Enhanced Oxygen Surface Exchange Kinetics and Stability on Epitaxial La_0.8Sr_0.2CoO_3−δ Thin Films by La_0.8Sr_0.2MnO_3−δ Decoration

Surface modification of perovskites is a new approach to develop highly active and stable cathodes for solid oxide fuel cells. Here, we report that La_0.8Sr_0.2MnO_3−δ (LSM82) surface decoration led to markedly enhanced activity and stability for surface exchange kinetics of the (001)_pseudocubic-oriented epitaxial La_0.8Sr_0.2CoO_3−δ (LSC82) thin films on (001)-oriented yttria-stabilized zirconia (YSZ). In-plane and out-of-plane strains of the LSC82 films at elevated temperatures determined from in situ high resolution X-ray diffraction were not influenced by LSM82 decoration. Atomic force microscopy and scanning electron microscopy analysis showed that the formation of secondary particles observed upon annealing on the undecorated LSC82 surface was eliminated by the LSM82 decoration, which was accompanied by increased strontium (Sr) on the surface as revealed by auger electron spectroscopy. The enhanced stability of LSM82-decorated LSC82 against surface decomposition is in good agreement with density functional theory (DFT) calculations that showed a considerable energy gain for Mn substitution in LSC82. The surface exchange coefficients (<i>k</i>^q) of LSC82, determined from electrochemical impedance spectroscopy, increased significantly with LSM82 coverage with average thicknesses up to ∼1 nm, while LSM82 thicknesses of ∼3.5 nm and greater reduced <i>k</i>^q. Moreover, LSM82 decoration increased the stability of LSC82 for oxygen surface exchange. Remarkably, the <i>k</i>^q of the LSC82 film with 0.9 nm-thick LSM82 coverage was not changed significantly over 70 h at 550 °C, after which time it exhibited activities 2 orders of magnitude higher than that of the undecorated LSC82. DFT calculations support the hypothesis that the enhanced oxygen surface exchange kinetics and stability of LSM82-decorated LSC82 can be attributed primarily to manganese (Mn) substitution in the LSC82 enabling the stabilization of higher Sr concentration in the perovskite structure near the surface as compared to the undecorated LSC82