Joseph Sfeir

LaCrO3-based anodes: stability considerations

LaCrO3 based materials have been studied as alternative anodes to the conventional Ni-YSZ cermet. Thermodynamic calculations as well as experimental work have been conducted in an attempt to analyze the stability of this system when doped with Mg, Ca, Sr, Mn, Fe, Co and Ni. Various gas atmospheres were simulated (air, humidified hydrogen, CO and CO2). It was possible to correlate some of the experimental results (electrochemical, XRD, XPS-Auger, SIMS and TEM-EDS) with the thermodynamic calculations. It was calculated that Sr and Mn substitution maintain the stability of the perovskite even in the severe reducing conditions used in SOFC, whereas the other substitutions destabilize the system. Experimentally, transition metal substituted LaCrO3 did not decompose readily in the reducing atmospheres containing wet hydrogen or methane indicating that the demixing is at least kinetically hindered.

Co-casting and co-sintering of porous MgO support plates with thin dense perovskite layers of LaSrFeCoO3

A tape casting co-sintering route is described in which thin dense layers of LaSrFeCoO3 (LSFC) have been formed on planar, porous MgO substrates 100- 200 micron thick. SEM analysis of the sintered structure showed that it was possible to eliminate most of the residual porosity in the LSFC layer, but maintain a porosity between 25 and 45% in the MgO support layer. The LSFC layer dit not reveal many cracks. The overall shrinkage of the co-sintered structure was about 25%. The LSFC layer topography was smooth and uniform with a metallic-like lustre. A good correlation was obtained between the observed microstructure and the gas permeability measurements made at room temperature.

Influence of microstructure on oxygen transport in perovskite type membranes

Abstract The influence of bulk microstructure (grain size distribution, grain boundary length) on the oxygen transport properties of permeation membranes has been investigated. For this purpose, La0.5Sr0.5FeO3-δ samples with different microstructures were prepared by varying sintering time and temperature. Average grain sizes, which ranged from 0.20 to 1.43 μm, were determined by SEM analysis. The oxygen transport properties of the samples were characterised by permeation measurements as a function of temperature in an air/argon oxygen partial pressure gradient. The fluxes presented a change in activation energy, which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (< 850°C) to surface limitation at higher temperature (> 900°C). Only transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. At 800°C, the fluxes were respectively 0.06, 0.03 and 0.01 μmol cm-2 s-1 through ≈ 1 mm thick samples with average grain sizes of 0.20, 0.63 and 1.43 μm respectively. This would imply that oxygen transport occurs more rapidly along grain boundaries than through the bulk. Grain boundary structure and composition were analysed by TEM.

Materials for Methane-fueled SOFC Systems

Abstract: A short overview of recent work on electroceramic materials relevant to methane-fueled SOFC systems is given. Various fuel feed options are considered, such as pure methane, biogas, and the addition of reforming agents. The principle and implementation into SOFC systems of mixed conducting ceramics for oxygen separation is described, as well as their characterisation by electrochemical methods. Ceramic anodes capable of operating with methane-rich fuel injection are presented. The document concludes with electrochemical results on planar anode supported ceramic fuel cells operating at reduced temperature.