Cation interdiffusion model for enhanced oxygen kinetics at oxide heterostructure interfaces

Abstract

An interface between the perovskite La_0.8Sr_0.2CoO_3−δ (LSC-113) and the K₂NiF₄-type (La_0.5Sr_0.5)₂CoO_4−δ (LSC-214) heterostructure was recently shown to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest to develop better optimized materials for solid-state electrochemical devices, e.g. solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ∼0.9 eV (∼1.3 eV) energy gain for exchanging a Sr from LSC-113(25%Sr) (LSC-113(40%Sr)) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that the Sr concentrations (in the LSC-214 phase) in a typical experimental temperature range of 500–600 °C and in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr), may be about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase (Vashook et al., Solid State Ionics, 2000, 138, 99–104), an Sr enrichment from x = 0.5 to x = 0.75 in (La_1−xSr_x)₂CoO_4−δ is expected to enhance the oxygen vacancy concentration by 2–2.5 orders of magnitude under typical experimental conditions. An increased vacancy concentration in LSC-214 near the interface can explain most of the enhanced oxygen kinetics observed up until now in these heterostructures.