Martin Søgaard

Kinetics and Mechanisms of Oxygen Surface Exchange on La0.6Sr0.4FeO3 − δ Thin Films

= �75.5 J/mol/K as found for bulk material. The thin film shows smaller apparent electrical conductivity than reported for bulk. This is due to imperfections in the film, which is not totally dense and contains closed porosity. Electrical conductivity relaxation was used to determine the surface exchange coefficient and its dependence on the temperature and oxygen partial pressure. Relaxation curves showed a good fit to a simple exponential decay. The vacancy surface exchange coefficient kV determined from Kchem shows a slope log kV vs log PO2 between 0.51 and 0.85. It is further found that kV is proportional to the product of the oxygen partial pressure and the vacancy concentration kV PO2 . Different reaction mechanisms that can account for the observed PO2 and -dependence of kV are analyzed. It is proposed that the vacancies are the active sites of adsorption of molecular oxygen and that the rate determining step for the exchange reaction is splitting of the adsorbed oxygen.

High Performance Cathodes for Solid Oxide Fuel Cells Prepared by Infiltration of La0.6Sr0.4CoO3−δ into Gd-Doped Ceria

Cathodes prepared by infiltration of La0.6Sr0.4CoO3-delta (LSC40) into a porous Ce0.9Gd0.1O1.95 (CGO10) backbone have been developed for low temperature solid oxide fuel cells. The CGO10 backbone has been prepared by screen printing a CGO10 ink on both sides of a 180 mu m dense CGO10 electrolyte-tape followed by firing. LSC40 was introduced into the CGO10 porous backbone by multiple infiltrations of aqueous nitrate solutions followed by firing at 350 degrees C. A systematic study of the performance of the cathodes was performed by varying the CGO10 backbone firing temperature, the LSC40 firing temperature and the number of infiltrations. The cathode polarization resistance was measured using electrochemical impedance spectroscopy on symmetrical cells in ambient air, while the resulting structures were characterized by scanning electron microscopy (SEM) and high temperature X-ray diffraction (HT-XRD). The firing temperature of 600 degrees C for the LSC40 infiltrate was found to provide a balance between LSC40 material formation and high surface area micro/nanostructure. The lowest polarization resistances measured at 600 and 400 degrees C were 0.044 and 2.3 Omega cm(2) in air, respectively. During degradation tests at 600 degrees C, the cathode polarization resistance levels out after about 450 h of testing, giving a final polarization resistance of 0.07 Omega cm(2)

Oxygen Permeation in Thin, Dense Ce0.9Gd0.1O1.95-δ Membranes II. Experimental Determination

Thin (~30 μm), dense Ce 0.9 Gd 0.1 O 1.95-δ (CGO10) membranes (5 × 5 cm 2 ) supported on a porous NiO/YSZ substrate were fabricated by tape casting, wet powder spraying and lamination. A La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ /Ce 0.9 Gd 0.1 O 1 . 95-δ (LSCF/CGO10) composite cathode was applied by screen printing. Oxygen permeation measurements and electrochemical characterisation of the cells were performed as a function of temperature with air and varying hydrogen/steam mixtures flowing in the feed and permeate compartments, respectively. The oxygen flux was found to reach 10 N mL min -1 cm -2 at ~1100 K and to exceed 16 N mL min -1 cm -2 at 1175 K. The measured oxygen flux was in good agreement with theoretical predictions from a model that takes into account the bulk transport properties of Ce 0.8 Gd 0.1 O 1.95-δ , the anode and cathode polarisation resistances, and the gas conversion and gas diffusion losses in the permeate compartment. The performance of the membrane was also investigated under varying CH 4 and H 2 O gas mixtures at 1106 K. The oxygen flux increased with decreasing steam to carbon ratio and was found to exceed 10 N mL min -1 cm -2 of O 2 for steam to carbon ratios below 4:3. Post-test analysis of the tested membrane did not reveal any significant microstructural degradation of the CGO10 membrane or the anode-support.

