Study of Reversible Electrode Reaction and Mixed Ionic And

STUDY OF REVERSIBLE ELECTRODE REACTION AND MIXED IONIC AND

ELECTRONIC CONDUCTION OF LITHIUM PHOSPHATE ELECTROLYTE FOR

AN ELECTROCHEMICAL CO2 GAS SENSOR

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate

School of The Ohio State University

Chong-Hoon Lee, M.S.

The Ohio State University

Dissertation Committee:

Professor Sheikh A. Akbar, Advisor Approved by

Professor Gerald S. Frankel

Professor Henk Verweij Advisor

Professor Charles E. Albright Dept. of Materials Science & Engineering ABSTRACT

An electrochemical CO2 gas sensor with lithium ion conductor was developed and characterized in order to examine the potential for real-life applications and understand

its sensing mechanism. Li2CO3 and Li2TiO3+TiO2 mixture were used as a sensing and a reference auxiliary phase, respectively. This electrochemical cell with a solid state

Li3PO4 electrolyte has shown good selectivity, sensitivity and linear response in

laboratory and automobile exhaust tests. However, the sensor response to CO2 gas showed a systematic deviation from the Nernst equation. Measured EMF did not agree with that calculated from the Nernst equation, even though it followed logarithmic behavior. Moreover, high sensitivity was observed for high CO2 concentrations (5~50%), compared to that for concentrations (500~5000 ppm). Two possible reasons for this deviation are: (1) reversibility of electrode reaction and (2) mixed ionic and electronic conduction of the electrolyte. Unless electrode reaction is fast enough, electrode polarization can easily induce overpotential. Pure ionic conduction of electrolyte is also necessary to avoid EMF loss during open circuit potential measurement.

EIS (Electrochemical Impedance Spectroscopy) was used to study electrode kinetics.

We found that Li2TiO3+TiO2 mixture reference electrode reaction is sluggish showing large electrode impedance. This impedance, however, was not affected by gas concentration change. On the other hand, that at the Li2CO3 sensing electrode is relatively

small and it increased with decreased CO2 and O2 concentration. It was also observed that these electrode impedances induced the overpotential when the current flowed through the sensor. This electrode overpotential problem was minimized by mixing gold powder or porous sputtered gold electrode increasing effective reaction sites of the electrode. New electrode design improved the sensor EMF closer to the Nernstian values, however,

ii the discrepancy still remained. Moreover, at higher sensor operating temperatures (T>500°C), the sensitivity deviated even further from the Nernstian value. Therefore, the temperature dependence of the current sensor clearly indicates that the non-Nernstian behavior is not just due to non-reversible electrode reaction.

More significant effect on the non-Nernstain behavior is due to mixed ionic and

electronic conduction of Li3PO4 electrolyte. Based on the EMF measurement and a modified Nernst equation, the transference number was estimated and the conduction domain boundary separating the n-type from the ionic conduction was constructed. This calculation predicted that the sensing side Li activity would be such that the electrolyte would be a mixed conduction (electronic and ionic) domain. Hebb-Wagner (HW) DC polarization measurement also confirmed a significant n-type electronic conduction of

Li3PO4 electrolyte. The transference numbers obtained from the EMF measurement and the HW DC polarization measurement were compared and the results confirmed that the origin of the non-Nernstian sensor behavior is mainly due to the mixed conduction of

Li3PO4 electrolyte at high temperatures (>500°C).

Dedicated to my parents

iv ACKNOWLEDGMENTS

I owe this dissertation to many people. First and foremost, I thank my advisor Dr. Sheikh Akbar for his support, guidance, and encouragement, especially for his patience. He is not only an advisor but also a mentor in my graduate school life. I thank Dr. Prabir Dutta and Dr. Henk Verweij for their advice and discussion we had. Especially, I acknowledge Dr. Verweij for his suggestion in finishing this dissertation. I am also grateful to Dr. Gerald Frankel for his advice in his class, candidacy exam, and final oral exam. Sensor characterization could not have been accomplished without the help of Cameron Begg and Hendrik O. Colijn. I am really thankful for all of their sincere help and advice. My special thanks have to go to Oswaldo Figueroa, who performed the sensor test in the engine and Dr. Ramasamy Ramamoorthy for intriguing discussion. My sincere thanks are due to Center for Industrial Sensors and Measurements (CISM) staff, Jin Wang and Kathy Honest. I also received a lot of help from all of my previous CISM colleagues, Dr. Nick Szabo, Dr. Shaestagir Chowdhury, Dr. Nancy Savage, Lian Chiang, Badri Narayanan, Adnan Merhaba, Sidharth Kapileshwar, Kunal Vaed, Yumin Lu, Samuel Shian and Santi Chrisanti. I cannot forget the help and friendship of current CISM students Sehoon Yoo, Jingyu Shi, Di Yu, Matthew Mottern, and Greg Quickel. I also thank visiting scholars, Dr. Jinsung Park, Dr. Sungpil Lee, Dr. Jonghwa Moon, Dr. Chong-Ook Park for their encouragement and discussion. I would not have finished this study without friendship and encouragement of other Korean students, Wonchul Lee, Eunguk Lee, Youngho Kim, Yuchae Yun, Eunwha

v Lee, Hyungchan Kim, Huyoung Lee, Jiho Kang, Youngsuk Kim, Junho Moon, Hongjin Kim, and Jin Nam who have been almost like my family. Above all, I cannot forget to express my heartful thanks to my family in Korea.

vi VITA

September 10, 1971…………………………… Born – Seoul, Korea

1998…………………………………………… B.S. Metallurgical engineering

Hanyang University, Seoul, Korea

2000……………………………………………..M.S. Materials Science and Engineering,

The Ohio State University,

Columbus, Ohio

2000 – present……………………………………Graduate Research Associate

The Ohio State University

Columbus, Ohio

PUBLICATION

Research Publications

1. N. Szabo, C. Lee, J. Trimboli, O. Figueroa, R. Ramamoorthy, S. Midlam-Mohler, A. Soliman, H. Verweij, P. Dutta, S. Akbar, “Ceramic-based chemical sensors, probes and field-tests in automobile engines.”, J. Mater. Sci., 38(21), 4239, (2003).

2. C.O. Park, C. Lee, S.A. Akbar and J. Hwang, “The origin of oxygen dependence in a

potentiometric CO2 sensor with Li-ionconducting electrolytes” Sensors and Actuators B, 88, 53, (2003)

vii 3. C. Lee, S.A. Akbar and C.O. Park, “Potentiometric type CO2 gas sensor with lithium phosphrous oxynitride electrolyte”, Sensors and Actuators B, 80, 234, (2001)

FIELDS OF STUDY

Major Field: Materials Science and Engineering

viii TABLE OF CONTENTS

Abstract...... ii

Dedication ...... iv

Acknowledgments...... v

Vita...... vii

List of Tables ...... xi

List of Figures...... xii

Chapters:

1. Introduction ...... 1

1.1 CO2 gas property and CO2 sensor application ...... 2 1.1.1 Physical properties of CO2 gas ...... 2 1.1.2 Biochemical properties of CO2 gas...... 3 1.1.3 Chemical properties of CO2 gas...... 4 1.2 CO2 gas sensors in the market and the literature ...... 5

2. Electrochemical CO2 gas sensor ...... 12

2.1 Solid state electrochemical CO2 gas sensors: Literature Review ...... 12 2.1.1 Type I sensor...... 15 2.1.2 Type II sensor ...... 16 2.1.3 Type III sensor ...... 18 2.1.4 Anion Conductor-based CO2 sensor ...... 24 2.2 Experimental...... 25 2.2.1 Sensor Fabrication ...... 26 2.2.2 Sensor Characterization ...... 27 2.2.3 Sensing Measurements...... 27 2.3 Results and Discussion ...... 28 2.3.1 CO2 sensor test in the lab...... 28 ix 2.3.2 Sensor test in automobile engine ...... 37 2.4 Summary...... 38

3. Reversibility for sensor electrodes...... 65

3.1 Reversible electrochemical reactrion...... 65 3.1.1 EIS (Electrochemical Impedance Spectroscopy) for electrode kinetic study...... 66 3.2 Experimental...... 74 3.2.1 Solartron 1260A Impedance Analyzer...... 74 3.2.2 Sample preparation ...... 75 3.2.3 EIS measurement ...... 76 3.3 Results and Discussion ...... 77 3.3.1 Sensor test with modified gold electrode...... 77 3.3.2 Impedance spectroscopy of sensor electrode materials ...... 78 3.4 Summary...... 83

4. The effect of mixed ionic and electronic conduction in the electrolyte to CO2 gas sensor ...... 105

4.1 Measurement of partial electronic or ionic conduction ...... 106 4.1.1 Conduction domain...... 106 4.1.2 Experimental Method to verify the Transference Number for a MIEC ...... 109 4.2 Experimental...... 114 4.3 Results and Discussion ...... 115 4.3.1 Total electrical conductivity measurement for Li3PO4 electrolyte ...... 115 4.3.2 EMF measurement...... 117 4.3.3 Hebb-Wagner (HW) Polarization Method...... 121 4.4 Summary...... 123

5. Conclusions and scope for future research ...... 135

Bibliography ...... 138

x LIST OF TABLES

Table Page

2.1 Solid state electrochemical CO2 gas sensors reported in the literature...... 14

2.2 Fitting equations for measured EMF vs. calculated EMF based on Nernst equation between 500 ppm and 5000 ppm CO2 concentration...... 30

2.3 Fitting equations for measured EMF vs. calculated EMF based on Nernst equation between 5% and 50% CO2 concentration...... 30

2.4 Standard formation energy of Li2CO3, TiO2, Li2TiO3 and CO2 at different temperatures [50]...... 31

2.5 IR drop and overpotential calculations based on open circuit potential measuring current and sensor impedance at 500 ppm CO2...... 32

2.6 Sensor response times when CO2 concentration was changed from 500 ppm to 1000 ppm ...... 33

2.7 Oxygen dependence at 500 ppm CO2 concentration for 400°C, 500°C and 600°C...... 35

3.1 Values of resistance and capacitance in the equivalent circuits of the testing cells...... 79

4.1 Total Conductivity from the AC measurement with gold ion blocking electrode (sputtered gold and gold paste)...... 115

4.2 Comparison of Ea and σ0 for Li3PO4 electrolyte of present study and literature...... 116

4.3 Calculated ionic transference numbers from EMF measurement at 400, 500, and 600°C under various CO2 concentrations...... 121

4.4 Plateau current, electronic conductivity and ionic transference number calculated from HW method and EMF measurement at 400, 500 and 600°C...... 122 xi LIST OF FIGURES

Figure Page

1.1 Man-made contributions to Greenhouse effect [6] (CFC: chlorofluorocarbons)...... 10

1.2 Fossil fuel-based CO2 emissions: 1860-1982 (Marland and Rotty 1983) [10]...... 11

1.3 Global atmospheric CO2 (solid line) and projection of simulated high growth rate of fossil fuel production since 1974 (dashed line) [10]...... 11

2.1 Illustration of electrochemical equilibrium of oxygen and YSZ at triple phase boundary...... 45

2.2 (a) Schematic diagram for solid state K2SO4 type II sensor [6]. (b) Schematic diagram for fused salt K2SO4 sensor [35]...... 46

2.3 Schematic of Type III sensor structure with Li2CO3 sensing electrode, Li2TiO3+TiO2 reference electrode and Li3PO4 lithium ion selective solid electrolyte...... 47

2.4 (a) Gold electrode design for fast ion conducting auxiliary phase. (b) Gold electrode design for non ion conducting auxiliary phase...... 48

2.5 Schematic of electrochemical, chemical, and electrical potential profile at equilibrium in the type III sensor structure...... 49

2.6 Type III sensor design with lithium phosphate lithium ion conductor...... 50

2.7 Planar type sensor design...... 50

2.8 XRD spectra for Li3PO4 +SiO2 5 mol% before sintering...... 51

2.9 XRD spectra for Li3PO4 +SiO2 5 mol% after sintering...... 52

2.10 SEM photo of Li3PO4+SiO2 5 mol%...... 53

xii 2.11 SEM photo of Li3PO4+SiO2 5 mol% ...... 53

2.12 SEM photo of Li2CO3 sensing electrode surface...... 54

2.13 SEM photo of Li2CO3 sensing electrode surface...... 54

2.14 SEM photo of Li2TiO3+TiO2 reference electrode surface...... 55

2.15 SEM photo of Li2TiO3+TiO2 reference electrode surface...... 55

2.16 Schematic of the sensor test assembly...... 56

2.17 Gas flow meter and tube furnace...... 56

2.18 Aging profile for sensor before sensing test...... 57

2.19 Typical sensing test data from 500 ppm CO2 to 50 % CO2 between 400°C and 600°C...... 57

2.20 Comparison of measured EMF and theoretically calculated EMF at 500°C, 550°C and 600°C...... 58

2.21 Comparison of measured EMF and theoretically calculated EMF at 400°C and 500°C...... 58

2.22 EMF comparisons of different internal impedance of multimeter at different temperatures...... 59

2.23 Typical sensor response time at 550°C...... 59

2.24 The time dependence of sensor EMF...... 60

2.25 Sensor EMFs from different 6 sensors...... 60

2.26 Oxygen dependence under 500 ppm and 3000 ppm CO2 at 400°C...... 61

2.27 Humidity interference during the gas concentration change from 500 ppm to 5000 ppm CO2 at 600°C. Humid gas was introduced between on and off...... 61

2.28 Humidity interference during the gas concentration change from 5% to 50% CO2 at 600°C. Humid gas was introduced between on and off...... 62

2.29 Measured EMF of the sensor vs. theoretically calculated EMF as a function of temperature for various CO2 partial pressures...... 62

xiii 2.30 Sensor test in Fiat diesel engine. Horiba commercial gas analyzer was used to verify CO2 concentration. (from Oswaldo Figueroa)...... 63

2.31 Sensor test in Fiat diesel engine for three different days. (from Oswaldo Figueroa)...... 63

2.32 Measured EMF with the PID controlled micro-furnace in the platform before and after engine test. (from Oswaldo Figueroa) ...... 64

3.1 Randles circuits for (A) a ideally polarizable electrode and (B) a non-polarizable electrode (Reversible electrode) [2]...... 88

3.2 Current-Voltage responses of (A) a polarizable electrode and (B) a non-polarizable electrode (Reversible electrode) [2]...... 88

3.3 The impedance Z plotted as a planar vector in the rectangular and polar coordinates [3]...... 89

3.4 General semicircles of impedance elements contributing overpotentials of solid state electrochemical cell in the impedance plane [6]...... 89

3.5 (A) Model A (Randles equivalent circuit) and (B) schematic impedance plot...... 90

3.6 Model B (Warburg Impedance in series with Rct)...... 90

3.7 Finite Warburg impedance in the intercalation system [3]...... 91

3.8 Infinite Warburg impedance in gas diffusion electrode [3]...... 91

3.9 Model C (Warburg Impedance in series with Randle circuit)...... 92

3.10 Schematic of transfer function analyzer [3]...... 92

3.11 Back scattered SEM photo of Li2TiO3+TiO2 with gold powder mixture (white: gold, dark:Li2TiO3+TiO2) (A) 400X (B) 1600X...... 93

3.12 Back scattered SEM photo of Li2CO3 with gold powder mixture. (A) Top of the Li2CO3 with gold powder electrode (faced to gas) (B) Bottom of the Li2CO3 with gold powder electrode (faced to electrolyte) ...... 94

3.13 Schematic of two different types of particle mixture...... 95

3.14 EMF comparison between sensors with and without gold powder...... 96

xiv 3.15 Backscattered SEM photo of sputtered gold on top of Li3PO4 electrolyte (white: gold, black: Li3PO4 electrolyte)...... 96

3.16 EMF comparison between sensors with Au paste and sputtered gold electrodes...... 97

3.17 Impedance spectra for different electrodes at 500°C...... 97

3.18 Impedance spectra for the cell-LC under different CO2 concentrations at 500°C...... 98

3.19 Impedance spectra for the cell-LT under different CO2 and O2 concentrations at 500°C...... 98

3.20 Impedance spectra for the cell-LT with and without gold powder at 500°C...... 99

3.21 Impedance spectra for the cell-LC with gold powder at 500°C...... 99

3.22 Impedance spectra for the cell-LT (without Au) and the cell-LT with gold powder and the cell-LT with sputtered gold at 500 ppm CO2 and 10% O2 at 500°C...... 100

3.23 Impedance spectra for the cell-LT (without Au) and the cell-LT with gold powder and the cell-LT with sputtered gold at 500 ppm CO2 and 10% O2 at 600°C...... 100

3.24 Impedance spectra for the cell-LC with sputtered gold under various CO2 and O2 concentrations at 400°C...... 101

3.27 Impedance spectra for the cell-LC with sputtered gold under various CO2 and O2 concentrations (5%~50% CO2) at 600°C...... 103

3.28 I-V curve under 5000 ppm CO2 and 10% O2 at 500°C...... 104

4.1 Schematic representation of partial electrical conductivity behavior for MaXb solid electrolyte [16]...... 128

4.2 Schematic representation of log σ surfaces over log PX2, 1/T space [13]...... 128

xv 4.3 Logarithmic I vs. V curve for AgI. (○) and (X) have the same indications to that of figure 3.1 [24]...... 129

4.4 I vs. V curve of AgBr. (○) indicates points taken with increasing and (X) with decreasing potential [24]...... 129

4.5 Transference number measurement by the Tubandt’s method [28]...... 130

4.6 Typical impedance plot for gold ion blocking electrode for Li3PO4 in air at 500°C...... 130

4.7 Impedance plot of gold ion blocking electrode for Li3PO4 under different gas environments...... 131

4.8 Arrhenius plot of the total conductivity of Li3PO4 electrolyte with gold ion blocking electrode...... 131

4.9 The electron conduction parameter boundary calculated from the measured EMF for various concentrations of CO2 at 400, 500 and 600°C...... 132

4.10 The sensitivity of CO2 sensing electrochemical cell measured using various Li-ion conducting electrolytes: LIPON (Li2.88PO3.73N0.14), Li3PO4+SiO2 (5 m/o) and Li3PO4+TiO2 (5 m/o)...... 132

4.11 The electron conduction parameter boundary calculated with different conditions for various concentrations of CO2 at 400, 500 and 600°C...... 133

4.12 A typical time response of the current in HW measurement for Li3PO4...... 133

4.13 HW curve for Li3PO4 : steady-state current as a function of the applied voltage at 500°C...... 134

4.14 HW curve for Li3PO4 : steady-state current as a function of the applied voltage at 600°C...... 134

xvi CHAPTER 1

INTRODUCTION

The development of chemical gas sensors is required due to concern over environmental pollution, flammable gases, efficiency in combustion and optimum control of industrial processes related to gas manufacturing [1-4]. The essential features utilizing the ceramic technology for this aim are much cheaper manufacturing cost, smaller devices than those of traditional analytical instruments with reasonable accuracy, and long term-stability [4].

Reliable CO2 gas sensors are needed for monitoring the environment, air quality in buildings, corrosion rate in chemical processing, carbonation of concrete and in modified atmospheres packaging (MAP) of food.

Solid state electrochemical sensors have attracted a lot of attention because of their simple design, low-cost manufacturing and potential for miniaturization. This dissertation focuses on the study of an electrochemical CO2 gas sensor based on a lithium ion conducting electrolyte with an emphasis on sensing performance and mechanism. Chapter 1 introduces the general properties of carbon dioxide gas with relevant applications and the available real life sensors in the market and literature. In Chapter 2 attention will be focused on sensor fabrication, mechanism, sensing characteristic and test results in the laboratory as well as in an automotive engine. Chapter 3 deals with electrical measurements, especially EIS, to study reversible electrode behavior. Mixed ionic and electronic conduction in lithium phosphate electrolyte is discussed in chapter 4.

1 1.1 CO2 gas property and CO2 sensor application

CO2 gas is the major carbon containing species in the troposphere; its partial pressure is about 300~340 ppm [5] and it is the fourth dominant gas species ranked next to Argon in the atmosphere. Carbon dioxide was discovered from the observation of a substance besides an ash after burning of charcoal and recorded by J. B. van Helmont at the beginning of 17th century [5].

