Direct electrochemical reactions

In direct electrochemical reactions, the substrate undergoes a heterogeneous redox reaction at the electrode surface within the Helmholtz layer. The reac-... 

Electrochemical fluorination 168,169> is a commercial process for perfluorina-tion of aliphatic compounds. The reaction is performed in liquid hydrogen fluoride -potassium fluoride at a nickel anode. The mechanism is not known free fluorine cannot be detected during electrolysis, so it seems probable that fluorination is a direct electrochemical reaction. Theoretically, hydrogen fluoride-potassium fluoride should be a very oxidation-resistant SSE, and it might well be that the mechanism is analogous to that proposed for anodic acetamidation of aliphatic compounds in acetonitrile-tetrabutylammonium hexafluorophosphate 44 K... 

Noble metal electrodes include metals whose redox couple M/Mz+ is not involved in direct electrochemical reactions in all nonaqueous systems of interest. Typical examples that are the most important practically are gold and platinum. It should be emphasized, however, that there are some electrochemical reactions which are specific to these metals, such as underpotential deposition of lithium (which depends on the host metal) [45], Metal oxide/hydroxide formation can occur, but, in any event, these are surface reactions on a small scale (submonolayer -> a few monolayers at the most [6]). 

Anodic processes may occur by direct electrochemical reaction of the waste at the anode, or by indirect oxidation when an oxidizing agent is generated in the waste stream. For example, oxygen and ozone may be generated from water in the waste stream, or the addition of sodium chloride to the waste stream can be used to generate free chlorine or hypochlorite at the anode. In addition, metal ions of higher valence states can be produced at the anode to oxidize wastes. 

In direct electrochemical reactions the substrate undergoes a heterogeneous redox reaction at the electrode surface within the Helmholtz layer. The thus formed reactive intermediate, i.e. a radical ion, undergoes the chemical follow-up reaction to the product in die reaction layer. There are steep concentration gradients near the electrode. Second-order reactions of the intermediates can thus be obtained at high current densities. 

It was shown previously, that oxygen presence is a main factor of possible structural and energy changes, which beside the direct electrochemical reaction may induce by-processes with the lost of catalytic activity of the materials of fuel cell electrodes. After the consideration of adsorption... 

A coulometric analysis can be performed utilizing a direct electrochemical reaction, or it can be accomplished by an indirect process using an electrochemically prepared intermediate. Either of these can be operated with controlled current or at controlled potential. All four variations are useful, and will be detailed below. 

The conversion of chemical energy to electrical energy in a PEM fuel cell occurs through a direct electrochemical reaction. It takes place silently without combustion. The key part of a PEM fuel cell, which is known as a membrane electrode assembly (MEA), consists of a polymer electrolyte in contact with an anode and a cathode on either side. To limction, die membrane must conduct hydrogen ions (protons) and separate either gas to pass to the other side of the cell. A schematic representation of a PEM fuel cell is shown in Figure 1.5. 

To summarize, the direct electrochemical reactions of the NAD" /NADH cofactors at metallic or carbon electrodes are highly irreversible, occur at large overpotentials, and can be affected by side reactions and fouling (adsorption) of cofactor-related products [26]. The development of BFCs based on NAD -dependent dehydrogenases, therefore, cannot rely on direct electrochemical reactions of the cofactor. As a result, a major focus for this field of research has been directed at finding suitable methods for the electrochemical oxidation of NADH. 

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