The Gas Diffusion Layer in High Temperature Polymer Electrolyte Membrane Fuel Cells

1.1 Polymer electrolyte membrane fuel cells. Operation at high temperature (120-200oC) 1.1.1 General overview Polymer Electrolyte Membrane Fuel Cells (PEMFC) can be considered as one of the most attractive type of fuel cells. They are able to produce efficiently high power densities. In addition, the use of a polymer electrolyte implies several advantages (Fuel Cell Handbook, 2004), such as low problems of sealing, assembling and handling. No corrosive acids, compared to Phosphoric Acid Fuel Cells (PAFC) are used, and the low temperature of the cell allows faster responses to changes in load demands. The characteristics of these cells make them especially suitable for automotive applications, even though they are also used for stationary generation, and currently, there is a great research effort for its application on portable devices (laptops, mobile phones, cameras, etc.). PEMFC are composed of the following basic elements: Ionic exchange membrane (PEM). Gas diffusion layer (GDL). Catalytic layer (CL). Monopolar/bipolar (in case of a stack) plates. The combination of the GDL+CL+PEM forms the membrane-electrode-assembly (MEA), which is the real heart of a PEMFC. This MEA can be formed by applying pressure and temperature to the (GDL+CL) in the anode side/PEM/(GDL+CL) in the cathode side (hot pressing procedure), or by directly depositing the CL onto the PEM, and subsequent hot pressing with the GDL. Ionic exchange membrane fulfils the role of allowing the transient of ionic charges from the anode to the cathode, closing the electrical circuit. It also possesses a low permeability to the gases, in order to avoid the depolarization of the electrode (Savadogo, 2004). A high mechanical and chemical stability is also required for these materials, due to the harsh operational conditions (oxidant and reducing gases in an acid medium). The most extended PEM material is Nafion®, a perflurosulphonated material, whose structure consists of a perfluorocarbon skeleton (Teflon-like), onto which, branch chains with pendant sulphonic acid groups are located, allowing the transient of ionic charges (see Figure 1).