Electronically conducting cation-exchange polymer

P. G. Pickup, C. L. Kean, M. C. Lefebvre, G. Li, Z. Qi, and J. Shan. 2000. Electronically conducting cation-exchange polymer powders Synthesis, characterization and applications in PEM fuel cells and supercapacitors. / New Mat Electrochem Systems 3(l) 21-26. 

The idea of using an ion-conductive polymeric membrane as a gas-electron barrier in a fuel cell was first conceived by William T. Grubb, Jr. (General Electric Company) in 1955. - In his classic patent, Grubb described the use of Amber-plex C-1, a cation exchange polymer membrane from Rohm and Haas, to build a prototype H2-air PEM fuel cell (known in those days as a solid-polymer electrolyte fuel cell). Today, the most widely used membrane electrolyte is DuPont s Nation... 

The need for high electronic conductivity has meant that work has focused on the use of the so called conducting polymers, exemplified by polypyrrole, polyaniline and polythiophene (Structures 1 to 3). These materials must be in their p-doped (partially oxidized) states (right hand side of eq. 1, for example) to exhibit sufficient electronic conductivity. Unfortunately, the p-doped polymers are cationic and will therefore tend to exclude the protons needed for the fuel cell reactions. To circumvent this problem, composites of conducting polymers with cation exchange polymers have been used. Thus p-doped polypyrrole / poly(styrene-4-sulphonate) (PSS), for example, exhibits both proton and electron conductivity (8). 

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum... 

The redox polymers, both of organic and inorganic origin (such as polyvinylpyridine modified by redox-active complexes of metals Prussian blue and related materials), can be considered as a version of electrodes of the second kind however, the equilibrium is usually estabhshed with respect to cations. Electron conducting polymers (polyanyline, polypyrrol, and so forth) also pertain, in the first approximation, to the electrodes of the second kind, which maintain equilibrium with respect to anion. Ion exchange polymer films on electrode surfaces form a subgroup of membrane electrodes. 

The most studied of all variables is the coimterion. Numerous workers " have shown that the counterion incorporated has a dramatic effect on the conductivity of the polymer. For a given counterion, the concentration employed also affects the conductivity of the resultant polymer. Maddison and Jenden even showed that counterion exchange after synthesis has an effect on the polymer conductivity. Conductivity decreases as the electron affinity of the counterion is increased. It has been reported that the degree of oxidation of the polymer does not vary appreciably as the coimterion is varied. The trend in conductivity is related to the nucleophilicity of the coimterion employed this may be due to some sort of anion-induced localization of the radical cation in the polymer. The effect of even slight changes in the molecular structure of the counterion on the conductivity of a range of PPys has been studied (Table 3.1). In some cases, the use of mixed counterion systems also has a marked effect on conductivity (Table 3.2). 

In three-dimensional mixed-valence systems, electron transfer can manifest itself as electrical conduction, thermally activated. Most work continues to focus on the better known semiconducting materials such as silicon-boron or silicon nitride " (at low temperature), or organic crystals of the anthracene type (at high temperature),or redox polymer-coated electrodes. In the last-mentioned case, the importance of ion migration as well as electron transfer has recently been emphasized. In the mixed-valent Tl(I)3Tl(III)Cl6, conductivity and isotopic exchange studies have been taken to indicate that cation transfer is the principal charge-carrying mechanism, and not electron transfer as such. " Mossbauer... 

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