DEVELOPMENT of CERAMIC Li-ELECTROLYTE BASED CO2 SENSORS for TEMPERATURES RANGING from AMBIENT to HIGH TEMPERATURE

DEVELOPMENT OF CERAMIC Li-ELECTROLYTE BASED CO2 SENSORS FOR TEMPERATURES RANGING FROM AMBIENT TO HIGH TEMPERATURE

DISSERTATION

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

By Inhee Lee, M.S. ***** The Ohio State University 2008

Dissertation Committee:

Professor Sheikh A. Akbar, Advisor Approved by

Professor Prabir K. Dutta ______

Professor Gerald S. Frankel Advisor Graduate Program Professor Patricia A. Morris in Materials Science and Engineering ABSTRACT

Solid-state electrochemical CO2 gas sensors composed of an electrolyte and two porous electrodes have been used extensively in the automobile and bio-chemical industry. Based on the field of application, the working temperature of the sensor

ranges from room temperature to 600 °C. Two potentiometric CO2 sensors that work at different temperature ranges were developed in this work.

A potentiometric CO2 gas sensor with Li3PO4 electrolyte and BaCO3 coated

Li2CO3 sensing electrode was developed and the sensing electrode was characterized in order to understand its sensing mechanism under humid conditions. This potentiometric CO2 sensor showed humidity-interference-free sensing response

for high CO2 concentrations (5~25%) at high temperatures (T > 400 °C). In addition, the sensor showed good reproducibility and long-term stability under humid

conditions. In the sensing electrode, the BaCO3 layer improved the resistance

against humidity as a chemical barrier, while the inner Li2CO3 layer was respon-

sible for the CO2 sensing. However, the sensor in which the eutectic layer covered the entire sensing electrode showed good sensing behavior under dry and humid conditions.

Lately, low-temperature CO2 sensors have been attracting attention due to their low power consumption and easy sensor miniaturization, since a heater is unnec-

essary. We have developed a low-temperature CO2 sensor based on lithium lanthanum titanate (LLT) electrolyte in dry conditions that requires further improvement. Lithium lanthanum titanate (LLT) electrolytes were prepared by a conventional solid-state method. The impedance of the LLT electrolyte was measured ii over the temperature range of 300 to 473 K and the frequency range of 5 Hz and 13 MHz. Activation energies for the Li ionic conduction for grain boundary and grain were estimated to be 0.47 and 0.31 eV, respectively. It was found that LLT is a good ionic conductor at low temperatures and a good candidate as an electro-

lyte for low-temperature electrochemical cells. A La2/3-xLixTiO3(LLT)-based CO2 sensor with a mixture of CeO2, Au, and Li2CO3 as the sensing electrode has been developed and shown to have relatively stable sensing behaviors at 200 °C under dry conditions. However, this sensor showed non-Nernstian behavior because electrochemical reactions were not fast enough on the sensing electrode and the solid electrolyte may have some electronic conduction. In addition, the observed sensitivity was less than the theoretical prediction. By adding K2CO3 on the sens-

ing electrode, the sensitivity of the low-temperature CO2 sensor was slightly improved. However, the sensing signals of the sensors were degraded by water vapor under humid conditions due to the formation of KHCO3 or K2CO3·mH2O (m=

2, 3, or 6). To optimize the low-temperature CO2 sensor, a more active sensing electrode is needed, which may be achieved by controlling the size of particles and their distribution on the electrode. In addition, a thinner electrolyte with pure ionic conduction is also required.

iii

Dedicated

to my loving wife Namshin and my loving son Jaejun:

The pursuit of my dream career would not have been possible

without your support and love.

I love you Namshin and Jaejun.

iv ACKNOWLEDGMENTS

I would like to express my greatest gratitude to my advisor, Professor

Sheikh Akbar for his academic insight and support throughout this research at the

Ohio State University.

I am also deeply indebted to Professor Prabir Dutta for his invaluable as-

sistance on research and willingness to share his academic insight and wisdom.

I cannot forget the help and friendship of current and previous CISM

member. Dr. Chonghoon Lee, Dr. Sehoon Yoo, Dr. Krenar Shqau, Dr. Jiun-Chan

Yang, Dr. Joe Obirai, Dr. John Spirig, Dr. Pengbei Zhang, Dedun Adeyemo, Julia

Rabe, Dr. Xiaogan Li, Ben Dinan, Haris Ansari, Mark Andio and Elvin Beach are

greatly acknowledged for their cooperation and friendship.

I would not have finished this study without friendship and encouragement of other Korean students, Dr. Hongjin Kim, Dr. Myung gyu Lee, Dr. Jin Nam, Dr.

Sungsik Hwang, Hojun Lim, Huyoung Lee, Dr. Insoo Park, Dr. Jihoon Kim, Ji- hyun Sung and Junro Yoon who have been almost like my family.

Finally, I would like to thank my parents, my wife Namshin and my son

Jaejun for being my solid support through this process. v VITA

February 21, 1975...... Born – Daegu, Korea

1997...... B.S. Materials engineering Hanyang University, Seoul, Korea

1999...... M.S. Metallurgical engineering Seoul National University, Seoul, Korea

1999 – 2004...... Materials Engineer, Samsung, Daejeon, Korea

2005 – present...... Graduate Research Associate, The Ohio State University, Columbus, Ohio

FIELDS OF STUDY

Major Field: Materials Science and Engineering

vi TABLE OF CONTENTS P a g e Abstract ...... ii

Acknowledgments ...... v

Vita ...... vi

List of Tables ...... x

List of Figures ...... xi

Chapters:

1. Introduction ...... 1 1.1 Carbon dioxide gas properties ...... 2 1.2 CO2 emission ...... 3

1.2.1 Vehicles ...... 3 1.2.2 Respiration ...... 3 1.3 The existing techniques of CO2 sensing ...... 4

1.3.1 Optical adsorption and emission ...... 5 1.3.2 Capacitive sensors ...... 5 1.3.3. Field effect transistor ...... 6 1.3.4 Electrochemical sensors ...... 6 1.4 Principles of potentiometric CO2 sensors ...... 8

1.5 Materials of potentiometric CO2 sensors ...... 12

1.5.1 Sodium ion conductor-based CO2 sensors...... 12

1.5.2 Lithium ion conductor-based CO2 sensors...... 15 1.6 Selectivity and sensitivity of electrochemical sensors ...... 16 References ...... 18

2. A potentiometric carbon dioxide sensor based on Li3PO4 electrolyte and BaCO3 coated Li2CO3 sensing electrode ...... 36 2.1 Experimental ...... 37 2.1.1 Sensor fabrication ...... 37 vii 2.1.2 Materials characterization ...... 40 2.1.3 Sensor testing setup ...... 41 2.2 Results ...... 42 2.2.1 Sensing electrode materials ...... 42 2.2.2 Reference electrode and Li ion electrolyte ...... 44 2.2.3 Sensor characteristics of sensors before eutectic reaction ...... 44 2.2.4 Sensor characteristics of sensors after eutectic reaction ...... 48 2.3 Discussion ...... 50 2.3.1 Sensing mechanism of CO2 ...... 50 2.3.2 Role of BaCO3 coating ...... 51 2.3.3 Role of eutectic reaction ...... 54 2.4 Conclusion ...... 56 References ...... 57

3. Lithium ion conductivity in perovskite lithium lanthanum titanate at low temperatures ...... 95 3.1 Brick-layer model ...... 97 3.2 Experimental ...... 98 3.2.1 Preparation of lithium lanthanum titanates ...... 98 3.2.2 Characterization of electrolyte ...... 99 3.3 Results ...... 101 3.3.1 Crystal structures by XRD...... 101 3.3.2 Microstructures ...... 102 3.3.3 Electrochemical characterization of LLT ...... 102 3.4 Discussion ...... 104 3.4.1 Crystal structure and Li content ...... 104 3.4.2 Relationships between microstructure and ionic conductivity ...... 106 3.5 Conclusions ...... 108 References ...... 109

4. A potentiometric sensor for the detection of low CO2 concentrations at low temperatures ...... 133 4.1 Experimental ...... 135 4.1.1 Preparation and characterization of materials for sensing electrode . . . . .135 4.1.2 Sensor fabrication ...... 137 viii 4.1.3 Gas sensing measurements ...... 139 4.1.4 Kinetic measurements of the sensing electrode ...... 139 4.2 Results ...... 140 4.2.1 Sensor characteristics ...... 140 4.2.2 Interferences ...... 143 4.2.2.1 Oxygen interference ...... 143 4.2.2.1 Humidity interference...... 144 4.2.3 Materials characterization of the sensing electrode ...... 145 4.2.4 Kinetics of the sensing electrode...... 146 4.3 Discussion...... 148 4.3.1 Choice of electrodes ...... 148 4.3.2 Non-Nerstian behavior of the sensor ...... 150 4.3.3 Strategies to improve sensor ...... 152 4.4 Conclusions ...... 153 References...... 155

5.Conclusions and scope for future research ...... 175

Bibliography...... 178

ix LIST OF TABLES

Table Page

Table 1.1 Typical concentrations of exhaust gas compositions. [6] ...... 34

Table 1.2 Classification of potentiometric type sensors...... 35

Table 2.1 Absorption band characteristics of CO2 gas and carbonate anion. [13] ...... 90

Table 2.2 Response times for changing CO2 concentration from 5% to 10% under dry and humid conditions...... 91

Table 2.3 Nernstian slope of the sensors with different molar ratios of Li2CO3 to BaCO3 at 500°C under dry and humid conditions after eutectic reaction ...... 92

Table 2.4 The variation of EMF values of the sensors before the eutectic layers in range of CO2 concentration from 5% to 20% under dry and humid conditions ...... 93

Table 2.5 The variation of EMF values of the sensors after the eutectic layers in range of CO2 concentration from 5% to 20% under dry and humid conditions ...... 94

Table 3.1 ICP Analysis Data for La0.5Li0.4TiO3 after sintering at various temperatures ...... 131

Table 3.2 Resistance of grain and grain boundary obtained from fitting using the Zview software...... 132

x LIST OF FIGURES

Figure Page

Figure 1.1 The Lewis structures of CO2. [2] ...... 23

Figure 1.2 The major gases of global warming (a) and the change of CO2 concentration in the atmosphere (b). [5] ...... 24

Figure 1.3 Schematic of capacitance-based carbon dioxide sensor (a) and capacitance change measured by sensor during exposure to 100% carbon dioxide and carbon dioxide-free air (b). [10] ...... 25

+ Figure 1.4 The schematic structure of FET-type CO2 sensor with Na exchange membrane. [16] ...... 26

Figure 1.5 The schematic of type III CO2 sensor structure developed by CISM. [1] ...... 27

Figure 1.6 The schematic of electrochemical, chemical, and electrical potential profile at equilibriums of the type III CO2 sensor structure. [1] ...... 28

Figure 1.7 Crystal structure of Na β-alumina, (a) unit cell structure and (b) conduction plane. [39] ...... 29

Figure 1.8 The sensor response to various CO2 concentrations at 450 - 550 °C (a) and a comparison of experimental sensitivity. [43] ...... 30

Figure 1.9 Schematic view of the NASICON structure in rhombohedral symmetry. [48] ...... 31

Figure 1.10. Phase stability diagram between Na2O and Na2CO3. [49] ...... 32

Figure 1.11. SEM image and EPMA characterization of the new phase after humid air treatment. [53] ...... 33 xi

Figure 2.1 Procedure for our CO2 sensor fabrication based on Li ion conductor with three different carbonate materials as the sensing electrode...... 60

Figure 2.2 Schematic structure of the sensor with three different carbonate materials as the sensing electrode...... 61

Figure 2.3 Schematic of the sensor test assembly (a) and Lindberg horizontal tube furnace (b)...... 62

Figure 2.4 X-ray diffraction patterns for (a) pure BaCO3 (b) BaCO3 coated on Li2CO3 and (c) pure Li2CO3...... 63

Figure 2.5 Phase diagram of Li2CO3-BaCO3 system. [10] ...... 64

Figure 2.6 SEM image of the surface of BaCO3 coated on Li2CO3 powder with the molar ratio of 1:1...... 65

Figure 2.7 SEM images of the surface of BaCO3 coated on Li2CO3 powder with different molar ratios of Li2CO3 : BaCO3. The molar ratio of (a) 3:1 and (b) 7:1...... 66

Figure 2.8 EDS mapping of Ba content on the surface of BaCO3 coated on Li2CO3 powder with different molar ratios of Li2CO3 : BaCO3. The molar ratio of (a) 3:1 and (b) 7:1...... 67

Figure 2.9 SEM images of the surface of (a) pure Li2CO3 and (b) a physical mixture of Li2CO3 and BaCO3...... 69

Figure 2.10 The cross-sectional images of BaCO3 coated on Li2CO3 by (a) FIB and (b) TEM...... 70

Figure 2.11 Infrared spectroscopy of BaCO3 coated on Li2CO3 (a) and pure Li2CO3 after exposure to H2O at 500°C for 3hr...... 71

xii Figure 2.12 SEM image of the surface of Li3PO4 electrolyte...... 72

Figure 2.13 SEM image of the reference electrode with Li2TiO3 and TiO2. . . . . 73

Figure 2.14 Response transient of the sensor with BaCO3 coated on Li2CO3 in dry and humid CO2 at 500°C...... 74

Figure 2.15 Response transient of the sensor with (a) pure Li2CO3 and (b) physical mixture of BaCO3-Li2CO3 in dry and humid CO2 at 500°C...... 75

Figure 2.16 Dependence of EMF on log [CO2 concentration] for the sensors with pure Li2CO3 (a) and BaCO3 coated on Li2CO3 (b) in dry and humid conditions at 500°C...... 76

Figure 2.17 Changes of the Nerstian slope of sensors C with different molar ratios of Li2CO3 and BaCO3 under humid conditions...... 77

Figure 2.18 Base-line shifts under humid conditions at the first humid test (a) and base line of second humid test for longer stabilization (b)...... 78

Figure 2.19 Typical response time of sensor B at 500°C...... 79

Figure 2.20 Long-term test of the sensor with BaCO3 coated on Li2CO3 (a) for 9 hr continuous test and (b) for 60 days test in humid condition at 500°C...... 80

Figure 2.21 Sensor EMF responses from different 7 sensors with sensing electrode C...... 81

Figure 2.22 Response transient of the sensor with BaCO3 coated on Li2CO3 with eutectic layer in dry and humid CO2 at 500°C. The molar ratio of Li2CO3 and BaCO3 is (a) 1:1, (b) 2:1, (c) 3:1, (d) 5:1 and (e) 7:1...... 82

Figure 2.23 SEM images of surface morphology of sensing electrode with different molar ratios. The molar ratio of Li2CO3 and BaCO3 is (a) 1:1 or 3:1, and (b) 5:1...... 85 xiii

Figure 2.24 EDS analyses of the selected points for Li2CO3-BaCO3 electrode. (a) a large particle (A in Figure 2.23 (a)), (b) needle-like region (B in Figure 2.23 (a))...... 86

Figure 2.25 SEM image of cross-section of sensors attached to the sensing electrode with the molar ratio of (a) 1:1, (b) 3:1 and (c) 5:1 after sensing tests. . 87

Figure 2.26 X-ray diffraction patterns of the mixture of sensing materials (Li2CO3-BaCO3) and electrolyte (Li3PO4) after heat-treatment at 650 °C for 2 hr...... 89

Figure 3.1. Arrhenius plots of conductivity of several well-known solid lithium ion conductors. [11] ...... 112

Figure 3.2 Crystal structure of La2/3-xLi3xTiO3 [11] ...... 113

Figure 3.3 Schematic of dual semicircles in impedance spectrum of electroceramics...... 114

Figure 3.4 Equivalent circuit representations for (a) the series brick-layer model (BLM), (b) the series/parallel BLM, and (c) the series/parallel-BLM with different electrical properties, parallel versus perpendicular to the grain boundary [17,18]...... 115

Figure 3.5 Schematic representation of the brick layer model (BLM). The unit is separated into a serial path of grain core and capping grain boundary by itself or in parallel with an outer grain boundary path [19]...... 116

Figure 3.6 Pixelation of the 3D-brick layer model. Each pixel has six orthogonal (RC) circuits extending from the finite difference node at its center, and is as- signed to either the grain core or the grain boundary [21]...... 117

Figure 3.7 Flow chart for the synthesis of lithium lanthanum titanate (LLT) electrolyte...... 118 xiv

Figure 3.8 Powder X-ray diffraction patterns of La(2-x)/3LixTiO3...... 119

Figure 3.9 The lattice parameter of the sintered LLTs based on cubic lattice with sintering temperatures...... 120

Figure 3.10 SEM micrographs of LLT samples at various sintering temperatures...... 121

Figure 3.11 Density of LLTs with x = 0.4 after sintering at various temperatures...... 124

Figure 3.12 Impedance diagrams of solid electrolyte and an equivalent circuit...... 125

Figure 3.13 The impedance plots of the LLTs with x = 0.4 at room temperature for (a) grain and (b)grain boundary part. The sintering temperatures are shown for identification. The approximate measuring frequencies are also shown. The inset in (b) shows the detailed view at the high frequency part...... 126

Figure 3.14 The grain conductivity (a) and the grain boundary conductivity (b) of LLTs with x = 0.4 at room temperature after sintering at various temperatures...... 127

