Crystallization overvoltage

The discussion of concentration polarization so far has centred on the depletion of electroactive material on the electrolyte side of the interface. If the metal deposition and dissolution processes involve metastable active surface atoms, then the rate of formation or disappearance of these may be the critical factor in the overall electrode kinetics. Equation (2.69) can be rewritten for crystallization overvoltage as... 

Reaction overvoltage is due to secondary reactions of, e.g., the electrolysis products (for example, the association of gaseous atoms into molecules, the escape of gas bubbles, the formation of a crystalline lattice in the case of metal deposition, also called crystallization overvoltage). 

If UPD and OPD processes of 2D and 3D Me phase formation are investigated under non-equilibrium conditions, then A// and, therefore, t] and A can be influenced by the reaction kinetics. For example, the charge transfer itself, mass transport, and chemical reaction steps which precede or follow the charge transfer can be kinetically hindered. Then, A/i, rj, and E are determined not only by crystallization overvoltage and underpotential as defined in eq. (1.3), but also contain charge transfer, diffusion, and/or chemical reaction contributions. 

Some basic aspects of alloy dissolution are best illustrated by the behavior of a liquid binary alloy A-B. This is due (1) to the absence of crystallization overvoltage and dissolution induced structural surface modifications [6] as well as (2) to the high diffusivity in the alloy phase that provides for the reactant supply at the alloy/electrolyte interface if one alloy component dissolves preferentially (at a higher rate than the other) (7). Provided that the standard electrode potential difference of the components, AE = E — El, is large AE > RT/F) and their charge transfer reactions are fast, one expects a schematic polarization curve as shown by Fig. 1(a). For Ea < E < Eb, only the less noble component. A, dissolves ( selective dissolution or deaUoying ), the partial anodic... 

Gerischer made an experimental attempt to measure the crystallization overvoltage of Ag deposition in a chronopotentiometric experiment. In this experiment two results were obtained. From the slope at t -> 0 the capacitance was determined. This capacitance was much larger than the double-layer capacitance and was interpreted as adsorption capacitance Qd- The ad-atom concentration was calculated from the adsorption capacitance... 

Vetter summarized the main results obtained up to 1967. The principal difficulty in the experimental investigation of metal electrodes in the past was the poorly reproducible preparation of the electrode surface. This problem could be avoided for some electrodes by dissolving metals in hquid mercury. Therefore, many results about the mechanism were measured with amalgam electrodes. On such an electrode no crystallization overvoltage is expected. When it was possible to suppress diffusion depletion, the charge transfer process could be investigated. The charge transfer mechanism could then be determined from the electrochemical reaction orders of the complex molecules or ions. 

Klapka V (1970) To the problem of crystallization overvoltage during electrocrystallization of metals. Coll Czechoslov Chem Comun 35 899-906... 

It is established that the first crystals start growing immediately after the appearance of the peak on the switching curves. The repeated switching-on of the current within a short period of time (5-10 s) does not give rise to peak formation. These facts indicate that the crystallization overvoltage is associated with three-dimensional nucleation. Based on the experimental results, we evaluated the crystallization overvoltage due... 

An increase in the melt temperature may complicate the crystallization process because of the interaction between the deposited components and the material matrix (Figure 4.9.10). For metals forming alloys with the deposited components, crystallization overvoltage is observed for a surface oxide film (Figure 4.9.11). After mechanical treatment of the surfaces of the working electrodes, they were electrochemically polished with simultaneous control of the substrate state in a microscope. The time of electrical polishing was determined with allowance made for the dissolution time of the Beilby layer [21]. 

It is characteristic that the height of the overvoltage maximum for the above metals is proportional to the reciprocal time of their formation. This seems to be associated with the penetration of the part of the deposited components into the substrate bulk due to solid-phase diffusion, which allowed us to qualitatively characterize the degree of inertness of the substrate material. Thus, crystallization overvoltage on the... 

The experimental study of the initial stages of M02C electrocrystallization from tungstate-molybdate-carbonate melts with electrodes prepared from various materials over a wide temperature range allows us to put forward the following concepts of nucleation. Thus, using inert substrates at r< 1073-1173 K, we observed considerable crystallization hindrances associated with the formation of three-dimensional nuclei. An increase in the electrolysis temperature facilitates the diffusion of atoms of the components into the substrate, which results in a decrease of crystallization overvoltage. Simultaneously, a transition from three- to two-dimensional nucleation is observed and, in some instances, to depolarization phenomena due to solid-phase saturation of the boundary layers of the electrode with the components (molybdenum and carbon) and the formation of an alloy with the material of the electrode. 

It has been emphasized already that atomic arrangements deviating from the bulk lattice, as well as crystals with non-equilibrium forms, can contribute to the overall nucleation rate. At low overvoltages, this contribution becomes appreciable [4.20]. 

Fig. 4.7 shows Hg droplets formed on a spherical Pt single-crystal electrode [4.37]. The nuclei are formed with a cathodic pulse with a given amplitude and duration, and are then grown at a lower overvoltage until of a visible size. The density of nuclei [nuclei cm ] is determined simply by counting the number of droplets on a given surface area of the electrode. 

The existence of a supersaturation or overvoltage threshold is a characteristic feature of nucleation-induced processes as, e.g., electrochemical phase formation. Based on this phenomenon, several experimental techniques for electrociystallization studies have been developed (cf. Section 4.2). Before going into further details, however, let us discuss some technical skills that can lead to the preparation of well-developed low dislocation density single crystal faces. 

Figure 5.4 Pyramids of growth on an Ag(lOO) face obtained by applying a short overvoltage pulse on an initially flat crystal face in the standard system Ag(100)/AgNO3 [5.7]. The pyramids mark the emergence points of the screw dislocations. The quadratic symmetry of the pyramids corresponds to the (100) nature of the face. Face areay4(ioo) = 2 x 10" cm. ...

Fig. 5.15 shows linear relations between the plateau current and the overvoltage, permitting the determination of the rate constant of step propagation xv. For this purpose, the step length is needed, which can be estimated if the step form and step orientation with respect to the rectangular face boundaries are known. To solve this problem, two orientations of the seed crystal with respect to the rectangular cross section of the capillary have been used. 

The sequence i)-iv) corresponds to increasing inhibition of the electrocrystallization process accompanied by increasing cathodic overvoltage [6.27, 6.28, 6.37]. Examples are shown in Fig. 6.1 [6.8]. A special texture type is produced by the so-called rhythmic-lamellar crystal growth, representing an oscillation reaction (Fig. 6.2) [6.38]. 

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