Gas Diffusion Layer (GDL)

Abstract

The GDL is important part of polymer electrolyte membrane PEM and direct methanol DM fuel cell. There are two gas diffusion layers (cathode and anode sides) which as the name suggests diffuse reactant gases to the catalyst layer (of the electrolyte membrane). Other main functions include removing excess water and heat away from the catalyst layer for the smooth operation and preventing flooding. GDL is capable of retaining some water to ensure good conductivity throughout the membrane. It provides mechanical strength and support to the catalyst and membrane. GDL is electrically conductive; it also provides pathway for current collection. Thus, the main requirements of GDL are permeability of gases and water, conductivity, and mechanical strength. Due to these requirements, GDL is most commonly made up of carbon fiber-based products, i.e., cloth and carbon paper. As carbon cloth is thicker than carbon paper, it has certain advantages compared to carbon paper. Carbon cloth is made by weaving carbon fibers and therefore more flexible and stronger than carbon paperbased GDL layers. New generation GDLmaterial combining the benefits of carbon cloth and paper is also being investigated by some researchers. Due to different considerations for a GDL, many factors such as type of carbon used, pore formation, and thickness play an important role in the fabrication of GDL for a particular application. In most cases GDL consists of two layers: macroporous diffusion medium (DM) and microporous layer (MPL) typically 200–400 mm thick. DM has pore size ranging from 10 to 30 mm, and MPL has pore size of 0.05–5 mm and is purposely made hydrophobic by treating with small amount of Teflon or PTFE (5–10 wt.%). The small pore size creates a capillary pressure gradient across the membrane for effective removal of water away from the reaction sites.