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Water Management in PEM Fuel Cells

Brooks R. Friess, Mehdi Shahraeeni, and Mina Hoorfar. School of Engineering, University of British Columbia, 3333 University Way, Kelowna, BC V1V1V7, Canada

Proton exchange membrane (PEM) fuel cells have drawn much attention in the last decade as a high-efficiency and low-emission source of energy. The PEM fuel cell consists of a membrane electrode assembly (MEA) sandwiched between two flow channels. The MEA contains a polymer electrolyte membrane (e.g., Nafion) embedded between two porous gas diffusion electrodes (GDE). The GDE is composed of a platinum catalyst and a gas diffusion layer (GDL) constructed from macro-porous substrates (i.e., carbon fiber paper or cloth impregnated with poly(tetrafluroethylene) (PTFE)) coated with one or more micro-porous layers (i.e., amorphous mixtures of carbon and PTFE). PEM Fuel cell technology has come a long way in the last few years. However, the performance of fuel cells still must be improved significantly before they can constitute a viable market. Recent experimental and numerical investigations identify water management as a critical factor in the design of robust and high-efficiency fuel cells. In essence, the ionic conductivity of the electrolyte is dependent on the hydration level of the membrane as water molecules transport hydrogen ions across the electrolyte. However, excessive water vapor condensation, due to lengthy operation or large output current, forms micro-droplets that cover the active sites on the catalyst layers, fill the pores of the GDL, and block the access of the reactant gas to the reaction site. Typically, this is the origin of the limiting current for PEM fuel cells. To understand and enhance the water transport and removal mechanisms, it is necessary to study droplet formation and multiphase flow in the internal network of the fuel cell, especially in the GDL. The GDL plays a critical role in water management within the fuel cell since humidification and water removal are both achieved through the GDL. The study of actual distribution of liquid water in the GDL requires accurate measurement of the surface properties of this porous medium. Despite various methods developed to illustrate the multiphase flow and transport in the GDL, the wettability of this layer is not well understood due to its structural complexity and lack of appropriate experimental techniques. Most of the studies conducted for measuring wettability of GDLs are based on the external contact angle measurements. However, the external contact angle does not describe adequately capillary forces acting on the water inside the GDL pores. In this paper, the capillary penetration technique is used to determine the penetration rate of the liquid (into the GDL) from the experimental weight increase. The complete form of the Washburn's equation (including gravity and inertia terms) is used to model the results and determine the internal contact angles. It has been observed that the experimental weight increase is a consequence of two simultaneous processes: the capillary penetration inside the GDL and the formation of meniscus outside the GDL. The weight increase due to the latter is also considered in the model. Finally, the contact angle results are used to determine the surface tension of the GDL using two approaches: the equation of state and surface tension components approaches.