Development and Characterization of Perovskites as Alternative Anode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Solid-oxide fuel cells (SOFCs) have the potential to meet the critical energy needs of our modern civilization and minimize the adverse environmental impacts from excessive energy consumption. They are highly efficient, clean and can run on a variety of fuel gases, including hydrocarbons and gasified coal or different types of ample carbonaceous solids. However, the conventional anode for an SOFC, a composite consisting of nickel metal and yttria-stabilized-zirconia (YSZ), is highly susceptible to carbon deposition and deactivation (poisoning) by sulfur contaminants commonly encountered in readily available fuels even in parts per million (ppm) levels. There is accordingly strong demand for development of alternative anode materials with tolerance to coking and sulfur poisoning. Among the novel anode electrodes, perovskite based materials (ABO3) are of great interest because they have been shown stable performance as redox stable anodes both in hydrocarbon and sulfur containing fuels. With an aim to improve the stability of perovskite related oxides and maximize the electrochemical performance, this dissertation focuses on perovskite-related oxide anode materials. The first part of this thesis describes the work done towards the development of a new Sc doped La0.8Sr0.2ScxMn1-xO3-δ and Y0.08Sr0.92Ti1-xFexO3-δ anode by infiltration on porous YSZ back bone. The composite anode exhibits enhanced electrochemical performance comparable to that of the conventional Ni-YSZ anode. Second, oxygen non stoichiometry and electrical conductivity of Sc doped La0.8Sr0.2MnO3-δ was measured by Coulometric titration at controlled temperature and oxygen partial pressure. The main goal of the work is to determine the oxygen vacancy formation and to provide a better understanding of the structural changes in perovskite related with a reduction of oxygen partial pressure. In this respect, a suitable defect chemical model is also proposed and verified with the oxygen non-stoichiometry in both oxygen excess and oxygen deficient regions in order to predict the oxygen defect formation. Also the correlation between defect formation and thermodynamic properties such as partial molar enthalpy and partial molar entropy of oxygen vacancy formation reaction is elaborated. Third, surfer tolerance of the conventional Ni-YSZ anode has been investigated by simple surface modification process. A single step infiltration of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) infiltration on Ni-YSZ anodes significantly improves the sulfur tolerance anode. The much-improved power output and sulfur tolerance of the BZCYYb modified Ni-YSZ anode were attributed to the adsorbed water uptake property of BZCYYb at microscopic levels and facilitated water-mediated sulfur removal reactions. The final part of this thesis outlined the work done towards the development of a new layered double perovskite PrBaMn2O5+δ anode materials. PrBaMn2O5+δ anode shows, superior electrochemical performance in both hydrogen and hydrocarbons fuels, with the high electrical conductivity in anode operating conditions. Transmission electron microscopy (TEM) analysis suggest that the most attractive properties of this material are the phase transition of disordered Pr0.5Ba0.5MnO3-δ perovskite, to A-site ordered PrBaMn2O5+δ perovskite, under SOFC anode operating condition, showing [MnO2] square sublattice is sandwiched between two rock salt layers, [PrO] and [BaO] layers, along the c axis. Towards the end, these findings contribute to understanding the electrochemical performance of perovskite anode materials in relation to oxygen non-stoichiometry and commercial viability of SOFCs that are driven by cost-effective and renewable fuels.