Fuel Cell Electrodes and Gas Diffusion Layers

Modern fuel cell electrodes are gas diffusion electrodes (GDEs) that consist of a gas porous layer of high surface area catalyst and a gas porous, electrically conducting gas diffusion layer, or electrode substrate (or electrode backing material) — compare Fig. 4.1. Two similar electrodes, anode and cathode, are in intimate contact with the polymer electrolyte membrane.

Views differ on whether the term "electrode'" denotes just the layered catalyst structure attached to the membrane or whether it comprises substrate and catalyst layer. Likewise, MEAs are currently sold that do not include the electrode substrates, which have to be purchased separately. In this context, it is better to talk of a CMA, a catalyzed membrane assembly. We will refer to the MEA as the complete entity shown in Fig. 4.1, and to electrodes or GDEs as gas diffusion layers in intimate contact with catalysts.

The various functions of fuel cell electrodes havebeensummarizedin 4.1. Briefly, anodeand cathode electrodes allow the respective reactants, hydrogenandair/oxygen, to reachthe reactive zone within the electrode. Electrons and heat are conductedthroughthe substratelayer, whichformsalink with the adjacent cells, cooling plates, or current collectorplates. Atthecathode, theremovalof (liquid) product water is an additional, particularlyimportanttask.

Why not let the reactants sweep directly past the catalystlayersforperfectgasaccessand effectivewater removal? Figure 4.13 shows why a substrate material is needed. The need arises ultimately from the compromise made between building a gas phase reactor and a unit with minimal ohmic losses. As Fig. 4.13 demonstrates, the substrate is needed as a spacer allowing gas access even to catalyst areas underneath the supporting ribs of the gas distribution structure or flow field (see Section 4.4.1.1). These regions are also known as the landing area. Likewise, electrons have to be conducted to or from catalyst regions underneath open gas channels. Here, the substrate provides a conductive path to the landing area of the flow field.

A number of materials have been used as substrates for fuel cell electrodes. Carbon fiber papers and woven carbon cloths are the most prevalent backing layers for electrodes. Carbon fiber papers are high-

FIGURE 4.13 Cross-section of a MEA (only membrane and one electrode are shown) in contact with a flow field plate. The schematic sketch shows the function of the porous electrode substrate. The open gas channels are needed for reactant supply, but only the landing areas can conduct electric current. Therefore, the substrate conducts the current laterally from electrode areas above the open channels to the landing areas. Likewise, gas is being conducted through the porous material to electrode sections above the landing areas. (Drawing courtesy of Elke Schnur, UmweltCampus Birkenfeld.)

FIGURE 4.13 Cross-section of a MEA (only membrane and one electrode are shown) in contact with a flow field plate. The schematic sketch shows the function of the porous electrode substrate. The open gas channels are needed for reactant supply, but only the landing areas can conduct electric current. Therefore, the substrate conducts the current laterally from electrode areas above the open channels to the landing areas. Likewise, gas is being conducted through the porous material to electrode sections above the landing areas. (Drawing courtesy of Elke Schnur, UmweltCampus Birkenfeld.)

Proton Exchange Membrane Catalyst Layer Electrode Substrate Open Gas Channel Flow Field Plate

Tanja Horn Birkenfeld
FIGURE 4.14 Micrographs of commercially available carbon fiber papers from (a) Toray Industries and (b) SGL Carbon, and (c) carbon cloth from E-Tek. Note the modification by fillers in (b) and (c). (Confocal microscopy images by Tanja Horn, Umwelt-Campus Birkenfeld.)

temperature-sintered, more-or-less rigid structures and offer excellent electronic conductivity (Campbell et al., 1999). Cloths are flexible materials, also with good conductivity (Campbell et al., 1999) and possible advantages in high power performance due to improved water management (Ralph et al., 1997). These materials are now available from several suppliers. Microscopic (confocal) pictures of two fuel cell carbon fiber papers and a substrate based on carbon cloth are shown in Fig. 4.14.

A potential low-cost approach consists of a poorly conducting carbon web (likely to be a non-woven material) filled with an electrically conducting filler, such as carbon black (Campbell et al., 1999).

As the substrate has to enable a two-phase flow (at least when used at the cathode), gas in and water out, it is usually covered with a wetproofing agent such as PTFE (Mosdale et al., 1994).

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment