Computational modelling of solid oxide eletrolytes and their interfaces for energy applications

Abstract

One of the greatest challenges we are currently facing is the development of efficient, clean and environmentally friendly methods of converting and storing energy from renewable resources. Solid oxide fuel cells and solid oxide electrolyser cells are devices which have been proposed for this purpose. The solid electrolyte in these systems is key to their operation as they transport oxide ions through the cell. A significant issue with these devices is the requirement of high operating temperatures for oxide ion conduction - leading to degradation of the device and causing costs to be high. In recent years there has been a push towards electrolytes which operate in the intermediate temperature range (600 - 800◦C). Ceria doped with trivalent cations has been suggested as it displays high ionic conductivities in the intermediate temperature range. This thesis primarily investigates trivalently doped ceria as a solid oxide electrolyte. The effect of a range of dopants on the structure and reducibility of ceria has been studied using density functional theory. The impact of the local defect structure will vary according to dopant as will the reducibility of CeO2 and so the best dopants for solid oxide electrolyte applications are identified. Force field-based molecular dynamics were used for the rest of the work in this thesis. We investigate the effect of both ignoring and including the polarisability on ionic conduction when modelling doped ceria. This was achieved by considering two interatomic potential models which were both derived from the same ab initio data, where one is a rigid ion model, and the other allows self-consistent solving of the dipoles in the system and using these to study bulk doped ceria. Biaxial tensile strain resulting from epitaxial growth has been seen to affect the ionic conductivity of thin films of doped ceria to varying degrees from little to no enhancements up to large enhancements. Here, we investigate the effect of strain on thin films of doped ceria along with the effect of different surface terminations on ionic conductivity. The impact of interfaces, on these materials, is essential to their performance as solid electrolytes. It has been suggested that the interfaces in these materials can result in reduced oxide ion conductivity; however, the majority of studies consider only the average effect of the interface and not the possible effects of different specifically defined interfaces. In this work, we discuss the effect of specific tilt grain boundaries on the performance of doped ceria as an oxide ion conductor. Finally, an alternative electrolyte which has also been suggested is Sr- and Mg-doped LaGaO3 (LSGM), is considered. To investigate the ionic conductivity in this material and its interfaces an interatomic potential is required. The derivation of a high-level polarisable interatomic potential for LSGM is discussed in detail in this thesis.