Experimental and Modeling Studies of Two-Phase Flow in Porous Media and Its Effects on the Performance of a PEM Fuel Cell

Abstract

An experimental investigation was conducted to study the two-phase flow properties of porous media used in proton exchange membrane (PEM) fuel cells. The liquid and gas phase relative permeability of porous media used in PEM fuel cells was measured at the University of Kansas and validated using the neutron imaging facility at the National Institute of Science and Technology (NIST). New correlations between the liquid saturation levels and relative permeability were identified. These correlations were further used to determine the liquid saturation levels in the electrodes of a PEM fuel cell during operation. The results showed that the 3rd-order power correlations between saturation levels and permeability developed for hydrophilic sands were unsuitable for the gas diffusion layers (GDLs) used in PEM fuel cells. The GDLs made of graphite fibers have different surface properties and structures than the well-sorted sands, causing a difference in the two-phase flow properties. One-dimensional two-phase flow models were developed to study the effect of the porous media on the liquid saturation levels, liquid water management strategies, and fuel cell performance. To address the saturation level discontinuity created at the interface of two materials with different wetting properties, a saturation jump condition was included in the models. This study showed that the hydrophobic part of the capillary curve was more important than the hydrophilic part because the zero capillary pressure (pc=0) condition at the gas channel/GDL interface bound the liquid saturation levels in the hydrophobic region. The properties of the GDLs affected the fuel cell performances greatly when the reactant transport in the porous media was the limiting step. The model including a micro-porous layer (MPL) in the cathode side showed that the zero-net-water-transport-across-the-membrane was achievable, which would eliminate the anode humidification requirement and improve the fuel cell performance. Hydrophobic catalyst layers (CLs) in the cathode and anode were required to prevent the CLs from being flooded, when the hydrophobic MPL was presented in the cathode side. The complete model consisting of both the cathode and anode showed that the liquid water transport rate from the cathode to the anode was higher when there was no MPL in the anode side. The complete model also showed that when the anode was treated as an interface instead of a complete porous electrode, over-prediction of the fuel cell voltage resulted, mainly from the omission of the ohmic losses in the anode.