The interest in CO2 gas has been growing because it plays an important role in affecting Earth’s climate and solar radiation balance as a greenhouse gas contributing to global warming, as evidenced from Figure 1.1 [6]. Even though there is little doubt of the

CO2 effect on global warming, it is important to monitor atmospheric CO2 concentration to understand its impact on climatic change.

1.1.1 Physical properties of CO2 gas

The triatomic molecule, CO2 has vibrational and rotational energy transitions that lead to absorption in the IR region like H2O, and O3 [7]. At a number of wavelengths the high

concentrations of water vapor and CO2 almost completely absorb the radiation emitted by Earth’s surface before it can be lost to space, thereby increasing the global average surface temperature by more than 30°C above the nominal temperature that would occur in the absence of such trace gases [7, 8].

Carbon dioxide is a natural species in the atmosphere and is also produced by activities such as the burning of fossil fuels for energy and the change of land-use. With the development of transportation and the machinery for industrial production, global energy use was increased by more than 10-fold from 770 million metric tons (1015) of coal equivalent (mmtce)* in 1900 to more than 9000 mmtce in 1984 [9]. The increase of fossil fuel release between 1860 and 1982 is shown in Figure 1.2 [10]. This tremendous

* To express emissions of different gases in a comparable way, atmospheric chemists often use a weighting factor called global warming potential. The heat-trapping ability of one metric ton (1,000 kilograms) of CO2 is taken as the standard, and emissions may be expressed in terms of metric tons of CO2 equivalent (abbreviated MTCDE). More commonly, emissions are expressed in terms of metric tons of carbon equivalent (MTCE). Carbon comprises 12/44 of the mass of carbon dioxide; thus to convert from CO2 equivalent to C equivalent, one multiplies by 12/44. Throughout this database, we use units of MTCE or million MTCE (MMTCE).

2 increase in energy consumption is affecting the change of content that composes the Earth’s atmosphere gases controlling global climate.

As can be seen in Figure 1.3, data collected in the Mauna Loa climate observatory in Hawaii at an altitude of about 11,000 feet revealed a systematic increase in atmospheric carbon dioxide [10]. It is known that a doubling of the CO2 concentration would raise the

average temperature by 5 or 6°C. Also, there is a hypothesis that the depletion of the CO2 concentration may have brought glacial periods. It was reported that the atmospheric CO2 concentration in 1900 was approximately 300 parts per million by volume (ppm) and it continuously increased showing the annual average concentration of 316 ppm in 1958 and 345 ppm in 1985. It is very clear that increasing CO2 concentrations have the possibility for significant impacts on global climate, even if these studies have not yet provided adequate evidence for the fundamental relationships between the benefits and impacts of various energy systems on society's activities [5].

Therefore, there is a need to develop the tools for monitoring CO2. Environmental monitoring is crucial for identification of the most dangerous sources of air pollution and for assessment of the effectiveness of anti-pollution measures in different environments, such as urban areas, technology lines and the exit of industrial gases to the atmosphere.

Hydrothermal circulation at the mid-ocean ridge is one of the fundamental process controlling the transfer of energy and matter from the interior of the Earth to the lithosphere, hydrosphere, and biosphere [11]. Hydrothermal interactions influence the composition of the oceanic crust and the chemistry of the oceans [11]. In addition, hydrothermal vent fields support diverse and unique biological communities by means of microbial populations that link the transfer of the chemical energy of dissolved chemical

species to the production of organic carbon [11]. A CO2 sensor is much needed for this study of mid-ocean Ridge hydrothermal system.

1.1.2 Biochemical properties of CO2 gas Carbon dioxide is a product of the metabolic activity of animals and a vital substance to the life of plants [5]. Therefore, its effect on plant life has been studied. The main effects on vital processes for plants are [5]:

3 1. Stimulating plant growth

2. The indirect fertilization of plants

3. Respiration control

Carbon dioxide gas is used as a component of MAP (modified atmospheres packaging). Conventional vacuum packaging system causes a drip loss of meat and degrades its color quality. MAP packaging system prevents the drip loss problem effectively and improves the color stability of meat. In addition, decreased oxygen and

increased CO2 concentrations reduce the respiration rate of fruit. As a result, the

processed food packaging industry has an interest in CO2 concentration control [12-14].

1.1.3 Chemical properties of CO2 gas Chemically carbon dioxide is not an active species and reactions between dry carbon and other compounds and elements can, in general, be promoted only at high temperatures [5]. In aqueous solutions, however, the situation is quite different. Because

of the acid properties of CO2 solutions, many reactions take place spontaneously and some of them are of considerable geological importance [5].

CO2 corrosion is very important in the oil and natural gas industry. CO2 has been in equilibrium between the three phases, water, oil and gas for millions of years in

hydrocarbon deposits [15]. The concentrations of CO2 present in each of these phases are therefore interrelated [15]. A certain partial pressure of CO2 in an oil field effluent leads

to a proportional dissolution of CO2 in the produced water, causing a certain degree of + acidification [15]. The dissolution of CO2 in the water introduces two species, the H ion and the molecule H2CO3 [15]. Furthermore, the primary corrosion product of steel pipes

in oil and gas production becomes ferrous bicarbonate Fe(HCO3)2, which is a relatively soluble salt [15]. For the prediction of CO2 enhanced corrosion, the monitoring of CO2 partial pressure seems to be necessary.

The carbonation of concrete causes shrinkage of the surface to bring about cracking and warping with rapid drying. Carbon dioxide not only combines with the calcium hydroxide of hydrated cement, but it attacks and decomposes the major constituents into

4 calcium carbonate and hydrated silica, alumina and ferric oxide. On the other hand, the

carbonation by CO2 gas improves the strength and hardness of cement. Therefore, the

control of CO2 concentration is very important for manufacturing concrete [16-18].

1.2 CO2 gas sensors in the market and the literature The major methods used to detect combustion gases fall short of practical application needs for in-situ measurements in aggressive industrial environments involving high temperature and chemical contaminants [1]. Fourier transform infrared spectroscopy

(FTIR) is the most popular technique for CO2 detection in the market because of its

accuracy, sensitivity and selectivity. CO2 gas is a triatomic molecule showing distinct absorption bands due to rotational transition of the molecules, resulting in a characteristic FTIR spectrum [19]. The response time and detection limits of the method are very good, but size, costly maintenance, and low temperature range are the disadvantages [1].

Gas chromatography/mass spectrometry (GC/MS) uses chromatography to separate mixtures and mass spectrometry to identify components. This method has excellent detection limits, but is suitable for low temperature ranges (<573K), and is costly and

complicated to maintain [1]. The SAW (Surface Acoustic Wave) CO2 gas sensor is very sensitive to small changes in surface mass but its humidity interference has delayed further development for commercialization [19].

There have been efforts to develop chemical sensors based on solid-state technology, either based on the surface characteristics or the bulk electrolyte properties of ceramics. However, such a sensor is not commercially available due to the lack of required stability. Only an expensive optical sensor (based on infra-red spectroscopy) has been used for gas analysis. It is important to develop solid-state chemical sensors that exhibit stable performance over an extensive period of time [1-4]. Ceramic-based sensors are promising because of their robust performance and their electric signals (electric current, capacitance or potential difference) directly connected to the chemical information (e.g., concentration, activity and partial pressure).

In the case of semiconducting gas sensors, charge transfer at the solid-gas interface by

adsorption of gases is utilized. BaTiO3 [21, 22], porous hydroxyapatite ceramics [23],

5 K2CO3-polyethylene glycol solution supported on Porous Ceramics [24, 25], La2O3-

loaded SnO2, CaO-loaded In2O3 [26, 27], sputtered indium tin oxide (ITO)

polycrystalline thin films [28] and Li2ZrO3 [29] have been examined for CO2 detection.

Capacitive-type sensors have the advantages of miniaturization, high reliability, and low cost. Gas adsorption brings on a change of depletion layer thickness. Namely, the electronic interactions between the adsorbed gas and the interface of two different phases

can change the charge density in the semiconductor. BaTiO3-PbO, CuO, NiO additive

oxide [31, 33], and thin film of aluminum phosphate (AlPO4) molecular sieves [34] were

studied for capacitive type CO2 sensors. However, the sensing mechanism and surface

characteristics are not yet fully understood or theoretically explained for CO2 detection. On the other hand, solid electrolyte sensors monitor the change in chemical potential of the sensing electrode. Thus, applying a so-called ‘auxiliary phase’ in equilibrium with the target gas on the sensing electrode enables the detection of a gas with excellent sensitivity and selectivity.

In conclusion, electrochemical type principles are the most suitable for CO2 sensor application because carbon dioxide (CO2) is not a redox active but an acid-based active gas, which mainly reacts with the ions of solids rather than the electrons [35]. Therefore,

this study will exploit the electrochemical CO2 gas sensor considering practical application and sensing mechanism.

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Figure 1.1: Man-made contributions to Greenhouse effect [6]. (CFC: chlorofluorocarbons)

Figure 1.2: Fossil fuel-based CO2 emissions: 1860-1982 (Marland and Rotty 1983) [10].

Figure 1.3: Global atmospheric CO2 (solid line) and projection of simulated high growth rate of fossil fuel production since 1974 (dashed line) [10].

11 CHAPTER 2

ELECTROCHEMICAL CO2 GAS SENSOR

2.1 Solid state electrochemical CO2 gas sensors: Literature Review

Electrochemical CO2 gas sensors have been developed because of their excellent selectivity, sensitivity and stability. One of the main advantages of an electrochemical sensor is that it monitors electrical potential change instead of current or resistance. Potential is an intensive property that is not changed by geometry or mass of sensor materials. Resistance and current are not intensive properties and hence they require more accurate control for sensor fabrication when the sensor utilizes such signals.

This section discusses the basics of electrochemical sensors. First of all, the word “electrode” should be clearly defined because there is some confusion regarding this expression in the solid state electrochemical sensor literature. “Electrode” is defined as an electronic conductor that can provide or consume electrons and/or holes [1]. Following the conventional definition, only metals or semiconductors can be considered as an electrode. But it is more convenient to consider an electrode as a structure that can provide a site for charge transfer in an electrochemical cell. For example, in the YSZ- based oxygen sensor, oxygen is electrochemically equilibrated through the Pt metal electrode with oxygen vacancies in YSZ. Therefore, the gas phase is often considered as an electrode.

Three different class solid state electrochemical sensors are generally considered in the literature. One is an equilibrium potential type sensor that will be discussed in this study. The second is a mixed potential type sensor which is controlled by the different kinetics

of two electrochemical reactions on the same electrode. An NOx sensor featuring this mechanism has been developed in CISM [2, 3]. In both equilibrium and mixed potential

12 sensors, an open circuit potential is monitored. The third type known as the amperometric type sensor is operated in the polarized condition where the electrochemical reaction is controlled by mass-transfer. More selective sensing is possible with this sensor design.

Equilibrium potential solid state gas sensors have been generally classified into three broad groups [4-6]. Type I sensors have an electrolyte containing mobile ions of the chemical species in the gas phase that it is monitoring. The most successful commercial product, YSZ oxygen sensor, is an example of type I. Type II sensors do not have mobile ions of the chemical species to be sensed, but an ion related to the target gas can diffuse in the solid electrolyte to allow equilibration with the atmosphere. Therefore, type I and type II sensors have the same design with gas electrodes combined with metal and an electrolyte where oxidized or reduced ions can be electrochemically equilibrated through the cell. In the third type of electrochemical sensor, auxiliary phases are added to the electrodes to enhance the selectivity and stability. Type III sensors make the electrode concept even more confusing. With respect to the design of a solid state sensor, the auxiliary phase looks as part of the electrode. But it cannot be an electrode because auxiliary phase materials are not generally good electronic conductors. In spite of this confusion, type III design offers more feasibility in terms of designing various sensors with different auxiliary materials and electrolytes.

Table 2.1 presents summary of the solid state electrochemical CO2 gas sensors reported in the literature. Substantial work has been done in this field with sodium ion conductors. Sodium β-alumina and NASICON (Na Super Ionic Conductor) are generally accepted as good solid state ionic conductors, however, their poor resistance to humidity led to the development of other electrolytes. Lithium ion conductor looks very promising due to

fast ionic conduction and less reactivity with water. The author has developed a CO2 sensor with lithium phosphorous oxynitride electrolyte. Its fabrication method and sensing behavior was discussed in author’s M.S. thesis [32]. In this work, the author addresses the fundamental basis of such sensors.

13 Reference Authors/ Electrolyte materials Sensing Electrode Electrode Year Gauthier et K2CO3 Pt, CO2, O2 Pt, CO2, O2 al. (1977) Type II K-conductor [7, 8] Au, CO2, O2 Cote et al. K2CO3 Au, CO2, O2 CaCO3+CaO (1984) [9] Maier et al. Au, Na ZrO +ZrO , Na-β-alumina Pt, Na CO , CO , O 2 3 2 (1986, 1993) 2 3 2 2 CO , O 2 2 [10, 11]

Pt, Na2CO3/BaCO3, Yao et al. NASICON Pt, O2 (Air reference) CO2, O2 (1990) [12]

Pt, Li2CO3/MCO3, CO2, Miura et al. NASICON Pt, O2 (Air reference) O2, (M=Ca, Sr, Ba) (1993) [13] Schettler et NASICON Pt, Na2CO3, CO2, O2 NaCoO2, NaxNiO2 al. (1993) Na-conductor [14] Au, Na Ti O +TiO 2 6 13 2 Maier et al. NASICON or or Au, Na CO , CO , O (1994, 1996) Na-β-alumina 2 3 2 2 Na Ti O +Na Ti O , 2 3 7 2 6 13 [15, 16] CO2, O2 Type NASICON Pt, Na2CO3/BaCO3, Lang et al. III Pt, O2 (Air reference) (sputter) CO2, O2 (1996) [17] NASICON Lecours et al (sputter and laser Pt, Na CO , CO , O Pt, O (Air reference) 2 3 2 2 2 (1996) [18] ablation) Imanaka et Au, Li CO , CO , O , LiTi (PO ) +0.2Li PO Au, Li CO , CO , O 2 3 2 2 al. (1990) 2 4 3 3 4 2 3 2 2 CaCO +CaO 3 [19] Au, CO , O Narita et al. Li CO +Li PO +Al O Au, CO , O 2 2 2 3 3 4 2 3 2 2 (Air reference) (1995) [20] Li-conductor Pt, LiCoO2-Co3O4, Zhang et al. Li2CO3+Li3PO4+Al2O3 Au, CO2, O2 CO2, O2 (1997) [21] Au, LiMn O Salam et al. Li CO +MgO Au, CO , O 2 4 2 3 2 2 (sealed) (1999) [22] Au, LiCoO2-Co3O4, Lee et al Li2.88PO3.73N0.14 Au, Li2CO3, CO2, O2 CO2, O2 (2001) [23] Maruyama et NASICON and YSZ Au, Na2CO3, CO2, O2 Au, CO2, O2 al. (1987) [24] NASICON and YSZ Chu et al. Au, Na CO , CO , O Pt, CO , O (Thick and Thin film) 2 3 2 2 2 2 (1992) [25] Miura et al. MSZ or LaF3 Au, Li2CO3, CO2, O2 Pt, CO2, O2 (1993) [26, Hybrid sensor 27] LiTi (PO ) +0.2Li PO Imanaka et ( alkali conductor 2 4 3 3 4 and Bi O -Y O , CeO - Au, Li CO , CO , O Pt, CO , O al. (1995) +anion conductor) 2 3 2 3 2 2 3 2 2 2 2 Gd2O3, ZrO2-Ln2O3 [28] Li2CO3+Li3PO4+Al2O3 Zhang et al. Au, CO2, O2 Pt, CO2, O2 and La0.9Sr0.1MnO3 (1997) [29] Tamura et al. Sc Zr (PO ) and YSZ Au, Li CO , CO , O Pt, CO , O 1/3 2 4 3 2 3 2 2 2 2 (2001) [30] Imanaka et Sc (WO ) or 2 4 3 Au, Li CO , CO , O Pt, CO , O al. (2001) Al (WO ) and YSZ 2 3 2 2 2 2 2 4 3 [31]

Table 2.1: Solid state electrochemical CO2 gas sensors reported in the literature. 14 2.1.1 Type I sensor Although type I oxygen sensors have been very successful solid state electrochemical devices, this type of sensor cannot be extended to other gases due to the restriction that the target gas species should be the same ion as the mobile species in the ionic conductor.

Therefore, CO2 sensor with type I structure does not exist.

The oxygen sensor has a very simple structure with YSZ electrolyte and two metal electrodes separating two different oxygen partial pressures. When the chemical potential, µRef , is fixed by a reference gas, typically air, µUnknown can be determined by O2 O2 measuring the EMF. The excellent catalytic and noble properties of Pt made it the most popular electrode for YSZ based sensors.

Ref Unknown O222 , Pt | ZrO | Pt, O (2.1) In the literature, the EMF measured by the above cell is understood based on the following electrochemical reaction at the three phase boundary where Pt electrode, gas and YSZ meet [33, 34]. 1 O+2e-O 2- (2.2) 2 2 (gas ) ( Adsorbed on Pt ) 2- ii O+VO(Adsorbed on Pt) O () YSZ O () YSZ (2.3) The porous Pt electrode enhances the oxygen reduction and maximizes the equilibrium

2- •• reaction of O and VO in YSZ.

So, the half cell reaction is 1 OV2e-O++ii (2.4) 2 2 (gas ) O( YSZ ) ( Pt ) O ( YSZ )

•• where the VO concentration is determined by Y2O3 doping into ZrO2, so the chemical

•• × potentials of VO(YSZ )and OO (YSZ ) are constant in YSZ ( ∆µ=•• 0 , ∆µ = 0 ). Figure VO(YSZ ) OO (YSZ ) 2.1 is a schematic illustration of the above electrochemical reaction. If electrochemical reaction (2.2) and chemical reaction (2.3) are very fast, the electrode potential is determined by the oxygen chemical potential in the gas phase only. The EMF follows the Nernst equation.

15 RT PUnknown E = lnO2 . (2.5) OC 4F Pref O2 where EOC represents EMF.

•• The key features of this successful sensor are the role of VO(YSZ ) as a buffer in the electrolyte and the fast electrochemical reaction at the three phase boundary to achieve equilibrium potential. Also, it requires a dense YSZ to prevent gas diffusion through it to maintain two different chemical potentials of oxygen.

2.1.2 Type II sensor

The first type II sensor was invented by Gauthier et al. using K2SO4 electrolyte for the detection of SO2 gas [7, 8]. It was assumed that electrochemical reaction with K2SO4 is similar to that of fused salt K2SO4 which was studied by Salzano et al. [35]. In their experiment, the following cell was constructed and tested.

Ref Ref Unknown Unknown SO22 , O , Pt | K 24 SO | Pt, SO 2 , O 2 (2.6) + where K2SO4 acts as a K conductor. Figure 2.2 (a) shows a schematic of Gauthier’s solid state SO2 sensor design and (b) shows the fused salt sensor that was proposed by Salzano et al. Both Gauthier and Salzano et al. assumed Pt electrode is an oxygen reversible electrode. In other words, the electrode potential of type II SO2 sensor is decided by the following reactions [7]: 1 (Electrochemical reaction) O+2e-O 2- (2.7) 2 2 (gas ) ( Pt ) ( Adsorbed on Pt ) 1 (Chemical reaction) SO + O SO (2.8) 2 (gas )2 2 ( gas ) 3 ( gas ) (Chemical reaction) SO + O2- SO 2- (2.9) 3 (gas ) ( Adsorbed on Pt) 4 ( K24 SO ) (Overall electrochemical reaction) SO + O +2e- SO2- (2.10) 2 (gas ) 2 ( gas ) ( Pt ) 4 ( K24 SO ) As can be seen, K+ ion doesn’t play any role in the electrochemical reaction, but it has to move to ensure charge balance. The fused salt model has a cation permeable membrane and it can keep the electrochemical equilibrium of K+ ion between the inside and outside solutions. On the other hand, solid state design does not have the membrane. Therefore, + + K ion concentration must be fixed in K2SO4 solid state electrolyte. If K ion

16 concentration in the bulk electrolyte is high enough and it is not varied depending on electrochemical reaction, gas concentration changes of SO2 and O2 determine directly the equilibrium potential of the following half cell reaction at the three phase boundary of electrolyte, Pt and gas.

i 2K+SO+O+2e-KSOi 2 ()2gas () gas () Pt 2 4 (2.11) where interstitial potassium ions are assumed to be the dominant charge carrier for this model. When O2 concentrations on both sides are same in Figure 2.2 (a), under the equilibrium condition, the EMF of the concentration cell is given by

PPUnknown() Unknown P Unknown RRTTSO22 O SO 2 EOC =ln= ln (2.12) 2FPPRef ( Ref ) 2F P Ref SO22 O SO 2

The requirements of type II sensors are exactly same as those for type I sensors.

