside solid displaced water D(): Dry weight weight of solid

Since the density of de-ionized water (ρ1) can be changed with temperature, corrected values were obtained from the table provided by the density kit manufacturer. The measured values are presented in Table 3.2. The measured density of the sintered

3 Li0.35La0.55TiO3 is calculated as 4.76 g/cm , which is 95% relative to theoretical value.

The result indicates that the optimized processes can form fairly dense Li0.35La0.55TiO3 electrolytes.

Table 3.2 Measured weights of sintered Li0.35La0.55TiO3 in various conditions and its computed densities

Density Corrected Relative DI water Weight D Weight W Weight S Density Corrected Relative Sample Density Temperature (g) (g) (g) (g/cm3) by Temp Density (%)8 (°C) (g/cm3)9 (%) 1 0.10622 0.10694 0.08466 4.76750 94.97021 21.40000 4.65776 92.78416 2 0.37113 0.37129 0.29593 4.92476 98.10281 21.40000 4.81136 95.84386 3 0.37438 0.37448 0.29879 4.94623 98.53044 21.40000 4.83233 96.26154

8 3 In order to calculate relative density, theoretical density (5.02 g/cm ) of tetragonal Li0.33La0.557TiO3 was used. 9 For calculating corrected density, equation 3.3 can be written as

D(0.97793 0.0012) 2 0.0012 (3.4) WS W:,:,: Wet weight S Suspended weight D Dry weight 0.97793g / cm3 : Density of the DI water at 21.4 C 0.0012g / cm3 : Density of air

3.2.2. Phase and microstructure of sintered Li0.35La0.55TiO3

The XRD patterns shown in Figure 3.4 confirmed the formation of a perovskite

Li0.33La0.557TiO3 having tetragonal (P4/mmm) structure (JCPDS #87-0935) up on heating a mixture of Li2CO3, La2O3, and TiO2 at 1100°C for 12 h. Subsequent sintering (1200°C-

12 h) of the calcined powders resulted in identical XRD patterns to those of the calcined powder. According to literature, tetragonal structured Li0.33La0.557TiO3 was reported from slow cooling process and our result is consistent with the literature [109]. Double stacking of the perovskite cubic unit cell along the c-axis results in the formation of tetragonal structure with ordering of La3+, Li+, and vacancies (the superstructure peaks are marked with an inverse triangle in Figure 3.4) [59, 109].

Figure 3.5 shows the microstructure of the sintered Li0.35La0.55TiO3 pellets via SEM and their grain size calculated by linear intercept method. The grains have rectangular shapes with minimal overall porosity which is consistent with the calculated density. Table 3.3 summarizes average grain sizes and standard deviations at each magnification. Higher magnification picture shows larger standard deviation; because higher magnification took more local area than lower magnification, the values at lower magnification, e.g. × 4,000, is a better representation of the sample. The mean grain size was calculated as 1.6 μm.

Table 3.3 Computed average grain size and standard deviation at various magnifications

Magnification × 10,000 × 7,000 × 4,000 Average size (µm) 1.501 1.532 1.608 Standard deviation (µm) 0.338 0.206 0.181

3.2.3. Characterization of grain and grain boundary conductivities of sintered

Li0.35La0.55TiO3 by impedance measurement

The impedance spectra from 25 to 100°C in nitrogen atmosphere are shown in

Figure 3.6 for the sintered Li0.35La0.55TiO3 pellet with sputtered Au electrodes. A high frequency (32-0.1 MHz, Figure 3.6(a)) semicircle and a low frequency semicircle (104-

102 Hz, Figure 3.6(b)) were clearly distinguished; the numbers in the figures indicate the exponent of 10 in frequency (Hz). The impedance semicircles in high frequency and low frequency can be correlated with conductivities from grain and grain boundary, respectively [110]. As the measurement temperature increases, the semicircles decreased and the intercept on real axis (Z’) changed from (i) to (iv) as shown in Figure 3.6(a) indicating thermally activated conduction. At temperature above 100°C, the semicircle at high frequency range was not identified due to reduction of the grain resistance and inductance effect.

For gas sensors, the electrical properties of an electrolyte have to be maintained in gas sensing environments to provide reliable sensing response. To investigate atmospheric effect on the electrical properties of Li0.35La0.55TiO3 electrolyte, the impedance measurement of the sintered electrolyte was carried out in various atmospheric conditions: N2, 4000 ppm CO2+21% O2/N2, and 21% O2/N2. As shown in

Figure 3.7, the impedance spectra from different atmospheric composition were similar with respect to imaginary values (-Z”) and intercept on real axis (Z’). Note that inductance effect caused negative value of imaginary part (-Z”) and increased at high frequency region; consequently the semicircle from grain was unable to be observed

62 above 100°C. This result indicates that the electrical property of Li0.35La0.55TiO3 electrolyte would be stable in different environments of O2, N2, and CO2.

From impedance spectra, the grain and grain boundary conductivities of the sintered Li0.35La0.55TiO3 was calculated from the impedance intercepts and sample dimensions10. Since it was not possible to identify the semicircle from grain part (high frequency range: 32-0.1MHz) because of inductance effect at above 100°C, the impedance intercepts (Z’) for grain and grain boundary were obtained from two different methods depending on the measurement temperature11.

Method 1 (From 25 to 75°C): The equivalent circuit shown below was used to fit the obtained impedance data since two semicircles were clearly observed. From this method, it was possible to estimate R and C values for grain and grain boundary.

R1 and R2: resistance values for grain and grain boundary, respectively C1 and C2: capacitance values for grain and grain boundary, respectively

10 The dimension of the sample was 0.08 cm thickness and 0.229 cm2 surface area. 11 *In order to get the best fitting data, automatic data fitting function in Z view program was additionally used. In the program, you can select data range and fit the data into circle. By combining equivalent circuit model and automatic fitting data, we found that the obtained semicircles were depressed about 8-10°. Note that the numbers from automatic data fitting can be varied depending on what data points we selected. 63

Method 2 (above 100°C):

- Grain part: the first intercept Z’ value was chosen when frequency swept from

low to high ranges; it was assumed that semicircles from grain were hidden due to

inductance effect12 at high temperature.

- Grain boundary part: the semicircles from grain boundary were observable up to

300°C. In order to fit the semicircles, the following equivalent circuit was used.

Figure 3.8, for example, shows the data fitting (55°C) by using the equivalent circuit for method 1. As mentioned in the footnote 11, the fitted semicircle slightly deviated compared to the experimental data due to its depressed nature.

The numbers obtained from fitting the impedance data are shown in Table 3.4.

The calculation of the grain and grain boundary conductivities of the sintered

Li0.35La0.55TiO3 was based on the following equation:

l(cm) 13  (S/cm)  2 (3.5) R( )A (cm )

12 This is due to the connection line between the sample and the instrument. As temperature increases, the impedance of the sample decreases and the effect of the induction from the line become significant. 13 l and A are thickness and surface area of the sample, respectively. 64

The grain and grain boundary conductivities of the sintered Li0.35La0.55TiO3 were computed as 1.35 × 10-4 S/cm and 2.69 × 10-6 S/cm at 25°C, respectively. These values at room temperature are in good agreement with those reported in literature [62, 63, 106].

Temperature dependence of grain and grain boundary conductivities of the sintered Li0.35La0.55TiO3 are drawn as Arrhenius plots of log σ vs. 1/T as shown in Figure

3.9 and the calculated values are tabularized in Table 3.4; to calculate total conductivity, resistance values of grain and grain boundary were combined. Since the semicircle from grain boundary was not able to be identified, the calculation for grain boundary conductivity was only available up to 300°C. The calculated conductivity values of the sintered Li0.35La0.55TiO3 indicate that the grain boundary dominates the total conductivity of a polycrystalline sample.

The activation energy for the grain and grain boundary conduction can be calculated from the slopes of the plots in Figure 3.9 by using Eq. 3.6.

E  expa (3.6) k T B

Where kB is the Boltzmann constant, Ea the activation energy, T the temperature, and σ the conductivities due to grain or grain boundary conductions, respectively. The slope for grain conductivity alters at temperature of 100, 250, and 375°C while the slope for grain boundary does not.

Table 3.4 Obtained resistance, capacitance 14 , calculated grain conductivity, grain boundary conductivity, and total

conductivity of sintered Li0.35La0.55TiO3 Grain Grain boundary Total Temperature R1 C1 R2 C2 Conductivity Conductivity Conductivity (°C) (Ω) (F) (Ω) (F) (S/cm) (S/cm) (S/cm) 25 2650 3.0E-11 1.35E-04 125000 3.7E-09 2.69E-06 2.63E-06 35 1770 4.0E-11 1.98E-04 75000 3.7E-09 5.53E-06 5.38E-06 45 1270 4.0E-11 2.84E-04 50000 3.2E-09 6.99E-06 6.82E-06 55 710 8.0E-11 4.97E-04 26000 4.0E-09 1.29E-05 1.26E-05 75 380 1.0E-10 9.28E-04 12500 3.0E-09 2.69E-05 2.61E-05 100 227 1.53E-03 7000 5.0E-09 4.99E-05 4.83E-05 125 126 2.75E-03 2600 3.9E-09 1.34E-04 1.28E-04 150 85 4.10E-03 1200 3.0E-09 2.79E-04 2.62E-04 175 58 6.01E-03 600 3.0E-09 5.82E-04 5.31E-04 200 46 7.47E-03 350 3.0E-09 9.98E-04 8.80E-04

250 36 9.98E-03 140 3.5E-09 2.50E-03 2.00E-03 300 35 1.06E-02 55 5.8E-09 6.35E-03 3.97E-03 350 34 1.03E-02 375 34 1.03E-02 400 30 1.16E-02 425 27 1.29E-02 450 24 1.46E-02 475 22 1.59E-02 500 20 1.75E-02 550 18 1.94E-02

14 The capacitance values in the table above were roughly estimated in order to fit the obtained data. More efforts have gone into resistance values. 66

In the temperature range between 25 to 300°C, the activation energy for grain boundary conduction was 0.41 eV and agrees well with literature [59]. For grain conduction, the activation energy changed depending on temperature as shown in Table

3.5.

Table 3.5 Activation energy for grain conduction at various temperatures

Temperature (°C) Activation energy value (eV) 25

This phenomenon has been observed by many research groups but the fundamental reasons are still controversial. Inaguma et al. suggested phase transition of

Li0.35La0.55TiO3 from tetragonal to cubic structure [59]. However Bohnke et al. disputed the claim based on XRD and Differential Scanning Calorimetry (DSC) measurements and proposed that the temperature dependency of grain conduction of LLTO can be explained by a Vogel-Tamman-Fulcher (VTF) behavior that was originally developed to deal with viscosity properties of glass or super-cooled liquids [69]; they claimed that conduction mechanism at higher temperature (T>127°C) can be attributed to tilting or rotating of the TiO6 octahedra [60, 69].

3.2.4. Investigation of electronic conductivity of sintered Li0.35La0.55TiO3 by Hebb-

Wagner DC polarization method

The total observed current is the sum of partial currents, corresponding to the currents carried by ions, electrons, and holes. From impedance measurement, it was possible to get the total conductivity of the sintered Li0.35La0.55TiO3. In order to determine the contribution of the individual contribution to the total current, additional experiment has to be carried out and Hebb-Wagner DC polarization was appropriate for the purpose.

Figure 3.10 shows time responses of the current for the asymmetric cell15 at 300,

400, and 500°C in 4000 ppm CO2 + 21% O2/N2. The steady-state current and calculated current density values at each temperature are summarized in Table 3.6. As temperature and applied voltage increased, measured current value increased as well, but more noise was observed.

