Strontium Titanate-based Anodes for Solid Oxide Fuel Cells
Abstract: The purpose for this work has been to develop new robust fuel electrodes for solid oxide fuel cells (SOFC). The aim was to find suitable ceramic materials or composites with promising properties for the use as SOFC anodes. Preferably, the electrode should not contain any metal (or at least a metal should not be a major component) in order to improve the redox properties. The present work focuses on trying to understand the electrocatalytical properties for hydrogen oxidation of a few interesting ceramic materials, such as strontium titanate-based materials in combination with doped ceria. The materials have been characterized with different experimental methods in order to determine which physical and chemical properties that govern the electrochemical performance and redox behavior. In the first section, the electrochemical properties for hydrogen oxidation of undoped Sr-deficient strontium titanate (Sr1-xTiO3) were investigated. In general, it was found that the undoped titanate materials were poor electrocatalysts for hydrogen oxidation and materials with higher electrochemical activity and higher electronic conductivity were needed. The benefits of using cone-shaped electrodes, as a relative method of comparing the electrochemical activity of different materials, were discussed. In the next section, a new wet chemistry based synthesis route was developed for fabricating homogeneous Nb-doped SrTiO3 particles with submicron particle sizes. The defect and electronic transport properties of Nb-doped SrTiO3 were investigated, where the theory about the general defect chemistry for n-doped titanates were discussed and confirmed with various experimental techniques. It was found that Nb-doped SrTiO3 is a ceramic material with high electrical conductivity (sigma > 120 S/cm at 1000 ºC in reducing atmosphere). The use of XANES (X-ray Absorption Near Edge Structure), for determining the oxidation state of Ti in the titanate, was also discussed. The electrochemical properties of Nb-doped SrTiO3 were subsequently investigated and it was concluded that Nb-doped SrTiO3 had promising redox stability properties. However, the electrocatalytic performance needed to be improved at least 100 times. In the last section, infiltration of active materials into a backbone structure of electronically conductive Nb-doped SrTiO3 was studied. It was found that a surfactant-assisted infiltration route promoted the formation of nano-sized ceria particles (in this case Gd-doped ceria, CGO) on the surfaces of the backbone material. The performance of the novel ceramic composite anode structure was similar or even improved in comparison with the current state-of-the-art Ni/YSZ anodes, when investigated on symmetrical cells. The ceramic composite fuel electrode was also found to be redox stable, the electrodes actually activated upon redox cycling at 650 ºC. CGO was suggested to act as the mixed conducting electrocatalytically active phase whereas the backbone structure, consisting of Nb-doped SrTiO3, mainly functioned as the electronic conductor.
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