La0.6Sr0.4CoO3–δ (LSC) thin‐film electrodes are prepared on yttria‐stabilized zirconia (YSZ) substrates by pulsed laser deposition at different deposition temperatures. The decrease of the film crystallinity, occurring when the deposition temperature is lowered, is accompanied by a strong increase of the electrochemical oxygen exchange rate of LSC. For more or less X‐ray diffraction (XRD)‐amorphous electrodes deposited between ca. 340 and 510 °C polarization resistances as low as 0.1 Ω cm2 can be obtained at 600 °C. Such films also exhibit the best stability of the polarization resistance while electrodes deposited at higher temperatures show a strong and fast degradation of the electrochemical kinetics (thermal deactivation). Possible reasons for this behavior and consequences with respect to the preparation of high‐performance solid oxide fuel cell (SOFC) cathodes are discussed.

Improvement of solid oxide fuel cells strongly relies on the development of cathode materials with high catalytic activity for the oxygen reduction reaction. Excellent activity was found for perovskite-type oxides such as LaSrCoO (LSC), but performance degradation, probably caused by surface composition changes, hinders exploitation of the full potential of LSC. This study reveals that the potentially very high activity of the LSC surface can be traced back to few very active sites. Already tiny amounts of SrO, for example, 4% of a monolayer, deposited on an LSC surface, lead to severe deactivation. Co, on the other hand, causes (re-)activation, suggesting that active sites are strongly related to Co being present at the surface. These insights could be gained by a novel method to measure changes of the electrochemical performance of thin film electrodes in situ, while modifying their surface: impedance spectroscopy measurements during deposition of well-defined fractions of monolayers of Sr-, Co- and La-oxides by single laser pulses in a pulsed laser deposition chamber.

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4FeO3-δ (LSF), and SrTi0.7Fe0.3O3-δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase.

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Exsolution generates stable and catalytically active metal nanoparticles via phase precipitation out of a host oxide. An ability to control the size and dispersion of the exsolution particles is desirable for design of nanostructured (electro)catalysts. Here, we demonstrate that tuning point defects by lattice strain affects both the thermodynamics and the kinetics of iron (Fe 0 ) exsolution on La 0.6 Sr 0.4 FeO 3 (LSF) thin film model. By combining in situ surface characterization and ab initio defect modeling, we show oxygen vacancy and Schottky defects to be the primary point defects formed upon Fe 0 exsolution. Lattice strain tunes the formation energy, and thus the abundance of these defects, and alters the amount and size of the resulting exsolution particles. In addition, we find that the density of exsolved nanoparticles matches the concentration of oxygen vacancy pairs, thus pointing to the surface oxygen vacancy pairs as preferential nucleation sites for exsolution. The tensile-strained LSF with a facile formation of these critical point defects results in a higher Fe 0 metal concentration, a larger density of nanoparticles, and a reduced particle size at its surfaces. These results provide important mechanistic insights and highlight the role of point-defect engineering in designing nanostructured catalysts in energy and fuel conversion technologies.

In the search for optimized cathode materials for high-temperature electrolysis, mixed conducting oxides are highly promising candidates. This study deals with fundamentally novel insights into the relation between surface chemistry and electrocatalytic activity of lanthanum ferrite based electrolysis cathodes. For this means, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were performed simultaneously on electrochemically polarized La0.6Sr0.4FeO3−δ (LSF) thin film electrodes. Under cathodic polarization the formation of Fe0 on the LSF surface could be observed, which was accompanied by a strong improvement of the electrochemical water splitting activity of the electrodes. This correlation suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may open novel paths in the search for electrodes with increased water splitting activity.

a Cation diffusion was investigated in La 0.6 Sr 0.4 CoO 3Àd (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 1C. Isotopic ( 86 Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi-and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains -diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size.A-and B-site cations showed very similar temperature dependencies with activation energies of B3.5 eV for bulk and B4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.

The temperature dependent conductivity of yttria stabilized zirconia (YSZ) exhibits a bending in Arrhenius' plots which is frequently discussed in terms of free and associated oxygen vacancies. However, the very high doping concentration in YSZ leads to such a strong defect interaction that the concept of free vacancies becomes highly questionable. Therefore, the temperature dependent conductivity of YSZ is reconsidered. The conductivity of YSZ with different doping concentration was measured in a broad temperature range. The data are analyzed in terms of two different barrier heights that have to be passed along an average path of an oxygen vacancy in YSZ (two barrier model). For 8-10 mol% yttria, the two barriers are in the range of 0.6 eV and 1.1-1.2 eV, respectively. The conductivity and thus the barrier heights also depend on the cooling rate after a high temperature pre-treatment. This indicates that different frozen-in distributions of dopants affect the vacancy motion by different energy landscapes. Temporarily existing defect configurations, possibly with a strong effect of repulsive oxygen vacancy interaction, are suggested as the reason of high barriers. Future dynamic ab-initio calculations may reveal whether this modified model of the YSZ conductivity is mechanistically meaningful. Yttria stabilized zirconia (YSZ) is among the most important ion conducting solids and acts as a kind of model material representing fast oxide ion conductors. Owing to this model character of YSZ, but also due to its application in solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs) and oxygen sensors, a vast amount of papers can be found dealing with its oxide ion conduction. The ionic conductivity is based on the motion of oxygen vacancies, introduced by Y 3+ ions replacing Zr 4+ . For concentrations above ca. 8 mol%, yttria doping also stabilizes the cubic structure down to room temperature. A detailed review of the science of YSZ and related materials is far beyond the scope of this paper but a few important facts regarding the ionic conductivity of zirconia-based solid electrolytes can be briefly summarized as follows:1-9 i) Doping concentrations above ca. 8 mol% Y 2 O 3 lead to a decrease of the conductivity, despite increasing oxygen vacancy concentration. ii) The conductivity not only depends on the vacancy concentration but also on the kind of dopant. For example, Sc-doped zirconia shows significantly higher conductivity than YSZ. iii) The temperature dependence of the conductivity cannot be described by a single activation energy (E act ) but shows higher E act values at lower temperatures.Numerous theoretical studies were performed in order to understand these experimental observations and to get a deeper insight into defect thermodynamics and kinetics of doped zirconia and of the closely related ceria-based ion conductors, see e.g. Refs. 10-29. Those model studies employed different simulation approaches such as molecular dynamics (MD), density functional theory (DFT) and kinetic Monte Carl...