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An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere; stars of low temperature might have outer atmospheres containing compound molecules. The atmosphere of Earth is composed of nitrogen (78 %), oxygen (21 %), argon (0.9 %), carbon dioxide (0.04 %) and trace gases. Most organisms use oxygen for respiration; lightning and bacteria perform nitrogen fixation to produce ammonia that is used to make nucleotides and amino acids; plants, algae, and cyanobacteria use carbon dioxide for photosynthesis. The layered composition of the atmosphere minimises the harmful effects of sunlight, ultraviolet radiation, solar wind, and cosmic rays to protect organisms from genetic damage. The c

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010

Micro-tubular Solid Oxide Fuel Cells with Nickelate Cathode-support

Henning Lübbe

The shortage of natural resources, rising energy demands and global warming have created an urgent need for more efficient energy conversion devices and different energy resources to achieve a more sustainable and efficient economy. Solid Oxide Fuel Cells (SOFC) are one of the most promising devices for the direct conversion from chemical to electrical energy in conjunction with high system efficiencies. Micro-tubular SOFC have been investigated since roughly 15 years. In contrast to planar systems, which produce electricity in the kW to MW range and target utilisation in stationary applications such as CHP plants, micro-tubular SOFC systems can be envisaged for mobile application due to quick start-up/shutdown operation. While most micro-tubular cells are mechanically supported by the electrolyte or anode, only few researches have investigated cathode-supported cells due to challenging fabrication. In this work, the new cathode material neodymium nickelate (NNO), Nd1.95NiO4+δ, was used as mechanical support, which is a novelty of this work, for micro-tubular SOFC. Different fabrication routes were used to create small tubes of 2-6 mm diameter. These tubes were evaluated with respect to their gas-diffusion properties by a new, self-manufactured diffusion setup and a complementary permeation setup. In order to achieve an economical fabrication process, the low-cost dip-coating method was investigated for the deposition of thin films from wet-chemical suspensions to create entire SOFC single cells. As SOFCs require gas-tight electrolyte layers, cofiring of tubular substrates and thin films must be executed to allow sufficient shrinkage of the deposited electrolyte thin layers to full density. Various electrolyte and electrode powders were characterised in different solvents, dispersants and binders with respect to low-temperature sintering feasibility. The shrinkage of scandia-stabilised zirconia (ScSZ) powder was decreased to 18% to still deliver sufficient sintered density by optimisation of the green density (usually zirconia requires shrinkage of 25% and more for densification). The cofiring temperature depends on the densification behaviour of powders but also on reactivity between adjacent layers. The reactivity between gadolinia-doped ceria (GDC)-interlayers and different zirconia materials was tested and showed lower compatibility of yttria-stabilised zirconia (YSZ) with GDC than for ScSZ/GDC reaction couples, which has not been reported so far. The interface of NNO cathodes and GDC interlayers was analysed by reactivity mapping and quantitative phase analysis. The performance optimisation of NNO/GDC interfaces by exchange current density measurement of NNO-cathodes sintered at different temperatures on GDC pellets is a further contribution of this work. The reactivity and performance investigations provide clear guidelines for maximum allowable cosintering temperature. Some electrochemical single cell tests showed competitive area-specific resistances (ASR) of ca. 1 Ωcm2 but were limited unfortunately by the counterelectrode microstructure and current collection methods, which were not the focus of this work. Open-circuit voltages (OCV) were 0.6-1.1 V at 700 °C in dry hydrogen atmosphere for tubes prepared by extrusion but only 0.3 V for tubes prepared by slip-casting. The electrochemical performances were modest with only 0.035 W/cm2 power density at 700 °C for tubes prepared by extrusion due to the elevated resistances of the counterelectrode and current collection.

EPFL2012