The obtained current or current density values from Hebb-Wagner method can be plotted as a function of applied voltage and the contribution from n-type electron conduction and p-type electron hole conduction [111]:

ETFR    Jnn=  1-exp    (3.7) RFTL   

ETFR    Jpp=  exp  1   (3.8) RFTL   

15 Dimension of Li0.35La0.55TiO3 pellet: 0.09 cm in thickness and 0.54 cm in diameter 68

Where L is the thickness of the sample, R the gas constant, E the potential, F the

Faraday’s constant, and σn and σp are the conductivities due to electrons and holes, respectively. With increasing voltage E, exponential term in Eq.3.7 saturates to 0 and the equation can be simplified while the same term in Eq. 3.8 goes to infinity. Figure 3.11 shows the contribution of electron and hole currents to the total current.

Table 3.6 Steady state current and current density at 300, 400, and 500°C from DC Hebb-Wagner polarization measurement

300°C 400°C 500°C Applied Current Current Current Voltage Current Current Current density density density (V) (A) (A) (A) (µA/cm2) (µA/cm2) (µA/cm2) 0.1 1.64E-08 0.07 8.93E-08 0.39 2.58E-06 11.27 0.15 2.81E-08 0.12 2.04E-07 0.89 2.53E-06 11.05 0.2 4.15E-08 0.18 2.57E-07 1.12 2.79E-06 12.18 0.3 7.53E-08 0.33 2.71E-07 1.18 3.76E-06 16.42 0.4 8.51E-08 0.37 4.09E-07 1.79 4.71E-06 20.57 0.5 7.05E-08 0.31 5.99E-07 2.62 5.24E-06 22.88 0.6 1.13E-07 0.49 7.39E-07 3.23 5.56E-06 24.28 0.7 1.85E-07 0.81 7.93E-07 3.46 5.84E-06 25.50 0.8 2.83E-07 1.24 8.86E-07 3.87 6.45E-06 28.16 0.9 3.32E-07 1.45 8.39E-07 3.66 7.58E-06 33.10 1.0 3.71E-07 1.62 9.00E-07 3.93 9.03E-06 39.43 1.1 3.82E-07 1.67 1.08E-06 4.72 1.09E-05 47.59 1.2 3.73E-07 1.63 1.44E-06 6.29 1.48E-05 64.62 1.3 4.38E-07 1.91 1.96E-06 8.56 1.97E-05 86.02 1.4 4.84E-07 2.11 2.90E-06 12.66 2.78E-05 121.39 1.5 5.38E-07 2.35 1.6 7.22E-07 3.15 1.7 1.04E-06 4.54

In Figure 3.12, the computed current density is presented as a function of the applied voltage at 300, 400, and 500°C. As the applied voltage increased, the current

69 density reached a plateau which is from n-type electronic conduction behavior and subsequent exponential increase can be explained by p-type hole conduction behavior.

Since the current density from n-type electrons, the sample dimension, and temperature are known, n-type electronic conductivity of the sintered Li0.35La0.55TiO3 can be estimated from Eq.3.7.

The transference number for lithium ion conduction (tLi+) is calculated as:

 Li+ t +  (3.9) Li  Li+ n

As mentioned earlier, total ionic conductivity in poly-crystalline material consists of grain and grain boundary conductivities. Because it was not possible to get the grain- boundary conductivity at 400 and 500°C from the linear curve shown in Figure 3.9, the values were obtained from extrapolation of the curve up to 500°C. The computed electronic conductivity and lithium ion transference number are given in Table 3.7. The data show that the electrolyte Li0.35La0.55TiO3 has high ionic transference number and almost negligible n-type electronic conductivity. Consequently, the electrolyte

Li0.35La0.55TiO3 can be considered a pure ionic conductor from 300 to 500°C, consistent with literature [60].

Table 3.7 Electronic conductivity, total conductivity, and Lithium ion transference number calculated from Hebb-Wagner method for Li0.35La0.55TiO3

Electronic conductivity Total conductivity Transference (S/cm) (S/cm) Number for Lithium ion (tLi+)

300°C 3.03 × 10-6 3.97 × 10-3 0.9992

400°C 5.79 × 10-6 7.21 × 10-3 0.9992

500°C 3.24 × 10-5 1.27 × 10-2 0.9975

3.2.5. Stability of Li0.35La0.55TiO3 with the sensor components and gases

3.2.5.1. Li2CO3 sensing electrode material

In order to investigate the reactivity of Li0.35La0.55TiO3 with Li2CO3 sensing electrode, mixtures of two materials were heated at 500, 600, and 700°C for 2 h in ambient air and the resulted products were analyzed by means of XRD, and the data is presented in Figure 3.13.

At 500°C, the diffraction peaks from Li0.33La0.557TiO3 (▼, JCPDS #87-0935) and

Li2CO3 (●, JCPDS #87-0728) were not altered and no new peaks were formed as shown in Figure 3.13(b). With heat-treatment at 600°C (Figure 3.13(c)), new peaks () were formed while the peak intensities from (101), (110), (112), (200), and (212) of

Li0.33La0.557TiO3 were lowered. As temperature increased to 700°C (Figure 3.13(d)), the intensity of the new peaks increased and the pattern is likely a perovskite structure, whereas the peaks from Li2CO3 and Li0.33La0.557TiO3 almost disappeared.

Borisevish et al. synthesized a new 1:2 ordered perovskite LaLi1/3Ti2/3O3 and reported its diffraction pattern with 2ɵ and I/Imax data [112]. The XRD patterns from the

Li2CO3-Li0.35La0.55TiO3 mixture heated at 700°C for 2 h showed excellent agreements in diffraction angles and intensities with LaLi1/3Ti2/3O3. According to the literature,

LaLi1/3Ti2/3O3 has a “doubled” perovskite structure and it seemed to be similar to tetragonal ABO3 which has “double stacked” ABO3. However, in the structure of

LaLi1/3Ti2/3O3 is A1(B1/3B2/3)O3, and lithium and titanium are located at the center of BO6 octahedra. Since there are only a few studies on LaLi1/3Ti2/3O3, its properties are not known [64, 112, 113].

3.2.5.2.Li2TiO3 and TiO2 reference electrode materials

Mixtures of Li0.35La0.55TiO3, Li2TiO3, and TiO2 were heated at 650 and 700°C in air. As shown in Figure 3.14(b), no new peaks were observed in XRD patterns at 650°C and this indicates that Li0.35La0.55TiO3 electrolyte is stable in contact with Li2TiO3+TiO2 reference material at 650°C. Anatase, however, reacted with Li0.35La0.55TiO3 at 700°C and formed a new phase while Li2TiO3 did not. The identification of the phase remains as future work.

Table 3.8 List of diffraction angle and relative intensities for the peaks in Figure 3.13

500°C – 2 h 600°C – 2 h 700°C – 2 h Borisevish et al. [112] 2ɵ I/I max 2ɵ I/I max 2ɵ I/I max 2ɵ I/I max 11.50 5.3 11.60 11.8 12.04 2.3 12.96 1 20.00 1.6 18.66 1.9 18.52 2.4 18.42 2 21.42 6.7 21.48 11.3 22.70 35.7 22.60 46 23.04 2.1 22.84 5.2 25.42 4.1 25.31 6 23.48 2.0 25.78 11.0 30.67 1.3 26.11 1 25.84 5.9 29.64 7.6 32.30 100.0 32.19 100 29.50 3.3 30.78 14.0 34.17 3.2 34.16 4 30.66 7.8 32.00 28.4 36.38 1.0 34.89 <1 31.86 11.2 32.82 100.0 38.08 1.6 36.10 1 32.74 100.0 34.26 7.0 39.80 30.9 37.95 2 34.22 3.2 34.92 5.1 41.46 2.0 39.70 32 34.74 2.8 36.24 3.7 43.72 13.0 41.47 2 36.22 2.1 37.16 8.6 46.26 25.8 43.04 1 37.00 3.9 39.79 6.5 47.80 4.2 46.18 28 73 39.72 2.3 40.42 20.6 48.39 0.6 47.67 5 40.34 19.1 42.16 1.4 50.68 0.7 50.68 2 41.70 0.7 43.63 2.2 52.06 11.9 52.03 13 41.94 1.3 45.78 4.2 53.19 4.4 53.33 6 42.19 1.3 46.19 4.0 53.40 2.9 57.43 32 43.58 1.4 47.04 35.2 57.48 31.4 58.72 1 46.94 39.8 48.56 4.4 48.66 2.7 48.88 4.4 52.90 2.2 52.78 1.7 54.20 1.7 54.36 2.6 54.58 0.8 56.61 2.3 58.36 42.6 56.88 3.6 57.40 4.5 57.94 8.2 58.44 35.4

3.2.5.3.Humidity and carbon dioxide

Generally, ambient air contains water vapor as well as 350 ppm CO2. If gas sensors are exposed to this condition, solid electrolytes as well as electrode materials should be stable. Otherwise new phases can form at the electrode-electrolyte interface and gas sensing reaction at TPBs can be influenced.

Figure 3.15(b) shows the XRD patterns of Li0.35La0.55TiO3 after being stored at

500°C for 72 h in presence of 4000 ppm dry CO2 and no new peak appears. After being stored at 50°C for 36 h in the presence of water vapor and 1% CO2, the formation of

Li2CO3 (●, JCPDS #87-0728) from Li0.35La0.55TiO3 was confirmed by XRD measurement. This observation agrees with Boulant et al. [74]. Because CO2 and water are present in ambient air, it will be necessary for the Li0.35La0.55TiO3-based devices to have water filter or waterproofing coating.

3.3. Conclusions

The composition of Li0.35La0.55TiO3 was synthesized by a conventional solid-state method and the sintered Li0.35La0.55TiO3 was characterized as a solid electrolyte for a potentiometric CO2 gas sensor. The optimization of the fabrication conditions resulted in the formation of intended shape of the sintered material without distortion or cracks. The structure of the sintered material was identified as tetragonal; average grain size and relative density were 1.558 µm and 95%, respectively. The electrical conductivity of sintered Li0.35La0.55TiO3 was revealed as pure ionic at 300, 400, and 500°C, which is appropriate for galvanic cell-based devices. The activation energy for grain conduction

74 depended on temperature while that for grain boundary conduction was constant for the measured temperature range. The phase stability of the sintered Li0.35La0.55TiO3 was examined with Li2CO3 sensing electrode and Li2TiO3+TiO2 reference electrode. It was found that Li0.35La0.55TiO3 electrolyte reacts with Li2CO3 depending on temperature. The

Li0.35La0.55TiO3-Li2CO3 reaction changes the interface structure by the formation of a new phase, LaLi1/3Ti2/3O3. Additionally, instability of Li0.35La0.55TiO3 in the simultaneous presence of water and CO2 was confirmed.

Figure 3.1 The structure of a CO2 sensor in this study.

Figure 3.2 Fabricated LLTO pellets under various conditions.

Figure 3.3 Evidence of reaction between LLTO pellets and alumina crucible during sintering at 1200-1350°C for 6-12 h.

Figure 3.4 Powder X-ray diffraction pattern of Li0.35La0.55TiO3 after calcination at 1100°C for 12 h. (▼: Superstructure peaks indicative of tetragonal structure)

(a) (b) (c)

Figure 3.5 Microstructure images of sintered Li0.35La0.55TiO3 at a magnification of (a) × 10,000, (b) × 7,000, and (c) × 4,000

(a) (b)

Figure 3.6 (a) High frequency region (32-0.1 MHz) and (b) Low frequency region (0.1 MHz-100 Hz) of the impedance plots

of the sintered Li0.35La0.55TiO3 as a function of temperature (25-100°C). The impedance was measured in N2 atmosphere (Numbers in the figure correspond to exponent of 10 in frequency, Hz) [104].

