Glass-Forming Exogenous Silicon Contamination in Solid Oxide Fuel Cell Cathodes

Spatially resolved analyses, by energy-dispersive X-ray spectroscopy (EDS) scanning electron microscopy (SEM), allowed the quantification of exogenous Si contamination in a solid oxide fuel cell (SOFC) cathode after operation. The Si quantification, taking into account the endogenous Si impurity level, correlated well with the expectation from the condensation of Si(OH)4 vapor, originating from upstream alloy components and saturated in the hot inlet air. At higher resolution, EDS-transmission electron microscopy (TEM) pointed out the deposition of Si vapor in the form of amorphous SiO2, blocking oxygen incorporation into the electrolyte phase within a composite SOFC cathode.

Self-Cleaning cotton textiles surfaces modified by photoactive SiO2/TiO2 coating

Danièle Laub

The photocatalytic activity of TiO2–SiO2-coated cotton textiles was investigated through the self-cleaning of red wine stains. It was shown that a TiO2–SiO2 species could be produced at temperatures of 100 °C with acceptable photo-activity on non-heat resistant materials. The most suitable Ti-content of the coating was found to be 5.8% and for SiO2, the content was 3.9% (w/w). The discoloration of red wine led to CO2 evolution that was more efficient for TiO2–SiO2-coated cotton for samples than of TiO2-coated ones. The reasons for these results are discussed. The textile surface did not show any change after several consecutive light-induced discoloration cycles of a red wine stain. By high-resolution transmission electron microscopy (HRTEM), the TiO2–SiO2 layer thickness on the cotton fibers was detected to 20–30 nm. The TiO2 and SiO2 were both observed to have particle sizes between 4 and 8 nm. Further electron microscopy work coupled with energy dispersive spectroscopy (EDS) showed that the Ti-particles were always surrounded by amorphous SiO2 and never alone by themselves. Infrared spectroscopy revealed that no modification of the cotton could be detected after photo-discoloration processes with TiO2–SiO2, taking a wine stain as model compound. The mixed TiO2 and SiO2 colloids lead during the dip-coating and subsequent thermal treatment on cotton to an organized structure of highly dispersed TiO2 particles always surrounded by amorphous silica.

2006

Ceramic Oxygen Separation Membranes and Mixed Conductors

Ferran Marqués, Peter Hugh Middleton, Jan Van Herle

A consortium was created to investigate electroceramic materials that show ionic or mixed ionic-electronic conduction (MIEC), for application as oxygen separation membranes and in solid oxide fuel cells (SOFC, electrolytes and cathodes). The focus is on dense structures as supported thin layers (5-20 µm) for reduced temperature operation (600-750°C). Under these conditions grain boundary effects are important. Zirconia, ceria and gallate are considered for SOFC electrolytes. The nature and the amount of the dopants will be varied, and sintering aids added. These parameters will influence the grain size and the grain boundary composition. The effects on microstructure, and conduction and strength properties will be examined by high resolution SEM/TEM, impedance spectroscopy and load testing under fast thermal cycling, respectively. A systematic study on the variation of the crystallite size of starting powders (prepared by ultrasonic spray pyrolysis - USP) and its effect on the grain boundary resistance is planned, for the case of zirconia electrolyte. Of special interest is also the compatibility between ceria and zirconia, when cofired at high temperatures: diffusion couples will be made, and studied by HR-SEM/EDX. Thin layers of these materials will be deposited on anode supports of planar shape, by tape casting, screenprinting or electrostatic spray pyrolysis (ESP). Dip coating and electrophoretic deposition (EPD), for example on tubular supports, is equally envisaged. Special attention is given to the conductivity measurement of such thin layers (in particular that of grain boundaries), and compared with that of bulky samples. Different results due to the thin layer microstructure have been observed. MIEC materials of the perovskite family La1-xSrx(Mn, Fe, Co)O3-x are considered as SOFC cathodes. Thin dense layers will be deposited by ESP on zirconia and ceria electrolytes. The effect of the annealing temperature, and therefore the grain size, on the electrochemical performance and the thermal-cycling performance of such cathode/electrolyte interfaces will be studied. Candidate MIEC Materials for oxygen separation membranes are intended to be fabricated as dense pellets (compaction) or tubes (extrusion). These can be single phase (e.g. La1-xBaxCo1-yFeyO3-d) or composites (e.g. ceria/La-chromite mixtures). They will be subjected to oxygen permeation measurements, i.e. under a large oxygen partial pressure gradient of for example CH4/air. Microstructural effects for permeation stability and mechanical stability will be of first importance: changes (grain size, grain boundaries, microporosity) will be undertaken. In view of their application, it is planned to try and deposit thin dense films of these materials on porous supports (e.g. of ceria): by ESP on porous plates, and by dip coating on porous tubes (made f.ex. by slip casting). Wherever possible, wet chemical synthesis routes are employed to obtain the ceramic starting powders, such as coprecipitation, citrate gel or dyanate complexation, colloidal synthesis or combustion synthesis.

