Doping/undoping processes

FIGURE 4-22 Typical response of polypyrrole detector to carbonate (S , 1 x 10-4M S2,2.5x lO M S3, 5 x 10 m) based on the doping-undoping process. (Reproduced with permission from reference 74.)... 

This contribution reviews recent results on [Si(Pc)0]n (Pc = phthalocyaninato) solid state electrochemistry and the structural interconversions that accompany electrochemical doping/undoping processes. In aceto-nitrile/(a-Bu)4N+BF4, it is found that a significant overpotential accompanies initial oxidation of as-polymerized [Si(Pc)0]n. This can be associated with an ortho rhomb ic- te tr agonal structural transformation. 

The necessary porosity for thicker layers was introduced by appropriate current densities [321-323], by co-deposition of composites with carbon black [28, 324] (cf. Fig. 27), by electrodeposition into carbon felt [28], and by fabrication of pellets from chemically synthesized PPy powders with added carbon black [325]. Practical capacities of 90-100 Ah/kg could be achieved in this way even for thicker layers. Self-discharge of PPy was low, as mentioned. However, in lithium cells with solid polymer electrolytes (PEO), high values were reported also [326]. This was attributed to reduction products at the negative electrode to yield a shuttle transport to the positive electrode. The kinetics of the doping/undoping process based on Eq. (59) is normally fast, but complications due to the combined insertion/release of both ions [327-330] or the presence of a large and a small anion [331] may arise. Techniques such as QMB/CV(Quartz Micro Balance/Cyclic Voltammetry) [331] or resistometry [332] have been employed to elucidate the various mechanisms. 

It has been shown [80] that organic semiconductors with the lowest redox potentials (which reflect the ionization potentials) of —0.2 V and 0.2 V vs SSCE for PPy and PANI, respectively, interact reversibly with ammonia. However, other polymers, such as PT (0.7 V), PA (0.7 V) and poly(para-phenylene) (PPP) (1.6 V) [81], all have higher redox potentials and their interaction is reported to be irreversible. The reversible changes of the transitions in the optical spectra correspond to the doping/undoping process. The results obtained on PT films, upon exposure to NO2, SO2 [82], or on PPy films during their exposure to NH3 [83], or on PPy and PPP films... 

As already mentioned, in addition to the estimation of the current efficiency of polymerization, EQCM is a convenient tool for measuring the doping level of CPs. It also gives information on the mechanism of the doping-undoping process. 

The perturbation of the p-doping-undoping process of the conjugated polymer upon the complexation effect may be caused either by electronic effects or by conformational changes in the potymer. Swager and co-workers have demonstrated that conforma-... 

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