Electrolytic reactions electrochemical polymerization

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Electrochemical polymerization is a fast and simple, widely used method to synthesize different conducting polymers. Electrodeposition enables film formation on surfaces with complicated patterns as well as control of film thickness [5]. Furthermore, the subsequent growth of the polymer film and the charging reactions can be followed in situ [6]. Parameters such as solvent media, electrolyte, electrochemical method used for polymerization and monomer material, all have a profound effect on film morphology, charge transfer and transport properties. Different from the powdery products prepared through chemical approaches, this method enables an easy one-step deposition of the film directly at the surface of the electrode substrate that can be further applied for electrochemical purposes. 

Fig. 29) Two adjacent microelectrodes were derivatized by stepwise electrochemical polymerization. First a polymer viologen film (BPQ )n (related to 4) on one electrode and then a polyvinylferrocene (PVFc) film (related to 11) on the other electrode were obtaineo from the corresponding monomers by reductive and oxidative depositions, respectively. Small spacing between the two microelectrodes is crucial because for these materials the maximum conductivity is much lower than that of polymers such as 14,17, and 20 The redox levels are as follows E (BPQ +/+) = —0.55 V, E°(PVFc /°) = 0.4 V vs SCE. The redox reaction at the interface immersed in an aqueous electrolyte containing LiCIO occurs at a good rate only in one direction because the reaction in Eq. (20) is thermodynamically downhill. 

The polymerization reaction, i.e., the reaction of two radical cations or the reaction of a radical cation with a neutral monomer molecule, depends on the conditions during electrochemical polymerization [58, 627, 629, 630]. During the electrocopolymerization of 3-methylthiophene and 3-thienylacetic acid, a radical cation (of 3-methylthiophene) attacks at a neutral monomer (3-thienylacetic acid). It is possible to produce the copolymer at a potential at which only one of the monomer species can be oxidized [108]. Fig. 19 shows a partial model of interfacial reactions taking place during the electrogeneration of PT or poly(pyrrole) from acetonitrile solutions containing the electrolyte LiClO. . The relative influence of each of these reactions depends on the chemical and electrical conditions of synthesis [629] ... 

The first example of the use of a polymer electrolyte as a medium for an electrochemical reaction is the electrochemical synthesis of conducting polymers [105,106]. Electrochemical polymerization of pyrrole, using ion-conducting polymers as a solid electrolyte, produced polypyrrole-polymer electrolyte bilayer composites in situ [106]. Figure 28 shows a schematic representation of the electrochemical polymerization of pyrrole (Py) by using a polymer electrolyte. The polymer electrolytes used were PEO network polymers in which several kinds of salts were dissolved. The polymer electrolyte film in which pyrrole had been incorporated was sandwiched between two electrodes, and the polymerization was carried out under galvanostatic conditions. When the electrodes were removed from the polymer electrolyte after polymerization, polypyrrole (PPy X") grew on the surface of the polymer electrolyte in contact with the anode (see Fig. 28). As a result, polypyrrole-polymer electrolyte bilayer composites were obtained. The... 

However, even if electrolytes have sufficiently large voltage windows, their components may not be stable (at least ki-netically) with lithium metal for example, acetonitrile shows very large voltage windows with various salts, but is polymerized at deposited lithium if this reaction is not suppressed by additives, such as S02 which forms a protective ionically conductive layer on the lithium surface. Nonetheless, electrochemical stability ranges from CV experiments may be used to choose useful electrolytes. 

Because the oxidation potential of the polymer is lower than that of the monomer, the polymer is electrochemically oxidized into a conducting state, kept electrically neutral by incorporation of the electrolyte anion as a counter-ion. This is an essential since precipitation of the unoxidized, insulating polymer would stop the reaction. Both coulometric measurements and elemental analysis show approximately one counter-ion per four repeat units. An important feature is the fact that the polymerization is not reversible whereas the oxidation of the polymer is. If the polymer film is driven cathodic then it is reduced towards the undoped state. At the same time neutrality is maintained by diffusion of the counter-ions out of the film and into the electrolyte. This process is reversible over many cycles provided that the film is not undoped to the point where it becomes too insulating. It is possible to use it to put new counter-ions into the film, allowing the introduction of ions which are too nucleophilic to be used in the synthesis. The conductivity of the film for a given degree of oxidation depends markedly on the counter-ion, varying by a factor of up to 105. 

Gas sensors usually incorporate a conventional ion-selective electrode surrounded by a thin film of an intermediate electrolyte solution and enclosed by a gas-permeable membrane. An internal reference electrode is usually included, so that the sensor represents a complete electrochemical cell. The gas (of interest) in the sample solution diffuses through the membrane and comes to equilibrium with the internal electrolyte solution. In the internal compartment, between the membrane and the ion-selective electrode, the gas undergoes a chemical reaction, consuming or forming an ion to be detected by the ion-selective electrode. (Protonation equilibria in conjunction with a pH electrode are most common.) Since the local activity of this ion is proportional to the amount of gas dissolved in the sample, the electrode response is directly related to the concentration of the gas in the sample. The response is usually linear over a range of typically four orders of magnitude the upper limit is determined by the concentration of the inner electrolyte solution. The permeable membrane is the key to the electrode s gas selectivity. Two types of polymeric material, microporous and homogeneous, are used to form the... 

The electrochemical behavior of lithium electrodes in a variety of polymeric electrolyte systems was studied extensively by a number of groups, including Scrosati et al. [390-392], Panero et al. [393], Abraham et al. [394-396], Osaka et al. [397-398], Watanabe et al. [399-401], Peled et al. [402], It is clear that there are surface reactions between the lithium and all of the polymeric systems mentioned above. It has already been clearly shown that the ether linkage is attacked by lithium, resulting in the formation of Li alkoxy species [149], Hence, it is expected the PEO-based polymers also react with Li surfaces. Spec-troelectrochemical studies of the Li-PEO system by Scherson et al. [177] provide some evidence for this possibility. Besides the polymers, the polymeric electrolyte systems contain salts with anions such as Aslv,, S03CF3, NlSOTTO),, ... 

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