However, it is not easy to get a dense K2SO4 pellet, so a practical design of type II sensor is almost impossible. Some electrolytes containing metal sulfates such as Ag2SO4-K2SO4

[36] and Na2SO4-Li2SO4-Y2(SO4)3-SiO2 [37] have been studied to provide fairly good

SO2 sensing properties, however, these still have the disadvantage of limited choice of the electrolyte.

The electrochemical cell for CO2 gas with solid reference electrode was fabricated with

K2CO3 by Cote et al. [9]. A solid reference electrode, an equimolar mixture of calcium carbonate and calcium oxide, provides equilibrium P at a certain temperature CO2 according to the following reaction.

CaCO32()⇔ CaO+ CO gas (2.13)

This design simply used the above mixture reaction for the reference CO2 gas, but still it is practically complicated because the CO2 concentration, which was equilibrated with

CaCO3 and CaO, should not be affected by the outside environment. Moreover, K2CO3 electrolyte does not have high ionic conductivity, or mechanical, chemical and thermal stability [6].

Carbonate based multi-phase (heterogeneous) solid electrolyte systems (Na2CO3:

ABO3 where, A=Li/K/Ba and B=Nb/Ti) were found to be useful materials for a CO2 gas 17 sensor [38]. When ferroelectric LiNbO3 is dispersed in a Na2CO3 host matrix, two significant interfaces participating in the conduction are homo-junction (Na2CO3/

Na2CO3) and hetero-junction (Na2CO3/LiNbO3) [38, 39]. An enhanced conductivity in such a system is caused by the formation of a highly disordered diffused space charge layer along the interface facilitating the ionic conduction [38]. This Na2CO3 with dispersed LiNbO3 is a possible candidate for a type II sensor.

The type II CO2 sensor with lithium carbonate as an electrolyte was suggested based on the premise that lithium carbonate is purely a lithium ion conductor and displays fairly good conductivity [21, 40, 41]. But the type II sensor structure is not practical for commercialization because it requires sealed reference gas electrode isolated from the sensing atmosphere.

2.1.3 Type III sensor

The type III sensor with NASICON and Na2CO3 sensing auxiliary phase was first proposed by Yao et al. [12]. Maier et al. proposed ideal designs for the type III CO2 gas sensors [15, 16]. They introduced TiO2+Na2Ti6O13 or Na2Ti6O3+Na2Ti3O7 as a reference electrode for the sodium β-alumina-based CO2 sensor. The advantages of this design are two-fold. First, this sensor does not need a reference gas because reference electrodes are not reactive with CO2 gas. Secondly, this sensor can avoid oxygen dependence due to its unique overall cell reaction. However, their design was based on pressed pellet electrode, which required mechanical support to make sure that the two electrodes are well bonded to the electrolyte. This design is still far from practical sensor device. Therefore, it encouraged author’s current study of Li2TiO3+TiO2 mixture reference electrode in the form of a thick film.

2.1.3.1 Thermodynamic analysis for solid state type III sensor

Figure 2.3 shows a schematic type III sensor structure with Li2CO3 sensing electrode,

Li2TiO3+TiO2 reference electrode and Li3PO4 lithium ion selective solid electrolyte. It also represents the 6 different interfaces where thermodynamic equilibria exist. Solid electrolyte is used as a membrane which can separate two different chemical potentials of lithium ions in the auxiliary phases. First, this type III sensor will be simply interpreted

18 by using ideal fused salt model. Therefore, auxiliary phases have to be considered as solutions that have dissolved mobile ions and gas species. One of the primary + assumptions is that the concentration and mobility of CO2, O2, and Li in the electrolyte and auxiliary phases are so high that small concentration changes do not change its chemical potentials in these phases. Dominant charge carriers are assumed to be interstitial cations in the both auxiliary phases because Li2CO3 is known as a Frenkel type intrinsic defect ionic conductor [42] and cation interstitial is common defects in many lithium ion compounds.

As shown in Figure 2.4, if the auxiliary phase is a good ionic conductor, design (a) can achieve equilibrium fast. Oxygen can be easily reduced on top of the gold electrode and lithium ions are equilibrated with O2 and CO2 transporting through the auxiliary phase. An auxiliary phase with poor ionic conduction probably does not work in such a design. Such auxiliary phases need porous design (b) with gold electrode located at the interface between the electrolyte and the auxiliary phase. This design helps to minimize the distance of lithium ion diffusion in the auxiliary phase. But porous auxiliary phase is necessary for oxygen and carbon dioxide to reach the gold electrode. Based on author’s study, Li2CO3 resembles former while Li2TiO3 the later.

In this analysis, design (a) is accepted for both Li2CO3 and Li2TiO3 auxiliary phases and they are considered as Li2+δ CO 3+1 2δ and Li2+δ TiO 3+1 2δ , slightly Li2O-rich or- deficient non-stoichiometric solid solutions.

• Electron equilibrium at Cu-Au interface 1 At this interface, electron equilibrium is established and their electrical potential difference results from the difference of chemical potential of electron. This junction potential is cancelled out by the other Cu-Au interface 6, so it does not appear in the overall potential difference.

µ=e, Cu I µ e, Au I (2.14)

µe, Cu I− FΦ Cu I= µ e, Au I− FΦ Au I (2.15)

19 1 ΦΦ−−=(µ µ ) (2.16) Au I Cu IF e, Au I e, Cu I

• Oxygen reduction and oxidation equilibrium at the interface 2 Oxygen reduction and oxidation equilibrium is established at the three phase boundary

(Au, Li2+δ CO 3+1 2δ , and Gas).

Li CO =( 2+δ )Li+ + CO2- (2.17) 2+δ 3+1 2δδ Li2+δδ CO 3+1 2 3+1 2 (2+δ ) CO2- = CO + O + (2+δ )e- (2.18) 3+1 2δ 24 2 (2+δ ) Li CO =( 2+δ )Li+ +CO + O + (2+δ )e- (2.19) 2+δδ 3+1 2 Li2+δδ CO 3+1 2 24 2

(2+ δ ) µ=+δµ+µ+µ++δµLi CO(2 )+ CO O (2 ) e (2.20) 2+δδ 3+1 2Li , Li2+δδ CO 3+1 2 24 2

µ=+δ⋅µ+Li CO (2 ) (++ RTa ln +Φ FLi CO ) 2+δδ 3+1 2 Li , Li2+δδ CO 3+1 2 Li , Li 2+ δδ CO 3+1 2 2+δδ 3+1 2 (2+δ ) (2.21) +µ(ln) +PTP + Rln(2)(F) + +δ⋅µ − Φ CO22 CO4 O 2 e, Au I Au I

-(µo +RTa ln ) Li++ , Li CO Li , Li CO ΦΦ− = 2+δ 3+1 2δ 2+δ 3+1 2δ Li2+δ CO3+1 2δ Au I F 1 (2.22) Rln+TP µ oo Oe,2 Au I(µLi CO−− µ CO lnP CO ) −+4 2+δ 3+1 2δ 22 F(2)F+δ

• Lithium ion exchange equilibrium at the interface 3 Junction potential due to a different chemical potential of lithium ion between

Li2+δ CO 3+1 2δ and Li3PO4 is established at this interface. Therefore, the sum of this junction potential and the other junction potential at the interface between Li3PO4 and

Li2+δ TiO 3+1 2δ becomes membrane potential between the two auxiliary phases.

µ=µ++ (2.23) Li , Li2+δ CO3+1 2δ Li , Li34 PO

20 µ+++RlnTa +Φ F Li , Li CO Li , Li CO Li2+δδ CO 3+1 2 2+δδ 3+1 2 2+ δδ 3+1 2 (2.24) o =µ++ +RTa ln +FΦLi PO Li , Li34 PO Li , Li 34 PO 34

Φ −Φ Li3 PO 4 Li 2+δδ CO 3+1 2 (2.25) 1 o =µ(µRlnRln)++ − +Ta + − Ta + F Li , Li2+δδ CO 3+1 2 Li , Li 3 PO 4 Li , Li 2+ δδ CO 3+1 2 Li , Li 3 PO 4

• Lithium ion exchange equilibrium at the interface 4

µµ++= (2.26) Li , Li34 PO Li , Li 2+δ TiO3+1 2δ

o µ+Rln+F++Ta Φ Li , Li PO Li , Li PO Li34 PO 34 34 (2.27) =µ++ +RlnTa + F ΦLi TiO Li , Li2+δδ TiO 3+1 2 Li , Li 2+ δδ TiO 3+1 2 2+δ 3+1 2δ

Φ−Φ Li2+δδ TiO 3+1 2 Li 3 PO 4 (2.28) 1 o =−µ+(µ++ RTa ln + − R Ta ln + ) F Li , Li3 PO 4 Li , Li 2+δδ TiO 3+1 2 Li , Li 3 PO 4 Li , Li 2+ δδ CO 3+1 2

• Oxygen reduction and oxidation equilibrium at the interface 5 As was discussed for the interface 2, same type of oxygen reduction and oxidation equilibrium exists at this interface (Au, Li2+δ TiO 3+1 2δ , and Gas). (2+δ ) (2+δ )Li+ + O2- + TiO = Li TiO (2.29) Li2+δδ TiO 3+1 2 2 2 2+δ 3+1 2δ (2+)δ (2+)δ O=2- O(2+)e-+ δ (2.30) 242 (2+δ ) (2+δδ )⋅⋅⋅ Li+ + O + TiO + (2+ ) e- = Li TiO (2.31) Li2+δδ TiO 3+1 2 4 2 2 2+δ 3+1 2δ

(2+δ ) ⋅ ( µ++ + RTa ln + F ΦLi TiO ) Li , Li2+δδ TiO 3+1 2 Li , Li 2+ δδ TiO 3+1 2 2+δδ 3+1 2 2+δ (2.32) +µo + RTP ln +(2+δ)(µ⋅=µ -FΦ ) TiO24 O 2 e, Au I Au I Li 2+δδ TiO 3+1 2

21 (µo +RTa ln ) Li++ , Li TiO Li , Li TiO ΦΦ− = 2+δ 3+1 2δ 2+δ 3+1 2δ Au II Li2+δ CO3+1 2δ F 1 (2.33) Rln+TP µ oo Oe,2 Au IIµµTiO− Li TiO ++4 22+δ 3+1 2δ F(2)F+δ

• Electron equilibrium at Cu-Au interface 6

µeAu, =µ eCuII, (2.34)

µ−Φ=µ−Φe, Au F Au e, Cu II F Cu II (2.35)

1 Φ−Φ=µ() −µ (2.36) Au II Cu IIF e , Au II e, Cu II

• Measured EMF Overall EMF is obtained from the difference of electrochemical potential of Cu I and II wire, which is not equilibrated in this system. High impedance voltmeter can measure it and the theoretical EMF can be calculated from the sum of all the junction potentials.

FV =µ−µ=µ−Φ−µΦe, Cu I e, Cu II e, Cu I F Cu I e, Cu II + F Cu II (2.37)

V = Φ−Φ= ( ΦΦ −)( +ΦΦ − ) Cu II Cu I Au I Cu I Li2+δ CO3+1 2δ Au I +−Φ+Φ−(Φ )( Φ ) (2.38) Li3 PO 4 Li 2+δδ CO 3+1 2 Li 2+ δ TiO 3+1 2 δ Li 3 PO 4 +−( ΦΦ )( +Φ−Φ ) Au II Li2+δ CO3+1 2δ Cu II Au II

oooo (µLi CO +µ TiO−− µ Li TiO µ CO ) RT =−2+δ 3+1 2δ 22+δ 3+1 2δ 2 ln P (2++δδ )F (2 )F CO2 (2.39) (µooo +µ−− µ µ o ) RT ≅−Li23 CO TiO 2 Li 2 TiO 3 CO 2 ln P 2F 2F CO2

The ideal behavior of a type III sensor is understood based on the schematic of electrochemical, chemical, and electrical potential profile relevant to all the above equilibria of the type III sensor structure as shown in Figure 2.5. In this figure, electrochemical potential of lithium ion is equilibrated ( ∆µ= 0 ), but electrochemical Li+

potential of electron is not equilibrated ( ∆µ≠ e- 0 ) because ideally the electrolyte is a 22 perfect ionic conductor (ti=1) in this model. It was observed from the experimentally measured EMF that electrochemical potential of Cu I is higher than that of Cu II under various CO2 concentration. Therefore, this diagram is constructed using this fact. Electrochemical potentials are located in the highest level to show that the electrochemical potential is the sum of chemical potential and electrical potential. The solid lines represent fixed potentials in this system and broken lines variable ones. Chemical potential of oxygen is a variable but both sides have the same oxygen partial pressure so that it does not appear in the overall electrochemical equilibrium. Fixed chemical potential of lithium carbonate, titania and lithium titanate are located in the order of their standard state chemical potential. If the system is in perfect equilibrium, overall EMF is only dependent of CO2 partial pressure.

From the thermodynamic equilibrium analysis, the overall EMF has two different kinds of potentials: reduction potential and membrane potential. Reduction potential is defined based on the electrochemical reaction in each auxiliary phase. On the other hand, membrane potential is due to the chemical potential difference of the transporting species (Li+ ions) in the two auxiliary phases. Similar to type I and type II sensors, the reduction potential is considered as an oxygen reduction potential. Therefore, it is reasonable to think oxygen reduction is a charge transfer limiting step in the auxiliary phases.

The existence of two different potentials can induce two different potential changes depending on CO2 concentrations. In other words, CO2 concentration can affect oxygen reduction potential, or lithium ion activity changing membrane potential, or both. The activity of lithium oxide is fixed by the reaction of Li2TiO3+TiO2 mixture at the reference electrode. If oxygen partial pressure is not changed, there is no potential change in this electrode. On the other hand, when CO2 concentration is changed, it affects the a in Li2 O

Li2CO3. Therefore, if lithium ion activity is fixed in the lithium carbonate phase, only oxygen reduction potential is changed.

Second possibility can occur when lithium ion activity is not a constant. The change of a causes the variation of chemical potential of lithium ion and the membrane potential Li2 O is affected. In the third case, CO2 influences both the oxygen reduction potential and the

23 junction potential, but the total amount of varied potential remains the same to reach thermodynamic equilibrium. Therefore, theoretical EMF is not dependent of these three possibilities in the type III sensor structure. This characteristic can provide more stable

EMF measurement of type III than that of type II. Type II sensing mechanism where CO2 change is directly related to electrolyte has more chances to lose sensor stability when cation activity is affected by CO2 concentration. It means high mobile ion concentration of electrolyte is more critical for type II sensor than for type III.

2.1.4 Anion Conductor-based CO2 sensor The EMF signal of sensors using carbonates as sensing electrodes always depends on not only the CO2 partial pressure but also the oxygen partial pressure due to the carbonate dissociation reaction. It was already discussed in the type III design that certain reference electrode such as Li2TiO3+TiO2 can eliminate oxygen dependence.

Another approach is where the oxygen conductor is combined with alkali metal carbonate phases as a reference to remove the oxygen dependence. Lithium carbonate was investigated in such a design with MSZ (Magnesium stabilized zirconia) [26, 27].

O22 , CO , Au | Li 23 CO | YSZ (or MSZ) | Au, CO 22 , O (2.40)

Miura et al. proposed [26] that a new phase of Li2ZrO3 as an ionic bridge is formed at the interface between Li2CO3 and zirconia according to the following reaction

Li23 CO + ZrO 2 = Li 2 ZrO 3 + CO 2 ()gas (2.41)

However, this oxygen conducting open-reference system needs more investigations because Näfe indicated that it requires a very dense Li2CO3 layer, otherwise the sensing electrode is also exposed to the same oxygen partial pressure as that of the reference electrode. So this sensor cannot work as a CO2 sensor [45]. Moreover, even if this sensor works, it does not seem to be stable because the Li2ZrO3 activity is also changed depending on CO2 concentration considering Miura’s expected reaction (2.41). Näfe introduced another layer to provide a fixed thermodynamic activity of sodium oxide which is not dependent of CO2 concentration such as NASICON [25, 46] or Na-β- alumina [47, 48]. The electrochemical cell of the system is expressed as 24 12 34

O22 , CO , Au | Na 23 CO | NASICON, Na-β -alumina or silicate | YSZ | Au, CO22 , O (2.42)

At the interface 1, the following reaction occurs.

1 Na CO = 2Na+- + CO + O + 2e (2.43) 2 3 Li23 CO 2 gas2 2 gas Au

Sodium ion migrates and reaches interface 3 through interface 2 and react with O2- ions supplied by YSZ and form Na2O in NASICON, Na-β-alumina or silicate.

+2- 2Na +O =Na2 O (2.44)

1 The oxygen reduction, O+2e=O-2- , takes place at the interface 4. 2 2, gasAu Adsorbed on Au

The EMF can be expressed as

()µ−µ−µoooRT EaP=−⋅Na23 CO Na 2 O CO 2 ln( ) (2.45) OC2F 2F Na22 O CO

The key issue of this design is how to keep the activity of Na2O in the buffer layer stable.

It means this design also requires sealed buffer layer to ensure the activity of Na2O to be independent of the oxygen atmosphere.

Based on literature survey, it is evident that type III structure with solid reference electrode is a promising sensor design for theoretical as well as practical considerations.

2.2 Experimental This sensor consists of three major parts: sensing electrode (lithium carbonate,

Li2CO3), electrolyte (Lithium phosphate electrolyte, Li3PO4), and reference electrode

(lithium titanate and titania mixture, Li2TiO3 + TiO2). Generally, sensor electrode materials are not good electronic conductors. Gold electrodes are used for electrical contact and gold wire is used as a lead wire. Platinum is generally known as a better catalyst than gold, but its oxidation in the carbonate melt was reported by Janz et al. [49]. Moreover, Pt paste curing temperature (about 1000°C) is much higher than gold paste curing temperature (600°C-850°C). This high temperature is not compatible with other

25 sensor elements, and hence, Au was chosen in this experiment. This section discusses the procedure of sensor synthesis/fabrication, characterization of sensors as well as sensing tests.

2.2.1 Sensor Fabrication Lithium phosphate (Alfa Aesar, 99.5%) was used as a lithium ion conductor. Lithium phosphate pellets did not sinter very well, so we added 5mol% SiO2 to enhance the sinterbility of these pellets. The powder mixture was ball-milled in ethanol for 8 hr and dried at 120°C. Two different designs of sensors were tested. One is the stack type design as shown in, and the other a planar type design in Figure 2.6 and Figure 2.7.

Lithium phosphate and silica mixture was cold pressed using a Carver model die press in a Carver hardened stainless steel die with a 12 mm inner diameter. A pressure of 1.2 kpsi (0.8 kpsi) was applied to the die for about 10 seconds and then the pressure was released. Pellets weighing 0.4g (0.1g), 11.9 mm (7.6 mm) diameter and 1.0 mm (1.15 mm) thickness were prepared. The numbers inside of parenthesis show the dimensions for small size pellets which were fabricated using a stainless steel die with an inner diameter of 8 mm. The green pellets were sintered at 800°C for 8 hr with a heating and cooling rate of 2°C/min. A Lindbergh furnace (Model 51732-B) was utilized for the heat treatment.

Gold paste (Heraeus Gold ink) was painted on both sides of the electrolyte with a diameter of 4 mm. It was cured at 700°C for 1 hr at a heating/cooling rate of 5°C/min in a Lindberg box furnace.

Lithium titanate (Li2TiO3, Lithium Corporation of America, Inc) mixed with 5 m/o of titania (TiO2, Alfa Aesar, 99.9%) was used as the reference electrode. The powder mixture was ball-milled in ethanol. It was mixed with α-terpineol organic binder (Fisher Chemicals) and painted on the surface of the lithium phosphate electrolyte. It was cured following the same heat treatment profile as that of gold paste curing.