Figure 3.15 Arrhenius plot of the conductivity for LLT at grain (■) and grain boundary (▲)...... 128

Figure 3.16 Activation energies of the grain (■) and grain boundary (▲) for LLT at various sintering temperatures...... 129

Figure 3.17 The grain boundary conductivity of LLT with x = 0.4 is plotted versus the grain size. The straight line is the linear fitting curve of the data...... 130

Figure 4.1 Illustration of the application of a low-temperature CO2 sensor in a spacesuit...... 159 xv

Figure 4.2 Schematic structure of the low-temperature potentiometric CO2 sensor...... 160

Figure 4.3 Symmetrical cells referred to as L (Li2CO3), LC (Li2CO3 + CeO2), and KLC (K2CO3 + Li2CO3 + CeO2) cell for kinetic studies...... 161

Figure 4.4 Response transients of sensors with (a) pure Li2CO3 and (b) Li2CO3 and Au on CeO2 as the sensing electrode to dry CO2 gas from 400 ppm to 2500 ppm at 300 °C...... 162

Figure 4.5 Response transients of sensors with (a) LC and (b) KLC (Li2CO3:K2CO3 molar ratio of 2:1), and (c) KLC (Li2CO3:K2CO3 molar ratio of 10:1) as the sensing electrode at 200 °C under dry conditions...... 163

Figure 4.6 Relationship between EMF and log [CO2 concentrations] from 400 to 2500 ppm at 200 °C under dry conditions for LC (■), KLC (Li2CO3:K2CO3 molar ratio of 2:1) (♦), and KLC (Li2CO3:K2CO3 molar ratio of 10:1) (▲)...... 165

Figure 4.7 Relationship between EMF and log [CO2 concentrations] of the sensor with KLC (Li2CO3:K2CO3 molar ratio of 10:1) sensing electrode to CO2 at 5% (♦), 10% (■) and 25% (●) O2 at 200 °C under dry conditions...... 166

Figure 4.8 (a) Response transients to CO2 concentration from 400 ppm to 2500 ppm at 80% RH for the sensor with LC sensing electrode and (b) Nerstian behavior plots under dry and humid conditions...... 167

Figure 4.9 Relationship between EMF and log [CO2 concentrations] of the sensor with KLC electrode of different Li2CO3:K2CO3 molar ratio, 2:1 (■), 5:1 (▲) and 10:1 (♦), at 200 °C under dry conditions...... 168

Figure 4.10 X-ray diffraction pattern of CeO2 powder prepared by a wet chemical method...... 169

Figure 4.11 SEM image of CeO2 particles calcined at 600 °C...... 170 xvi

Figure 4.12 TEM image of nano-sized Au deposited on CeO2 particles...... 171

Figure 4.13 (a) Impedance spectra of symmetrical cells with different sensing electrodes to 500 ppm CO2 and 10 % O2 at 200 °C and (b) Randles equivalent circuit model...... 172

Figure 4.14 Impedance spectra of the cells with 2500 ppm CO2 concentrations...... 173

Figure 4.15 Impedance plots of the cells with L, LC or KLC by increasing the concentration of O2 from 5% to 20%...... 174

xvii

CHAPTER 1

INTRODUCTION

This dissertation focuses on the development and study of solid-state elec-

trochemical CO2 gas sensors based on a lithium ion conducting electrolyte. It is anticipated that such sensors could be beneficially used in various applications,

such as combustion-based systems in automobiles at high temperatures (T ≥

500 °C) and respiration-controlled systems in space suites at low temperatures (T

≤ 200 °C). Chapter 1 introduces carbon dioxide chemistry as well as the general

principles and issues of electrochemical CO2 sensors. Chapter 2 concentrates on

the development of a potentiometric CO2 sensing system that is free of humidity interference and employs a Li electrolyte with an emphasis on sensing performance and operation mechanisms at high temperatures. This is an extension of a previously developed CO2 sensor by Chong-Hoon Lee [1]. In Chapter 3, the basic

transport properties study of the lithium ion electrolyte at low temperatures is dis-

cussed. Chapter 4 deals with the effects of modifying the surface of the sensing

electrodes by wet chemical processes on the sensing performance enhancement at

1 low temperatures. Chapter 5 addresses unresolved issues and future work needed to improve electrochemical CO2 sensors for low-temperature applications.

1.1 Carbon dioxide gas properties

Carbon dioxide gas is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. As shown in Figure 1.1 [2], the central carbon atom is connected to the two oxygen atoms, and there are no lone pairs of electrons around the central atom. Therefore, carbon dioxide has a linear geometry, and the O−C−O bond angle is 180 °. The carbon-oxygen bonds in this molecule are polar, and the oxygen atom, which is the more electronegative atom, has a partial negative charge. The carbon atom, which is the less electronegative atom, has a partial positive charge. Even though the bonds are polar, their orienta- tions to one another make CO2 molecule non-polar.

Carbon dioxide was discovered from the observation of a substance be- sides an ash after burning of charcoal and recorded by J. B. van Helmont at the

th beginning of 17 century [3]. Recently, the interest in CO2 gas has been growing because it is the major gas to cause global warming as evidenced from Figure 1.2

[4]. Because the partial pressure of CO2 gas is continuously increasing from 300 to 370 ppm [5], it is important to monitor atmospheric CO2 concentration to understand its impact on climate change.

2 1.2 CO2 emission

Despite widespread concern about climate change, annual carbon dioxide

emissions from burning fossil fuels have increased 39 percent since 1992, from

6.1 billion metric tons of carbon to 8.5 billion metric tons in 2007 [5].

1.2.1 Vehicles

Exhaust gases from a typical spark-ignition engine during high power use can

reach temperatures as high as 900 °C. The temperature of the exhaust gases generally

falls in the range of 400 to 600 °C [6]. The CO2 concentration in exhaust gas from engines is typically about 13%, depending on the efficiency of combustion and the air/fuel ratio [7]. In addition to CO2, CO, H2, NOx, water, and hydrocarbons are also formed. Table 1.1 lists the typical concentrations of these gases in automotive exhaust [7].

1.2.2 Respiration

Carbon dioxide is a product of the metabolic activity of humans and ani-

mals and a vital substance to the life of plants [3]. However, our direct contribu-

tion to atmospheric carbon dioxide concentrations via respiration is relatively in-

significant, even though the worldwide human population now exceeds six billion.

However, to control respiration rate in a closed system such as a space suit,

monitoring increases of CO2 concentrations due to human respiration is very important. By monitoring CO2 concentration, in humans and other mammals, the

3 respiratory system facilitates oxygenation of the blood with a concomitant re-

moval of carbon dioxide and other gaseous metabolic wastes from the circulating blood. Through the efficient removal of carbon dioxide from the blood, the system also helps to maintain the acid-base balance of the body.

1.3 The existing techniques of CO2 sensing

CO2 gas sensors have been developed to monitor the environment because

CO2 is primarily responsible for global warming. In addition, CO2 gas signifi- cantly influences many industrial areas through such processes as affecting the corrosion rate of equipment during chemical processing and promoting the enhancement of combustion efficiencies in automotive applications. Because of these numerous applications, small, inexpensive, and reliable solid-state gas sensors with low power consumption have been in high demand for several years.

Although many methods are available to improve gas sensors, the general requirements for all of them are sensitivity, selectivity, and stability [8].

There are many types of solid-state gas sensors that measure changes in optical properties [9], capacitance [10,11], work function [12], or electrical poten-

tial by means of the gas/solid interactions [13]. Despite various approaches, solid-

state gas sensors share a common feature, i.e., a sensing material chemically re-

acts with a specific target gas.

4 1.3.1 Optical adsorption and emission

The major methods used to detect combustion gases fall short of practical

application needs for in-situ measurements in harsh industrial environments in-

volving high temperatures and chemical contaminants [14]. Fourier transform infrared spectroscopy (FTIR) is the most popular technique for CO2 detection be-

cause of its accuracy, sensitivity and selectivity. CO2 gas is a tri-atomic molecule

that has distinct absorption bands due to the rotational transition of the molecules,

resulting in a characteristic FTIR spectrum [15]. The response time and detection

limits of the method are very good, but the method has the disadvantages of large

size, costly maintenance, and a narrow operational temperature range [14].

1.3.2 Capacitive sensors

Capacitive-type CO2 sensors are based on changes in the dielectric proper-

ties of the sensing electrode [10,11]. Figure 1.3(a) shows the schematic view of

such a sensor. Several sensing electrodes have been reported, such as organic ma-

terials and ceramic p-n junctions. For example, the fluoropolymer, Teflon AF

TM 2400 [10] or ceramic p-n junctions with BaTiO3 and CuO [11] were used as

sensing electrodes. The sensor with organic sensing layers is operated at room

temperature. When the sensor is exposed to CO2, the capacitance of the sensor

changes, as shown in Fig. 1.3(b). Although the mechanism has not been discussed,

this sensor has the advantage of a low working temperature. However, humidity

interference is anticipated.

5 1.3.3. Field effect transistor

This type of sensor is generally based on the enhancement mode metal-

oxide-semiconductor field effect transistor (MOSFET) structure. In the ion selec-

tive field effect transistor (ISFET), the gate metal is replaced with a sodium ion

exchange membrane (Figure 1.4), and the device is immersed in a solution con-

taining Na ions. The Na ions in the membrane interact with CO2 gas. When there

is a high concentration of Na ions in the membrane, many of the ions will accu-

mulate on the gate, widening the channel between the source and the drain. With a

low concentration of Na ions, the channel will be narrow. Therefore, the FET-

type CO2 sensor with NASICON instead of gate metal oxides exhibited good CO2 sensing properties [16]. However, the processes to fabricate this sensor are com- plicated and require expensive and high-vacuum equipment.

1.3.4 Electrochemical sensors

Although several solid-state gas sensors for CO2 gas exist, the electro-

chemical type is more suitable than others because CO2 is not a redox gas. It is an

acid-based active gas that usually reacts with the ions of solids rather than elec-

trons [17].

Electrochemical gas sensors measure the electrical potential generated by

an electrochemical reaction. Unlike the resistance measured in semiconducting

sensors, the potential is an intensive property that is independent of the sensor ge-

ometry. In general, electrochemical gas sensors can be divided into two main

6 categories according to their sensing mechanism: (1) potentiometric sensors,

which measure potential at equilibrium and (2) mixed-potential sensors, which

measure potential based on kinetic differences in the electrodes [18]. Depending on the interaction between the target gas and the solid electrolyte, potentiometric gas sensors are fundamentally classified into type I, type II and type III sensors

[19-22], as presented in Table 2.1. Their sensing behaviors are observed as a

change in the potential due to an electrochemical reaction at the triple-phase

boundary (TPB), which involves the electrode, the electrolyte, and the target gas

at equilibrium.

In type I sensors, the electrolyte has a mobile ion in common with the tar-

get gas [23]. Due to this restriction, type I sensors cannot be used for certain gases.

Therefore, this type of sensor cannot function as a CO2 sensor.

The first type II sensor proposed for CO2 gas detection utilized K2CO3 as

an electrolyte [24, 25]. In type II sensors, although the mobile ion is not the same

as the target gas, the electrolyte limitation still exists. Also, there are concerns

about the sensor’s long-term stability because the electrolyte reacts directly with

the target gas.

To overcome the limitations of type I and type II sensors, type III sensors utilize auxiliary phases as a sensing electrode and have no direct relationship be-

tween the electrolyte and the target gas. The auxiliary phase (or sensing electrode)

should be a mixed ionic and electronic conductor and must involve both the mo-

bile ion of the electrolyte and the target gas. For CO2 sensors, carbonate materials

7 are usually used as the sensing electrode. Many researchers have examined the

potential for detecting various gases, such as CO2 [26, 27], NO2 [28], Cl2 [29] and

SO2 [30] using type III sensors. The principles of potentiometric sensors and vari-

ous sensing materials of type III CO2 sensors are described in the next section.

1.4 Principles of potentiometric CO2 sensors

Electrochemical sensors are fabricated using a dense solid electrolyte with

high ionic conductivity and two porous electrodes that are able to provide triple phase boundaries. To understand the mechanistic model, it is helpful to consider the electrochemical potential that exists at the interfaces. The electrochemical potential (μi) is defined as the combination of chemical and electrical potentials to

which a charged species is exposed [31]:

μi = μi + zi F φ (1.1)

where μi, zi, F, and φ are the chemical potential of i, the electrical valence of i,

the Faraday constant, and the electrostatic potential, respectively. For potenti-

ometric sensors, the potential difference is more accurately expressed as the interfacial potential difference, which can be derived through reactions involving charge transfer at equilibrium [32] and can be measured as the electrostatic potential difference. At the equilibrium interface between an electrolyte (EL) and an electrode (ED), the electrochemical potential of each component must be equal:

EL EL ED ED μφμφ + zF = + zF ii ii (1.2)

8 and the potential difference at the interface is:

EL ED EL ED φ−φ=−μ−μ( iii) zF (1.3)

The potential difference is measured across an electrochemical cell, not between one electrode and an electrolyte [33]. Therefore, to measure the potential difference by changing CO2 concentrations, one electrode is defined as a sensing electrode with a variable chemical activity of mobile ionic species depending on CO2 partial pressure, while the other electrode serves as a reference electrode with a constant chemical activity of the mobile ionic species [1]. Therefore, electrochemical sensors must consist of an electrolyte and two electrodes.

Figure 1.5 shows the schematic of a type III CO2 sensor structure developed by CISM that consists of a Li2CO3 sensing electrode, a Li2TiO3+TiO2 reference electrode, and a Li3PO4 lithium-ion-selective solid electrolyte [34]. First, this type III sensor will be simply interpreted by using ideal fused-salt model. There- fore, auxiliary phases have to be considered as solutions that have dissolved mobile ions and gas species. One of the primary assumptions is that the concentra-

+ tions of CO2 and O2 in auxiliary phases and mobility of Li in the electrolyte are so high that small concentration changes do not change the chemical potentials in these phases. Dominant charge carriers are assumed to be interstitial cations in the auxiliary phase, because Li2CO3 is known as a Frenkel-type intrinsic defect ionic conductor [35] and cation interstitials are common defects in many lithium ion compounds. Figure 1.5 shows six different interfaces where thermodynamic equi-

9 libria exist [34]. At each interface, different equilibrium conditions are established as follows:

Electron equilibrium at Cu-Au interface 1

μ=μe, Cu1 e, Au1 (1.4)

Oxygen reduction and oxidation equilibrium at interface 2

μ=μ+μ+μ+μ221 (1.5) Li2CO3 Li+, Li2CO3 CO22 O2 e

Lithium ion exchange equilibrium at interface 3

μ=μLi+, Li2CO3 Li+, Li3PO4 (1.6)

Lithium ion exchange equilibrium at interface 4

μ=μLi+, Li3PO4 Li+, Li2TiO3 (1.7)

Oxygen reduction and oxidation equilibrium at interface 5

22μ+μ+μ+μ=μ1 (1.8) Li+, Li2TiO32 O2 TiO2 e Li2TiO3

Electron equilibrium at Cu-Au interface 6

μ=μe, Au2 e, Cu2 (1.9)

Therefore, overall EMF is obtained from the difference of electrochemical potential of Cu I and Cu II wire, which is not equilibrated in this system. The theoretical EMF can be calculated from the sum of all the junction potentials.

0000 ()μ+μ−μ−μLi2CO3 TiO2 Li2TiO3 CO2 RT EMF=− ln P (1.10) 2F 2F CO2

The ideal behavior of a type III sensor is understood based on the schematic of the electrochemical, chemical, and electrical potential profile relevant to

10 all the above equilibria of the type III sensor structure as shown in Figure 1.6 [34].

In this Figure, the electrochemical potential of the lithium ion is equilibrated

(0Δμ = ), but the electrochemical potential of the electrons is not equilibrated Li+

( Δμ ≠ 0 ) because, ideally, the electrolyte is a perfect ionic conductor (t = 1) in e− i this model. It was observed from the experimentally measured EMF that electrochemical potential of Cu I is higher than that of Cu II at various CO2 concentrations. Therefore, this diagram was 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 present variable potentials.

The chemical potential of oxygen is a variable, but, since both sides have the same oxygen partial pressure, it does not appear in the overall electrochemical equilibrium. The fixed chemical potentials of lithium carbonate, titania, and lithium titanate are located in the order of their standard state chemical potential. If the system is in equilibrium, overall EMF is only dependent on the partial pressure of CO2.

1.5 Materials of potentiometric CO2 sensors

1.5.1 Sodium ion conductor-based CO2 sensors

In a sodium-based potentiometric CO2 sensor, Na2CO3 is used as the sensing electrode. Na2ZrO3/ZrO3 [36] or Na2Ti6O13/TiO2 [37] has been proposed as a reference electrode. However, because sensing properties at high temperatures

11 primarily depend on the properties of the electrolyte, research has focused on the electrolyte.

