Deposition overpotentials

During metal deposition processes the addition of adsorbable species has been found to cause an increase in the deposition overpotential [71 Lou]. Evaluation of the data results in the calculation of an adsorption isotherm. (Data obtained with this method are labelled CT.)... 

Properties of thin layers of lead electrodeposited on vitreous carbon have been found identical with that of metallic lead [304]. Therefore Pb and Pb02 coated reticulated vitreous carbon (RVC) electrodes [185] can be applied as electrodes in lead-acid batteries, as reviewed in [305]. The deposition of lead on carbon is through the diffusion-controlled process with instantaneous or progressive nucleation, for high and low Pb + concentration, respectively, and three-dimensional growth mechanism. The number of nucleation sites increases with deposition overpotential, as shown for vitreous [306] and glassy carbon [307] electrodes. The concentration dependence of the nucleation... 

Additives that increase the deposition overpotential at a given current density, for instance, by altering the Tafel constants, can be considered deposit-leveling additives. Since additives are typically present in very small concentrations, their transport to the electrode is nearly always under diffusion control and sensitive to flow variations. 

The better understanding of the concept effective overpotential can be realized by taking into account the fact that the time of dendritic growth initiation depends on used deposition overpotentials. Increasing deposition overpotentials lead to decreasing times for... 

For large cathodic current densities smaller than the limiting current density, the deposition overpotential increases with increasing current density and vice versa, according to the Bulter-Volmer equation ... 

The formation of the smaller (and less different to each other) dendrites could be obtained by the increase of deposition overpotential. Unfortunately, the increased overpotential produces the hydrogen evolution in this system and the formation of degenerate dendrites and honeycomb-like deposits.76,77 Nevertheless, the dendritic growth in this system at larger overpotentials is possible by the application of appropriate square-wave pulsating overpotential (PO) regime. For example, the well-developed dendrites were formed with amplitude overpotential of 1,000 mV, deposition pulse of 10 ms, and pause of 100 ms (the pause to pulse ratio 10) (Fig. 26a). They can be well approximated by the cones shown in Fig. 23. Also, superficial holes due to attached hydrogen bubbles were formed between these dendrites, as can be seen from ref.78... 

In the solutions containing gelatin, the morphology of the electrodeposits was also of the unorientated dispersion type. Consequently, the (002) orientation index of the deposited Zn depended on the overpotential for Zn deposition as shown in Fig.5(b). This result can be explained well by the Pangarov s two-dimensional nucleation theory (9) that the crystal orientation of the deposited metals is determined by the deposition overpotential of the metals. 

Certainly additives are usually capable of adsorption on the cathode surface and in some cases organic matter is occluded into the deposit, especially when the plated metal has a high surface energy (high melting point). Many additives also increase the deposition overpotential and change the Tafel slope. This may be due to the need for electron transfer to occur through the adsorbed layer or due to complex formation at the electrode surface. 

Morphology is probably the most important property of electrodeposited metals. It depends mainly on the kinetic parameters of the deposition process and on the deposition overpotential or current density. The morphology of an electrodeposited metal also depends on the deposition time until the deposit has attained its final form. 

Fig. 2.3 (a) The model of an electrode for diffusicm-contiolled electrodeposition of metals and (b) cross section of Cu deposit obtained from 0.50 M O1SO4 in 0.50 M H2SO4 in a system from (a). The thickness of the agar diffusion layer was 1.0 mm. Deposition overpotential 300 mV deposition time 120 min. The substrate is a piece of a phmiograph disk negative (Reprinted from Refs. [7, 11, 13] with kind permission from SpringCT)... 

Fig. 2.6 Copper deposits obtained from 0.30 M CUSO4 in 0.50 M F12S04 by electrodeposition under mixed activation-diffusion control. Deposition overpotential 220 mV. Quantity of electricity (a) 40 mA h cm, (b) the same as in (a), (c) 10 mA h cm, (d) 10 mA h cm, (e) 20 mA h cm , and (f) the root of the carrot from (e) (Reprinted from Refs. [7, 8, 13] with kind permission from Springer and Ref [20] with permission from the Serbian Chemical Society)...

