Scaling the Water Percolation in PEM Fuel Cell Porous Transport Layers

A typical polymer electrolyte membrane fuel cell (PEMFC) consist of a series of non-wetting porous layers comprised between the bipolar plates: the porous transport anode and cathode layers with their catalyst layer and the proton exchange membrane. The cathode porous transport layer (PTL),also known as gas diffusion layer, has the dual role of facilitating the access of the reactants to the catalyst layer while removing the generated water. Water percolation through the PTL will evolve on one of the drainage flow patterns, (either capillary fingering or stable displacement), depending on the injection flow rate. In this paper, using a specially designed ex-situ experimental setup, images of the water percolation and the pressure required to inject the water on a PTL are simultaneously recorded. From the image post-processing analysis, the area covered by the water or wetted area is calculated. The time evolution of wetted area and the percolation pressure are indicative of the drainage flow pattern taking place. Through a proper scaling of the problem it was found that, in addition of the capillary number and the viscosity ratio, a non-dimensional number was defined correlating the pressure-area data; two variables which are usually analyzed separately. Using this non-dimensional number a simple logarithmic dependence for all injection flow rates was obtained with a PTL sample. When a PTL sample with different morphological and wetting properties was used, a new curve was obtained for all injection flow rates. Each curve has a unique slope and, based on preliminary results, that slope can be used as a PTL characterization technique. This non-dimensionalization not only can be used as a characterization technique but also as a validation method for numerical simulation of drainage in porous media.