First-Principles Analysis of Oxygen Incorporation into Sr(Ti1-xFex)O3-D : Using External Dopants to Tune the Incorporation Mechanism

Abstract

Although solid oxide fuel cells have been under intensive investigation for several decades, their practical development is hindered still by several fundamental challenges. One in particular is related to the kinetics of the electrode reactions, especially with respect to oxygen exchange at the cathode. Oxygen exchange requires chemisorption, dissociation, and incorporation, and can involve a complicated set of competing processes: for example, it may occur through directly via gas phase oxygen interaction with a surface vacancy through an active site-mediated mechanism, or it may occur directly onto a pristine surface with vacancies in the interior serving only as the final sink for the incorporated oxygen species. Often, resolving the rates of the relevant processes is challenging. The material Sr(Ti1-xFex)O3- d(STF) is a model system for the study of oxygen exchange mechanisms on electrodes of solid oxide electrodes. The perovskite material is stable in a wide range of oxygen partial pressures, and exhibits a flexible defect chemistry and electronic structure. The B-site substitution of Ti for Fe results in fast oxygen exchange kinetics. Even so, there remain several questions about the rate limiting steps to oxygen exchange in these materials, with contradictory suggestions that oxygen incorporation is limited by either electron transfer to the adsorbed molecules at the electrode surface or by the vacancy concentration at the surface. In this work, we will present the results of our first-principles calculations to provide some clues to the nature of oxygen incorporation in STF. We use density functional theory to consider several incorporation mechanisms under different environmental conditions. In particular, we consider how the presence of external dopants (eg A-site doping with La) might allow access to different ranges of oxygen vacancy concentrations and carrier concentrations in the material. We will show (i) the results of a defect chemistry equilibrium model to calculate the equilibrium defect concentrations and carrier concentrations in different chemical environments and under different degrees of extrinsic doping, and (ii) the reaction mechanism for oxygen molecule adsorption at the surface for different degrees of doping. The defect chemistry model shows that the concentration of oxygen vacancies and the Fermi level of the material can be tuned by the incorporation of external dopants. Further, according to our simulations, we predict that the most favorable pathway for oxygen incorporation can be controllably changed from an active-site mediated mechanism to a direct adsorption mechanism by appropriately tuning the dopant concentration.