SYNTHESIS AND CHARACTERIZATION OF La2Mo2O9-BASED ELECTROLYTES FOR SOLID OXIDE FUEL CELL

Abstract

The global energy demand is increasing at a very steep rate. To fulfill the energy requirements, non-conventional fossil fuels viz., coal, crude oil, petroleum products are significant contributors. But these traditional sources cause a lot of pollution in the past and even in the present and thus having a negative impact on ecology and society. Therefore, it is very much necessary to switch to clean energy sources. In this regard, it is required to develop alternative renewable and clean energy sources like solar, wind, and hydroelectric power and fuel cells intensively at a wide scale. Regarding clean energy sources or energy conversion devices, still, a lot of effort has to be made to make the clean energy generation economical. In the current scenario, progress is being made on a wide scale for a clean energy generation. Solid oxide fuel cells (SOFCs) are considered to be one of the clean energy conversion devices. It has water as its by-product, which can be utilised again in the system to regenerate fuel. A SOFC unit mainly consists of two electrodes: anode, cathode, and a solid electrolyte. The fuel plays an important part in the cell operation and is independent of the SOFC unit as it is often converted into hydrogen through reforming process. The function of the SOFC relies mainly on O2- (oxide) ion transport from cathode to anode side through the electrolyte. The main function of both the electrodes is to initiate the reaction between the reactants (fuel and oxygen) through the electrolyte, without being consumed or corroded during the reaction. The SOFC unit should also bring the three phases, i.e., the electrode, electrolyte, and the fuel into contact of each other. The discovery of LAMOX based electrolytes in the year 2000 has attracted much attention in the SOFC area and may be seen as a potential competitive candidate to traditionally YSZ and GDC based electrolytes. It shows a superior oxygen ion conductivity of (0.06 S/cm and 0.4 S/cm for doped La2Mo2O9 samples at 800 °C) as compared to yttria-stabilised zirconia (YSZ). However, its applicability is limited by its instability in reducing atmosphere, abrupt volume expansion due to the phase transition from monoclinic to cubic phase; high thermal expansion and chemical reactivity towards electrode components. The first chapter of the thesis is dedicated to the introduction of fuel cell technology. The chapter includes an introduction to basic concepts of fuel cells, their history, and development through the years, various types of fuel cells classified according to the types of electrolyte used, and their operating temperatures, theory of fast oxide ion conductors, components of SOFCs viz., electrode, electrolyte, interconnects sealants and reforming agents. Materials used as i cathode/anode/electrolyte, the necessity of (La2MO2O9) LAMOX based electrolytes and its superiority over other electrolyte materials, structural study of LAMOX, limitations of La2MO2O9 as an electrolyte material, doped LAMOX members as a substitute for the parent compound for overcoming its limitations, suitable electrode material for LAMOX family members have also been discussed in the first chapter. Various kinds of electrolyte systems employed in solid oxide fuel cell systems have been studied in the past years. The most extensively researched electrolyte is yttria-stabilised zirconia (YSZ) and gadolinia doped ceria (GDC) electrolyte, which in the present time are still used commercially. The operation of YSZ at 1000 °C makes it a very robust system for high-temperature applications, and the highest conductivity shown is 0.1 S/cm at 1000 °C. The pristine LAMOX possesses some serious limitations viz., Mo reduction in reducing (Ar-H2) atmosphere, low electrical conductivity at room temperature, large volume expansion at the phase transition temperature, and suitable compatible electrodes. Solutions in the form of doping at various elemental sites have been proposed in a way such that the limitations of pure La2Mo2O9 could be overcome; then, it may very well serve as a potential candidate for electrolyte materials for intermediate temperature (600- 800 °C). The second chapter of the thesis deals with the synthesis of the required compositions by solid state synthesis route and details of various instruments required for characterisation of the samples prepared. Specifications of all the instruments and their standard reference data have been mentioned in this chapter. The third chapter deals with the effect of air and reducive atmosphere treated La2Mo2O9 at 900 °C, which has been studied by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), x- ray photoelectron spectroscopy (XPS) and complex impedance spectroscopy studies (IS). The results revealed and validated with the previously published results and have shown detailed study on grain boundary resistance and XPS study of pristine LAMOX. The main purpose of the LAMOX study in this context is for electrolyte in fuel cells. However, the low value of electrical conductivity due to grain boundary resistance and monoclinic phase, Mo reduction in Ar-H2 atmosphere, and high thermal expansion during the phase transition may lead to an overall degradation of the fuel cell. The fourth chapter of the thesis is dedicated to the effect of various dopants in the LAMOX to overcome the disadvantages of pristine LAMOX like Mo reduction, thermal expansion coefficient mismatch due to phase change. The first section of the chapter deals with W doping ii at Mo site, which has a solubility of around 60-80 mol percent at Mo site. The structural, morphological, x-ray photoelectron spectroscopy (XPS) and ion dynamics along with conductivity studies of La2Mo2-xWxO9 (0.25 ≤ x ≤ 1.5) both in air and reducive atmospheres have been presented and discussed. Though the W doping in LAMOX successfully stabilises the cubic phase at room temperature, it was not able to entirely suppress the Mo6+ reduction to its lower oxidation states as clearly revealed by comprehensive FE-SEM and x-ray photoelectron spectroscopic studies. No other elements in the doped LAMOX compositions were reduced. The second section of the chapter deals with the effect of K doping in LAMOX. The composition La1.9K0.1Mo2O9-δ shows the stabilisation of the high-temperature cubic phase of La2Mo2O9 at room temperature, as confirmed by XRD and Rietveld analysis. La1.8K0.2Mo2O9-δ composition shows the formation of K doped LAMOX phase with the appearance of K2O as a secondary phase in both of the K doped LAMOX compositions. The microstructural analysis revealed some intra-granular pores suggesting incomplete sintering in the Ar-H2 atmosphere of the doped compositions. X-ray photoelectron spectroscopic study has been done after microstructural characterisation and shows the Mo6+ stability in the reducive atmosphere. The third and last part of the chapter has shown the effect of rare earth hand W doping simultaneously in LAMOX at La and W site respectively. X-ray photoelectron spectroscopy study shows that there is not much stabilisation of the Mo6+ in the reducing atmosphere. The fifth chapter of the thesis is dedicated to the compatibility study of cathode/anode material La2NiO4+δ (LN) and NiWO4, respectively, with pristine and W doped LAMOX as electrolyte in the air as well as reducive atmosphere. The XPS study is thoroughly exploited to study the effect of fuel (diluted hydrogen gas) on the electrode/electrolyte interface at elevated temperatures. Also, the cationic diffusion of the interspecies is also studied. The sixth chapter of the thesis is concluding remarks of the present work, which shows the findings of the study. Future work and suggestions are also mentioned, which are required to carry out to make the present study more comprehensive and meaningful.