Catalyst coated membrane hydrophilic

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). 

New reactor designs and immobilisation methods have been used to extend the lifetime of lipases in scCC>2 (Lozano et al., 2004). Ceramic membranes have been coated with hydrophilic polymers and the enzyme covalently attached to these. In SCCO2, activities and selectivities were excellent and the half-life of the catalyst was enhanced. It is thought the hydrophilic layer of the membrane protected the enzyme. Operational stability of enzymes has also been increased by using ionic liquid/scC02 biphasic systems (Lozano et al., 2002 Reetz et al., 2003). 

In some of the cases listed in Table 3.1 catalytic membranes are used. For instance the catalytic membranes prepared by Peters et al. (2007) have a catalyst layer deposited on a composite hydrophilic membrane. The dual-layer structure allows the independent optimization of the selective and the catalytic properties and partially satisfies contradicting material properties , such as the necessity to achieve a low permeability of the reactants together with a high availability of the same reactants on the catalyst. The value of 5 can be varied by changing the number of dip-coat... 

Membrane-based Hydrophilic Catalyst Layer. Wilson and Gottesfeld [8, 21-23] suggested an ionomer-bonded hydrophilic catalyst layer prepared with the decal transfer method. The so-called decal transfer process includes two key steps (1) coating catalyst ink onto a blank substrate (e.g., PTFE film) then (2) transferring the coat onto the membrane (as shown in Figure 19.6). A typical preparation procedure is as follows ... 

There are many sol-gel derived materials with chemical function, such as catalyst, membrane, chromatograph column, and water repellent and hydrophilic films. There are also coating films which protect substrates from chemical attacks, such as corrosion and oxidation. Barrier coatings have a role in suppressing the passing of Oaand H2O through the plastic films. The sol-gel derived materials with chemical functions mentioned above are listed in Table 1-7. 

Zhu et al ° reported the esterification of acetic add and ethanol in a tube-and-sheU type reactor, using a polymer-ceramic composite membrane obtained by dip-coating a ceramic support tube with a hydrophilic poly-etherimide. Reactants, along with sulfuric add as homogeneous catalyst, were fed in the tube side while a vacuum was applied to the shell side. 

---