Page 55 - Introduction to Transfer Phenomena in PEM Fuel Cells

44 Introduction to Transfer Phenomena in PEM Fuel Cells
1.3.1.3. The gas diffusion layers
The gas diffusion layers (GDL) are a fibrous porous medium that has two
main functions: to ensure a uniform distribution of reactive gases on the
surface of the electrodes, and the transport of electrons to or from the
external electrical circuit. They are made of thin carbon fibers, a porous and
hydrophobic material. The thickness of these fibers is between 250 and
400 µm, and the perforations or pores available are 4 to 50 μm in size [NGU
10]. These gas diffusion layers must also serve as a support for the
electrolyte and its structure must facilitate the evacuation of water that would
prevent the reaction from occurring.
1.3.1.4. Bipolar plates

Bipolar plates are made of graphite or metal; they evenly distribute the
fuel and the oxidant to the cells of the fuel cell. They also collect the
generated electric current at the output terminals.

In a single-cell fuel cell, there is no bipolar plate; however, there is a
single-sided plate that provides the flow of electrons. In fuel cells that have
more than one cell, there is at least one bipolar plate (flow control exists on
both sides of the plate). Bipolar plates provide several functions in the fuel
cell.
Some of these functions include the distribution of fuel and oxidant
inside the cells, the separation of the different cells, the collection of the
electric current produced, the evacuation of the water from each cell, the
humidification of the gases and cooling of the cells. Bipolar plates also have
channels which allow the passage of reactants (fuel and oxidant) on each
side. They form the anode and cathode compartments on opposite sides of
the bipolar plate. The design of the flow channels may vary; they may be
linear, coiled, parallel, comb-like or evenly spaced as shown in Figure 1.19.

The materials are chosen based on chemical compatibility, corrosion
resistance, cost, electrical conductivity, gas diffusion ability, impermeability,
ease of machining, mechanical strength and their thermal conductivity.