Influence of ionic conductivity of the nano-particulate coating phase on oxygen surface exchange of La0.58Sr0.4Co0.2Fe0.8O3−δ

Abstract

The oxygen surface exchange kinetics of mixed-conducting perovskite La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) ceramics coated with a porous nano-particulate layer of either gadolinea (Gd₂O₃), ceria (CeO₂) or 20 mol% Gd-doped ceria (GCO) was determined by electrical conductivity relaxation (ECR). The measurements were performed in the temperature range 700–900 °C, following pO₂-step changes between 0.2 and 0.4 atm. The apparent value of the surface exchange coefficient, k_chem, is found to vary with the loading amount and ionic conductivity of the coated phase whilst, as expected, the chemical diffusion coefficient D_chem remains invariant with the applied coating. Partial coverage of the LSCF surface with non-ionic conductive Gd₂O₃ or CeO₂ lowers the value of k_chem relative to that observed for bare LSCF, which is attributed to surface blocking effects. In contrast, partial coverage of LSCF with GCO electrolyte particles enhances the apparent value of k_chem up to a factor of ∼6 compared to bare LSCF. The data of pulse isotope exchange (PIE) measurements show that the surface exchange reaction on bare LSCF is predominantly limited by dissociative adsorption of O₂. Different mechanisms for the improved oxygen surface exchange kinetics after partially covering the LSCF surface with GCO are discussed.