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In the high current density regime, the performance of SOFCs is limited by concentration overpotentials, which are particularly significant at the anode side where a competition occurs between incoming hydrogen and outgoing water. In standard SOFCs where the anode is made of YSZ and Ni, this becomes crucial when the demand for reactants exceeds the capacity of the porous cermet anode to supply them by gas diffusion mechanisms. A delicate compromise should be found between maintaining a high level of activation in the anode, i.e. high density of triple phase boundaries (TPB), favored by a distribution of small interconnected pores with high tortuosity and small grains size, and high gas permeability, favored by a distribution of large interconnected pores with low tortuosity, while the percolation of both electronic and anionic conductivity phases is maintained. The present work aims at determining experimentally anodes gas permeability as a function of the experimentally determined percolation, tortuosity and volume fraction of the pores, assuming that the diffusion of gases is described by the Darcy Law. Anodes with varying porosity ensured by different quantities of the pore former were obtained by cold pressing and sintering. SEM image analysis, mercury intrusion porosimetry and permeability measurements for different gases were performed. The obtained results show that permeability increases with the volume fraction of the pore former. However, non-linearity as a function of the gases molecular weight is observed and discussed as a function of the mean free path and thus of possible mixed Darcy and Knudsen diffusion.

International audience; Recently a new design of solid oxide fuel cell (SOFC) named dual membrane fuel cell (dmFC) has been introduced and proved. It is based on the introduction of three independent compartments for hydrogen, oxygen and water, which eliminate the dilution of the fuel or the oxidizer respectively in the SOFC construction or in its proton conducting (pSOFC) modification. Due to the registered sufficient mixed ionic conductivity of BaCe0.85Y0.15O2.925 (BCY15), a simplified modification in respect to shaping technology and construction is introduced. It's performance is better than the one of pSOFC cell produced with the same material and shaping procedure: 76 mW/cm2 (dmFC thickness 1.1 mm) against 71 mW/cm2 (pSOFC thickness 0.66 mm) at 700 °C. Impedance measurements carried out down to 1 mHz ensure information about the water formation and behaviour in the separate central membrane compartment of the dmFC design, which brings to performance improvements, registered also in electrolyser mode of operation.

The effect of tap water contaminants on the kinetics of the hydrogen evolution reaction on a nickel electrode in 1 mol dm(-3) KOH was investigated by galvanostatic polarization and electrochemical impedance spectroscopy techniques. It was found that the tap water contaminants lead to an increase in the overpotential of the hydrogen evolution reaction, especially at low temperatures. The combination of electrochemical techniques, as well as physicochemicals such as SEM and EDAX ones, confirmed that the contaminants are specifically adsorbed and blocked the available electrode surface for the reaction. It was concluded that they do not participate in an electrochemical reaction in the potential region where HER occurs. Besides the short term negative impact on the rate of hydrogen evolution, a 55 h test revealed that the overpotential shows a steady increase over time in presence of tap water contaminants, while in absence of these contaminants the overpotential is constant. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

This paper presents results for the conductivity of BaCe0.85Y0.15O2-δ (BCY15) measured by electrochemical impedance spectroscopy in wet hydrogen and in Air, as well as test results from the application of the material in a new dual membrane fuel cell configuration (“IDEAL Cell”), in which the water is produced and evacuated through a separate chamber. The conductivity of dense BCY15 in wet hydrogen atmosphere at 700 °C is 2.0 10-2 S/cm. The measured values in air are of the same order. Preliminary tests of the material in the new cell design, where the three types of conductivity (protonic, oxide ion and mixed) are used in different cell compartments, are successfully performed.

IDEAL-Cell is a new concept of a high temperature fuel cell operating in the range 600-700 °C. It is based on the junction between the anode part of a PCFC and the cathode part of a SOFC through a mixed H+ and O2- conducting porous ceramic membrane. This concept, extensively described in the present paper, aims at avoiding all the severe pitfalls connected with the presence of water at the electrodes in both SOFC and PCFC concepts. Spark Plasma Sintering samples were designed specifically for proving the IDEAL-Cell concept. The first electrochemical results obtained at 600 °C under hydrogen on millimeter thick samples show that IDEAL-Cell behaves like a high temperature fuel cell. It is estimated that the overall efficiency of this new concept should greatly surpass that of standard SOFCs and PCFCs and that the material constraints, especially in the case of interconnect materials, should significantly decrease.

The IDEAL-Cell concept is based on the junction between a PCFC anode/electrolyte section and a SOFC cathode/electrolyte section, through a mixed proton and oxide ion conducting porous ceramic membrane, operating in the temperature range 600-700{degree sign}C. Recombination of ions takes place within the junction, or central membrane (CM), and water vapor is evacuated through open porosity. The first results on the fabrication of multilayered samples reproducing the IDEAL-Cell structure are reported here, together with electrochemical tests carried out on selected samples in order to evaluate the performances of the chosen materials and to demonstrate the feasibility of this innovative concept of fuel cell. Stable OCV and power generation were obtained in the multilayered structures. Anomalies on the I/V curves and impedance measurements under large perturbation could be considered as proofs of water formation inside the central membrane.