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TECHNICAL PAPERS: Two-Phase Flow and Heat Transfer

Multiphase Transport Phenomena in the Diffusion Zone of a PEM Fuel Cell

[+] Author and Article Information
S. M. Senn

Department of Mechanical Engineering, Laboratory of Thermodynamics in Emerging Technologies,  Swiss Federal Institute of Technology(ETH Zurich), CH-8092 Zurich, Switzerland

D. Poulikakos1

Department of Mechanical Engineering, Laboratory of Thermodynamics in Emerging Technologies,  Swiss Federal Institute of Technology(ETH Zurich), CH-8092 Zurich, Switzerlanddimos.poulikakos@ethz.ch

1

Corresponding author.

J. Heat Transfer 127(11), 1245-1259 (Jun 20, 2005) (15 pages) doi:10.1115/1.2039108 History: Received November 23, 2004; Revised June 20, 2005

In this paper, a thorough model for the porous diffusion layer of a polymer electrolyte fuel cell (PEFC) is presented that accounts for multicomponent species diffusion, transport and formation of liquid water, heat transfer, and electronic current transfer. The governing equations are written in nondimensional form to generalize the results. The set of partial differential equations is solved based on the finite volume method. The effect of downscaling of channel width, current collector rib width, and diffusion layer thickness on the performance of polymer electrolyte membrane (PEM) fuel cells is systematically investigated, and optimum geometric length ratios (i.e., optimum diffusion layer thicknesses, optimum channel, and rib widths) are identified at decreasing length scales. A performance number is introduced to quantify losses attributed to mass transfer, the presence of liquid water, charge transfer, and heat transfer. Based on this model it is found that microchannels (e.g., as part of a tree network channel system in a double-staircase PEM fuel cell) together with diffusion layers that are thinner than conventional layers can provide substantially improved current densities compared to conventional channels with diameters on the order of 1 mm, since the transport processes occur at reduced length scales. Possible performance improvements of 29, 53, and 96 % are reported.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic drawing of the cathode diffusion layer (DL) and the cathode channel (CH). The catalyst layer (CL) is treated as a boundary condition. The computational domain is indicated by the dotted rectangle, where b is the diffusion layer width, L is the diffusion layer thickness, and h is the channel width.

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Figure 2

Standard case: (a) Oxygen mole fraction xO2, (b) water vapor mole fraction xH2O, (c) liquid water saturation S, (d) temperature difference T̃ξ6−ξ6(=T−Th) (K), and (e) electric potential ϕ̃

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Figure 3

Standard case: (a) volumetric condensation heat source Q̃K̃6 and (b) volumetric Joule heat source K̃5(∇̃ϕ̃)2

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Figure 4

Standard case: absolute values of molar fluxes, (a) ∣ÑO2∣=(eyÑO2)2+(ezÑO2)2, (b) ∣ÑH2O∣=(eyÑH2O)2+(ezÑH2O)2, and (c) 104∣ÑL∣=104(eyÑL)2+(ezÑL)2

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Figure 5

Standard case with different effective electrical conductivities (i.e., χ=62.5,125,250,500,1000,2000,4000,8000Ω−1m−1). Distributions at the reaction boundary z̃=0: (a) current density j(y)∕j0, (b) irreversible reaction heat fluxes −K̃13j̃ϕ̃ (solid lines) and reversible reaction heat fluxes −K̃14j̃T̃ (dashed lines), and (c) liquid water saturation S(y∕b).

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Figure 6

Standard case with different contact angles [i.e., θ=95,100,110,120,130,140,150,160,170deg]. Note that the curves also represent the standard case with different absolute permeabilities (i.e., κ=0.077,0.31,1.2,2.6,4.2,6.0,7.6,9.0,9.9×10−13m2, respectively). Distributions at the reaction boundary z̃=0: (a) current density j(y)∕j0 and (b) liquid water saturation S(y∕b).

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Figure 7

Standard case with different drag coefficients (i.e., nd=0.5,1.0,1.5,2.0,2.5). Distributions at the reaction boundary z̃=0: (a) current density j(y)∕j0 and (b) liquid water saturation S(y∕b).

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Figure 8

Standard case with different condensation rate constants (i.e., γ=90,9000s−1): (a) water vapor mole fraction xH2O(γ=90s−1), (b) liquid water saturation S(γ=90s−1), (c) condensation heat source Q̃K̃6(γ=90s−1), (d) water vapor mole fraction xH2O(γ=9000s−1), (e) liquid water saturation S(y∕b)(γ=9000s−1), and (f) condensation heat source Q̃K̃6(γ=9000s−1).

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Figure 9

Standard case with different electric potentials (i.e., ϕh=0.6,0.7,0.8,0.9,1.0V). Distributions at the reaction boundary z̃=0: (a) current density j(y)∕j0 and (b) liquid water saturation S(y∕b).

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Figure 10

Standard case with different electrode widths b (i.e., b=2.0,1.0,0.5mm) and electrode thicknesses L̃ (i.e., L̃=132,232,332,432,532,632,732,832). Distributions at the reaction boundary z̃=0: (a) current density j(y)∕j0(b=2.0mm), (b) liquid water saturation S(y∕b)(b=2.0mm), (c) current density j(y)∕j0(b=1.0mm), (d) liquid water saturation S(y∕b)(b=1.0mm), (e) current density j(y)∕j0(b=0.5mm), and (f) liquid water saturation S(y∕b)(b=0.5mm).

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Figure 11

(a) Standard case with different electrode thicknesses L̃ (i.e., 132⩽L̃⩽832) and electrode widths b (i.e., b=2.0,1.0,0.5mm), subject to h=b∕2. Average current densities javg∕j0 for b=2mm (bottom curve), b=1mm (middle curve), and b=0.5mm (upper curve). (b) Standard case with different channel widths h̃ (i.e., 0.12⩽h̃⩽0.88) and effective electrical conductivities (i.e., χ=500,1000Ω−1m−1). Average current densities javg∕j0 for χ=500Ω−1m−1 (bottom curve) and χ=1000Ω−1m−1 (upper curve).

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