Polypyrrole modelling

Figure 12. Model of interfacial reactions proposed for the electrogeneration of polypyrrole from aqueous and acetonitrile solutions. (Reprinted from T. F. Otero and J. Rodriguez, Electrochim. Acta 39, 245, 1994, Figs. 2, 7. Copyright 1997. Reproduced with permission from Elsevier Science.)...

Figure 38. Evolution of the proposed surface aspect of a polypyrrole film during an oxidation reaction initiated from high cathodic potentials (E < -800 mV vs. SCE). The chronoamperometric response is shown at the bottom. Experimental confirmation can be seen in the pictures in Ref. 177. (Reprinted from T. F. Otero and E. Angulo, Oxidation-reduction of polypyrrole films. Kinetics, structural model, and applications. Solid State Ionics 63-64, 803, 1993, Figs. 1-3. Copyright 1993. Reprinted with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055, KV Amsterdam, The Netherlands.)...

Otero and co-workers208,212 have visually observed nuclei of oxidized polymer in thin polypyrrole films on electrodes. They attribute these to sites of counter-ion and solvent ingress. A nucleation model based on the growth of ionically conductive zones provides good agreement with experimental chronoamperometric responses. 

Mandelbrot, on fractal surfaces, 52 Mao and Pickup, their work on the oxidation of polypyrrole, 587 Marcus model, inapplicability for interfacial electron transfer, 513 Mechanical breakdown model for passivity, 236... 

Polyanaline, oxidation of, 563 Polycrystine electrodes, models of the double layer for, 49 Polypyrrole... 

As expected, the impedance responses obtained in practice do not fully match that of the model of Fig. 9.13. However, as shown by the typical case of Fig. 9.14 which illustrates the response obtained for a 5 mol% ClO -doped polypyrrole electrode in contact with a LiC104-propylene carbonate solution (Panero et al, 1989), the trend is still reasonably close enough to the idealised one to allow (possibly with the help of fitting programmes) the determination of the relevant kinetics parameters, such as the charge transfer resistance, the double-layer capacitance and the diffusion coefficient. 

In general it has been found that polypyrrole is extremely poorly crystalline, limiting the information that can be obtained from direct structural techniques such as X-ray crystallography, and hence much of our knowledge has been obtained from indirect measurements and/or experiments on model compounds (e.g. X-ray studies on pyrrole trimers and dimers). It is now generally accepted that the ideal structure of the polymer is a planar (a or )-bonded chain in which the orientation of the pyrrole molecules alternates. 

Figure 3.77 (a) The cycloaliphatic monomer subunils employed by Wegner and Riihe (1989). n varied between 3 and 10. (b) Schematic description of the polypyrrole backbone wrapped in a layer of poorly conducting methylene moieties of the alkyiene chains fused (o the pyrrole units in the 3,4 positions. The minimum separation distance, K, between adjacent chains can be estimated from molecular models. From Wegner and Riihe (1989). 

Fig. 11.12. Equivalent circuits considered for use in modeling the polypyrrole/eiectrolyte interphase. (Reprinted from D. L. Miller and J. O M. Bockris, Structure of the Polypyrrole/Solution Interphase, J. Electro-chem. Soc. 139 970-975, 1992. Reproduced by permission of The Electrochemical Society, Inc.)...

Toward an understanding of the conduction properties of polythiophenes (26) and polypyrroles (25) a large number of soluble oligomers has been prepared. Oligothiophenes, indeed, represent the most common model compounds for electrically conducting polymers [149]. Thereby, lower oligomers... 

The origin of the conduction mechanism has been a source of controversy ever since conducting polymers were first discovered. At first, doping was assumed to simply remove electrons from the top of the valence band (oxidation) or add electrons to the bottom of the conduction band (reduction). This model associates charge carriers with free spins (unpaired electrons). However, the measured conductivity in doped polyacetylene (and other conducting polymers such as polyphenylene and polypyrrole) is r greater than what can be accounted for on the basis of free spin alone. 

The next group of materials comprises conducting polymers (ICP). Systems with identical polymers have often been reported for polyacetylene. It is known that this ICP forms insertion compounds of the A and D types (see Section 6.4, and No. 5 in Table 12). Cells of this Idnd were successfully cycled [277, 281-283]. However, the current efficiency was only 35% heavy losses were observed due to an overoxidation of the PA [284]. In other cases as for polypyrrole (PPy), the formation of D-PPy was anticipated but did not occur [557, 558]. Entry (6) in Table 12 represents some kind of ideal model. A PPy/PPy cell with alkyl or aryl sulfates or sulfonates rather than perchlorates is claimed in [559]. Similar results were obtained with symmetric polyaniline (PANI) cells [560, 561]. Symmetric PPy and RANI cells yield about 60% current efficiency, much more than with PA. An undoped PPy/A-doped PPy combination yields an anion-concentration cell [562, 563], in analogy to graphite [47], (cf. No. 7). The same principle can be applied with the PPy/PT combination [562, 563] (cf. No. 8). Kaneto et al. [564] have reported in an early paper the combination of two pol54hiophene (PT) thin layers (< 1 pm), but the chargeability was relatively poor (Fig. 40, and No. 9 in Table 12). A pronounced improvement was due to Gottesfeld et al. [342, 343, 562, 563], who employed poly[3-(4-fluoro-phenyl)thiophene], P-3-FPT, in combination with a stable salt electrolyte (but in acetonitrile cf. Fig. 40 and No. 10 in Table 12). In all practical cases, however, Es.th was below 100 Wh/kg. 

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