Electrochemistry of mixed oxygen ion and electron conducting electrodes in solid electrolyte cells

Mixed ion and electron conductors (MIECs) have garnered increased attention as active components in the electrodes of solid oxide electrolyzers (for electricity to fuel conversion) and especially of solid oxide fuel cells (for fuel to electricity conversion). Although much of the work in the literature is directed toward the understanding of oxygen electroreduction on the surfaces of MIECs, more recent studies also explore the role of these materials in fuel electrooxidation. In both cases, the rich chemical and electronic behaviors of MIECs imply a broad range of possible reaction pathways. We highlight the significant progress that has been made in elucidating these pathways through well-designed experimental and computational studies. At the macroscopic level, patterned electrode studies enable identification of active sites, whereas at the microscopic level, surface-sensitive techniques in combination with atomistic-level simulations are beginning to reveal the nature of the rate-determining step(s) and enable rational design of materials with enhanced activity.