(a) (b) (c)

Figure 3.7 The impedance plots of the sintered Li0.35La0.55TiO3 in various atmospheric conditions at (a) 25°C, (b) 100°C, (c) 200°C continued. 80

(d) (e) (f)

Figure 3.7 continued at (d) 300°C, (e) 400°C, and (f) 500°C 81

Figure 3.8 Impedance data fitting by using equivalent circuit. Original data was from 55°C.

Figure 3.9 Arrhenius plots of the conductivity of sintered pellet of Li0.35La0.55TiO3, ■: grain region, ○: grain boundary region [104].

Figure 3.10 Time response of the current in Hebb-Wagner measurement for sintered pellet of Li0.35La0.55TiO3 at (a) 300, (b) 400, and (c) 500°C in 4000 ppm CO2 + 21% O2/N2. 83

Figure 3.11 Dependence of the electron and hole currents upon the relative potentials of the reference and reversible electrodes [111].

Figure 3.13 XRD patterns after heating (a) a mixture of Li0.35La0.55TiO3 and Li2CO3 at (b) 500, (c) 600, and (d) 700°C for 2 h in ambient air (▼: Li0.33La0.557TiO3, ●: Li2CO3, :

LaLi1/3Ti2/3O3, and ?: Unknown).

Figure 3.14 XRD patterns after heating (a) a mixture of Li0.35La0.55TiO3, TiO2 and, Li2TiO3 at (b) 650°C for 2 h, a mixture of Li0.35La0.55TiO3 and (c) TiO2, and (d) Li2TiO3 at 700°C for 12 h in ambient air (▼: Li0.33La0.557TiO3, ♦: Li2TiO3, ○: TiO2 (Anatase), and ?: Unknown).

Chapter 4

A study of Performance and Limits of a Potentiometric CO2 Sensor With

Li0.35La0.55TiO3 Electrolyte, Li2CO3 Sensing and Li2TiO3+TiO2 Reference Electrodes

In chapter 3, the sintered Li0.35La0.55TiO3 was characterized as a solid electrolyte for a potentiometric CO2 gas sensor. Its purely ionic conductivity and the magnitude of it along with high density (95% theoretical) are sufficient for being an electrolyte of a galvanic cell-based sensor. Since potentiometric sensors measure voltage difference between two electrolyte-electrode interfaces, the design of the gas sensors requires proper selection of electrodes that is in contact with the electrolyte to provide satisfactory response and stability [88, 114, 115]. After electrolyte and electrode materials are selected strategically, fabrication and operation conditions for the sensor should be carefully optimized since new phases can be formed at the electrolyte-electrode interface during such processes. Different crystal structure or electrical properties of the newly formed phases can affect the performance of the devices.

It has been accepted that any new phases should not be formed at the interface between an electrolyte and an electrode because new phases can block ionic motion, however increasing the number of components complicates the sensors and such complications often make it difficult to understand experimental results. It may not be 89

easy to build general rules about the effect of the second phase formation or new interfacial layer because of these complications [46, 86, 116, 117]. For example, Kida et al. claimed that the formation of interfacial phase could exert positive effect on the gas sensing performance of their sensors due to high ionic conductance of the interfacial layer [86]. Aono et al., on the other hand, found that the reaction of sensing and electrolyte materials could cause decrease in EMF of their sensors with time [117].

Therefore investigation of the effect of second phases on CO2 sensing should be examined for new electrolyte-electrode systems.

Preliminary results in chapter 3 revealed chemical reactions between

Li0.35La0.55TiO3 with Li2CO3 sensing material (T≥600°C) and TiO2 reference material

(T≥700°C). In order to unveil the effect of the reaction on the Li0.35La0.55TiO3-based potentiometric CO2 sensor, systematic investigation needed to be conducted. Chapter 4 emphasizes the electrolyte-electrode interface reaction in the development of

Li0.35La0.55TiO3-based CO2 sensor and also addressed its limitations and possible solutions for optimum sensing performance.

4.1. Experimental

4.1.1. CO2 Sensor fabrication

Figure 4.1 shows the fabrication flow diagram of the sensor: i) gold paste

(Heraeus, C5789 or C5729) was painted on both sides of a sintered Li0.35La0.55TiO3

electrolyte and was fired at 850°C for 5-10 min16 in a box furnace; ii) to fabricate the reference electrode, bi-phase powders of 95 wt% Li2TiO3 (99%, Lithium Corporation of

America Inc.) and 5 wt% TiO2 (99.6%, Anatase, Alfa Aesar) was mixed with α-terpinoel organic binder (Fisher Chemicals) and was painted on one side followed by a heat- treatment at 650°C for 2 h in ambient air; iii) the sensing electrode was formed by painting of Li2CO3 paste mixed with α-terpinoel organic binder and firing in the temperature range between 350 and 700°C for 2 h in ambient air. A photograph of the fabricated sensor is shown in Figure 4.2.

4.1.2. Gas preparation and EMF measurements

Gas sensor test setup is presented in Figure 4.3. In order to make synthetic air

(21% O2/N2), nitrogen (99.99%), oxygen (50% balanced by N2), and CO2 (1% balanced by N2) were mixed and flown at the rate of 160 sccm. The synthetic gas maintained 21%

O2/N2 background, and the introduction of CO2 gas was controlled from 500 to 4000 ppm during the test. In order for the sensor to be placed in the middle of a quartz tube inside a furnace (Lindberg Blue M furnace, Thermo Scientific), the sensor was attached to a sensor holder as shown in Figure 4.4 and was inserted into a quartz tube. The gold wires connected to the sensor were connected to an HP 34970A data acquisition system for measuring the emf of the sensor. The data was recorded every 5-10 seconds by Agilent

BenchLink Data Logger 3 (Ver.3.10.00).

16 The firing time for Au paste was obtained from manufacturer’s manual. 91

4.1.3. Nernstian slope calculation

As mentioned in chapter 2, the Nernst equation for CO2 reaction is,

0 2.303RT EMF  E  log p(CO 2 ) (4.1) 2F

Gas constant, R = 8.314 J/K•mol

1 Volt = 1 J/Coulomb

Faraday constant, F = 96485 C/mol

1 mV = 0.001 V

2.303RT The slope of E versus log p(CO2) should be same as for ideal Nerstian 2F behavior. For example, the theoretical Nernst value at 537°C (810K) is:

J 8.314 810K RT K mol 2.303  2.303   0.0801J/C  0.0801 V  80.1 mV (4.2) C 2F 2 96485 mol

The problem is how to determine EMF value from experimental results. Figure

4.5 shows a schematic of CO2 sensing time traces and two different methods to calculate

Nernstian slopes. One is the emf magnitude measurement (Figure 4.5(a)); total emf change is measured on each switching of CO2 concentration from 0 ppm (or 300-350 ppm in ambient air) to an intended value. The other one is the end point emf measurement; final emf value is measured at each CO2 concentration within a certain 92

CO2 introducing time (in this study, CO2 was introduced and allowed to equilibrate for 30 minutes). Two methods show almost similar results with less than 5% error in the case of no baseline drift, whereas deviation between results from two methods increases with baseline drift. In this study, the Nernstian slopes were calculated based on the magnitude of emf change from the baseline (21% O2/N2) at each CO2 concentration, because more than 100% Nernstian behavior was obtained by the end point method, as shown in Figure

4.6, which would be meaningless.

4.2. Results

4.2.1. Optimization of Li2CO3 fabrication of Li0.35La0.55TiO3 based CO2 sensor

The XRD data shown in Figure 3.13 confirms that the reaction between

Li0.35La0.55TiO3 and Li2CO3 starts around 600°C in air. In order to investigate the effect of the reaction on CO2 sensing, the Li2CO3 paste was painted on Li0.35La0.55TiO3 electrolyte as a sensing electrode and heated at 350, 400, 500, 650, and 700°C for 2 h in air. The reference electrode was first heat treated at 650°C to minimize the number of variables to be considered for the analysis. The sensors were tested at 350°C in 21%

O2/N2 background with the change of CO2 gas concentrations from 450 to 4000 ppm or

1000 to 4500 ppm.

Figure 4.7 shows CO2 sensing time traces of Li0.35La0.55TiO3–based sensors at

350°C. For the sensing electrode heat-treated for temperatures between 350 and 500°C, the EMF values increased with increasing CO2 concentration. In the case of Li2CO3

sensing electrode heat-treated at 650 and 700°C, in contrast, the of EMF (ΔE) changes for different CO2 concentrations were almost negligible.

17 Figure 4.8 presents the percent of Nernstian behavior of the sensors with Li2CO3 sensing electrode fabricated at different temperatures 18 . The calculated slopes are normalized to the ideal value at 350°C (61.83 mV/decade). The performance and the reproducibility of the sensors are substantially changed when the sensing electrode is fabricated at around 500°C. As the fabrication temperature increased from 350 to 500°C, the percent Nernstian behavior improved from 67 to 81%. On the other hand, the sensors fabricated at 650 and 700°C exhibited poor Nernstian slopes, e.g. 17% and 11% of theoretical values, respectively.

Morphological changes of the surface of Li2CO3 and Li0.35La0.55TiO3 were examined by SEM. A paste of Li2CO3 was painted on the surface of Au/Li0.35La0.55TiO3 and heated at 400, 500, 650, and 700°C for 2 h in air. In Figures 4.9 to 4.12, the SEM image at the top left-side shows the entire sensor (magnification of ×35), while (a), (b), and (c) focus on details of the different locations as indicated in the figure caption. After heating at 400°C for 2 h, Li2CO3 powders formed particle-particle contact and covered entire Li0.35La0.55TiO3 surface as shown in Figure 4.9(a) and (b); on the Li0.35La0.55TiO3 surface, only scratches from surface polishing were observed (Figure 4.9(c)). With heat-

17 In order to evaluate the performance of gas sensors, several parameters have been used such as response & recovery times, the percent of Nernstian, stability, etc. In this dissertation, the percent of Nernstian was chose since the response & recovery times can be influenced by instrumental set-up (e.g. test chamber size). The percent of Nernstian was calculated by the equation: Slope from measured value Percent Nernstian behavior = 100 Slope from theoretical value 18 Note: The sensors were tested at 350°C. 94

treatment at 500°C for 2 h, Li2CO3 particles were interconnected and covered the

Li0.35La0.55TiO3 surface as shown in Figure 4.10(a) and (b). The surface of the electrolyte was similar to that at 400°C. Significant morphological changes were observed at 650°C; the interconnected Li2CO3 became a continuous and somewhat porous film with large size pores as seen from Figure 4.11(b). On the Li0.35La0.55TiO3 surface, newly formed precipitates are noteworthy features in Figure 4.11(a) and (c); precipitates in the size range of 0.8-1 µm were observed where Li2CO3 layer was in contact with the electrolyte.

By heating Li2CO3 at 700°C for 2 h, the Li2CO3 film was rarely observed and the popping marks shown in Figure 4.12(a) seemed to be formed by melting of Li2CO3 layer during heat-treatment. The size of the precipitates on the Li0.35La0.55TiO3 surface has grown (0.8-3 µm) compared to that observed after heating at 650°C as seen in Figure

4.12(b) and (c). Energy-dispersive X-ray spectroscopy (EDS) analysis was tried to identify the composition of the precipitates but the resolution of the equipment and beam broadening effect limited the analysis. The phase of the precipitates was not identified by experimental methods but we have assumed that there are two possible phases for the precipitates. One possible phase is LaLi1/3Ti2/3O3; based on the changes in XRD patterns

(Figure 3.13), the precipitates can be correlated with LaLi1/3Ti2/3O3, which was from the reaction between Li0.35La0.55TiO3 and Li2CO3. The other possible phase is for the precipitates to be Li2CO3 and such morphology was from melting and subsequent re- crystallization.

