The Proton Exchange Membrane

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

The actually developed PEMFCs have a Nafion membrane, which partially fulfills these requirements, since its thermal stability is limited to 100 °C and its proton conductivity decreases strongly at higher temperatures because of its dehydration. On the other hand, it is not completely tight to liquid fuels (such as alcohols). This becomes more important as the membrane is thin (a few tens of micrometers). Furthermore, its actual cost is too high (more than 500 m ), so that its use in a PEMFC for an electric car is not cost competitive. 

The investigation of new electrocatalysts, particularly Pt-based catalysts, that are more active for oxygen reduction and fuel oxidation (hydrogen from reformate gas or alcohols) is thus an important point for the development of PEMFCs [16,17, 22, 23]. 

The realization of the MEA is a crucial point for constructing a good fuel cell stack. The method currently used consists in hot-pressing (at 130 °C and 35 kg cm ) the electrode structures on the polymer membrane (Nafion). This gives non-reproducible results (in terms of interface resistance) and this is difficult to industrialize. New concepts must be elaborated, such as the continuous assembly of the three elements in a rolling tape process (as in the magnetic tape industry) or successive deposition of the component layers (microelectronic process) and so on. 

The bipolar plates, which separate both electrodes of neighboring cells (one anode of a cell and one cathode of the other), have a triple role  

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The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

As with batteries, differences in electrolytes create several types of fuel cells. The automobile s demanding requirements for compactness and fast start-up have led to the Proton Exchange Membrane (PEM) fuel cell being the preferred type. This fuel cell has an electrolyte made of a solid polymer. 

The Proton Exchange Membrane Fuel Cell (PEMFC)... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

In the case of 50 kW power, the rate of hydrogen supply needed (LH) is around 1.69 X 103 (mol/h) at the energy-conversion-efficiency level of 45% for the proton exchange membrane fuel cell (PEM-FC) [38]. 

The proton exchange membrane (PEM) allow only the protons to pass through to the cathode. The electrons must travel along an external circuit to the cathode, creating an electrical current. 

Would the preferential CO oxidation reaction be needed if the proton-exchange membrane fuel cell (PEMFC) with Pt anode catalyst were able to work at temperatures higher than about 403 K ... 

A potential problem with the proton exchange membrane (PEM) fuel cell, which is the type being developed for automobiles is life span. Internal combustion engines have an average life span of 15 years, or about 170,000 miles. Membrane deterioration can cause PEM fuel cells to fail after 2,000 hours or less than 100,000 miles. 

Most automotive fuel cells use a thin, fluorocarbon-based polymer to separate the electrodes. This is the proton exchange membrane (PEM) that gives this type of fuel cell its name. The polymer provides the electrolyte for charge transport as well as the physical barrier to block the mixing... 

In a proton exchange membrane (PEM) fuel cell, protons travel through a film 18 microns thick which is the proton exchange membrane. Electrons are blocked by the film and take another path which provides the electric current flow. Over time and usage tiny holes can form on the film which reduces fuel cell performance. If the film is made thicker and stronger, then performance suffers. 

Since CO acts as a poison to the proton exchange membrane fuel cell in the 50 ppm range, it has to be removed before feeding the H2 enriched gas to the fuel cell. This CO removal occurs in the PROX reactor, where Pt/Al203 catalysts are common, even though some interest in Au-based catalysts is growing due the lower cost of the active phase [48]. 

We discuss both the Proton Exchange Membrane as well as the Solid Oxide Fuel Cells in this chapter (PEMFC and SOFC). Both types are in full development, the PEMFC for mobile and stationary applications, and the SOFC for stationary applications as well as for auxiliary power generation for transport. 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

Catalyst layer ink can be deposited on gas diffusion layers to form a CCGDL, as discussed in the previous section. Alternatively, the catalyst ink can be applied directly onto the proton exchange membrane to form a catalyst-coated membrane (CCM). The most obvious advantage of the CCM is better contact between the CL and the membrane, which can improve the ionic connection and produce a nonporous substrate, resulting in less isolated catalysts. The CCM can be classified simply as a conventional CCM or as a nanostructured thin-film CCM. 

Gamburzev, S., and Appleby, A. J. Recent progress in performance improvement of the proton exchange membrane fuel cell (PEMFC). Journal of Power Sources 2002 107 5-12. 

Chao, W. K., Lee, C. M., Tsai, D. C., Chou, C. C., Hsueh, K. L., and Shieu, F. S. Improvement of the proton exchange membrane fuel cell (PFMFC) performance at low-humidity conditions by adding hygroscopic y-Al203 particles into the catalyst layer. Journal of Power Sources 2008 185 136-142. 

G. J. M. Janssen and M. L. J. Overvelde. Water transport in the proton-exchange-membrane fuel cell Measurements of the effective drag coefficient. Journal of Power Sources 101 (2001) 117-125. 

Because of its lower temperature and special polymer electrolyte membrane, the proton exchange membrane fuel cell (PEMFC) is well-suited for transportation, portable, and micro fuel cell applications. But the performance of these fuel cells critically depends on the materials used for the various cell components. Durability, water management, and reducing catalyst poisoning are important factors when selecting PEMFC materials. 

Fuel cells are an attractive alternative for replacing single-use and rechargeable batteries in mobile communication gadgets like cell phones, PDAs and laptops. Particularly the Proton Exchange Membrane (PEM) cell... 

New membrane materials for PEM fuel cells must be fabricated into a well-bonded, robust membrane electrode assembly (MEA) as depicted in Figure 1. In addition to the material requirements of the proton exchange membrane itself as outlined above, the ease of membrane electrode assembly fabrication and the resulting properties of the MEA are also... 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

After rehearsing the working principles and presenting the different kinds of fuel cells, the proton exchange membrane fuel cell (PEMFC), which can operate from ambient temperature to 70-80 °C, and the direct ethanol fuel cell (DEFC), which has to work at higher temperatures (up to 120-150 °C) to improve its electric performance, will be particularly discussed. Finally, the solid alkaline membrane fuel cell (SAMFC) will be presented in more detail, including the electrochemical reactions involved. 

In addition, in a DAFC, the proton exchange membrane is not completely alcohol tight, so that some alcohol leakage to the cathodic compartment will lead to a mixed potential with the oxygen electrode. This mixed potential will decrease further the cell voltage by about 0.1-0.2 V. It turns out that new electrocatalysts insensitive to the presence of alcohols are needed for the DAFC. 

An additional problem arises from ethanol crossover through the proton exchange membrane. It results that the platinum cathode experiences a mixed potential, since both the oxygen reduction and ethanol oxidation take place at the same electrode. The cathode potential is therefore lower, leading to a decrease in the cell voltage and a further decrease in the voltage efficiency. 

The ionic monomer that forms the proton exchange membrane (PEM) separating and ionically connecting the two gas diffusion electrodes can be dissolved in isobutyl alcohol or other organic solvents, such as isopropanol. This circumstance opens the way for improving the ionic contact between the catalyst particles of a gas diffusion electrode and the proton-conducting membrane and electrolyte. 

The proton exchange membrane has to be improved so as to reduce resistance across the membrane and the transfer of sulphur species across the membrane that results in the deposition of sulphur on the cathode thereby poisoning the cathode. The focus will initially be on testing membranes developed for fuel cells. The NWU will be acquiring and testing fuel cell membranes which include the three top membranes in meeting the minimum requirements as set by US DOE for 2008, from Professor P. Pintauro (Vanderbilt University, Nashville, USA), Dr. R. Wycisk (Case Western Reserve University, Cleveland, USA) and Giner Electrochemical Systems Inc. 

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