Preparation of Silicate Based Glasses and their Interaction Study with Different Electrolytes for Solid Oxide Fuel Cells

Abstract

The increasing demand of energy due to the increasing world population and their needs. Hence, new sustainable and environmentally friendly energy sources are required. Moreover, the fossil-based energy sources are also depleted with passage of time. Thus, a lot of research is going on to search for an alternative, renewable, sustainable, and clean source of energy. Many energy devices and their development are being progressed. These energy sources can be categorized as batteries, fuel cells and capacitors. Each source has advantages and disadvantages. Out of these sources, the fuel cells could be more environmental viable with better efficiency particularly solid oxide fuel cells. Solid oxide fuel cells (SOFCs) qualify all of the above-mentioned requirements and they could be considered as the future sustainable and renewable source of energy. SOFC is an electrochemical device in which chemical energy change into electrical energy without combustion of fuels because of the reaction of fuel with the oxidant that is passing through the ionic conductor (solid electrolyte) at high temperature. High operating temperature SOFC is not only required to achieve the good catalytic activity of the electrodes (cathode and anode) usually made of ceramic materials in SOFC, but also to enhance the ionic and overall mixed conductivity of electrolyte and electrodes, respectively. The SOFC is different from conventional fuel cell technologies in numerous ways. They are entirely made of solid-state components and majorly by the ceramics. Secondly, the cells have a higher operating temperature range than any other fuel cell i.e. 800°C-1000°C. Third, there are no essential limitations on the cell configuration as well as fuel flexibility due to high temperature operation of SOFC. On the other hand, the high operating temperature of SOFC increases the interdiffusion among the components lead to corrosion and form undesirable crystalline phases during operation of SOFC. The formation of these crystalline phases ultimately reduced the life time as well as the efficiency of this device. There are two major ways in which cells are xviii designed: as rolled tubes, which have been created by Westinghouse electric corporation since the late 1950s, or as flat plates, which have just been adopted by many other companies and are currently used by the electronics industry. Planar design (P-SOFC) is usually constructed using flat plates of two electrodes and a thin dense ionic conducting electrolyte. Generally, a unit of SOFC is connected with another unit of SOFC with the help of interconnects or separators, to increase its overall output. Interconnects are made of bipolar plates of metallic alloys or perovskite structured ceramic materials. It allows the flow of electrons from one cell’s anode to an adjacent cell’s cathode while acting as a physical barrier for the streams of fuel and oxygen that enter the anode and cathode chambers, respectively. Therefore, to avoid the mixing of fuel and oxygen, interconnects should be non-porous. In P-SOFC stacking, sealing materials are applied at the edges of every component. Hence, sealing plays a key role in avoiding any leakages of either air or fuel from cathode and anode chambers, respectively. High operating temperature of SOFC, steep change in oxygen partial pressure, forming an interface with cathode, anode, electrolyte, and interconnect materials, put a big challenge to develop a suitable robust sealant for P-SOFC. So, conventional seal can not work at operating temperature of SOFC. The glass could be good choice for this purpose. In the present thesis, Sr and Ba mixed alkaline earth glasses are synthesized using melt quench technique. This work mainly focuses on developing a sealant for planar design of solid oxide fuel cell. The prepared glasses were characterized for their structural, thermal, optical and mechanical properties to check their suitability as sealants. All glass samples are used to make the diffusion couples with common interconnect (Steel: SS-430) and electrolytes (Yittria stabilized zirconia and Mg doped BiVO11-δ). These as prepared diffusion couples are explored for different temperature and different time duration like 1, 10, 100, and 1000 hours (h) in air to study the nature of interfaces, and crystalline phase using FESEM, XRD and hardness tester etc. The thesis contains five chapters and the details are summarized below. xix Chapter 1: This chapter contains basic of solid oxide fuel cells with a brief account on its working. The solid oxide fuel cells are classified on the basis of electrolytes used in them. The properties of the different SOFC components such as anode, cathode, electrolyte, interconnects and sealants are also discussed in this chapter. The different types of sealants used in SOFC are also discussed and given in this chapter. Chapter 2: The literature related to alkaline earth metal containing vary glass sealants is discussed in this chapter. Apart from the modifiers such as alkaline earth metal oxide, the role of various other additives when added to BaO and SrO containing borosilicate glasses are also provided. The different crystalline phases formed during heat treatment (exposed) of a glass diffusion couple with other components of SOFC play a major role in deciding the glass role as sealants. On the basis of the above literature review, the motivation of the present study is given followed by the objectives of the present research work. Chapter 3: The details of glass synthesis such as their chemical composition used, purity and melting temperature are given in this chapter. The technical details of various experimental techniques are given which were used to characterize the as-prepared glasses. These techniques are differential thermal calorimetry (DSC), X-ray diffraction (XRD) analysis for crystal structure, FTIR studies for molecular structure analysis, Vicker’s hardness to calculate microhardness of all the glasses. Field emission scanning electron microscopy (FESEM) for microstructural and surface analysis of the diffusion couples formed between glass sealants and SS-430, YSZ and doped bismuth vanadate. Chapter 4: This chapter discusses the results obtained from various experimental techniques. Moreover, for various diffusion couples formed the results related to between glasses and interconnect and electrolytes are discussed on length. SrO containing glasses (40SrO-45SiO2- 10B2O3-5ZrO2) do not form any interface with the SS-430 metal interconnect due to its very high CTE than steel SS-430. SB-30/SS-430 is the best sealant followed by SB- 20/ SS-430 and xx SB-40/SS-430 respectively. Overall, it is concluded that the presence of SrO/BaO in glass sealant when ratio is equal to or 0.67 is very good sealing materials for SOFC applications. On the other hand, SrO containing glasses i.e. SB-0 and SB-10 glasses formed the best interface with 8YSZ electrolyte. Interestingly, high BaO contained glass sealants are formed the interface with doped bismuth vanadate. Though it contained a lot of pores around the interface with cracks. Chapter 5: This chapter accounts for the conclusion obtained from the results discussed in the previous chapters on sealants for their interaction with SS-431, 8YSZ and doped bismuth vanadate. In all the series, SB-20 and SB-30 shows the best results in terms of smooth interface without having pores and cracks even 1000 h testing crystalline phases formed, microstructure, diffusion of ions among the components of SOFC. This implies that SB-30 can be a potential candidate as sealants for interconnect (SS430). On the other hand SrO contained glass sealant is good for YSZ electrolyte. The chapter concludes with a discussion for the future perspectives and suggestions.