Multi-Fuel oxidation in Solid Oxide Fuel Cells: Model anodes and system studies

With the evolution of renewable energy technologies it has become necessary that a balance is found between power production with conventional energy sources and other long term solutions. SOFCs offer an alternative for utilising conventional fossil fuels as well as sustainable biomass derived fuels at a high efficiency. For use with such fuels, materials like ceria offer higher performance and improved carbon tolerance as compared to conventional Ni-YSZ anodes. The mixed ionic electronic conductivity of ceria helps to spread the electrochemically active area beyond the interface between nickel and ceramic which is expected to improve performance. While it is known that H2 and CO oxidation can occur on ceria, there are very few studies and no consensus about the exact mechanism or the rate limiting steps. This is mainly because of the 3D interpenetrating microstructure of the anode which makes reactions difficult to localise and identify individually. Pattern anodes can be made with well-defined geometry which helps to localise reactions and study elementary reactions with interference from phenomena such as diffusion. Deriving kinetics and rate parameters from this data can be used to extend knowledge gained into other studies. This thesis is aimed at studying fundamental electrochemistry of H2 and CO oxidation using pattern anodes of nickel and ceria and extend this knowledge to macroscopic simulations and system studies. The thesis comprises of three main parts 1) Preparation and testing of Ni and Ce pattern cells with H2 and CO 2) Elementary kinetic modeling of oxidation on ceria 3) Effect of H2S on Ni and Ce 4) SOFC-Gas Turbine systems with different biofuels. The following observations are made from this study. 1) Pattern anodes of nickel and ceria can give important indications about oxidation of H2 and CO and H2-CO mixtures. Preferential H2 oxidation on ceria was observed in addition to nickel. 2) Elementary modelling of H2 and CO oxidation on ceria is carried out using fitting of impedance spectra. This model can be used with macroscopic simulations which require kinetic data 3) H2S has a deleterious effect on ceria as well as nickel. Elementary modelling can capture the poisoning effect of H2S on ceria 4) System studies indicate that high efficiency SOFC – GT systems need to be adapted for different fuels after a thorough understanding of exergy losses associated with subsystems – particularly the fuel cell subsystem.Process and EnergyMechanical, Maritime and Materials Engineerin