Interfacial Transport Phenomena of the Non-Wetting Phase in Porous Media: Applied to Polymer Electrolyte Fuel Cell Gas Diffusion Media

Traditional methods for mathematical modeling of water transport in porous media lead to a sudden, sharp discontinuity in liquid saturation at the interface between two porous media of different properties. The goal of this study was to determine if this discontinuity is a physical phenomenon that truly occurs or a mathematical anomaly that does not manifest physically at porous interfaces through which a non-wetting phase flows. The study uses mathematical models and experiments applied to water transport in the gas diffusion layer of polymer electrolyte fuel cells (PEFCs). Evidence of a region of gradual property change between the microporous layer and macroporous layer of bi-layer diffusion media is presented, and a mathematical model describing the effects of this gradual interfacial region is developed by adapting the methods used in the sudden interface model. The revised model results in a continuous liquid water saturation distribution across the diffusion media, compared to the sharp discontinuity in the liquid phase saturation predicted by the earlier sudden interface models, despite utilizing the same physical assumptions. High resolution neutron radiography is used to measure the liquid content profile across the diffusion media, and the results are compared with the model predictions. When the instrumental broadening attributed to the limited resolution of the measurement is considered, the neutron radiography results are found to be similar to model predictions. An additional neutron radiography experiment is conducted with a new, higher spatial resolution neutron detector. A PEFC is constructed with half of its active area supported by a diffusion media that features a microporous layer and half of its active area supported by a media without the microporous layer. By capturing images from these regions independently, effects of the microporous layer on the liquid water profile are able to be observed directly. An argument based on the pore network model method is presented to elucidate the behavior of an interface between porous media at the pore scale, and this is shown to agree with the results of the macro-averaged model. It is determined that the discontinuity in liquid saturation that occurs in the model is a physical phenomenon when the media is considered at the macroscopic level in the case of an ideal interface, but it is unlikely that this discontinuity occurs in the porous media of the fuel cell, due to the fact that a more gradual property change occurs in the material than originally anticipated.