Study of transition metals doped strontium zirconate/manganite for solid oxide fuel cell applications

Abstract

Environment-friendly, better efficiency, and fuel flexibility have made solid oxide fuel cells (SOFCs) a popular alternative to conventional combustion energy generating systems. However, despite having many advantages, the wide-scale commercialisation of SOFCs is still hampered by its cost and incompatibility of the different components. High operating temperature (≥1000℃) leads to a higher start-up time with increased electrode kinetics. Therefore, research nowadays is focused on the development of materials for intermediate-temperature (600-800℃) and low-temperature (<600℃) range SOFCs. Among all the components of SOFC, the cathode is a vital component. It should have mixed conductivity, i.e. electronic and ionic conductivity. Mixed conducting materials have a pronounced effect on the oxygen reduction reaction since their presence increases the triple-phase boundary, thus, lowering the operating temperature, forming the core of this thesis. Mainly, perovskite-based materials have been studied for use as SOFC cathode materials for a long time due to their ability to accommodate a range of elements from the periodic table, inherent vacant sites and ability to accommodate multiple cations as dopants. Therefore, in this thesis, the effects of transition metal (Cu2+ and Ni2+) doping on the structural, thermal and conducting properties of strontium based perovskite (ABO3) materials (SrZrO3 and SrMnO3) are studied. The doping has been done at the B-site of the ABO3 type materials to enhance their conducting properties, match coefficient of thermal expansion (CTE), and increase the sinterability of Sr-based perovskites. Selected samples exhibiting comparable structural and thermal properties with a boro-silicate glass sealant and a standard interconnect material are also analysed for structural and thermal compatibility of developed materials. The thesis is divided into seven chapters with an overview in the beginning and a list of cited references at the end of each chapter. Chapter 1 describes the background of fuel cells, types and geometry of the SOFCs. The advantages and disadvantages of the different fuel cells, their working principle, and different designs are also discussed. The focus of the thesis is primarily to develop cathode materials for SOFC; therefore, the different processing methods are discussed in detail and their effects on the reaction mechanism. Further, the role of doping transition metals is discussed to understand their effect on the structural, thermal and conducting properties of perovskite-structured materials. Chapter 2 gives a detailed account of the literature related to SrZrO3, SrMnO3 and other perovskite structured materials, along with interaction/interfacial study of different SOFC components. The literature is based on the role of different dopants, structural, thermal and conducting properties. The motivation of the present research work is also discussed in this chapter. Towards the end of the chapter, the objectives of the thesis are defined based on the motivation and literature survey. Chapter 3 describes the detailed synthesis procedure of SrM1-xAxO3-δ (M: Zr, Mn; A: Cu, Ni; 0.00≤x≤0.20) and the interaction study of the selected cathode with a glass-sealant and a interconnect (Crofer 22 APU). The samples are characterised using various techniques to study structural, thermal, and conducting properties of synthesised materials. The principle and technical details of the different characterisation techniques, namely X-ray diffraction (XRD), Rietveld refinement, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), dilatometry, thermogravimetric-differential thermal analysis (TG-DTA), impedance analyser, ultraviolet-visible spectroscopy (UV-Vis), photoluminescence spectroscopy (PL) and Vickers hardness are discussed in this chapter. Chapter 4 deals with the effect of doping Cu and Ni with different concentrations in place of Zr in SrZrO3. This chapter is further divided into two sub-sections. In the first sub-section, doping of Cu2+ is discussed, whereas, in the second section, Ni2+ doping is discussed in detail. Doping of transition metals in SrZrO3 induces oxygen vacancies in the crystal lattice necessary for ionic motion. The effect of doping on the structural, thermal and conducting properties is studied. The results are discussed in light of the disordering created by the dopants and their concentrations. The monophasic nature of the samples enhances the conducting properties of the system. Cu2+ enhanced the thermal and conducting properties of SrZrO3; however, due to the brittleness of the Ni-doped samples, the conductivity measurement could not be carried out. Both dopings had thermal expansion in the tolerance limit required for SOFCs. Chapter 5 discusses the effect of different dopings in SrMnO3. The first part gives a detailed account of the effect of doping Cu2+ in SrMnO3, and the second part gives information about the effect of Ni2+ doping in SrMnO3. The samples are studied in light of the structural transformations, vacancy generation on doping lower valence cation, the presence of secondary phases and thermal changes due to ordering or disordering of the vacancies. The presence of minor phases is detrimental in determining as well as enhancing the conductivity of the samples. However, the thermal expansion of the doped samples is well within the prescribed limit for SOFCs. Chapter 6 gives a detailed account of the chemical interaction of selected cathode material, i.e. SrZr0.85Cu0.15O3-δ with a glass sealant and a standard interconnect material. This chapter is divided into two sections. The first section deals with the changes observed at the cathode-sealant interface, while the second section elaborates on the changes at the cathode-interconnect interface. The structural and thermal changes occurring at the interface are studied based on XRD, SEM, and dilatometry results. The interaction with the sealant was successful; however, due to incompatibility of CTE at high temperatures, chemical bonding between the cathode and interconnect could not form. Chapter 7 accounts for the conclusions drawn from the obtained results on SrM1-xAxO3-δ (M: Zr, Mn; A: Cu, Ni; 0.00≤x≤0.20) and the interaction study. In all the series, the SrZr1-xCuxO3-δ (0.05≤x≤0.20) is the best cathode material in terms of monophasic nature, CTE and conductivity. The conductivity values vary from 10-7 to 10-4 S cm-1 with increasing Cu dopant concentration and the composition, SrZr0.85Cu0.15O3-δ, has the highest conductivity at 800℃. Interestingly, the CTE of the diffusion couple after exposure of 500 hrs is within the required limit for SOFCs. Additionally, the future scope and suggestions are also given at the end of this chapter.