Na β-alumina

Numerous solid-state electrochemical sensors use Na β-alumina as the electrolyte because of its high ionic conductivity, small electronic conduction, and thermal stability [38]. This ionic compound has two different phases: β-alumina with ideal stoichiometry, such as NaAl11O17, and β″-alumina with a non- stoichiometric composition. Stoichiometric β-alumina has a hexagonally layered structure with two spinel blocks bridged by oxygen atoms at widely spaced inter- vals, as shown in Figure 1.7 [39]. The oxygen ions and Al ions are closely packed in the spinel blocks, while the Na ions and bridging oxygen ions are located on the conducting plane between the spinel blocks with vacant sites. The ionic conduction of the Na ions occurs through the plane of the bridging oxygen ions [39], which results in anisotropic ionic conduction in Na β-alumina. Na β″-alumina is a thermodynamically metastable compound in the binary system of Na2O-Al2O3

[40]. However, Na β″-alumina can provide higher ionic conductivity because it has a greater number of Na ions than Na β-alumina. Because of the thermodynamic instability of Na β″-alumina, Na β-alumina has largely been used as the electrolyte in electrochemical CO2 sensors.

However, Näfe reported electronic conduction using Hebb-Wagner polarization measurements for Na β-alumina [41, 42], an experimental technique used

12 to measure the transference number by means of the DC polarization method.

Näfe believed that electrochemical CO2 gas sensors with Na β-alumina do not respond to changes in CO2 concentration up to 700 °C due to electronic conduction.

This electronic conduction may be the origin for the neutralization of defects, such as interstitial ions and vacancies with electrons or holes in the ionic crystal.

Contrary to Näfe’s prediction, Maier et al. [43,44], Liu et al. [45], and

Kim et al. [46] proposed CO2 sensors using Na β-alumina that demonstrated sensitivity to CO2 through the formation of Na2CO3 on the surface of Na β-alumina electrolytes at 500 °C, as shown in Figure 1.8(a). This difference between Näfe’s prediction and real sensors reveals that the Hebb-Wagner polarization measurement may not provide exact information about electronic conduction in an open- circuit device. The CO2 sensors with the Na β-alumina electrolyte exhibit good sensitivity at high temperatures, where the measured value (n = 2.01 at 500 °C) is almost identical to the theoretical value (n = 2), as depicted in Figure 1.8(b).

However, to improve the CO2 sensors using Na β-alumina as the electrolyte, some problems, such as anisotropic ionic conduction and small electronic conduction, must be solved.

NASICON (Na Super Ionic Conductor)

In Figure 1.9, the rhombohedral structure of NASICON has a three- dimensional network channel for ionic conduction that can solve the problem of anisotropic conduction of Na β/β″-alumina [47]. Although the Na ion conductivi-

13 ties of Na β/β″-alumina and NASICON are very similar [48], a CO2 sensor with a

NASICON electrolyte might show more stable sensing behavior at high CO2 partial pressure because the activity of Na2O in NASICON is less than that of Na β- alumina, as shown in Figure 1.10 [49]. Consequently, several potentiometric CO2 sensors with NASICON electrolytes and Na2CO3 sensing electrodes have been reported at elevated temperatures above 500 °C [50-52]. Sadaoka et al. reported a

NASICON electrolyte-based sensor with fast response times and good sensitivity, which is similar to the theoretical sensor [50]. However, these investigators identified drift in the sensor and humidity interference in a long-term test. They sur- mised that the degradation of the performance of the sensors due to humidity re- sulted from the formation of Na2HCO3 and NaOH at the grain boundary of the

NASICON electrolyte, due to the easy diffusion of water molecules and CO2 into

NASICON [50]. The sodium ion activity may be altered by the formation of these new compounds in the electrolyte, which results in permanent drifts in the sensing characteristics. However, corroborative evidence was not observed experimentally using characterization techniques. Recently, Kida et al. reported that the

Na3PO4 phase is formed at grain boundaries of NASICON during the heat cycle after the humid treatment [53]. The formation of Na3PO4, which shows a defi- ciency in Si and Zr, was analyzed using an Electron-Probe Micro Analyzer

(EPMA), as shown in Figure 1.11. Although the direct relationship between the formation of Na3PO4 and the degradation of the sensors in humid conditions was not examined, these investigators believed that the formation of Na3PO4 could

14 induce sensor drift under such conditions. However, their studies of humidity interference were focused on the electrolyte and failed to consider the Na2CO3 sensing electrode. Because carbonates also are active materials for humidity, in order to clarify the humidity influence on sensor behavior, the auxiliary sodium carbonate phase and the sodium ion conductor must be investigated simultaneously. Re- cently, several researchers improved the sensor stability in humid conditions by using binary carbonate as the sensing electrode: however, the mechanism itself remains unclear [54-56].

1.5.2 Lithium ion conductor-based CO2 sensors

Lithium ion conductors have been of interest with respect to their application in solid-state lithium batteries because of their low equivalent weight and high electropositive characteristics [57, 58]. Potentiometric sensors with lithium ion conductors also are very promising because the lithium ion is fast and less reactive with water than any other alkali metal ion. In the early stages, Salam et al. proposed type II potentiometric CO2 sensors that used lithium carbonate as an electrolyte at 400 °C [24]. However, the type II sensor is limited by its poor long- term stability, as mentioned in the previous section. Moreover, the poor sinterability of the Li2CO3 electrolyte may cause sensor degradation. In an attempt to overcome such disadvantages, Lee et al. suggested a type III sensor using

Li2.88PO3.73N0.14 as the electrolyte. The following electrochemical cell represents the sensor assembly [12]:

15 O2, Au | LiCoO2-Co3O4 | Li2.88PO3.73N0.14 | Li2CO3 | Au, CO2, O2

3Li2CO3 + 2Co3O4 + 1/2O2 ↔ 6LiCoO2 + 3CO2 (1.11)

Using the overall reaction, the EMF of the cell can be expressed as

EMF = –∆Gºf(rxn)/6F + {RT/12F} lnPO2 – {RT/2F} lnPCO2 (1.12) where, ∆Gºf(rxn) = 6∆Gºf(LiCoO2) + 3∆Gºf(CO2) – 3∆Gºf(Li2CO3) – 2∆Gºf(Co3O4)

This sensor exhibits a very fast and reproducible response to CO2 gas changes at

500 °C. Also, it is not necessary to seal its reference electrode because it is not reactive with CO2. However, its sensitivity to CO2 gas, expressed by the Nern- stian slope, is less than the theoretical value. These deviations may result from electrode kinetics or the electronic conduction behavior of the lithium electrolyte, as addressed in the earlier discussion of the electronic conduction characteristics of Na β-alumina [41, 42].

1.6 Selectivity and sensitivity of electrochemical sensors

The word “reversible” in thermodynamics usually describes an equilibrium condition. Equilibrium is defined generally as a balance of two opposing forces [59]. 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.

1 -2-⎯⎯⎯⎯⎯⎯⎯⎯→Reduction or cathodic reaction O22 (gas)+ e (Pt)←⎯⎯⎯⎯⎯⎯⎯⎯ O (absorbed on Pt) (1.13) 2 Oxidation or anodic reaction

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

2- dependence on the partial pressure of O2 gas and the concentration of O adsorbed on Pt, as dictated by the Nernst equation. Electrochemical sensors utilize this equilibrium potential to measure the gas concentration, and, therefore, reversible electrodes are necessary. Practically, the electrode reaction is considered to be electrochemically reversible if the measured EMF follows the Nernst equation [60].

However, measured EMF might deviate from the Nernst equation due to a polarizable electrode and ohmic potential drop by electrolyte resistance. Therefore, a non-polarizable or reversible electrode is critical for a gas sensor electrode. Ad- ditionally, ohmic potential drop can be avoided by minimizing the distance between the reference electrode and the working electrode. This distance can be the thickness of the electrolyte when only two electrodes are used in the measurement.

Thus, a thin solid electrolyte is required for potentiometric sensors.

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Figure 1.1 The Lewis structures of CO2. [2]

Figure 1.2 The major gases of global warming (a) and the change of CO2 concentration in the atmosphere (b). [5]

Figure 1.3 Schematic of capacitance-based carbon dioxide sensor (a) and capacitance change measured by sensor during exposure to 100% carbon dioxide and carbon dioxide-free air (b). [10]

+ Figure 1.4 The schematic structure of FET-type CO2 sensor with Na exchange membrane. [16]

Figure 1.5 The schematic of type III CO2 sensor structure developed by CISM. [1]

Figure 1.6 The schematic of electrochemical, chemical, and electrical potential profile at equilibriums of the type III CO2 sensor structure. [1]

Figure 1.7 Crystal structure of Na β-alumina, (a) unit cell structure and (b) conduction plane. [39]

Figure 1.8 The sensor response to various CO2 concentrations at 450 - 550 °C (a) and a comparison of experimental sensitivity. [43]

Figure 1.9 Schematic view of the NASICON structure in rhombohedral symmetry. [48]

Figure 1.10. Phase stability diagram between Na2O and Na2CO3. [49]

Figure 1.11. SEM image and EPMA characterization of the new phase after humid air treatment. [53]

33 Table 1.1 Typical concentrations of exhaust gas compositions. [6]

Typical Concentrations of Exhaust Gas Constituents

CO2 13.5% H2 0.23 vol%

H2O 12.5% NOx 1050 ppm

CO 0.68 vol% HC 750 ppm

O2 0.51 vol%

34 Table 1.2 Classification of potentiometric type sensors.

CHAPTER 2

A POTENTIOMETRIC CARBON DIOXED SENSOR BASE ON Li3PO4

ELECTROLYTE AND BaCO3 COATED Li2CO3 SENSING ELECTRODE

Carbon dioxide sensors are becoming increasingly important for monitor-

ing air-quality, measuring metabolic activity of animals and controlling combus-

tion [1-4]. While there are commercial sensors for air-quality monitoring, there is

a vital need for reliable sensors for high-temperature combustion-related applica-

tions. Potentiometric type sensors that consist of Na+ or Li+ electrolyte and alkali

metal carbonate sensing electrodes are promising because they show satisfactory

EMF signals over a wide range of gas concentrations and temperatures [5, 6].

Potentiometric sensors based on Na+ conductors such as NASICON [5]

with Na2CO3 sensing electrodes are reported to respond in a Nernstian behavior to

CO2 concentration with response times of several minutes at ~500-700°C. The major problem with these sensors is that they suffer from significant interference from humidity.

Successful attempts to improve sensor performance by modification of the sensing electrode material have been described [7, 8]. With a binary carbonate

36 (Na2CO3-BaCO3) electrode, a faster response was observed in comparison to a

Na2CO3 electrode [7]. It was found that sensors with sensing electrodes of mix-

tures of alkali and alkaline earth carbonates exhibited better stability against hu-

midity. Previous work has been reported on sensors with binary carbonates, but

with limited results of short-term tests at two or three CO2 concentrations under

humid conditions. Moreover, the exact role of the binary mixture as the sensing

electrode is unknown.

In this work, we have focused on the long-term stability of the sensor in

humid conditions as well as a novel structure of the sensing electrode with the

goal of elimination of humidity interference.

We report herein that the water interference associated with the Li2CO3 electrode is eliminated by coating with BaCO3, whereby, the response and the recovery become faster in humid conditions. The structure of the coated carbonate electrode as well as the CO2 detection mechanism of this sensor is also examined.

2.1 Experimental

2.1.1 Sensor fabrication

Figure 2.1 shows the procedure for our CO2 sensor fabrication based on Li

ion conductor with three different carbonate materials utilized as a sensing electrode.

37 Electrolyte

A schematic structure of the fabricated sensor is shown in Figure 2.2. Lith-

ium phosphate (Li3PO4, Alfa Aesar, 99.5%) added with 5 mol% SiO2 to enhance

the sinterability was used as an electrolyte. The powder mixture was ball-milled

in ethanol for 8 hr and dried at 120 °C. The dried mixture was compacted into a

disc at 1.5 kpsi and sintered at 800 °C for 8 hr with a heating and cooling rate of

5 °C/min. A Lindbergh furnace (Model 51732-B) was utilized for the heat treat-

ment. On both sides of the Li3PO4 electrolyte disk of 1.2 mm in diameter and 0.8

mm thick, gold paste (Heraeus Gold ink) was painted 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.

Reference electrode

Lithium titanate (Li2TiO3, Lithium Corporation of America Inc., 99%)

mixed with 5 mol% titania (TiO2, Alpha Aesar, 99.9%) was used as the reference

electrode. The powder mixture was ball-milled in ethanol. It was mixed with an α-

terpineol organic binder (Fisher Chemicals) and painted on the surface of the

Li3PO4 electrolyte. It was cured following the same heat treatment profile of the

gold paste curing.

38 Sensing electrode with Ba2CO3 coating layer

In order to compare the sensing behaviors, the sensing electrode was fab-

ricated with three different materials: (A) pure lithium carbonate; (B) a physical

mixture of lithium carbonate and barium carbonate with a molar ratio of 1:1; and

(C) lithium carbonate powder with a coating layer of barium carbonate. For the sensor with electrode A, lithium carbonate (Li2CO3, Alpha Aesar, 99%) was ball-

milled in ethanol and painted on the surface of the Li3PO4 electrolyte by hand-

painting and cured at 600 °C for 1 hr at a heating and cooling rate of 5 °C/min.

For the sensor with electrode B, lithium carbonate and barium carbonate (BaCO3,

J.T. Baker Chemical CO., 99%) was mixed in a 1:1 molar ratio, ball-milled in ethanol and painted by hand-painting. In order to avoid the eutectic reaction (at

609(±4) °C) of Li2CO3 and BaCO3, the sensor was cured at 500 °C for 3 hr at a

heating and cooling rate of 5 °C/min. For the sensor with electrode C, the coating

layer of BaCO3, the powder of BaCO3 coated on Li2CO3 was painted and cured

under the same condition as that described for electrode B. The following process

was used to coat Li2CO3 with BaCO3.

In the coating process, barium nitrate (Ba(NO3)2, Johnson Matthey,

99.999%) was used as a BaCO3 precursor. A 0.02 M Ba(NO3)2 aqueous solution

was prepared in deionized water using magnetic stirring, and 2 wt % Li2CO3 powder was added into the Ba(NO3)2 solution, then water was removed by heat-

ing in a rota-evaporator. For Ba(NO3)2 decomposition and reaction with CO2 gas,

the dried powder was heat-treated at 580 °C for 3 hr under a CO2 atmosphere [9].

39 During the heat treatment, BaCO3 was coated on the surface of the Li2CO3 pow-

der. The molar ratio of Li2CO3 and BaCO3 was changed from 1:1 to 7:1.

Sensing electrode after eutectic reation

First of all, the sensors were fabricated by attaching sensing electrode C.

In order to investigate the effect of the eutectic layer in the sensing electrode on

eliminating humidity interference, the sensors with BaCO3 coated on Li2CO3 as a

sensing electrode were heat treated for the complete eutectic reaction (eutectic

temperature ~ 609(±4)°C) at 650 °C for 2 hr under CO2 atmosphere.

2.1.2 Materials characterization

The microstructural observation and phase analyses of the coated samples

were done by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), focused ion beam (FIB) and X-ray diffraction (XRD). The surface image of the sensing electrode was observed by SEM (Quanta, FEI), and the cross-sectional images of the sensing electrode and the thickness of the BaCO3 coating layer was investigated by FIB (Strata DB235, FEI) and TEM (CM 200,

FEI). The phase analyses of pure Li2CO3 and BaCO3 coated on Li2CO3 system

were determined using a Scintag PAD-V diffractometer with Cu Kα radiation at

45 kV and 20 mA in 2θ ranging from 10 to 70o. JCPDS standard data were compared to the spectra for phase identification.

40 CO2 and H2O co-adsorption was characterized by Fourier transform infra-

red spectroscopy. A Bruker IFS 66/S spectrometer equipped with a liquid-

nitrogen cooled MCT detector was used in this study. After exposing samples to

5 % CO2 and 80 % R.H. at 500 °C for 3 hr, samples were immediately observed

by IR spectroscopy. IR spectra were collected at a spectral resolution of 4 cm-1

and were converted to Kubelka-Munk units for quantitative comparison.

2.1.3 Sensor testing setup

A schematic of the sensor test assembly is shown in Figure 2.3 (a). For sensing tests, the sensor was located in the central uniform temperature zone of a

Lindberg horizontal tube furnace (Figure 2.3 (b), Model TF 55035A). Three gases

including air, N2 and CO2 were mixed for the sensing tests. Sample gases were

prepared from 100 % pure CO2, diluted in nitrogen by mixing it with air. The arti-

ficial air was used as a background gas in all measurements flown at the rate of

210 ml/min. The CO2 gas concentration from 5 % to 20 % was controlled by mix-

ing CO2, N2 and air. The flow of gases used was determined with digital mass

flow controllers manufactured by Sierra. 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. Humid gas was prepared by bub-

bling the gas mixture through water at room temperature. In humid tests, the rela-

tive humidity was 80 %. The test temperature was 500 °C. The EMF values of the

sensor were measured by a two–probe technique with a model HP 34401A volt-

41 meter which was linked to a computer via the HP Benchlink data logger software

for data acquisition for every 2 s. With this software, the EMF was simultane-

ously monitored and shown on the computer screen.