Lithium carbonate (Li2CO3, Alpha Aesar, 99.999% and 99%) was used as the sensing electrode. It was also mixed with α-terpineol organic binder and painted on the surface of

26 the lithium phosphate electrolyte by hand-painting and heat-treated at 600°C for 1 hr at a heating and cooing rate of 5°C/min.

2.2.2 Sensor Characterization

XRD (Scintag XDS 2000 X-ray diffractometer, Cu Kα radiation at 45 kV and 20 mA current) was used for phase analysis for Li3PO4 electrolyte. JCPDS standard data were compared to the spectra for phase identification. Figure 2.8 and Figure 2.9 show the XRD spectra of lithium phosphate and 5 mol% SiO2 before and after sintering. As can be seen in these spectra, the electrolyte is a two-phase mixture after sintering.

SEM (Philips, XL30 ESEM) enabled the microstructure investigation of the electrolyte pellet and analysis of the sensor electrode surface. Before taking the SEM micrographs, the samples were coated by sputtering from a palladium-gold target. Figure 2.10 and 2.11 show the SEM pictures for the surface of lithium phosphate electrolyte. Grain size was about less than 500 nm. Figure 2.12and 2.13 show the porous electrode structure of

Li2CO3 sensing electrode and Figure 2.14 and 2.15 show the Li2TiO3 reference electrode.

At 600°C, Li2CO3 slightly melted and sintered very well. On the other hand,

Li2TiO3+TiO2 mixture looks almost powder-like due to its high melting point (about

1500°C). Therefore, Li2CO3 generally shows better adhesion to the electrolyte than

Li2TiO3+TiO2.

2.2.3 Sensing Measurements

2.2.3.1 Experimental setup A schematic of the sensor test assembly is shown in Figure 2.16. Sensors were located in the central uniform temperature zone of a Lindberg horizontal tube furnace (Figure

2.17, Model TF 55035A). Basically, three gas species N2, air, and CO2 were mixed for the sensing tests. CO2 gas concentration from 500 ppm to 5000 ppm was controlled by mixing gas from a cylinder of 1% CO2 concentration with N2. 100% pure CO2 cylinder was used for CO2 concentration from 5% to 50%. A mixture of ultra high purity nitrogen and artificial air was used as a background gas in all measurements flown at the rate of 210 ml/min. Omega Fl-1461-s flow meters and a Sierra 900 5-channel digital flow unit with Sierra 840L mass flow controllers were used to control gas flow rates for the test. 27 Flow meters were calibrated by a digital soap bubble flow meter (Fisher Scientific, Model 520).

The gases were vented out through a water bubbler into the exhaust pipe. The test temperatures were confined primarily in the range of 400°C to 600°C.

2.2.3.2 EMF measurements The EMF was measured by a HP 34970A voltmeter which was linked to a computer via the HP Benchlink data logger software for data acquisition for every two seconds. With this software, the EMF was simultaneously monitored and shown on the computer screen. A Gamry PC4/300 and a Keithley 1567A high resistance meter also were utilized to check the accuracy of the HP voltmeter.

2.3 Results and Discussion

2.3.1 CO2 sensor test in the lab General sensing behavior - Each sensor was aged before testing. Even though this process is not usually mentioned in literature, based on our empirical observation, aging is necessary for stabilization of sensor response. The aging profile is shown in Figure 2.18. Sometimes fast heating to the operating temperature decreased the absolute sensor EMF. Therefore, 5°C/min heating rate was used up to 300°C where it dwelled for 1 hr. After this, the temperature was increased to 600°C at a rate of 3°C/min and the sensor was kept for 5 hrs at this temperature. Finally, the temperature was brought to 500°C for testing. The testing temperature of 500°C was chosen because reliable sensor performance had been observed between 500°C and 550°C. Without the 600°C heat treatment, sometimes the sensor showed a different response at 500°C before and after a 600°C heat-treatment.

Figure 2.19 shows the typical sensing tests for various temperatures (400°C to 600°C) with EMF vs. Time. The sensor was tested for CO2 concentrations from 500 ppm to

500,000 ppm (50 %). O2 concentration was fixed at 10% during CO2 response tests. CO2 concentration was changed every 20 min in this experiment. When the temperature was lower than 450°C, the noise was severe due to high sensor impedance. This periodic

28 noise was probably due to interference from other electric instruments in the lab. Sensor response was stable within 1mV drift at each CO2 concentration, however, at high CO2 concentration and low temperature, its response showed drift. At 500°C, when the CO2 gas concentration reached 20 %, the EMF went down lower than the steady state value and then it recovered. This was not observed all the time, but in many experiments. This behavior could be minimized by decreasing the speed of changing gas concentration.

Even though it showed unusual response at 20 % CO2 concentration, the result was reproducible and stable at 500°C. When the sensor was operated at 450°C and 400°C and the gas concentration was set to 20 % CO2, the EMF kept decreasing for 20 min similar to other steps. Moreover, the sensor didn’t show the same EMF value when the CO2 concentration was changed from 50 % back to 5 %. This behavior was consistently found at low temperatures. Therefore, this sensor may not be suitable for high CO2 concentration environment (higher than 20 %) at temperatures lower than 500°C. The application for high concentration of CO2 requires at least 550°C operating temperature.

First, Li2CO3 formation on the reference electrode, Li2TiO3+TiO2, was suspected based on following reaction.

Li23 TiO + CO 2 ()23g = Li CO + TiO 2 (2.46)

Based on our thermodynamic study, reaction (2.46) is unlikely because the free energy of this reaction is positive and also our sensor test result confirms that free energy for reverse reaction is negative. This means that the unstable behavior has no thermodynamic origin. Probably, slow kinetics of electrode reaction at low temperature caused this unstable behavior. But the exact reason is not known at this moment.

Sensitivity to CO2 gas - Compared to our previous CO2 gas sensor [22], which adopted lithium phosphorous oxynitride electrolyte and lithium cobalt oxide and cobalt oxide mixture reference electrode, the sensor exhibit higher EMF values with a comparable sensitivity. For the EMF vs. log P , data points for every last two minutes CO2 of each concentration of CO2 were averaged. Measured data were fitted by linear least squares fit and were compared with theoretical calculations (Table 2.2 and Table 2.3). Theoretical equations were calculated from previous section. Standard formation energies 29 Measured EMF R2 Theoretical EMF EMF= 370.20− 53.38 log( P /[ppm]) EMF= 498.1− 66.7 log( P /[ppm]) 400°C CO2 0.999 CO2 EMF= 430.34− 46.99 log( P /[ppm]) EMF= 566.9− 71.6 log( P /[ppm]) 450°C CO2 0.996 CO2 EMF= 532.53− 60.38 log( P /[ppm]) EMF= 634.2− 76.6 log( P /[ppm]) 500°C CO2 0.999 CO2 EMF= 607.85− 68.97 log( P /[ppm]) EMF= 701.3− 81.6 log( P /[ppm]) 550°C CO2 0.998 CO2 EMF=−−623.05 66.70 log( P /[ppm]) EMF= 767.1− 86.5 log( P /[ppm]) 600°C CO2 0.998 CO2

Table 2.2: Fitting equations for measured EMF vs. calculated EMF based on Nernst equation between 500 ppm and 5000 ppm CO2 concentration.

Measured EMF R2 Theoretical EMF EMF=−491.79 78.15 log( P /[ppm]) EMF= 498.1− 66.7 log( P /[ppm]) 400°C CO2 0.999 CO2 EMF=−550.18 75.49 log( P /[ppm]) EMF= 566.9− 71.6 log( P /[ppm]) 450°C CO2 0.999 CO2 EMF=−664.7 88.79 log( P /[ppm]) EMF= 634.2− 76.6 log( P /[ppm]) 500°C CO2 0.996 CO2 EMF= 709.27− 89.64 log( P /[ppm]) EMF= 701.3− 81.6 log( P /[ppm]) 550°C CO2 0.999 CO2 EMF=−679.17 76.616 log( P /[ppm]) EMF= 767.1− 86.5 log( P /[ppm]) 600°C CO2 0.968 CO2

Table 2.3: Fitting equations for measured EMF vs. calculated EMF based on Nernst equation between 5% and 50% CO2 concentration.

for this calculation at given temperatures are found in Table 2.4 [50]. The data points representing 50% CO2 concentration at 400°C and 450°C were not included in the measured EMF because the fitting coefficient (R2) was too low when these data were included.

As can be seen in Figure 2.20 and 2.21, the sensor showed clearly different sensitivity

under lower CO2 concentration and higher CO2 concentration at all temperatures. Lower

sensitivity was observed at low CO2 concentration and sensitivity increased with CO2 concentration. Moreover, measured EMFs were consistently lower than the theoretical values. Experimental error ascribed with gas leakage in the testing setup and flow meter is almost negligible. So, clearly, these sensors do not follow the Nernstian behavior. 30 ∆ G J/mol ∆ G J/mol ∆ G J/mol ∆ G J/mol f (Li23 CO ) f (TiO2 ) f (Li23 TiO ) f (CO2 ) 600K -1045224 -827753 -1485520 -395182 673K -1024180 -814579 -1462299 -395340 700K -1016366 -809706 -1453710 -395398 723K -1009917 -805600 -1446424 -395441 773K -995831 -796642 -1430584 -395535 800K -988225 -791810 -1422030 -395586 823K -981833 -787724 -1414775 -395623 873K -967938 -778842 -1399004 -395704 900K -960434 -774046 -1390488 -395748

Table 2.4: Standard formation energy of Li2CO3, TiO2, Li2TiO3 and CO2 at different temperatures [50].

IR Drop and Electrode Overpotential – The measured EMF changed substantially when the internal impedance of the voltmeter changed. This is an indication that the electrochemical reaction at the electrodes may not be reversible in the current sensor design. The HP 34970A multimeter has two internal impedance values of 10 MΩ and 10 GΩ. When the test result of 10 GΩ internal impedance is compared to that of Gamry (10 GΩ), it showed less than 1 mV difference. Also Keithley 6517A high resistance meter (20 TΩ) was used to make sure that correct open circuit potential was being recorded. Test results of 10 GΩ and 20 TΩ internal impedances showed the same results. Therefore, 10 MΩ internal impedance of multimeter is not enough to measure the open circuit potential of the current sensor. The differences between 10 MΩ and 10 GΩ internal impedance are presented in Figure 2.22. In this figure, it is recognized that EMF did not show much different values at 600°C. The difference of EMF between 10 MΩ and 10 GΩ internal impedances is about 20 mV in the entire range of CO2 concentrations. However, as temperature decreased, the difference became larger. About 80 mV and 150 mV differences were observed at 500°C and 400°C, respectively, at 500 ppm CO2. As the

CO2 concentration increased, the EMF difference became less.

31 Overpotential due IR Drop due to Open circuit measuring to total electrolyte current at 500 ppm CO2 Total impedance of Electrolyte resistance Temperat impedance sensor Resistance ure of cell Internal Internal Internal Impedance (MΩ) (MΩ) Impedance Impedance 10MΩ 10GΩ 10MΩ 10GΩ 10MΩ 10GΩ 21.54 0.625 171.25 4.968 400°C 7.756 nA 0.225 nA 2.777 22.08 mV mV mV mV 6.003 4.749 60.86 48.15 500°C 28.49 nA 22.54 pA 0.2107 2.136 mV µV mV µV 1.549 1.626 14.73 15.47 600°C 42.08 nA 44.19 pA 0.0368 0.350 mV µV mV µV

Table 2.5: IR drop and overpotential calculations based on open circuit potential measuring current and sensor impedance at 500 ppm CO2.

The IR drop in the electrolyte is believed to be a source of this difference. The impedance of the whole sensor structure (7.6 mm diameter pellet) is shown in. The electrolyte resistance was determined by a Solarton 1260 impedance analyzer. The total impedance was measured by the DC I-V measurement. More detailed discussion of impedance data is presented in chapter 3. The IR drop was calculated from the sensor impedance and the current used to measure the open circuit potential. This current was obtained from the measured EMF and the internal impedance of the voltmeter. The EMF of the lowest CO2 concentration was the largest value at a given temperature, which means IR drop is the most severe. Table 2.5 also defines the amount of potential change due to IR drop. The maximum potential drop due to electrolyte resistance of 21.54 mV was found at 400°C. Therefore, IR drop due to only electrolyte resistance was not significant. Overpotential due to whole sensor impedance was calculated and it was well in accord with the EMF differences depending on internal impedance in. This overpotential changed as CO2 concentration changed. Therefore, the higher overpotential at lower CO2 concentration results in a smaller sensitivity to CO2 concentration than 32 predicted by the Nernst equation when a voltmeter with 10 MΩ internal impedance was used. However, this overpotential is almost negligible at 600°C. Moreover, the usage of 10 GΩ internal impedance can prevent overpotential at all temperature ranges during open circuit potential measurement.

From the IR drop and overpotential study, we found that not only the electrolyte resistance but also the slow electrochemical reaction at the electrodes can change the EMF when we use an inappropriate voltmeter. Therefore, presented data were collected with the usage of 10GΩ internal impedance. Whenever 10 MΩ internal impedance was used, it was clarified. This electrode kinetics will be discussed in chapter 3. However, another question is why non-Nernstian behavior was still observed even when 10 GΩ internal impedance was used. This non-Nernstian behavior simply cannot be explained by the IR drop and electrode overpotential during open circuit potential measurement.

Sensor response time - Response time is represented by the time corresponding to the

90% change of EMF when CO2 concentration is changed. Figure 2.23 shows a typical response of a CO2 sensor at 550°C. Table 2.6 as a function of time represents the response times when CO2 concentration was changed from 500 ppm to 1000 ppm for various temperatures. Response time is closely related to the flow rate and gas mixing

Response time for CO2 concentration change from 500 ppm to 1000 ppm 400°C 26s 450°C 26s 500°C 40s 550°C 40s 600°C 46s

Table 2.6: Sensor response times when CO2 concentration was changed from 500 ppm to 1000 ppm.

33 time. When we controlled the gas flow meter to change the gas concentration, it took time for changed gas to reach the sensor. Therefore, true response time is hard to measure by the testing setup used in this work.

Another interesting result was observed with respect to the response time. Surprisingly, low temperature response time was shorter than that of high temperature. One possibility is that current sensor is not in thermodynamic equilibrium at T < 500°C. Therefore, when

CO2 concentration is changed, EMF is changed from one steady state to another. This activation energy might be lower than that of the equilibrium potential change. Each sensor shows a little bit different response time but such a tendency is found in every sensor.

Sensor aging – Figure 2.24 presents the time-dependent sensor tests. Over a four day period, the sensor was kept in the furnace at 500°C. During this period, the sensor seemed to be stable and it showed about 5 mV change in the EMF. After 4 days of test, the sensor was cooled down and it was removed from the testing setup. When it was reloaded in the furnace after 9 days, it showed a decreased EMF at 500 ppm and 1000 ppm CO2 concentration. The next day (10th day), it exhibited almost the same EMF as that of the previous day. The sensor must be fully aged after cooling and heating cycles. What we observed in the automobile engine test is similar to the aging behavior. When the sensor was tested again in the lab after three weeks of field test, the sensor response showed very similar behavior before and after the engine test. The sensor aging will not be discussed in this work but it has to be studied for the real life sensor applications of the sensor.

Even though several sensors were fabricated using the same procedure, they did not show exactly the same behavior. The EMF of 6 different sensors showed about 10 mV deviations (Figure 2.25). The deviation was lower at higher CO2 concentration is increased. This deviation could be minimized by the use of high internal impedance voltmeter because the current sensor does not have fully reversible electrodes.

Oxygen dependence - Oxygen dependence was tested from 4% to 10% O2. One of the problems for the previous sensor [32] was a strong oxygen dependence. Therefore,

34 Li2TiO3+TiO2 mixture was introduced as a reference electrode into the current sensor structure. Thermodynamically, this sensor is supposed to have no oxygen dependence under equilibrium condition as discussed in chapter 2.1. Figure 2.26 shows the oxygen dependence at 400°C. The sensor showed almost negligible oxygen dependence at 3000

ppm of CO2. However, this sensor showed oxygen dependence at 500 ppm CO2 background. Similar results were observed at 500°C and 600°C. When O2 concentration was set back to 10% from 4% (), the sensor EMF did not fully recover to the original 10% level and it reached 4~5 mV higher than the original value at 400°C. It means oxygen dependence at 400°C was not quite reversible. At 500°C and 600°C, this oxygen dependence was stable. Oxygen dependence is not a serious practical problem because

low CO2 concentration application usually has almost constant 21% oxygen. Furthermore, combustion applications such as automobile exhaust have various oxygen

concentrations but the CO2 background is already at the % level. Therefore, this sensor would be independent of oxygen content.

Maier et al. also found oxygen dependence at relatively low temperature (less than

500°C) and low CO2 concentration (0.195 mbar) in their sodium based CO2 gas sensor study [15]. They thought that slow kinetics of the electrode reaction contributed to the oxygen dependence. Our experimental results are in accordance with their observations

with respect to CO2 concentration effect. But oxygen dependence of our CO2 sensor was observed at all temperatures. Table 2.7 shows the EMF value depending on oxygen

concentration at 500 ppm CO2 for various temperatures. Therefore, it indicates that

Sensor EMF (mV) Oxygen 4% O 6% O 8% O 10% O concentration 2 2 2 2 400°C 202.44 198.54 195.78 194.30 500°C 349.48 347.32 344.88 343.16 600°C 443.60 439.16 437.39 435.83

Table 2.7: Oxygen dependence at 500 ppm CO2 concentration for 400°C, 500°C and 600°C.

35 lithium titanate reference electrode reaction may not be reversible at all temperature ranges and the oxygen reduction potential is kept constant at the reference electrode. The EMF changes due to the change of the sensing electrode potential.

Humidity interference - The humidity resistance of the lithium ion conductor is one

of the attractive features in CO2 sensor application. Humidity interference was tested by utilizing water vapor pressure. Test gas mixture flowed through gas tight flask containing water in order to use saturated water vapor pressure at 25°C. Figure 2.27 shows 600°C test result. Gas concentration was changed from 500 ppm to 5000 ppm every 10 minutes. After humidity was introduced humidity, the sensor EMF increased about 10 mV at 500

ppm CO2 and also response time became more sluggish. A similar result was found at

500°C. The same test was also performed between 5% and 50% CO2 at 600°C and the results are shown in Figure 2.28. The sensor nearly didn’t change the EMF responding to

humidity. But sensor signal of 500°C test changed at 50% CO2. The humidity interference became more prominent at 400°C. So generally speaking, this sensor

exhibits some humidity interference at low operating temperature and high CO2 concentration. In the literature, the reason for the humidity effect has not been made clear. A carbonate sensing electrode might cause the interference of humidity due to the - formation of HCO3 ion [51-53]. However, the stability of these types of hydro-carbonate phase is not confirmed due to limited thermodynamic data at the sensor operating temperature between 400°C and 600°C.

So far, according to our sensor tests, it is believed that the current sensor doesn’t have reversible electrodes ensuring the equilibrium potential. On the other hand, Figure 2.29

shows the EMF as a function of temperature for various CO2 concentrations. As can be seen in this graph, it was found that the EMF deviated more at high temperatures. At about 500°C, the measured EMF was the closest to the theoretical Nernst equation at all

CO2 concentrations. If it is due to only non-reversible electrode reaction, this deviation has to be minimized with increasing sensor operating temperature. It indicates that the current sensor behavior cannot be explained by slow kinetics of the electrode reaction only. Considering the nature of solid state ionic conductor in the literature, mixed ionic

36 and electronic conduction of Li3PO4 electrolyte might be another reason for this. This issue will be discussed in chapter 4.

2.3.2 Sensor test in automobile engine The sensor was tested in an automobile engine to examine its practical utility. Oswaldo Figueroa, a CISM graduate student, designed a PID (Proportional Integral Derivative) temperature controller and the platform for the automobile exhaust test. The sensor was integrated with a commercial NTK heater and packaged similar to a commercial oxygen sensor. The sensor operating temperature was kept at 450°C by the heater which was controlled by the PID controller. A thermocouple was also attached on top of the sensor surface in order to give feedback to the PID controller, which controlled temperature to an accuracy of ±0.2°C.

