Overpotentially deposited copper

The reversible potential [Eq. (11.1)] of copper in conditions of unit activity for copper ions is 0.34 V, and for zinc ions, 0.77 V. It is clear that if a solution contains unit activities of Cu and Zn ions, zinc will not codeposit with copper unless the overpotential for copper deposition is high enough to compensate for this large difference in deposition p>otentials. 

The copper deposits obtained under activation and mixed controls as those shown in Fig. 2.4 are shown in Fig. 4.8c, d. A considerable decrease in the grain size of deposit obtained at the low current densities (in the activation controlled region Figs. 2.4ac and 4.8) due to the increase of the amplitude of the overpotential relative to the corresponding value in constant overpotential deposition can be seen. There is no qualitative change, however, in the structure of the deposit. 

The method is applicable to mixtures if correct conditions are chosen. Suppose, for example, that a copper-nickel solution is to be analysed. Copper deposits readily at a cathode potential close to the equilibrium value for copper, E (Cu, Cu) = +0.34 V E for nickel is -0.25 V, but there is an appreciable activation overpotential (q.v.) for nickel deposition, so that a still more negative potential will be needed. An applied current will thus first deposit copper exclusively at a cathode potential of about +0.3 V. Towards the end of the deposition the concentration of copper ions will begin to fall steeply, and the cathode potential will have to become more negative to maintain deposition. However, a cathode potential of -0.25 V would correspond to a copper ion concentration given by... 

Therefore, criteria in the selection of an electrode reaction for mass-transfer studies are (1) sufficient difference between the standard electrode potential of the reaction that serves as a source or sink for mass transport and that of the succeeding reaction (e.g., hydrogen evolution following copper deposition in acidified solution), and (2) a sufficiently low surface overpotential and rate of increase of surface overpotential with current density, so that, as the current is increased, the potential will not reach the level required by the succeeding electrode process (e.g., H2 evolution) before the development of the limiting-current plateau is complete. 

Figure 3a is an illustration of the effect of surface overpotential on the limiting-current plateau, in the case of copper deposition from an acidified solution at a rotating-disk electrode. The solid curves are calculated limiting currents for various values of the exchange current density, expressed as ratios to the limiting-current density. Here the surface overpotential is related to the current density by the Erdey Gruz-Volmer-Butler equation (V4) ... 

It is clear from the calculated limiting-current curves in Fig. 3a that the plateau of the copper deposition reaction at a moderate limiting-current level like 50 mA cm 2 is narrowed drastically by the surface overpotential. On the other hand, the surface overpotential is small for reduction of ferri-cyanide ion at a nickel or platinum electrode (Fig. 3b). At noble-metal electrodes in well-supported solutions, the exchange current density appears to be well above 0.5 A/cm2 (Tla, S20b, D6b, A3e). At various types of carbon, the exchange current density is appreciably smaller (Tla, S17a, S17b). 

A steep rise of current to the plateau. This rise would be complete within approximately 200 mV, if surface overpotential were negligible. (See, e.g., Fig. 3a, illustrating cathodic deposition of copper in which the experimental current-voltage curve indicates an appreciable surface overpotential, and Fig. 3b, illustrating cathodic reduction of ferricyanide in which the surface overpotential is negligible.)... 

Termination of the plateau at a sufficiently high overpotential. The potential at which a consecutive electrode reaction sets in (e.g., hydrogen evolution in cathodic reactions) is determined by the composition of the electrolyte (specifically, the pH) and by the nature and state of the electrode surface (hydrogen overpotential). The reduction of ferricyanide in alkaline solution on nickel also provides a better-defined plateau in this respect than the deposition of copper in acid solution. 

In many cases mass transfer is not the sole cause of unsteady-state limiting currents, observed when a fast current ramp is imposed on an elongated electrode. In copper deposition, in particular, as a result of the appreciable surface overpotential (see Section III,C) and the ohmic potential drop between electrodes, the current distribution below the limiting current is very different from that at the true steady-state limiting current. 

A correlation between the spacing of striae and convection downstream of protrusions does not fully describe the process. The initial protrusions arise far from transport control and cannot be attributed to a diffusive instability of the type described in the previous section. Jorne and Lee proposed that striations formed on rotating electrodes by deposition of zinc, copper and silver are generated by an instability that arises only in systems in which the current density at constant overpotential decreases with increasing concentration of metal ion at the interface [59]. 

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 . ... 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

An experimental study of zinc electrodeposition on copper wire from 0.1 M zincate solution in 1.0 M KOH was presented by Simicic et al. [232]. A possible mechanism of the formation of spongy zinc deposits was considered. Also, it was shown that in the case of a square-wave pulsating overpotential regime, the deposit was less... 

The rate determining step need not always be, as in this case, one of the reduction steps. Thus at low overpotential, slow surface diffusion was rate determining for the deposition of copper.7... 

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