2.2 Results

2.2.1 Sensing electrode materials

Figure 2.4 shows X-ray diffraction patterns obtained for BaCO3 coated on

Li2CO3 powder. The XRD patterns for pure BaCO3 and Li2CO3 are also shown for

comparison. All the peaks observed for BaCO3 coated on Li2CO3 powder are identified as either BaCO3 or Li2CO3 phase. These results suggest that lithium and

barium carbonates do not form substitutional solid solutions. No solid solution

between Li2CO3 and BaCO3 based on XRD data is consistent with the phase dia-

gram of Li2CO3 and BaCO3 as shown in Figure 2.5 [10]. Such an observation is

expected taking into account the differences in crystal structures (Li2CO3-

+ 2+ monoclinic, BaCO3-orthorhombic), ionic radii (Li 0.59Å, Ba 1.36 Å) and va-

lences of both ions [11].

Figure 2.6 shows BaCO3 coated on Li2CO3 powder with rough and porous

surface morphology at the molar ratio of 1:1. This suggests that BaCO3 coating

layer is not continuous and doesn’t cover the entire surface of Li2CO3. Figure 2.7

shows the change of surface morphology of sensing electrode C with the molar

ratios of Li2CO3 to BaCO3. As the molar ratio of Li2CO3 : BaCO3 increased,

BaCO3 particles were sparsely deposited on the surface of Li2CO3, as shown in

42 Figure 2.7. The distribution of the Ba element on the surface of the BaCO3 coated

on Li2CO3 sensing electrode with different molar ratios of Li2CO3 : BaCO3 was

examined using EDS element mapping on the SEM, as shown in Figure 2.8. The

data reveal a homogenous distribution of Ba on the surface at the molar ratio of

1:1. Therefore, the number of triple phase boundaries (TPBs) between Li2CO3 and

BaCO3 decreased with the molar ratio of Li2CO3 to BaCO3. The surface mor-

phologies of the sensing electrodes with pure Li2CO3 or a physical mixture of

Li2CO3 and BaCO3 is shown in Figure 2.9.

Figure 2.10 (a) shows the cross-sectional image of the sensing electrode

by FIB. Due to different contrast of materials, two layers are revealed by the ion

beam. The outer layer is BaCO3 and the inner layer is Li2CO3. A representative

TEM image of BaCO3 coated on Li2CO3 powder is shown in Figure 2.10 (b). The

Li2CO3 surface is covered with a BaCO3 coating layer. The thickness of BaCO3 coating layer is approximately 300 nm. However, it is difficult to obtain better images of TEM because of the sample preparation of TEM. The size of Li2CO3 particle is too large to be observed by TEM. In addition, the surface of a com- pressed disc of BaCO3 coated on Li2CO3 is so rough that it is difficult to get a thin

sample of TEM by FIB.

Generally, infrared (IR) spectroscopy is used to determine the structures of

compounds or to identify molecules on surfaces. Different functional groups or

different bond types should absorb infrared radiation at characteristic wavelengths

of rotation, stretch, bend, and vibration [12]. Since multiple atoms in CO2 gas en-

43 able various vibration modes, CO2 gas is easily detected by IR spectroscopy as

presented in Table 2.1 [13]. In this study, IR spectroscopy was carried out to de-

termine the nature of the adsorbed species after exposure to CO2 and H2O. The

data are presented in Figure 2.11. We focused primarily on the 1500 − 3500 cm-1 region, where the bands due to CO2 and H2O species are observed. Both pure

Li2CO3 and BaCO3 coated on Li2CO3 show a similar intensity of CO2 adsorption

–1 at 2350 cm . However, the IR spectrum of BaCO3 coated on Li2CO3 powder

shows little adsorption of water vapor at 3000 − 3500 cm–1.

2.2.2 Reference electrode and Li ion electrolyte

Figure 2.12 shows the SEM image for the surface of Li3PO4 electrolyte.

After sintering, Li3PO4 electrolyte shows a dense surface without pores. The grain

size of Li3PO4 electrolyte was not uniform and was about 5 ~ 10 μm. Figure 2.13

displays the porous electrode structure of the reference electrode with Li2TiO3 and

TiO2.

2.2.3 Sensor characteristics of sensors before eutectic reaction

General sensing behavior and humidity interference

Figure 2.14 shows the response transients to dry and humid CO2 gas at

500 °C for the sensor with electrode C (BaCO3 on Li2CO3). The sensor showed

almost identical response to CO2 over the concentration range of 0 % to 20 % under both dry and humid conditions. The difference in sensitivity between dry and

44 humid conditions was less than 2 %. The 90 % response time of the sensor was 14 s under dry conditions, and 54 s under humid conditions. In addition, the sensor showed fast recovery under humid conditions.

For comparison, Figure 2.15 shows the response transients of the sensors with electrode A (pure Li2CO3) and electrode B (the physical mixture of Li2CO3-

BaCO3) at 500°C with dry and humid CO2 gas. The sensors responded well to dry

CO2 gas. However, the response was affected by the presence of water vapor. For the sensor with pure Li2CO3 in Figure 2.15 (a), there was a difference in sensitivity of about 4.1 % ~ 8.5 % between the dry and humid tests. Under humid conditions, the response time was longer than 1 min. In addition, the recovery time under humid conditions exceeded twofold or more when it was compared with the recovery time under dry conditions, although the sensor with the physical mixture of Li2CO3-BaCO3 demonstrated a better recovery and a smaller difference in sensitivity compared to the sensor with pure Li2CO3, as shown in Figure 2.15 (b).

Sensitivity to CO2 gas

Figure 2.16 shows the Nernstian behavior of the sensors with electrode A

(pure Li2CO3) and electrode C (BaCO3 on Li2CO3) at 500 °C under dry and humid conditions. For sensor C, the EMF values changed linearly with the logarithm of CO2 concentration in the entire range tested under both dry and humid conditions with a Nernstian slope of 73.9 mV/decade and 73.6 mV/decade, respectively.

These values of the Nernstian slope are very similar to the theoretical value at

45 500 °C, 76.6 mV/decade. For sensor A, the Nernstian slope for the dry condition

is 74.6 mV/decade and for the humid condition is 67.7 mV/decade. Figure 2.17 shows the change of the Nernstian slope of sensors C with different molar ratios of Li2CO3 and BaCO3 under humid conditions. As more BaCO3 was added in the

sensing electrode C, the Nernstian slope is closed to the theoretical value (76.6

mV/decade). At the molar ratio of 1:1, the Nernstian slope is much closed to the

theoretical value. However, the sensitivity to CO2 gas decreased as excess BaCO3 was added above the molar ratio of 1:1. Therefore, BaCO3 minimized the humid-

ity interference in the sensor at the optimum molar ratio of 1:1.

Base-line shift

The base-line of the sensor with electrode C (BaCO3 on Li2CO3) shifted

up from 285 mV to 301 mV under humid condition as shown in Figure 2.18 (a).

Therefore, the sensor is not suitable for practical applications where humidity

level changes, although the sensitivity does not change between dry and humid

conditions. The shift of base-line likely results from the slow stabilization. In the

reference electrode, initial stabilization is so slow because of the slow diffusion of

gas and ions for electrochemical equilibrium [26]. Especially, the initial stabilization of the sensor may take longer time under humid conditions than under dry

conditions because water vapor may be hindered to achieve the electrochemical

equilibrium in the reference electrode. However, Figure 2.18 (b) exhibits that the

sensor in second and third humidity tests showed smaller deviations of base line

46 between dry and humid tests. Therefore, for the practical application of the sensor, pre-treatment under humid condition for long time is necessary.

Sensor response time

Response time is represented by the time corresponding to the 90 %

change of EMF when CO2 concentration is changed. Figure 2.19 shows a typical

response of a CO2 sensor at 500 °C. Table 2.2 represents the response times when

CO2 concentration was changed from 5 % to 10 % under dry and humid condi-

tions. Response time under dry condition was shorter than that observed under

humid condition. The rate of equilibration under dry condition might be faster

than that under humid conditions [14]. Each sensor shows a small variation in re-

sponse time but such a tendency is found in every sensor. In addition, response

time is closely related to the flow rate and gas mixing time. When we controlled

the gas flow meter in order to change the gas concentration, it took time for this

changed gas to reach the sensor. Therefore, true response time presents some limi-

tations in measurements by the testing setup used in this work.

Long-term stability and reproducibility

In order to check long-term stability, sensing behaviors were investigated

at 500 °C for 9 hrs continuously and for 60 days with an interval of 2-3 days un-

der humid conditions. Figure 2.20 (a) shows the sensing behavior of the sensor

with BaCO3 coated on Li2CO3 for 9 hrs under dry and humid conditions. During

47 the 9 hrs continuous humidity test, the EMF values showed a good correlation with those measured for the dry test. Figure 2.20 (b) shows the EMF values at 5 % and 10 % CO2 for 60 days under humid conditions. As shown in Figure 2.20 (b), it can be seen that the sensor signals were stable during the test period of about 60 days.

Seven different sensors of electrode C were fabricated using the same procedure. The Nernstian slope of the 7 different sensors showed a standard deviation of approximately 1.3, as shown in Figure 2.21. Under humid conditions, the maximum Nernstian slope was 73.6, and the minimum was 70.4. The sensors demonstrated very good reproducibility.

2.2.4 Sensor characteristics of sensors after eutectic reaction

In Figure 2.22, the sensing behaviors of the sensor with the BaCO3 on

Li2CO3 sensing electrode after eutectic reaction are presented under dry and humid CO2 gas at 500 °C. For the sensor with the molar ratio of 1:1, there was a big difference in sensitivity between dry and humid test, as shown in Figure 2.22 (a).

Under dry and humid conditions, sensing signals were not fully recovered in the concentrations of CO2 from 5% to 20%. In Figure 2.22 (b) and (c), the sensors with molar ratios of 2:1 and 3:1, showed good sensing behavior in dry CO2 gas, while interference under humid conditions was observed. In Figure 2.22 (d) and

(e), the sensors showed small variation in EMF values between dry and humid conditions over the CO2 concentration range from 0 % to 20 %, when the sensing

48 electrode had the molar ratio of Li2CO3 and BaCO3 such as 5:1 and 7:1, respec-

tively. In addition, the sensor with the molar ratio of 7:1 showed relatively good recovery under humid conditions. However, this response time and recovery were

slightly longer than those measured for the sensor with the BaCO3 on Li2CO3 sensing electrode before the eutectic reaction. In Figure 2.23, the surface morphology of sensing electrodes, with different molar ratios, was investigated by

SEM. At the molar ratio of 1:1 and 3:1, the two different regions of electrodes were observed as shown in Figure 2.23 (a). One is a large particle that is labeled

A, and the other is micro-needles labeled B. EDS on the SEM revealed that the particles do not have Ba element, while the micro needles have Ba element as shown in Figure 2.24. Therefore, the large particles are pure Li2CO3 and micro

needles are the mixture of Li2CO3 and BaCO3 that was formed by eutectic reaction [16]. At the high molar ratio of Li2CO3 and BaCO3, the sensing electrode was

covered by the eutectic structure as shown in Figure 2.22 (b).

In Table 2.3, the Nernstian slope of the sensors with different molar ratios at 500°C under dry and humid conditions after eutectic reaction is presented. For

a sensor with molar ratios of 5:1 or 7:1, the difference of the Nernstian slope un-

der dry and humid conditions was less than 3%. In addition, the Nernstian slopes under dry and humid conditions were very similar with the calculated theoretical

values.

49 2.3 Discussion

2.3.1 Sensing mechanism of CO2

The current sensor is composed of the following solid-state electrochemi-

cal cell :

Air, CO2, Au|BaCO3 coated on Li2CO3|Li3PO4|Li2TiO3 + TiO2|Au, CO2, Air

Liu et al. [17] and Schettler et al. [18] reported a potentiometric CO2 sensor based

on a sodium ion conductor with a sealed reference electrode. However, in our

CO2 sensor, a reference electrode is chemically inert against CO2, so that the en-

tire sensor can be exposed to the ambient gas atmosphere without sealing the ref-

erence electrode.

The sensing electrode composed of a binary carbonate system such as

Li2CO3 and BaCO3 has been investigated by earlier researchers [8, 16, 19-21].

Since Li+ conductor is used as the electrolyte, the sensing electrode reaction in-

volving Li+ has been proposed as

+ - 2Li + CO2 +1/2O2 +2e = Li2CO3 (2.1)

The reference electrode reaction is written as

+ - Li2TiO3 = 2Li + 1/2O2 +2e + TiO2 (2.2)

The EMF that is measured between the two electrodes can be expressed as

Re ++GGoo− 1 Re Se RTaLi+ (1) (2) RT EP=−() μLi+ −μ Li+ =−lnSe = − ln CO 2 (2.3) FFaFFLi+ 22

In the reference electrode, the lithium activity is constant because the reaction is independent of the CO2 partial pressure, while the lithium activity in the sensing

50 electrode is dependent on the CO2 partial pressure. Moreover, the effect of O2 is compensated at equilibrium since both electrodes see the same O2 content. There- fore, the EMF is only dependent on the partial pressure of CO2. Figure 2.15 shows the experimental dependence of E vs. log PCO2. For the sensor with pure Li2CO3, the slopes of the Nernstian equation are different between dry and humid conditions. This may result from the deterioration of Li2CO3 to other compounds such as LiOH, LiHCO3 and Li2CO3·xH2O in the presence of water vapor [22, 23]. In the case of pure Li2CO3, unexpected reactions formed the above mentioned products as well as reaction (2.1) may occur in the sensing electrode. Therefore, the slope of the Nernstian equation can deviate from the theoretical value. The sensor with BaCO3 coated on Li2CO3 electrode is shown to give the same Nernstian slopes of dry and humid conditions, which are very close to the theoretical value for the number of electrons (n=2) for sensing electrode reaction.

2.3.2 Role of BaCO3 coating

In the BaCO3 coating on Li2CO3 electrode with the molar ratio of 1:1, humidity interference was drastically reduced leading to almost humidity- interference free response. In this sensing electrode, the outer BaCO3 layer plays an important role as a chemical barrier of hindering adsorption of water vapor, while the inner Li2CO3 layer does CO2 sensing. The BaCO3 layer did not affect the activity of Li ion because the sensor with BaCO3 coated on Li2CO3 showed the same sensitivity with the sensor with pure Li2CO3 under dry condition. There-

51 fore, BaCO3 layer was not involved in the sensing electrode reaction. The difference in the affinity of water between BaCO3 and Li2CO3 provides the effect of chemical barrier. The solubility of Li2CO3 in water is 1.3g/l at 20 °C, while

BaCO3 in water is insoluble [24]. Such low affinity of BaCO3 to water is likely the reason for high resistance against humidity. In addition, the IR spectra show that the powder of BaCO3 coating on Li2CO3 adsorbs less H2O than pure Li2CO3 powder.

The BaCO3 coating layer on the sensing electrode may have contributed to the increase in the resistance against humidity. However, in the case of the mixture of BaCO3 and Li2CO3 as a sensing electrode, humidity interference was reduced although humidity interference was not completely eliminated. Therefore, the sensors with the BaCO3 coating layer that did not cover the entire Li2CO3 sensing electrode showed better sensing behaviors than the sensor with pure

Li2CO3 under humid conditions. Table 2.4 presents the variation of EMF values of the sensors with electrode C of different molar ratios of Li2CO3 to BaCO3 between dry and humid conditions. As the ratio of BaCO3 decreased in the sensing electrode, the sensor had the small number of triple phase boundaries (TPBs) between Li2CO3 and BaCO3 and showed large variation of EMF values between dry and humid conditions. Thus, the chemical barrier against humidity at the TPB between Li2CO3 and BaCO3 is important in order to eliminate the humidity interference. Therefore, humidity interference can be eliminated as the area of TPBs between Li2CO3 and BaCO3 increased by adding more BaCO3 with small particle

52 size. However, the exact role of BaCO3 is still unclear. Several possible effects of

BaCO3 are proposed.

– Preventing bicarbonate formation.

Based on the change of Gibbs free energy, the reaction for formation of LiHCO3 is more easy than that for the formation of Ba(HCO3)2 under humid conditions.

Therefore, by adding BaCO3 in the sensing electrode, the formation of LiHCO3 may be prevented.

– Surface effect by doping BaCO3.

As reported in literature, by adding Cr2O3 to TiO2 resulting in the formation of

Cr2–xTixO3 that showed stable performance with minor humidity effect because of the change of surface defect and chemistry [27]. In our sensor, the surface of the sensing electrode may be changed by doping BaCO3 in Li2CO3. Due to the change of surface chemistry, the reactivity toward humidity may be changed.

Other binary systems such as CaCO3-Li2CO3 and SrCO3-Na2CO3 were also found to show similar stability in the presence of water vapor because CaCO3 and SrCO3 have no solubility in water [7, 20].

2.3.3 Role of eutectic reaction

In Figure 2.22, except for the sensor with the molar ratio of 1:1, most sensors after the eutectic reaction showed better resistance against humidity than the sensor with pure Li2CO3 and the sensor with physical mixture of Li2CO3 and BaCO3

[16].

