Study of Undoped and Doped Vanadium Oxides for Application In Solid Oxide Fuel Cell (SOFC)

Abstract

The present thesis describes the synthesis of V2-xMxO5-δ (M=Li2O, MgO, Al2O3 and TiO2 (x=0.05, 0.10, 0.15, 0.20, 0.25, 0.30) systems by melt-quench technique. The physical, structural, optical, thermal and conducting properties of the prepared samples are studied using various experimental techniques to determine their suitability as an electrolyte for IT-SOFC applications. Based on above properties, some selected samples are used for interaction study with interconnect materials, which is being used in solid oxide fuel cells. The research work carried out for Ph. D. thesis is divided into six chapters along with a list of cited references at the end of each chapter.  Chapter 1 describes the background and introduction of various fuel cells particularly SOFCs. The role of different components of solid oxide fuel cell has been discussed. Since the present work is related to develop electrolyte materials, therefore, the main focus has been given on the electrolyte materials of SOFCs. The suitability of materials as electrolyte is described in context to specific properties such as thermal stability at fuel cell operating temperature in oxidizing and reducing medium and electrical conductivity. The present study reveals that the development of newer electrolyte materials with high conductivity at a lower temperature and thermal stability is required.  Chapter 2 deals with the literature survey and gives an idea about the composition- based structure-properties of vanadium and others materials. The effect of various dopants on the different properties of V2O5 synthesized by various experimental techniques has been studied. From the literature survey it can be concluded that the ionic conductivity and thermal stability is great concerned at high operating temperature of solid oxide fuel cell (SOFC) in oxidizing and reducing medium. The high operating temperature of SOFC is responsible to degradation, coefficient of thermal expansion (CTE) mismatch, electrodes sintering and catalyst poisoning among the SOFC components. Based on the available literature survey, the

gaps in study and objectives of the present thesis are also given in the end of this chapter.

 Chapter 3 gives the detail about source of raw materials, synthesis parameters and experimental method employed for samples preparation to achieve the proposed work objectives. The technical details of the used experimental characterizations are also given. It explains the techniques used for structural, optical, thermal and electrical characterization for as-synthesized samples. These techniques comprise X-rays diffraction (XRD) for phase formation, Fourier-transform infrared (FTIR) and Raman spectroscopy for structural vibrations analysis, UV-Visible spectroscopy for optical band gap analysis and scanning electron microscopy (SEM) for morphological study of the interface of diffusion couples. Differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA) are used to analyze thermal stability of the as-synthesized samples. The two-probe impedance analyzer is used to study the electrical properties of the prepared samples.  Chapter 4 describes the results and discussion of the prepared samples. In this chapter, interpretations of the data obtained from various characterization techniques have been discussed. This chapter is further divided into five sections. The first section represents the properties of V2-xLixO5-δ (x=0.15-0.30) systems. X-ray diffraction (XRD) patterns confirm the formation of three different crystalline phases (orthorhombic Li0.04V2O5, monoclinic and monoclinic Li0.30V2O5). FTIR and Raman spectra indicate that the doping of Li2O into V2O5 leads to a transition from VO5 into VO4 structural unit. The optical band gap (Eg) decreases from 2.2 to 2.08 eV while Urbach energy (EU) increases (0.31-0.41 eV) with the addition of Li2O content in place of vanadium. Thermal stability is found to decrease with the Li2O addition in place of V2O5. The DC conductivity is increased from 0.08 to 0.12 Scm-1 at 450 ᵒC with Li2O doping. In second section, V2-xMgxO5-δ (x=0.0-0.30) systems has been explained. Density, inter-ionic distance, and fragility index are found to decrease with the addition of MgO content. The IR

study suggests that the glass network is made up of VO4 polyhedra by changing VO5 structural units in the presence of MgO. Higher content of MgO improves the glass formation tendency and reduces the thermal stability of the samples. VM-0.30 glass shows the highest glass transition temperature (274 ºC) and Hruby’s parameter (0.34) as compared to other samples due to its highest rigidity and glass formation ability. The conductivity of the samples decreases to ̴ 10-4 Sm-1 for x=0.30 at 300 °C, while activation energy increases to 0.38 eV with MgO content. The third section describes the properties of V2-xAlxO5-δ (x=0.0-0.30) systems. Density and molar volume are decreased with the doping of Al2O3 in place of V2O5. X-ray diffraction (XRD) pattern confirmed the formation of single phase (orthorhombic V2O5). The shifting of X-ray diffraction peak (010) to a higher angle revealed a compressive strain in the present samples due to the size difference of dopant (Al3+ ion) and host (V5+ ion). The estimated crystallite size was found in the range of 143.25-124.92 nm. Spectroscopic investigations revealed a transformation of the [VO5] polyhedra into [VO4] polyhedra on the doping of Al2O5 into V2O5. Thermal stability of the samples was found to decrease on the doping of Al2O3. The optical band gaps of the samples are found in the range of 2.31-2.35 eV. While DC conductivity lies in the range of 0.22-0.36 Sm-1 at 400 ᵒC. The fourth section describes the properties of V2-xTixO5-δ (x=0.0-0.30) systems. Density decreases whereas molar volume increases as the Al2O3 doping increases on the cost of V2O5. The X-ray diffraction peaks reveal a single-phase formation i.e. orthorhombic V2O5. Spectroscopic investigation clearly suggests structural changes on the doping of TiO2 on the cost of V2O5 as [VO5] groups are transformed into [VO4] groups with formation of V-O-Ti chains. The optical band gap exhibits a decreasing trend as TiO2 doping concentration increases and lies in the semiconductor range (2.28-2.20 eV). The DC conductivity of the samples increases with an increase in doping of TiO2. The activation energy of the samples was found

in the range of 0.275 to 0.231 eV. All the samples exhibit good DC conductivity (0.34 to 1.12 Scm-1) at 400 ºC and good thermal stability (small weight change < 0.6%) in the temperature range of room temperature to 500 ºC. This chapter also presents the interaction study between V2-xMgxO5-δ (x=0.20, 0.25) glass samples and AISI 430 steel as interconnect. The interaction between MgO doped V2O5 glass samples and SOFC interconnect material at 620 ºC for different time durations has been investigated using SEM experimental technique. V2-xMgxO5-δ (x=0.20, 0.25) glass samples exhibit good interface with AISI 430 steel at 620 ºC for 1, 10, and 100 h. In addition, the X-ray dot mapping also has been studied for better understanding of the morphology of the interfaces and diffusion of the elements from both sides. The formations of crystalline phases during heat treatment duration have also been studied. Chapter 5 describes the overall conclusion drawn from the physical, structural, optical, thermal, and electrical properties along with interaction study of the prepared samples. To enrich this work, future scope of the present study has also been given at the end of this chapter.