García Briones, Juan

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    Numerical study of transport processes in the gas diffusion media of a fuel cell
    (2009-12) García Briones, Juan; Leonardi, Stefano; College of Engineering; Raj Pandya, Vikram R.; Ruiz, Orlando E.; Department of Mechanical Engineering; Mehta, Narinder K.
    Fuel cells, widely regarded as the 21st century energy-conversion devices, convert chemical energy directly into electricity with high energy density at low operating temperature, good performance operation, and high environment compatibility. Proton Exchange Membrane Fuel Cells (PEMFCs), a kind of fuel cell that is considered in this study, are receiving much attention from energy-related researchers and industries world-wide due to its many promising applications in stationary power plants, automobile power systems, and portables power sources. However, the efficient operation of this type of fuel cell requires complex water management into the cathode gas diffusion media because one of its greatest performance/durability limitations is centered mainly on the liquid water transport essentially at the cathode porous media. At the heart of PEMFCs is the membrane electrode assembly (MEA). The MEA is sandwiched by two flow field plates that are often mirrored to make a bipolar plate when cells are stacked in series for greater voltages. The MEA consists of a proton exchange membrane (PEM), two catalyst layers (CLs), and two gas diffusion media (GDM). The cathode GDM, in which this study is focused, is a key component because of its central role in liquid water removal. The present study is focused on the liquid water transport across the PEMFC cathode gas difussion media. The water is presented at the cathode of this type of fuel cell due to the constant production of water at the cathode CL’s surface and the electro-osmotic drag of water across the PEM. In this study, the liquid water saturation distribution profile within a hydrophobic bilayer cathode diffusion media (GDL-MPL) was determined in one- and two- dimensions using Leverett model. Moreover, the 2-D saturation distribution profile through four different types of GDLs considering the effects of active area reduction in the CL also have been determined, but herein using Van Genuchten model. Two second-order non-linear partial differential equations which govern the liquid water transport across the GDM have been discretized in an orthogonal coordinate system using the finite volume method. In this numerical analysis the second order approximation was considered and the saturation term was treated explicitly. The resolution of the domains was 300x300 for the normal and streamwise direction respectively, and the convergence criteria for the iteration method used, maximum residual, set as a tolerance 1x10−6 between the previous value and the calculated value. The results show that the porous media liquid water saturation is significantly decreased at the cathode GDM-CL interface when using a thin and highly hydrophobic micro-porous layer. Thus, the presence of a finer MPL at the cathode side plays a crucial role in improving the cell performance. Conversely, among the four different GDLs which were considered, Toray 120A shows better conditions in the water removal.