53 The phase diagram for the Li2CO3-BaCO3 system shows a physical mixture of Li2CO3 and BaCO3, with a eutectic temperature of 609(±4) °C at about 55 mol % Li2CO3 as shown in Figure 2.5. For the sensor with the molar ratio of 1:1, most of the sensing electrode melted and formed eutectic layer. Since this composition has a large amount of the eutectic liquid, the eutectic liquid can flow into the electrolyte and react with the Li3PO4 electrolyte [25]. Therefore, a new interface was formed between the electrolyte and sensing electrode as shown in Figure

2.25 (a). As the composition of the sensing electrode is far away from the eutectic composition, the thickness of the new interface became thinner, as shown in Fig- ure 2.25 (b), because of the small amount of the eutectic liquid. In Figure 2.25 (c), the new interface was not observed. In order to explain the formation and the structure of the new phase between the electrode and electrolyte, XRD studies were performed. The mixture of Li2CO3, BaCO3 and Li3PO4 with 5% SiO2 was ball-milled and heat-treated at 650 °C for 2hr. The XRD pattern of the mixture is shown in Figure 2.25. Thus, the sensing behavior of this sensor had deteriorated by the new phases such as barium silicate (Ba4Si6O18) and barium phosphate

(Ba3P4O13) at the interface under dry and humid conditions. Therefore, in the case of low molar ratios of Li2CO3 and BaCO3 such as 1:1, 2:1 and 3:1, the sensors with the eutectic layer cannot be directly compared with the sensors without the eutectic layer because of the formation of new phases.

For the sensors with a large amount of Li2CO3, a small amount of eutectic liquid didn’t form the new phases, and covered most of the area of remaining

54 solid Li2CO3 as the eutectic structure as shown in Figure 2.23 (b). Therefore, by comparing sensors with the high molar ratio of Li2CO3 and BaCO3, the effect of the eutectic layer on eliminating humidity interference can be explained. Table

2.5 presents the variation of EMF values of sensors with electrode C of different molar ratios after the heat treatment above the eutectic temperature. For molar ratios of Li2CO3 and BaCO3 such as 5:1 and 7:1, the variation of EMF values of the sensors with the eutectic layer between humid and dry conditions became smaller. Therefore, the humidity interference decreased after the heat treatment above the eutectic temperature.

From these results, the eutectic layer on the sensing electrode is considered important to remove the humidity interference. This result is consistent with the sensing tests of sensors without the eutectic layer. Because the eutectic layer consists of Li2CO3 and BaCO3, these are providing many TPBs. The TPBs in the eutectic layer should reduce the effect of humidity on our CO2 sensor as a chemical barrier against humidity [16].

2.4 Conclusion

A potentiometric CO2 gas sensor with lithium phosphate electrolyte was fabricated with a sensing electrode of BaCO3 coated on Li2CO3 and it showed ex- cellent performance toward CO2 monitoring at 500°C under dry tests as well as under humidity tests. In addition, the sensor showed good reproducibility and long-term stability under humid conditions. All sensors showed the Nernstian be-

55 havior in good agreement with the theoretical response. To form the BaCO3 layer on Li2CO3, a wet chemical process was used. The coating layer of BaCO3 was an effective approach to improve the sensor stability against humidity. In the layered structure of the sensing electrode, the outer BaCO3 layer improved the resistance against humidity as a chemical barrier, while the inner Li2CO3 layer was responsible for the CO2 sensing. After heat treatment above the eutectic temperature, the sensor in which the eutectic layer covered the sensing electrode showed good sensing behavior under dry and humid conditions. Thus, the TPB in a binary carbonate system is important to eliminate humidity interference.

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Figure 2.1 Procedure for our CO2 sensor fabrication based on Li ion conductor with three different carbonate materials as the sensing electrode.

60 Au lead Sensing electrode (Sensor A, B, C)

Li3PO4 Au

Reference electrode Au lead (Li2TiO3 + TiO2)

Figure 2.2 Schematic structure of the sensor with three different carbonate materials as the sensing electrode.

Figure 2.3 Schematic of the sensor test assembly (a) and Lindberg horizontal tube furnace (b).

62 L

L L L L L B L L L B L B

L: Li2CO3, B: BaCO3

Figure 2.4 X-ray diffraction patterns for (a) pure BaCO3 (b) BaCO3 coated on Li2CO3 and (c) pure Li2CO3.

Figure 2.5 Phase diagram of Li2CO3-BaCO3 system. [10]

64 10μm

Figure 2.6 SEM image of the surface of BaCO3 coated on Li2CO3 powder with the molar ratio of 1:1.

65 (a)

10μm

(b)

μ 5 m

Figure 2.7 SEM images of the surface of BaCO3 coated on Li2CO3 powder with different molar ratios of Li2CO3 : BaCO3. The molar ratio of (a) 3:1 and (b) 7:1.

66 (a)

(b)

Continued

Figure 2.8 EDS mapping of Ba content on the surface of BaCO3 coated on Li2CO3 powder with different molar ratios of Li2CO3 : BaCO3. The molar ratio of (a) 3:1 and (b) 7:1.

67 Figure 2.8 continued

(c)

68 (a)

10μm

(b)

μ 10 m

Figure 2.9 SEM images of the surface of (a) pure Li2CO3 and (b) a physical mixture of Li2CO3 and BaCO3.

(a)

300 nm (b)

Figure 2.10 The cross-sectional images of BaCO3 coated on Li2CO3 by (a) FIB and (b) TEM.

Figure 2.11 Infrared spectroscopy of BaCO3 coated on Li2CO3 (a) and pure Li2CO3 after exposure to H2O at 500°C for 3hr.

71 10μm

Figure 2.12 SEM image of the surface of Li3PO4 electrolyte.

72 10μm

Figure 2.13 SEM image of the reference electrode with Li2TiO3 and TiO2.

73 1.2

Air Air 1

0.8 5%

0.6 15%

E/E0 5% 10% 10% 15% 20% 20% 0.4 dry 0.2 humid

0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 Time (min)

Figure 2.14 Response transient of the sensor with BaCO3 coated on Li2CO3 in dry and humid CO2 at 500°C.

74 1.2 (a) Air Air 1

0.8 5%

0.6 15%

E/E0 5% 10% 10% 15% 20% 20% 0.4 dry 0.2 humid 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 Time (min)

(b) Air Air

15% 5% 10% 10% 15% 20% 20%

dry humid

Figure 2.15 Response transient of the sensor with (a) pure Li2CO3 and (b) physical mixture of BaCO3-Li2CO3 in dry and humid CO2 at 500°C.

Figure 2.16 Dependence of EMF on log [CO2 concentration] for the sensors with pure Li2CO3 (a) and BaCO3 coated on Li2CO3 (b) in dry and humid conditions at 500°C.

76 220 y = -56.255x + 460.84 (Li:Ba=1:3) y = -63.287x + 504.43 (Li:Ba=1:2) 210 y = -73.631x + 555.51 (Li:Ba =1:1) y = -71.139x + 545.85 (Li:Ba=2:1) 200 y = -70.048x + 541.43 (Li:Ba=3:1) y = -69.666x + 535.1 (Li:Ba=5:1) 190 ] y = -69.498x + 538.38 (Li:Ba=7:1) mV [ y = -67.777x + 510.15 (pure Li2CO3)

EMF 180

170

160

150 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 log (CO2 conc.) [ppm]

Figure 2.17 Changes of the Nernstian slope of sensors C with different molar ratios of Li2CO3 and BaCO3 under humid conditions.

77 350 (a)

Air

300

250 EMF [mV]

200

5% CO 150 2

10% CO2 dry humid

100 0 10203040506070 Time (min)

350 nd (b) 2 humid test Air 300

250 ] mV [ EMF 200

3rd humid test

150

5% CO2 5% CO2

100 0 40 80 120 160 200 240 Time (min)

Figure 2.18 Base-line shifts under humid conditions at the first humid test (a) and base line of second humid test for longer stabilization (b).

Figure 2.19 Typical response time of sensor B at 500°C.

79 1.2 Air (a)

0.8

5% 0.6

E/E0 10% 15% 20% 0.4 dry 0.2 humid 0 0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 Time (min)

250 (b)

200

) 150 mV (

EMF 100 Under humid condition

50 5% CO2

10% CO2 0 0 102030405060 Time (days)

Figure 2.20 Long-term test of the sensor with BaCO3 coated on Li2CO3 (a) for 9 hr continuous test and (b) for 60 days test in humid condition at 500°C.

Figure 2.21 Sensor EMF responses from different 7 sensors with sensing electrode C.

81 Air

5% 10% 15% 20% dry humid

Air

5% 10% 15% 20%

dry humid

Continued

82 Figure 2.22 continued

Air

5% 10% 15% 20%

dry humid

Air

5% 10% 15% 20% dry humid

Continued

83 Figure 2.22 continued

Air

5% 10% 15% 20% dry humid

84 B

(b)

10µm

Figure 2.23 SEM images of surface morphology of sensing electrode with different molar ratios. The molar ratio of Li2CO3 and BaCO3 is (a) 1:1 or 3:1, and (b) 5:1.

Figure 2.24 EDS analyses of the selected points for Li2CO3-BaCO3 electrode. (a) a large particle (A in Figure 2.23 (a)), (b) needle-like region (B in Figure 2.23 (a)).

Continued

Figure 2.25 SEM image of cross-section of sensors attached to the sensing electrode with the molar ratio of (a) 1:1, (b) 3:1 and (c) 5:1 after sensing tests.

87 Figure 2.25 continued

88 ●

●

● ▲ ● ● ● ■ ▲ ■ ▲▲ ▲ ▲ ●●●●

●: Ba Si O , ▲: Ba P O , ■ : Li PO 4 6 18 3 4 13 3 4

Figure 2.26 X-ray diffraction patterns of the mixture of sensing materials (Li2CO3-BaCO3) and electrolyte (Li3PO4) after heat-treatment at 650 °C for 2 hr.

89 Table 2.1 Absorption band characteristics of CO2 gas and carbonate anion. [13]

Wavenumber Vibration type [cm-1]

CO2 Symmetric stretching 1380 Raman active Bending out of plane 666 IR active Asymmetric stretching 2350 IR active 2- CO3 Symmetric stretching 1065 Raman active Bending out of plane 880-850 IR active Asymmetric stretching 1450-1410 IR and Raman active Bending in plane 720-680 IR and Raman active

90 Table 2.2 Response times for changing CO2 concentration from 5% to 10% under dry and humid conditions.

Sensor A Sensor B Sensor C Dry 12 s 16 s 14 s Humid 96 s 64 s 54 s

91 Table 2.3 Nernstian slope of the sensors with different molar ratios of Li2CO3 and BaCO3 at 500°C under dry and humid conditions after eutectic reaction.

1:1 2:1 3:1 5:1 7:1 (Li2CO3 (Li2CO3 (Li2CO3 (Li2CO3 (Li2CO3 : BaCO3) : BaCO3) : BaCO3) : BaCO3) : BaCO3) Dry 75.3 70.4 71.2 74.5 73.8 condition

Humid 68.8 66.2 69.8 72.3 72.5 condition

92 Table 2.4 The variation of EMF values of the sensors without the eutectic layers in range of CO2 concentration from 5% to 20% under dry and humid conditions.

Sensing electrode Variation of EMF values

BaCO coated on Li CO 3 2 3 1.5 ~ 2.3% (Li2CO3 : BaCO3 = 1:1) BaCO coated on Li CO 3 2 3 1.7 ~ 3.1% (Li2CO3 : BaCO3 = 2:1) BaCO coated on Li CO 3 2 3 2.5 ~ 3.6% (Li2CO3 : BaCO3 = 3:1) BaCO coated on Li CO 3 2 3 2.8 ~ 4.5% (Li2CO3 : BaCO3 = 5:1) BaCO coated on Li CO 3 2 3 3.4 ~ 4.5% (Li2CO3 : BaCO3 = 7:1)

93 Table 2.5 The variation of EMF values of the sensors with the eutectic layers in range of CO2 concentration from 5% to 20% under dry and humid conditions.

Sensing electrode Variation of EMF values

BaCO coated on Li CO 3 2 3 ~ 10% (Li2CO3 : BaCO3 = 1:1) BaCO coated on Li CO 3 2 3 1.6 ~ 4.9% (Li2CO3 : BaCO3 = 2:1, 3:1) BaCO coated on Li CO 3 2 3 ~ 2.5% (Li2CO3 : BaCO3 = 5:1, 7:1)

CHAPTER 3

LITHIUM ION CONDUCTIVITY IN PEROVSKITE LITHIUM

LANTHANUM TITANATE AT LOW TEMPERATURES

The phenomenon of ionic transport in solids has scientifically been studied over many years. At the end of the 19th century, Warburg and Tegetmeier [1,2] measured charge transfer to prove Faraday’s law for solid ionic conductors, also called solid electrolytes. Scientific research related to ionic motion in solids has became an attractive field to understand the mechanisms of ion transport in solids, and research is also being conducted to identify potential technological applications of solid electrolytes, such as chemical sensors, rechargeable batteries, and fuel cells [3]. The increased knowledge and understanding of the properties of materials, as well as the discovery of several solid electrolytes such as NASICON

(Na Super Ionic Conductor) [4] with high ionic conductivity, have provided opportunities to use these materials in several high-temperature applications.

Although solid electrolytes are important for some electrochemical devices, such as batteries and chemical sensors, most solid electrolytes have ex-

95 tremely low ionic conductivities at low temperatures. Therefore, many research

groups have focused on improving the poor performance of electrolytes at low

temperatures in order to develop electrochemical devices that function better at

low temperatures [5−10].

Among various solid electrolytes, Li solid electrolytes are very important

because of their application for lithium ion secondary batteries. Until now, vari-

ous metal oxides have shown high bulk lithium ion conductivity in a wide tem-

perature range for the application of lithium ion secondary batteries as shown in

Figure 3.1 [11]. Using operational temperature as a criterion, they can be divided

mainly into two groups: (i) high-temperature ionic conductors, for example,

Li2SO4[12], Li4SiO4[13], and Li14ZnGe4O16 [14] and (ii) low-temperature ionic

conductors, for example, γ-Li3.6Ge0.6V0.4O4 [5], Li3N [6,7], Li- β-alumina [8],

Li1+xTi2-xMx(PO4)3 (M = Al, Sc, Y or La) [9], and Li0.34La0.5TiO2.94 [10].

Among these Li ion conductors at low temperatures, a perovskite-type ox-

ide, La2/3-xLixTiO3 (LLT), is an attractive solid Li ion conductor because La2/3-

xLixTiO3 with the optimum composition, i.e., La0.55Li0.3TiO3, shows the highest Li

ion conductivity of the bulk part, which is about 10-3 S/cm at room temperature

[10]. In general, La, Li, and vacancies are distributed in the A-site, and Li ions migrate through the conduction path composed of Li and vacancies in this electrolyte, as shown in Figure 3.2 [11]. Although the Li ion transport mechanism in this electrolyte has been studied, the exact dimensionality (2D or 3D) of the lithium

96 mobility in LLT is still controversial. Therefore, much more detailed studies are

needed.

In this work, we studied the relationship between Li ion conductivity and microstructure in lithium lanthanum titanates (LLT) sintered at various temperatures. During sintering at higher temperatures, more Li loss in the Li ion conductor is expected. Thus, the effects of Li content and microstructure on Li ion conductivity are discussed in this chapter.

3.1 Brick-layer model

In the late 1960s, the brick-layer model (BLM) was developed to describe the electrical properties of ion-conducting ceramics with polycrystalline structures

[15]. Bauerle [15] suggested an equivalent circuit with two parallel resistor–

capacitor (RC) combinations to represent two semicircles of impedance response

of electro-ceramics, as shown in Figure. 3.3. Beekmans and Heyne [16] conceived

an equivalent circuit model that was simpler than the one suggested by Bauerle,

and their model later was called the ‘‘brick layer model’’ by Burggraaf [17,18]. In

this model, conducting grains are surrounded by resistive grain boundaries. There-

fore, there are two conduction paths, i.e., the series path and the parallel path, as shown in Figure 3.4. However, the parallel grain boundary path is ignored. Näfe

[19] modified this model to the series/parallel-BLM by considering the side-wall

grain boundary path, as shown in Figure 3.5. This model was first used for model-

ing oxygen ion conduction in nano-structured ceramics by Hwang et al. [20]. Re-

97 cently, a 3D-BLM has been developed such that grains and grain boundaries are

divided into pixels with 3D interconnectivity, as shown in Figure 3.6 [21,22].

These BLMs cannot successfully explain the electrical conduction of nano-

structured ceramics, since the widths of the grain boundary are comparable in size

to grains, and the grains are not cubular in shape. However, the grain and grain

boundary conductivity of a solid electrolyte with micron-sized grains is explained

well by BLM.

3.2 Experimental

3.2.1 Preparation of lithium lanthanum titanates

Lithium lanthanum titanate was synthesized according to the following re-

action.

x Li2CO3 + (2 - x)/3 La2O3 + 2 TiO2 = 2 La2/3-xLixTiO3 + x CO2 (3.1)

According to the literature [11], perovskite oxide La0.57Li0.3TiO3 has

shown the highest ionic conductivity, and the loss of Li during sintering was ex-

pected. Belous et al. [23] reported that La2/3-xLixTiO3 (LLT) in the range of 0.12 <

x < 0.51 shows high ion conductivity. Therefore, in order to obtain La0.57Li0.3TiO3, samples with different compositions of raw materials were prepared by the conventional solid-state reaction. More Li was added than the theoretical amount to account for Li2O evaporation at high temperatures. After analyzing the composi-

tion of the LLT that was heat-treated for the solid-state reaction, LLT with x = 0.4

was studied since the literature indicates that this LLT has the highest ionic con-

98 ductivity [11]. The compositions of the products were analyzed by inductively

coupled plasma (ICP) spectroscopy. The starting compositions and experimental

compositions are given in Table 3.1.

