Studies on Lanthanum based Ionic Conductors as Electrolyte Materials

Abstract

Solid oxide fuel cell (SOFC) is the most efficient fuel cell where different fuels can be used without any external reformers. Additionally, they are portable and cost-effective as compared to other cells. However, the high operating temperature of SOFC leads to decrease its life and efficiency as material used in SOFC start degrading with time. Because of chemical reactions within the components of SOFC at high temperature reduce its life, the need is felt to reduce its operating temperature without compromising the properties of the SOFCs. Therefore, material development, particularly electrolyte is very critical for the commercialization of intermediate temperature (IT) SOFCs. Among all the electrolyte materials, perovskite based oxide ion conductors, particularly, Sr- and Mg-doped lanthanum gallate (LSGM) are of great interest. The ionic conductivity of LSGM is higher than those of YSZ and scandia stabilized zirconia (SSZ) and lower than that of gadolinia doped ceria (CGO). Moreover, LSGM does not have an easily reducible ion, like Ce 4+ . Therefore, LSGM is better than Gd doped CeO 2 . The conductivity of LSGM depends on dopant concentration, particularly with transition element doping that shows good conductivity, especially at temperatures, upto 600 ºC. Moreover, the performance of these electrolytes is still a major issue. Lanthanum based perovskites including LaScO 3, LaInO 3 , LaYO3 and LaAlO 3 are used as oxygen ion conductors and are suitable electrolyte materials for SOFC. Considering these facts, it has been planned to investigate the effect of systematic doping of SrO, BaO and CaO in LaGaO 3 and LaInO 3 system. The work carried out in this Ph.D thesis is divided into five chapters. Chapter 1 contains introductory aspects of fuel cells (FCs) and describes a brief overview of different categories of FC types, their design and role of different components. Since, the present work is on development of electrolyte material for IT-SOFCs, so, emphasis is given on the factors that influence ionic motion. The origin of perovskite structure and their properties are explained. Study reveals that systematic investigation is required to find suitable electrolyte, which can be used in IT-SOFCs. Chapter 2 describes the detailed literature review on various types of solid electrolytes used. It includes the review of structural, thermal, morphological, and electrical study of yttria stabilized zirconia (YSZ), ceria based systems, bismuth based system, pyrochlore based systems and lanthanum based perovskites. From the literature review, it was concluded that lanthanum based xiii electrolytes are most suitable for their use in IT-SOFCs. However, systematic study of uniform doping in LaGaO3 and LaInO 3 synthesis has not been studied in detail. The reason for selecting the dopants to achieve the required properties has been explained and accordingly the objectives of the present work have been defined. Chapter 3 describes the source of raw materials used and synthesis route followed to prepare the samples. It also describes the techniques utilized for structural, electrical and thermal characterization of solid electrolyte. These techniques include X-ray diffraction (XRD) for phase identification, Raman spectroscopy to analyze structural modification with doping, scanning electron microscopy (SEM) for morphological study and energy dispersive spectroscopy (EDS) for semi-quantitative elemental analysis. Besides this, thermal expansion coefficient (TEC) of each sample was measured to study the lattice expansion behavior of electrolyte at higher temperatures. Thermogravimetric analysis (TGA) is used to analyze thermal stability at higher temperatures. Two probe impedance spectroscopy (IS) technique was used to study the electrical conduction behavior of the synthesized materials. Chapter 4 describes the results and discussion part of the synthesized samples. This chapter is divided into six sections. In the first section, La 1- SrGaO3- (where, =0.0-0.20) system was analyzed. The substitution of Sr 2+ at La-site of LaGaO 3 forms solid solulion upto 10 mol%. The strain increases uniformly with Sr-substitution for all the synthesized samples. 5 mol% Srsubstitution leads to vacancy ordering within the system. It resulted in lesser electrical conductivity of this sample. However, 10 mol% Sr-content shows highest conductivity (2.37 mS/cm at 800 C) among all the synthesized samples. Higher substitution (15 and 20 mol%) leads to formation of secondary phase (LaSrGa 3O7 ) which adversely affected the overall conductivity of the system. Moreover, the Sr-content decreased the grain size in comparison to unsubstituted sample. TEC values of this system are in the required range for SOFC applications. In the second series, La 1- BaGaO3- (where, =0.0-0.20) system was analyzed. Phase identification followed by Rietveld refinement of selected samples was done. This study revealed that Ba 2+ substitution leads to stabilization of rhombohedral phase at room temperature, which facilitates the increase in overall conductivity upto 15 mol% Ba-content. Moreover, the activation energy values of Ba-substituted samples (0.65 eV from 600-800 C) indicates that conduction in this system is mainly due to oxygen ions. xiv In the third series, La 1- CaGaO3- (where, =0.0-0.20) system was analyzed. Morphological features indicated that Ca-content facilitated better sinterability and grain growth in the synthesized samples. Ca-substituted samples formed single phase upto 10 mol%, above which secondary phase (LaCaGa 3O7 ) formation takes place. The highest conductivity achieved is 3.8 mS/cm at 800 C for LCG-10 sample. The activation energy values (1.15 eV from 600-800 C) are higher than Sr and Ba-substituted systems. In the fourth series, La 1- SrInO 3- (where, =0.20-0.50) system was analyzed. The solid solubility limit was achieved upto 20 mol% Sr-content. LSI-23 possesses highest conductivity (0.16mS/cm at 800 C) in this series. However, increased Sr-content also increases porosity in the sample. At higher temperature, the conductivity phenomenon changes from protonic to oxide ion conductivity in all the samples. The LSI-23 is most stable at higher temperature in reducing atmosphere. The thermal expansion of LSI-23 sample is also in the required range of SOFCs. In the fifth series, La 1- BaInO 3- (where, =0.0-0.20) system was analyzed. Ba-substituted system was single phase up to 5 mol%, above which polymorphic cubic perovskite phase (pm m) was co-formed. Raman analysis indicated A-site substitution of Ba-ion and formation of oxygen vacancies with substitution. LBI-10 shows the highest ionic conductivity among all the investigated samples. The SEM analysis indicated the formation of well packed grains. In the last series, Ca-substituted lanthanum indate (La 1- BaInO 3-, where =0.0-0.20) was investigated. The LaInO3 formed single phase up to 10 mol% of Ca-substitution. Ca-content leads lattice expansion due to partial Ca 2+ occupancies at In-sites. It is also confirmed from the Raman bands, particularly 180-300 cm -1 shifted to the lower wave number. The decrease in intensity and an increase in FWHM in Raman bands with substitution indicated the disorderness in the system. The Arrhenius plots of all LCI samples show almost linear behavior. The activation energy values indicated that the conduction is dominated by ion movement. Chapter 5 describes the overall conclusion drawn from the entire study. Based on the present work, the scope for the future work is also provided at the end of this section.