Jérôme Dillet

Size determination by use of two-dimensional Mueller matrices backscattered by optically thick random media

Here we are concerned with the systematic study of polarized light transport in thick, isotropic, homogeneous random media and of the associated inverse problem. An original spatial and intensity rescaling of the polarization transport allows one to account implicitly for the volume fraction. This parameter elimination permits a complete exploration, by means of Monte Carlo simulations of the dependence of polarized light transport on microscopic parameters. Analysis of the Mueller matrices obtained from the simulations show that additional correlations (with respect to scalar transport) are obtained between the microscopic parameters and the spatial distribution of specific elements of the Mueller matrix. As a consequence, using carefully chosen polarization states, one can determine an average particle size independently of the volume fraction of particles, with only the knowledge of the refractive-index ratio being required. This analysis is validated with experimental Mueller matrices obtained for emulsions of various size, concentration, and polydispersity.

Effect of Oxygen Depletion Along the Air Channel of a PEMFC on the Warburg Diffusion Impedance

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 (PEMFC). 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 (2007); 154, B770 (2007)] brought that they influence significantly the shape and the size of the measured impedance spectra