(Invited) How Surface Ionic & Electronic Point Defects Control Oxygen Exchange Reactions

Abstract

Surface electrochemical reactions are ubiquitous in many energy conversion systems. Of immense interests are solid-state electrochemical reactions that occur at the solid/gas interface in oxide electro-catalysts, such as those in oxygen-ion-conducting solid-oxide fuel cells and electrolyzers. Unlike their liquid counterparts, these electrochemical reactions involve the simultaneous transfer of ions and electrons between the solid and the gas. As such, the electro-catalysts need to catalyze both the oxygen-ion-insertion and electron-transfer reactions. Under reaction conditions, perovskite oxides such as lanthanum ferrites and fluorite oxides such as cerium oxides gain and lose significant amounts of oxygen. It is well known that oxygen nonstoichiometry control a vast number of properties in oxides, such as magnetism, the formation of 2-D electron gas at heterojunction, and catalysis. Yet, the role of surface oxygen nonstoichiometry in electrochemical reactions remains largely a mystery due to difficulties in controlling and quantifying oxygen and valence electrons on the surface. The fundamental question here is how does the surface oxygen nonstoichiometry control the electro-catalytic activity of oxides. In this talk, I will present operando spectroscopic investigations of cerium oxide and transition metal perovskite oxide surfaces using X-rays. We directly probed occupied and unoccupied electronic states using surface sensitive techniques under reaction conditions, that is, under high temperature, pressure and bias.