Catalyst coated membrane hydrophobic

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

To date, there are two major types of catalyst layer fabrication techniques. One is to cast or spray the catalyst ink onto the gas diffusion layer to form a catalyzed GDL (CGDL), which is hydrophobic and has a thickness of about 20-50 pm the other is to deposit or spray catalyst ink onto the proton exchange membrane to form a catalyst coated membrane (CCM), a hydrophilic layer with a thickness of 5-10 pm. 

Unlike hydrophobic CLs, hydrophilic CLs use a hydrophilic perfluorosulfonate ionomer (PFSI) such as Nafion as a binder instead of PIPE. Hence, this kind of CL can be called an ionomer-bonded hydrophilic CL. During preparation, the catalyst powder (e.g. Pt/C), PFSI (e.g. Nafion ), and solvent (e.g. ethanol or isopropanol) are mixed thoroughly to form a uniform hydrophilic catalyst ink/paste that is then transferred to a GDL or a membrane. Hydrophilic CLs can be classified into two groups, according to the transfer method GDL-based hydrophilic CL and catalyst coated membrane (CCM). 

The GDL of PEM fuel cells connects between the bipolar plate/flow channel and catalyst layer. It provides the pathways for mass, heat, and electron transport, and also acts as the physical support of CL and membrane. The present state-of-the-art GDL consists of both a macroporous substrate and a micropo-rous layer (MPL). The macroporous substrate is a porous network of graphite fibers, which is usually carbon paper, carbon felt or carbon cloth. The substrate is often coated with hydrophobic materials such as PTFE for better liquid water removal. The MPL is coated on one side of the substrate which faces the catalyst layer. A typical MPL is a composite layer mixed by carbon particles and PTFE. The MPL usually has lower porosity and permeability but higher hydrophobicity and electrical/thermal conductivity than its substrate layer. The MPL enhances the water management and thermal/electricity connection between the substrate layer and CL to better cell performance. Unlike the membrane and catalyst layer, researches focusing on developing new materials and/or revolutionary design are scarce. Therefore, this subsection only focuses on the macroporous substrate/MPL GDL. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

---