Polyaniline conducting polymer support

The most widely studied conducting polymer support is polyaniline (PANl), which has been shown to decrease the poisoning of Pt by COads [88]. Gharibi et al. have recently explored the factors responsible for the enhanced formic acid oxidation activity of Pt supported on a carbon/PANI composite [89]. They concluded that improvements in both electron and proton conductivities, as well as the increased methanol diffusion coefficient and decreased catalyst poisoning, could be involved. A carbon nanotubes/PANI composite [90], poly(o-methoxyaniline) [91], and polyindole [92] have recently been reported as effective supports for formic acid oxidation at Pt nanoparticles, while polycarbazole [93] has also been used to support PtRu nanoparticles. 

The use of conducting polymer supported metal catalysts for the main fuel cell reactions (oxygen reduction, hydrogen oxidation, and methanol oxidation) is reviewed and new results are presented for oxygen reduction at polyaniline supported Pt and methanol oxidation at poly(3,4-ethylenedioxythiophene) supported Pt-Ru. It is shown that polymer supported electrodes can provide performances that proach those of comma cial carbon supported catalysts, but tlmt stability problems curr tly make applications in fuel cells impractical... 

Poly(Af-acetylaniline) (PAANI), one of the substituted polyaniline conducting polymers, was successfiilly used as a support for fuel cell catalysts [84]. PEDOT/ PSS has also been reported as support material, with activities for oxygen reduction of Pt/PEDOT/PSS comparable with that obtained with commercial carbon-supported catalysts. However, long-term stability of PEDOT/PSS has to be improved and conductivity is still lower compared with carbon materials [85]. Choi et al. electrodeposited Pt-Ru catalysts on poly(Af-vinyl carbazole) (PVK) and poly(9-(4-vinyl-phenyl)carbazole) (P4VPCz). The performance of DMFCs with carbon-supported Pt-Ru showed better performance than that of the Pt-Ru/PVK composite due to its low electronic conductivity [86]. 

Though conductivity of polyaniline is not as high as that of some other polymers, it is emerging as the material of choice for many applications. It is stable in air and its electronic properties can be easily tailored. It is one of the oldest synthetic polymers, and probably it is the cheapest conducting polymer used in devices. It can be easily fabricated as thin films or patterned surfaces. Polyaniline will never replace the materials which have extremely high conductivity. However, it will be useful for certain specific applications. Andy Monkman has a program to extrude the polymer braids and lay the insulation of the coaxial cables in a single step. The work is supported by a cable company [3], Properties of polyacetylene are discussed in detail in Chapter 2. 

Polypyrrole was often used as support for platinum particles. Similarly to the case of polyaniline, the activity of such electrodes for the oxidation of methanol depends both on the amount of platinum and on the thickness of the polymer film [43]. In the same study, by using in-situ infrared spectroscopy, it was confirmed that linearly adsorbed CO species are the only detectable species present at the electrode surface. The authors attributed the enhancement of the overall activity observed to the high and uniform dispersion of the metallic particles with, possibly, an effect of the conducting polymer matrix itself. The same conclusions were drawn from another study [44] where the size of the particles obtained by electrodeposition was estimated at 10 nm. In this study, the Pt particles were entrapped into the polymer layer and showed a better activity than particles only deposited on the polymer surface. The authors interpreted their results as a decrease of the poisoning phenomenon in the 3D film in comparison to the only 2D deposit. 

Koleli et al. studied conducting polymer films as electrode materials for CO2 reduction. " They showed that polyaniline supported by a Pt sheet reduces CO2 to HCOOH, HCHO and CH3COOH in methanol based electrolyte at -0.4 V vs. SCE under ambient to elevated pressures up to 20 kbar. The current density is 1 to 2 mA cur with the total faradaic efficiency of the products close to 100%. They obtained similar results in polypyrrol electrode as well. 

Since 1986, there has been no International Conference on Science and Technology of Synthetic Metals in which the question—solution or dispersion —did not raise exciting discussions. Most of the scientists support the position [43] that clear (colored) mixtures of (organic) solvents with intrinsically conductive polymers (OM) are solutions. (The first reports of soluble and moldable conducting polymers were published by Elsenbaumer et al. [44a,b].) Only a few support our position [42b], which considers the mixtures to be dispersions. In the recent years, the debate concerning polyaniline and solvent systems has attracted growing interest. 

Polyaniline has only recently been employed to support Pt particles for oxygen reduction (10,11). In one approach, polyaniline films were cast onto glassy carbon electrodes fi om a solution of camphorsulfonic acid doped polyaniline, and then Pt was electrochemically deposited firom a H2PtCl5 solution. The pol iline film was found to provide a higher surfoce area for Pt deposition but exhibited limited permeability to oxygen. Use of a composite of polyaniline and Nafion (a proton conducting polymer commonly used in the fabrication of fiiel cell electrodes) resulted in better oxygen permeabilty (11). 

Hydrogen oxidation has been studied at electrodes coated with Pt supported on electrochemically deposited films of polypyrrole (5, 17), polyaniline (17, 18), and poly(pyrrole-co-dithiophene) (19). It appears that the kinetics and mechanism of H2 oxidation are similar to those on bulk Pt (18), as long as the Pt is deposited on the surfece of the polymer to avoid mass transport limitation in the polymer matrix (17) and the polymer is sufficiently conductive at the IT/H2 formal potential (17). The latter point is particularly important since the electronic conductivities of all these p-doped conducting polymers decrease with decreasing potential, and can... 

The relatively high operating potentials of methmiol anodes, and slowness and mechanistic complexity of the medianol oxidation reaction provide considerable incentive to develop polymer supported catalysts, and this has resulted in much research activity. Polypyrrole has been most widely used as a support, presumably because its conductivity extends to lower potentials than for most other conducting polymers. Polyaniline has also attracted significant attention, and some polythiophenes are attractive for their enhanced stability. 

A similar problem was encountered when conductive polyaniline/PSS was used as the support. Fortunately, conducting poly(3,4-ethylenedioxythiophene) (PEDOT)/PSS composite was found to be much more stable. Before Pt deposition, a PEDOT/PSS composite possessed an electronic conductivity of 9.9 S cm while the conductivity of Vulcan XC-72 was 3.0 S cm measured under the same conditions. In other words, the conductive polymer composite was more than three times as conductive as the commercial carbon black. Pt nanoparticies with ca. 4 nm diameter were deposited onto PEDOT/PSS composite particles through the... 

Melt mixing has been also used to disperse conductive polymers in SBS rubber. Polyaniline doped with dodecylbenzenosulfonic acid was mixed with SBS rubber in a Brabender mixer [119]. A conductivity of 2 S cm was achieved with a loading of 50% of the conductive polymer. According to the authors, this blend could be extruded. However, no supplementary data supporting this conclusion were provided. 

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