The samples were prepared by a conventional solid-state reaction method.

Appropriate amounts of La2O3 (99.9%, Aldrich Chemical), Li2CO3 (99.999%,

Alfa Aesar), and TiO2 (99.9%, Alfa Aesar) were mixed and ball milled in ethanol

with zirconia balls for 12 hr. The mixed powder was calcined at 800 °C for 4 hr and 1000 °C for 6 hr in air using a Lindberg furnace (Model TF 55035A). The calcined powders were ground and then pressed into pellets under a hydraulic pressure of 3.5 kpsi. The disc-shaped pellets weighed 0.4 g each, and they had diameters of 12 mm and thicknesses of 2 mm. The green pellets were sintered at various temperatures, ranging from 1100 °C to 1350 °C for 6 hr in air with a heating/cooling rate of 5 °C /min. Figure 3.7 shows the procedure for preparing lithium lanthanum titanate electrolyte.

3.2.2 Characterization of electrolyte

The phase was identified with X-ray diffraction (XRD) with CuKα radia-

tion at 45 kV and 20 mA in the 2θ range from 10 to 70o (Scintag PAD-V diffrac-

tometer, Japan.) by comparing with JCPDS standard data. The lattice parameters were calculated from XRD data. The samples were polished to observe the microstructure with a scanning electron microscopy (SEM, Quanta, FEI), and the grain

99 sizes were determined from SEM micrographs of the surface by using the linear intercept method.

For the impedance test, a symmetrical test cell was fabricated using sputtering of gold and gold paste (Heraeus Gold ink).

Au | La2/3-xLi3xTiO3 (LLT) | Au

For the gold–ion–blocking electrode, a CrC-150 TORR International, Inc. sputtering system and gold target (99.99% purity, 3-inch diameter, 0.5 mm thick, Sput- tering Materials, Inc.) were used for sputtering in Ar gas. Pre-sputtering for 3 min, 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. For fully dense gold electrode, a 10-min sputtering time was used. The sputtering rate is about 30 nm/min. The vacuum condition could not be controlled perfectly, and it varied from 1.2×10−6 torr to 6.4×10−6 torr. The Ar pressure was kept about

4.5×10−3 torr. The operating current was set at 105 mA. After the sputtering of gold, the sample was heat treated at 700 °C for 5 hr with 5 °C/min heating and cooling rates. Gold wire was attached on both sides of the LLT electrolyte with gold paste (Heraeus Gold ink) by hand painting, and cured at 600 °C for 1 hr with heating and cooing rates of 5 °C /min.

The electrochemical properties of the electrolyte were measured using a

Solartron 1260A impedance analyzer over the frequency range of 5 Hz to 13 MHz and the temperature range from 300 to 473 K. Li ion conductivities were calculated by analyzing the impedance data. The activation energies for the samples

100 were calculated by measuring the conductivities over the temperature range from

300 to 473 K.

3.3 Results

3.3.1 Crystal structures by XRD

Figure 3.8 shows the XRD patterns of different compositions of La2/3-

xLixTiO3. Only the perovskite phase was observed in the composition range from

x = 0.2 to 0.5. In the literature [27], it was indicated that, if the sintering tempera-

tures and times were not suitable, impurity phases, such as lithium titanium oxide

and lanthanum oxide would be found. However, under our experimental condi-

tions, impurity phases were not observed in the XRD patterns, although XRD is

generally limited to crystalline materials with at least 1% volume of the phase.

Since La2/3-xLixTiO3 is well known as a good solid electrolyte of lithium

ions [10], the crystal structure of La2/3-xLixTiO3 has already been studied by many

researchers [24–26]. According to the literature [25, 26], (Li, La)TiO3 has a crystal structure in which La ions, Li ions, and vacancies are distributed in perovskite

A site as shown in Figure 3.2.

The chemical compositions of La2/3-xLixTiO3 from ICP spectroscopy are given in Table 3.1. About 25% of the initial lithium content was lost due to evaporation during the sintering process at 1350°C.

Figure 3.9 shows the lattice parameter, a, of the samples with various sintering temperatures, based on the XRD results. For sintered LLTs with x = 0.4,

101 the lattice parameter decreased with increasing sintering temperatures above

1200 °C. The reason the lattice parameter decreased with increasing sintering

temperatures is that the amount of Li ion evaporation is increased at high sintering

temperatures. Due to the loss of Li, the lattice might be contracted.

3.3.2 Microstructures

Figure 3.10 shows surface morphologies of the sintered samples for LLT

with x = 0.4 by SEM. The grain sizes of the samples increased steadily with the

sintering temperature. For sintered LLTs at various sintering temperatures, ab-

normal grain growth was not observed. After sintering at 1100 °C and 1150 °C,

the samples still had many pores and low relative density, as shown in Figure 3.11.

After sintering above 1250 °C, very few pores were observed, and the densities of the samples increased. The LLTs sintered above 1250 °C showed similar densities, as shown in Figure 3.11.

3.3.3 Electrochemical characterization of LLT

A typical impedance plot for a solid electrolyte and an equivalent circuit are shown in Figure 3.12, which shows three semicircles related to grain, grain boundary, and electrode. Therefore, impedance spectroscopy could be used to separate grain and grain boundary conductivities. Figure 3.13 shows the impedance patterns measured at 300 K for LLT sintered at various temperatures. The impedance plots show the same pattern with the typical pattern that consists of

102 three responses from the grain, grain boundary, and the electrode. Figure 3.13 (a) and (b) highlight the grain and the grain boundary parts, respectively. The significant difference in the impedance between the grain and the grain boundary is the different axis scale and frequency range. At the high frequency range, grain resistance, the size of semicircle, decreases slightly with sintering temperatures below

1200 °C. However, after full densification with the relative density above 90%, grain resistance increases steadily, as shown in Figure 3.13 (a). The resistance of the grain boundary decreases continuously with the sintering temperature, as shown in Figure 3.13 (b) and in the inset. Table 3.2 shows values of the resistance of grain and grain boundary obtained from fitting using the Zview software

(Scribner Associates, Inc) and the two RC parallel circuit as shown in Figure 3.12.

Figure 3.14 shows the dependence of grain boundary and grain conductivities for sintered LLTs on sintering temperatures measured at 300 K. The conductivities were calculated by analyzing the impedance patterns using the equivalent circuit model. The grain boundary conductivity continuously increased with increasing sintering temperatures. However, grain conductivity decreased slightly above 1200 °C.

Figure 3.15 shows the Arrhenius plot of grain and grain boundary ionic conductivities for LLT sintered at 1350 °C. Figure 3.16 shows the activation energy of the grain and grain boundaries for LLT at various sintering temperatures.

The activation energy (Egb) for the grain boundaries decreased from 0.53 to 0.47 eV with increasing sintering temperatures, while the activation energy (Eg) of the

103 grain remained constant as 0.31 eV. This means that the lithium ion mobility of

the grain remained constant and that of the grain boundary increased. However,

Egb has been reported from 0.4 eV to 0.68 eV [11]. Therefore, Egb did not signifi-

cantly change with the sintering temperature.

3.4 Discussion

3.4.1 Crystal structure and Li content

In general, La2/3-xLixTiO3 with the perovskite structure is stable over a

wide range of compositions. The crystal structure depends on the number of lat-

tice vacancies, the composition (Li/La ratio), substitution (A site, B site, or both),

synthesis method and sintering conditions [11]. The simple cubic unit cell, hex-

agonal, tetragonal and orthorhombic perovskite-type distorted cells were reported

in the literature [11]. For our experimental conditions for solid-state reaction and

sintering, La2/3-xLixTiO3 has a simple cubic perovskite structure, as shown in Fig- ure 3.2.

The lattice parameter decreased with increasing sintering temperatures, al-

though the rate of decreasing lattice parameter was dependent on the range of sintering temperatures. This observation may be attributed to the increased amount

of lithium loss at high temperatures. The lattice contracts as the Li2O evaporates

at high temperatures. In other compositions with x = 0.35 in La2/3-xLixTiO3, the

lattice parameter increased with the sintering temperature [11]. Therefore, it is

hard to determine the sole relationship between the lattice parameter and Li con-

104 tent. The lattice parameter of LLT might be dependent on the ratio of Li and La and the number of vacancies in the lattice.

However, Li ion conductivity is highly sensitive to the Li content, because conductivity is the product of the concentration and mobility of charged carriers.

Therefore, we deduced that reduction of the Li ion conductivities of the grain and grain boundary were due to the decrease in the lithium ion concentration after sintering at high temperatures. However, grain conductivity was also dependent on the density of the samples, as well as on their Li content. As the densities of the samples increased at higher sintering temperatures before full densification, grain conductivity increased rapidly. However, after full densification, grain conductivity showed almost the same value, because the density was almost constant. How- ever, it decreased slightly as the sintering temperature increased due to the Li loss during the sintering process. By considering Li content and the densities of the samples, it was found that the sample sintered at 1250 °C for LLT with higher Li content and density had the highest grain conductivity (log σg ~ 3.10) among the samples. The conductivity value is similar to the value 2.99 reported by Inaguma et al. [3]. In summary, the grain conductivity for the samples can be determined by two major factors such as the density of electrolyte and Li content.

3.4.2 Relationships between microstructure and ionic conductivity

The grain boundary conductivity for LLT rapidly increased as the sintering temperature increased, as shown in Figure 3.12 (b). Thus, grain boundary

105 conductivity was determined to a great extent by grain size rather than by differ-

ences in the composition of the grain boundary. Aono et al. [6] reported similar

results for the relationship between conductivities and microstructure in the lith-

ium titanium phosphate system. They ascribed this phenomenon to the increase in

the conductive path area and the tunnel fitting (grains in close contact) induced by

the increase in density and grain size [6].

Further assuming the brick-layer model is applicable for the samples, true

grain boundary conductivity can be easily calculated from following equations:

(true) σgb = σgb / D (3.2)

(true) σgb = σgb (1+ lg / lgb) (3.3)

(true) where σgb , D, lg, and lgb are the true grain boundary conductivity, the line den-

sity of the grain boundary, the grain size, and the grain boundary thickness, re-

spectively. Equation (3.3) predicts that the grain boundary conductivity, σgb, is

(true) proportional to the grain size if σgb and lgb are constant during the sintering

processes. Figure 3.17 shows that the σgb of LLT samples is proportional to the

grain size although there is some deviation from the linear relationship between

the grain size and conductivity. Equation (3.3) shows that the intercept of the

(true) (true) curve at lg =0 is σgb , and the slope is σgb × lgb, as shown in Figure 3.17.

(true) –5 Therefore, σgb and lgb can be estimated as 1.3 × 10 S/cm and 0.92 µm, re-

(true) spectively. The estimated the value of σgb is approximately 1/100 of the σg value, and is extremely low. The estimated, relatively thick grain boundary layer may include the thickness of the space charge region at the grain boundary.

106 Therefore, both the highly resistive grain boundary and the thick grain boundary

layer may be responsible for the low σgb value. For our LLT, the total ionic con-

ductivity was lower (~10–5 S/cm) than the value of total conductivity from 10–3 to

10–4 S/cm in literature [11] due to the high resistance of the grain boundary. To

increase the total conductivity, the grain boundary resistivity should be reduced.

In addition, in our LLT system, as the grain size of LLT increases after full densi-

fication, increase of the grain conductivity were not observed, while the grain

boundary conductivity increased. Therefore, it is more effective to increase the

grain boundary conductivity, and one approach for accomplishing this is to de-

crease the thickness of the grain boundary by improving the sinterability of LLT.

In the sintering process, the driving force for filling up the gaps between the

grains is the presence of chemical potential gradients and chemical diffusion. In

LLT, other components increasing the sinterability will lead to inhomogeneous

distributions or unexpected segregations in the grain boundary. Therefore, more

detailed studies must be conducted to identify which other components can be

added to LLT and the amounts that are effective. Another approach is to obtain the grain boundary with lower resistance by modifying their composition.

3.5 Conclusions

The conclusions are summarized as follows:

(1) Lithium lanthanum titanate (LLT) electrolytes were prepared by a conven-

tional solid-state method.

107 (2) By X-ray diffraction measurement of the prepared LLT, it was found that its

crystal structure was exactly the same as the cubic perovskite, regardless of the

sintering temperature.

(3) The impedance of the LLT electrolyte was measured over the temperature

range of 300 to 473 K and in the frequency range of 5 Hz and 13 MHz. Activation energies for the Li ionic conduction for grain boundary and grain were estimated to be 0.47 and 0.31 eV, respectively.

(4) It was found that LLT is a good ionic conductor at low temperatures and a good candidate as an electrolyte for low-temperature electrochemical cells.

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[23] A.G. Belous, G. N. Novitskaya, S.V. Polyanetskaya, Y. Gornikov, “Study of complex oxides with the composition La2/3-xLi3xTiO3,” Inorg. Mater. 23 (1987) 412

[24] O. Bohnke, H. Duroy, J.L. Fourquet, S. Ronchetti, D. Mazza, “In search of + the cubic phase of the Li ion-conducting perovskite La2/3−xLi3xTiO3: structure and properties of quenched and in situ heated samples,” Solid State Ionics 149 (2002) 217

[25] J.L. Fourquet, H. Duroy, M.P. Crosnier-Lopez, “Structural and Microstruc- tural Studies of the Series La2/3−xLi3x□1/3−2xTiO3,” J. Solid State Chem. 127 (1996) 283

110

[26] D. Mazza, S. Ronchetti, O. Bohnke, H. Duroy, J.L. Fourquet, Solid State Ion- ics 149 (2002) 81

[27] M. Itoh, Y. Inaguma, W.H. Jung, L. Chen, T. Nakamura, Solid State Ionics 70/71 (1994) 203

111

Figure 3.1. Arrhenius plots of conductivity of several well-known solid lithium ion conductors. [11]

112

Figure 3.2 Crystal structure of La2/3-xLi3xTiO3. [11]

113

Figure 3.3 Schematic of dual semicircles in impedance spectrum of electroceramics.

114

115

116

117

Figure 3.7 Flow chart for the synthesis of lithium lanthanum titanate (LLT) electrolyte.

118

x = 0.5

x = 0.4 Intensity

x = 0.3

x = 0.2

2θ

Figure 3.8 Powder X-ray diffraction patterns of La(2-x)/3LixTiO3.

119

Figure 3.9 The lattice parameter of the sintered LLTs based on cubic lattice with sintering temperatures.

120 (a) T=1100 °C 10um

(b) T=1150 °C 10um

Continued

Figure 3.10 SEM micrographs of LLT samples at various sintering temperatures.

121 Figure 3.10 continued

(d) T=1250 °C 10um

Continued

122 Figure 3.10 continued

(e) T=1300 °C 10um

(f) T=1350 °C 10um

123

Figure 3.11 Density of LLTs with x = 0.4 after sintering at various temperatures.

124

Figure 3.12 Impedance diagrams of solid electrolyte and an equivalent circuit.

125

Figure 3.13 The impedance plots of the LLTs with x = 0.4 at room temperature for (a) grain and (b) grain boundary part. The sintering temperatures are shown for identification. The approximate measuring frequencies are also shown. The inset in (b) shows the detailed view at the high frequency part.

126

Figure 3.14 The grain conductivity (a) and the grain boundary conductivity (b) of LLTs with x = 0.4 at room temperature after sintering at various temperatures.

127

Figure 3.15 Arrhenius plot of the conductivity for LLT at grain (■) and grain boundary (▲).

128

Figure 3.16 Activation energies of the grain (■) and grain boundary (▲) for LLT at various sintering temperatures.

129

Figure 3.17 The grain boundary conductivity of LLT with x = 0.4 is plotted versus the grain size. The straight line is the linear fitting curve of the data.

130 Table 3.1 ICP Analysis Data for La0.5Li0.4TiO3 after sintering at various temperatures.

Starting composition La0.5Li0.4□0.1TiO3

Sintering conditions Experimental composition

at 1100 °C for 6hr La0.49Li0.406□0.124TiO2.938

at 1150 °C for 6hr La0.49Li0.403□0.127TiO2.937

at 1200 °C for 6hr La0.49Li0.401□0.129TiO2.935

at 1250 °C for 6hr La0.49Li0.375□0.155TiO2.923

at 1300 °C for 6hr La0.47Li0.342□0.248TiO2.876

at 1350 °C for 6hr La0.49Li0.317□0.213TiO2.894

131 Table 3.2 Resistance of grain and grain boundary obtained from fitting using the Zview software.

Sintering temperature Rg (kΩ) Rg.b (kΩ)

1100 °C 5.4 1512

1150 °C 1.8 384

1200 °C 0.95 72

1250 °C 0.87 54

1300 °C 0.89 36

1350 °C 0.90 25

132

CHAPTER 4

A POTENTIOMETRIC SENSOR FOR THE DETECTION OF LOW CO2

CONCENTRATIONS AT LOW TEMPERATURES

Potentiometric CO2 sensors based on solid electrolytes, such as NASICON

and Li3PO4 with an alkaline metal carbonate sensing electrode, have been shown

to exhibit good CO2 sensing properties at elevated temperatures (400 °C or above)

[1,2]. However, to expand the scope of their application fields, low-temperature

CO2 sensors are attracting attention due to their low power consumption and easy

sensor miniaturization, since a heater is unnecessary. Recently, CO2 monitoring has been required for measurements of respiratory or bio-related activities. Spe- cially, since an astronaut exhales out CO2 in a closed spacesuit, CO2 concentra-

tions would build up to deadly levels (25% concentration of CO2 in a few minutes). In order to remove excess CO2 from the spacesuit atmosphere, CO2 concen-

trations must be monitored at low temperatures, as shown in Figure 4.1.

