Fluorite type oxide-ion conductors : new approaches

Abstract

The development of electrolyte materials for the high temperature (800-1000 ∘C) solid oxide fuel cell (SOFC) is mostly based on anion-deficient fluorite oxides, especially cubic stabilised zirconia and ceria. The 8 mol % yttria stabilised zirconia (8YSZ) is so far the most commonly used one owing to its fairly high ionic conductivity and good stability at high temperature cell operation. However, new materials with higher ionic conductivities are demanded to reduce the cell ohmic loss at high temperatures. Although doped ceria provides a higher ionic conductivity than doped zirconia, the reduction of Ce⁴⁺ in the fuel cell reducing atmosphere introduces electronic conduction and lattice expansion which can be detrimental to cell performance. The perovskite structured strontium-magnesium doped lanthanum gallate (LSGM) has high ionic conductivity that is comparable to doped ceria. The limitation for this material comes from its interaction with the traditional Ni-YSZ anode. This means developments of new anode materials, or interlayer designs are required. The scandia stabilised zirconia (SSZ) in the doped zirconia family is a very attractive candidate as it offers the highest ionic conductivity amount the doped zirconia systems. However, 11SSZ, which offers the highest ionic conductivity in the Sc₂O₃ − ZrO₂ system that is comparable to doped ceria and LSGM in the high temperature operating range, undergoes a rhombohedral-cubic phase transition at about 600 ∘C, with the cubic phase existing only at T > 600 ∘C. Apart from this phase stability issue, most SSZ compositions show a significant conductivity degradation behaviour over time.

Accordingly, this thesis is to investigate new electrolyte materials, with a particular focus on the co-doped 11SSZ systems, that may offer higher ionic conductivities, and improved phase and conductivity stabilities for the high temperature fuel cell application. The material's properties are all related to its underlying chemistry. As a matter of fact, this thesis provides new approaches in evaluating the observed properties and conductivity behaviours in the fluorite materials, links the experimental evidence to its underlying chemistry. This thesis aims to provide a deep level understanding on the fundamental of zirconia and stabilised zirconia: its chemistry and defect structure, in order to uncover the fundamental of phase stabilisation, factors that limit the maximum ionic conductivity and driving forces for the conductivity degradation, etc., in doped zirconia systems. This is extended to all the fluorite-based and related systems. It follows that the electrolyte performance is closely related to its microdomain structural changes. In particular, the problem of conductivity degradation is tackled by the elimination of short-range ordering of oxygen vacancies. Apart from the microdomain structure, the total ionic conductivity is also closely related to the crystal phase assembly and microstructures.

The elimination of conductivity degradation in one of the Mg, In co-doped zirconia composition (IMSSZ) significantly improves the long-term conductivity stability, together with a stable, simple crystal structure and a high ionic conductivity (0.14 S cm⁻¹ at 850 ∘C and the ionic conductivity can reach 0.4 S cm⁻¹ at 1000 ∘C). This will contribute to an overall improved cell performance when integrating into the SOFC. The theory, concepts and characterisation methods developed in this study for fluorite and fluorite-related materials, especially those related to microdomain structural studies and characterisations, can be applied further to any energy material with a certain adaptation. This is hoped to provide some insights into new material design, in particular the electrolyte.