Electrochemical deposition anodic reaction

The formation of colloidal sulfur occurring in the aqueous, either alkaline or acidic, solutions comprises a serious drawback for the deposits quality. Saloniemi et al. [206] attempted to circumvent this problem and to avoid also the use of a lead substrate needed in the case of anodic formation, by devising a cyclic electrochemical technique including alternate cathodic and anodic reactions. Their method was based on fast cycling of the substrate (TO/glass) potential in an alkaline (pH 8.5) solution of sodium sulfide, Pb(II), and EDTA, between two values with a symmetric triangle wave shape. At cathodic potentials, Pb(EDTA)2 reduced to Pb, and at anodic potentials Pb reoxidized and reacted with sulfide instead of EDTA or hydroxide ions. Films electrodeposited in the optimized potential region were stoichiometric and with a random polycrystalline RS structure. The authors noticed that cyclic deposition also occurs from an acidic solution, but the problem of colloidal sulfur formation remains. 

The electrolyte is made by in situ chlorination of vanadium to vanadium dichloride in a molten salt bath. Higher valent chlorides are difficult to retain in the bath and thus are not preferred. The molten bath, which is formed by sodium chloride or an equimolar mixture of potassium chloride-sodium chloride or of potassium chloride-lithium chloride or of sodium chloride-calcium chloride, is contained in a graphite crucible. The crucible also serves as an anode. Electrolysis is conducted at a temperature about 50 °C above the melting point of the salt bath, using an iron or a molybdenum cathode and a cathode current density of 25 to 75 A dnT2. The overall electrochemical deposition reaction involves the formation and the discharge of the divalent ionic species, V2+ ... 

The anodic reaction is an oxidation reaction producing electrons in the anode, while the cathodic reaction is a reduction reaction consuming electrodic electrons at the cathode interface. We shall consider, as an example, an electrochemical cell consisting of a metallic zinc electrode and a metallic copper electrode, in which the anodic reaction of zinc ion transfer (zinc dissolution) is coupled with the cathodic reaction of copper ion transfer (copper deposition) as shown in the following processes ... 

Kaba and Hitchens (1989) found that electrolysis of a mixture of urine and feces produced C02, N2, and H2. Some HOC1 is generated this eliminates the pathogens and bleaches the contents. The anodic reactions at 90 °C consume the biomass the cathode evolves hydrogen and can be assumed to deposit the small metal content. The residuum is sodium chloride from the urine. Most of the electrochemical studies that establish the basis of a practical process for electrochemical sewage treatment have been carried out on packed-bed electrodes as shown in Fig. 15.29. 

Electrochemical deposition of lithium usually forms a fresh Li surface which is exposed to the solution phase. The newly formed surface reacts immediately with the solution species and thus becomes covered by surface films composed of reduction products of solution species. In any event, the surface films that cover these electrodes have a multilayer structure [49], resulting from a delicate balance among several types of possible reduction processes of solution species, dissolution-deposition cycles of surface species, and secondary reactions between surface species and solution components, as explained above. Consequently, the microscopic surface film structure may be mosaiclike, containing different regions of surface species. The structure and composition of these surface films determine the morphology of Li dissolution-deposition processes and, thus, the performance of Li electrodes as battery anodes. Due to the mosaic structure of the surface... 

Nucleation and growth kinetics — Nucleation-and-growth is the principal mechanism of phase transformation in electrochemical systems, widely seen in gas evolution, metal deposition, anodic film formation reactions, and polymer film deposition, etc. It is also seen in solid-state phase transformations (e.g., battery materials). It is characterized by the complex coupling of two processes (nucleation and phase growth of the new phase, typically a crystal), and may also involve a third process (diffusion) at high rates of reaction. In the absence of diffusion, the observed electric current due to the nucleation and growth of a large number of independent crystals is [i]... 

A good overview on the various passivation and deposition processes can be found in Refs. [267-269]. In Table 1.5 the resulting Dit trap densities for the various possible passivation techniques are shown. Thermal passivation yields the highest interface quality, that is the lowest Dit can be achieved. Quality wise the electrochemical passivation is next. However, electrochemical reactions at a semiconductor surface are only possible in the accumulation mode. Therefore, anodic reactions only take place at p-type doped Si electrodes (accumulation of majority charge carriers, i.e. holes), whereas on n-Si only reduction reactions are possible. Consequently, only p-type doped Si can be anodically passivated. This can be changed by an illumination... 

Electrodeposition or electrochemical deposition (of metals or alloys) involves the reduction of metal ions from electrolytes. At the cathode, electrons are supplied to cations, which migrate to the anode. In its simplest form, the reaction in aqueous medium at the cathode follows the equation ... 

Continuous and semicontinuous electrochemical reactors are normally employed for effluent metal ion remediation, where the anode reaction is usually oxygen evolution from water [compare with Equation (26.4)]. After the metal contaminant is captured on the cathode, the cathode can be discarded, the collected metal can be resold, or the deposited metal can be chemically or elecfro-chemically etched into a small volume of a suitable leaching liquor (e.g., water) so as to increase its concentration substantially. 

Apart from platinum s intermediate nature on bonding, another point in platinum s favor is availability platinum can be purchased in various suitable forms at a reasonable price some noble metals are difficult to find and purchase. The word noble means here stable and of course that is a first point one wants in an electrocatalyst. It must be a catalyst, not enter into the reaction. It is meant to accelerate the reaction. It must itself be stable, thermally and electrochemically. On the last point, platinum is only fairly good because oxide-free platinum does start itself to dissolve around 1.0 V on the normal hydrogen scale. By using it in anodic reactions in a potential range anodic to 1.0 V, Pt(II) is likely to get into the solution and may be deposited on the cathode. 

Equations (2.17)-(2.19) represented by the lines 1, 2, and 3 are stable electrochemical reactions. Equations (2.20) and (2.21) represent chemical reactions. Zinc will dissolve at the suitable potential to form soluble zinc ions, i.e., Zn when the solution pH is less than 7. When the solution pH is in between 8 and 11, anodic reactions produce zinc hydroxide Zn(OH)2 which is less soluble in water and may deposit on the metal surface as a protective film. This zone is designated as passivity region. When the pH value is more than 11 corrosion of zinc is possible by forming complex zincate ions Zn02. The bottom region of the diagram represents thermodynamically stable solid zinc. Line 6 and... 

In EMST, electrochemical reactions based on Faradaic reactions such as metal deposition, anodic dissolution, various oxide formation, and anodic polymerization are common. Ion transfer reactions (ITR) from the electrolyte to the solid or solid to electrolyte can be used for formation of positive or negative structure by deposition or dissolution. ITR can also be performed by electron transfer reaction (ETR) in chemical reactions in the bulk electrolyte. Pure ETR cannot be utilized for microstructuring. Local field distributions at the interface and inside the microstmcrnre play an important role during vertical structure formation by depositions or removal. It also depends on the ionic conductivity of the materials to be deposited or dissolved such as metal, semiconductor, and oxides. 

The catalytic problems associated with the anodic oxidation of methanol are very similar to those associated with the anodic oxidation of CO-containing technical hydrogen. At present, the standard catalyst for both reactions is a mixed Pt-Ru catalyst (with about 50 at% of each metal) obtained by their joint chemical or electrochemical deposition from solutions of simple or complex compounds on carbon black. 

It is important to distinguish between three types of electrochemical process for the deposition of metals (1) electroplating (2) immersion plating and (3) electroless plating (Table 8.6). They differ in the nature of their anodic reaction. Considering the simple case of discharge of a metal ion, the common cathodic... 

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