Based on the performance of the sensors at 350°C, optimum condition for fabricating Li2CO3 sensing layer was chosen as 500°C for 2 h in air. Further 95

investigations were carried out for the Li0.35La0.55TiO3-based sensors with Li2CO3 sensing electrode fabricated under the optimum condition.

4.2.2. The sensor performance in 21% O2/N2

The Li0.35La0.55TiO3 gas sensor with Li2CO3 sensing electrode fabricated at 500°C for 2 h was examined from 250 to 550°C; the tests were conducted in 21% O2/N2 background gas atmosphere and the level of CO2 was controlled from 1000 to 4000 ppm or 500 to 4000 ppm. Figure 4.13 shows the response traces of the sensor. The EMF increased as CO2 was introduced without any base-line drift up to 400°C. The baseline started to drift at 475°C and the EMF decreased after reaching maximum values. At

500°C, the response behaviors were similar to that at 475°C but the EMF values were more significantly reduced. The sensor degradation was clearly observed at 550°C; all

EMF values seemed to be saturated even at different CO2 partial pressures. The degradation at 500-550°C seemed to be irreversible; as shown in Figure 4.14, the sensor was not fully recovered after testing at 500°C while the responses were recovered after examining at 450°C. At 500°C, the baseline was altered by about 200 mV and the values of EMFs at different CO2 partial pressure were similar.

The percent Nernstian behavior of the sensor are calculated based on the magnitude method and are presented with error bars in Figure 4.15. At 300°C, the sensor showed average 62% of theoretical value but the deviation was ±10%. As test temperature increased, the percent Nernstian behavior was improved to 95±4% at 450°C.

However the percent Nernstian behavior dropped to 90% at 475°C and eventually decreased to 85% with ±15% deviation at 500°C.

From the time traces and the percent Nernstian behaviors of the sensors, it was confirmed that the sensor degraded around 475°C during the test in 21% O2/N2 atmosphere. However this temperature is approximately 125°C lower than the temperature of fabrication at 600°C (2 hours). Since the sensor degradation at 475°C can limit its applicable temperature range, it was necessary to investigate reasons of the discrepancy between the two temperatures.

4.2.3. The effect of background CO2 on Li0.35La0.55TiO3-Li2CO3 reaction

Experimental conditions for Figure 3.13 and Figure 4.13 were different; one was the presence of 400 ppm CO2 and the other was the heating time. The powder samples used in Figure 3.13 were heated for 2 h in ambient air19 while the samples in Figure 4.13 were tested for approximately 95 h in the temperature range between 250 and 550°C with

21% O2/N2 background gas. At each CO2 sensing test temperature, the sensors were exposed to 21% O2/N2 without CO2 for about 7 h while total time in the presence of CO2 was 2-2.5 h. In total, the sensors experienced 21% O2/N2 without CO2 atmosphere for approximately 74 h during entire test20.

Due to the different conditions, it seemed that the degradation of the sensors at

475°C is not only correlated with the presence of 400 ppm CO2 but also with thermal

19 In general, ambient air contains 380-400 ppm CO2 in its composition. 20 Note: A sensor was tested at 250, 300, 350, 375, 400, 425, 450, 475, 500, and 550°C in a straight way; 7 h of no CO2 atmosphere × 10 different temperature step ≈ 70-74 h total 97

history caused by long exposure to high temperature21. Considering the effect of thermal history and the presence of CO2, the experiment was designed as follows; i) prepare fresh sensors fabricated under identical process; ii) form Li2CO3 layer at 500°C for 72 h

22 with/without 4000 ppm CO2; iii) test both sensors at 350°C in 21% O2/N2 background gas. If thermal history is a critical factor for the sensor degradation, both sensors would show degraded performance at 350°C. On the other hand if the presence of 4000 ppm

CO2 controlled the degradation, one sensor would show good CO2 sensing performance over the other.

Figure 4.16 shows the EMF time traces of the sensors during the experiment designed as mentioned above. In the presence of CO2 heat treatment (Figure 4.16(a): bold line), the sensor showed stable EMF at around -200 mV. In the absence of background

CO2 (Figure 4.17(a): dotted line), however, the EMF of the sensor was stable at -325 mV during first 15 hours heating and then changed drastically for next 30 hours from -325 to

-50 mV. The results of subsequent CO2 sensing tests at 350°C are shown in Figure

4.17(b). The sensor fabricated with 4000 ppm CO2 successfully detected CO2 at 350°C

(Nernstian value: 85%), whereas the one fabricated without 4000 ppm CO2 performed poorly for CO2 sensing test; practically 0% Nernstian value and drifty baseline. The results clearly showed that the absence of CO2 in the background gas was the reason for the degradation of the Li0.35La0.55TiO3-based CO2 gas sensors with Li2CO3 sensing layer, but thermal history was not.

21 C.T.E.s for Li0.35La0.55TiO3 and Li2CO3 are unknown. 22 4000 ppm CO2 was introduced instead of 400 ppm to show the effect of CO2. 98

4.3. Discussion

The Li0.35La0.55TiO3-based CO2 gas sensors with Li2CO3 sensing layer were fabricated under the optimum condition and were examined from 300 to 550°C. It was found that the sensor degraded at 475°C during testing in 21% O2/N2 background gas.

However the sensor degradation temperature was 125°C lower than the Li2CO3-

Li0.35La0.55TiO3 reaction (in ambient air) temperature revealed by XRD study. The investigation revealed that the temperature discrepancy was due to the presence of CO2 in the background gas. In this discussion below we start with CO2 sensor working principle and then focus on the Li2CO3-Li0.35La0.55TiO3 reaction and the role of CO2 in the background gas for sensor degradation. At the end of the discussion, we propose possible solutions to prevent the Li2CO3-Li0.35La0.55TiO3 reaction during the sensor fabrication.

4.3.1. Principle of CO2 sensor operation

The solid-electrolyte-based potentiometric CO2 sensor used in this study has the following structure.

CO2, O2, (Li2TiO3 + 5wt% TiO2) | Au | Li0.35La0.55TiO3 | Au | Li2CO3, O2, CO2 (4.3)

reference electrode |solid electrolyte| sensing electrode

In the presence of O2 and CO2, the following electrochemical reactions can take place at

TPBs and the sensor achieves an equilibrium.

1 ● Sensing electrode: 2Li+- +CO (g) O (g)+2e Li CO (s) (4.4) 22 2 2 3

1 ● Reference electrode: 2Li+- +TiO (s) O (g)+2e Li TiO (s) (4.5) 22 2 2 3

● The overall reaction: Li2 TiO 3 (s) + CO 2 (g) Li 2 CO 3 (s) + TiO 2 (s, Anatase) (4.6)

Since the sensor in this study has the open reference electrode, the sensing and reference electrodes are exposed to the same gas atmosphere. This design feature necessitates that the reaction (4.4) should not occur at the reference electrode. The changes of the Gibbs free energy (∆G) of reaction (4.4) and (4.5) are calculated, and shown in Figure 4.17(a). At the reference electrode, reaction (4.5) is more favorable than reaction (4.4) above 278°C; even if both electrodes are exposed to the same gas atmosphere, reaction (4.4) will not happen at the reference electrode. However, below

278°C, reaction (4.4) is possible at the reference electrode23.

However, the reference material Li2TiO3 can be unstable under certain conditions.

Figure 4.17(b) shows the change of the stability of Li2TiO3 depending on CO2 partial

24 pressure . According to the thermodynamic calculation, by decreasing CO2 partial pressure, Li2TiO3 reference material can react with CO2 and can form Li2CO3 and TiO2.

If this could be realized, some of the reference material would transform to Li2CO3 as depicted in Figure 4.18(a) and sensing electrode potential would partially be compensated by the reaction (4.4) from the reference electrode. However the XRD data shown in

23Below 200-250°C, it may change the reference materials. See future work in chapter 5. 24 It also shows the Gibbs free energy (∆G) for overall reaction (4.6) depending on temperature and CO2 partial pressure. 100

Figure 4.18(b) identified that Li2TiO3 was stable even when the material was heated at

100°C for 120 h in 1% CO2. This indicates a large kinetic barrier for the reaction although thermodynamically the reaction is favorable. Based on the test conditions e.g.

250°C ≤T≤550°C and 400 ppm≤CO2 concentration≤4000 ppm, we expected there would be no formation of Li2CO3 from CO2 and Li2TiO3; the reference electrode would provide stable-reference potential during CO2 sensing test.

4.3.2. The reaction between Li2CO3 and Li0.35La0.55TiO3

The XRD results in Figure 3.13 revealed that a mixture of Li0.35La0.55TiO3 and

Li2CO3 reacted at 600°C in air and the reaction product was identified as LaLi1/3Ti2/3O3.

The reaction seemed to correlate with the degradation of the sensor when Li2CO3 sensing electrode was fabricated at or above 650°C in air (typically 2 hours) on the

Li0.35La0.55TiO3 electrolyte. However it could not explain why the sensors with Li2CO3 sensing layer fabricated at 500°C permanently degraded at 475°C in 21% O2/N2 background gas because the degradation temperature was about 125-175°C below the

Li2CO3-Li0.35La0.55TiO3 reaction temperature. As described earlier, it was revealed that the presence of CO2 in 21% O2/N2 background was a critical factor for the sensor degradation.

In Figure 4.16(a), the EMF change during heating Li2CO3 in 21% O2/N2 for 72 h gave us a clue that constituents at the electrolyte-electrode interface were changed without CO2 while the interface was stable with CO2, because potentiometric sensors measure the activity change of lithium at the electrolyte-electrode interface. The results in 101

Figure 4.16(b) convinced us that the degradation of the sensors at 475°C in 21% O2/N2 should have correlation with the Li2CO3-Li0.35La0.55TiO3 reaction; the CO2 sensing time traces from the sensor fabricated at 700°C in air was similar to that from the sensor fabricated at 500°C in 21% O2/N2 without CO2. In both cases, the EMF change was the same with respect to different CO2 levels. This motivated us to investigate the effect of

CO2 on the reaction between Li0.35La0.55TiO3 and Li2CO3.

In order to account for these observations, we propose the following hypotheses, followed by rationalizations, thereof.

Hypothesis 1. Depending on the CO2 partial pressure, Li2CO3 can decompose and react with Li0.35La0.55□0.1TiO3 around 475-500°C, and leads to sensor degradation.

+ Hypothesis 2. As the Li inserts into Li0.35La0.55TiO3 beyond the solid solution limit

(Li=0.48), the Li0.35La0.55TiO3 structure becomes unstable; it changes Li/La ratio and initiates TiO6 octahedra tilting in ABO3 structure. Eventually ABO3 structure loses its crystal symmetry, e.g. P4/mmm for tetragonal, as the reaction proceeds. The distorted

LLTO transforms into new phase, for example, LaLi1/3Ti2/3O3 at 700°C.

Hypothesis 1 is needed to explain the temperature discrepancy between the irreversible sensor degradation at T ≥ 475°C in 21% O2/N2 atmosphere (Figure 4.13) and the reaction between Li0.35La0.55TiO3 and Li2CO3 at or above 600°C in air (Figure 3.13).

