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Research Papers

Reaction Kinetics at the Triple-Phase Boundary in PEM Fuel Cells

[+] Author and Article Information
P. Berg

Faculty of Science, UOIT, 2000 Simcoe Street N, Oshawa, ON, L1H 7K4, Canadapeter.berg@uoit.ca

A. Novruzi

Department of Mathematics and Statistics, University of Ottawa, 585 King Edward Avenue, Ottawa, ON, K1N 6N5, Canadanovruzi@uottawa.ca

O. Volkov

Department of Mathematics and Statistics, University of Ottawa, 585 King Edward Avenue, Ottawa, ON, K1N 6N5, Canadaovolkov@uottawa.ca

J. Fuel Cell Sci. Technol 5(2), 021007 (Apr 11, 2008) (10 pages) doi:10.1115/1.2821599 History: Received July 03, 2007; Revised July 10, 2007; Published April 11, 2008

A mathematical model is developed to describe the reaction dynamics in the vicinity of the triple-phase boundary (TPB), which is an important part of the pore scale structure of the catalyst layer in proton exchange membrane fuel cells. The model incorporates coupled diffusion, migration, and reaction phenomena of the chemical components in an undersaturated air pore and ionomer. One challenging feature of the work is the description of the TPB by a system of nonlinear partial differential equations (PDEs), coupling bulk, and surface-diffusion phenomena, which offers an approach to study the rarely investigated proton surface diffusion along the air pore surface. A numerical technique is implemented, taking into account the particular form of the domain, in order to solve the nonlinear PDE system efficiently. Several numerical results are discussed, including a sensitivity analysis with respect to the physical reference case and geometric parameters. The results indicate that surface diffusion might play a major role for the reaction kinetics, but only if the air pore is void of liquid water. In contrast, the formation of liquid water in the gas pores will turn surface diffusion into bulk diffusion, with the latter resembling the Grotthus mechanism.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic picture of a C∕Pt particle in the CL and geometric modeling of the triple-phase contact point

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Figure 2

Diffusion fluxes considered in this model

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Figure 3

Surface diffusion

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Figure 4

Reference case solution. Ω1 and Ω2 each occupy a right angle sector. Solutions of oxygen, vapor, water and protons are given in a dimensionless form, the electric potential in Volt.

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Figure 5

Dimensionless electrochemical reaction term S¯c (see Eq. 17) along the TPB Γ01∪Γ02 for different current densities i0ref. The coordinate −1 corresponds to the boundary Γ1, and the coordinate +1 corresponds to the boundary Γ2. Triangles and circles correspond to high and low current densities, respectively.

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Figure 6

Dimensionless reaction term c¯Oc¯+exp(αcφ¯) along the ionomer-C∕Pt interface. Comparison between the (oxygen-only) model of O’Hayre (5) and the results calculated with the full model presented in this article for different values of the kinetic reaction parameter k0.

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Figure 7

Current as a function of different reference values

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Figure 8

Current as a function of the ionomer fraction angle α

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Figure 9

Water sorption isotherm on the air pore/ionomer interface Γ12 and in the air pore Ω1

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Figure 10

Flux balance at the origin

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