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Research Papers

Two-Phase Transport in PEM Fuel Cell Cathodes

[+] Author and Article Information
Vladimir Gurau1

Chemical Engineering Department, Case Western Reserve University, Cleveland, OH 44106vladimir.gurau@case.edu

Thomas A. Zawodzinski, J. Adin Mann

Chemical Engineering Department, Case Western Reserve University, Cleveland, OH 44106

1

Corresponding author.

J. Fuel Cell Sci. Technol 5(2), 021009 (Apr 11, 2008) (12 pages) doi:10.1115/1.2821597 History: Received July 24, 2007; Revised August 29, 2007; Published April 11, 2008

To date, multiphase computational fluid dynamics models for proton exchange membrane (PEM) fuel cells failed to provide even a qualitative depiction of the fuel cell water management. This was primarily due to the inability to capture two-phase phenomena in the cathode catalyst layer and the water saturation equilibrium at the interface between the fuel cell components. A model without the cathode catalyst layer cannot capture dominant mechanisms of water transfer and cannot explain correctly the fuel cell performance. We propose a multifluid, multiphase model consisting of separate transport equations for each phase. The model accounts for gas- and liquid-phase momentam and species transport in the cathode channel, gas diffusion layer (GDL), and catalyst layer and for the current density, ionomer-phase potential, and water content in the catalyst coated membrane. The model considers water produced at cathode by (I) electrochemical reaction, (II) change of phase, and (III) parallel, competing mechanisms of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores. Liquid water is transported in the GDL and the catalyst layer due to liquid pressure gradient and in the channel due to gravity and two-phase drag. We have developed a transport equation for the water content. The source/sink terms of the transport equation represent the parallel, competing mechanisms of water transfer between the ionomer phase and the catalyst layer pores. They are (I) sorption/desorption at nonequilibrium and (II) electro-osmotic drag by the secondary current. Another distinguishing feature of this model is the capability to capture water saturation equilibrium at channel-GDL and GDL–catalyst layer interfaces. The computational results are used to study the dynamics of water transport within and between the fuel cell components and the impact of the GDL and catalyst layer properties on the amount of water retained in the fuel cell components during operation. A new dominant mechanism of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores is identified. The amount of water retained in GDL is determined by GDL permeability and its pore size at the interface with the channel. The amount of water retained in the cathode catalyst layer is determined by the saturation equilibrium at the interface with the GDL. Models based on the two-phase mixture model are not applicable to PEM fuel cell electrodes.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

The mesh and the computational domain

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Figure 2

Assumed drainage curves for the catalyst layer (p0=6240Pa, sgmin=0.2, α=–0.025) and GDL (p0=6260Pa, sgmin=0.2, α=–0.05)

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Figure 3

The radius of the attached bulge, R, decreases to a minimum equal to the capillary radius rcap and then increases again. Liquid water accumulates in the GDL until at interface pl=plmax=2σlv∕rcap. The pendant drop detaches when its weight exceeds 2πrcapσlv.

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Figure 4

Primary and secondary current densities

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Figure 5

Gas-phase pressure field and gas-phase streamlines after the first 10s of operation

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Figure 6

The liquid water streamlines after: (a) −10s, (b) −20s, (c) −30s, and (d) −40s of operation

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Figure 7

Liquid saturation profiles after 0s, 10s, 20s, 30s, 40s, 50s, and 60s of fuel cell operation along a streamline in the symmetry plane, 1cm downstream of the inlet. The case is for a GDL with the following permeability coefficients: (kV,in-plane=2.0×10−12m2, kV,through-plane=2.5×10−12m2).

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Figure 8

Liquid water saturation profiles after 10s, 30s, and 50s of fuel cell operation for the case of a GDL with relatively low permeability coefficients (kV,in-plane=2.0×10−12m2, kV,through-plane=2.5×10−12m2) and a GDL with relatively high permeability coefficients (kV,in-plane=6.0×10−12m2, kV,through-plane=7.5×10−12m2).

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Figure 9

Liquid water saturation profiles after 10s, 20s, 30s, and 40s of fuel cell operation for the case of two GDLs with different radii of the largest pores at the interface with the channel

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Figure 10

Residuals for all iterations after 22s of simulated time

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