Modélisation numérique tridimensionnelle des processus physico-chimiques dans une pile à combustible à membrane échangeuse de protons.

Abstract

In the current situation of global warming and depletion of oil reserves, the use of proton exchange membrane fuel cells (PEMFC) needs further improvements. In this work, two numerical 3D models have been developed; single-phasic and two-phase water transport coupled with heat transfer and electrochemical kinetics in a PEMFC with parallel, straight channels, characterized by three different shapes: rectangular, triangular and elliptical. The single-phase model describes the coupled transport of gaseous species in the electrodes of the cell and the transport of water in the membrane. The two-phase model considers liquid water in porous diffusers. The equations governing fluid and transfer phenomena vary between the continuous medium in the gas channels and the porous medium in the diffusion and catalytic layers. The models are implemented into the commercial computational fluid dynamics (CFD) software package Fluent®6.3, with its user-defined functions (UDFs). The use of the models makes it possible to study the influence of the geometrical parameters and the impact of the cell potentiel and the humidification of the reactants on the performance of the cell. It is shown, on the basis of the monophasic model, that the geometry of the gas channels and the dimensions of the channel have a great influence on mass and heat transfer phenomena. The best performance is obtained for a triangular channel with 0.1 mm width. The performance of the cell can be greatly improved at low voltage and high relative humidity. The two-phase model indicates good hydration of the membrane at high current density. The liquid water saturation in the porous layer is the highest in the rectangular channel which generates a low current density. The simulations reveal that oxygen consumption at the catalytic surface is much lower in the two-phase model due to the accumulation of liquid water that blocks O2 access to the catalytic layer active sites. The consideration of the liquid water generates a lower limit current than in the monophasic case. In view of the results obtained, taking into account liquid water in the modelling of a PEMFC is essential. The geometrical and operating parameters in the different parts of an elementary cell affect greatly the performance of the fuel cell.