Substrate and Thickness Effects on the Oxygen Surface Exchange of La_0.7Sr_0.3MnO₃ Thin Films

Substrate- and thickness-related effects on the oxygen surface exchange of La_0.7Sr_0.3MnO₃ (LSM) thin films were investigated to understand better cathode reactivity in solid oxide fuel cells. Epitaxial (100)-oriented LSM films were fabricated on (100)-SrTiO₃ and (110)-NdGaO₃ substrates and were characterized using electrical conductivity relaxation. A strong substrate effect on the chemical surface exchange coefficient (<i>k</i>_chem) was observed, with a higher <i>k</i>_chem found for films on SrTiO₃ than those on NdGaO₃. Two distinct activation energies (<i>E</i>_a) were observed for <i>k</i>_chem, which were assigned to two parallel exchange processes; the relative contributions from each depended on the substrate, film thickness, and temperature. For films coherently strained to the substrates, <i>k</i>_chem values differed by almost an order of magnitude, whereas <i>E</i>_a was ∼1.5 (± 0.1) eV on both substrates. For relaxed films, <i>k</i>_chem values differed only by a factor of 2, and <i>E</i>_a was ∼0.75 (± 0.1) eV on both substrates. We discuss the strain effect relative to the native surface exchange and the thickness effect relative to the extended defect populations in the films. The outcome of this study sheds light on how microstructural features affect surface chemistry in modified cathodes