Elucidating Composition-Structure-Property Relationships in Oxide Ion Conductors for Energy Applications

Abstract

This thesis describes the study and development of a number of oxide ion conductors, with the aim of understanding the links between chemical composition, structure and conductivity.

Chapter 1 gives an introduction to oxide ion conductivity, its major applications, challenges with current technology, and a discussion of some of the relevant oxide ion conducting materials.

Chapter 2 reviews the theory behind the scientific and data analysis methods used in this work, with a focus on X-ray and neutron scattering techniques and impedance spectroscopy.

Chapter 3 describes the structural and dynamical characterisation of brownmillerite-type SrScGaO, a material identified as a potential pure oxide ion conductor. The measured conductivity is discussed in terms of the local structure, phonon modes and a structural phase transition, providing insight into the oxide ion migration mechanism and showing it is not comparable to that in other brownmillerite compounds. A series of novel Zn-substituted derivatives are reported, showing significantly improved conductivity and, for some compositions, the apparent adoption of a cubic perovskite structure.

Chapter 4 explores the properties of the cubic Zn-substituted compound, SrScZnGaO, in more detail. It is a highly-disordered perovskite on average, but neutron total scattering reveals a local structure markedly different from the average, and modelling suggests it contains an unusual terminal oxygen site and a high proportion of tetrahedra units, described approximately by the formula AOhTdO. These structural properties lead to the reversible incorporation of water, a significant proton conductivity at low temperature, and an increase in oxide ion conductivity of two orders of magnitude relative to SrScGaO.

Chapter 5 reports the first investigation into a family of AOhTdO materials in the context of their oxide ion conductivity. The structure, oxide ion and proton conductivity and water uptake behaviour of the compound BaYGaO is presented, along with the subsequent characterisation of the BaSrYGaO solid solution as a function of composition and temperature. The structural differences across the series are linked to variations in conductivity behaviour, including differing proton contributions. An exploratory doping study based on these findings is described, resulting in the new compound BaLaYGaO, which is a pure oxide ion conductor with the highest conductivity of any material reported in this thesis.

Chapter 6 discusses the controversial phase `LaSiO$_{27}', whose existence is critically examined through an investigation into the structure and elemental composition of a single crystal sample. We present evidence to suggest the phase is not stable, containing La and Si vacancies instead of interstitial oxide ions, and show that an alternative structural model and composition can explain all of our experimental data. [math mode missing closing $]