Diffusion coefficient charge transport

Tables 5.4 and 5.5 provides the homogeneous charge transport diffusion coefficients, Dct, for osmium polymers with different loadings in HCIO4 and H2SO4. Further information about the nature of these processes can be obtained by determining the thermodynamic parameters. These parameters are also summarized...

Cyclic voltammograms obtained for various m values are displayed in Figs. 3.3 and 3.4. In Fig. 33 k — ks where kg is the rate coefficient of charge transfer, while in Fig. 3.4 k = Djd, where D is the charge transport diffusion coefficient and d is the layer thickness. 

Chronoamperometry [1,2] is used to determine the charge transport diffusion coefficient, and also to study phase formation, phase transitions, and relaxation. Chronocoulometry is applied to determine the total charge consumed as well as to determine Q vs. E functions. 

The charge transport diffusion coefficient, which can be determined by transient techniques, is characteristic of the rate-hmiting step (either the electron or the ionic charge transport). However, it is possible to decouple the electron and ion trans-... 

The kinetics of the electron transfer at the electrode-polymer film interface, which initiates electron transport in the surface layer, is generally considered as a fast process, which is not rate limiting. It was also presumed that the direct electron transfer between the metal substrate and the polymer involves only those redox sites situated in the layer immediately adjacent to the metal surface. As follows from the theory (Eq. 8) the measured charge transport diffusion coefficient should increase linearly with c, whenever the contribution from the electron-exchange reaction is important therefore the concentration dependence of D may be the test of theories based on the electron-exchange reaction mechanism. Despite the fact that considerable efforts have been made to find the predicted linear concentration dependence of D, it has been observed only in a few cases and for a limited concentration range. 

In a homogeneous solution the hopping contribution in Eqn. 31 or in Eqn. 31a is very small due to the fact that the physical diffusion contribution is very large, typically 10 cm s . In contrast the opposite applies in polymer media. In such systems the observation of a linear increase in the apparent charge transport diffusion coefficient with redox... 

The interesting point to note here is that from knowledge of both the slope and the intercept, we can evaluate both the charge transport diffusion coefficient Dcr and the total redox center concentration in the layer. This is done using the following expressions ... 

The apparent charge transport diffusion coefficient can readily be determined from the latter expression. 

In a recent communication, Fenner, Van Dyke, and Martin report a variant of the chronopotentiometric current pulse technique, which involves examining the time variation of the open-circuit electrode potential. The open-circuit potential recorded after applying a current pulse varied in a linear manner with The charge transport diffusion coefficient is evaluated by comparing simulated and experimental oc transients. Martin and coworkers show that the concentration of diffusing species at any distance x in the film at any time t after terminating the current pulse is given by... 

It is not always recognized that potential sweep voltammetry can be used to obtain quantitative information on charge percolation in polymer films. We now briefly indicate how apparent charge transport diffusion coefficients can be extracted by analyzing such voltammetric parameters as peak current, peak width, and peak potential as a function of scan rate. This analysis is diffusional in concept and perhaps best applied to polymer materials where redox conduction is the predominant mechanism of charge transport. 

FIGURE 1.60. Simulated Nyquist plots exhibiting the effect of a distribution in charge transport diffusion coefficients on the impedance response recorded at a low frequency. We assume in this case a log-normal distribution in Dcr with variance where = log... 

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