PNNL

Abstract

In mixed ionic electronic conductors (MIECs) the electrochemistry of oxygen reduction and evolution is complicated by the competition between surface adsorption/desorption, interaction of these surface species with bulk vacancies and diffusion of these species to the MIEC/electrolyte interface. Charge transfer may occur by diffusion of oxygen adsorbates to the triple phase boundary or by diffusion of vacancies from the electrolyte with subsequent exchange with oxygen adsorbed on the porous MIEC wall. A model describing the competition between these two charge transfer pathways is developed. Key to the model is treatment of the boundary condition at the MIEC/electrolyte interface. The absolute potential across this interface is used to relate the surface overpotential of each pathway in terms of the other and terms arising from the concentration overpotentials. Results show the conditions under which one process may dominate over the other.