Charge transport rate

For investigating the nature of the charge-transport process, thermodynamic studies are particularly useful. In these studies, the charge transport rate, Dct, is measured over a range of temperatures. Its temperature-dependence can be described by using the Arrhenius equation, as follows ... 

Gratzel and co-workers [5] have studied the mechanism for this electron transport process. Their approach is based on the measurement of charge-transport rates for a range of modified surfaces using chronoabsorptiometiy. In this technique, the absorbance of the modified surface is followed as a function of time upon oxidation (or reduction) of the redox center. A typical example of the absorption changes observed upon oxidation of the tiiarylamine shown in Figure 6.31 is presented in... 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

The electrochemical response of this system will depend on the timescale of the involved electrochemical experiment. Thus, if the charge transport rate is signili-cantly faster than the experimental timescale, the oxidized/reduced site concentration ratio, [ Ox ] [Rcd -nM j, will be uniform throughout the microporous layer and in thermodynamic equilibrium with the applied potential. Thus, the concentration profiles (see Figure 2.5) for the oxidized and reduced forms of the electroactive... 

Values determined for diffusion coefficients/charge transport rates using both cyclic voltammetry techniques were essentially the same (see Fig. 8.8), and these are denoted as DcriCV), These charge transport rates typically ranged from 10r i-io-i (see Tables 8.2 and 8.3). 

Sampled current voltammetry (SC V) involves applying a potential-step wave form of increasing amplitude, covering a potential window where initially no redox reaction occurs, to one where the current response is diffusion-controlled. The current decay from the exciting pulses is then recorded instantaneously at several different times. In the work under discussion, plots of /nm versus were linear in accordance with the Cottrell equation. Charge transport rates evaluated using this technique were essentially the same, within experimental error, as those evaluated using chronamperometry. 

The influence of sulphuric acid concentration on charge transport rates for a series of osmium loadings, 25 > > 5, is shown in Table 8.2. As noted previously, the rate of charge transport can be limited by one of three processes ... 

Many researchers have focused on the preparation and catalytic properties of polymer-bound ruthenium and osmium species because of their proven ability to catalyze homogeneous reactions and the vast synthetic chemistry available for their preparation. A series of preformed polymers of [M(bpy)2(pol)nCl]Cl, where M can be a Ru(II) or Os(II) metal center coordinated to 2,2 -bipyridine ligands (bpy) and anchored to a pyridine or imidazole nitrogen of a PVP or poly(N-vinylimidazole) polymer (pol), have been prepared and characterized with respect to charge transport rates and mechanisms in drop-coated films on electrode surfaces. Electrodes coated with films of the ruthenium polymer have been shown to mediate the oxidation of nitrite, and nickel bis(2-hydroxyethyl)dithiocarbamate. ... 

This phenomenon is akin to thermodynamic phase transitions in other branches of physical chemistry. The abrapt deterioration of the charge transport rate in poly(tetracyanoquinodimethane Fig. 6.22) or poly(vinylferrocene) films [23] at high electrolyte concentrations (10moldm LiCl or 5moldm CaCh) and its temperature dependence (Fig. 6.23) can be interpreted based on thermodynamic theory [20,23,222]. In a more compact stracture the rate of electron hopping may increase since the concentration of redox sites is high however, a deterioration in the film s permeability to the counterions due to the decrease in the free volume is expected at the same time. The maximum observed in the peak current versus salt concentration curve is the result of the balanced effects of the enhanced electron-... 

Penner, R. M., and Martin, C. R., Electrochemical investigations of electronically conductive polymers. 2. Evaluation of charge-transport rates in polypyrrole using an alternating current impedance method, J. Phys. Chem., 93, 984-989 (1989). 

The advantage in such a situation is that cation mobilities can be much larger, especially in the case of ions such as Li. This has been demonstrated in a series of experiments in which charge transport rates with different supporting electrolytes such as LiBp4 and n-(C4H9)4BF4 were monitored via chronocoulometry [61]. The... 

These experiments yield an apparent diffusion coefficient, D pp, which is a quantitative measure of the charge-transport rate (19-24). 

Because large amplitude electrochemical methods have proven useful for evaluations of charge-transport rates in redox polymers(19-24), analogous... 

Finally, we assume that the rate of this diffusional charge-transport in pol3Tpyrrole is controlled by counterion motion rather than by the actual self-exchange event. This assumption has been experimentally demonstrated for other redox poljoners (44) and, as noted above, is supported by the available data on polypyrrole (14-18,45). Note, however, that even if this assumption proves to be incorrect, the D pp values obtained here are still valid quantitative measures of the charge-transport rate. 

The theory developed here predicts that, if thin films have the same chemical and morphological composition as thick films, D pp should be independent of film thickness. In contrast, while excellent matches between experimental and simulated transients were obtained for all of the film thickness reported here (Figure 9), D pp clearly decreases with film thickness (Table I). These data are in agreement with the often reported qualitative observation that charge transport rates decrease with film thickness (45). Furthermore, Osaka et al. observed a similar trend in their data (29). 

Hillman and Bruckenstein have used this type of analysis to discuss break-in effects, charge and mass trapping, structural evolution with redox cycling, kinetic decoupling of ion and solvent transfer, and variations in apparent charge transport rate and formal potential with experimental time scales. The reader is referred to the original paper for further details. 

In all those experimental procedures, the authors reported a fast charge-transport rate at the electrode/electrol3fe interface, which is desirable for electroanal3fical applications. However, as we present in the next section, slight changes in the experimental growth procedure can produce very different materials. 

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