The sensor was tested three times over three weeks. The probe was mounted in the exhaust pipe of a 2.4L-5 cylinder 1997 Fiat diesel engine. The exhaust gases were monitored with a Horiba Motor Exhaust Analyzer, MEXA-7500. In this test, the torque was varied while keeping the engine speed constant. Increasing and decreasing the engine

torque changed the partial pressure of CO2 in a controlled manner.

Figure 2.30 shows the test result and Figure 2.31 shows the reproducibility in test results. Even though there is some difference for the three different days, generally it showed reproducible results. Figure 2.32 shows the laboratory tests in the PID controlled micro-furnace before and after the engine test. Considering the harshness of the engine environment, the measured EMF did not change significantly, which showed promising results for commercialization.

On the other hand, we found that the sensor response in the lab and in the engine do not coincide with each other. In fact, the experimental conditions in the lab test and engine exhaust stream are quite different. First of all, engine exhaust has various unburned

hydrocarbons species and H2O possibly present in the exhaust stream. Although CO, CH4 and NOx interference was tested and found to be negligible in the previous study [32], the humidity effect on this sensor was already observed and discussed. The engine test

37 showed decreased EMF, which is in the opposite direction to the humidity effect. More careful interference testing is required for understanding different calibration problem.

Secondly, flow rates are tremendously different between two tests. In the lab, all the tests were conducted with 210 ml/min total flow rate. But automobile exhaust has 105 times faster flow rate than the lab test. Such a fast flow rate can evacuate the small chamber holding the sensor probe. So the operating condition probably is much less than 1 atmosphere. The performance of the sensor in such a low pressure condition has never been examined in the lab.

The other possibility is a temperature fluctuation effect. The exhaust gas temperature was fluctuating from 100°C to 300°C during the change of torque in the engine. Even though the PID controller was adopted to keep the temperature constant during the test, it was observed that the thermocouple could not read the right temperature of the sensor surface. In other words, the sensor surface and thermocouple are not thermally equilibrated. When the thermocouple was kept at 450°C, the sensor EMF shows the value that corresponds to a temperature of about 520°C~530°C. This observation was also found from another sensor which was tested on the same platform.

The design of attached thermocouple on top of the sensor surface probably is not a good idea for application in a large temperature fluctuation environment. A lot of commercial sensors have RTD (Resistance Temperature Detector) to monitor the temperature for sensor operation. RTD seems to control temperature more accurately than an attached thermocouple. Even though three possible reasons for the calibration problem were discussed here, more investigations are required to reveal the exact origin of this discrepancy.

2.4 Summary

An electrochemical CO2 gas sensor with lithium phosphate electrolyte was fabricated

and it showed an excellent performance toward CO2 monitoring in the lab as well as in the engine tests. The sensor response, however, showed a systematic deviation from the Nernstian behavior. Two possible reasons for this deviation were further investigated as described in chapters 3 and 4. These two possible reasons are: (1) non-reversible

38 electrode reaction and (2) mixed ionic and electronic conduction. Observations associated with each effect are summarized below.

• Non-reversible electrode reaction

⇒ Unstable response to CO2 gas at low temperatures (T< 500°C) and high CO2 concentrations (higher than 20%).

⇒ Lower sensitivity and lower absolute EMF than predicted by the Nernst equation.

⇒ IR drop and overpotential due to high sensor impedance.

⇒ Shorter response time at low temperature than at high temperature.

⇒ Difference in EMF values of similar sensors.

⇒ Oxygen dependence at low CO2 background concentration.

• Mixed ionic and electronic conduction

⇒ More deviation of the measured EMF from the Nernst equation at temperatures higher than 500°C.

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Figure 2.1: Illustration of electrochemical equilibrium of oxygen and YSZ at triple phase boundary.

Figure 2.2: (a) Schematic diagram for solid state K2SO4 type II sensor [6]. (b) Schematic diagram for fused salt K2SO4 sensor [35].

Figure 2.3: Schematic of Type III sensor structure with Li2CO3 sensing electrode, Li2TiO3+TiO2 reference electrode and Li3PO4 lithium ion selective solid electrolyte.

Figure 2.4: (a) Gold electrode design for fast ion conducting auxiliary phase. (b) Gold electrode design for non ion conducting auxiliary phase.

ical potential profile at ochemical, chemical, and electr III sensor structure. Figure 2.5: equilibrium in the type Schematic of electr

Figure 2.6: Type III sensor design with lithium phosphate lithium ion conductor.

Figure 2.7: Planar type sensor design.

5 mol% before sintering. 2 +SiO 4

PO 3 Figure 2.8: XRD spectra for Li

5 mol% after sintering. after 5 mol% 2 +SiO 4 PO

3 Figure 2.9: XRD spectra for Li

Figure 2.10: SEM photo of Li3PO4+SiO2 5 mol%.

Figure 2.11: SEM photo of Li3PO4+SiO2 5 mol%

Figure 2.12: SEM photo of Li2CO3 sensing electrode surface.

Figure 2.13: SEM photo of Li2CO3 sensing electrode surface.

Figure 2.14: SEM photo of Li2TiO3+TiO2 reference electrode surface.

Figure 2.15: SEM photo of Li2TiO3+TiO2 reference electrode surface.

Figure 2.16: Schematic of the sensor test assembly.

Figure 2.17: Gas flow meter and tube furnace.

Figure 2.18: Aging profile for sensor before sensing test.

Figure 2.19: Typical sensing test data from 500 ppm CO2 to 50 % CO2 between 400°C and 600°C.

Figure 2.20: Comparison of measured EMF and theoretically calculated EMF at 500°C, 550°C and 600°C.

Figure 2.21: Comparison of measured EMF and theoretically calculated EMF at 400°C and 500°C.

Figure 2.22: EMF comparisons of different internal impedance of multimeter at different temperatures.

Figure 2.23: Typical sensor response time at 550°C.

Figure 2.24: The time dependence of sensor EMF.

Figure 2.25: Sensor EMFs from different 6 sensors.

Figure 2.26: Oxygen dependence under 500 ppm and 3000 ppm CO2 at 400°C.

Figure 2.27: Humidity interference during the gas concentration change from 500 ppm to 5000 ppm CO2 at 600°C. Humid gas was introduced between on and off.

Figure 2.28: Humidity interference during the gas concentration change from 5% to 50% CO2 at 600°C. Humid gas was introduced between on and off.

Figure 2.29: Measured EMF of the sensor vs. theoretically calculated EMF as a function of temperature for various CO2 partial pressures.

Figure 2.30: Sensor test in Fiat diesel engine. Horiba commercial gas analyzer was used to verify CO2 concentration. (from Oswaldo Figueroa)

Figure 2.31: Sensor test in Fiat diesel engine for three different days. (from Oswaldo Figueroa)

Figure 2.32: Measured EMF with the PID controlled micro-furnace in the platform before and after engine test. (from Oswaldo Figueroa)

64 CHAPTER 3

REVERSIBILITY FOR SENSOR ELECTRODES

3.1 Reversible electrochemical reaction The word “Reversible” in thermodynamics usually describes the equilibrium condition. Equilibrium is defined generally as a balance of two opposing forces [1]. In electrochemistry, this expression can be directly related to the reversible potential where anodic and cathodic currents are balanced at equilibrium. The half cell reaction of oxygen reduction on the three phase boundary of Pt, YSZ and air is used as an example.

Reduction or Cathodic Reaction 1 2- O2 (gas )+ 2e- ( Pt ) R O ( Adsorbed on Pt ) (3.1) 2 Oxidation or Anodic Reaction

At equilibrium, there is no net reaction on this electrode because cathodic and anodic currents in the opposite directions are same. However, there is an exchange current which represents the absolute magnitude of cathodic or anodic current. At equilibrium, electrode should exhibit the reversible potential with a dependence on the O2 (gas ) partial pressure

2- and the concentration of O (Adsorbed on Pt) dictated by the Nernst equation. Electrochemical sensors utilize this equilibrium potential to measure the gas concentration and, therefore, reversible electrodes are necessary.

Even though it is generally accepted that reversible electrode can read exact reversible potential, in other words, thermodynamic property of the system such as partial pressure, concentration or activity, it is hard to define what kinds of electrodes have such capability. Practically, reversible electrodes are non-polarizable because charged species at the electrode/electrolyte interface can easily cross the interface giving or taking

65 electrons [2]. Therefore, the electrochemical reaction should be very fast on the reversible electrode.

Figure 3.1 shows two equivalent circuits representing typical electrode/electrolyte interface (Randles circuits) of two extreme conditions. They are composed of charge transfer resistance and double layer capacitance. Depending on the nature of the electrodes, different current-voltage responses are depicted in Figure 3.2. At the non- polarizable electrode, current is changed with a small overpotential. On the other hand, polarizable electrode requires large overpotential for a small current change. In the open circuit potential measurement, the inverse effect is observed because small current flow during the measurement can induce a large overpotential if the sensor electrode is polarizable. Therefore, a non-polarizable or reversible electrode is critical for a gas sensor electrode.

There are not many experimental techniques available to verify whether an electrode process is reversible or irreversible. Practically, the electrode reaction is considered electrochemically reversible if a measured EMF follows the Nernst equation [3]. Fouletier et al. [4, 5] indicated polarization due to very low oxygen concentration can make measured EMF deviate from the Nernst equation. Macdonald stated that for a reversible electrode, the exchange current should be much higher than the measuring current [3].

Electrode reactions are Reversible reaction at the electrode can be achieved with fast kinetics such as charge transfer, diffusion and adsorption/desorption, which can be represented by impedance. This section will discuss these kinetic effects in an electrochemical sensor by making use of the EIS technique.

3.1.1 EIS (Electrochemical Impedance Spectroscopy) for electrode kinetic study

3.1.1.1 Basics of electrical Impedance Electrical impedance is one of the transfer functions of a material, Z ()ω , which ascribes the response to a perturbation in the system [3].

66 Vt() Z ()ω= (3.2) I()t

Vt( ) = Vm sin(ω t ) (3.3)

It( ) = Im sin(ω+θ t ) (3.4) where ω is the angular frequency and θ is the phase difference between the voltage and the current [3]. Therefore, when the impedance is expressed as a planar vector using polar coordinates in Figure 3.3, the real part of Z is in the direction of the real axis x, and the imaginary part of Z is along the y axis [3].

Z (ω=) ZjZ′ + ′′ (3.5)

Re(ZZZ )= ′ =θ | | cos( ) (3.6)

Im(ZZZ )= ′′ =θ | | sin( ) (3.7)

θ=tan−1 (Z′′′ /Z ) (3.8)

Obtained impedance has a meaning when following prerequisites are satisfied. Impedance must be independent of the magnitude of the applied voltage. In electrochemical system, small perturbation, approximately 10 mV, is applied to keep the system within the linear region of the I-V curve. Also, system has to be stable before and after perturbation. Therefore, open circuit potential monitoring before and after EIS measurement is important. The impedance should be finite when frequency approaches 0 or ∞ and be continuous at all the frequencies [3].

3.1.1.2 Sensor impedance and Overpotential Overpotential is an additional potential to drive a reaction at a certain rate [3] or the deviation from the thermodynamic equilibrium potential. Overpotential is not significant when the open circuit potential is monitored in the reversible system. However, if system is not reversible, small measuring current during open circuit potential measurement can induce the overpotential and it appears as a deviation form reversible potential. EIS is a powerful technique for studying electrochemical behavior at the interface controlled by

67 charge transfer, chemical reaction, and mass transport across electrolyte/electrode interface, even though this technique cannot directly determine whether electrochemical reaction is reversible or not. EIS has an advantage for electrochemical sensor that measures an open circuit potential. Compared to DC measurements, which can induce large polarization, it is possible to study the kinetics of electrochemical reaction close to equilibrium by EIS.

Another area of solid state electrochemistry that has seen extensive use of EIS is fuel cells. Energy losses at solid electrolyte/electrode interfaces due to overpotentials are thermodynamically irreversible and their origin must be understood in order to minimize their effects [6]. Figure 3.4 shows the typical impedance spectrum of an arbitrary solid state electrochemical cell when the relaxation times of the three circuit elements are so different that they are resolved in the corresponding impedance plots.

In this figure, the first semicircle in the high frequency region represents the bulk electrolyte effect associated with a parallel RC combination [8]. Two semicircles can also appear when impedance spectra resolves grain and grain boundary phases. The bulk electrolyte resistance can induce ohmic potential drop during the open circuit potential measurement. It can be avoided by a high internal impedance voltmeter to decrease the current or by minimizing the distance between the reference electrode and the working electrode. This distance can be the electrolyte thickness when only two electrodes are used in the measurement.

The second semicircle is related to the charge transfer resistance which is in reciprocal relationship with the exchange current i0 in equation (3.9) [3], if charge transfer reaction is a one-electron process.

RT Rct = (3.9) Fi0

The exchange current is directly related to any system’s ability to allow a net current without a large activation [3]. Therefore, high exchange current, in other words, small Rct cannot be overemphasized for open circuit potential measurement. Berkel [9] pointed out that the effective cross-sectional area of a porous Pt electrode attached to an oxygen 68 conductor, which can be translated to large numbers of three-phase boundaries (TPB), plays a very important role for voltage loss. Even though exchange current density depends on the catalytic ability of electrode materials, the geometry of the electrode can still control exchange current.

The third arc is assigned to the lowest frequency domain. At this frequency range, electrode behavior is related to diffusion or adsorption/desorption process. Frequency dependent “Warburg impedance” is well known typical diffusion related impedance. Its mathematical derivation was first introduced by Warburg from a semi-infinite diffusion boundary condition. This impedance is usually shown as a straight line at a 45° angle from the real axis in the complex plane.

The fuel cell and battery industry have endeavored to minimize these overpotentials, which result from the above sensor impedances in order to maximize power generation. Electrochemical sensors do not require high current density, but equilibrium potential can be easily achieved when all the sensor impedances are negligible. The author believes such a kinetic consideration can provide a clue for the non-Nernstian behavior of the current sensor.

3.1.1.3 EIS for solid state system Solid state systems have different properties from liquid state systems. In liquid systems, the interesting impedance information is usually limited within relatively low frequency (104 ~10-3 Hz), but solid state systems require high frequency of up to 107 Hz in order to resolve the impedance components of bulk electrolyte such as grain and grain boundary effects.

Also, depressed semicircles are frequently observed in the solid, which can be explained by CPE (Constant Phase Element) due to a frequency-dependent resistor at constant capacitance [3, 6]. It can be also understood by relaxation distributions using ideal parallel resistor and capacitor couples. This transmission-lines model represents complicated individual steps in possible overall electrode reaction at the electrolyte/electrode interface [3, 6]. Even though CPE is interpreted mathematically and

69 it helps data fitting process, unfortunately, complicated CPE has not been understood except for Warburg impedance that is accepted as one of the CPEs.

In a liquid system, usually counter ion species (e.g. Cl-) move in opposite direction to electro-active species (e.g. H+), however, counter charge carriers usually do not move during solid state electrochemical reaction unless the electrolyte is a mixed conductor. This situation is called “unsupported” [3, 10]. It is unusual to find cation and anion conductor with similar conductivities. On the other hand, mixed ionic and electronic conduction is quite common in solid electrolytes due to a small deviation from stoichiometry.

3.1.1.4 EIS analysis and equivalent circuit Construction of an equivalent circuit is useful to extract meaningful information from EIS data. There is, however, some risk in suggesting only mathematical fitting since multiple equivalent circuits can fit the same data. The key is to use a physically meaningful model rather than simply choosing the best fitting model.

In the literature, different equivalent circuits have been commonly suggested to explain different systems. Basic circuit elements are used for equivalent circuits: the resistance (R), the capacitance (C) that represents, for example, double layer capacitance, and the inductance (L). The inductive component is not a common physical element in electrochemical system, however, it comes out as an erroneous measurement from lead wire or adsorption process in an electrochemical reaction [10]. Sometimes these circuit components are not enough to explain EIS results. An additional component called CPE is required for non-ideal behavior. CPE is usually understood as a distorted capacitance and resistance component as discussed before, but it is usually difficult to associate with any physical/chemical process.

Warburg impedance can be derived from the Fick’s Law under a semi-infinite diffusion boundary condition [3].

∂Cxt(,) jt () D | = (3.10) ∂x x=0 zF

70 Here, current is determined by diffusion and the relationship between concentration and electric potential is acquired from the Nernst equation. For an ideal solution [3]

dER T = (3.11) dCzF C

From equations (3.10) and (3.11), the Warburg impedance is obtained as

dE 11 Z = ⋅⋅ (3.12) W ,∞ dC zF jDω

Utilizing these circuit elements, the equivalent circuit represents the physical model of the electrochemical system. It is very important for impedance data analysis and a lot of different models have been proposed in the literature. Even though each system has different components of electrochemical reactions, their equivalent circuits can be generalized under the following three groups.

• Model A (Simplified Randles Equivalent Circuit) Figure 3.5 (A) shows two simple parallel RC circuits connected in series, which is the simplified Randle equivalent circuit. The first RC is composed of a bulk capacity (Cb) arising from the finite dielectric constant of the solid electrolyte [3] and a bulk electrolyte resistance (Rb). In the second RC, Rct is known as the charge transfer resistance and Cdl double layer capacitance. It is considered as a reasonable model when charge transfer is the only limiting step in the kinetics. As can be seen in Figure 3.5 (B), its impedance plot appears as two semicircles in the complex plane. The first semicircle corresponds to bulk electrolyte RC component and the second to the electrode polarization. Therefore, in an ideal reversible electrode, the second semicircle can be very small or can disappear because of negligible Rct, which means that the charge transfer step is very fast.

Bauerle et al. [12] proposed this model for an YSZ electrolyte and porous Pt electrode interface. They observed that the resistance of the first semicircle, which can be assigned to Rct and Cdl in the admittance plane, changed depending on porosity of the Pt electrode and P change. The activation energy extracted from this resistance was about 2-2.5 eV. O2 This thermally activated process could represent the molecular dissociation step or the

71 electron transfer step [12]. EIS can provide such kinetic information but it is still ambiguous to decide which kinetic step can determine dominant effect by itself. Therefore, additional spectroscopy is necessary to confirm their results.

Armstrong [13] explained four different electrode conditions in the metal electrode/solid electrolyte systems. This model A equivalent circuit represents the non- blocking electrode condition such as Ag|Ag4RbI5 [14]. Villain et al. [15] studied the impedance behavior of Cu|CuBr interface. Such systems showed impedance behavior similar to Figure 3.5 (B). Generally, reversible electrode shows relatively smaller second semicircle than the first one in the complex plane and the sum of the Rb and Rct from the AC measurement has to correspond to the DC resistance.

On the other hand, Machowski et al. [16] reported characterization of a Au|AgI-Ag2O-

V2O5-P2O5 system by EIS. Even though this interface is not reversible with respect to Ag+ because the metal electrode and cation in the electrolyte are not the same species, the impedance spectrum shows negligibly small second semicircle at low frequency as the content of V2O5 is increased. High content of V2O5 was believed to increase the electronic conduction of this system, because aliovalent vanadium ions V4+ and V5+ act as centers for electron hopping [16]. Therefore, reversible interface cannot be simply determined based on the impedance plot.

• Model B (Randles Equivalent Circuit) This model is similar to the simple Randles circuit, but the Warburg impedance is connected with Rct in series (Figure 3.6). Therefore, the electrochemical reaction is controlled by both activation and mass transport process. It has been observed in intercalation reaction of thin film electrodes and also gas phase diffusion through electrodes. Figure 3.7 shows a typical finite Warburg impedance in an intercalation system. As can be seen, the phase angle begins to increase due to the finite length effects of the thin film electrode at low frequencies. The semi-infinite boundary condition does not hold anymore in this system.

Ho et al. [16] first developed this model with the following cell

72 Li | LiAsF6y32 (0.75M) in propylene carbonate | Li WO thin film on SnO (3.13)

Warburg impedance is due to chemical diffusion of lithium in the LiyWO3 thin film cathode. When the open circuit potentials were decreased with more lithium insertion to

LiyWO3, electrochemical reaction kinetics was found to move from diffusion controlled to charge transfer controlled at high frequencies [16].

Similar observation was reported by Paul et al. [18] studying MnO2 growth on the rotating carbon electrode. In this system, diffusion proceeds not through aqueous layer in

MnO2 surface but through porous solid layer which grows on the outer surface of the

MnO2 deposit. Lithium diffusion in the intercalation electrodes such as La2/3-xLi3xTiO3

[19], graphite [21], LiCoO2 [22] and LiMn2O4 [23] has also been studied by EIS.

