Classification of MEA Materials

As mentioned above, there are three main components in an MEA, including membrane, CL and GDL. 

1) Membrane. Perfluorosulfonic acid (PFSA) is the most commonly used membrane material [4], PFSA membranes are relatively strong and stable in both oxidative and reductive environments, since the structure of PFSA is based on a PTFE backbone. The conductivity of a well-humidified PFSA membrane can be as high as 0.2s cm. As is well known, fuel cell operation at elevated temperatures can increase the rates of reaction, reduce problems related to catalyst poisoning, reduce the use of expensive catalysts, and minimize problems due to electrode flooding. Unfortunately, a PFSA membrane must be kept hydrated to retain its proton conductivity. Moreover, a PFSA membrane is alcohol permeable if it is used in DAFCs. Because of the disadvantages of PFSA membranes, many alternatives have been proposed [106]. Five categories of membranes are classified (1) perfluorinated, (2) partially fluorinated, (3) non-fluorinated, (4) non-fluorinated composite, and (5) others. 

2) Catalyst layer. In intimate contact with the membrane and the backing layer is the CL. This CL, integral with its binder, forms the electrode. Although we know that the reaction of oxygen and hydrogen to form water is a thermodynamically spontaneous reaction, the reaction rate is extremely slow at normal conditions. That is why the catalysts should be used to accelerate the reactions. 

Non-Pt catalysts are extremely interesting, due to limited Pt sources and high cost. Pd-based [57,1011], Ag-based [55] and WC promoted [57] cathode catalysts 

3) Gas diffusion layer (GDL). The polymer membrane is sandwiched between two sheets of porous backing media, which are referred to as gas diffusion layers (or current collectors). The GDLs are usually made of a porous carbon paper or carbon cloth, typically 1000 pm thick. The GDL acts as a gas diffuser, provides mechanical support and an electrical pathway for electrons, and transfers water away from the electrodes. The porous structure allows the gas to spread out as it diffuses so that the gas will be in contact with the entire surface area of the catalyzed membrane. The GDL is typically water-proofed to ensure the rapid gas diffusion necessary for good fuel cell performance. 

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