Electrochemistry cathodic processes

L. I. Krishtalik, Hydrogen overvoltage and adsorption phenomena Part III Effect of the adsorption energy of hydrogen on overvoltage and the mechanism of the cathodic process, in Advances in Electrochemistry and Electrochemical Engineering. P. Delahay, editor, Vol. 7, Wiley-Interscience, New York, 1963, pp. 283-340. 

Sterten, A., Solli, P.A., Cathodic process and cyclic redox reactions in aluminium electrolysis cells. Journal of Applied Electrochemistry, 25, pp. 809-816, 1995. 

From the literature it follows that the electro-deposition of molybdenum from the binary MeF-Me2Mo04 mixtures is impossible. However, a small addition (1 mole %) of boron oxide or Si02 to the electrolyte facilitates the electro-deposition of molybdenum. The presence of boron or silicon oxide most probably modifies the structure of the melt, which results in changes in the cathode process. The survey of electrochemistry of molybdenum deposition was given by Danbk et al. (1997). 

The mechanism depicted in Scheme 18.3 which involves the disproportionation of HO2 radicals was the more accepted one at that time [3]. Posteriorly, the role of a proton source in the oxygen reduction reaction was evaluated in a similar ionic liquid, [C2mim][BF4] (Scheme 18.2), in the presence of2.1 mM and 2.64Mof water [11]. The increase in water concentration modified the electrochemistry of the oxygen reduction reaction from a reversible reduction process corresponding to the 02/02 redox couple to an irreversible cathodic process. In summary, the main features observed upon addition of water were (1) an increase of the current density due to more favourable mass transport condition (increased fluidity and conductivity in the medium), (2) shift in potential for the reduction process to more positive values caused by changes to the protonation equilibria and the solvation of the electrogenerated species [13], and (3) loss of reversibility for the reduction process. 

In aprotic nonaqueous media, the organic electrochemistry of anodic and cathodic reactions is concerned predominantly with radical-ion chemistry in many cases involving aromatic substances, the radicals are of sufficient stability for them to be characterized spectroscopically by conventional absorption spectrophotometry and by esr spectroscopy. Linear relations are found between the cathodic and anodic half-wave potentials and the ionization potentials or electron affinities determined in the gas phase. The oxidation and reduction potentials can also be related to the theoretically calculated energies of the highest occupied (anodic process) or lowest vacant (cathodic process) molecular orbitals. 

Magnesium exhibits a very strange electrochemical phenomenon known as the negative-difference effect (NDE). Electrochemistry classifies corrosion reactions as either anodic or cathodic processes. Normally, the anodic reaction rate increases and the cathodic reaction rate decreases with increasing applied potential or current density. Therefore, for most metals like iron, steels, and zinc etc, an anodic increase of the applied potential causes an increase of the anodic dissolution rate and a simultaneous decrease in the cathodic rate of hydrogen evolution. On magnesium, however, the hydrogen evolution behavior is quite different from that on iron and steels. On first examination such behavior seems contrary to the very basics of electrochemical theory. 

Saltykova, N.A. and Baraboshkin, A.N. (1969) About pecuharities of cathodic processes during electrodeposition of solid metals from molten salts, Physical Chemistry and Electrochemistry of Molten Salts (in Russian), Naukova Dumka. Kiev, Vol. 2, pp. 202-209. 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

The processes of cathodic protection can be scientifically explained far more concisely than many other protective systems. Corrosion of metals in aqueous solutions or in the soil is principally an electrolytic process controlled by an electric tension, i.e., the potential of a metal in an electrolytic solution. According to the laws of electrochemistry, the reaction tendency and the rate of reaction will decrease with reducing potential. Although these relationships have been known for more than a century and although cathodic protection has been practiced in isolated cases for a long time, it required an extended period for its technical application on a wider scale. This may have been because cathodic protection used to appear curious and strange, and the electrical engineering requirements hindered its practical application. The practice of cathodic protection is indeed more complex than its theoretical base. 

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

The slow protonation rate of the conjugated anion of the sulphone (1st step) leads to the obtainment of a pseudo one-electron process. However, no self-protonatiori process exists in the presence of an excess of a proton donor of lower pKa than that of the electroactive substrate and Figure 6a, curve 2 shows evidence for a two-electron step. Full substitution on the a carbon, as in the case of phenyl 2-phenylbut-2-yl sulphone, does not allow one to observe any deactivation (Figure 6b, curve 1). It is worth mentioning that cathodic deactivations of acidic substrates in aprotic solvents are rather general in electrochemistry, e.g. aromatic ketones behave rather similarly, showing deprotonation of the substrate by the dianion of the carbonyl compound39. 

We call the electrode of interest - that at which the electrochemical changes of interest occur - the working electrode (WE). When we cite an overpotential, we cite the potential of the working electrode with respect to the potential of the reference. The overpotential t] is seen to be positive during anodic electrochemistry and negative during cathodic electrode processes. 

In the mid-1960s, Dessy and coworkers [12, 13] provided an extensive survey of the anodic and cathodic reactions of transition metal organometallic species, including binary (homoleptic) carbonyls, and this provided a stimulus for many later detailed studies. Whereas the electrochemistry of heteroleptic transition metal carbonyls is covered elsewhere in this volume, that of the binary carbonyls, which is covered here, provides paradigms for the electrochemistry of their substituted counterparts. A key aspect is the generation of reactive 17-electron or 19-electron intermediates that can play key roles in the electrocatalytic processes and electron-transfer catalysis of CO substitution by other ligands. 

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