Bazan et al. [24] observed this type of diffusion impedance in the Ag|α-AgI system which is a well known reversible metal | cationic conductor structure. They explained this Warburg impedance behavior describing activated Ag*+ species diffusion in the Ag electrode.

When gas species diffuse through the electrode and this process is the limiting step for electrochemical reaction, at the beginning of low frequencies, it shows a straight line at 45° with the real axis but at lower frequencies it shows the semicircle, like parallel RC circuit behavior as can be seen in Figure 3.8. Oxygen diffusion through La1-xSrxCoO3-δ and LaMnO3 electrodes were studied by Adler [25] and Brichzin [26]. From the Warburg impedance they could calculate the oxygen diffusion coefficient in these electrodes.

Moghadam [27] observed that Ag and Pt electrodes for oxygen ion conductors have different diffusion mechanisms (bulk vs. surface) from impedance measurements. The activation energies for oxygen diffusion through Pt and Ag were extracted from the Warburg impedance. In the case of the Pt electrode, the calculated diffusion coefficient was much higher than that for the bulk diffusion. Therefore, the diffusion controlled electrode reaction in the Pt electrode is diffusion on the surface of the Pt film. On the other hand, their results showed that calculated activation energy from the Warburg impedance in the Ag film agrees well with that of the bulk diffusion coefficient.

73 Therefore, it was revealed that the oxygen transporting mechanism is bulk diffusion through the Ag electrode as opposed to surface diffusion in the Pt electrode.

• Model C (Warburg Impedance series with Randle circuit) Macdonald [6] simplified the equivalent circuit for a solid state electrochemical cell to three parallel RC combinations in series (Figure 3.9). Lee et al. [28] calculated the ionic conductivity of Co1-δO from the finite length Warburg impedance at the low frequency region. They constructed a cell of the form

O21-2 , Pt | YSZ | Coδ O | YSZ | Pt, O (3.14) where YSZ was used as an electron blocking electrode (ion probe). In this cell, the bulk resistances of Co1-δO and YSZ are very small, so they found almost negligible bulk RC at high frequency. However, the second semicircle was not well understood in this experiment because this system has two different types of interfaces: O,2 Pt | YSZ and

YSZ | Co1-δ O. Even though the equivalent circuit has some controversy of parallel combination for charge transfer related elements, they successfully interpreted the finite length Warburg impedance as the ionic response of Co1-δO. They were also able to determine the oxygen ion conductivity of YBa2Cu3Ox with a similar method [29].

3.2 Experimental

3.2.1 Solartron 1260A Impedance Analyzer A Solartron 1260A impedance Analyzer was utilized for impedance measurements in this experiment. Solartron 1260A adopted a direct method for the acquisition of impedance data. The direct method can measure the phase angle from an oscilloscope directly [3]. FRA (Frequency Response Analyzer) determines the impedance from the cell response, St(), with two synchronous reference signals [3]. One is a sine-wave perturbation and the other cosine-wave perturbation as can be seen in Figure 3.10 [3]. When a sine-wave perturbation function Pt( ) is applied to the cell, it is represented as

Pt()= P0 sin(ω t ) (3.15)

74 where P0 is the amplitude and ω is the frequency [3]. In the same way, the cell response is written as

0 St()=ωω+φω+ P Z ( )sin[ t ( )] ∑ Amm sin( m ω−φ+ t ) Nt () (3.16) m where Z()ω is the transfer function of the cell that we want to obtain [3]. The response also contains a second and a third term. The second term represents harmonics due to the nonlinear nature of the electrochemical system [3]. The third term is added because of noise [3]. The real and imaginary components of the impedance are given by the integrals

1 T Re : Z′ (ω= )St ( )sin( ω t ) dt (3.17) T ∫0

1 T Im : Z′′ (ω= )St ( ) cos( ω t ) dt (3.18) T ∫0

Therefore, impedance is determined by the correlation between sin(ωt ) in phase with St() and cos(ωt ) out of phase by 90°.

3.2.2 Sample preparation For the impedance test, symmetrical testing cells,

Au | LiPO34 | Au (3.19)

Li23 CO , Au | Li 34 PO | Au, Li 23 CO (3.20)

Li23 TiO +TiO 2 , Au | Li 34 PO | Au, Li 23 TiO +TiO 2 (3.21) were fabricated using the same method for the sensor fabrication described in chapter 2.

Gold powder (Alfa Aesar, Spherical, APS 5.5-9.0 micron) was mixed with the auxiliary phases (Li2CO3 and Li2TiO3+TiO2) in 50:50 volume ratio in an agate mortar and pestle. α-terpineol organic binder (Fisher Chemicals) was used to make the mixture as a paste. Before applying this mixture, gold wire was attached on the both sides of the

Li3PO4 electrolyte with gold paste (Heraeus Gold ink). The electrode paste was hand- painted and heat-treated at 700°C for 1 hr at a heating/cooling rate of 5°C/min for

75 Li2TiO3+TiO2 mixture and at 600°C for 1hr at a heating and cooing rate of 5°C/min for

Li2CO3.

For the cell with sputtered gold design, a CrC-150 TORR International, Inc sputtering system and gold target (99.99% purity, 3 inch diameter×0.5 mm thick, Sputteringmaterials Inc) were used for sputtering in an Ar gas. Pre-sputtering for 3 minutes, in which the Ar plasma was lit with the gold target shielded from the electrolyte pellet by a shutter, ensured a clean target surface for film deposition.

Sputtering time was set from 4.5 to 7.5 minute for the porous gold electrode. For the gold-ion-blocking electrode to become fully dense, a 10 minute sputtering time was used. The vacuum condition was not exactly controllable, and it varied from 1.2×10-6 to 6.4×10-6. The Ar pressure was kept about 4.5×10-3. Depending on the vacuum condition, operating current was also changed. It was set between 100 and 107 mA.

After the sputtering of gold, the sample was heat-treated at 700°C for 5 hrs with a 5°C/min heating and cooling rate. Gold wire was attached by using gold paste. It was cured at 700°C with a 5°C/min heating and cooling rate. Each auxiliary phase material was painted on top of the sputtered gold electrode. It was heat-treated with the same procedure of gold powder and auxiliary phase mixture electrode.

3.2.3 EIS measurement The testing cells were loaded inside of a quartz tube and it was located in the central uniform temperature zone of a Lindberg horizontal tube furnace. The quartz tube was shielded by aluminum foil to avoid electric noise. Aluminum foil was grounded to furnace body. Pt lead wires were connected between testing cells and impedance measurement instrument.

Solartron 1260A model was used for the impedance measurement. It can sweep frequency from 10 µHz to 32 MHz. Unreliable current measurement became serious at the low frequency range (<103), which was dependent of the sample resistance value. If the sample resistance was about 10 MΩ, the scattering of data started at 1 kHz. For high impedance sample, Gamry was used for more reliable data collection at low frequency where scattering in data shows up. Gamry has a limitation of high frequency range sweep 76 (f>0.1 MHz). So, impedance measurements were performed by using Solartron 1260A and Gamry to have reliable data in the entire frequency rage of interest.

3.3.1 Sensor test with modified gold electrode As discussed in chapter 2, different internal impedance of voltmeter showed significant differences in the EMF measurement due to overall sensor impedance. This result was believed to be due to overpotentials of a non-reversible electrode reaction rather than a simple IR drop. Therefore, the auxiliary phases were modified with two different designs. One is the sensor electrode with gold powder and the other the sputtered gold.

Gold powder sensor: In the first design, we mixed the auxiliary phases with gold powder following the work of Maier et al. [30, 31]. They reported they observed the theoretical EMF using the following cell

O2223 , CO , Na CO , Au | Na-β -alumina | Au, Na 26323722 Ti O + Na Ti O , CO , O (3.22)

The interesting feature of this design is mixing gold powder with electrode pellets. The gold powder mixture seemed to increase the exchange current and it helped the electrode become more reversible.

Figure 3.11 shows a SEM picture of the Li2TiO3+TiO2 electrode morphology.

Li2TiO3+TiO2 particles and gold powders are well distributed because their particle sizes are similar (5~10 µm). The adhesion of the electrode to the electrolyte was good.

However, delamination of Li2CO3 with gold powders from the electrolyte was found in the sensing electrode. In Figure 3.12, Li2CO3 particles and gold powders are shown sintered separately and their ratio looked close to 50:50. However, more gold particles were found at the bottom of this electrode, which is the interface between the electrolyte and the sensing electrode. Figure 3.13 shows the schematic cartoon of these two different types of electrode mixture. In the case of Li2TiO3+TiO2 particles and gold powders, they can be homogeneously mixed. But the different particle sizes of Li2CO3 and gold forced segregation of gold from Li2CO3 and heavier gold powder agglomerated at the bottom.

77 When this sensor was tested at 500°C (Figure 3.14), the modified sensor EMF shifted closer to the theoretical values. But the values were still less than that calculated from the Nernst equation. This might be due to a problem of two different particles distribution. So, the next design was studied to solve this problem.

Sputtered gold electrode - Gold was sputtered on top of the electrolyte and annealed at 700°C for 5 hours. After annealing, a porous gold layer was formed on top of the electrolyte (Figure 3.15). Auxiliary phases were painted on the porous gold layer and heat treated following the sensor fabrication method described earlier. The sensor with sputtered gold electrode was tested at 500°C. Figure 3.16 shows the EMF comparison for sensors with gold paste and sputtered gold electrodes with 10 MΩ and 10GΩ internal impedance of voltmeter. It didn’t improve the absolute sensor EMF much, but its dependence on the internal impedance of the voltmeter was minimized indicating that the total sensor impedance was decreased minimizing overpotential.

3.3.2 Impedance spectroscopy of sensor electrode materials For the impedance measurement, symmetrical cells were constructed and tested. For convenience, the following abbreviations will be used for the cells with different auxiliary phases.

GB: Gold blocking electrode without the auxiliary phases

LC: Gold electrode with Li2CO3 auxiliary phase

LT: Gold electrode with Li2TiO3+TiO2 auxiliary phase

These electrochemical cells were constructed with two symmetric electrode designs.

Regular design: In this design, 12 mm diameter electrolyte pellets were prepared and 4 mm diameter hole was punched in a commercial scotch tape. By using this tape as a mask, gold was sputtered on top of the electrolyte to make the same size of gold electrodes for the different auxiliary phases. After this procedure, auxiliary phases were hand-painted on both sides of the electrolyte as usual.

Figure 3.17 shows impedance spectra of different auxiliary phases at 500°C. 500 ppm 7 CO2 and 10% O2 was used for testing. Frequency was swept from 10 to 0.01 Hz with 10 78 mV applied voltage amplitude. In Figure 3.17, it was considered that impedance plots have two distinctive semicircles even though second one was not seen fully in this frequency range. Therefore, model A, which has two parallel RC combinations, was used for impedance analysis.

These tested cells had clearly bulk electrolyte property in the first semicircle in the high frequency region because resistances of these semicircles did not change depending on gas concentration change. When we compare the capacitance value (~10-11 F) of the bulk properties with that reported literature, it is in good agreement with the values found in other solid state electrochemical systems, grain (~10-12 F) and grain boundary (~10-10 F) -5 -7 of β-alumina [3]. Cdl was about 10 F, which is higher than that of β-alumina (~10 F). Table 3.1 shows values of the elements obtained from fitting using the Zview software (Scribner Associates, Inc).

Generally, GB shows a very large resistance element in the second semicircle corresponding to low frequencies. In this figure, LC showed the smallest charge transfer resistance, which means it is close to a reversible electrode. On the other hand, gold electrode was an ion blocking electrode, which is a totally non-reversible electrode. LT shows the behavior similar to GB, but with a larger charge transfer resistance indicating a non-reversible electrode.

Bulk electrolyte properties Electrode polarization

Rb (Ω) Cb (F) Rct (Ω) Cdl (F) GB 43044 1.40×10-11 3.47×106 1.65×10-5 LT 29490 1.14×10-11 275770 2.29×10-5 Sensor 22258 2.57×10-11 LC 11536 4.35×10-11 2437 4.02×10-5

Table 3.1: Values of resistance and capacitance in the equivalent circuits of the testing cells. 79 In these tests, the bulk resistance changed depending on the auxiliary phase material even though the same dimension of pellets were used with the same size of gold electrode. Gold electrode without the auxiliary phase showed the largest resistance and then the cell with LT, sensor (non symmetrical design) and LC followed in decending order. This is not an erroneous observation because the resistance of the sensor structure was located between the cell with LC and that with LT. Same observations were also found in other cells consistently. This seems to be related to mixed ionic and electronic conduction of Li3PO4 electrolyte depending on lithium activity in the auxiliary phase. It is plausible that Li2CO3 auxiliary phase with the highest lithium activity increased the electronic conductivity of the electrolyte and this will be discussed in chapter 4.

These cells were also tested under different gas environments. Figure 3.18 shows the impedance spectra of the LC cell with varied CO2 concentrations from 200 ppm to 50%.

It clearly showed that charge transfer resistance increased with CO2 concentration. At ppm level CO2 concentrations, it was hard to distinguish this change but it became prominent at higher CO2 concentrations.

Oxygen reduction mechanism has been studied for MCFC (Molten Carbonate Fuel

Cell) [33-35]. Dave et al. [33, 35] found that increased CO2 concentration can decrease the concentration of peroxide, which decreases the exchange current density of oxygen reduction. The existence of peroxide in the solid state lithium carbonate is not reported, but increased Rct with CO2 concentration seems to be related to oxygen reduction in the cell-LC.

On the other hand, LT cell was also tested in different CO2 and O2 concentrations. As can be seen in Figure 3.19, the impedance behavior did not change as a function of gas concentration. LT is a good reference electrode which is inactive with CO2 but oxygen dependence of current sensor might be due to the slow kinetics of the reference electrode.

The cell with gold powder: The cell with gold powder was also studied with EIS. Figure 3.20 shows the impedance spectra of the cell-LT with and without gold powder. As can be seen, gold powder decreased the electrode impedance compared to the cell without gold powder showing smaller second semicircle. In the sensor structure, the

80 reference electrode overpotential is larger than that of the sensing electrode. Therefore, previous discussion that the sensor with gold powder showed higher EMF than that without gold powder agrees with increased exchange current of the LT cell with gold powder.

On the other hand, the cell-LC with gold powder showed quite a different impedance behavior compared to cell-LC without gold powder. In Figure 3.20, it is recognized that the impedance spectra looks more complicated at low frequencies than that of the cell-LC without gold powder. In the previous discussion, simple Randles circuit model was used to understand impedance behavior for the LC cell. If charge transfer resistance is large enough, low frequency processes such as diffusion or adsorption do not appear within the same frequency range [3]. So, it is possible that very low frequency processes were missing in the previous LC without gold powder. After gold powder was mixed with lithium carbonate auxiliary phase, exchange current is increased. Therefore, charge transfer resistance was not clearly observed but diffusion or gas adsorption related impedance became distinguished in this frequency region. Similar behavior was also clearly observed in the LC with sputtered gold electrode.

As can be seen in Figure 3.21, low frequency arcs are not perfect semicircles and also they do not seem to represented a single process. Therefore, in order to understand this behavior, it is necessary to introduce model B or C, which contains Warburg impedance elements. However, its phase angle was not exactly at 45° from the real axis. Therefore, it does not seem to be a diffusion-related impedance. From the impedance measurement of the modified electrodes, it is clear that the Li2CO3 electrode impedance is clearly affected by CO2 concentration.

Electrochemical cell with sputtered gold: The impedance spectra of the cell-LT with sputtered gold electrode are shown in Figure 3.22 and 3.23. Cell-LT with sputtered gold seemed to increase the electrode area compared to other LT cells because bulk electrolyte resistance of this cell showed the smallest value with the same size of electrolyte pellet. However, impedance behavior of this LT with sputtered gold showed a similar behavior to that of the cell with Au paste. This result is due to small number of pores in the

81 sputtered gold electrode. Therefore, the effective surface area for electrochemical reaction of the sputtered gold electrode is not as much as that with gold powder.

The number of pores and their size were significantly changed by sputtering condition such as vacuum pressure and sputtering time. To make a similar structure of porous gold electrode, the same sputtering time was used, however, gold target thickness and vacuum condition were not constant. This changed the gold layer thickness even though same sputtering time was used. Therefore, sputtering time had to be slightly increased for subsequent sputtering run to achieve similar thickness. Also, the vacuum condition was carefully monitored.

But, in general, the electrode impedance of the LT cell is quite higher than that of cell- LC. Even though gold powder and sputtered gold enhance the reversibility, its kinetics of electrochemical reaction at the operating temperature seemed to be limited due to its low lithium ion conductivity [32].

The impedance spectra of the cell-LC with sputtered gold are shown at different temperatures, 400°C, 500°C and 600°C in Figure 3.24 - Figure 3.27. Solartron and Gamry equipment were used to get these impedance spectra. Data points from Solartron covered the frequency range from 3980 Hz to 10M Hz and Gamry covered the lower frequency range.

The DC resistance was also measured for comparison and was deduced from the linear region of I-V curve. The potential was swept from 0 to +30 mV and it was cycled to 0 mV. At 500°C, straight linear I-V curve was observed as shown in Figure 3.28, but the I- V curves showed hystereses at other temperatures. The DC resistances are marked on the real axis in the impedance plane plots.

From the tests under different gas concentrations, it is recognized that electrode impedance is significantly affected by CO2 and O2 concentrations at all temperatures.

Low frequency arcs expanded with decreasing CO2 and O2 concentrations. However, no indication of Warburg type impedance behavior was observed in the cell-LC with sputtered gold electrode. Therefore, these impedances are believed due to gas adsorption processes.

82 The impedance spectrum of the cell-LC with sputtered gold has two distinct semicircles at high temperatures but they became lumped at low temperatures. At high temperatures, adsorption/desorption kinetics plays more important role, while charge transfer effect become more dominant at lower temperature. This type of polarization was explained by typical adsorption isotherms [12, 36] in literature.

3.4 Summary

Electrochemical CO2 gas sensor was examined by the EIS technique and the results can be summarized as follows.

• Li2TiO3+TiO2, reference electrode

⇒ Gold powder and sputtered porous gold electrode enhanced the exchange current decreasing overall sensor impedance.

⇒ Its electrode impedance was not affected by CO2 or O2 gas concentration. In other words, electrochemical reaction of the reference electrode seemed to be very slow.

⇒ Even though the exchange current is increased, this reference electrode still is the main source of polarization in the sensor structure.

• Li2CO3 sensing electrode

⇒ Compared to the reference electrode, it shows relatively small overpotential in

the CO2 sensor.

⇒ Its overpotential is significantly changed depending on CO2 and O2 concentration.

⇒ Higher sensitivity at high CO2 concentrations probably is due to the much

smaller overpotential than that at low CO2 concentrations.

⇒ At high temperature, charge transfer kinetics seemed to be fast and overpotential is mainly due to the diffusion or adsorption process. But charge transfer kinetics became more dominant as temperature decreased.

83 Fine gold electrode structure such as gold powder and sputtered porous gold electrode enhanced the general electrode kinetics, but still non-Nernstian behavior was not fully understood by electrode kinetics study. Therefore, chapter 4 will discuss the possible other reason, mixed ionic and electronic conduction of Li3PO4 electrolyte.

84 References

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10. D. R. Franceschetti, J. R. Macdonald and R. P. Buck, “Interpretation of Finite- Length-Warburg-Type Impedances in Supported and Unsupported Electrochemical Cells with kinetically Reversible Electrodes”, J. Electrochem. Soc., 138, 1368, (1991).

11. K. Jonscher, “Analysis of the alternating current properties of ionic conductors”, J. Mater. Sci., 13, 553, (1978).

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13. R. D. Armstrong, “The metal-solid electrolyte interphase”, Electroanal. Chem., 52, 413, (1974).

85 14. R. D. Armstrong, T. Dickinson and R. Whitfield, J. Electroanal. Chem., 39, 257, (1972).

15. S. Villain, M. A. Desvals, G. Clugnet and P. Knauth, “Study of polycrystalline CuBr and the interface Cu|CuBr by impedance spectroscopy”, Solid State Ionics, 83, 191, (1996).

