PEM cell

In 1997, Ford announced that it would invest 420 million in a global alliance with what was then Daimler-Benz and Ballard Power Systems. This provided Ballard with an important infusion of capital. As a result of these investments, Ford owned 15% of Ballard and DaimlerChrysler 20%. It was a critical moment for fuel cells since the total investment was reaching almost 1 billion, including the 450 million by DaimlerChrysler. The alliance of Ford, Volvo, and DaimlerChrysler was pushing the leading edge of fuel cell innovation. Ballard has focused on PEM cells with a goal to have commercial fuel cells available by 2010. 

DaimlerChrysler s fuel cell was in the Mercedes-Benz B-class car. The fuel cell is a sandwich design with the polymer PEM cell between... 

Toyota began to work with PEM cells in 1989 and produced a methanol reformed car, the FCEV, in 1997. This car was based on Toyota s electric RAV4. 

PEM fuel cells can convert about 55% of the fuel energy fed into them into actual work. The comparable efficiency for IC engines is in the range of 30%. PEM cells also offer relatively low temperature operation at 80°C. The materials are used to make them reasonably safe with low maintenance requirements. 

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... 

A relatively new member of the fuel cell family, the DMFC is similar to the PEM cell in that it uses a polymer membrane as an electrolyte. The DMFC is a special form of low-temperature fuel cell. It can be operated at 355 75 K temperatures depending on the fuel feed and type of electrolyte used. In a DMFC, methanol is fed directly into the fuel cell without the intermediate step of reforming the alcohol into hydrogen (Collins, 2001). 

The polymer electrolyte membrane or proton exchange membrane (PEM) cell. 

The SOFC will be the major concern here although it will be helpful to consider first the elements of the PEM cell since not only is it a very strong contender for large-scale use, but its basic science is simple. 

The PEM cell is the cleanest fuel cell since the fuel is hydrogen, the oxidant oxygen and the product water. Although it clearly falls outside the scope of a text focused on electroceramics there are good reasons for prefacing the present discussion with a brief outline of those elements of the science and technology basic to it and common to the ceramics-based fuel cells. Also, for an intelligent... 

The elements of the PEM cell are shown in Fig. 4.26 together with the reactions occurring at the anode and cathode. 

It is inappropriate to pursue here optimization of the electrolyte design for a PEM cell. The essentials of the cell are a thin polymer membrane coated on each surface with carbon mixed with platinum particles acting as the catalyst. 

The intermediate temperature SOFC offers an advantage over the PEM cell of a predicted higher efficiency, 45-50% compared to 30%. It can also be integrated with a reformer which, utilizing some of the waste heat, produces useable fuels, (H2 and CO) from a hydrocarbon fuel. 

Fig. 3. PEM cell components and reaction showing the positive anode and negative cathode...

Electrolytes are a critical material in the performance of electrolyzers. Low-temperature electrolysis of water relies on proton exchange membrane (PEM) cells using sulfonated polymers for the electrolytes. Key issues for all electrolyzers are the kinetics of the system that is controlled by reaction and diffusion rates. Catalysts such as platinum, Ir02, and RUO2 are used to improve the reaction kinetics, but they also contribute to the cost of the system, which is also an issue. Steam electrolysis is also a possibility at a temperature of about 1,000°C using ceramic membranes. 

For use in proton exchange membrane (PEM) fuel cells (see section 3.6), the CO contamination in the hydrogen produced must be below 50 ppm (parts per million). This is due to the poisoning limit of typical platinum catalysts used in PEM cells. The implication is the need for a final CO cleaning treatment, unless the main reaction steps (2.1) and (2.2) can be controlled so accurately that all reactants are accounted for. This CO cleaning stage may involve one of the following three techniques preferential oxidation. 

In general, because PAFCs are proton conductors like the proton exchange membrane (PEM) and the subcategory direct methanol fuel cells (sections 3.5 and 3.6), there is a continuous conceptual transition between them. The polymers used in PEM cells usually contain weakly acidic components such as HSO3, but may be reinforced with a stronger acid in order to increase conductivity or allow operation at higher temperatures. 

Figure 3.48. Distributions of local current density along the membrane of a PEM cell with either straight (left) or interdigitated (right) channel design, at maximum average current density (=0.8 A cm" ). The membrane thickness is 0.16 mm. The figure co-ordinate system is x-z (cf. Fig. 3.36) rather than y-z as used in several previous figures. (From M. Hu et al. (2004). Three dimensional, two phase flow mathematical model for PEM fuel cell Part II. Analysis and discussion of the internal transport mechanism. Energy Conversion Management. 45, 1883-1916. Used with permission from Elsevier.)...

Figure 3.49. Slice of a PEM cell showing gas diffusion layer (A), catalyst layer (B) and membrane layer (C), at a magnification factor of 200 (a). Tunnelling electron microscope pictures of catalyst layer at a magnification factor of 500 (b), 18 400 (c) and in (d) 485 500. (From N. Siegel, M. EUis, D. Nelson, M.v.Spakovsky (2003). Single domain PEMFC model based on agglomerate catalyst geometry. J. Power Sources 115, 81-89. Used with permission from Elsevier.)...

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