Experimental Characterization of In-Plane Permeability of Gas Diffusion Layers: Influence of the Saturation Level
Date
2009-05
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Membrane Fuel
Cell (PEMFC) is a promising energy conversion device due to its clean and efficient
operation. Gas Diffusion Layer (GDL), a thin porous material, is one of its key
components. It has been shown that by tailoring the GDL properties one can increase the
PEMFC performance significantly. Characterizing the material properties is very
important for material selection in a design process. The focus of this work is to
experimentally measure an important property of the GDL material: permeability (or
more specifically in-plane permeability, since the material is typically not isotropic).
Water is the byproduct of the fuel cell reaction. During the PEMFC operation,
water often condenses within the GDL pores thus hindering the gas flow and blocking the
gas access to the reaction sites (catalyst layers). The main goal of our experiments is to
measure the influence of the water content within the porous GDL (or, saturation) on the
in-plane permeability.
Convection is often a key mode of reactant transport, and its influence varies
depending on the channel configuration. As the materials are porous, one can use the
porous media theory of Darcy’s law to describe the convective transport for which one
should first characterize the permeability of the material which is an input required in
Darcy’s law which relates the flow rate through a porous material with the pressure drop
experienced by the fluid. So convection is important and to model convection, one needs to measure permeability of the GDL. GDL is anisotropic; hence one must characterize
in-plane (and through-plane) permeability. Water condenses in the GDL pores, so one
must be able to describe permeability as a function of the saturation level.
For the purpose of this investigation, the in-plane permeability of the dry GDLs
was compared to the in-plane permeability of the wet (i.e. water-saturated) GDL’s for
different saturation levels. . For future work, different types GDLs are to be compared to
each other in addition to the effect of coating GDL with Teflon, to change the
hydrophobicity level of the GDL, and its permeability and saturation level.
Findings and results of this project include the measurements for plain carbon
paper (TGP-H-060 by Toray Industries, Inc.). Due to several major problems, primarily
coming from the complexity of the experimental setup, accruing more reliable data for
the above GDL type, as well as comparing the data for different GDL types, has been
postponed for the future. During the project, we have encountered several difficulties as
follows: Scale malfunction, test stand inaccuracy, a number of hidden leaks in the
plumbing of humid air, and condensation of water in the chamber manifolds. All the
former problems have been addressed and the only problem left to solve to get very
accurate data is to find out where water condenses in the tubes. This would help
explaining less water collected at the end of the experiments compare to how much it is
put in. The condensation problem is greatly reduced by running experiments with dry
inlet air.
Exponentially increase of permeability in the beginning of the run could be due
to the water being pushed out by force rather than drying over time which is actually desired. Result curves start to smooth out for permeability around 40% saturation and
flatten out until GDLs gets dry.