102

In general, solids equilibrate with surrounding gases and this causes oxidation or reduction of materials. For Li2CO3, it equilibrates with surrounding CO2. Depending on the partial pressure of CO2, Li2CO3 can decompose via the following reaction:

Li2 CO 3 (s) Li 2 O (s) +CO 2 (g) (4.7)

1 Li CO (s) + O (g) Li O (s) + CO (g) (4.8)25 2 32 2 2 2 2

The ∆G of reaction (4.7) depends on temperature and CO2 partial pressures and results of the calculation are shown in Figure 4.19. At a fixed temperature, the reaction becomes favorable as the CO2 partial pressure decreases; Li2CO3 decomposition is favorable. According to the calculation, the decomposition of Li2CO3 can start at 495°C in the presence of 100 ppb CO2. Since background gas of 21% O2/N2 in this study was synthesized by mixing 50% O2/N2 and 99.99% N2, the concentration of CO2 in the background gas would be at low ppb level. Therefore it is reasonable to assume that atmospheric conditions in Figure 4.13 were thermodynamically favorable to decompose

Li2CO3 around 500°C.

To validate the CO2 dependence of the Li2CO3-Li0.35La0.55TiO3 reaction, a mixture of Li0.35La0.55TiO3 and Li2CO3 powders was heated at 500°C for 55 h with/without 4000 ppm CO2 in 21% O2/N2 background and XRD patterns of the samples are shown in Figure 4.20(a). In the absence of CO2, the diffraction peaks from

25 Since Li2O2 is decomposed to Li2O at about 450°C, the reaction (4.7) will be only considered in this discussion.

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Li0.33La0.557TiO3 evidently disappeared and the new peaks (?) are observed after 55 h heating and the peak positions of the new peaks are in between Li0.33La0.557TiO3 and

LaLi1/3Ti2/3O3. In contrast, there are no significant changes on the diffraction peaks or intensities of the patterns after heating in 4000 ppm CO2 for 55 h. The same effect by the presence of CO2 on the Li2CO3-Li0.35La0.55TiO3 reaction was confirmed as well at 650°C

(Figure 4.20(b)); the reaction was prevented with pure CO2 while new phase () was observed and identified as LaLi1/3Ti2/3O3. These results proved that the reaction between

Li0.35La0.55TiO3 and Li2CO3 is possible below 600°C depending on CO2 partial pressure.

In support of Hypothesis 2, a mixture of Li0.35La0.55TiO3 and Li2CO3 powders was heated at 500°C for 2, 10, and 55 h in 21% O2/N2 background and the change of XRD patterns of the samples are presented in Figure 4.21. As heating time increased, the intensity of the diffraction peaks for Li0.33La0.557TIO3 decreased; intensity reduction of

Li0.33La0.557TIO3 peaks was larger than that of Li2CO3 and this may be due to different

26 lithium mole number in the materials: 2 for Li2CO3 and 0.35 or less for Li0.35La0.55TiO3 .

The diffraction intensities from (110) of Li0.33La0.557TIO3 were normalized to that of (002) of Li2CO3 and its changes are shown in Figure 4.22 as a function of heating

27 time . The peak intensities from Li0.33La0.557TiO3 exponentially decreased.

Our interpretation is that those reduction of diffraction peak intensity of

Li0.33La0.557TiO3 arise from the change of Li/La ratio in the LLTO structure by insertion

26 For powder reaction, mixing ratio was 1:3=Li2CO3:Li0.35La0.55TiO3 in weight. This is same as 1:1.42= Li2CO3:Li0.35La0.55TiO3 in mole. The amount of lithium from the decomposition of 1 mole of Li2CO3 is same as that from 5.7 moles of Li0.35La0.55TiO3. 27 Note (110) peak from Li0.33La0.557TiO3 was shifted but author counted the peak for the calculation. 104

+ of excess Li (x>0.16 into Li3xLa((2/3)-x)□((1/3)-2x)TiO3), which came from Li2CO3 decomposition. As the inserted excess Li ions fill the vacancies and interstitial sites in

Li0.33L0.57TiO3, the Li/La ratio increases. Because of the ionic radii difference between

Li+ (0.60 Å , Pauling radii) and La3+ (1.15Å , Pauling radii), the crystal structure becomes unstable and initiates TiO6 octahedral tilting. Due to the tilting, Li0.33La0.557TiO3 can lose its crystal symmetry (P4/mmm) and the Li0.33La0.557TiO3 diffraction peaks were weakened. If high temperature or long reaction time are applied, the distorted structure would transforms to new phase such as LaLi1/3Ti2/3O3 [64].

Experimental support for this hypothesis comes from the XRD of Li1.75La0.55TiO3 which was formed by heating the corresponding amounts of Li2CO3 (excess), La2O3, and

TiO2 at 1100°C (12 h). It was expected that excess amount of lithium was intentionally introduced in Li3xLa(2/3)-x□(1/3)-2xTiO3 and would have to form the same patterns as the reaction product of Li0.35La0.55TiO3 and Li2CO3 formed at 700°C. As expected, two patterns were identical as shown in Figure 4.23(b) and (c). The new pattern was identified as disordered LaLi1/3Ti2/3O3 [112, 113]. The disordered LaLi1/3Ti2/3O3 seemed to take the maximum lithium in its structure because the pattern remained the same even after adding more Li2CO3 and heating the mixture at 700°C for 12 h in air.

4.3.3. The effect of the formation of LaLi1/3TiO3 on sensor performance

As the reaction between Li2CO3 and Li0.35La0.55TiO3 proceeds, a new phase

(LaLi1/3Ti2/3O3) forms at original interface composed of Li2CO3 and Li0.35La0.55TiO3. The

105

formation of the new phase can affect morphology and electrical properties at the interface.

The morphological changes of the interface are schematically presented in Figure

4.24. When Li2CO3 forms below 500°C for 2 h in air or higher temperature in the presence of CO2, the electrode-electrolyte interface will be clearly defined as shown in

Figure 4.24(a): TPBs are not blocked and reaction (4.4) for CO2 sensing will occur. If the reaction can be prevented, higher Li2CO3 fabrication temperature will improve the adhesion of sensing layer due to better physical bonding. This can explain why the best

CO2 sensing performance at 350°C was produced by the sensor heated at 500°C as shown in Figure 4.8.

When the reaction between Li2CO3 and Li0.35La0.55TiO3 can occur during Li2CO3 electrode fabrication, LaLi1/3Ti2/3O3 or intermediate phase between Li0.33La0.557TiO3 and

LaLi1/3Ti2/3O3 are formed as presented in Figure 4.24(b). The new phase or intermediate phases can block TPBs and consequently reduce accessible TPBs for CO2 sensing reaction or act as a kinetic barrier for transporting lithium ion to TPBs. As the reaction continues, it will consume all Li2CO3 sensing layer as we already saw in Figure 4.12 and

CO2 sensing will be no longer be possible. Under thermal fluctuation, the sensor can experience interface delaminating due to different coefficient of thermal expansion (CTE) as seen in Figure 4.24(c) and Figure 4.25.

The formation of the new phase at the interface can change not only the interface structure but also the electrical properties. The electrical properties of LaLi1/3Ti2/3O3 have not been studied yet. 106

Since the chemical reaction occurs at Li2CO3-Li0.35La0.55TiO3 contact, it is difficult to obtain reproducible reaction result due to different particle contact area, particle size and reaction sites. Consequently, the Li0.35La0.55TiO3-based sensor can suffer from lack of reproducibility when the interface reaction occurs.

4.3.4. Practical implications of sensor fabrication and performance

It is clear that the interface reaction at Li2CO3 sensing layer and Li0.35La0.55TiO3 electrolyte has to be avoided; it degrades the sensor performance and causes reproducibility issues. Our investigation showed the reaction is related to Li2CO3-CO2 equilibrium, and introduction of high concentration of CO2 (~99.99%) can prevent the reaction between Li2CO3 and Li0.35La0.55TiO3 even at high temperatures. This will also facilitate good adhesion between the electrode and the electrolyte.

The Li2CO3 sensing layer was fabricated by heating at 650°C for 2 h in the presence of 99.99% CO2 and the sensor was tested from 250 to 475°C in 21% O2/N2 background gas. Figure 4.26 shows its CO2 sensing time traces. Due to slow kinetics,

CO2 sensing at 250°C was not good; response time was long, and equilibrium emf was not achieved after introducing CO2. Above 300°C, however, the sensor showed improvements in response time and sensitivity. The performance of the sensors with different Li2CO3 preparation conditions is compared in Figure 4.27. Nernstian behavior of the sensor with Li2CO3 fabrication at 650°C was significantly improved by introducing

99.99% CO2; the poor performance of the sensor prepared in air (■) enhanced from 20%

107

to 80% at 350°C and from 18% to 85% at 375°C because the electrode-electrolyte reaction was prevented for heat treatment in the presence of CO2.

For practical sensor operation, the presence of 400 ppm CO2 in the atmosphere will extend long term operation of the sensor at 475°C as well as at 300-450°C. In Figure

4.28, the sensor showed stable CO2 sensing responses at 350°C as expected. Figures 4.29

(a) and (b) show CO2 sensing responses of Li0.35La0.55TiO3-Li2CO3 sensors at 475°C with and without background 400 ppm CO2 (mimicking an ambient environment), respectively. In the absence of background CO2, the sensor’s performance deteriorated.

The baseline drifted and the emf change was not sensitive to different CO2 levels; these are attributed to the Li2CO3-Li0.35La0.55TiO3 reaction. In the presence of 400 ppm CO2, the baseline drifted 25 mV over a week and the Nernstian slope only changed from 91% to 88%. Therefore the sensor system as outlined in this study based on Li0.35La0.55TiO3-

Li2CO3 will be suitable for detecting changes of CO2 in ambient air, or other high CO2 environments such as combustion processes, and will not be appropriate for long term sensing in zero CO2 background environments.

4.4. Summary

A potentiometric CO2 gas sensor with Li0.35La0.55TiO3 electrolyte was examined with Li2CO3 sensing electrode and Li2TiO3+TiO2 reference electrode. XRD study revealed that Li0.35La0.55TiO3 electrolyte reacts with Li2CO3 and the crucial factors controlling this reaction are CO2 partial pressure as well as temperature. The

Li0.35La0.55TiO3-Li2CO3 reaction does not only change the interface structure but also the 108

electrical properties. These changes can deteriorate the performance of the sensor. From a sensor fabrication point of view, the introduction of high concentrations of CO2 can prevent the Li0.35La0.55TiO3-Li2CO3 reaction and this made high temperature fabrication possible; consequently electrolyte-electrode adhesion can be improved. As long as appropriate level of CO2 is present in the sensing environment, this sensor can monitor

CO2 concentration (>500 ppm) in the temperature range of 250 and 450°C.

109

Figure 4.1 CO2 sensor fabrication flow diagram.

Figure 4.2 A fabricated CO2 sensor in this study. 110

Figure 4.3 A photograph of gas sensor test set-up.

Figure 4.4 Installation of a gas sensor (a) sensor connection and (b) measuring leads outlets.

111

Figure 4.5 Two methods to calculate the Nernstian slope: (a) magnitude and (b) end point measurements

Figure 4.6 Two methods to calculate the Nernstian slope: (a) magnitude and (b) end point measurements. 112

Figure 4.7 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode heat treated at (a) 350, (b) 400, (c) 500, (d) 650, and (b) 700°C for 2 h in air, respectively.

113

Figure 4.8 Percent Nernstian behaviors at 350°C versus fabrication temperature of

Li2CO3 sensing electrode on Li0.35La0.55TiO3 (reference electrode was fabricated at 650°C for 2 h prior to deposition of Li2CO3 sensing electrode) [104].

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118

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Figure 4.16(a) CO2 sensing time traces at 350°C of Li0.35La0.55TiO3 sensors with Li2CO3 sensing electrode heat-treated with/ and without CO2 during Li2CO3 layer fabrication.