Oxygen Permeation in Thin, Dense Ce0.9Gd0.1O1.95-δ Membranes I. Model Study

A model of a supported planar Ce 0.9 Gd 0.1 O 1.95-δ oxygen membrane in a plug-flow setup was constructed and a sensitivity analysis of its performance under varying operating conditions and membrane parameters was performed. The model takes into account the driving force losses at the catalysts at the feed and permeate side of the membrane, related to the gaseous oxygen reduction . and fuel oxidation, respectively, as well as the gas conversion and gas diffusion resistances in the porous support structure at the permeate side. The temperature and oxygen activity dependence of the oxide ionic and electronic conductivity and the oxygen nonstoichiometry of Ce 0.8 Gd 0.1 O 1.95-δ were described based on literature data. The performance of the membrane was characterised by the delivered oxygen flux and the membrane voltage. The dependence of the performance on the various membrane and operating parameters was analyzed by a separation of the various losses. The chemical expansion of Ce 0.9 Gd 0.1 O 1.95-δ under operation was estimated from the calculated oxygen activity and nonstoichiometry profiles inside the membrane.

Determination of Oxygen Transport Properties from Flux and Driving Force Measurements

We demonstrate that an electrolyte probe can be used to measure the difference in oxygen chemical potential across the surface, when an oxygen flux is forced through an oxygen permeable membrane disk. The oxygen flux as well as the total oxygen chemical potential difference is carefully controlled by an oxygen pump. The developed method is tested on a (La 0.6 Sr 0.4 ) 0.99 CO 0.2 Fe 0.8 O 3-δ membrane. An La 0.75 Sr 0.25 MnO 3 /Y 0.16 Zr 0.84 O 1.92 /La 0.75 Sr 0.25 MnO 3 oxygen pump was attached to one side of the membrane. A conical Ce 0.9 Gds 0.1 O 1.95 (CG10) electrolyte probe was pressed against the other side of the membrane. The voltage difference between the base and the tip of the CG10 probe was recorded with an applied oxygen flux through the membrane. This voltage was used to extract precise values of the surface exchange rate constant, ko. Using these values of k o , the vacancy diffusion factor, D°, could be extracted from data of the flux and the oxygen chemical potential difference across the membrane measured with the oxygen pump. Furthermore, upon a gas change, the transient voltage signals of the oxygen pump and the probe could be fitted to give values of D 0 v k o .

Integration of mixed conducting membranes in an oxygen–steam biomass gasification process

Oxygen–steam biomass gasification produces a high quality syngas with a high H2/CO ratio that is suitable for upgrading to liquid fuels. Such a gas is also well suited for use in conjunction with solid oxide fuel cells giving rise to a system yielding high electrical efficiency based on biomass. However, high costs for both oxygen supply equipment and operation are significant challenges for the commercial implementation of this technology. Mixed ionic and electronic conducting (MIEC) membranes can be used for oxygen separation from air at a lower energy consumption compared to cryogenic distillation, especially for small to medium scale plants. This paper examines different configurations for oxygen production using MIEC membranes where the oxygen partial pressure difference is achieved by creating a vacuum on the permeate side, compressing the air on the feed side or a combination of the two. The two configurations demonstrating the highest efficiency are then thermally integrated into an oxygen–steam biomass gasification plant. The energy demand for oxygen production and the membrane area required for a 6 MWth biomass plant are calculated for different operating conditions. Increasing the air feed pressure increases the energy consumption but decreases the membrane area. As an example, for the highest efficiency configuration working at a membrane temperature of 850 °C, 6 bar of air feed pressure and 0.3 bar of permeate side pressure, 150 m2 are needed to generate the oxygen for the 6 MWth plant at an energy consumption of 100 kW h per tO2.

Gd0.6Sr0.4Fe0.8Co0.2O3 − δ : A Novel Type of SOFC Cathode

The fabrication and electrochemical activity of a type of solid oxide fuel cell (SOFC) cathode is described in this paper. In search of new cathodes a Gd 0.6 (Sr 0.4 Fe 0.8 Co 0.2 O 3-δ compound was synthesized using the glycine-nitrate method. It turned out that this was a two-phase compound consisting of two perovskite phases, a cubic and an orthorhombic phase, as shown by Rietveld refinements. These two phases were synthesized and a cone-shaped electrode study was undertaken. It was shown that the composite cathode had an electrochemical activity superior to that of the two single-phase perovskites, indicating that the unique microstructure of this type of cathode is essential for achieving high electrochemical activity toward the reduction of oxygen in a SOFC.