Electronic structure of perovskite oxide surfaces at elevated temperatures and its correlation with oxygen reduction reactivity

Abstract

The objective is to understand the origin of the local oxygen reduction reaction (ORR) activity on the basis of the local electronic structure at the surface of transition metal oxides at elevated temperatures and in oxygen gas. This goal presents a key challenge to traditional surface science approaches, and is important for enabling high performance electrochemical energy conversion and storage systems, such as fuel cells, batteries, super-capacitors. Firstly, this thesis identified the correlation between the surface chemistry and electronic structure on SrTi1Fe,03 (STF), as a model perovskite oxide. Angleresolved x-ray photoelectron spectroscopy showed that that Sr enrichment increases on the STF thin films with increasing Fe content. In situ scanning tunneling microscopy and spectroscopy (STM/STS) demonstrated that the apparent energy gap increases with Fe fraction. This trend is opposite to the dependence of the bulk STF band gap on Fe fraction and is attributed to the formation of SrO that deteriorates oxygen reduction kinetics. The second case study in this thesis aimed to obtain a microscopic level understanding and control of the vastly faster ORR kinetics near the La0.8Sr0.2CoO 3/(La0.5Sro. 5 )2CoO 4 (LSC113/214) hetero-interfaces. We implemented a novel combination of in-situ STM/STS and focused ion beam milling to probe the local electronic structure at nanometer resolution in model multilayer superlattices. At 200-300 °C, the LSC2 14 layers are electronically activated through an interfacial coupling with LSC113 . Such electronic activation is expected to facilitate charge transfer to oxygen and enable enhanced reactivity near the LSC113 214 interfaces. Our results contribute to an improved understanding of oxide hetero-interfaces at elevated temperatures and identify electronically coupled oxide structures as the basis of novel cathodes. This thesis was then able to explain the mechanism behind the electronic activation of LSC2 14 by the neighboring LSC113 at high temperatures based on the exchange of electron and oxygen defects across the interfaces, by performing hard xray photoelectron spectroscopy and high resolution X-ray diffraction measurements. Lastly, a vertically aligned nano-composite structure made of LSCU324 was successful synthesized by the combinatorial pulsed laser deposition and was tested as a novel SOFC cathode.