Bredikhin et al. demonstrated the potential of potentiometric CO2 sensing

at room temperature [3]. Similar devices, e.g., a design involving a NASICON

electrolyte in conjunction with a mixture of sodium carbonate and a semiconduct-

133 ing metal oxide as the sensing electrode, have been reported by other researchers

[4,5]. A mechanism involving water has been proposed, based on the assumption

that hydroxyl groups will be adsorbed at the surface, followed by a reaction with

- CO2 to form HCO3 groups on the surface of the semiconducting metal oxide.

In this mechanism, the presence of water vapor has been shown to be nec-

essary for the CO2 sensing capability at low temperatures. Unfortunately, Nernst’s

correlation between EMF and CO2 concentration for these sensors shifts up or

down rather extensively with changes in relative humidity. Thus, a new design is

required for an appropriate CO2 sensor that is insensitive to humidity at low tem-

peratures.

Research related to low-temperature potentiometric CO2 sensors has focused on choosing the best metal oxide as the sensing electrode rather than on understanding the role of the metal oxides. Therefore, many metal oxides, such as

SnO2, ZnO, In2O3, and ITO, have been investigated as potential candidates [3-8].

Due to slow electrochemical reactions at low temperatures, the kinetics of the re-

actions that occur at the sensing electrode must be studied carefully.

In this work, the CO2 sensing properties of a La2/3-xLixTiO3-based potenti-

ometric sensor, alternatively using Au, CeO2, K2CO3, and Li2CO3 as the sensing

electrode, was investigated under dry conditions. In the sensing electrode, Au and

CeO2 acted as catalysts for the reduction of O2, and K2CO3 improved the adsorp-

tion of CO2 gas. In addition, to improve low-temperature potentiometric sensors,

134 kinetic studies, such as the charge transfer reaction on the surface of the sensing

electrode, were conducted using electrochemical impedance spectroscopy (EIS).

4.1 Experimental

4.1.1 Preparation and characterization of materials for sensing electrode

CeO2 in sensing electrode

Many similar procedures have been reported to prepare nano CeO2 parti-

cles by using wet chemical methods for the application of catalysts. Our experi-

ment follows one typical procedure from the literature [9].

Various chemical reagents were used, including ammonium cerium (IV) nitrate ([(NH4)2Ce(NO3)6], Aldrich) as the cerium precursor, ethylene glycol (EG,

Panreac) as a dispersant, and polyvinylpyrrolidone (PVP, Mw=10000, Aldrich) as

a lubricant. The cerium nitrate was dissolved in 30 mL of ethylene glycol contain-

ing 0.16 M PVP while stirring until a homogeneous solution was obtained. The

solution was heated under reflux to the boiling point of ethylene glycol (~ 190 °C)

for 4 hr. The obtained solid was centrifuged and washed with deionized water and ethanol to remove excess ethylene glycol and PVP. After drying in an oven at

120 °C for 12 hr, the product was calcined at 600 °C for 4 hr for crystallization.

Au nano-particles on CeO2 in sensing electrode

The experimental method was a modification of conventional sodium

borohydride (NaBH4, Fisher chemicals) reduction of hydrogen tetrachloroaurate

135 hydrate (HAuCl4, Strem chemicals) solution [10]. Nano Au particles were depos-

ited on the surface of CeO2 by precipitation in the wet chemical method. CeO2 particles (10 g) were suspended in 100 mL of a 2.4x10-3 M aqueous solution of

HAuCl4, which was being stirred at a temperature at 25 °C. Next, 1 mL of the

-3 prepared NaBH4 (2.4×10 M) in ethanol solution was added to the HAuCl4 solu-

tion. The reacting solution was aged and vigorously stirred for 20 min at 25 °C,

after which the solution was centrifuged at 12,000 rpm for 30 min to separate the

supernatant and the product. The product was washed with distilled water and

ethanol three times. Finally, the product was dried in an oven at 120 °C for 12 hr.

Characterization

X-ray diffraction analysis was performed by a Scintag PAD-V diffracto-

meter with Cu Kα radiation at 45 kV and 20 mA. For particle size and morphol-

ogy of CeO2, scanning electron microscope (SEM) images were obtained by an

FEI Quanta SEM at an accelerating voltage of 5.0 kV. Particle sizes were calcu-

lated from SEM images that included more than 500 particles. For morphology of

Au deposited on CeO2, transmission electron microscope (TEM) images were ob-

tained by an FEI CM 200 operating at 100 kV, and the back-scattering images of

the SEM were observed.

136 4.1.2 Sensor fabrication

Figure 4.2 shows a schematic structure of the potentiometric sensor that

consists of an electrolyte, a reference electrode, and a sensing electrode.

Electrolyte

As mentioned in Chapter 3, lanthanum lithium titanate (La2/3-xLixTiO3) prepared by the solid-state reaction of lanthanum oxide (La2O3, 99.9%, Aldrich

Chemical), lithium carbonate (Li2CO3, 99.999%, Alfa Aesar), and titanium diox-

ide (TiO2, 99.9%, Alfa Aesar) at 800 °C for 4 hr and at 1000 °C for 6 hr was used

as the electrolyte. The heat-treated powder was ball-milled in ethanol for 8 hr,

dried in an oven at 120 °C, and pressed into green pellets under a hydraulic pres-

sure of 3.5 kpsi. The electrolyte disc was fabricated by sintering green pellets at

1350 °C for 6 hr. On both sides of the electrolyte, gold paste was painted for elec-

trical contact and gold wires were attached by curing at 600 °C for 1 hr at a heat-

ing/cooling rate of 5 °C/min in a Lindberg box furnace.

Reference electrode and sensing electrode

The mixture of lithium titanate (Li2TiO3, Lithium Corporation of America

Inc., 99%) and titanium dioxide (TiO2, Alpha Aesar, 99.9%) was used as the ref-

erence electrode. It was mixed with α-terpineol (Fisher Chemicals), painted on

one surface of the electrolyte and cured at 600 °C for 1 hr at a heating/cooling rate

of 5 °C/min.

137 In order to compare the sensing behaviors, the sensing electrode was fab-

ricated with three different materials: (A) pure lithium carbonate (referred to as

L); (B) a physical mixture of lithium carbonate and nano-sized Au deposited on

CeO2 (referred to as LC); and (C) K2CO3 added to the LC electrode (referred to as

KLC). For the sensor with electrode L, ball-milled lithium carbonate (Li2CO3, Al-

pha Aesar, 99%) was mixed with α-terpineol, painted on the surface of the elec-

trolyte, and cured at 600 °C for 1 hr at a heating and cooling rate of 5 °C/min. For

the sensor with electrode LC, lithium carbonate with 10 wt.% nano-Au deposited

on cerium oxide (CeO2) was used as the sensing electrode to promote the electro-

chemical reaction. It was mixed with α-terpineol, painted on the surface of the

electrolyte, and cured at 400 °C for 3 hr at a heating and cooing rate of 5 °C/min.

For the sensor with electrode KLC, a 0.2 M aqueous solution of potassium car-

bonate (K2CO3, Mallinckrodt chemicals) was dropped on Li2CO3 and heat-treated

at 200 °C for 2 hr under a CO2 atmosphere. The molar ratio of Li2CO3 and K2CO3 was changed such as 2:1 and 10:1. The mixture of Li2CO3 and K2CO3 was mixed

with 10 wt.% nano-Au deposited on cerium oxide and α-terpineol. The mixture

was painted and cured under the same conditions as that described for electrode

LC.

4.1.3 Gas sensing measurements

For sensing tests, the same set-up was used, as mentioned in Chapter 2.

Three gases, N2, air, and CO2, were mixed for the sensing tests. Sample gases

138 were prepared from 1% CO2 diluted in nitrogen by mixing it with air. The CO2 gas concentrations from 400 ppm to 2500 ppm were controlled by mixing. The test temperatures were 200 and 300 °C. In humid tests, humid gas was prepared by bubbling the gas mixture through water at room temperature, and the relative humidity was 80%. The EMF values of the sensor were measured by a two-probe technique with a model HP 34401A voltmeter.

4.1.4 Kinetics measurements of sensing electrode

For the impedance test, symmetrical testing cells were fabricated using the same method as described in Chapter 2.

Li2CO3, Au|La2/3-xLixTiO3|Au, Li2CO3 (4.1)

Li2CO3 + CeO2, Au|La2/3-xLixTiO3|Au, Li2CO3 + CeO2 (4.2)

Li2CO3+K2CO3+CeO2, Au|La2/3-xLixTiO3|Au, Li2CO3+K2CO3+CeO2 (4.3)

As shown in Figure 4.3, these symmetrical cells are referred to as L, LC, and

KLC cells, respectively. To prepare these symmetrical cells, α-terpineol organic

binder (Fisher Chemicals) was mixed with the electrode materials (Li2CO3 or

Li2CO3 + CeO2, or Li2CO3 + K2CO3 + CeO2). Before applying this mixture, Au

wire was attached on both sides of the La2/3-xLixTiO3 electrolyte with Au paste

(Heraeus Gold ink) by curing at 600 °C for 1 hr. The electrode paste was hand- painted and cured at 600 °C for 1 hr with a heating/cooing rate of 5 °C/min.

The symmetrical test cells were located in the central uniform temperature zone of a Lindberg horizontal tube furnace. In order to avoid electrical noise, the

139 quartz tube was shielded by aluminum foil that was grounded to the furnace body.

Pt lead wires were connected between test cells and the impedance measurement

instrument. Solartron 1260A model was used for the impedance measurement in

the frequency range from 10 μHz to 32 MHz.

4.2 Results

4.2.1 Sensor characteristics

General sensing behavior

Since the work of Inaguma et al. [11], who reported a bulk Li ion conduc-

-3 tivity of 1x10 S/cm at room temperature, La2/3-xLixTiO3(LLT) compounds have been well known as the fastest lithium ion-conducting solid electrolyte. In Chap- ter 3, our investigation of Li ion conductivity of LLT showed 1x10-5 S/cm at room

temperature. Though our electrolyte is not as good as Inaguma’s, the conductivity

value is large enough for the operation of an electrochemical sensor at 200 -

300 °C.

Figure 4.4 shows the response transients to dry CO2 gas at 300 °C for the

sensor with different sensing electrodes. In Figure 4.4 (a), the sensor with the pure

Li2CO3 sensing electrode did not work at 300 °C because it was not chemically

active enough to work at this temperature. However, the sensor with catalysts in the sensing electrode, such as CeO2 on which Au had been deposited, show satis-

factory performance in the CO2 concentration range of 400 to 2500 ppm under

dry conditions, as shown in Figure 4.4 (b). Although the sensor with CeO2

140 showed sensing performance at 300 °C, the sensor with Au deposited on the CeO2 showed better sensitivity.

Figure 4.5 shows the CO2 sensing behaviors of the sensors with LC sens-

ing electrode and the KLC sensing electrode at 200 °C under dry conditions.

Since K2CO3 improves the adsorption of the CO2 on sensing electrode, the sensor

with the KLC sensing electrode shows a little better sensitivity than the sensor

with the LC electrode. However, these sensors showed slow recovery or did not

attain full recovery. In addition, the sensor with a Li2CO3:K2CO3 molar ratio of

2:1 showed serious baseline drift. Therefore, by adding more K2CO3 to the sens-

ing electrode, the sensitivity of the sensor was improved, while the recovery and stability of the sensor deteriorated. At a Li2CO3:K2CO3 molar ratio of 10:1, the

sensor showed better recovery than the sensor with the molar ratio of 2:1 as

shown in Figure 4.5 (c). Therefore, by adding more K2CO3 to the sensing elec-

trode, the sensitivity of the sensor was improved, while the recovery and stability

of the sensor deteriorated.

Figure 4.6 shows the EMF response in the range of CO2 concentrations

from 400 to 2500 ppm at 200 °C. At this temperature, the EMF response of the

sensors deviated slightly from linear relationship with the logarithm of CO2 con-

centration. This deviation became serious at higher CO2 concentrations. The sen-

sor with KLC showed a smaller deviation at 2500 ppm CO2 at 200 °C than the sensor with LC. Therefore, this deviation might result from saturation by the adsorbed CO2.

141 In the literature, a sensing mechanism involving water was proposed at

low temperatures, based on the assumption that hydroxyl groups adsorb on the

- surface, followed by reaction with CO2 to form HCO3 groups on the surface of

the semiconducting metal oxide [3-8]. In our sensor, a high adsorption rate for the

target gas was obtained by using the mixture of Li2CO3, K2CO3, and Au deposited

on CeO2 as the sensing electrode. Hence, it was demonstrated that, the sensor can work at 200 °C without the aid of water vapor.

Response time

From response transients in Figure 4.4, 90% response time was measured with a gas flow rate of 210 ml/min at 200 °C and 300 °C. The response time was longer than 2 min. This holds for the entire CO2 concentration range from 400

ppm to 2500 ppm.

Sensitivity

Figure 4.6 indicates the slopes of the Nernstian plots (sensor EMF re-

sponse vs. log PCO2) of the sensors with different sensing electrodes. The sensitivi- ties of sensors (9.8mV/decade and 3.1mV/decade for KLC and LC electrodes, re-

spectively) were smaller than the theoretical value (46.8 mV/decade) at 200 °C

because of slow electrochemical reactions and low CO2 adsorption on the sensing electrode.

142 Drift

The sensor with the LC sensing electrode was subjected to a drift test with

a CO2 concentration of 2500 ppm at 300 °C in 10 % O2. Data were collected

every 5 s for 72 consecutive hours. During this test, the EMF signal of the sensor

exhibited a deviation of 0.93 mV, corresponding to 3.4% of the baseline EMF

value. The minimum EMF was measured as 4.23 mV and the maximum response

was 5.16 mV. The other sensor with the KLC electrode showed more serious drift,

with an EMF deviation of 1.2 mV.

4.2.2 Interferences

4.2.2.1 Oxygen interference

It has been reported that, thermodynamically, the sensing behavior of potentiometric CO2 sensors at high temperatures is not supposed to be influenced by

the concentration of oxygen at equilibrium conditions [12]. However, Maier et al.

reported O2 dependence at 450 °C and a low CO2 concentration (0.195 mbar) in

potentiometric CO2 sensors based on a Na electrolyte [13]. Figure 4.7 shows the

relationship between EMF values and log PCO2 of the sensor under 5%, 10% and

20% O2 at 200 °C under dry conditions with the KLC (Li2CO3:K2CO3 molar ratio

of 10:1) sensing electrode. The slopes of the Nernstian plots were slightly in-

creased when the partial pressure of the O2 increased. In addition, the variation of

EMF values of the sensor was observed by changing O2 concentration because of

CeO2-x in the sensing electrode. Moreover, slow kinetics of the electrode reaction

143 to CO2 might contribute to the oxygen interference since adsorbed oxygen can

easily react with Li ion on the sensing electrode to form lithium oxides. However,

oxygen interference is not a serious problem because low CO2 concentration ap-

plications usually have almost constant exposure to a gas mixture that contains

21% O2. Therefore, this sensor should be practically independent of oxygen con-

tent.

4.2.2.1 Humidity interference

Almost all low-temperature potentiometric CO2 sensors are able to sense

CO2 only under high relative humidity such as above 40% RH [3-5]. However,

the sensing behaviors of these sensors are dependent on the relative humidity.

Therefore, to monitor the concentration of CO2 at low temperatures, humidity

sensor should be combined with the CO2 sensor. Moreover, the sensors are degraded after the sensors are exposure to high humid conditions.

Figure 4.8 (a) shows the response transients to CO2 concentrations from

400 ppm to 2500 ppm at 200 °C under 80% RH for the sensor with the LC sens-

ing electrode. Under humid conditions, the sensor showed a relatively good recovery. In addition, the dependence of the response time on relative humidity was negligible. As shown in Figure 4.8 (b), the slope of the Nernstian behavior plot slightly increased under humid conditions, although the sensor showed a devia-

tion from the linear relationship between EMF values and the logarithm of CO2 concentration. In Figure 4.9, the sensor with the KLC sensing electrode was ex-

144 tensively dependent on relative humidity, because K2CO3 is more reactive with

moisture than other carbonates. Therefore, the slope of the Nernstian behavior

plot became steeper with increasing amounts of K2CO3 in the sensing electrode.

The large EMF shifts in the sensor with K2CO3 might be attributed to the instabil-

ity of K2CO3 or the formation of KHCO3. Based on the change in the Gibbs free

energy, the reaction for the formation of KHCO3 occurs more easily at low tem-

peratures than the expected reaction for the auxiliary phase in sensing electrode,

+ i.e., 2Li + CO2 + 1/2O2 + 2e- = Li2CO3.

