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

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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 character- ized 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 addi- tion, 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 lan- thanum titanate (LLT) electrolyte in dry conditions that requires further improve- ment. Lithium lanthanum titanate (LLT) electrolytes were prepared by a conven- tional 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 im- proved. However, the sensing signals of the sensors were degraded by water va- por 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 tem- peratures . 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 . ..
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