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Research Papers

Impact of Microchannel Boundary Conditions and Porosity Variation on Diffusion Layer Saturation and Transport in Fuel Cells

[+] Author and Article Information
Kenneth M. Armijo, Van P. Carey

Department of Mechanical Engineering,  University of California at Berkeley, 6123 Etcheverry Hall, Mailstop 5117, Berkeley, CA 94720–1740

J. Fuel Cell Sci. Technol 9(4), 041008 (Jun 15, 2012) (8 pages) doi:10.1115/1.4006476 History: Received January 13, 2012; Revised February 14, 2012; Published June 15, 2012; Online June 15, 2012

Polymer electrolyte membrane (PEM) fuel cell flooding can be detrimental to energy generation performance due to its role in reducing gas diffusion layer (GDL) oxygen transport. Previous transport models have made use of a zero-saturation boundary condition at the GDL/oxygen gas channel (GC) interface. However, the physical accuracy of this boundary condition is still unclear and further investigation is needed to lead to a more robust model of the GDL saturation distribution. This work provides a half-cell two-phase transport model for saturation as well as liquid water and gaseous oxygen pressure distributions. This work focuses on the impact of nonzero saturation boundary conditions at the GDL/GC interface, and its impact on GDL two-phase transport. Saturation boundary conditions at this location are determined based on GDL interfacial liquid coverage of water droplets that form as a result of product water that diffuses through the porous medium and blocks oxygen transport paths. The results indicate that nonzero saturation boundary conditions, which are a consequence of GDL/GC liquid droplet coverage, increase cathode saturation by as much as 4% and 12% for low and high current density conditions respectively. It is also shown that cathode saturation dependence on the interfacial liquid coverage fraction α is reduced with an increase in porosity ɛ. As ɛ increases from 0.3 to 0.5, the cathode saturation difference is reduced by 22%. This investigation also evaluated the inclusion and optimization of a microporous layer (MPL) within the half-cell system. It was found that cathode saturation reductions were more significant for increasing MPL porosity than for GDL porosity. The results suggest that its inclusion was able to reduce cathode saturation by up to 90% at the GDL/MPL interface for near zero α values.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Liquid water droplet emersion from the cathode/GDL interface to the oxygen gas channels

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Figure 2

Gas diffusion layer model domain and transport phenomena profiles

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Figure 3

Transport model comparison for liquid saturation across the GDL

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Figure 4

GDL/gas channel interfacial liquid coverage

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Figure 5

Illustration for required GDL thickness based on approximated liquid coverage area πr2 to facilitate regional lateral gas diffusion and transport

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Figure 6

Saturation boundary condition analysis for SBC 0 and SBC  > 0 for varying current densities, considering ɛ = 0.5

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Figure 7

Saturation distributions across GDL membrane for ɛ = 0.5 and sBC  = 0.37 for current densities: (a) I = 0.5 A/cm2 , (b) I = 1.0 A/cm2 , (c) I = 1.5 A/cm2 , and 0 ≤ α ≤ 0.8

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Figure 8

Cathode saturation for varying GDL/GC liquid surface coverage for (a) ɛ = 0.3 and sBC  = 0.11, (b) ɛ = 0.4 and sBC  = 0.19, and (c) ɛ = 0.5 and sBC  = 0.37 for 0.5 ≤ I ≤ 1.5 A/cm2

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Figure 9

Half-cell domain with MPL and GDL layers for water and oxygen transport

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Figure 10

GDL/MPL saturation distribution with α liquid coverage, considering ɛGDL  = 0.5 with ɛMPL  = 0.3 for I = 1 A/cm2

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Figure 11

GDL/MPL saturation distribution with α liquid coverage, considering ɛGDL  = 0.5 with ɛMPL  = 0.8 for I = 1 A/cm2

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Figure 12

GDL/MPL cathode saturation with alpha liquid coverage, considering ɛGDL  = 0.3 with ɛMPL  = 0.3 and ɛMPL  = 0.8, for I = 0.5–1.0 A/cm2

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Figure 13

GDL/MPL cathode saturation with alpha liquid coverage, considering ɛGDL  = 0.5 with ɛMPL  = 0.3 and ɛMPL  = 0.8, for I = 0.5–1.0 A/cm2

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