Chemical Interactions in Ionomers and Ionomer Membranes Containing Metal particles and Ions
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typically is Pt/carbon particles are in contact with each other and the electrode and they are surrounded by a proton conducting material in which they initially were dispersed as a paste. This widely used type of structure has provided for a great deal of recent progress, and it has lead to other ideas about how interfacial region can be constructed. This region is complicated, of course, in part because it is the area in which it is necessary to deliver reactants, remove products, manage water, and accomplish a number of other engineering goals. One approach to providing more design flexibility and to addressing some additional chemical issues is to attempt to build some more of the chemistry into the membrane structure itself One form of this is shown in Figure 3, in which metal particles and ions in the membrane are drawn as having the same functions as those in the more current one. The drawing is schematic, but it presents the notion that metal ions, metal particles comprising one element, and metal species comprising two or more metal species can be built in the ionomer membrane. Their compositions and forms will dictate the roles they can play in the fuel cell, and their roles will be related, of course, their spatial distribution within the membrane. There are at least three roles that particles or ions in the ionomer membranes are designed to play. One, of course, is that of the Pt/C particles now used in the interfacial region. As such, they must function as catalysts, and they must be capable of being connected to the current collecting electrode and accessible to the reactant gases. A second is that they could be used to catalyze reactions to help to control or eliminate fouling of the fuel cell, especially in the 201 Mat. Res. Soc. Symp. Proc. Vol. 575 © 2000 Materials Research Society
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form of catalyst poisoning by CO. The third role addresses the aspect of water management related to membrane hydration. In this case the catalysis of the H2 -0 2 reaction by migrating reactants, or the reactions of H2 0 and CO, or some involving methanol are useful to consider. Catalysis by Pt is of central importance for H2-0 2 cells, and catalysis by Ru in combination with Pt or other metals is potentially valuable in direct methanol cells. That makes it clear that the preparation of particles with these metals in ionomers, and their reactions with H 2, 02, and CO, at least, will be interesting to consider. Since Rh also is well known to catalyze a num
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