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Title: In-situ analysis of La0.6Sr0.4Co0.2Fe0.8O3-d surface in ambient atmospheres
Author: Niania, Mathew
ISNI:       0000 0004 6348 2262
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The LSCF material system is a desirable material for a Solid Oxide Fuel Cell (SOFC) cathode due to its Mixed Ionic-Electronic Conducting (MIEC) properties. It has been utilised commercially for many years, however, the cell lifetime is inhibited by degradation processes passivating the surface of the electrode. A-site cation segregation is believed to be a primary degradation mechanism due to the formation of electronically insulating secondary phase particles reducing the active surface area that facilitates the oxygen reduction reaction. Numerous reports have studied the overall effect that degradation has on SOFC cell performance, however, it is still unclear the extent to which microstructural changes affect a material's oxygen exchange properties. To date, many studies measuring the oxygen exchange rate utilised pure oxygen atmospheres in order to isolate the effect of oxygen. However, for cost and practicality reasons, the desired gas stream for SOFC is ambient air. Multiple oxygen-containing gaseous components and impurities (such as CO, CO2, H2O, SO2 and NOx) are contained within ambient air and have been shown to alter the oxygen exchange rate or enhance the degradation of MIEC materials. This work focuses upon characterising the effect ambient air has upon the surface microstructure and oxygen exchange rate of the LSCF system. In-situ High-Temperature Environmental Scanning Electron Microscopy (HT-ESEM) was used to analyse the growth rate and growth behaviour of strontium-based particles in pure O2, pure H2O, ambient air and vacuum environments. The HT-ESEM data was directly compared to Electron Backscattered Diffraction crystal orientation data in order to understand what effect the LSCF domain structure had upon the segregation of strontium. It was observed that the surface microstructure has a strong influence on the growth behaviour and growth kinetics of the particles. A common methodology for measuring oxygen self-diffusivity and surface exchange rates is Isotopic Exchange Depth Profiling (IEDP). This traditionally has used pure oxygen as the anneal environment to isolate the exchange properties of the O2 species, however, in the presence of other oxygen containing species the exchange process will be more complicated. In order to analyse the surface exchange rate in ambient air (or any other atmosphere containing a consistent oxygen partial pressure), the novel 'back-exchange' technique was developed. Initial development of the technique has demonstrated its validity and confirmed enhancement of the oxygen exchange rate in ambient air over pure oxygen. Time-of-Flight (ToF) SIMS was used to measure isotopically exchanged diffusion profiles. For materials with a high oxygen self-diffusivity, such as LSCF, the 'line-scan' method must be employed instead of a standard depth profile. The ToF-SIMS utilises a statistical method in order to correct for detector 'dead time', however, this method relies upon the total ion count to remaining constant across each pixel of the raster area. The line-scan method relies upon analysis of a surface perpendicular to the original exchange surface and as such will not have a constant ion count near the sample edge. Errors associated with the measured diffusion profile are discussed and an optimised sample preparation has been proposed.
Supervisor: Kilner, John ; Skinner, Stephen Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral