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In-Plane Effective Diffusivity in PEMFC Gas Diffusion Layers

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Abstract

The in-plane effective diffusion coefficients in gas diffusion layers typically used in fuel cell electrodes were measured as a function of compression and hydrophobic polymer loading. This method was based on the transient diffusion of oxygen from air into an initially nitrogen purged porous sample and has proven to be accurate, fast, and straightforward. As anticipated, with higher compressions and higher PTFE loadings, effective diffusivity decreased, as a result of less pore space available for transport and because tortuosity increased. When plotted against compressed porosity, the effective diffusivity of untreated and treated materials for a given type of sample collapsed on top of each other, despite the simultaneous impact of PTFE loading and compression. It was possible to distinguish between the impact of PTFE and compression by plotting the data as tortuosity against compressed thickness. High compressions on the sample lead to irreversible damages to the fiber structure, resulting in decreased or unexpectedly low tortuosity. Finally, a percolation model was fitted through one of the tested materials and a reasonable agreement was observed for lower compression, but a fit to the entire data could not be achieved. This was attributed to fundamental structural changes occurring in the sample upon high compressions, an observation that helps to explain the general inability of theoretical tortuosity models to describe GDLs.

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Abbreviations

C :

Concentration (%)

D :

Diffusion coefficient (\(\hbox {cm}^{2}\hbox { s}^{-1}\))

\(D'\) :

Normalized effective diffusivity (–)

d :

Diameter (m)

l :

Length domain (m)

t :

Time (s)

w :

PTFE loading (wt%)

z :

Spatial coordinate (m)

\(\alpha \) :

Fitting parameter (–)

\(\delta \) :

Thickness (M)

\(\varepsilon \) :

Porosity (–)

\(\varepsilon _p \) :

Percolation threshold (–)

\(\rho \) :

Density (\(\hbox {kg/m}^{3}\))

\(\tau \) :

Tortuosity (–)

0:

Initial

1:

Final

b :

Bulk

eff:

Effective

PTFE:

Polytetrafluoroethylene

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Acknowledgments

The funding provided by the Automotive Fuel Cell Cooperation (AFCC) and the Natural Sciences and Engineering Research Council (NSERC) of Canada is greatly appreciated. The authors also wish to thank SGL Group for donating GDL sample materials.

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Authors and Affiliations

  1. Department of Chemical Engineering, McGill University, Room 3060, Wong Building, 3610 University Street, Montreal, QC, Canada

    Rinat R. Rashapov & Jeff T. Gostick

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  1. Rinat R. Rashapov
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  2. Jeff T. Gostick
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Correspondence to Jeff T. Gostick.

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Appendix

Appendix

See Figs. 10, 11 and 12.

Fig. 10
figure 10

Normalized effective diffusivity \(D^{\prime }\) versus compressed porosity \(\varepsilon \) of untreated and treated SGL 34, SGL 35, TGP-H-060, and TGP-H-060

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Fig. 11
figure 11

Tortuosity \(\tau \) versus compressed porosity \(\varepsilon \) of untreated and treated SGL 34, SGL 35, TGP-H-060, and TGP-H-060

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Fig. 12
figure 12

Tortuosity \(\tau \) versus thickness ratio \(\delta _0 /\delta \) of untreated and treated SGL 25, SGL 35, TGP-H-060, and TGP-H-060

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Rashapov, R.R., Gostick, J.T. In-Plane Effective Diffusivity in PEMFC Gas Diffusion Layers. Transp Porous Med 115, 411–433 (2016). https://doi.org/10.1007/s11242-016-0648-4

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