Electrocatalysis electronic-conducting polymers

Like other ion-exchange polymers, conducting polymers have been used to immobilize electroactive ions at electrode surfaces. Often the goal is electrocatalysis, and conducting polymers have the potential advantage of providing a fast mechanism for electron transport to and from the electrocatalytic ions. 

Among the several fields in which electronically conducting polymers are useful or may be so the future, are electrocatalysis, prosthetics, and electrodes suited for use with biomaterials, (d) Consider each of these areas and state the reasons you think electronically conducting compounds (those now available and those that may be synthesized) would have characteristic properties of special use in the areas mentioned. (Bockris)... 

In Chapter 24, Duo and coworkers discuss metal oxide nanoparticle reactivity on synthetic boron-doped diamond surfaces. Lamy and Leger treat electrocatalysis with electron-conducting polymers in the presence of noble metal nanoparticles, and new nanostructure materials for electrocatalysis are the subject of the final chapter, by Alonso-Vante. 

Electrocatalysis with Electron-Conducting Polymers Modified by Noble Metal Nanoparticles... 

C. Lamy and J.M. Leger, Electrocatalysis with electron conducting polymers modified by noble metal nanoparticles. In Catlysis and Electrocatalysis at Nanoparticle Surfaces, ed. A. Vieckowski, E. Saviniva, and C. Vayenas, Marcell Dekker, New York, 2003. 

The modification of electrode surfaces with electroactive polymer films provides a means to control interfacial characteristics. With such a capability, one can envisage numerous possible applications, in areas as diverse as electronic devices, sensors, electrocatalysis, energy conversion and storage, electronic displays, and reference electrode systems [1, 2]. With these applications in view, a wide variety of electroactive polymeric materials have been investigated. These include both redox polymers (by which we imply polymers with discrete redox entities distributed along the polymer spine) and conducting polymers (by which we imply polymers with delocalised charge centres on the polymer spine). 

Tetracyanoquinodimethane (TCNQ) and many of its derivatives are easily reduced to anions of the type TCNQ-, which form salts with various cations. With many cations, e.g., tetrathiafulvalene cations (TTF+), and N-methyl phenazinium cations (NMP+), the TCNQ- anions form electronically conducting salts (- molecular metals, -> charge-transfer complexes) that can be used as electrodes, especially because of their electrocatalytic properties (- biosensors, -> electrocatalysis, -> molecular metals) [i,ii]. TCNQ undergoes insertion electrochemical reactions (-> insertion electrochemistry) leading to TCNQ salts [iii, iv]. Polymers... 

Lefebvre, M. Qi, Z. Pickup, P. G. (1999). Electronically conductive proton-exchange polymers as catalyst supports for proton-exchange membrane fuel cells electrocatalysis of oxygen reduction, hydrogen oxidation, and methanol oxidation. J. Electrochem. Soc., 146, 2054-2058. 

The electrochemical polymerization of Ti-electron-rich aromatics, such as aniline, pyrrole and thiophene, to obtain electrically conducting polymers is well-known. Some reports describe the polymerization of amino-, pyrrolyl- and hydroxy-substituted tetraphenylporphyrins and suitable substituted phthalocyanines (for reviews see [230,231]) (anodic electropolymerization of 2,9,16,23-tetraaminophthalocyanine (M = Co(II), Ni(II)) [231,232] and 2,9,16,23-tetra(l-pyrrolylalkyleneoxy)phthalocyanines (M = 2H, Zn(II), Co(II) [232])) under formation of polymers 53 and 54 shown as idealized structures. Depending on the reaction conditions the film thicknesses are between around 50 nm and several pm. The films remain electroactive at the electrochemical potential so that oxidation or reduction current envelope grows with each successive potential cycle. Electrochromism, redox mediation and electrocatalysis of the electrically conducting films are summarized in [230,231]. 

In addition to studies focusing exclusively on the catalyst surface, the catalyst support (when employed) can play a major role in enhacing the activity/selectivity via morphologic, electronic, and physico-chemical effects. These factors have been extensively explored in the case of thermochemical heterogeneous reactions where a variety of compounds and structures have been successfully used on an industrial scale as catalyst supports (e.g., oxides, sulfides, meso- and microporous materials (molecular sieves), polymers, carbons [251-256]). In electrocatalysis, on the other hand, the practical choice of support in gas diffusion electrodes has been largely limited thus far to carbon black particles. The high electronic conductivity requirement, combined wifli electrochemical stability and cost effectivness, imposes serious restrictions on the type of materials that could be employed as supports in electrocatalysis. 

Conducting polymers have been extensively studied in recent years due to a great variety of possible applications in several fields, such as in energy storage systems, electrocatalysis [88, 89], electrodialysis membranes [90, 91], sensors [92, 93], biomedical applications [94, 95], and anticorrosion coatings [96-100]. They exhibit different oxidation states and behave as electronic or mixed conductors [101]. These new materials, also known as synthetic metals, can reach high electrical conductivity, very close to the value of some metals. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

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