Figure 4.16(b) CO2 sensing time traces of Li0.35La0.55TiO3 based CO2 sensor with Li2CO3 heat treated with/ and without 4000 ppm CO2 for 72 h [104].

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Figure 4.17(a) Calculated ∆G (free energy) for the electrochemical reactions at the sensing and reference electrodes in standard states.

Figure 4.17(b) The change of Gibbs free energy for the overall reaction (Li2TiO3 + CO2  Li2CO3 + TiO2) depending on CO2 partial pressure and temperature. 122

Figure 4.18(a) The change of the composition of the reference electrode during sensor test at low temperature according to thermodynamic result shown in Figure 4.17 (b).

Figure 4.18(b) Stability of Li2TiO3 in the presence of 10000 ppm CO2 at 100°C for 120 h (♦: Li2TiO3). 123

Figure 4.19 Calculated ∆G (free energy) with varying CO2 partial pressure for Li2CO3 decomposition [104].

124

(a) (b)

Figure 4.20 The effect of CO2 on the reaction between Li0.35La0.55TiO3 and Li2CO3 at (a) 500 and (b) 650°C.

125

Figure 4.21 The change of XRD patterns for the mixture of Li0.35La0.55TiO3 and Li2CO3 after heating at 500°C for 0, 2, 10, and 55h in 21% O2/N2.

126

Figure 4.22 The change of intensity ratio between (110) peak of Li0.33La0.557TiO3 and (002) peak of Li2CO3 in Figure 4.21 as a function of heating time.

127

128

Figure 4.24 Schematics of the cross-section for Li2CO3-Li0.35La0.55TiO3 sensor (a) as fabricated, (b) formation of LaLi1/3Ti2/3O3, and (c) delaminated Li2CO3 sensing layer.

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Figure 4.26 CO2 sensing time traces of Li0.35La0.55TiO3 based sensor with Li2CO3 heated at 650°C for 2 h in 99.99% CO2. The sensor was tested from 250 to 475°C in 21% O2/N2 background gas.

Figure 4.27 Percent Nernstian behaviors of the sensors with Li2CO3 fabricated at 650°C-2h in 99.99% CO2 (□), at 500°C-2h in air (●), and at 650°C-2h in air (■); tested in 21% O2/N2 atmosphere.

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Figure 4.28 Long term stability of the Li0.35La0.55TiO3 sensor at 350°C in 400 ppm +21% O2/N2; the sensor fabricated at 500°C for 2 h in air.

Chapter 5

Modification of Li2CO3 sensing electrode for improving CO2 gas sensing:

Preliminary Results

In chapter 4, results were reported from a new potentiometric CO2 gas sensor using Li0.35La0.55TiO3 electrolyte with Li2CO3 sensing electrode and Li2TiO3+TiO2 reference electrode. The sensor exhibited good CO2 sensing behavior from 250 to 450°C, but its performance below 250°C was not reproducible and suffered from slow response time. These problems are suspected to originate from slow electrochemical reaction kinetics of CO2 on the sensing electrode. In general, the efficiency of electrochemical reactions becomes lower at low temperatures, particularly under dry condition.

Under humid condition, however, CO2 sensing at room temperature already has been reported in literature. Bredikhin et al. reported room temperature operating CO2 gas sensor based on NASICON electrolyte and SnO2 electrode in the presence of humidity as shown in Figure 5.1 [118]; the response time was less than 1 minute and this behavior held from 15°C to 45°C. Several other semiconducting metal oxides, such as In2O3, ZnO,

ITO and Co3O4, showed similar room temperature CO2 sensing properties under wet conditions [119]. The role of the metal oxide has not been well understood but it is

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believed that the surface hydroxyl group can play an important role for the following reaction [118, 120]:

+- Na +OH +CO23 =NaHCO (5.1)

Obata et al. intentionally added NaHCO3 and Na3PO4 to metal oxide and the sensor showed CO2 sensing behavior at room temperature because Na2CO3 was formed spontaneously from Na3PO4 in the presence of water [121]. However Obata et al. found that EMF of the sensor with NaHCO3 and Na3PO4 electrode shifted with a change in relative humidity (RH) [121]. Such dependence on RH was effectively reduced by replacing the mixture of metal oxide, (NaHCO3 and Na3PO4) with Li2CO3-BaCO3 binary mixture as presented in Figure 5.2 [119]. Long-term stability of the sensors in humid conditions has not been investigated in detail.

Although room temperature CO2 sensors are available in the presence of humidity, the scope of our research project was the development of low temperature CO2 sensors in dry atmosphere. Therefore a different approach was pursued.

At low temperature (T < 250 C) in dry atmosphere, the reaction kinetics is a limiting factor for the CO2 sensing reaction and hence new sensing materials that exhibit high catalytic activity to lower the activation energy barrier for the reaction has to be selected. A great deal of research has been conducted toward lowering sensor working temperature, but it is far from being solved because the detailed mechanism by which the

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electrode equilibrium is established is not known [122]. The establishment of the electrode equilibrium may involve complex multi-step mechanisms:

1 O (g) 2e O2- (5.2a) 2 2

+2 2Li (from a solid-electrolyte) O (adsorbed) Li2 O (5.2b)

CO2 (g) Li 2 O Li 2 CO 3 (5.2c)

(5.3a)

2 2 CO 2 (g)  O (adsorbed)  CO 3 (adsorbed) (5.3b)

2+ CO3 (adsorbed) 2Li (from a solid-electrolyte) Li 2 CO 3 (5.3c)

or may just be achieved by a single-step mechanism:

1 Li CO (s) 2Li (s)  CO (g)  O (g)  2e (5.4) 2 3 22 2

Due to this reason, most research on lowering operation temperature of CO2 sensors has relied on empirical studies and this approach seemed to be main reason why low temperature potentiometric CO2 sensors have not been developed.

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Another aspect is the mechanical interaction and way to enhance adhesion between a carbonate electrode and Li0.35La0.55TiO3 electrolyte and way to increase the number of TPBs. More TPBs can provide more CO2 reaction sites and therefore the performance of the gas sensor can be improved in terms of sensitivity. Good adhesion at electrolyte-electrode interface does not only improve sensor durability but also can form more TPBs. Chapter 5 is aimed at providing some ideas to form good interface bonding and more TPBs; by modifying Li2CO3 sensing electrode, preliminary data were obtained.

5.1. Mixture of carbonates as a sensing electrode

Binary and ternary carbonate sensing electrodes exhibit faster response time and improved EMF during CO2 sensing in dry and wet atmosphere as compared to single carbonates around 350-550°C [78, 123, 124]. The mechanism of binary and ternary carbonate sensing electrodes has not yet been elucidated in detail, but one possible reason is the reduction of melting temperature due to their eutectic compositions [123, 125]. The eutectic melt of carbonate mixture can improve adhesion at the electrolyte-electrode interface and can result in the formation of more TPBs. Other possibility may come from the change of morphology of the sensing electrode by applying binary carbonate [123]; the role of difference in electrode morphology has not been investigated for carbonate mixtures. In spite of ambiguity of reasons for improved sensing mechanism by binary or ternary carbonates, binary carbonates are used in potentiometric CO2 gas sensors for enhancing gas sensing performance. In this sense, it was reasonable to apply carbonate mixture for Li0.35La0.55TiO3-based CO2 sensors to improve TPB structure. In this section, 135

results from Li2CO3-BaCO3 binary carbonate and Li2CO3-K2CO3-Na2CO3 ternary carbonates sensing electrodes are presented.

5.1.1. Li2CO3-BaCO3 binary carbonates sensing electrode

According to Pasierb et al., Li2CO3 and BaCO3 form eutectic solution at 500°C at

1 to 1 molar ratio [126]. It means that this eutectic melting can be possible on

Li0.35La0.55TiO3 surface during heat-treatment at 500°C. This eutectic reaction makes adhesion strong between an Au electrode, carbonate film, and Li0.35La0.55TiO3 electrolyte.

In addition, eutectic phase may be dominant at TPBs but this may be helpful for enhancing CO2 detection as reported in many literatures [17, 86, 127]. However, the eutectic reaction may promote Li0.35La0.55TiO3- Li2CO3 reaction because of the presence of the liquid phase leading to possible formation of additional phases that would deteriorate CO2 detection.

In order to prevent the reaction between Li0.35La0.55TiO3 and Li2CO3, binary carbonate layer was formed in two steps. Solution of Ba(NO3)2 was first coated on

Li0.35La0.55TiO3 electrolyte as a precursor and subsequently Li2CO3 paste was painted and heated in a CO2 environment. During the heat-treatment, Ba(NO3)2 layer transforms to

BaCO3 and makes a contact with Li2CO3 layer. As a result, the Li2CO3 - BaCO3 eutectic reaction can occur selectively. If we can control the kinetics of the eutectic reaction by optimizing temperature (or time) for electrode fabrication, BaCO3-Li2CO3 eutectic layer would only form in between Li2CO3 sensing layer and Li0.35La0.55TiO3 solid electrolyte but would not react with Li0.35La0.55TiO3 as illustrated in Figure 5.3. This structure can 136

provide good interface bonding avoiding the reaction between Li2CO3 electrode and

Li0.35La0.55TiO3 electrolyte.

Prior to fabrication of sensor sample, it was necessary to check the formation of

BaCO3 from Ba(NO3)2. A solution of Ba(NO3)2 was heated at 630°C for 2 h in the presence of 1% CO2 with 90 sccm flow and its XRD pattern is shown in Figure 5.4;

Ba(NO3)2 (JCPDF #: 24-0053) diluted in DI water was transformed to BaCO3 (JCPDF #:

45-1471).

To fabricate a sample, 0.02 M of Ba(NO3)2 was dropped on the sensing electrode

28 side and subsequently Li2CO3 paste was painted and then heated at 630°C for 2 h with

29 5°C/min ramp rate in a tube furnace with 90 sccm flow of 1% CO2 . Figure 5.5 shows

CO2 sensing time trace of the sensor from 180 to 400°C. The sensor showed good response behaviors from 300 to 400°C; equilibrium EMFs were achieved. As test temperature decreased, magnitude of EMFs reduced and EMFs didn’t reach equilibrium values within 30 minutes. In the temperature range between 300 and 400°C, as seen in

Figure 5.6, the percent Nernstian behavior of the sensor with binary carbonate were comparable to those from the sensor with Li2CO3 fabricated at 500°C – 2 h in air; the sensor with binary carbonate showed sensing behaviors about 10% of theoretical Nernst slope even at 180°C.

Although the sensor with binary carbonate showed some promising results below

250°C, the concern was the reproducibility of the results; other sensors fabricated under

28 Based on temperature calibration, actual temperature was 610°C. 29 Li0.35La0.55TiO3 electrolyte, Li2TiO3+TiO2 reference electrode, and gold metal electrode were fabricated in the same manners as described in chapter 4. 137

the same fabrication condition did not show similar values in the percent Nernstian behavior, response time, and CO2 sensing time traces. In order to reproduce the results shown in Figure 5.5, different heating temperatures (400, 500, 600, and 650°C) and different mole concentrations of Ba(NO3)2 (0.04M and 0.1M) were applied but the sensing performances were not reproducible from sample to sample.

The reproducibility problem in the sensors with binary carbonate might have come from the reaction between Li2CO3-BaCO3 binary carbonate and Li0.35La0.55TiO3; the thickness of BaCO3 layer was too thin to prevent the reaction between

Li0.35La0.55TiO3 and the eutectic melts of Li2CO3-BaCO3 binary carbonate. Figure 5.7 shows the surface of Li0.35La0.55TiO3 electrolyte after heating binary carbonate at 630°C for 2 h in a tube furnace with 90 sccm flow of 1% CO2; the change of morphology was observed as seen in Figure 5.7(a) and precipitates were identified on the electrolyte surface. The shape of the precipitates in Figure 5.7(b) was different from that in Figure

4.11 and 4.12; it seemed to grow in one specific direction.

