La1-xSrxMnO3±δ as a nonstoichiometric model system for the catalysis of oxygen evolution reaction

The main issue of electrochemical water splitting is the search for suitable catalyst materials for the kinetically hindered oxygen evolution reaction (OER) at the anode. The state-of-art precious metal catalysts suffer from their high price and insufficient long term stability in the common electrolytes. Binary or multinary transition metal oxides in alkaline medium are an cost-efficient alternative, since they can achieve both lower overvoltages as well as better long-term stability than precious metal oxides. One of the best-studied materials for SOFC applications is the perovskite system La1-xSrxMnO3±δ (LSMO). Due to its general nonstoichiometry and the large variety of possible defect species it is a perfect model system for studying the influence of defect chemistry on the catalytic activity in OER. We have systematically investigated LSMO as a nonstoichiometric model catalyst system for OER in alkaline media. Nanocrystalline powders over the whole composition range have been prepared by a sol-gel based auto combustion method. XPS and XRD analysis verified the presence of pure phase materials with a continuous change of manganese oxidation state from Mn3+ to Mn4+. Measurements in an electrochemical RDE setup showed a clear trend in catalytic activity in OER with the highest values at medium La/Sr compositions. An equivalent trend could also be observed in the electrical conductivity of the powders, leading to the assumption of a higher polaron hopping probability at medium La/Sr compositions. Additional annealing of pristine powder samples in oxidizing and reducing atmospheres caused a further change in manganese oxidation state and ongoing electrochemical measurements should reveal whether the defined adjustment of the nonstoichiometry will lead to an improvement of catalytic activity compared to the untreated catalysts

Dorfzerstörung und Relokalisierung durch Braunkohletagebau in konzeptionellen biografischen und kollektiven Kontexten von Raum, Ortsbezügen und Nachbarschaft

Magdeburg, Univ., Fak. für Geistes-, Sozial- und Erziehungswiss., Diss., 2009von Heidrun Gode-Luerße

Electrochemical activation of molecular nitrogen at the Ir/YSZ interface.

Nitrogen is often used as an inert background atmosphere in solid state studies of electrode and reaction kinetics, of solid state studies of transport phenomena, and in applications e.g. solid oxide fuel cells (SOFC), sensors and membranes. Thus, chemical and electrochemical reactions of oxides related to or with dinitrogen are not supposed and in general not considered. We demonstrate by a steady state electrochemical polarisation experiments complemented with in situphotoelectron spectroscopy (XPS) that at a temperature of 450 °C dinitrogen can be electrochemically activated at the three phase boundary between N2, a metal microelectrode and one of the most widely used solid oxide electrolytes—yttria stabilized zirconia (YSZ)—at potentials more negative than E = −1.25 V. The process is neither related to a reduction of the electrolyte nor to an adsorption process or a purely chemical reaction but is electrochemical in nature. Only at potentials more negative than E = −2 V did new components of Zr 3d and Y 3d signals with a lower formal charge appear, thus indicating electrochemical reduction of the electrolyte matrix. Theoretical model calculations suggest the presence of anionic intermediates with delocalized electrons at the electrode/electrolyte reaction interface. The ex situSIMS analysis confirmed that nitrogen is incorporated and migrates into the electrolyte beneath the electrode

Auxin-induced WUS expression is essential for embryonic stem cell renewal during somatic embryogenesis in Arabidopsis

Somatic embryogenesis requires auxin and establishment of the shoot apical meristem (SAM). WUSCHEL (WUS) is critical for stem cell fate determination in the SAM of higher plants. However, regulation of WUS expression by auxin during somatic embryogenesis is poorly understood. Here, we show that expression of several regulatory genes important in zygotic embryogenesis were up-regulated during somatic embryogenesis of Arabidopsis. Interestingly, WUS expression was induced within the embryonic callus at a time when somatic embryos could not be identified morphologically or molecularly. CorrectWUS expression, regulated by a defined critical level of exogenous auxin, is essential for somatic embryo induction. Furthermore, it was found that auxin gradients were established in specific regions that could then give rise to somatic embryos. The establishment of auxin gradients was correlated with the induced WUS expression. Moreover, the auxin gradients appear to activate PIN1 polar localization within the embryonic callus. Polarized PIN1 is probably responsible for the observed polar auxin transport and auxin accumulation in the SAM and somatic embryo. Suppression of WUS and PIN1 indicated that both genes are necessary for embryo induction through their regulation of downstream gene expression. Our results reveal that establishment of auxin gradients and PIN1-mediated polar auxin transport are essential for WUS induction and somatic embryogenesis. This study sheds new light on how auxin regulates stem cell formation during somatic embryogenesis