Charge propagation

Charge propagation within the film is in principle slower than charge injection (or consumption) at the electrode/film interface Whether electrons are transported... 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

In the theoretical treatment of ion exchange polymers the roles of charge propagation and of migration of ions were further studied by digital simulation. Another example of proven 3-dimensional redox catalysis of the oxidation of Ks[Fe(CN)5] at a ruthenium modified polyvinylpyridine coated electrode was reported... 

Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

The charge propagates in the film by electron hopping between the polymer Red/Ox couples. This is controlled by the electrode potential only in a close proximity of the electrode in more distant sites, the charge transport is driven by a concentration gradient of reduced or oxidized mediators. The observed faradaic current density, jF, is a superposition of... 

When the characteristic time for charge diffusion is lower than the experiment timescale, not all the redox sites in the film can be oxidized/reduced. From experiments performed under these conditions, an apparent diffusion coefficient for charge propagation, 13app> can be obtained. In early work choroamperometry and chronocoulometry were used to measure D pp for both electrostatically [131,225] and covalently bound ]132,133] redox couples. Laviron showed that similar information can be obtained from cyclic voltammetry experiments by recording the peak potential and current as a function of the potential scan rate [134, 135]. Electrochemical impedance spectroscopy (EIS) has also been employed to probe charge transport in polymer and polyelectrolyte-modified electrodes [71, 73,131,136-138]. The methods... 

From a theoretical point of view, charge propagation in films containing space-distributed redox centers can be achieved either by the physical displacement of the sites or by the transference (hopping) of electrons from neighboring reduced to oxidized sites or by the combination of both processes. In the case of free diffusing couples immobilized in oppositely charged polyelectrolytes, both processes occur and an apparent diffusion coefficient can be defined and measured [136, 142, 143] ... 

The nature of the LbL film structure puts the NPs into close proximity to one another. Thus, for cases in which the NPs are electroactive, charge propagation through the films seems most likely to occur by electron exchange, first between the... 

The gap is obviously temperature independent and furthermore it is expected that transport at energies E < 5 will be strongly suppressed due to the absence of electronic states to suport charge propagation. As a result,... 

The box-like shape of the CV curves of PFA-AN8-H is characteristic of charging the EDL with quick charge propagation (Figure 3.23). Figure 3.24 shows the current density dependence of the... 

Conductivity (electronic, ionic), and the mechanism of charge propagation. 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

---