J. Van herle

Low temperature fabrication of (Y,Gd,Sm)-doped ceria electrolyte

A general procedure based on the oxalate coprecipitation route was developed to produce very sinterable ceria (CeO2) powder doped with Gd2O3, Sm2O3 or Y2O3. The method is simple, reliable and very reproducible. Without any milling step, powder of 20 mol% GdO1.5, SmO1.5 or YO1.5-CeO2 could be fired translucent dense (97% relative to theoretical) at 1300 °C (4 h) already, as pressed compacts. This compares to typical values of 95% relative density obtained after firing at 1500–1600 °C in the standard literature. Cast tapes from the intensively milled powders were fired equally dense at 1400 °C (2 h), among the best results yet reported for tape cast doped ceria. Owing to their high density, excellent ionic conductivity values were observed for the doped ceria electrolytes, on the order of 5–7 S m−1 at 750 °C in air.

A study on the La1-xSrxMnO3 oxygen cathode

The impedance responses and current-overpotential characteristics were studied for oxygen reducing cathodes, stoichiometric La1 − xSrxMnO3 (x = 0.16 − 0.20) in solid oxide fuel cells, under varying conditions of temperature (700–900 °C) and oxygen partial pressure (1 − 10−4atm) for two distinctly different electrode morphologies (porous and dense structure). An electrode densified at the interface with the yttria-stabilized zirconia solid electrolyte was more efficient in this study. The reaction at the finely porous LSM proceeds via the triple phase boundary and ineffectively uses the mixed conductivity property of the material. Reaction mechanisms for both cases (porous and dense structure) are discussed and compared with the literature. A new, two-layered cathode structure is proposed.

Anode supported solid oxide fuel cells with screen-printed cathodes

To increase power density at reduced temperature operation of SOFC, thin film 8YSZ electrolytes were deposited on Ni–YSZ anode support plates by tape casting and cofiring. Cathode behaviour, limiting cell output at lower temperature, was studied in more detail. Cathodes were deposited by screenprinting and firing. With consecutive layers of doped lanthanum manganite and cobaltite, power density of 0.5 W cm−2 at 750°C was obtained using hydrogen fuel. On cells of 100 cm2, no fuel diffusion limitation above 70% conversion occurred, and electrical efficiency of 35% was achieved. Actual cell temperature increases significantly above a current density of 0.3 A cm−2. This effect causes erroneous electrode and cell characteristics.

Lanthanide co-doping of solid electrolytes: AC conductivity behaviour

Abstract Solid electrolytes of cubic structure employed in Solid Oxide Fuel Cells (SOFC) like ceria (CeO 2 ) and zirconia (ZrO 2 ) are typically doped with a single selected element from the (Y, lanthanide)-series. Co-doping of ceria with several elements is investigated here in terms of its influence on ionic conductivity. It is found that, for a same total dopant concentration, mixtures of dopants give a higher total ionic conductivity (by 10–30%) than the best singly doped material.

Oxygen diffusion through silver cathodes for solid oxide fuel cells

Abstract The oxygen reduction mechanism at the interface O2,Ag/YSZ (yttria-stabilized zirconia) was investigated by impedance spectroscopy and current-overpotential recording. Oxygen partial pressure was varied between 10−4 and 1 atm, and temperature between 600 and 850°C. Both dense and porous silver electrodes were studied. Bulk diffusion of oxygen atoms through the solid silver was identified as the rate-determining mechanism. A second, smaller contribution is ascribed to dissociative adsorption of oxygen molecules on the silver electrode. Good agreement with known data for the diffusion and solubility of oxygen in silver is obtained.

Conductivity measurements of various yttria-stabilized zirconia samples

Samples of yttria-stabilized zirconia manufactured by the following fabrication procedures, were obtained from commercial sources: (i) hot isostatic pressing; (ii) tape casting; (iii) vacuum plasma spraying, and (iv) calendering. The ionic conductivities of these samples were measured by (a) impedance spectroscopy; (b) the four-point probe method; (c) the current-interruption technique, and (d) the van der Pauw technique. The tape-cast and hot pressed samples showed good and very reproducible conductivity values. The vacuum plasma sprayed samples showed an anisotropy in their conductivity, with the cross-plane value being several times lower than the in-plane value. A simple model based on the porous microstructure of these samples can explain this observation. Sintering of the plasma sprayed samples minimized the anisotropy and significantly improved their conductivity values. The calendered samples also showed a similar anisotropy in their conductivity data when they were inadequately sintered.

Thermo-Economic Optimization of a Solid Oxide Fuel Cell, Gas Turbine Hybrid System

Large scale power production benefits from the high efficiency of gas-steam combined cycles. In the lower power range, fuel cells are a good candidate to combine with gas turbines. Such systems can achieve efficiencies exceeding 60%. High temperature Solid Oxide Fuel Cells (SOFC) offer good opportunities for this coupling. In this paper, a systematic method to select a design according to user specifications is presented. The most attractive configurations of this technology coupling are identified using a thermo-economic multi-objective optimization approach. The SOFC model includes detailed computation of losses of the electrodes and thermal management. The system is integrated using pinch based methods. A thermo- economic approach is then used to compute the integrated system performances, size and cost. This allows to perform the optimization of the system with regard to two objectives: minimize the specific cost and maximize the efficiency. Optimization results prove the existence of designs with costs from 2400

Modeling and Study of the Influence of Sealing on a Solid Oxide Fuel Cell

/kW for a 44% efficiency to 6700

Electrocatalysis in solid oxide fuel cell electrode domains

/kW for a 70% efficiency. Several design options are analysed regarding, among others, fuel processing, pressure ratio or turbine inlet temperature. The model of a pressurized SOFC-microGT hybrid cycle combines a state-of- the-art planar SOFC with a high speed micro gas turbine sustained by air bearings.

Oxygen reduction at porous and dense cathodes for solid oxide fuel cells

The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFCs). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this study, simulation is used to investigate the effects of an imperfect seal behavior on the performance and reliability of SOFCs. Diffusion through the sealing material and inherent local combustion of fuel are added to the computational fluid dynamics (CFD) repeat-element model, which also allows us to compute the flow field, the electrochemical reactions, and the energy equations. The results are in good agreement with experiments. The zones of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of reoxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The modeling approach presented here, which includes the imperfections of components, allows us to reproduce experiments with good accuracy and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability.