Electrodeposition reactions

Electrochemical oscillation during the Cu-Sn alloy electrodeposition reaction was first reported by Survila et al. [33]. They found the oscillation in the course of studies of the electrochemical formation of Cu-Sn alloy from an acidic solution containing a hydrosoluble polymer (Laprol 2402C) as a brightening agent, though the mechanism of the oscillatory instability was not studied. We also studied the oscillation system and revealed that a layered nanostructure is formed in synchronization with the oscillation in a self-organizational manner [25, 26]. 

Kim and Jorne [37] have used a rotating zinc hemisphere to study the kinetics of zinc dissolution and deposition reactions in concentrated zinc chloride solutions. The electrodeposition reaction of cadmium on mercury was used by Mortko and Cover [43] in their investigation of a rotating dropping mercury electrode their data behaved according to Eqs. (74)-(76). 

When the charge-transfer step in an electrodeposition reaction is fast, the rate of growth of nuclei (crystallites) is determined by either of two steps (I) the lattice incorporation step or (2) the diffusion of electrodepositing ions into the nucleus (diffusion in the solution). We start with the first case. Four simple models of nuclei are usually considered (a) a two-dimensional (2D) cylinder, (b) a three-dimensional (3D) hemisphere, (c) a right-circular cone, and (d) a truncated four-sided pyramid (Fig. 7.2). 

It is clear from Fig. 6 that the concentration of a reacting species decreases at the electrode surface as the current is increased. The minimum concentration is zero at the surface, which corresponds to the maximum rate at which the electrodeposition reaction can proceed. The current density corresponding to this maximum rate is called the limiting current density i, which can be approximated by... 

The following derivations are based on fundamental principles and will clearly illustrate how the potential dependence of electrochemical reaction rates, characterized by experimentally determinable transfer coefficients, arises in generalized reaction schemes, and the constraints that the required limiting assumptions impose upon this potential dependence. This approach is required because the simple transfer coefficients of B R are really only of use for assigning mechanisms if they are properly applied this is actually not so trivial a point given the above-mentioned confusion that has arisen in the kinetic analysis of, e.g., the A1 electrodeposition reaction. Hence, attention will be given in the following material to completeness. 

With this in mind, a forward equilibrium rate constant for the electrodeposition reaction Eq. (9) under nonstandard conditions is... 

All solid surfaces exhibit structural features that can have significant effects on the kinetics of charge transfer reactions and on the stability of the interfacial region. In the case of metals, the most significant structural features for "smooth" surfaces are emergent dislocations, kink sites, steps, and ledges. It has long been known, for example, that the kinetics of some electrodissolution and electrodeposition reactions depend on the density of such sites at the surface, but the exact mechanisms by which the effects occur have not been established. The role of "adion" in these processes is also unclear, as is the sequence of the dehydration-electronation-adsorption-diffusion-incorporation processes, even for the simplest of metals. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

The interplay of different dye molecules present during the electrodeposition reactions of dye-modified ZnO was studied for zinc complexes of tetrasulfonated phthalocyanine (TSPcZn, Figure 4.4a) and tetraphenylporphyrin (TSTPPZn) in which case both dyes (Figure 4.4b) were simultaneously adsorbed to form a hybrid material with ZnO [258]. The typical absorption bands for both dyes were detected. Films of TSPcZn/ZnO consisted of larger particulate domains when compared with TSTPPZn/ZnO or (TSTPPZn + TSPcZn)/ZnO. The presence of the porphyrin stabilized the phthalocyanine on the ZnO since a greater amount of TSPcZn was adsorbed. In the photosensitization of ZnO, TSPcZn and TSTPPZn worked... 

So far we have dealt with reactions in which a solid has been consumed. When a solid surface is built up by a reaction, transport of reactants in the fluid phase is destabilizing, just as is the case during solidification (Seshan, 1975). An example of considerable practical importance is electrodeposition. In this case, instability is normally undesirable because it leads to an irregular surface that does not have the preferred bright appearance. It has been found that instability can sometimes be prevented by including small quantities of certain surface active organic additives in the electrodeposition bath (Edwards, 1964). The additives diffuse to the solid-liquid interface and are either incorporated into the developing deposit or consumed by an electrochemical reaction. Since they are surface active, they adsorb at the interface and produce a decrease in the rate of the electrodeposition reaction, possibly because they block some sites where metal ions would otherwise deposit. 

The polarization curves without including the ohmic potential drop for different /o/iu ratio values for both, one and two electron reactions, are shown in Fig. 1.3 [21]. They are obtained by using Eq. (1.36) for the different /o/t ratios and/c and/a values obtained for different rj, b, and b in the dependence of the mechanism of electrodeposition reactions. From Fig. 1.3, it is a clear that these dependencies are similar to each other for the large values of the /qAl ratio and at any low value of overpotential. Because of this, the polarization curves without including the ohmic potential drop are not suitable and, for that reason, will be not treated further. 

The expression above describes appropriately experimental current transients obtained in studies of electrodeposition processes for numerous systems. The electrodeposition of Hg may be considered as a model system for the study of the fundamentals of electrochemical phase formation, and is briefly discussed here (see Sect. 5.3.4.4). Analysis of current transients allows determining the values of the nucleation rate and the number density of active sites. The experimentally observed dependence of the nucleation rates on overpotential and the charge transferred during the electrodeposition reaction are in agreement with fundamental theoretical postulates. The number density of nuclei observed microscopically on the surface, on the other hand, are in good agreement with those predicted from the nucleation rates obtained. Overall, experimental studies show the dominant role of deposit substrate interactions during the electrochemical phase formation phenomena. 

In general, experimental studies of the kinetics of formation of metalhc phases on electrodes are performed potentiostati-cally, stepping the potential from a positive to a negative value with respect to the reversible potential of the electrodeposition reaction. Under potentiostatic conditions, it is generally assumed that both the steady state nucleation rate Js and the growth rate... 

These benefits, evident from the introductory remarks, include wide limits of electrochemical stability, high ionic conductivity, good solvent properties for inorganic feedstocks, especially oxide ores, and relatively small cathodic deposition overvoltages. Two disadvantages occurring from these properties include the solubility of many refractory container materials and the frequent mass-transfer-controlled rate to the electrodeposition reaction. The former problem may be overcome in most cases, reverting to metallic vessels if necessary the latter problem will be discussed below. 

This mechanism was proposed by Diard et al. [1992] to evaluate EIS for cathodic production of hydrogen by the metallic electrodeposition reaction and anodic oxidation reaction of metals (e.g. iron) close to their corrosion potentials. A similar mechanism was found by Rerolle and Wiart [1996] studying the oxygen evolution during zinc electro winning. 

The ions resulting from anodic metal dissolution are present at higher concentration near the electrode surface than in the bulk, and this leads to a locally higher density of the electrolyte. In an unstirred cell with vertical electrodes, the resulting buoyancy force (Figure 4.31) causes the electrolyte in the anodic diffusion layer to move downwards. In an analogous way, an electrodeposition reaction results in an upward movement of the electrolyte because it lowers the metal ion concentration in the vicinity of the electrode. 

Fig. 1. Overpotential vs current density dependence (light gray) for electrodeposition reaction whereto — 10 Am aa = Oic = 0.5 — 0.05 A m . Black line is vsj dependence...

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