2001

Advanced analytical electron microscopy applied to Solid Oxide Cell materials and their degradation

Stéphane François Jean Poitel

Electron microscopy offers a wide range of techniques to study materials. Environmental Scanning Electron Microscopy (ESEM), in particular, allows the characterisation of samples at high-temperature using a laser heating stage, and under various gaseous atmospheres leading to direct observation of the sample surface morphology dynamics in conditions that are close to those in real operating devices (i.e. at high temperature in various gases). Solid Oxide Fuel Cells (SOFC) operating around 800°C in both oxidising and reducing gases, with proven co-generation efficiencies of 90%, offer very promising solutions in the ongoing energy transition. However, production costs and materials degradation in those demanding operating conditions remain significant barriers to their large-scale deployment. Studying SOFC materials' degradation through a combination of ESEM with other advanced microscopy techniques fulfilled the double aim of demonstrating the power of an ESEM and improving the understanding of SOFC materials' degradation during operation. Two materials from the SOFC stack were selected for degradation case studies at around 800°C in oxidising/reducing gases: coated ferritic steel interconnect and Ni-YSZ anode. The objectives were to explore the parameters' influence (temperature, heating ramp, pressure, gas exposure...), set the boundaries and define the guidelines of the ESEM technique while demonstrating its capabilities. The two different materials sets were selected for their relevance both for the SOFC field and for their different specific characteristics allowing a wide range of experiments to be explored. A specific method for the study of the oxidation of coated steel was developed combining ESEM, Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM). Post-test segmentation of the in-situ observed images demonstrated the possibility to obtain quantitative data. FIB sample preparation from the exact site of ESEM observation allowed to correlate the underlying microstructure with the in-situ surface observation. Applying this method, oxidation mechanisms of several coatings were characterised and specific behaviours observed and explained such as the appearance of oxide ridges and the behaviour of a cerium barrier layer in case of cobalt-cerium coated steel. The heating ramp speed and the oxidation temperature were demonstrated to be crucial parameters to obtain the desired microstructure. The nitriding pretreatment of interconnect was investigated too and shows promising results for improving the steel oxidation resistance. Another combination of techniques was developed for the study of the Ni-YSZ anode by complementing the ESEM observation with continuous Mass Spectrometer (MS) acquisition. The first reduction of the NiO-YsZ anode was analysed under gas flows of hydrogen and hydrocarbons. Redox cycling effect on the anode microstructure was investigated and the resulting observations were in line with models from the literature for the reduction and re-oxidation kinetics. Finally, the demonstration of an active solid oxide cell in the microscope operating at more than 700°C under mixed gas paves the way to further possible studies approaching the device's observation in real operating conditions. Limitation of the ESEM techniques are discussed and recommendations for further improvements and perspectives given.