16. P. Machowski, M. Opallo, J. E. Garbarczyk and M. Wasiucionek, “Cyclic voltammetry and impedance spectroscopy studies of silver vanadate phosphate glasses”, Solid State Ionics, 157, 287, (2003).

17. Ho, I. D. Raistrick and R. A. Huggins, “Application of A-C techniques to the study of lithium diffusion in tungsten trioxide thin films”, J. Electrochem. Soc., 127, 343 (1980).

18. R. L. Paul and A. Cartwright, “The mechanism of the deposition of manganese dioxide Part II. Electrode impedance studies”, J. Electroanal. Chem., 201, 113, (1986).

19. W. C. West, K. Sieradzki, B. Kardynal and M. N. Kozicki, Equivalent circuit modeling of the Ag|As0.24S0.36Ag0.40|Ag system prepared by photodissolution of Ag”, J. Electrochem. Soc., 145, 2971, (1998).

20. Bohnke, C. Bohnke and J. L. Fourquet, “Mechanism of ionic conduction and electrochemical intercalation of lithium into the perovskite lanthanum lithium titanate”, Solid State Ionics, 91, 21, (1996).

21. P. Yu, B. N. Popov, J. A. Ritter and R. E. White, “Determination of the lithium ion diffusion coefficient in graphite”, J. Electrochem. Soc., 146(1), 8, (1999).

22. K. Dokko, M. Mohamedi, Y. Fujita, T. Itoh, M. Nishizawa, M. Umeda and I. Uchida, Kinetic characterization of single particles of LiCoO2 by AC impedance and potential step methods, J. Electrochem. Soc., 148 (5), A422, (2001).

23. K. Dokko, M. Mohamedi, M. Umeda and I. Uchida, “Kinetic characterization of Li- ion extraction and insertion at LiMn2O4 single particle electrodes using potential step and impedance methods, J. Electrochem. Soc., 150 (4), A425, (2003).

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86 26. V. Brichzin, J. Fleig, H. U. Habermeier, G. Cristiani and J. Maier, “The geometry dependence of the polarization resistance of Sr-doped LaMnO3 microelectrodes on yttria-stabilized zirconia”, Solid State Ionics, 152-153, 499 (2002).

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Figure 3.1: Randles circuits for (A) a ideally polarizable electrode and (B) a non-polarizable electrode (Reversible electrode) [2].

Figure 3.2: Current-Voltage responses of (A) a polarizable electrode and (B) a non-polarizable electrode (Reversible electrode) [2].

Figure 3.3: The impedance Z plotted as a planar vector in the rectangular and polar coordinates [3].

Figure 3.4: General semicircles of impedance elements contributing overpotentials of solid state electrochemical cell in the impedance plane [6].

89 (A)

Figure 3.5: (A) Model A (Randles equivalent circuit) and (B) schematic impedance plot.

Figure 3.6: Model B (Warburg Impedance in series with Rct).

Figure 3.7: Finite Warburg impedance in the intercalation system [3].

Figure 3.8: Infinite Warburg impedance in gas diffusion electrode [3].

Figure 3.9: Model C (Warburg Impedance in series with Randle circuit).

Figure 3.10: Schematic of transfer function analyzer [3].

92 (A)

Figure 3.11: Back scattered SEM photo of Li2TiO3+TiO2 with gold powder mixture (white: gold, dark:Li2TiO3+TiO2) (A) 400X (B) 1600X.

93 (A)

Figure 3.12: Back scattered SEM photo of Li2CO3 with gold powder mixture. (A) Top of the Li2CO3 with gold powder electrode (faced to gas) (B) Bottom of the Li2CO3 with gold powder electrode (faced to electrolyte)

Figure 3.13: Schematic of two different types of particle mixture.

Figure 3.14: EMF comparison between sensors with and without gold powder.

Figure 3.15: Backscattered SEM photo of sputtered gold on top of Li3PO4 electrolyte (white: gold, black: Li3PO4 electrolyte).

Figure 3.16: EMF comparison between sensors with Au paste and sputtered gold electrodes.

Figure 3.17: Impedance spectra for different electrodes at 500°C.

Figure 3.18: Impedance spectra for the cell-LC under different CO2 concentrations at 500°C.

Figure 3.19: Impedance spectra for the cell-LT under different CO2 and O2 concentrations at 500°C.

Figure 3.20: Impedance spectra for the cell-LT with and without gold powder at 500°C.

Figure 3.21: Impedance spectra for the cell-LC with gold powder at 500°C.

Figure 3.22: Impedance spectra for the cell-LT (without Au) and the cell-LT with gold powder and the cell-LT with sputtered gold at 500 ppm CO2 and 10% O2 at 500°C.

Figure 3.23: Impedance spectra for the cell-LT (without Au) and the cell-LT with gold powder and the cell-LT with sputtered gold at 500 ppm CO2 and 10% O2 at 600°C.

Figure 3.24: Impedance spectra for the cell-LC with sputtered gold under various CO2 and O2 concentrations at 400°C.

Figure 3.25: Impedance spectra for the cell-LC with sputtered gold under various CO2 and O2 concentrations at 500°C.

concentrations at 600°C. 2 and O 2

-LC with sputtered gold under various CO Figure 3.26: Impedance spectra for the cell

concentrations 2 and O 2 LC with sputtered gold under various CO LC ) at 600°C. 2 Figure 3.27: Impedance spectra for the cell- (5%~50% CO

Figure 3.28: I-V curve under 5000 ppm CO2 and 10% O2 at 500°C.

104 CHAPTER 4

THE EFFECT OF MIXED IONIC AND ELECTRONIC CONDUCTION IN THE ELECTROLYTE TO CO2 GAS SENSOR

In the previous chapter, electrode kinetics were studied to understand the non- Nernstian sensor response. As discussed earlier, the other possible reason for this behavior is mixed ionic and electronic conduction of Li3PO4 electrolyte. Näfe has studied electronic conduction of Na-β-alumina [1-3] and discussed the non-Nernstian behavior of

a CO2 sensor due to a p-type conduction of Na-β-alumina [4, 5]. If the electrolyte of a galvanic cell is not a perfect ionic conductor, the Nernst equation must be considered with the appropriate ionic transference number.

The transference number or transport number, ti , is defined by the fraction of the total current carried by a particular charged species [6]

ii σ i ti = = (4.1) ∑i j ∑σ j j=1 j=1

where ii is the current and σ i is the conductivity by mobile charged species i.

The total conductivity σT becomes the sum of all the contributions from the mobile species in the electrolyte expressed as

σ T = ∑ µi qici (4.2) i

where µi , qi and ci correspond to the electrochemical mobility, the amount of charge per carrier, and the concentration of mobile species i contributing to the current flow, respectively. If the mobility is independent of concentration, the concentration of charge

105 carriers in the electrolyte plays an important role in determining the relative conductivity contribution [7].

The significance of the transference number, ti , is that it can change the EMF in the electrochemical sensor [8]. Moreover, it is not a constant but a variable changed by the activity of mobile species in the electrolyte [8]. The Nernst equation is valid only when the ionic transference number is 1. Considering transference number, the generalized Wagner cell voltage equation for an oxygen conductor is expressed as [9-10]

µµ′′ ′′ 111OO22 EMF=− t d µ =− d µ (4.3) ∫∫ion OO22σ 4Fµµ′′ 4F e OO221+ σion where µ′ and µ′′ denote the oxygen chemical potentials at the two different electrodes O2 O2

of the galvanic cell. If tion =1, the above equation reduces to the Nernst equation.

4.1 Measurement of partial electronic or ionic conduction

4.1.1 Conduction domain The solid-state electrolyte plays an important role in an electrochemical device. Ionic conductivity or electronic conductivity of the electrolyte depends on the presence of local deviations from the perfect crystalline order, in other words, non-stoichiometry [10, 11].

If each conductivity, σ ion and σ el , of a material does not differ by more than 2 orders of magnitude, we can call this material MIEC (Mixed Ionic and Electronic Conductor) [12].

When σ ion of a certain material is higher than σ el satisfying the above condition, it can be called an ionic conductor. It is important to note that even the well-known solid electrolytes have dominant ionic conductivity only in certain ranges of temperature and chemical potential of the mobile species [13, 14].

The ionic conduction of a solid electrolyte is mainly related to the crystal structure, while the electronic conduction is determined by the band gap, if it does not contain any impurity [15]. Ionic crystals have intrinsic or extrinsic defects such as vacancies or interstitial ions and impurities. These point defects are described following the Kröger-

106 Vink notation, which assigns a charge relative to a perfect crystal instead of absolute charge of the defect.

Here, we pick the example of a pure binary compound, MX. Intrinsic ionic and electronic defect species would be produced by Schottky, Frenkel, anti-Frenkel or intrinsic electronic disorder in thermodynamic equilibrium.

'' •• '' •• (Schottky Disorder) O = VMX +V KSMX =[V ][V ] (4.4)

'' •• '' •• (Frenkel Disorder) MMMi = V+M K[V][M]FMi= (4.5)

•• '' •• '' (Anti-Frenkel Disorder) XXXi = V+X K[V][X]AF= X i (4.6)

'• ' • (Electronic excitation) O = e + h K[e][h]i = (4.7)

where K,S K,F KAF and Ki are the mass action constants for Schottky, Frenkel, anti- Frenkel and intrinsic electronic disorder, respectively [2].

These defects must satisfy the electro-neutrality condition (ENC) generally expressed as in equation (4.8) equating the total positive charge to the total negative charge.

++ -- ∑ nniizz= ∑ ii (4.8) ii

For instance, the ENC for electrolytes with Frenkel disorder can be written as

•• • '' ' 2 [M iM ]+= [hVe ] 2 [ ] + [ ]. From the ENC and the equation (4.5), Frenkel defect concentration can be represented as a function of the chemical activity of the metal component in the electrolyte. The partial conductivity for each species can also be determined if we know the mobility of the charged species [2]. Figure 4.1 shows the schematic partial electrical conductivities for an MX compound [16].

Paterson et al. [13] proposed conduction domains for an ionic compound MaXb. At high temperatures, the electrolyte has an ionic conductivity independent of P . X2 However, excess electron and positive hole conductivities, σ and σ , depend on P n p X2 1 with an exponent of ± , respectively [13, 16]. At constant P , all three conductivities n X2

107 exhibit Arrhenius-type temperature dependence with activation energies Qion , Qn and Q , which are P and T-independent [13]. The total conductivity σ in the solid p X2 T electrolyte is defined as follows

σ T = σ ion +σ p +σ n (4.9)

D  Qion  σ ion = σ ion exp−  (4.10)  RT 

1/ n  Q  σ = σ D P exp− p  (4.11) p p X 2    RT 

D −1/ n  Qn  σ n = σ n PX exp−  (4.12) 2  RT  where n is determined by the predominant defect equilibrium of the MaXb solid electrolyte, σD , σD , σD are also P and T-independent [16]. At a certain temperature, ion p n X2 conduction versus P graph shows that the total conductivity is dominated by only one X2

of the above conduction modes [16]. As can be seen in Figure 4.2, logσ T and logσ p are

nearly identical in region A because σ p is much higher in magnitude than either σ ion or

σ n . For the same reason, logσ T approaches logσ ion in region B and to logσ n in region C.

The transition boundary between σ ion and σ n , or σ ion and σ p or σ n and σ p , is defined as the points where the two conductivities are equal. Combining equations (4.10), (4.11) and (4.12), one can derive the following relationships

QQ− 1 σD logPn=− ion p + nlogion = log P (4.13) X 2  D ⊕ 2.303RT σ p

QQ−σ1 D logPn=+ ion n − nlog ion = log P (4.14) X 2  D − 2.303RT σn

108 where P⊕ and P− are the p- and n-type electron conduction parameters or the Schmalzried

parameters, which indicate activities where conductivities σ n and σ p respectively are

equal to the σ ion . The region near the border of each dominant conduction domain in Figure 4.1 represents MIEC [13].

4.1.2 Experimental Method to verify the Transference Number for a MIEC

4.1.2.1 EMF measurement Schmalzried found the theoretical inference of the ionic transference number from the measured voltage of a galvanic cell with an oxygen conductor exhibiting mixed ionic and electronic conductivity [7, 18]. When we assume that the mobilities of the charge carriers are independent of oxygen partial pressure, from equations (4.11) - (4.14), σσ= (/)PP1/n and σσ= (/)PP−1/n can be obtained. Hence, t in the mixed pOo 2 − 2 ⊕ nOo 2 − 2 − ion conducting oxygen electrolyte can be written as

PP t =++=++σσσσ/[ ] [1 (OO22 )1/nn ( )−− 1/ ] 1 (4.15) OOO222−−−pn PP⊕−

Substituting equation (4.15) into equation (4.3) and integrating, the general cell emf is

PP1/nn++′ 1/ PP 1/ n′′ 1/ n nTR ⊕−OO22 EOC =−[ln + ln ] (4.16) 4F PP1/nn++′′ 1/ PP 1/ nn′ 1/ ⊕−OO22 where P ′′ corresponds to PO at the reference electrode and P ′ at the sensing O2 2 O2 electrode.

Depending on conditions of oxygen partial pressure on the electrodes, equation (4.16) can be modified to a simpler form. Very low oxygen partial pressure is a common reducing environment, which is important for fuel cell application. In this type of EMF measurement, air is used as a reference electrode that keeps the electrolyte within the ionic conduction domain. Reversible electrodes such as Pt with a buffer gas H2+H2O or metal-metal oxide (Fe-FeO, Ni-NiO, and Mn-MnO) are used to provide the low oxygen

109 partial pressure. Therefore, the relationship of oxygen partial pressure and electronic conduction parameters are represented by equation (4.17).

PPPP′″<<<<<<− ⊕ (4.17) OO22

Using equation (4.17), equation (4.16) yields the following expression

11  1 44 PP− + ″ 4 RRTTO2 P ″ E ==ln ln O2 (4.18) OC FF11 1 44 4 PP− + ′ P− O2 and this leads to

4FEOC PP− =−″ exp (4.19) O2 RT

Equation (4.19) produces an n-type electronic conduction parameter of this oxygen conducting cell from the measured EMF.

Tretyakov et al. [19], Hardaway et al. [20] and Levine et al. [21] studied the electronic conduction parameter for the ThO2-Y2O3 oxygen conductor and Schieltz et al. [22] investigated the electronic conduction of Y2O3 doped-HfO2 by using equation (4.19).

For the EMF measurement, reversible electrodes are required. If electrode polarization effect changes the EMF, reliable electronic conduction parameter cannot be obtained.

4.1.2.2 Hebb-Wagner Polarization Method Hebb and Wagner developed an experimental technique to measure the transference number by using the DC polarization method, which suppresses either the electronic or ionic conductivity with respect to the terminal electrode [9]. Wagner measured the electronic conductivity of the cuprous halides constructing the cell,

Cu (Reversible electrode) | CuX | Graphite (Non-reversible electrode) (4.20)

where X denotes Cl, Br, or I [23].

110 When a voltage is applied (positive right-hand side) below the decomposition potential of CuX to the cell, the copper ions in CuX migrate from the graphite electrode, to the copper electrode while electrons move to the opposite direction [9]. At the graphite side, copper ion concentration is decreased because copper transports to the Cu side of the cell. Under a steady-state condition, migration of copper ions resulting from the electrical field is balanced by the diffusion flux due to the concentration gradient as shown in equation (4.21).

 ∂µi   ∂ϕ  ji = −ci Bi   + zi e  (4.21)  ∂x   ∂x 

where ci is the number of particles and Bi the mobility. In a binary system, electronic conduction depends on the chemical potential of metal ion under the polarization condition. Thus the partial conductivities due to excess electrons and electron holes are proportional to the respective concentrations. Then the electronic conductivity is obtained as follows

 DD  (µ−µMe Me) −µ−µ( Me Me ) σ=σDDexp  +σ exp  (4.22) npzTRR zT  11 

D D where σ n and σ p are the respective partial conductivities in the mixed conductor coexisting with pure metal, Me .

Therefore, the total current density is expressed as

DDEETFFR    J =σnp1exp − −  +σ exp  − 1  (4.23) RRFLTT    where E is the applied voltage and L is the thickness of the MIEC.

Increasing potential E, the first term, excess electron conduction in equation (4.23)

D approaches σ n , while the second term corresponding to electron hole conduction increases exponentially.

111 Wagner assumed that there is no concentration gradient of interstitial ions or vacancies under these conditions and no potential gradient within the mixed conductor because ions do not move after polarization reaches the steady state. Consequently, migration of excess electrons and electron holes is due to a concentration gradient rather than a potential gradient [9]. The voltage applied to the cell is essentially used in order to increase the potential differences across the interface between the electronic conductor and the mixed conductor. It makes the concentration of excess electrons lower or the concentration of electron holes of mixed conductor higher than those at the reversible electrode [9].

According to equation (4.23), a plot of J versus E is expected to show a saturation

D RT D D plateau, J = σ , in a certain range of E, if σ >> σ . n FL n p

Such a plateau has been found for a sample of AgBr between 333°C and 372°C as shown in Figure 4.4 [24]. This behavior indicates that the electronic conductivity of silver bromide is predominantly n-type in this voltage range. From this plot, we can

D immediately calculate the electronic conductivity σ n of silver bromide in equilibrium with silver.

For an AgI single crystal, the current vs. potential plot does not show a saturation plateau; its behavior follows a logarithmic function [24]. In other words, the second term in equation (4.23) prevails.

D RFTE logJ ≅σ log p + (4.24) FL 2.3RT

D RT  RT  σ n if E >> and E >>  ln D (4.25) F  F  σ p

F Thus a plot of log J versus E should produce a straight line with the slope as 2.30RT

D can be seen in Figure 4.3. The hole conductivity, σ p , in equilibrium with silver can be obtained by extrapolating the exponential function to E = 0.

112 For polarization measurements on Zr0.85Ca0.15O1.85 and Th0.85Y0.15O1.925, Patterson et al. [25] used a solid gold non-reversible electrode to block the oxygen, and various metal+metal oxide reversible electrodes. Rapp et al. [16] indicated that thermoionic emission may result in electronic transport around the sample for these dc measurements of very high resistances at temperatures of 800°C and higher. For this reason vacuum should not be used in the system [16].

This Hebb-Wagner polarization measurement technique is very useful and well defined theoretically. However, well-defined reversible electrode at high temperature is rarely available for a lithium ion conductor.

The limitations of HW polarization measurement are:

⇒ Decomposition of the sample [26, 27] can provide incorrect information because I-V relation due to sample decomposition is not clearly distinguished from the p-type conduction behavior, which shows exponential current increase.

⇒ Non-reversible electrode can induce an electrode overpotential [1].

4.1.2.3 Faradaic efficiency measurement (Tubandt or Hittorf method) Transference number can be calculated by the amount of charged particles when constant DC is passed through an ionic conductor [1]. Tubandt et al. [28] developed this technique to measure the ionic transference number of a AgI sample. They connected three pellets of AgI and let a current flow through this system as can be seen in Figure 4.5. The change of each mass can show whether this system follows the Faraday’s law. It was observed that part A and D showed the same amount of mass change and part B and C had no mass change. From these observations, they concluded that AgI is a pure ionic conductor. Matsumoto et al. [29] used this method to confirm lithium ionic conduction in LiLaSiO2. They used three similar pellets to measure the weight change of the anodic and cathodic part of the pellet because of limitation of pure lithium alloy.

Kharton et al. developed this measurement for oxygen conductors such as LaCo(M)O3

(M=Ga, Cr, Fe or Ni) [30], Bi2O3-ZrO2-Y2O3 and Bi2O3-NbO5-H2O3 [31]. In their

113 experiment, oxygen was pumped into the internal volume by a direct current. The current - value was kept constant with an accuracy of 0.2%. The oxygen flux density J O (t) (mol/s 1⋅cm-2) pumped through the specimen is defined by the equation:

1 dν (t) J (t) = S − ⋅ O (4.26) O    dt 

2 where S (cm ) is the electrode area, ν O (mol) is the amount of oxygen pumped into the cell, and t is the time after starting the measurement cycle. The quantity ν O is calculated from the water-column manometer reading with consideration of atmospheric pressure and temperature of the system. Therefore, the transference number of oxygen ion conduction is obtained by

4F ⋅ S ⋅ J (t) t = O (4.27) O I(t)

The drawbacks of the Faradaic efficiency measurement are:

⇒ It is difficult to measure the accurate weight change.

