(146g) Effect of Wall Adhesion On Liquid Water Transport through the Gas Diffusion Layer of a PEM Fuel Cell

The gas diffusion layer (GDL) is one of the key components which play a crucial role in the successful operation of PEM fuel cell. It assists in the transportation of the reactants from the flow channels uniformly over the catalyst layers as well as in draining out the liquid water from the catalyst layer back to the channels in addition to conducting the electrons with low resistance. Generally it is a carbon-based porous medium that allows for the transport of electrons through the fibers and allows the reactants and the liquid water to flow through the void space to/from the catalyst layer in a PEM fuel cell. This carbon fiber paper is often coated with PTFE to give a nature of hydrophobicity to the medium, and this is expected to facilitate the liquid water transport. Liquid water transport through the porous structure is found to be greatly affected by the nature of the porous medium, specifically, its hydrophobicity which is indicated by the wall adhesion angle or contact angle made by water on the porous medium. Contact angle is a quantitative measure of the wetting of a solid by a liquid. The actual mechanism of the flow of liquid water through the porous structure is not very well understood. And it is essential to understand the liquid water transport through the porous gas diffusion layer which helps in taking good measures to mitigate flooding of the cell, thereby improving the cell performance. In this paper, the dynamic behaviour of the liquid water transport through an idealized GDL is simulated numerically using computational fluid dynamics (CFD) techniques along with Volume of Fluid (VOF) method [1] and Continuum Surface Force (CSF) formulation [2], subject to hydrophilic/hydrophobic boundary condition at the fibre-fluid interface. VOF method has been used in the area of fuel cells recently to simulate liquid water emergence in the flow channels [3-5]. A simple three-dimensional structure with three layers each containing cylindrical fibres of 8 micron diameter and 40 micron length with a layer to layer gap of 1 micron is used to represent a small portion of the GDL. These dimensions are typical of the values observed in the carbon paper which is a commonly-used GDL in PEM fuel cells. Liquid water is injected at one place in the bottom surface with a very low velocity (Re ~ 0.4) to simulate the situation of liquid water entering into the GDL from the catalyst layer. The Volume of fluid (VOF) method is then used to track the air-water interface subject to a specified wall adhesion angle at fibre-fluid interface. Comparative simulations have been performed for two cases with different contact angles, each representing a hydrophilic and a hydrophobic fibre. The transport of the liquid water is tracked in the fibrous domain and the water evolution is shown at different time intervals in the Fig. 1 for both hydrophilic and hydrophobic GDLs. It is observed that the action of the interfacial tension in hydrophilic GDL is to spread the liquid water along the fibre and fills the bottom surface, thus blocking the flow of reactants to the catalyst layer. In case of hydrophobic GDL, liquid water is prevented from spreading laterally. Instead of spreading along the fibre or bottom surface, the water droplet grows bigger and bigger in size and touches the fibre in the next layer. The growth of the water droplet in the hydrophobic GDL with time is shown in figure 2. From there, the water may channel through the gap between the fibres and get evacuated from the GDL. This prevention of lateral spreading allows the pathway for the reactants to enter in the counter direction. References: 1. C.W. Hirt, B.D. Nichols. Volume of Fluid (VOF) method for the dynamics of free boundaries, J. Comput. Phys. 39 (1981) 201-225. 2. J.U. Brackbill, D.B. Kothe, C. Zemach, A continuum method for modeling surface tension, J. Comput. Phys. 100 (1992) 335-354. 3. A. Theodorakakos, T. Ous, M. Gavaises, J.M. Nouri, N. Nikolopoulos, H. Yangaihara. Dynamics of water droplets detached from porous surfaces of relevance to PEM fuel cells, J. Colloid Interface Sci. 300 (2006) 673-687. 4. X. Zhu, P.C. Sui, N. Djilali. Dynamic behavior of liquid water emerging from a GDL pore into a PEMFC gas flow channel, J. Power Sources 172 (2007) 287-295. 5. X. Zhu, P.C. Sui, N. Djilali. Three-dimensional numerical simulations of water droplet dynamics in a PEMFC gas channel, J. Power Sources 181 (2008) 101-115.