Defect equilibria and electrode kinetics in Prx̳Ce1̳-̳x̳O2̳-̳[̳d̳e̳l̳t̳a̳]̳ mixed conducting thin films : an in-situ optical and electrochemical investigation

Abstract

An improved fundamental understanding of oxygen defect equilibria and transport kinetics in oxides is essential for achieving enhanced performance and longevity in many oxide-based practical applications. The ability to diagnose a material's behavior in a thin film structure under operating conditions (in operando), ideally in situ, is therefore of importance. In this dissertation, a novel experimental technique capable of simultaneously performing in situ and in operando optical absorption and electrochemical impedance spectroscopy (EIS) measurements was developed and utilized, for the first time, over a range of temperatures and controlled atmospheres. The technique was applied to the Prx̳Ce1̳-̳x̳O2̳-̳[̳d̳e̳l̳t̳a̳]̳, (PCO) model thin film system. PCO shows mixed ionic and electronic conducting (MIEC) characteristics at relatively high pO2 regimes (e.g. air), which is beneficial for solid oxide fuel cells (SOFCs) cathode performance. The Pr impurity levels in PCO allow for optical transitions (2.0 - 3.3 eV), leading to the red coloration of oxidized samples. A change in the redox state of Pr results in a color change and so serves as a means of investigating the Pr oxidation state and thereby oxygen non-stoichiometry. Pr⁴⁺ concentrations, derived independently from optical and electrochemical measurements, and their corresponding trends, were found to be self-consistent, confirming that the oxygen reduction enthalpy in thin film 10PCO is lower than that in the bulk. The derived extinction coefficient, . . . , can now be utilized to study defect equilibria of PCO or other relevant oxide films by optical means alone. The oxygen surface exchange reaction kinetics, driven by chemical and electrical driving forces, were investigated and correlated to each other, with the aid of the thermodynamic factor. The impact of surface chemistry and metal current collector on the reaction kinetics was discussed. A specially designed cell structure enabled the extension of the oxygen diffusion pathway, allowing for the monitoring of color front migration in PCO films. Such optical color front motion experiments offer the opportunity for in situ, more rapid and reversible investigation of oxygen diffusion kinetics in thin films and open new opportunities to study materials' spatially distinguishable properties.