The precipitates in Figure 5.7(b) seemed to be correlated to the reaction product between Li0.35La0.55TiO3 and Li2CO3-BaCO3 binary carbonate. Figure 5.8 showed the change of XRD pattern of a mixture of Li0.35La0.55TiO3, Li2CO3, and BaCO3 depending on heating temperature in the presence of 1% CO2. In spite of the introduction of CO2, the reaction between the electrolyte and the binary carbonate could not be prevented. The phases observed at 630 and 650°C were not identified (Figure 5.8(b) and (c)) but at

700°C one phase was identified as BaTiO3 or LaTiO3 (Figure 5.8(d)). Therefore the diffraction patterns observed at 630 and 650°C were likely to be intermediate phases 138

between Li0.33La0.557TiO3 and BaTiO3 (or LaTiO3); the peak position at Figure 5.8(d) was different from that of LaLi1/3Ti2/3O3, which was the product of Li0.35La0.55TiO3-Li2CO3 reaction.

For Li2CO3-BaCO3 binary carbonate, the reaction with Li0.35La0.55TiO3 electrolyte seemed to be a critical factor to fabricate reliable CO2 gas sensors. The reproducibility can depend on how to achieve good interface bonding from eutectic reaction avoiding the electrolyte-binary carbonate electrode reaction. To achieve this goal, one needs to consider several factors. Firstly, at 630°C it was not possible to prevent the reaction between the electrolyte and the binary carbonate by introducing 1% CO2; as seen in

Figure 4.20, the introduction of high purity CO2 (~100%) will be appropriate at that temperature range. Secondly, coated Ba(NO3)2 solution could not be formed uniformly on the electrolyte surface; the concentration of the solution was not enough to cover the entire electrolyte surface; as a result, bare-Li0.35La0.55TiO3 surface could be exposed to

Li2CO3 and the interfacial reaction could occur. Lastly, unlike Li2CO3-Li0.35La0.55TiO3 reaction, the eutectic melt itself could be a fundamental reason to prompt the interfacial reaction even in the presence of 1% CO2.

5.1.2. Li2CO3-Na2CO3-K2CO3 ternary carbonates sensing electrode

According to Imanaka et al., ternary carbonates which consisted of Li2CO3,

K2CO3, and Na2CO3 worked well as a CO2 sensing electrode; it showed 100% Nernstian behavior at 350°C. They claimed the result was due to low melting temperature (390°C) of the ternary carbonates (Li2CO3:K2CO3:Na2CO3=47.6:25.4:27.0). We thought low 139

melting temperature can prevent the reaction between Li0.35La0.55TiO3 and ternary carbonate and examined one composition (No.1 in table 5.1) from the table in literature

[125].

Since the reaction between Li0.35La0.55TiO3 and ternary carbonates should be prevented, it was necessary to check phase change after heat-treatment of the mixture of the electrolyte and ternary carbonates. The mixture was heated at 390°C for 30 min and the heated powder was examined by XRD as shown in Figure 5.9; there was no reaction between the electrolyte and ternary carbonates at 390°C30.

Table 5.1 Ternary eutectic mixtures [125]

No Eutectic salt mixtures (mol %) Melting point (°C) 1 43.5%Li2CO3-31.5%Na2CO3 -25%K2CO3 400 2 49.5%Li2CO3-44.5%Na2CO3 -6%K2CO3 468 3 39%Li2CO3-27.9%Na2CO3 -33.1%K2CO3 349 4 58%Na2CO3-3%K2CO3-39%Rb2CO3 557 5 5 22%Li2CO3-38%Na2CO3-40%Rb2CO3 410 6 39%Li2CO3-38.5%Na2CO3-22.5%Rb2CO3 400 7 50%Li2CO3-29%Na2CO3-21%Rb2CO3 412 8 14%CaSO4-6%BaSO4-80%Li2SO4 660

The CO2 gas sensors with ternary carbonates were fabricated with heating the sensing electrode at 390°C for 30 min. Figure 5.10 presents the CO2 sensing time trace of the samples; two sensors were tested in 21% O2/N2 background. The CO2 sensing performances of the two sensors were similar above 300°C but the discrepancy between two sensors started below 250°C; the CO2 sensing time traces (Figure 5.10) and the

30 This result indirectly proved that the eutectic melt itself does not react with Li0.35La0.55TiO3. 140

percent Nernstian behavior (Figure 5.11) were very different. Since there was no chemical reaction between the Li0.35La0.55TiO3 electrolyte and ternary carbonate electrode, this can indicate that CO2 sensing reaction itself is a problem at low temperature. Possible reason of reproducibility problem is ambiguous at this moment.

5.2. Li2CO3 formation from LiOH solution

In order to fabricate carbonate sensing electrode, painting paste of powder mixture may not be an ideal method in terms of the fabrication of thin film and the formation of good adhesion between a sensing layer, an electrolyte, and a metal electrode at TPBs. By painting carbonate paste, it is difficult to achieve uniformly thin carbonate layer as presented in Figure 5.12(a); the thick sensing layer can inhibit fast diffusion path of target gas molecules and this can result in slow response. In addition, the fabricated carbonate film by powder paste could not be tightly bonded to metal electrode and a solid- electrolyte due to raw powder size as seen in Figure 5.13(a); TPBs cannot work as a site for electrochemical reaction.

In order to fabricate thin carbonate film with good uniformity and tight adhesion at

TPBs, carbonate powder route was changed to liquid precursor using lithium hydroxide

(LiOH). The solution method was expected to provide several advantages: (i) tight film bonding at TPBs as presented in Figure 5.12(b), (ii) film thickness control by solution concentration as shown in Figure 5.13(b) and (iii) possibility of use of ink-jet printing.

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To fabricate a sample, 2.15 M of LiOH was painted on the sensing electrode side

31 and subsequently heated at 500°C for 2 h in the presence of 4000 ppm CO2 . Figure 5.14 shows the CO2 sensing time trace of the sample from 250 to 500°C in 21% O2/N2 background gas. The responses of the sensor were not stable at 250°C and were degraded from 400°C; the degradation temperature of the sensor was 75°C lower than that of the sensor which was fabricated from Li2CO3 powder.

The sensors were fabricated with different molar ratio of LiOH to DI water32 and were heated at 200°C or 650°C in the presence of 1% CO2. Figure 5.15 shows CO2 sensing time traces at 300 and 350°C for the sensor with LiOH heated at 200°C. During the CO2 sensing test at 300°C, the EMF of the sensor drifted but reached equilibrium quickly. At 350°C, the degree of EMF drift was alleviated a little. For the sensor with

LiOH heated at 650°C, CO2 sensing time traces at 350 and 400°C are presented in Figure

5.16. In spite of the EMF changes for different CO2 concentrations, the percent of

Nernstian behavior of the sensor in Figure 5.16 was less than 15% at 350 and 400°C.

When Li2CO3 sensing layer was formed from LiOH electrode, the fabrication temperature seemed to be a critical factor for the electrode morphology. By heating LiOH solution at 200°C for 12 h, dense carbonate layer was formed as shown in Figure 5.17(a); the carbonate layer seemed to inflate during the fabrication as seen on Figure 5.17(b).

However when LiOH solution was heated at 650°C for 2 h, the formed carbonate layer did not inflate and the morphology was likely dense.

31 Li0.35La0.55TiO3 electrolyte, Li2TiO3+TiO2 reference electrode, and gold metal electrode were fabricated in the same manners as described in chapter 4. 32 The volume ratio LiOH to DI water was 1 to 1. 142

For future studies, it is necessary to investigate the correlation between sensor performance and electrode morphology as well as interfacial reaction between LiOH and

Li0.35La0.55TiO3 during heat-treatment.

5.3. Enhancement of electrical conductivity of Li2CO3

In order to improve CO2 sensing, the investigation has focused on how to enhance adhesion between a carbonate electrode and Li0.35La0.55TiO3 electrolyte. The eutectic melts of binary or ternary carbonates and hydroxide solution were applied to achieve good bonding at the interface between an electrode and an electrolyte. These methods showed good preliminary data, but some of the methods are likely to promote the interface reaction. As an alternative approach to improving CO2 sensing behavior, it is plausible to increase number of TPBs by modifying electrical property of electrode materials.

An ideal electrode must have high ionic conductivity and high electronic conductivity because an electrode is a place where the conductivity changes from electronic to ionic or vice versa [11]. For electrochemical devices, high electrical conductivity can provide two benefits: (1) better sensitivity: the reaction can occur through the entire volume of the electrode and therefore more TPBs are available; (2) faster response time: the electrical potential between an electrode and an electrolyte can reach equilibrium quickly. However lithium carbonate, which is used in potentiometric

CO2 gas sensors as a sensing electrode, has poor electronic conductivity as well as lower

143

ionic conductivity. By trying to modify each property, the effects of enhancement of electronic and ionic conductivity were examined.

5.3.1. Ionic conductivity of Li2CO3

According to Mizusaki and Tagawa, the ionic conductivity of Li2CO3 could be enhanced by doping with Li3PO4 [128]. They melted a mixture of Li3PO4 and Li2CO3 and grew crystal along the (002) plane of the monoclinic Li2CO3 structure. The ionic conductivity was measured parallel and perpendicular to (002) of Li3PO4-doped Li2CO3.

As Li3PO4 was doped more the ionic conductivity of Li3PO4-doped Li2CO3 increased as shown in Figure 5.19. The doped Li2CO3 has been used as a solid-electrolyte for type II

CO2 gas sensors but the sensing performance below 300°C were not good [34, 54] due to low ionic conductivity. Instead of Li3PO4, addition of MgO enhanced ionic conductivity of Li2CO3 as well [129]. Salam et al. tested Li2CO3-MgO electrolyte for potentiometric

CO2 gas sensor and achieved 84% Nernstian behavior at 300°C [129]; the LLTO-based sensor with Li2CO3 sensing electrode showed 60% of percent Nernstian behavior at

300°C.

A sensing electrode with 9 mol % Li3PO4 doped Li2CO3 was examined with

Li0.35La0.55TiO3 electrolyte. Powder of Li3PO4 was mixed with Li2CO3 and the mixture was painted on the electrolyte surface. The sensor was fabricated at 650°C for 2 h in the presence of 99% CO2. The sensor was tested from 150 to 475°C in 21% O2/N2 background. Figure 5.20 shows CO2 sensing time traces of Li0.35La0.55TiO3 sensor with

Li3PO4 doped Li2CO3 sensing electrode; the baseline was stable from 450 to 475°C but it 144

drifted below 450°C. Furthermore the response below 250°C was difficult to be analyzed.

Figure 5.21 presents the percent Nernstian behavior of the sensor with Li3PO4 doped

Li2CO3 compared to that with Li2CO3. As CO2 sensing temperature decreased, the percent Nernstian behavior of the sensor with Li3PO4 doped Li2CO3 sensing electrode was reduced more than that of the sensor with Li2CO3 sensing electrode. Since the phase of the Li3PO4 doped Li2CO3 in this study has not been analyzed, it is unknown whether

Li3PO4 was really doped into Li2CO3 or not. If the doping did not occur, the presence of

Li3PO4 could block TPBs. This needs to be investigated in the future.

5.3.2. Electronic conductivity of Li2CO3

For lithium ion battery, conducting carbon (i.e. acetylene black) is added to cathode electrode [67]. As mentioned earlier, the carbon additive can enhance the electronic conductivity of cathode material such as lithium oxide compound (i.e. LiCoO2) and the charge transfer reaction can be possible over the entire electrode at low temperature (T≈100°C). As mentioned earlier, the electronic conductivity of Li2CO3 is poor. Therefore it was reasonable to add conducting additive to Li2CO3 sensing material.

