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Title: Study of Alkaline Earth Metals Substitution in Bismuth based Mixed ion Conductors
Authors: Samita
Supervisor: Singh, Kulvir
Pandey, O. P.
Keywords: Mixed conduxtors;Thermal analysis;Impedance;XRD;SPMS;Physics
Issue Date: 29-Sep-2014
Abstract: Solid oxide fuel cells (SOFCs) have gain ample interest in research community due to its high efficiency and fuel flexibility without any requirement of external reforming. Despite of advantages aspects of SOFCs, still it cannot compete with conventional combustion systems in cost and durability. The current interest of researchers is to reduce the operating temperature of SOFCs to make them durable and cost effective. However, lower operating temperature of SOFC (below 800 °C) leads to some other problems such as reduction in electrode kinetics. The most pronounced effect of reduced temperature is felt on oxygen reduction reaction at the cathode side. It is believed that use of mixed electronic-ionic conducting oxides as a cathode could serve the purpose, because, the active sites for oxygen reduction will extend from triple phase boundary region to entire electrode surface in this case. In order to achieve this, the bismuth based mixed ion conductors have been investigated in the present work as these perovskites exhibit high electrical conductivity. The Bi1-xMxMnO3±δ, Bi1-xMxFeO3±δ, Bi1-xMxYO3±δ and Bi1-xMxCrO3±δ (M=Ca, Sr; 0.40≤x=0.55) systems have been synthesized by solid state reaction method and investigated for their structural, thermal and electrical properties. Some of the selected samples were also investigated for their chemical compatibility with YSZ (8 mol % Y2O3 stabilized zirconia) electrolytes. The entire work in this thesis is organized in the six chapters: • Chapter 1 describes the background and fundamentals of different types of fuel cells. The merits, demerits and the potential applications of the fuel cells in different areas have been presented. The role of different components in solid oxide fuel cells has also been explained briefly. Since the present work is confined to develop materials for cathode. Therefore, the main emphasis is given on the cathode electrode of SOFCs. The overall electrode kinetics and the basic structural requirements of a material to be used as a cathode have been discussed. Perovskite and Ruddlesden-Popper structure are also explained as the majority of cathode materials are based on these crystal structures. • Chapter 2 provides a detailed literature survey in perspective of different cathode materials that are being investigated for SOFCs. As both A and B-site cations in perovskite (ABO3) structure play significant role, so, the study carried out so far for both categories of doping in different systems have been presented. In this chapter, the different cathode materials investigated so far have been categorized on the basis of different A-site and B-site cations. The cell output, limitations and the compatibility of different cathode materials with the other components of SOFC have also been presented. The current state of art of other developed cathodes based on Ruddlesden-Popper structure and composites are also presented in this chapter. The last section of this chapter presents the gaps in the study so far with the main objectives and the plan of undertaking research work. • Chapter 3 describes the experimental procedure followed during the course of work to achieve the objectives of the proposed work. The details of the raw materials used and the synthesis procedures followed for different systems have been given in this chapter. The fundamental details and mechanisms of different techniques viz. X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Iodometric titration, Raman spectroscopy, Fourier transform infrared spectroscopy, Scanning electron microscopy (SEM), X-ray dot mapping, Differential thermal analyzer, Thermogravimetric analyzer, Dilatometery and ac impedance spectroscopy employed to characterize the samples for their structural, thermal and electrical properties have been discussed in detail. • Chapter 4 deals with the results of the different systems studied in this work. This chapter is further divided into four sections. First section represents the results of the study carried out on Bi1-xAxMnO3±δ (A=Sr, Ca; 0.40≤x≤0.55) system. The effect of different substituents Sr2+/Ca2+ on the structural, thermal and electrical properties have been discussed. Second, third and fourth section represents the results of Bi1-xAxFeO3±δ (A=Sr, Ca; 0.40≤x≤0.55), Bi1-xAxYO3±δ (A=Sr, Ca; 0.00≤x≤0.20) and Bi1-xAxCrO3±δ (A=Sr, Ca; 0.40≤x≤0.55) systems, respectively. The effect of different substituents (Sr2+/Ca2+) and B-site cation (Mn, Fe, Y, Cr) on different properties have also been discussed. The structural, thermal and electrical properties have been discussed in the light of structural transformation, ordering/disordering created in the systems due to the conversion of B-site cation into higher oxidation state and generation of oxygen vacancies by the lower valence cation. • Chapter 5 presents the chemical interaction study of some selected samples of the different series with YSZ electrolyte. The best samples of each series were chosen on the basis of their electrical conductivity and thermal expansion coefficient. The chemical interaction between present studied systems and electrolyte was investigated with the help of XRD, SEM and X-ray dot mapping techniques. The interaction study indicates the formation of SrZrO3 and m-ZrO2 phases in all the samples which increase with the increase in the duration of heat treatments. The formation of these phases has been explained on the basis of tolerance factor and stability of B-site cations of these perovskite oxides. • Chapter 6 presents the summary of the results obtained from structural, thermal, electrical and interaction study of the different series. At the end of this chapter, future scope of this study has also been given to enrich this work.
Description: PhD-SPMS-Thesis
Appears in Collections:Doctoral Theses@SPMS

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