Crystal-Orientation-Dependent Oxygen Exchange Kinetics on Mixed Conducting Thin-Film Surfaces Investigated by In Situ Studies

Abstract

The oxygen exchange kinetics and the surface chemistry of epitaxially grown, dense La 0.6 Sr 0.4 CoO 3−δ (LSC) thin films in three different orientations, (001), (110), and (111), were investigated by means of in situ impedance spectroscopy during pulsed laser deposition (i-PLD) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). i-PLD measurements showed that pristine LSC surfaces exhibit very fast surface exchange kinetics but revealed no significant differences between the specific orientations. However, as soon as the surfaces were in contact with acidic, gaseous impurities, such as S-containing compounds in nominally pure measurement atmospheres, NAP-XPS measurements revealed that the (001) orientation is substantially more susceptible to the formation of sulfate adsorbates and a concomitant performance decrease. This result is further substantiated by a stronger increase of the work function on (001)-oriented LSC surfaces upon sulfate adsorbate formation and by a faster performance degradation of these surfaces in ex situ measurement setups. This phenomenon has potentially gone unnoticed in the discussion of the interplay between the crystal orientation and the oxygen exchange kinetics and might have far-reaching implications for real solid oxide cell electrodes, where porous materials exhibit a wide variety of differently oriented and reconstructed surfaces.

Structural Characterization of La0.6Sr0.4CoO3−δ Thin Films Grown on (100)-, (110)-, and (111)-Oriented La0.95Sr0.05Ga0.95Mg0.05O3−δ

“…Here, it is not clear whether or not the ordering is present in the [010] direction since the [0-11] view direction only allowed us to examine the [100] direction. The similarity of all thin films in terms of oxygen content and ordering (see also the EELS analysis below) in the upper film regions is in agreement with our electrochemical results [15], which found no significant differences between the (oxygen vacancy-mediated) oxygen exchange kinetics of pristine, differently oriented films. Furthermore, in the LSC/(111) LSGM sample, there was a 10 nm thick region in which there was no brownmillerite ordering present.…”

“…LSGM is a perovskite that allows the epitaxial growth of LSC with moderate tensile in-plane strain. Furthermore, LSGM is an excellent oxygen ion electrolyte, and thus the system LSGM|LSC also enables a direct electrochemical characterization, which was performed in a separate study [15]. In that study, we could show that the oxygen exchange kinetics on pristine LSC surfaces did not strongly depend on crystal orientation.…”