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

The oxygen exchange kinetics and the surface chemistry of epitaxially grown, dense La0.6Sr0.4CoO3−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.


S.I. 2. LSC grown on YSZ/GDC
HRTEM images of LSC grown on (001) and (111) oriented YSZ with a GDC buffer layer reveal epitaxial growth for the (001) direction with no indications of grain boundaries. Signs of a brownmillerite phase are visible, which fade around 8-10 nm from the interface. For (111) oriented YSZ, the GDC buffer layer again grows epitaxially, however, LSC exhibits different domains, indicating columnar growth with in-planed rotated grains.

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Crystal orientation dependent oxygen exchange kinetics on mixed conducting thin film surfaces -S-4/8

S.I. 3. Surface Morphology
The surface morphology of LSC grown in three orientations on LSGM (001), LSGM (110) and LSGM (111) directly after deposition. The macroscopic structures that can be seen on the surface originate from twin structures and crystal domains in the substrate.

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Crystal orientation dependent oxygen exchange kinetics on mixed conducting thin film surfaces -S-5/8

S.I. 4. Sample comparison
To warrant compaptibility of in-situ and ex-situ measurements, the surface exchange resistance of 40 nm thick LSC thin films grown on different substrates at 600 • C during i-PLD measurements in 0.04 mbar O 2 atmosphere was compared. Average resistance values and the corresponding standard errors are shown for several measurements (6 for LSC grown on YSZ, 3 for LSC grown on GDC/YSZ and 2 for LSC grown on LSGM). All three thin films exhibit similar surface exchange resistance values with the fastest kinetics observed for LSC on LSGM . We strongly suspect that this is caused by the comparatively high tensile strain for LSC grown on LSGM which is known to accelerate oxygen exchange 1 . S.I. 6. XPS details II Figure S 6. left: Fermi edge region of pristine, 001-oriented and 110-oriented LSC thin film surfaces, as well as of a polycrystalline (preferentially 110-oriented) surface for comparison. All three measurements show very similar Fermi edge regions, indicating the similar band structure in all three cases. right: S2p region for different LSC orientations before and after exposure to 1 mbar O 2 and subsequent sulphate adsorbate formation. It is noteworthy that quantification of SO 4 species via the O1s area is much more precise, as the signal intensity is 8 times as high as for S2p (2x cross section and 4x stoichiometric factor).

S.I. 7. LSC cation composition and surface reconstructions
When we compare the surface compositions of differently oriented LSC films we find that all surfaces right after deposition exhibit more Sr and La, and less Co (36-39 %), compared to 50 % in the nominal stoichometry. Substantial Co depletion of the LSC surface due to A-site cation segregation was already observed before in literature. Interestingly, the surface composition is very stable regarding the different substrate orientations and atmospheric annealing steps -presumably this is linked to the short 1 mbar annealing time of 20-30 minutes -for longer annealing times, strong Sr segregation on polycrystalline LSC films was observed previously in literature 2 . Possibly, the atomic composition of the surface layer does not correspond to a perovskite bulk-plane, and it also might be similar for all three film orientations, accounting for the consistent work functions and XPS compositions.

Figure S 7.
Composition of the topmost 2.2 nm of differently oriented LSC thin films; Peak areas were normalized to 100%, and corrected for the inelastic mean free path (IMFP) length (KE exponent 0.7) and experimentally derived sensitivity factors by Christ 3 . For better quantification, peaks with similar kinetic photoelectron energy (La 4d, Sr 3d, Co 3p) were chosen, in order to minimize the effects of IMFP variation and the analyser transmission function.