(Invited) Optically and Chemically Modulated Oxygen Transport across Interfaces

Abstract

Materials that conduct oxygen are central to many technologies that promise more highly efficient chemical to electrical and electrical to chemical energy conversion, as exemplified by solid oxide fuel and electrolysis cells (SOFC/SOEC), permeation membranes for partial oxidation of hydrocarbons and solar thermal driven chemical conversions. We begin by describing recent insights achieved in controlling the oxygen reduction reaction (ORR) on mixed ionic electronic conductors (MIEC). We have been able to demonstrate the ability to reversibly depress or enhance the oxygen exchange reaction at MIEC surfaces by orders of magnitude by infiltrating the surfaces with binary oxides with controlled acidity. This we demonstrate can be correlated with the ability of the surface additives to deplete or accumulate electrons needed for the ORR. With these insights, we have been able to understand more fundamentally the source of chromia-induced poisoning of SOFC cathodes and the means to recover performance following poisoning. We next turn our attention to the well known blocking of oxygen ions at grain boundaries in solid oxide electrolytes where grain boundary resistances can drop the overall oxygen ion conductance in polycrystalline solid electrolytes by orders of magnitude. The barriers to oxygen transport across grain boundaries are largely attributed to space charge barriers that form at the grain boundary due to the positive core charge of the grain boundaries. We have demonstrated the ability to modulate the barrier potentials by illumination with above band gap light, illustrating the ability to enhance ionic conduction in solid electrolytes at reduced temperatures, while at the same time demonstrating a novel method for interrogating the distribution of barrier heights at grain boundaries and the dynamics associated with the resultant redistribution of oxygen vacancies in space charge regions under conditions of photo-induced electron-hole generation and recombination. This phenomenum is yet another example of the rapidly developing field of opto-ionics.