Polypyrrole anode

Loss of electroactivity, mechanical strength, and formation of carbonyl groups along with the formation of carbon dioxide were observed [175-181] when polypyrrole anode was cycled between charging and discharging in different electrolytic systems. However, in modified polypyrroles, the type of substituents at N, substituents on the pyrrole ring, and the conditions of synthesis also play an important role in deciding the stability of materials in electrochemical systems [175,182-185]. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (— )camphoric anhydride or (S)-phenylalanine methyl ester299. Much better results were obtained with the poly(L-valine) coated platinum electrodes300. For example, t-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

The flow of an anodic current oxidizes the conducting polymer and the film swells. At the polypyrrole/tape interface, electrochemically stimulated conformational changes in the polymer promote an expansion that... 

Figure 23. Artificial muscle formed by a three-layer polypyrrole-nonconducting tape-polypyrrole. The consumed charge works two times in this device when polypyrrole I is oxidized (anodic process), pushing the free end of the layer, polypyrrole II is reduced (cathodic process), trailing the layer. Stresses at the polymer/polymer interfaces are summarized in the box. (Reprinted from Handbook of Organic Conductive Molecules and Polymers, H. S.Nalwa,ed., Vol. 4,1997, Figs. 10.13,10.15a, 10.18,10.36. Reproducedwithpermission of John Wiley Sons, Ltd., Chichester, UK.)...

Figure 39. Semilogarithmic representation of / vs. anodic potential from a series of potential steps. Each series was performed between a cathodic potential and different anodic potentials in a 0.1 M UClCtyPC solution. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, A new model for electrochemical oxidation of polypyrrole under conformational relaxation control. 7. Electroanal. Chem. 394,211, 1995, Figs. 2-5. Copyright 1995. Reprinted with permission from Elsevier Science.)...

Figure 50. Semilogarithmic plot of cathodic (Ec) and anodic (E) potentials against values of 1 IQ [cPQityd ] extracted from Figs. 52 and 53. Following Eq. (48), values of the coefficient of electrochemical relaxation (zr) and the coefficient of cathodic polarization (ze) can be deduced from the slopes. (Reprinted from T. F. Otero and H.-J. Grande, Reversible 2D to 3D electrode transition in polypyrrole films. ColloidSurf. A. 134,85, 1998, Figs. 4-9. Copyright 1998. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 Amsterdam, The Netherlands.)...

In 1979, Diaz et al. produced the first flexible, stable polypyrrole (PPy) film with high conductivity (1(X) Scm ). The substance was polymerized on a Pt-electrode by anodic oxidation in acetonitrile. The then known chemical methods of synthesis " usually produced low conductivity powders from the monomers. By contrast, electropolymerization in organic solvents formed smooth and manageable films of good conductivity. Thus, this technique soon gained general currency, stimulating further electropolymerization experiments with other monomers. In 1982, Tourillon... 

Polyacetylene proved qnite incapable of working in a realistic battery context, and MacDiarmid did not mention this application in his Nobel lectnre of October, 8 2000. However, other materials have proven their worth, and prototype batteries made with polypyrrole and polyaniline as cathodes (positives), and metal or lithiated carbon materials as anodes (negatives), have been demonstrated in dne conrse by the Japanese and German indnstry, for instance. Novdk et al. (1997) have reviewed the field in detail. 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

Novak, P., P. A. Christensen, T. Iwashita, and W. Vielstich, Anodic oxidation of propylene carbonate on platinum, glassy carbon and polypyrrole, /. Electroanal. Chem., 263, 37 (1989). 

Burgmayer and Murray [40] reported electrically controlled resistance to the transport of ions across polypyrrole membrane. The membrane was formed around a folded minigrid sheet by the anodic polymerization of pyrrole. The ionic resistance, measured by impedance, in 1.0 M aqueous KC1 solution was much higher under the neutral (reduced) state of the polymers than under the positively charged (oxidized) state. The redox state of polypyrrole was electrochemically controlled this phenomenon was termed an ion gate, since the resistance was varied from low to high and vice versa by stepwise voltage application. 

Thus, it appears that the transition represented by the anodic peak in the cyclic voltammogram of polypyrrole is due to a changeover in the dominant carrier type and is accompanied by a dramatic contraction of the film. The authors strongly suspected that this contraction was due to electro-striction associated with bipolaron formation. As a further test they also carried out experiments intended to test if proton expulsion from the film occurred on oxidation. They found that it did indeed occur but monotonically at alt potentials > -0.6 F, in agreement with the extremely elegant work of Tsai et at. (1987), and so could not be responsible for the relatively sudden contraction at potentials > —0.2 V. 

Several groups have recently shown (36,42,43,44) that photoanode materials can be protected from pRotoano3ic corrosion by an anodically formed film of "polypyrrole".(45) The work has been extended (46) to photoanode surfaces first"Treated with reagent that covalently anchors initiation sites for the formation of polypyrrole. The result is a more adherent polypyrrole film that better protects n-type Si from photocorrosion. Unlike the material derived from polymerization of I, the anodically formed polypyrrole 1s an electronic conductor.(45) This may prove ultimately important in that the rate of ionTransport of redox polymers may prove to be too slow... 

Electrochemical communication between electrode-bound enzyme and an electrode was confirmed by such electrochemical characterizations as differential pulse voltammetxy. As shown in Fig. 11, reversible electron transfer of molecularly interfaced FDH was confirmed by differential pulse voltammetry. The electrochemical characteristics of the polypyrrole interfaced FDH electrode were compared with those of the FDH electrode. The important difference between the electrochemical activities of these two electrodes is as follows by the employment of a conductive PP interface, the redox potential of FDH shifted slightly as compared to the redox potential of PQQ, which prosthetic group of FDH and the electrode shuttling between the prosthetic group of FDH and the electrode through the PP interface. In addition, the anodic and cathodic peak shapes and peak currents of PP/FDH/Pt electrode were identical, which suggests reversibility of the electron transport process. 

Cyclic voltammetry was performed with the ADH-NAD-MB/polypyrrole electrode in 0.1 M phosphate buffer (pH 8.5) at a scan rate of 5 mV s l. The corresponding substrate of ADH caused the anodic current at +0.35 V vs. Ag/AgCl to increase. These results suggest a possible electron transfer from membrane-bound ADH to the electrode through membrane-bound NAD and MB with the help of the conductive polymer of polypyrrole. 

Fig. 9.12 Cyclic voltammetry of the p-doping(anodic)-undoping(cathodic) process of a polypyrrole electrode in LiClO -PC solution. Pt substrate, Li reference electrode, scan rate 50 mV s .

Many industrial and academic laboratories have investigated doped polymers as improved positive electrodes in rechargeable lithium batteries. A common example is the battery formed by a lithium anode, a liquid organic electrolyte (e.g. LiC104-PC solution) and a polypyrrole film... 

The quaternary groups in viologens can be derivatised to produce compounds capable of chemically bonding to a surface, especially electrode surfaces. These include symmetrical silanes, e.g. (1.93), which can bond to the oxide lattice on the electrode surface, and a viologen with pyrrole side chain (1.94) that undergoes anodic polymerisation to form a film of the viologen bearing polypyrrole on the electrode. Polymeric bipyridilium salts such as (1.95) have also been prepared for use in polymeric electrolytes. ... 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

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