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Research Papers

Interfacial Water Transport Effects in Proton-Exchange Membranes

[+] Author and Article Information
Brian Kientiz

Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Haruhiko Yamada

 Toyota Central R&D Labs, Inc., Nagakute, Aichi, 480-1192, Japan

Nobuaki Nonoyama

Higashifuji Technical Center, Toyota Motor Corporation, Susono, Shizuoka, 410-1193, Japan

Adam Z. Weber1

Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720azweber@lbl.gov

1

Corresponding author.

J. Fuel Cell Sci. Technol 8(1), 011013 (Nov 04, 2010) (7 pages) doi:10.1115/1.4002398 History: Received October 21, 2009; Revised November 16, 2009; Published November 04, 2010; Online November 04, 2010

It is well known that the proton-exchange membrane is perhaps the most critical component of a polymer-electrolyte fuel cell. Typical membranes, such as Nafion® , require hydration to conduct efficiently and are instrumental in cell water management. Recently, evidence has been shown that these membranes might have different interfacial morphology and transport properties than in bulk. In this paper, experimental data combined with theoretical simulations that explore the existence and impact of interfacial resistance on water transport for Nafion® 21x membranes will be presented. A mass-transfer coefficient for the interfacial resistance is calculated from experimental data using different permeation cells. This coefficient is shown to depend exponentially on relative humidity or water activity. The interfacial resistance does not seem to exist for liquid/membrane or membrane/membrane interfaces. The effect of the interfacial resistance is to flatten the water content profiles within the membrane during operation. Under typical operating conditions, the resistance is on par with the water transport resistance of the bulk membrane. Thus, the interfacial resistance can be dominant especially in thin, dry membranes and can affect overall fuel cell performance.

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Figures

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Figure 1

Schematics of the experimental (a) liquid water and (b) water vapor permeation cells

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Figure 2

Water flux across a LE/VE cell with a Nafion® 212 membrane as a function of liquid water pressure in the first chamber with gas streams having average water activities of 0.54, 0.70, and 0.85 in the second chamber at 80°C

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Figure 3

Resistance to water transport in Nafion 21x membranes as a function of membrane wet thickness for different activities

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Figure 4

Derived interfacial resistance coefficient as a function of water activity

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Figure 5

Modeled water flux (lines) compared with experimental water flux data (points) as a function of average water activity for three Nafion 21x membrane configurations

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Figure 6

Fraction of total resistance due to interfacial effects as a function of both membrane dry thickness and water activity

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Figure 7

Water concentration profiles for several representative cases. In all cases, the water activity at length zero is set to 0.2. In LE/VE cases, the LE side is at 1 bar water pressure with no interfacial resistance. In VE/VE cases, the high activity side is at unit activity. (I) LE/VE cell with high liquid water permeability. (II) LE/VE cell with low liquid water permeability. (III) VE/VE cell with no interfacial resistance. (IV) VE/VE cell with interfacial resistance.

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Figure 8

Water activity at the low-humidity side as a function of current density (going in the positive direction) across the membrane for positive, negative, and zero net water fluxes for both simulations with (solid) and without (dotted) interfacial effects

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