4.2.3 Materials characterization of sensing electrode

Figure 4.10 shows the X-ray diffraction patterns of CeO2 powders cal-

cined at various temperatures in air for 90 min. All peaks of CeO2 are identified as

the cubic fluorite structure, and all the peaks became stronger and sharper as the

calcination temperature increased. Figure 4.11 shows the SEM images of the

CeO2 particles calcined at 600 °C for 90 min. It can be seen that the particles of

ceria display a spherical-shaped morphology and large sizes that exceed 100 nm.

However, the nano CeO2 particles are loosely agglomerated. Figure 4.12 shows

the TEM image of the CeO2 particles on which Au had been deposited. For each

CeO2 particle, nano Au particles were uniformly deposited on the surface of the

CeO2 by the wet chemical method.

145 4.2.4 Kinetics of sensing electrode

Figure 4.13 shows impedance spectra of symmetrical cells with different

sensing electrodes to 500 ppm CO2 and 10 % O2 at 200 °C in the range of fre-

quency from 0.01 to 107 Hz with an amplitude of applied voltage of 10 mV. In

Figure 4.13, it was considered that impedance plots have two distinctive semicircles. Therefore, this impedance plot was analyzed by using the Simplified

Randles Equivalent Circuit, which has two RC combinations in series as shown in

Figure 4.13 (b). The first RC is composed of a bulk capacity (Cb) arising from the

finite dielectric constant of the solid electrolyte [14] and a bulk electrolyte resis-

tance (Rb). In the second RC, Rct is known as the charge transfer resistance, and

Cdl is the double-layer capacitance. It is considered as a reasonable model when

charge transfer is the only limiting step in the kinetics. These test cells clearly had

bulk electrolyte properties in the first semicircle in the high frequency region, because the resistances of these semicircles did not change when gas concentrations

changed. However, in these tests, the bulk resistance changed depending on the

auxiliary phase material, even though pellets with the same dimensions were used

and the size of gold electrode was the same.

In Figure 4.13, the L sensing electrode showed the largest resistance, fol-

lowed by the cells with the LC and KLC sensing electrodes in descending order.

In Figure 4.13, the KLC electrode showed the smallest charge transfer resistance,

which means it is close to a reversible electrode. On the other hand, the L elec-

trode was a totally non-reversible electrode. The LC electrode showed a behavior

146 similar to the KLC electrode, but with a slightly larger charge transfer resistance.

From this impedance spectroscopy, it can be concluded that the KLC electrode is

the best sensing electrode for CO2 detection at 200 °C.

These cells were also tested under different gas environments. Figure 4.14

shows the impedance spectra of the cells with the CO2 concentration of 2500 ppm.

Figure 4.14 clearly shows that charge transfer resistance increased with CO2 con-

centration. The oxygen reduction mechanism has been studied for molten carbon-

ate fuel cell (MCFC) [15, 16]. Dave et al. [16] found that increased CO2 concen-

trations 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 has not been reported, but increased Rct with CO2 concentration

seems to be related to oxygen reduction in the cell. As can be seen in Figures 4.13

and 4.14, the impedance behavior did not change as a function of gas concentra-

tion in the case of the L electrode, which is not a good sensing electrode due to its

inactivity toward CO2 at low temperatures.

As shown in Figure 4.15, the resistance of the cells with LC or KLC elec-

trodes to charge transfer decreased as the concentration of O2 increased. From

these impedance plots, it can be seen that CeO2 plays an important role in the re-

duction of O2 in our CO2 sensors. At high O2 concentrations, large amounts of O2 might be reduced, and the exchange current should increase. Therefore, the cell with CeO2 showed a smaller resistance to charge transfer at higher O2 concentra-

tions.

147 If charge transfer resistance is large enough, low frequency processes,

such as diffusion or adsorption, do not appear within the same frequency range

[17]. However, in our CO2 sensors, the sensing electrodes included nano-sized Au

particles. Therefore, charge transfer resistance was relatively small, but, even so,

diffusion or gas adsorption was not clearly observed in this frequency region. The

reason is that adsorption/desorption kinetics plays a more important role at high

temperatures, while the charge transfer effect become more dominant at lower temperatures [18, 19].

4.3 Discussion

4.3.1 Choice of electrodes

The role of CeO2

Cerium dioxide (ceria, CeO2) and ceria-based materials are becoming im-

portant materials as catalysts in electrochemical devices [20-22]. The catalytic

activity of ceria is mainly related to its existence in two oxidation states, i.e., tri-

4+ 2- valent and tetravalent, and the Ce -O charge transfer. On the surface of CeO2,

- 4+ adsorbed O2 gas might be reduced to O due to charge transfer from Ce to O2 as

following:

OO2(gas )→ 2 ( ad ) -- OeO()ad+→ () ad (4.4) -- OO()ad→ ( TPB )

148 - In the non-stoichiometric oxides (CeO2-x), an oxygen vacancy or O is formed due to reduction of Ce4+ ions into Ce3+ ions. Therefore, O- can react with adsorbed CO2 gas at triple phase boundaries. Nanocrytalline CeO2 powders can be expected to show better catalytic activity and redox properties in comparison to those of the microcrystalline CeO2 because of the higher mobility primarily of the oxygen ions.

The role of Au

Although Au exhibits high stability under typical conditions and the most extreme conditions, recent studies have shown that the nano-sized Au deposited on metal oxides can be highly active as a catalyst [23]. For example, Haruta et al.

[24] reported that the catalytic activities of nano-sized Au deposited on metal oxides supports low-temperature CO oxidation. The catalysts of Au deposited on metal oxides have also been shown to be active in a number of other oxidation reactions [25-28]. This chemistry is likely driven by the formation of highly reac-

2- 2- tive superoxo (O ) or peroxo (O2 ) species [29-31].

In our sensing electrode, the chemisorption of O2 on Au/CeO2 occurred.

2- 2- and nano-sized Au on CeO2 activated O2, superoxo (O ) or peroxo (O2 ). The

2- adsorbed CO2 on Li2CO3-K2CO3 was oxidized to CO3 , which reacted with Li ions, resulting in a change in the activity of Li on the sensing electrode.

149 The role of K2CO3

Mixtures of metal oxides and K2CO3, such as TiO2-K2CO3, MgO-K2CO3 and CeO2-K2CO3 showed a strong CO2 adsorption characteristic that was believed to be the result of the formation of potassium bicarbonate species and physical

CO2 adsorption [32-34]. Therefore, the sensors showed improved sensitivity when

K2CO3 was added, since K2CO3 is a more active material to CO2 adsorption than

Li2CO3. However, the sensors were degraded by the KHCO3 phase.

4.3.2 Non-Nernstian behavior of the sensor

According to the sensing mechanism of the potentiometric CO2 sensors,

the EMF values are dependent on the partial pressure of CO2 gas and are in

agreement with the Nernst equation:

RT EE=+o ln P (4.5) nF CO2

−ΔGo E o = , (4.6) RT

Where n is the number of electrons that participated in the reactions, PCO2 is the

o partial pressure of the CO2 gas, and ΔG is the standard free energy of the follow-

ing reaction:

Li2TiO3 + CO2 → Li2CO3 + TiO2 (4.7)

However, the experimental relationship between EMF values and the partial pres-

sure of CO2 gas showed discrepancies from the theoretical relationship given by

Eq. (4.5), as shown in Figure 4.6. In our low-temperature CO2 sensors, the ob-

150 served discrepancies in the experimental dependencies of E vs PCO2 from theoretical relationship may be explained in several ways:

– Local equilibrium of the reactions on the sensing electrode may be achieved. In other words, the equilibrium between CO2 gas and Li ions is not established on the entire surface of the electrode. This explanation may be valid at lower operating temperatures. A gradual change from non-Nernstian to Nernstian behavior is to be expected with increasing operation temperatures.

– On the sensing electrode, several competitive reactions, rather than one particu- lar reaction, occur simultaneously. This phenomenon is known as ‘mixed potential.’

– The solid electrolyte, La2/3-xLixTiO3 might not be a pure Li ion conductor. If the solid electrolyte has a non-negligible component of the electronic conductivity, the following equation should be considered [35]:

EtEexperimental= ion theoretical , (4.8) where Eexperimental, Etheoretical, and tion are experimental and theoretical values of the

EMF and the transference number of Li ions, respectively. According to Näfe et al. [36–38], even a low electronic transference number, such as 5×10−3, in Na-β- alumina leads to a non-Nernstian behavior. In addition, this influence of the electronic transport component is more significant at low temperatures.

– The solid electrolyte might support the ionic conduction of two or more types of ions. In hydrogen sensors at high temperatures, non-Nernstian behavior was ob-

151 served due to the transport of both hydrogen and oxygen ions through proton conductors [39, 40].

– Surface effects connected with contact potential difference [41].

Although the LLT electrolyte shows the highest ionic conductivity at room temperature, LLT is not stable in direct contact with elemental lithium and undergoes easy and fast Li insertion with consequent reduction of Ti4+ to Ti3+.

– The influence of high electrode polarization resistance.

Under high polarization resistance, equilibrium cannot be held since the high polarization resistance prevents charged species from passing through triple phase boundaries. Therefore, the electrochemical reaction is slow and cannot reach equilibrium.

4.3.3 Strategies to improve sensor

In order to improve the potentiometric sensors, there are two main issues.

One is to achieve the most active sensing electrode, and the other is related to the ion conductivity of the solid electrolyte.

In potentiometric sensors, the particle size of electrodes considerably af- fects sensing behaviors, such as the sensitivity and the response time of the sensor.

In NO2 potentiometric sensors with LaFeO3 as the sensing electrode, it was observed that the sensitivity and response time were improved at 400 °C as the size of LaFeO3 particles decreased [42-44]. As the surface area of an electrode in-

152 creases, more sites to adsorb target gases are provided on the sensing electrode.

Therefore, the sensitivity should be enhanced.

In our CO2 potentiometric sensors, Au, CeO2, K2CO3, and Li2CO3 provided available sites for adsorption of CO2 gas and electrochemical reactions on the sensing electrode. In order to increase triple phase boundaries in the sensing electrode, the size of particles should be decreased and components of the sensing electrode are uniformly distributed. By increasing the porosity of the sensing electrode, CO2 gas can easily reach the reaction sites. Therefore, the porosity of the sensing electrode and the size of particles of the component should be considered at the same time for the optimization of sensor performance.

As the thickness of the solid electrolyte decreases, the ohmic overpotential of the electrochemical cells decreases. Therefore, a dense and thin electrolyte might beneficially impact the kinetics of the electrochemical reaction due to the small resistance of ionic transport through the electrolyte. Practically, the best combination of solid electrolyte and sensing electrode is critical to the optimization of the sensor performance.

4.4 Conclusions

We have developed a low-temperature CO2 sensor based on Li ion conductor in dry conditions that requires further improvement. The following conclusions can be drawn in the present study:

153 1. A La2/3-xLixTiO3(LLT)-based CO2 sensor with CeO2, Au, and Li2CO3 as the sensing electrodes has shown to have relatively stable sensing behaviors at 200 °C under dry conditions.

2. In the sensing electrode, CeO2 and nano-sized Au played important roles as catalysts in reducing the working temperature of the sensor. These catalysts reduced adsorbed oxygen in the sensing electrode. The reduced oxygen reacted with

2- adsorbed CO2 on Li2CO3-K2CO3 and was oxidized to CO3 . Due to the formation of lithium carbonate on the sensing electrode, the sensor can respond to CO2 changes.

3. By adding K2CO3 on the sensing electrode, the sensitivity of the low- temperature CO2 sensor was slightly improved. However, the sensing signals of the sensors were degraded by water vapor under humid conditions due to the formation of KHCO3 or K2CO3·mH2O (m= 2, 3, or 6).

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158 Pulse oximetry sensor Respiratory monitor (CO2 sensor)

Skin temperature sensor

Figure 4.1 Illustration of the application of a low-temperature CO2 sensor in a spacesuit.

159

Figure 4.2 Schematic structure of the low-temperature potentiometric CO2 sensor.

160

Figure 4.3 Symmetrical cells referred to as L (Li2CO3), LC (Li2CO3 + CeO2), and KLC (K2CO3 + Li2CO3 + CeO2) cell for kinetic studies.

161

Figure 4.4 Response transients of sensors with (a) pure Li2CO3 and (b) Li2CO3 and Au on CeO2 as the sensing electrode to dry CO2 gas from 400 ppm to 2500 ppm at 300 °C.

162

Continued

163 Figure 4.5 continued

164

165

166

Figure 4.8 (a) Response transients to CO2 concentration from 400 ppm to 2500 ppm at 80% RH for the sensor with LC sensing electrode and (b) Nernstian behavior plots under dry and humid conditions.

167

168

Figure 4.10 X-ray diffraction pattern of CeO2 powder prepared by a wet chemical method.

169

Figure 4.11 SEM image of CeO2 particles calcined at 600 °C.

170

Figure 4.12 TEM image of nano-sized Au deposited on CeO2 particles.

171 (a) 0.1 200 °C , 500 ppm CO2, 5% O2

L+K+C L+C L 0.0 0.0 0.1 0.2 0.3

(b)

Figure 4.13 (a) Impedance spectra of symmetrical cells with different sensing electrodes to 500 ppm CO2 and 10 % O2 at 200 °C and (b) Randles equivalent circuit model.

172 0.1 300 °C , 2500 ppm CO2, 5% O2

L+K+C 0.0 L+C 0.0 0.1 0.2 0.3

Figure 4.14 Impedance spectra of the cells with 2500 ppm CO2 concentrations.

173 0.1 300 °C , 500 ppm CO2, 5% O2

L+K+C 0.0 L+C 0.0 0.1 0.2 0.3 0.1 300 °C , 500 ppm CO2, 10% O2 L

L+C L+K+C 0.0 0.0 0.1 0.2 0.3

300 °C , 500 ppm CO2, 20% O2

L+C 0.0 L+K+C 0.0

Figure 4.15 Impedance plots of the cells with L, LC or KLC by increasing the concentration of O2 from 5% to 20%.

174

CHAPTER 5

CONCLUSIONS AND SCOPE FOR FUTURE RESEARCH

Potentiometric CO2 sensors with Li3PO4 electrolyte, BaCO3 coated on

Li2CO3 sensing electrode and the mixture of Li2TiO3 and TiO2 as a reference elec-

trode was developed for high-temperature applications. This sensor showed satis-

factory performance with fast response time, appreciable sensitivity and selectiv-

ity against humidity.

In addition, by using lithium lanthanum titanate (LLT) electrolyte with

high ionic conductivity and by adding nano-sized Au, CeO2 and K2CO3 to the sensing electrode, the working temperature of the sensor was reduced from

500 °C to 200 °C. The following summary presents the key results from this work

and highlights the unresolved issues in this area as a guide for future work.

Summary of key results

1. As the sensing electrode was modified by adding BaCO3 coating layer or small

particles of BaCO3, humidity interference was eliminated. The sensor showed values close to the Nernstian slope under both dry and humid conditions.

175 2. Lithium lanthanum titanate (LLT) electrolytes prepared by a conventional

solid-state method showed high Li ion conductivity at room temperatures. LLT is

a good candidate as an electrolyte for low-temperature potentiometric CO2 sen-

sors.

3. By using LLT as the electrolyte and by adding Au, CeO2 and K2CO3 to the

sensing electrode, the working temperature of the sensor was reduced from

500 °C to 200 °C under dry conditions. In the sensing electrode, Au and CeO2 acted as catalysts to reduce O2 and K2CO3 improved CO2 adsorption.

4. Although the possibility of low-temperature potentiometric CO2 sensor was

shown by an appropriate selection of electrolyte and electrode materials, the

working temperature of the sensor cannot reach room temperature without the aid

of water vapor.

Major contributions of this work

This work has two major contributions: (1) development a of humidity-

interference free CO2 sensor for high-temperature applications; and (2) demon-

stration of the basis for the development of a low-temperature CO2 sensor under

dry conditions.

1. Humidity-interference free CO2 sensor at high temperatures

To eliminate the humidity interference of the potentiometric CO2 sensor, TPBs

are important in the sensing electrode. By controlling TPBs by adding BaCO3 to

the sensing electrode, humidity-interference free CO2 sensor was developed.

176 2. Low-temperature CO2 sensor under dry conditions

Until now, water vapor is necessary for all electrochemical sensors to detect CO2 at low temperatures. The possibility that a potentiometric CO2 sensor can work

under dry conditions at relatively low temperatures (~200 C) was investigated in

this work by adding catalysts such as Au, CeO2 and K2CO3.

Suggested future work

1. A La2/3-xLixTiO3(LLT)-based CO2 sensor with CeO2, Au, and Li2CO3 as the

sensing electrodes has been developed at 200 °C under dry conditions. However,

this sensor showed non-Nernstian behavior. The origin of the non-Nernstian behavior is still unresolved.

2. To optimize the low-temperature CO2 sensor, a more active sensing electrode is

needed, which may be achieved by controlling the size of particles and their dis-

tribution on the electrode. In addition, a thinner electrolyte with pure ionic con-

duction is also required.

3. Potentiometric sensor is able to work by electrode redox reactions. Therefore,

we need further basic studies on the catalytic activity of both chemical and elec-

trochemical reactions taking part in the sensor. Moreover, further studies are re-

quired to better understand the sensing mechanism.

4. Finally, to develop a room-temperature CO2 sensor, non-oxide-based platform

such as a polymer-based device needs to be explored. By using polymer-based

sensors, the application fields can be extended because of their flexibility.

177

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