Fig. 2.8 (a) Schematic representation of the formation of an indestructible reentrant groove and the cadmium deposits obtained by deposition from 0.10 M CdS04 in 0.50 M H2SO4 onto a cadmium electrode. Deposition overpotential 50 mV. Depositirai times (b) 2 min, (c) 2 min, and (d) 10 min (Reprinted from Refs. [7, 11, 13] with kind permission from Springer and Ref. [33] with permission from Elsevier)... 

Substitution of /l,c from Eq. (2.62) into Eq. (1.13), after rearranging, gives the Eq. (2.37) as in the case of spherical diffusion control, and the further derivation remains the same. Hence, the dendritic growth can be initiated at the same deposition overpotential under conditions of both spherical and cylindrical diffusion. 

The slopes of these dependencies are similar to each other and independent of the deposition overpotential during the non-dendritic amplification of the surface coarsening according to the Eq. (2.34). 

Fig. 2.16 The dependence of the overpotential on time during cadmium electrodeposition on a spiral platinum cathode (electrode surface area 1.5 cm ) from 0.50 M CdS04. The deposition current was 65 pA, is the nucleation, rjcr is the crystallization, and rj is the deposition overpotential (Reprinted from Ref. [47] with permission from Bulgarian Chemical Communications and Ref. [13] with kind permission from Springer)...

The difference in overpotential between the curves for a given supersaturation (nucleation on an inert substrate) and the curve for a supersaturation equal to unity (deposition on a native substrate) gives the value of the crystallization overpotential, ijcr [48]. It is equal to the difference in the overpotential at point c and at point e in Fig. 2.16. If the current is switched off at point e, the electrode potential will approach to the reversible potential of the deposited metal (point g) after switching on the current again at point g, the overpotential returns to the same value as at point e, i.e., the deposition overpotential, r/, meaning that a new phase is formed. On the contrary, if current is switched off before point c, the electrode potential will approach the initial stationary potential of the inert electrode, meaning that new phase has not been formed [47]. 

Fig. 2.19 Cadmium deposits obtained from 1.0 M CdS04 in 0.50 M H2SO4 solution onto a copper plane electrode (a) deposition overpotential 10 mV deposition time 24 min, (b) deposition overpotential 40 mV deposition time 4 min, (c) deposition overpotential 60 mV deposition time 2 min, and (d) deposition overpotential 110 mV deposition time 80 s (Reprinted from Refs. [13, 54] with kind permission from Springer)...

In the case of Cu, due to the lower exchange current density value, a surface film is practically formed by a smaller quantity of electricity (Fig. 2.20b). The value of the deposition overpotential is larger than in the case of Cd, and the crystallization overpotential is lower, resulting in a decrease of the zero nucleation zone radiuses. In the case of Cu, it is clear that a considerably larger nucleation rate is observed. 

It is obvious that the larger nucleus density, the thinner is the thickness of the metal film required to isolate the substrate from the solution. At the same time, a thinner surface film will be less coarse than a thicker one. This means that a smoother and thiimer surface film will be obtained at larger deposition overpotentials and nucleation rates, i.e., by electrodeposition processes characterized by high cathodic Tafel slopes and low exchange current densities. 

Fig. 2.23 Silver deposits electrodeposited from 0.50 M AgNOs in 100 g dm NaN03 (a) in the potentiostatic regime of electrolysis onto Ag wire electrode without the addition of H3PO4. Quantity of electricity 100 mA h cm. Deposition overpotential 120 mV. The exchange current density 26 mA cm and (b) in the galvanostatic regime of electrolysis onto Pt wire electrodes with the addition of 6 g dm H3PO4. Current 30 mA. Time 2 s. The exchange current density 5 mA cm (Reprinted from Refs. [13, 65, 66] with kind permission from Springer)...

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