Analysis of Water Transport in Proton Exchange Membranes Using a Phenomenological Model

[+] Author and Article Information
P. C. Sui

Institute for Integrated Energy Systems, Department of Mechanical Engineering,  University of Victoria, Victoria, B.C., Canada

Ned Djilali1

Institute for Integrated Energy Systems, Department of Mechanical Engineering,  University of Victoria, Victoria, B.C., Canadandjilali@uvic.ca


Corresponding author.

J. Fuel Cell Sci. Technol 2(3), 149-155 (Jan 28, 2005) (7 pages) doi:10.1115/1.1895945 History: Received October 04, 2004; Revised January 28, 2005

An investigation of water transport across the membrane of a proton exchange membrane fuel cell is performed to gain further insight into water management issues and the overall behavior of a representative phenomenological model. The model accounts for water transport via electro-osmotic drag and diffusion and is solved using a finite volume method for a one-dimensional isothermal system. Transport properties including the water drag and diffusion coefficients and membrane ionic conductivity are expressed as functions of water content and temperature. An analytical solution based on a generalized form of the transport properties is also derived and used to validate the numerical solutions. The effects of property variations on the water flux across the membrane and on the overall membrane protonic conductivity are analyzed. The balance between transport via electro-osmotic drag and diffusion depends not only on operating conditions, such as current density and relative humidity at the membrane boundaries, but also on 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 the 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 humidification conditions and current densities. Implications for the resolution of membrane transport in CFD-based models of proton exchange membrane fuel cells are also discussed.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Computational domain and BC

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Figure 2

Sorption isotherm of Nafion

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Figure 3

Diffusion coefficients used in calculation

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Figure 4

Protonic conductivity of Nafion by Springer (7) and Sone (20)

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Figure 5

Comparison of analytical solution and numerical solution

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Figure 6

Predicted water-content profiles using different diffusion-coefficient expressions, membrane thickness 50μm at I=10,000A∕m2

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Figure 7

Water-content profiles at different current density for membrane thickness 50μm

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Figure 8

Flux, membrane resistance, and IR loss at different current density for zm=50μm, cw,a=1 and cw,c=14

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Figure 9

Flux and membrane resistance at different membrane thickness

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Figure 10

Water-content profiles at different boundary conditions, I=5000A∕m2

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Figure 11

Flux predictions (mol∕m2s) for different relative-humidity conditions on both sides of membrane for I=10,000A∕m2 and membrane thickness (a) 175μm and (b) 50μm

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Figure 12

Relative error of approximated solutions for cw,a=1, cw,c=14, I=10,000A∕m2 for membrane thickness of (a) 175μm (Nafion 117) and (b) 50μm (Nafion 112)




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