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The usual expression of the finite Warburg element used for analyzing electrochemical impedance spectroscopy spectra is obtained assuming that the reaction occurs at the electrode/membrane interface and that the oxygen concentration at the gas channel/gas diffusion layer interface is constant. A simple improvement of this expression consists in the oxygen concentration depletion along the gas channel. This pseudo-two-dimensional approach should be more appropriate for the investigation of mass transfer limitations in membrane-electrode assemblies of polymer electrolyte membrane fuel cells ͑PEMFC͒. Starting from experimental data from the literature, numerical simulations show that conclusions about the mass transfer limiting layer and its main characteristics can be significantly modified, which can contribute to a better understanding of oxygen transport in fuel cells. The results also put forward the existence of a critical value of the air stoichiometry below, which, close to the air channel exit, no oxygen can access to the active layer. However, the diffusion impedance model does not take into account time-dependent current variation effects on the gas concentration, although experimental proofs by Schneider et al. ͓J. Electrochem. Soc., 154, B383 ͑2007͒; 154, B770 ͑2007͔͒ brought that they influence significantly the shape and the size of the measured impedance spectra.

Electrochemical Impedance Spectroscopy (EIS) is a well-established technique for studying Polymer Exchange Membrane Fuel Cells (PEMFC) but data interpretation remains delicate, mostly because impedance models are either based on oversimplified equations or conversely, include too many correlated parameters. It is thus crucial to carefully choose the models to interpret impedance data, according to FC materials and operation conditions. Most of PEMFC impedance spectra are composed of two loops in Nyquist plot that can be perfectly represented by classical Randles Electrical Equivalent Circuit (EEC). However, several spectra show a straight line at high frequencies associated with proton conduction in the cathode catalyst layer. Assuming an interface electrode, the Randles EEC is poorly adapted to such spectra and one will rather use Transmission Line Models (TLM). However, since TLM do not usually consider mass transport, it is necessary to adapt the EEC, especially at the cathode. Such EEC can then be used as general FC models independently of the occurrence of the straight line at high frequencies, i.e. independently of the ratio between proton conduction and reaction kinetics limitations. These TLM EECs are then used to analyze the layer(s) at the origin of oxygen transport limitations: catalyst and/or the gas diffusion layer.

International audienceFrom the structural point of view a simple Cauchy relation is not expected to hold for isotropic materials. Such a Cauchy relation would imply the reduction of independent elastic stiffness constants for the isotropic state from two to one. However, high frequency elastic data of glasses and viscous liquids show a linear transformation between the shear and the longitudinal elastic stiffness which is called a generalized Cauchy relation. It seems, that the parameters of the linear transformation are related to the global and local symmetry and/or order. Brillouin investigations on the elastic stiffness coefficients of a consolidated nano-crystalline material (CeO2) and of DGEBA/SiO2 nano-composites are used in order to elucidated the role of the discrepancy between local and global symmetry and/or order

High performance Brillouin microscopy (BM) has been used to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a LiNbO3 single crystal. It is demonstrated that under suitable conditions BM is able to detect microwave-induced bulk as well as surface acoustic waves. Brillouin spectroscopy is able to probe sound wave intensities of induced phonons, which are as small as those of thermal phonons.

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