Carbon black (Alfa Aesar, Super P conductive, 99+% metal basis) was added to

Li2CO3 in various molar ratios: Li2CO3:C=6.7:4.1 or 3:1). The mixture was painted on the Li0.35La0.55TiO3 surface and subsequently heated at 500°C for 2 h in the presence of

99% CO2. Figure 5.22(a) and (b) showed the CO2 sensing traces from the sensors with

Li2CO3:C=6.7:4.1 and 3:1, respectively. Both sensors did not show good sensing performance during the tests. The investigation of the failure remains as a future work. 145

5.4. Summary

In order to improve CO2 sensing performance of the Li0.35La0.55TiO3 electrolyte- based gas sensor, several methods were approached to make better adhesion between a carbonate electrode and Li0.35La0.55TiO3 electrolyte and to increase the number of TPBs and their preliminary results are reported. Through Li2CO3-BaCO3 binary carbonate formation, the sensor showed CO2 sensing performance below 200°C, as well as faster response time and quicker equilibrated EMFs. By using LiOH solution, Li2CO3 sensing layer was formed in the presence of CO2 and the sensor was examined; however further investigation is necessary to reveal correlation between fabrication parameters (i.e. temperature, concentration, and materials) and optimizing microstructure. Modification of electrical properties of Li2CO3 sensing material needs to be optimized for better CO2 response. Since the results described in chapter 5 are preliminary, further studies are needed to obtain reproducible performance.

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Figure 5.1 CO2 sensing time traces of Sb-doped SnO2 electrode-based gas sensor at 25°C in wet condition [118]

Figure 5.2 Nernstian slopes at 30, 50 and 70% RH for devices using (a) Li2CO3- BaCO3 and (b) NaHCO3 at 30°C [119].

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Figure 5.3 Schematic of gas paths in (a) thick and (b) thin Li2CO3 sensing layer.

Figure 5.4 Change of XRD pattern from Ba(NO3)2 (black line) to BaCO3 (red line) after heating at 630°C for 2 h in 1% CO2 with 90 sccm flow; Ba(NO3)2 was diluted in DI water.

148

149

150

(a)

(b)

Figure 5.7 SEM images of (a) edge of binary carbonate layer (1: binary carbonate, 2:

Li0.35La0.55TiO3 surface, 3: precipitates) and (b) Li0.35La0.55TiO3 surface where originally contacted with binary carbonate layer.

151

152

Figure 5.9 XRD pattern of a mixture of Li0.35La0.55TiO3, Li2CO3, K2CO3, and Na2CO3 after heating at 390°C for 30 min in air (▼:Li0.33La0.557TiO3, ●: Li2CO3, ○: Na2CO3, and ■: K2CO3).

153

154

Figure 5.11 The change of percent Nernstian behavior for the Li0.35La0.55TiO3 based CO2 sensor with Li2CO3 sensing electrode fabricated at 500°C for 2 h in air (○), with Li2CO3-K2CO3-Na2CO3 ternary sensing electrode fabricated at 390°C for 2 h in air (■ and▲); tested in 21% O2/N2 background.

155

Figure 5.12 Schematic of gas paths in (a) thick and (b) thin Li2CO3 sensing layer.

Figure 5.13 Schematic for Li2CO3 film morphology based on fabrication methods: (a) Li2CO3 powder paste and (b) LiOH solution coating.

156

Figure 5.14 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with Li2CO3 sensing electrode which was fabricated from LiOH solution heat treated at 500°C for 2 h in 4000 ppm CO2.

(a) (b)

158

(a) (b)

Figure 5.19 Ionic conductivity (a) parallel and (b) perpendicular to (002) of Li3PO4- doped Li2CO3 [128]. 159

Figure 5.20 CO2 sensing time traces of Li0.35La0.55TiO3 sensor with 9 mol % Li3PO4 doped Li2CO3 sensing electrode heat treated at 650°C for 2 h in 99% CO2.

Figure 5.21 Comparison of Percent Nernstian behaviors of the sensor with Li2CO3 sensing electrode (■) and with 9 mol % Li3PO4 doped Li2CO3 sensing electrode (▼). 160

161

Chapter 6

Conclusions and Future Work

New solid-electrolyte-based potentiometric CO2 gas sensors were developed with

Li0.35La0.55TiO3 solid-electrolyte, Li2CO3 sensing and Li2TiO3+TiO2 reference electrodes.

The solid-electrolyte and electrode materials were prepared by conventional ceramic processes. The deterioration of the performance of the sensor under specific conditions motivated author to explore the interfaces between the solid-electrolyte and the electrodes, and the equilibrium between the sensing electrode and CO2. The following observations have been made.

6.1 Summary of key results

1. The optimum conditions were established to fabricate the solid-electrolyte with

Li0.35La0.55TiO3 composition without physical distortion and phase segregation after synthesis; calcinations of the mixture of raw materials at 1100°C for 12 h with 5°C/min ramp rate and sintering of the green pellets at 1200°C for 12 h with 3°C/min ramp rate.

2. The fabricated Li0.35La0.55TiO3 pellet with the optimum conditions was identified as a perovskite Li0.33La0.557TiO3 having P4/mmm tetragonal structure, which is the result 162

of double stacking of cubic perovskite cell along the c-axis with ordering of La3+, Li+, and vacancies. The optimum conditions produced the Li0.35La0.55TiO3 pellet with 94% of theoretical density and 1.5-1.6 µm average grain size.

3. Electrical properties of the sintered Li0.35La0.55TiO3 pellet were investigated by means of impedance spectroscopy and Hebb-Wagner polarization method. At room temperature, the grain and the grain boundary conductivities were determined to be

1.35×10-4 S/cm and 2.69×10-6 S/cm, respectively. The activation energy for grain boundary conduction was calculated as 0.41 eV in the temperature range from 25 to

300°C while one for grain conduction depended on temperature: 0.35 eV for

25°C

Li0.35La0.55TiO3, was determined to be practically a pure ionic conductor for the temperature range of 300 to 500°C.

4. The reaction temperature between Li2CO3 sensing electrode and Li0.35La0.55TiO3 electrolyte depended on CO2 partial pressure. As the level of CO2 decreases, the reaction temperature decreases as well. This is due to Li2CO3 decomposition in low CO2 partial pressure; thermodynamic calculation showed the decomposition temperature of Li2CO3 can be lowered depending on CO2 concentration. Experimental evidence confirmed the reaction at 500°C in ppb level of CO2 but the author believes that the reaction is also possible at 475°C based on the sensor degradation temperature. 163

5. As a result of the Li2CO3-Li0.35La0.55TiO3 reaction, the formation of new phase was confirmed. A hypothesis was proposed and was validated by experiments. The decomposition of Li2CO3 can promote the insertion of lithium ion into Li0.35La0.55TiO3 structure. The insertion increases Li/La ratio in the structure and initiates TiO6 octahedra tilting. Due to the tilting, the electrolyte eventually loses its crystal symmetry and the lost symmetry is resulted in the reduction of the diffraction peaks in XRD data.

6. The lithium excess LLTO can transform into other structure depending on temperature or reaction time. At 700°C-2 h or 650°C-2 h in air, a new phase formed from the Li2CO3-Li0.35La0.55TiO3 reaction and was identified as LaLi1/3Ti2/3O3, which has

AB1/3B2/3O3 structure At 500°C-55 h in 21% O2/N2, the diffraction peaks of the lithium excess LLTO were placed in between Li0.33La0.557TiO3 and LaLi1/3Ti2/3O3.

7. The solid-electrolyte was stable with Li2TiO3 even at 700°C while

Li0.35La0.55TiO3 reacted with TiO2 (anatase) at that temperature. At 650°C, the reference materials, Li2TiO3 and TiO2, did not experience any chemical reaction with

Li0.35La0.55TiO3.

8. In the presence of CO2 and water vapor, the formation of Li2CO3 was found in

Li0.35La0.55TiO3 at 50°C. Without water vapor, the solid-electrolyte was stable in 4000 ppm CO2 up to 500°C.

164

9. The reaction between Li2CO3 and Li0.35La0.55TiO3 deteriorates the performance of

Li0.35La0.55TiO3-based potentiometric CO2 gas sensors. As a result of the reaction, the sensors resulted in poor Nernstian behavior and poor reproducibility. According to our experimental results, the optimum sensor fabrication conditions in air for Li2CO3 sensing electrode and Li2TiO3+TiO2 reference electrode were found as 500°C – 2h and 650°C –

2h, respectively.

10. From a sensor fabrication point of view, the introduction of pure or high concentration of CO2 can increase the fabrication temperature of Li2CO3 electrode up to

650°C without any degradation of the sensor performance. High temperature fabrication can form better bonding with the solid-electrolyte and eventually can enhance the durability of the sensor.

11. For practical application of new sensors, as long as CO2 is present in the measuring environment, the sensor performs well. For example this sensor can be used to monitor fluctuations in CO2 in the ambient atmosphere in the temperature range of 250 and 450°C. This will hold true for measuring changes of CO2 in combustion streams.

165

6.2 Future work

1. From SEM analysis, it was difficult to determine the composition of the precipitates on the Li0.35La0.55TiO3-electrolyte after heating Li2CO3 at 700°C – 2h in air.

This was due to the low resolution of the equipment itself. Transmission electron microscope (TEM) on the precipitates, therefore, is recommended to investigate the composition of the precipitates; the sample preparation in cross-section will be proper for the analysis.

2. The Li0.35La0.55TiO3-based potentiometric CO2 gas sensors showed good performance from 250 to 450°C but some parameters for the gas sensors has not been specified such as selectivity and humidity interference. It is, therefore, suggested to investigate the selectivity in the presence of other gases such as CO and the humidity interference in the temperature range from 250 to 450°C.

3. Below 250°C, the sensors with Li2CO3 sensing electrode showed poor performances in terms of deviation from the Nernstian behavior. For practical applications, it may not be important whether CO2 sensing responses of sensors follow

Nernstian behavior or not as long as the EMF shows linear-dependence on CO2 partial pressure change. The CO2 sensing electrochemical reaction seemed to be sluggish below

250°C. It is, therefore, recommended to investigate the CO2 sensing mechanism in dry atmosphere and develop catalytic electrode to promote low temperature CO2 gas sensing.

166

4. In order to improve CO2 sensing performance of the Li0.35La0.55TiO3-electrolyte with Li2CO3 electrode, the sensing material or the sensing electrode fabrication method were modified. Preliminary data showed low temperature CO2 sensing (T ≤ 250°C) as well as faster response time at around 350-400°C. However, there is reproducibility problem and a systematic research has to be conducted. One can investigate the effect of modification more extensively from the preliminary data.

167

Symbols33

Greek  Electrical potential. ~ Chemical potential, inclusive of any q term.  Particle | molar chemical potential, exclusive of any q term.

Special | Indicates an interface between, or composite of different phases; it can be omitted for variables where the interfacial nature is evident.

Abbreviations, Chemical Symbols, Constants, Functions, and Operators s Dense crystalline, amorphous or quasi-homogeneous solid. g Gas phase. e Normal metal mobile electron. F Faraday’s constant. a Activity. R Gas constant.

Roman Variables l Mobile species. z Charge of ionic or electronic species, expressed in unit of q el . T Temperature. E0 EMF of the cell in standard conditions. P Gas pressure

33 Symbols in this paper follow “Guidelines for Technical Writing” by W.L. Chriman, J.R. Pepperney, and H. Verweij (Oct-2006). 168

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