Computational studies of interstitial-type oxide ion conductors for applications in solid oxide fuel cells

Abstract

Fundamental advances in solid-state ionics are crucial for the development and commercialisation of cleaner energy conversion and storage technologies, such as solid oxide fuel cells (SOFCs). Modern computational techniques play a valuable role in this field by providing insight into the properties of materials on the atomic scale. In this thesis we examine three types of material as potential interstitial-type oxide-ion conductors for application in intermediate-temperature SOFCs. Firstly, we investigate neodymium nickelate, Nd2NiO4+δ, which is a mixed ionic/electronic conductor with a Ruddlesden–Popper-type structure. We show that interstitial ions are introduced into the structure via Frenkel or oxidation processes. Furthermore, we find an interstitialcy-type conduction mechanism for these ions, as previously found for the La2NiO4+δ and Pr2NiO4+δmaterials. Molecular dynamics simulations reveal a reduction in migration energy for Nd deficient systems, which is consistent with tracer diffusion data. Secondly, we use both atomistic and density functional theory techniques to examine SrFeO2, a recently-discovered material that is isostructural with the ‘infinite-layer’ cuprates. We find a favourable oxidation energy for the material, in accordance with experiment. The migration of the resulting interstitial oxide ions is predicted to occur in a two-step process very similar to the vacancy-hopping mechanism of the parent perovskite. Lastly, we examine the site selectivity and compensation mechanisms of a wide range of cation dopants in the La9.33(GeO4)6O2 apatite material. We find that small, highly charged dopants (e.g., Ti4+) are more favourable on the Ge site, whilst larger dopants with lower charge (e.g., Sr2+) prefer the La site. We also find that subvalent doping is not predicted to reduce the oxide ion content of the material, which is significant for interstitial ion conduction behaviour.

Date of Award	1 Jul 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Muhammed Islam (Supervisor)