Electrochemical characteristics charge transport

The overall formation mechanism of PS must involve the fundamental electrochemical reactions in three essential aspects 1. nature of reactions, reactants, products, intermediates, number of steps, and their sequences, 2. nature and rate of charge transport in the different phases at silicon/electrolyte interface, 3. spatial and temporal distributions of reactions and the cause of such distributions. The first and second aspects, which governs the properties of a uniform and flat surface and do not involve geometric factors, have been characterized in previous Sections and the major characteristics are summarized in Table 5. This Section deals with the third aspect, that is, spatial and temporal... 

In this chapter we will attempt to provide a brief but illustrative description of the various aspects of the research and technology of conducting polymers. To appreciate fully the diverse range of operations that these materials may fulfil, it is crucial to understand their basic properties. Therefore, particular attention will be devoted here to the description of the mechanism of charge transport and to the characteristics of the electrodic processes in electrochemical cells. 

Thin-film ideal or Nemstian behavior is the starting point to explain the voltammetric behavior of polyelectrolyte-modified electrodes. This condition is fulfilled when (i) the timescale of the experiment is slower than the characteristic timescale for charge transport (fjD pp, with Ithe film thickness) in the film, that is all redox within the film are in electrochemical equibbrium at any time, (ii) the activity of redox sites is equal to their concentration and (iii) all couples have the same redox potential. For these conditions, anodic and cathodic current-potential waves are mirror images (zero peak splitting) and current is proportional to the scan rate [121]. Under this regime, there exists an analytical expression for the current-potential curve ... 

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... 

The performance of PEMFC is often presented by the polarization curve that shows the voltage output as a function of current density. Fig. 8 shows a typical polarization curve of PEMFC. As the PEMFC processes charge-transfer reactions and the diffusion of the reactants to and products from the electrochemical interface, the transport and kinetics within the cell determine the polarization characteristics of PEMFC. In the practical PEMFC, the terminal cell potential V... 

We have already mentioned several effects that are connected with the polymeric nature of the layer. It is evident tliat all the charge transport processes listed are affected by the physicochemical properties of the polymer. Therefore, we also must deal with the properties of the polymer layer if we wish to understand the electrochemical behavior of these systems. The elucidation of the stracture and properties of polymer (polyelectrolyte) layers as well as the changes in their morphology caused by the potential and potential-induced processes and by other parameters (e.g., temperature, electrolyte composition) set an entirely new task for electrochemists. Owing to the long relaxation times that are characteristic of polymeric systems, the equilibrium or steady-state situation is often not reached within the time allowed for the experiment. 

A characteristic feature of an electrochemical cell is that the electronic current, which is the movement of electrons in the external circuit, is generated by the electrochemical processes at the electrodes. In contrast to the electronic current, the charge is transported between the positive and the negative electrode in the electrolyte by ions. Generally the current in the electrolyte consists of the movement of negative and positive ions. 

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