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The changes of the surface morphology and the surface chemistry of LSC thin films grown on different substrates were tracked for 100 hours under SOFC operation conditions. Atomic force microscopy was used to monitor the formation of particles at the LSC surface. Depending on the thin film structure (polycrystalline vs. epitaxial), different particle formation dynamics were observed. Electron microscopy was employed to investigate the chemistry of the segregated particles and revealed that the particles were Srand S-rich. Secondary ion mass spectrometry and X-ray photoelectron spectroscopy measurements were performed on degraded LSC thin films, which also found significant amounts of sulfur on the LSC surface, despite no deliberate addition of sulfur compounds, as well as A-site cation enrichment. Impedance spectroscopy was used to track the polarization resistance of LSC grown on YSZ over the same degradation period and a strong increase inthe polarization resistance and in its activation energy was revealed (1.09 to 1.73 eV). The experimental results indicate that sulfur adsorption on LSC surfaces is omnipresent in the investigated conditions and even trace amounts of sulfur compounds present in nominally pure measurement gases account for particle formation and multiple degradation effects under operating conditions. [ABSTRACT FROM AUTHOR]

The polarization resistance of La0.6Sr0.4FeO3–δ (LSF) thin film electrodes was investigated by electrochemical impedance spectroscopy. Different states were considered, ranging from pristine films right after deposition to films degraded under various conditions. Impedance measurements performed inside the pulsed laser deposition chamber (in-situ) revealed the existence of a very facile, low resistive state of as-deposited thin films with up to two orders of magnitude lower resistances compared to conventional ex-situ measurements and literature data. Ex-situ and in-situ measurements show different pO2 dependencies of polarization resistances, suggesting a different oxygen exchange reaction mechanism on the as-deposited films. The time dependence of the polarization resistance indicates that two degradation mechanisms with different timescales are responsible for the much higher resistances found in ex-situ measurements. Degradation mechanisms are discussed and conclusions with respect to the rate determining step of oxygen exchange on different surfaces are drawn. [ABSTRACT FROM AUTHOR]

Reduction-stable mixed ionic and electronic conductors such as Sr(Ti,Fe)O3-δ (STF) are promising materials for application in anodes of solid oxide fuel cells. The defect chemistry of STF and its properties as solid oxide fuel cell (SOFC) cathode have been studied thoroughly, while mechanistic investigations of its electrochemical properties as SOFC anode material are still scarce. In this study, thin film model electrodes of STF with 30% and 70% Fe content were investigated in H2+H2O atmosphere by electrochemical impedance spectroscopy. Lithographically patterned thin film Pt current collectors were applied on top or beneath the STF thin films to compensate for the low electronic conductivity under reducing conditions. Oxygen exchange resistances, electronic and ionic conductivities and chemical capacitances were quantified and discussed in a defect chemical model. Increasing Fe content increases the electro-catalytic activity of the STF surface as well as the electronic and ionic conductivity. Current collectors on top also increase the electrochemical activity due to a highly active Pt-atmosphere-STF triple phase boundary. Furthermore, the electrochemical activity depends decisively on the H2:H2O mixing ratio and the polarization. Fe0 nanoparticles may evolve on the surface in hydrogen rich atmospheres and increase the hydrogen adsorption rate. [ABSTRACT FROM AUTHOR]

The defect chemistry and charge transport properties of La0.95Sr0.05Ga0.95Mg0.05O3-δ (LSGM) single crystals, grown by the Czochralski technique, were studied by impedance spectroscopy performed over a wide temperature range (180-800°C) and oxygen partial pressures from 0.21 to 1013 mbar. Owing to ion blocking Pt thin film electrodes impedance data showed a Warburg type 45° slope in the complex impedance plane. The spectra were analyzed by means of a generalized transmission line model which yielded the electronic and ionic conductivity, chemical and dielectric capacitance and the oxygen chemical diffusion coefficient. Activation energies for electronic (0.89 eV) and ionic charge transport (0.95 eV at low and 0.56 eV at high temperatures) were determined. Oxygen partial pressure dependent measurements revealed that the p-type conductivity is exactly proportional to pO21/4 and that the ionic transference number approaches 1 for low oxygen partial pressures and low temperatures. Additional Hebb-Wagner type DC polarization experiments were carried out to determine the n- and p-type conductivities in a broad chemical potential range and to verify the results obtained by the AC impedance measurements. [ABSTRACT FROM AUTHOR]

Owing to its mixed ionic and electronic conductivity and high thermochemical stability, La0.6Sr0.4FeO3-δ (LSF64) is an attractive electrode material in solid oxide fuel/electrolysis cells (SOFCs/SOECs). Well defined thin film microelectrodes are used to compare the electrochemical properties of LSF64 in oxidizing and reducing conditions. The high electronic sheet resistance in hydrogen can be overcome by the use of an additional metallic current collector. With the sheet resistance being compensated, the area specific electrode resistance is similar in humidified hydrogen and oxygen containing atmospheres. Analysis of the chemical capacitance and the electrode resistance for current collectors on top and beneath the LSF64 thin film allow mechanistic conclusions on active zones and bulk defect chemistry. Cyclic gas changes between reducing and oxidizing conditions, performed on macroscopic LSF64 thin film electrodes with top current collector, reveal a strong degradation of the surface kinetics in synthetic air with very fast recovery in reducing atmosphere. Additional in-situ high-temperature powder XRD on LSF64 demonstrates the formation of small amounts of iron oxides in humidified hydrogen at elevated temperatures. [ABSTRACT FROM AUTHOR]

Pulsed laser deposited La0.6Sr0.4CoO3-δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) single crystals were investigated by impedance spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. At high deposition temperatures and after thermal annealing an inhomogeneous cation distribution was detected in the surface-near region, most manifest in a significant Sr enrichment at the surface. An activating effect of chemical etching of LSC is described, which can lower the polarization resistance by orders of magnitude. Chemistry behind this activation and thermal degradation was analyzed by ToF-SIMS and ICP-OES measurements of in-situ etched LSC films. The latter allow quantitative depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate the accompanying changes in surface morphology. All measurements suggest that stoichiometric LSC surfaces intrinsically exhibit very high activity towards oxygen reduction. [ABSTRACT FROM AUTHOR]

In the context of the search for fundamental size effects regarding electrical conduction, we analyze the transition from semi-infinite or isolated to overlapped depletion layers in appreciably dense nanocrystalline SrTiO3 ceramics occurring if the grain size becomes smaller than ∼ 100 nm. At these small grain sizes, the bulk impedance signal that is seen clearly for microcrystalline samples disappears, leaving only one impedance response, which can be attributed to the space-charge zones. Space-charge potentials and Mott—Schottky lengths are calculated and shown to be consistent with the assumption of the mesoscopic condition. In accordance with the increased homogeneity of the mesoscopic situation in an 80 nm sample, and unlike for microcrystalline samples, the impedance response is characterized by almost an ideal capacitance that no longer originates from space-charge polarization. These conclusions are corroborated by detailed numerical calculations using linear irreversible thermodynamics. [ABSTRACT FROM AUTHOR]