Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material

Siebenhofer, Matthäus; Nenning, Andreas; Wilson, George E.; Kilner, John A.; Rameshan, Christoph; Kubicek, Markus; Fleig, Jürgen; Blaha, Peter

doi:10.1039/d3ta00978e

Cited by 7 publications

(10 citation statements)

References 83 publications

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“…Upon exposure of pristine surfaces to considerable gas phase pressures (> 10 −3 mbar), a new oxygen species appears at 532 eV, alongside a weak corresponding S species 19 . Consistent with earlier studies, where SO 2− 4 adsorbates induced a significant increase in the work function of LSC 13 , this phenomenon could be reproduced for PCO, albeit with a less pronounced increase. This is in line with the quantification of SO 2− 4 adsorbates from XPS data, revealing higher concentrations on LSC surfaces (55 % coverage on LSC vs. 25 % coverage on PCO), indicating the strong affinity of the basic LSC surface for acidic adsorbates.…”

Section: Tuning the Work Function With Ultra-thin Oxide Layerssupporting

confidence: 91%

“…While previous studies have used the Smith acidity of the decoration as a descriptor to predict these changes 9,13,14 , we introduce the ionic potential of surface cations as an alternative metric to describe surface acidity. Based on the definition by Cartledge 15 , we define the surface ionic potential φ as the ratio of ionic charge q and ionic radius r, averaged over the surface cation stoichiometry (with cation fractions x i )…”

Section: Tuning the Work Function With Ultra-thin Oxide Layersmentioning

confidence: 99%

“…2 for Sr 2+ in SrO). The value of pristine LSC was estimated from LEIS measurements 13 , for PCO the nominal stoichiometry (90 % Ce and 10 % Pr) was assumed.…”

Section: Tuning the Work Function With Ultra-thin Oxide Layersmentioning

confidence: 99%

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Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

Siebenhofer,

Nenning,

Rameshan

et al. 2023

Preprint

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Improving materials for energy conversion and storage devices is deeply connected with an optimization of the surfaces of these materials. Surface modification has therefore emerged as a very promising strategy on the way to enhance modern energy technologies. This study shows that surface modification with ultra-thin oxide layers allows for a systematic tailoring of surface properties, in particular of the surface dipole and the work function, and it introduces the ionic potential of surface cations as a readily accessible descriptor for these effects. The combination of X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) illustrates that basic oxides with a lower ionic potential than the host material induce a positive surface charge and reduce the work function of a mixed conducting host material and vice versa. The emerging surface dipoles are caused by changes in the energy landscape, leading to redistribution of electronic charge density and/or a geometric reorientation of the surface. As a proof of concept that this strategy is widely applicable to tailor surface properties, we examined the effect of ultra-thin decoration layers on the oxygen exchange kinetics of pristine thin films of structurally and chemically different mixed conducting oxides in very clean conditions by means of in-situ impedance spectroscopy during pulsed laser deposition (i-PLD). The extended study confirms that basic decorations with a reduced surface work function lead to a substantial acceleration of the oxygen exchange on a material's surface. Computational results suggest that the underlying mechanism of the kinetic improvement relies on a modification of the energetics of charged O2 adsorbates and substantiate their importance for the oxygen exchange reaction.

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Section: Tuning the Work Function With Ultra-thin Oxide Layerssupporting

confidence: 91%

Section: Tuning the Work Function With Ultra-thin Oxide Layersmentioning

confidence: 99%

See 1 more Smart Citation

Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

Siebenhofer,

Nenning,

Rameshan

et al. 2023

Preprint

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“…Hence, the evolution of charge carrier concentrations in such heterointerfaces is highly nontrivial and requires further computational investigation, going beyond the scope of this study. At this point, we also want to refer to a recent study on the effects of acidic adsorbates on LSC surfaces, combining experimental and computational approaches . Similar to acidic decorations, acidic adsorbates cause a significantly increased surface exchange resistance as well as an increased work function (negative surface charge) and charge redistribution in this system has largely been associated with oxygen atoms and charge transfer toward the adsorbate.…”

Section: Mechanistic Discussionmentioning

confidence: 99%

“…As a last noteworthy aspect, we suspect that surface decorations also influence reaction energetics such as adsorption equilibria, 49 further complicating the situation. This claim is again supported by recent computational results on acidic adsorbates on LSC, 51 where calculations revealed that acidic adsorbates cause strongly increased adsorption barriers for O 2 molecules and render O 2 adsorbates in surface vacancies energetically unfavorable.…”

Section: ■ Mechanistic Discussionmentioning

confidence: 99%

Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics

Riedl

Siebenhofer

Nenning

et al. 2023

ACS Appl. Mater. Interfaces

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The oxygen exchange kinetics of epitaxial Pr 0.1 Ce 0.9 O 2−δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO 2 , TiO 2 ) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO 2 dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

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Investigation of atomic-scale decorations on mixed conducting oxides via time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Fahrnberger,

Siebenhofer,

Hutter

et al. 2023

Applied Surface Science

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Electronic and ionic effects of sulphur and other acidic adsorbates on the surface of an SOFC cathode material

Abstract: A combined experimental and computational approach reveals electronic and ionic effects of acidic adsorbates on mixed conducting oxide surfaces.

Cited by 7 publications

References 83 publications

Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

Engineering surface dipoles on mixed conducting oxides with ultra-thin oxide decoration layers

Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics

Investigation of atomic-scale decorations on mixed conducting oxides via time-of-flight secondary ion mass spectrometry (ToF-SIMS)

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