First Principles Insights Into Oxygen Transport in Solid Oxide Fuel Cell Cathode Materials Based on Lanthanum Strontium Cobalt Ferrite

Abstract

Lowering operating temperatures below 800 °C is critical for the viability of solid oxide fuel cell (SOFC) technology. Mixed ion-electron conducting (MIEC) cathodes increase the active region of the cathode by enabling bulk transport of oxygen ions. La1-xSrxCo1-yFeyO3 (LSCF) is the most common MIEC cathode; however, the properties of LSCF vary dramatically based on its elemental composition. Understanding the link between LSCF’s composition, electronic structure, and defect chemistry enables rational material design. This thesis applies density functional theory (DFT)-based methods to members of the LSCF family, linking oxygen ion transport to their electronic structure.

We show that creating oxygen vacancies in LaFeO3 is more endothermic than in LaCoO3 because Fe-O bonds are stronger than Co-O bonds and reducing Fe3+ to Fe2+ releases less energy than Co3+ to Co2+. Holes introduced by La vacancies in LaFeO3 increase the oxygen vacancy concentration by several orders of magnitude. The low-spin state of Co3+ in LaCoO3 leads to the lowest oxygen vacancy formation energy among its near-degenerate magnetic configurations.

Holes arising from Sr substitutions in La1-xSrxFeO3 (LSF) and La1-xSrxCoO3 (LSC) partially delocalize over the Fe/Co and O sublattices. The extent to which the oxygen sublattice accepts the electrons left behind during oxygen vacancy formation is greater in LSF than LSC, leading to lower oxygen vacancy formation energies in LSF than in LSC.

Modeling LSCF yields the important insight that oxygen vacancies preferentially form between one Fe and one Co. The least favorable sites for oxygen vacancies are between two Co ions. Hence, too much Co in LSCF leads to too many unfavorable sites for oxygen vacancy formation and to lower ionic conductivity. Insufficient Co in LSCF will yield a semiconducting electronic structure like LSF instead of the half-metallic behavior we predict for yCo = 0.25. Fe-rich compositions should be preferred for LSCF and the commonly used La0.6Sr0.4Co0.2Fe0.8O3 is close to the optimal composition.

This thesis establishes important guidelines for optimizing LSCF as a SOFC cathode. Choosing an Fe-rich composition of LSCF, employing A-site substoichiometry, and finding a way to encourage low-spin Co3+ to form are critical directions enabling development of improved intermediate-temperature SOFC devices.