Lanthanum and nickel co-doped strontium titanates for solid oxide fuel cell anodes
Creators: name: Stevenson, Graham Robert
Publisher: name: Imperial College London
Year: 2020

description: Fuel cells are a technology that can potentially revolutionise the means by which we store and convert energy. High-temperature solid oxide fuel cells can do so at higher efficiencies than conventional fuel cells and can utilise the waste heat to further boost overall efficiency, while acting on a variety of fuels that could ease the transition to a clean energy infrastructure. Solid oxide fuel cell materials are currently limited by anode degradation and overall lifetime. In order to make the technology viable, these issues need addressing. One means of doing so is investigating alternative anode materials. Perovskite materials, in particular lanthanum doped strontium titanates, have been of interest recently due to their respectable electronic conductivity and stability in sulfur- and carbon-containing fuel sources. Furthermore, doping of this material has led to further functionalisation through use of exsolution: the growth of socketed catalytic nanoparticles to enhance material performance. This thesis aims to investigate the technology, literature and answer the following questions on the lanthanum-doped strontium titanate materials: 1) Is it possible to synthesise the materials by a non-solid-state route? 2) Can the number of processing steps in electrode formation be reduced? 3) Can an understanding of the exsolution process be further developed? 4) Can these materials be further improved through our understanding? The material is shown to be able to form the required phase at temperatures as low as 1000 °C through a modified synthesis route, still exhibiting the exsolution phenomenon that makes this class of materials of interest, and is also shown to produce structures with inherent porosity. This leads to the formation of one-step processed microstructures which perform slightly worse than conventionally manufactured samples of the same material. Through RC-circuit fitting and Gerischer element fitting of electrochemical impedance spectroscopy data, this change in performance is attributed to be due to the difference in porosity between the two microstructures. The formation of nanoparticles on the surface is also shown to improve performance compared to similar materials in similar conditions, thus a sensitivity study into the exsolution behaviour is undertaken. The factors that control exsolution are briefly investigated and an understanding of how these may improve the exsolution profile is developed, leading to the creation of a new composition of doped-titanate. This composition shows a considerably more nanoparticle-dense exsolution profile than the predecessors treated in the same conditions and also shows an approximately 22% improvement in electrochemical performance measured through electrochemical impedance spectroscopy. Thus, this body of work shows that the titanates can indeed be synthesised at lower temperatures, the inherent microstructures formed this way may be functionalised for use as electrodes – where our understanding of the interpretation of electrochemical impedance spectroscopy data is greatly improved – and the conditions that produce consistent, nanoparticle-dense, exsolution arrays are elucidated on, leading to demonstration of our understanding by improvement of the material.