Abstract
A numerical investigation on the water transport across the membrane of a proton exchange membrane fuel cell is carried out to gain insight into water management issues, which are crucial to the efficient operation of such fuel cells. The transport equation of water content based on a phenomenological model, which includes an electro-osmotic drag term and a diffusion term, is solved using the finite volume method for a 1-D configuration with the assumption of a uniform temperature distribution. Transport properties including the drag coefficient and diffusion coefficient of water in the membrane and the ionic conductivity of the membrane are expressed as functions of water content and temperature. The effects on the water flux across the membrane and on overall membrane protonic conductivity due to variations of these properties are studied. The numerical results show that water transport in the membrane is mainly determined by the relative strength of electro-osmotic drag and diffusion, which are affected by operating conditions such as current density and relative humidity at the membrane surface, and design parameters such as membrane thickness and membrane material. Computed water fluxes for different humidity boundary conditions indicate that for a thick membrane, e.g. Nafion 117, electro-osmotic drag dominates transport over a wide range of operating conditions, whereas for a thin membrane, e.g. Nafion 112, diffusion of water becomes equally important under certain conditions. Implications of the one-dimensional investigation on comprehensive CFD based modelling of proton exchange membrane fuel cell are also discussed.
Published Version
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