General Design Features

The proton exchange membrane fuel cell, PEMFC, takes its name from the special plastic membrane used as the electrolyte. Robust cation exchange membranes were originally developed for the chlor-alkali industry by DuPont and have proved instrumental in combining all the key parts of a fuel cell, anode and cathode electrodes and the electrolyte, in a very compact unit. This membrane electrode assembly (MEA), not thicker than a few hundred microns, is the heart of a PEMFC and, when supplied with fuel and air, generates electric power at cell voltages around 0.7 V and power densities of up to about 1 Wcm-2 electrode area. Thin gas-porous noble metal electrode layers (several microns to several tens of microns) on either side of the membrane contain all the necessary electrocatalysis, which drives the electrochemical power generation process. The membrane relies on the presence of liquid water to be able to conduct protons effectively, and this limits the temperature up to which a PEMFC can be operated1. Figure 4.1 shows a schematic of an MEA.

1 This, of course, also depends on operating gas pressure. But operating pressures of more than approximately 0.1 to 0.3 MPa (1-3 bar or 15-45 psi) above ambient are usually ruled out because of the high compression energies required.

Gregor Hoogers

Trier University of Applied Sciences, 4.4

Umwelt-Campus Birkenfeld

Membrane

i Catalyst layer Gas diffusion layer/Substrate e

H2 in

O2 in

H2O out

FIGURE 4.1 Schematic of a membrane electrode assembly (MEA) consisting of catalyst layers, gas diffusion layers, and proton exchange membrane. The whole unit is no thicker than a few hundred microns and generates power densities of up to 1 Wcm-2 of electrode area.

FIGURE 4.1 Schematic of a membrane electrode assembly (MEA) consisting of catalyst layers, gas diffusion layers, and proton exchange membrane. The whole unit is no thicker than a few hundred microns and generates power densities of up to 1 Wcm-2 of electrode area.

Pemfc Air Cooling

FIGURE 4.2 Fuel cell stack made up of flow field plates (or bipolar plates) and MEAs (shown in the insert).

The MEA is typically located between a pair of current collector plates with machined flow fields for distributing fuel and oxidant to the anode and cathode, respectively, as shown in Fig. 4.2. A water jacket for cooling is often placed at the back of each reactant flow field followed by a metallic current collector plate. The cell can also contain a humidification section for the reactant gases, which are kept close to their saturation level in order to prevent dehydration of the membrane electrolyte.

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