Electrodeposition overpotentials

The last effect can be qualitatively discussed as follows. In the Eq. (1.31), the activation part of electrodeposition overpotential required for the charge transfer, /act. is given by Eq. (2.89), while the rest of the overpotential is due to mass transfer limitations, given by Eq. (6.1) ... 

The future of cathodic metal removal seems to be fruitful, since it is a mature technology, and there is a wide variety of cell designs commercially available nowadays. A promising field of application is recovery of precious metals from, e.g., spent catalysts and printed circuit boards, in which cell design and cell potential are usually not critical due to the value of the metal. Selective metal electrodeposition from a mixture of different ions is still a challenge, especially when the electrodeposition overpotentials are very close. 

The activation part of electrodeposition overpotential required for the charge transfer, 7act. is given by Eq. (3.20) ... 

If the electrodeposition overpotential and the overpotential of dendritic growth initiation from the examined electrolyte are known, it is obvious that the specific surface of copper powder can be calculated by Eq. (3.31) using this value of y and the corresponding value of the current efficiency for copper electrodeposition. The upper limit of the value of the copper powder specific surface can be estimated as follows. Assuming that electrodeposition is carried out at 1.0 V with the current efficiency for copper electrodeposition of 0.5, 5sp = 2,800 cm g is obtained using Eq. (3.31) which is in accordance with the data of Calusaru. In this way, one of the most important characteristics of copper powder is related to overpotential of the electrodeposition and hence to the electrodeposition conditions. 

There is an important difference between disperse deposits formed in galvanostatic and potentiostatic conditions. In potentiostatic electrodeposition, the properties of disperse deposits mainly depend on overpotential of electrodeposition. The electrodeposition overpotential remains constant during deposition time, as well as the real current density, and the structure of disperse deposits does not change with the electrodeposition time. Hence, it can be expected that both the stmcture of powder particles and the properties of powder (as association of different powder particles) on the macrolevel do not depend on the electrodeposition time. 

These considerations have been based entirely on thermodynamics and take no account of the overpotential, which is dependent on the rate of the process and the nature of the surface at which the reaction occurs. For this reason, the rate of reduction of HjO or HjO is usually low, and remains so to potentials from 0-5 to 1-OV below that given in equation 12.1. Even so, the instability of water is an insuperable obstacle to electrodepositing... 

Electrodeposition deposition of a metal or alloy onto a substrate by electrochemical reduction of its ions from an electrolyte under the application of a cathodic overpotential. 

Murase K, Uchida H, Hirato T, Awakura YJ (1999) Electrodeposition of CdTe films from ammoniacal alkaline aqueous solution at low cathodic overpotentials. J Electrochem Soc 146 531-536... 

Ishizaki T, Ohtomo T, Fuwa A (2004) Electrodeposition of ZnTe film with high current efficiency at low overpotential from a citric acid bath. J Electrochem Soc 151 C161-C167... 

The electrolysis temperature affects the electrolyte conductivity, the overpotential, and the solubility of the electrodeposit in aqueous as well as in molten salt systems. The effect of temperature is particularly important in the latter case. The lower limit of the temperature of operation is set by the liquidus temperature of the bath and the solubility of the solute. Generally, the temperature chosen is at least 50 °C above the melting temperature of... 

This technique is applied to mixtures of metal ions in an acidic solution for the purpose of electroseparation only the metal ions with a standard reduction potential above that of hydrogen are reduced to the free metal with deposition on the cathode, and the end of the reduction appears from the continued evolution of hydrogen as long as the solution remains acidic. Considering the choice of the cathode material and the nature of its surface, it must be realized that the method is disturbed if a hydrogen overpotential occurs in that event no hydrogen is evolved and as a consequence metal ions with a standard reduction potential below that of hydrogen will still be reduced a classic example is the electrodeposition of Zn at an Hg electrode in an acidic solution. 

Growth formation in epitaxial electrodeposition. Recently, Sheshadri113 observed that at small overpotentials caused by faradaic rectification, growth formation occurs in the epitaxial electrodeposition of copper on various copper single-crystal planes. 

Codeposition produces some of the better II-VI electrodeposits and, as can be seen in Table 1, has been used and studied extensively. Aqueous codeposition of CdTe serves as a good example of the method. The deposition is usually performed at an underpotential for Cd, at a potential where the Cd deposits exclusively on previously deposited Te. Te, on the other hand, is more noble than Cd and is thus deposited at an overpotential. The tellurite concentration, however, is kept far below that of the Cd+ so there is a large excess of Cd+l As soon as Te deposits, Cd quantitatively underpotentially deposits on top, providing control over deposit stoichiometry. 

All of these effects combine to provide enhanced yield and improved electrical efficiency. Other benefits which will become apparent include increased limiting currents [7,8], lower overpotentials and improved electrodeposition rates [9]. (Efficiency is defined as the amount metal deposited divided by the amount that should be deposited according to Faraday s laws of electrolysis.)... 

Technetium metal can be electrodeposited from an acidic solution of pertechnetate using a platimun, nickel or copper cathode. Electrolysis of neutral, unbuffered solutions, alkaline solutions, and sulfuric acid solutions lower than 2 N yield a black deposit of hydrated TcOj The current efficiencies are generally poor but the deposition is reasonably quantitative. The deposition requires the application of relatively negative cathode potentials and is therefore non-selective. Polaro-graphy indicates that the overpotentials for the evolution of hydrogen on technetium are rather low hence, electrolysis from acidic media will always include concurrent discharge of hydrogen . ... 

Figure 6.12. Overpotential characteristic of transition from compact to powdery deposit in electrodeposition of Cu from CUSO4 (0.1 M) + H2SO4 (0.5 M) solutions. (From Ref. 22, with permission from Wiley.)...

Equations (7.16) and (7.17) are used in an analysis of experimental data. Eor example, R5mders and AUdre (32) used these equations to analyze copper electrodeposition on platinum. They concluded that at the intermediate overpotentials (120 and 170 mV), the dimensionless current transients are consistent with the theoretical predictions for progressive nucleation, Eq. (7.17). At overpotentials higher than 220mV, nucleation shifted to the instantaneous nucleation theory. 

Figure 7.18. Current-potential curve showing the correlation between overpotential 17 and growth forms of electrodeposited copper from WCUSO4 and WH2SO4 at 25°C. (From Ref 40, with permission from Elsevier.)...

Seiter et al. (40) found a correlation between overpotential tj and growth forms of electrodeposited copper on copper sheet substrate with (fOO) texture, shown in Figure 7.18. 

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

Ru is easily oxidized anodically but the oxide is not stable and dissolution occurs under O2 evolution both in acid and in base [43, 56]. Nevertheless, if Ru oxide is electrodeposited during anodic polarization of aqueous solutions of RUCI3, the electrodeposited Ru oxide is catalytically active for O2 evolution, as shown by the decrease in anodic overpotential. However, such a configuration is impractical for water electrolysis since the liquid phase should contain RUCI3, which would be deposited everywhere in the cell circuit. 

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