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Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb

A SOFC, with a solid electrolyte layer and at least one anode layer constituting a stratified structure. The anode layer incorporates a twice substituted lanthanide chromite, substituted by Sr on A sites and by Ni or Cu on B sites. The stratified structure comprises a composite layer comprising a major amount of the substituted lanthanide chromite and a minor amount of the material of the electrolyte layer.

The content of this report is property of HTceramix and subject to restrictive distribution The goal of the project "Oxygen membranes for the partial oxidation of natural gas" was the pre-industrial evaluation of the concept of an electroceramic reactor, capable of converting natural gas into synthesis gas (H2+CO) by partial oxidation with pure oxygen. This project was the continuation of the pre-project on the fundamental principle (FOGA 085) accomplished with success at EPFL and which was co-financed by FOGA (50'000 CHF). In the present second Phase, FOGA supported with 250'000 CHF the spin-off company HTceramix SA, which is specialised in the development and fabrication of electroceramics for an effective conversion of natural gas. HTceramix has managed the project in close collaboration with EPFL. The goal of this second Phase was to give proof of the feasibility of a future reactor. This ambitious project was divided into 5 stages defined by milestones, all clearly fulfilled. The first milestone gave the proof of the mechanical, thermo-mechanical and chemical stability of a material selected in the 1st Phase, stability required for an operational reactor. During the next stage, ceramic tubes of this material were fabricated in collaboration with external partners. Both compaction and extrusion were successfully applied for the product processing; extrusion was preferred because it lead to a thinner wall thickness, a better mechanical stability and a reduced raw material consumption. In parallel with the tube fabrication, a catalyst compatible with the ceramic membrane was developed for the promotion of partial oxidation (POX, stage 3). A noble-metal-free oxide satisfied the requirements of 90% conversion of methane and 98% CO-selectivity. For the proof of concept, the natural gas should be converted into synthesis gas by the combination of ceramic tubes and the POX-catalyst (stage 4). The tubular reactor operated over 1'400 hours with a methane conversion of 95% and a CO-selectivity of 92%. The oxygen permeation flux across the tubular membrane was enhanced by a factor 10 compared to Phase 1 project. In stage 5, the concept of a heat integrated tubular reactor was studied. It was calculated that 1 kWth natural gas can be converted in a 0.15 litre volume reactor (consisting of 12 tubes 20cm long and of 5mm diameter each). The required surface for integrated heat exchanger is 70 cm2/kW. All formulated goals were fulfilled, respecting both the financial and time limits.

Electrocatalytic properties of Ca, Sr, Mg, Fe, Co and Ni substituted lanthanum chromites deposited on 8YSZ lelectrolytes were studied using I/V and impedance techniques. The studied parameters were: the sintering temperature, the polarization, the gas and the anode composition. The polarization resistance depended much on these parameters. Also, a direct relationship between the electric conductivity, the catalytic and the electrocatalytic activities was evidenced. The electrocatalytic behavior followed the trend of LaCrO3 ?LaCr0.9Co0.1O3-[delta] < LaCr0.9Ni0.1O3- [delta] < La0.85Sr0.15CrO3-[delta] < La0.85Sr0.15Cr0.9Ni0.1O3-[delta] which corresponded to the measured activities for steam-reforming and oxidation catalysis of the same powders used as catalysts. The best electrochemical result was obtained with a La0.85Sr0.15Cr0.9Ni0.1O3-[delta] - anode (450 mW/cm2 in H2 and 300 mW/cm2 in CH4, T=877°C, 150 µm YSZ)

The effect of the bulk microstructure (grain size distribution, grain boundary composition on the oxygen transport properties of La0.5Sr0.5FeO3membranes was investigated. For this purpose, samples with different microstructures were prepared by modifying the sintering duration and/or temperature. The average grain sizes, ranging from 0.20 to 1.43μ m, were determined from SEM analysis. The oxygen transport properties of the samples were characterised by permeation measurement. The fluxes presented a change in the activation energy which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (900°). Only the transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. This would imply that oxygen transport occurs more rapidly along the grain boundaries that through the bulk. Grain and grain boundary compositions were analysed by TEM.

Progress on anode supported cell stacks (SOFCONNEX design, 50 m2 per cell) is presented. A 6-cell stack and a 8 cell-stack were mounted and tested with hydrogen fuel at 800 degre C, yielding 100 W el and 140 W el, corresponding to a power density of 1kW el/L (0.34 W/cm2). Fuel utilisation was 50% and electrical efficiency 25%. A one-cell stack delivered 0.4 W/cm 2 at 70% fuel utilisation and 33 % electrical efficiency, and showed a performance increase over its 450 h test period. Another one-cell stack was monitored and variable conditions (20-50 % fuel utilisation, 0.2-0.5 A/cm 2) for 5500 h including several thermal cycles, with -5%/1000 h degradation