⇒ The availability of pure metal for alkali ion conductor is limited.

⇒ Gas tight experimental setup is necessary for anion conductor which is reversible to gas electrodes. (e.g. Oxygen conductor)

4.2 Experimental

For the total conductivity measurement, a Li3PO4 electrolyte pellet was prepared by the cold pressing and sintering process described in chapter 2. Gold paste (Heraeus Gold ink) or sputtered gold was used as an ion blocking electrode. The whole pellet surface was painted by gold paste and it was cured at 700°C for 1hr. This procedure was repeated twice to obtain a perfect ion blocking electrode. In the case of sputtered gold, gold film was deposited following the procedure described in chapter 3. A sputtering time of 10 min was used to get a fully dense film on top of the electrolyte. It was annealed at 700°C for 5hr. Gold wire was attached on top of the electrode surface by using gold paste. Solartron 1260A was used to measure the impedance for total conductivity measurement.

114 Hebb-wagner polarization measurement requires one reversible electrode and one ion blocking electrode. Gold paste was also used for the ion blocking electrode following the above procedure. On the other side of the pellet, a reversible electrode was made from lithium carbonate and gold paste mixture. This electrode was cured at 600°C for 1hr. In order to measure the steady state current, potentio-static mode was chosen with the Gamry instrument. Potential was increased in 100 mV steps. Each step was maintained for 2 hr to collect the steady state current.

4.3.1 Total electrical conductivity measurement for Li3PO4 electrolyte

Sputtered Gold (Ω-1cm-1) Gold paste (Ω-1cm-1) 400°C 5.254×10-7 2.061×10-7 450°C 2.184×10-6 1.165×10-6 500°C 7.682×10-6 4.570×10-6 550°C 2.105×10-5 1.615×10-5 600°C 5.379×10-5 4.369×10-5

Table 4.1: Total Conductivity from the AC measurement with gold ion blocking electrode (sputtered gold and gold paste).

Three polymorphs of β, γ, α-Li3PO4 are known to occur at progressively higher

temperatures [32]. The phase transformation temperatures are 520°C (β→γ-Li3PO4) and

1170°C (γ→α-Li3PO4) [32]. In this experiment, γ-Li3PO4 was used as a test sample.

Total conductivity was measured by gold ion blocking electrode. Figure 4.6 shows a typical impedance spectra of gold ion blocking electrode in air at 500°C. High frequency

115 impedance plot showed a depressed semicircle which can be resolved into two semicircles, corresponding to bulk and grain-boundary contributions. The total conductivity including ionic and electronic nature was obtained from the first semicircle. shows the total conductivity obtained from the AC measurement. The conductivity data is plotted as log (σT ) vs. reciprocal absolute temperature in Figure 4.8. The data showed good fit to the Arrenius equation

Ea σ=σT 0 exp  - (4.28) kT

where Ea is the activation energy, σ0 is the pre-exponential constant and k is the Boltzmann constant.

Electrolyte conductivity using sputtered gold is higher than that using gold paste. Probably, it is due to better contact of sputtered gold electrode to the electrolyte than that of gold paste. Moreover, their activation energies were different. Calculated activation energy (1.24 eV) of sputtered gold electrode from the Arrenius plot was lower than that of gold paste electrode (1.42 eV), but it agreed well with that proposed by Wang et al.

-1 Ea (eV) σ0 (S·cm ·K)

Present Study 1.24 6.57×105

Wang et al. [32] 1.24 1.0×106 (J. Solid State Chem. 1995) Hu et al. [33] 1.30 2.5×106 (J. Electrochem. Soc. 1977)

Table 4.2: Comparison of Ea and σ0 for Li3PO4 electrolyte of present study and literature.

116 [32] in Table 4.2, even though there is a small difference in σ0 . They reported the measured conductivity as the lithium ion conductivity, however, it has to be considered as a total conductivity rather than the ionic conductivity. The AC measurement with ion blocking electrode cannot separate ionic and electronic conduction. Therefore, if the electrolyte is not a perfect ionic conductor, this measurement can only provide total conductivity.

Figure 4.7 shows the impedance plot with different gas concentrations. As can be seen, impedance behavior did not change as a function of gas concentrations. It indicates that the conduction of Li3PO4 electrolyte is chemically stable under various environments. Therefore, it is believed that oxygen does not affect possible electrochemical reaction forming lithium oxide at the triple phase boundary of gold electrode, electrolyte and gas, which can disturb the ion blocking electrode condition.

4.3.2 EMF measurement As shown in chapter 2, the sensor assembly can be represented as the following electrochemical cell.

O2223 ,CO , Li TiO+ TiO 2 , Au | Li 34 PO | Au, Li 2322 CO , CO , O (4.29) aaLi′′′ Li

The measured EMF gives the Li activity difference between the two interfaces, I

(reference electrode) and II (sensing electrode), of Li3PO4 electrolyte.

RT aLi′′ EOC =− ln  (4.30) F aLi′

I II where aLi and aLi are the activities of Li at the reference and measuring electrodes, respectively.

The sensitivity of CO2 electrochemical cells was measured for various Li-ionconducting electrolytes as shown in Figure 4.10. The slopes of the emf with respect to logarithmic concentration of CO2 are generally lower than the theoretical value of -76.7

117 RT mV/decade −2.3 at 500°C. Since the reversible open-circuit potential of a mixed 2F conducting cell is expressed as

EtEOC= ion ⋅ th [34] (4.31)

where tion and Eth correspond to the average ionic transference number and the theoretical emf value of the cell, respectively. The degree of deviation from the theoretical slope indicates the degree of the electronic conduction involved in the electrolyte during the emf measurement. This argument suggests that some electronic conduction is present in all the electrolytes featured in Figure 4.10.

Schmalzried [7] was the first to treat the theoretical analysis of the interference of the electronic conduction from the measured emfs of a galvanic cell involving an oxygen-ion conducting electrolyte exhibiting mixed ionic and electronic conductivity. Recently, Näfe [5] extended that treatment involving Na-ion conducting electrolyte of Na β-alumina, where the electronic conduction occurs at the reference electrode or at the working electrode or at both.

The Wagner cell voltage equation is expressed for lithium ion conductor as

RT aa+ ′′ aa+ ′ E =−[ln⊕ Li + ln− Li ] (4.32) OC F ′ ′′ aa⊕−++Li aa Li

In our experiments, the measured emf shows that the activity of Li at the measuring side is higher than that at the reference electrode. Therefore, assuming that the reference electrode resides in a regime where the conduction occurs only by Li ions, aa⊕− Li′ a, it is most probable that the activities of Li at the working electrode resides in a regime where electronic conduction occurs, aa⊕ Li′′ [13]. In this case, the emf of the cell is given [5] as

  RT aLi′′ a− EOC =−ln  ⋅ (4.33) ′′′ F aaaLi Li + − 

118 where aLi′ and aLi′′ are the activity of Li at the reference and at the working electrode, respectively and a− is the so called electron conduction parameter, which is the Li activity where the conductivity due to electrons become the same as that due to Li ions.

According to equation (4.33), the electron conduction parameter, a− can be written as

11FEOC 1 =−exp (4.34) aa− Li′ R T a Li′′

where aLi′ can be calculated from the equilibrium reaction of the reference material

1 Li TiO =2Li+TiO + O (4.35) 23 22 2

At a given oxygen and CO2 pressure, aLi′′ is also obtained from the equilibrium reaction occurring at the working electrode,

1 Li CO =2Li+ O +CO (4.36) 232 2 2

Figure 4.9 shows the electron conduction parameters as a function of temperature for the Li3PO4 + 5m/o SiO2 electrolyte calculated from equation (4.33) at 400°C to 600°C for various concentrations of CO2 with a constant oxygen concentration of 10%. Above this boundary, the electron conduction becomes dominant (t− > 0.5 ) and below the ionic conduction is dominant (tion > 0. 5 ). The calculated electron conduction parameters fall within a very narrow range of the boundary line for a wide range of CO2 concentrations, approximated as

10000 loga− =⋅ 1.38 − 41.06 (4.37) T

The activity of Li at the working electrode (Li2CO3) corresponding to 200 ppm CO2 is also plotted in Figure 4.9 as a function of temperature. In this figure, the working electrode resides in the ionic conduction region at lower temperature of 400°C but it goes into the mixed conduction region as temperature increases. Even though the dependence is small, the electron conduction parameter definitely changes with the activity of Li 119 (CO2 concentration at the working electrode) so that the electrolyte contacting the working electrode exhibit different values for the electron conduction parameter as the

CO2 concentration at the working electrode changes. Thus electrolyte interface would adjust itself to the atmosphere as Näfe [5] pointed out for Na-conducting β-alumina.

However, the calculated electronic conduction parameter was so low that the electrical conduction of Li3PO4 at 500°C and 600°C is mostly by n-type conduction based on Figure 4.9. Moreover, the lithium activity of the reference electrode also resides in the n- type conduction region, which contradicts our first assumption that the lithium activity of the reference electrode is in the ionic conduction region. This result indicates that one of our conditions chosen is not appropriate and needs to be modified.

When equation (4.32) was simplified to (4.33), the condition aa⊕ Li′′ and aa⊕− Li′ a were used. But the second condition is not valid based on the resultant electronic conduction parameter. Therefore, we defined a new condition of aa⊕ Li′ , which means that the lithium activity of the reference electrode also can be close to n- type electronic conduction boundary, instead of second condition in the previous assumption. In this case, equation (4.32) can be modified to

  RT aaaLi′′ Li′ + − EOC =−ln  ⋅ (4.38) ′′′ F aaaLi Li + − 

Using equation (4.38), new electronic conduction boundary was calculated and is represented in Figure 4.11. Its calculated electronic conduction parameter is

10000 loga− =− 2.214 ⋅ + 9.8825 (4.39) T

As can be seen in Figure 4.11, the lithium activities at the sensing electrode are located in the n-type conduction region slightly above the electronic conduction parameter, but the lithium activity at the reference electrode is in ionic conduction region. These results are consistent with our assumptions.

Transference number was also calculated from the electronic conduction parameters and the lithium activities at the lithium carbonate electrodes. 120 aLi σ=σ⋅nion (4.40) a−

σion tion = (4.41) σnion+ σ

These results are shown in table 4.3. Generally, the ionic transference numbers at 500°C are higher than those at 600°C and they also increased with CO2 concentration due to the decreased lithium element activities at all temperatures. On the other hand, they become smaller at higher than 20% CO2 concentration. This behavior is more significant at 400°C. Unstable EMF measurement is the reason of this as described in chapter 2.

4.3.3 Hebb-Wagner (HW) Polarization Method For the HW measurement, the HW cell was constructed as following

Li23 CO++ Au | Li 34 PO 5 m/o SiO 2 | Au (ion blocking electrode) (4.42)

Typical time responses of the current in the HW measurements at 500°C for Li3PO4 pellet (1.15 mm thickness and 7.6 mm D) at all potentials are shown in Figure 4.11. We see a very fast response at the beginning, followed by a much slower change in current at

CO2 Concentration Ionic transference number at the Li2CO3 electrode (ti) (ppm) 400°C 500°C 600°C 500 0.199 0.418 0.292 1000 0.217 0.457 0.326 2000 0.229 0.489 0.352 3000 0.239 0.510 0.366 5000 0.244 0.534 0.380 50000 0.319 0.673 0.525 100000 0.322 0.646 0.650 200000 0.246 0.559 0.638 500000 0.083 0.554 0.613

Table 4.3: Calculated ionic transference numbers from EMF measurement at 400, 500, and 600°C under various CO2 concentrations.

121 a given voltage. Steady state currents were monitored for 2 hr at each voltage. The current vs. voltage curve was fitted by exponential decay function and the steady state current value was obtained from the fitting parameter.

Figure 4.13 and 4.14 present the steady-state current as a function of the applied voltage at 500°C and 600°C. A plateau of current in the range of 0.3-0.9 V and a subsequent fast increase of the current are seen at 500°C. On the other hand, a plateau is observed in the rage of 1.1-1.3V at 600°C. These plateaus represent n-type electronic conduction behavior and the plateau current is carried by excess electron minority charge carrier. The increase in current above 0.9 V at 500°C and 1.3 V at 600°C might be due to hole conduction or decomposition of the sample.

According to the HW theory (equation (4.23)), n-type electronic conductivity can be obtained from the plateau current as

D FL σ=nJ plateau  (4.43) RT

The dotted lines in Figure 4.13 and 4.14 show the plateau current and the electronic conductivity was calculated using equation (4.43) at different temperatures. Table 4.4

Electronic Total conductivity t (measured Plateau t (measured i conductivity measured i by EMF current (A) by HW) (S·cm-1·K) from HW method) (S·cm-1·K)

400°C - 5.254×10-7 - - 0.199

500°C 1.349×10-6 7.682×10-6 4.545×10-6 0.408 0.418

600°C 1.193×10-5 5.379×10-5 4.020×10-5 0.253 0.292

Table 4.4: Plateau current, electronic conductivity and ionic transference number calculated from HW method and EMF measurement at 400, 500 and 600°C. 122 shows the plateau current, electronic conductivity and calculated transference number compared to the EMF measurement.

The total conductivities measured by gold ion blocking electrode were used for the calculation of ionic transference number. At high temperatures, HW method and EMF measurement show very similar results. But the EMF measurement shows the very low ionic transference number at 400°C, moreover, we couldn’t observe the plateau current from HW method at 400°C. It is believed that the electrode reaction is not reversible, so the measured EMF is affected by not only electronic conduction but also non-reversible electrode reaction at 400°C. Therefore, ionic transference number from EMF measurement was not accurate at this temperature. Also, the lithium carbonate and gold paste electrode in HW cell might not be reversible at 400°C.

4.4 Summary

Electrochemical CO2 gas sensors were examined by the EIS and DC technique and the following summarizes the results.

• Total conductivity measurement

⇒ Total conductivity measured with gold ion blocking electrode from AC measurement agrees well with the values reported in literature.

• EMF measurement

⇒ Electronic conduction parameter calculated from the EMF measurement provides a conduction domain near the boundary between n-type conduction and ionic conduction region.

⇒ EMF measurement showed that the lithium activity of Li2CO3 sensing electrode is located in the n-type conduction region but the lithium activity at the reference electrode is in ionic conduction region.

⇒ However, the ionic transference number was too low in previously proposed conduction domain diagram due to an inappropriate assumption.

123 ⇒ The ionic transference number calculated from the modified electronic conduction parameter agrees well with that measured from the HW method at high temperatures, but it didn’t agree well with the values at low temperature due to non-reversible electrode reaction.

• Hebb-Wagner DC polarization measurement

⇒ In all temperature range, the HW DC polarization measurement showed significant n-type conduction behavior.

⇒ This result combined with the EMF measurement shows that the current Li3PO4 electrolyte has significant electronic conduction.

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Figure 4.1: Schematic representation of partial electrical conductivity behavior for MaXb solid electrolyte [16].

Figure 4.2: Schematic representation of log σ surfaces over log PX2, 1/T space [13].

Figure 4.3: Logarithmic I vs. V curve for AgI. (○) and (X) have the same indications to that of figure 3.1 [24].

Figure 4.4: I vs. V curve of AgBr. (○) indicates points taken with increasing and (X) with decreasing potential [24].

Figure 4.5: Transference number measurement by the Tubandt’s method [28].

Figure 4.6: Typical impedance plot for gold ion blocking electrode for Li3PO4 in air at 500°C.

Figure 4.7: Impedance plot of gold ion blocking electrode for Li3PO4 under different gas environments.

Figure 4.8: Arrhenius plot of the total conductivity of Li3PO4 electrolyte with gold ion blocking electrode.

Figure 4.9: The electron conduction parameter boundary calculated from the measured EMF for various concentrations of CO2 at 400, 500 and 600°C.

Figure 4.10: The sensitivity of CO2 sensing electrochemical cell measured using various Li-ion conducting electrolytes: LIPON (Li2.88PO3.73N0.14), Li3PO4+SiO2 (5 m/o) and Li3PO4+TiO2 (5 m/o).

Figure 4.11: The electron conduction parameter boundary calculated with different conditions for various concentrations of CO2 at 400, 500 and 600°C.

Figure 4.12: A typical time response of the current in HW measurement for Li3PO4.

Figure 4.13: HW curve for Li3PO4 : steady-state current as a function of the applied voltage at 500°C.

Figure 4.14: HW curve for Li3PO4 : steady-state current as a function of the applied voltage at 600°C.

134 CHAPTER 5

CONCLUSIONS AND SCOPE FOR FUTURE RESEARCH

Electrochemical CO2 sensors with Li3PO4 electrolyte, Li2CO3 sensing electrode and

Li2TiO3+TiO2 mixture reference electrode were prepared by sintering and thick or thin

film technology. These sensors showed reproducible, stable and linear response to CO2 concentrations with good sensitivity and selectivity in the lab as well as in the automobile engine tests.

A systematic deviation from the Nernst equation motivated current study of reversible electrode reaction and mixed ionic and electronic conduction studies. The following summary presents the key results from this study and highlights the unresolved issues in this area as a guide for future research.

Summary of key results

1. Reversible electrode reaction

⇒ Higher sensitivity was observed for high CO2 concentrations (5~50%), compared to that for low concentrations (500~5000 ppm). Oxygen dependence was not

negligible at low CO2 background (500~1000 ppm). Overpotential due to polarization at the electrodes influenced the EMF value during open circuit potential measurement when the voltmeter used a low internal impedance (10 MΩ).

⇒ From the EIS measurement, it was concluded that the polarization at the Li2CO3

sensing electrode is relatively small compared to the Li2TiO3+TiO2 mixture

reference electrode at high CO2 concentrations and it significantly increased with

decreased CO2 and O2 concentrations.

135 ⇒ Li2TiO3+TiO2 mixture reference electrode reaction is sluggish causing large overpotential. This overpotential, however, was not affected by the gas concentration change.

⇒ Electrode overpotential was minimized by mixing gold powder or porous sputtered gold electrode increasing effective reaction sites of the electrode. New electrode design improved the EMF closer to the Nernstian value.

2. Mixed ionic and electronic conduction

⇒ Various Li-ion-conducting electrolytes are used to check the non-Nernstian

behavior of the electrochemical CO2 sensor. The sensitivity of various sensors deviates from the Nernstian slope, which indicates the presence of electronic conduction in the electrolyte.

⇒ The sensitivity deviated even further from the Nernstian value at higher sensor operating temperatures (T>500°C). It is due to an increased electronic conduction of the electrolyte.

⇒ Based on the EMF measurement and a modified Nernst equation, the transference number was estimated and the conduction domain boundary separating the n-type from the ionic conduction was constructed. The proposed electronic conduction

parameter for Li3PO4 +5m/o SiO2 electrolyte is represented as

10000 loga− =− 2.214 ⋅ + 9.8825 . T

According to this electronic conduction parameter, the reference electrode is in the ionic conduction domain and the sensing electrode in the electronic conduction domain.

⇒ Hebb-Wagner (HW) DC polarization measurement also clearly confirmed a

significant n-type electronic conduction of Li3PO4 electrolyte. The transference numbers obtained from the EMF measurement and the HW DC polarization measurement were compared and they are in good agreement. In conclusion, the

136 origin of the non-Nernstian sensor behavior is mainly due to the mixed conduction

of Li3PO4 electrolyte.

Suggested future work

1. Based on our results, the choice of the proper electrolyte which has high mobile ion concentration and pure ionic conduction at high temperature is very important.

Therefore, it is worth developing such a lithium ion conductor for CO2 sensing

2. Fast electrochemical reaction must be achieved at the sensor electrodes. The number

of choice for the sensing electrode is limited because carbonate is necessary for CO2 sensing and lithium carbonate is inevitable when a lithium ion conductor is used. On

the other hand, reference electrode needs a material which is inactive toward CO2 gas, but active in oxygen reduction.

3. Even though the basic sensing mechanism related to the Nernst equation was understood by the present study, it still showed humidity interference at low

temperature (<500°C) and high CO2 concentration. The origin of humidity interference is yet to be resolved.

4. From the engine test, it appears that more controlled temperature monitoring is required. Also, further examination of interference from other gas species will provide a clearer understanding of sensor performance in the engine test. Moreover, for long-term stability in engine environment, sensors need to be tested at much higher pressures than the atmospheric pressure.

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