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

The oxygen exchange kinetics and the surface chemistryof epitaxiallygrown, dense La0.6Sr0.4CoO3-delta (LSC) thin films in three different orientations, (001), (110),and (111), were investigated by means of in situ impedancespectroscopy during pulsed laser deposition (i-PLD) and near-ambient-pressureX-ray photoelectron spectroscopy (NAP-XPS). i-PLD measurements showedthat pristine LSC surfaces exhibit very fast surface exchange kineticsbut revealed no significant differences between the specific orientations.However, as soon as the surfaces were in contact with acidic, gaseousimpurities, such as S-containing compounds in nominally pure measurementatmospheres, NAP-XPS measurements revealed that the (001) orientationis substantially more susceptible to the formation of sulfate adsorbatesand a concomitant performance decrease. This result is further substantiatedby a stronger increase of the work function on (001)-oriented LSCsurfaces upon sulfate adsorbate formation and by a faster performancedegradation of these surfaces in ex situ measurementsetups. This phenomenon has potentially gone unnoticed in the discussionof the interplay between the crystal orientation and the oxygen exchangekinetics and might have far-reaching implications for real solid oxidecell electrodes, where porous materials exhibit a wide variety ofdifferently oriented and reconstructed surfaces.

Automated summary

The oxygen exchange kinetics and surface chemistry of dense LSC thin films on different orientations were investigated using in-situ impedance spectroscopy and NAP-XPS. The results showed that the exchange kinetics on pristine LSC surfaces were fast and independent of orientation. However, in the presence of acidic, gaseous impurities, the (001) orientation was more susceptible to the formation of sulfate adsorbates and performance degradation. This finding has implications for solid oxide cell electrodes with porous materials.