Solution Interphase

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

We show that the electric field in the metal-solution interphase is very high (e.g., 10 or lO V/cm). The importance of understanding the structure of the metal-solution interphase stems from the fact that the electrodepKJsition processes occur in this very thin region, where there is a very high electric field. Thus, the basic characteristics of the electrodeposition processes are that they proceed in a region of high electric field and that this field can be controlled by an external power source. In Chapter 6 we show how the rate of deposition varies with the potential and structure of the double layer. 

Charging of Interphase. Let us consider a case where a metal M is immersed in the aqueous solution of its salt, MA. Both phases, metal and the ionic solution, contain ions, as discussed earlier. At the metal-solution interface (physical boundary) there will be an exchange of metal ions M+ between the two phases (Fig. 4.2). 

Fundamentals of Electrochemical Deposition, Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

in this case the solution side of the interphase acquires opposite and equal charge, qX (the charge per unit area on the solution side of the interphase). At equilibrium the interphase region is neutral  

In Chapter 6 we show how the rate of deposition varies with the potential and the structure of the double layer. 

Some M+ ions from the crystal lattice enter the solution, and some ions from the solution enter the crystal lattice. Let us assume that conditions are such that more M+ ions leave than enter the crystal lattice. In this case there is an excess of electrons on the metal and the metal acquires negative charge, qM (charge on the 

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Techniques are described which obtain the IR absorption spectra of species, either adsorbed or free In the electrode/electrolyte solution Interphase. Applications slanted towards topics relevant to electrocatalytic processes are discussed to Illustrate the capabilities of the methods In probing molecular structure, orientation and Interactions. 

In principle, therefore, these valuable techniques can provide all of the information needed to specify the molecular structure of the electrode/electrolyte solution interphase, the dynamics of adsorption/... 

In the electrode-solution interphase, the adsorption of these substances is also affected by the influence of the electric field in the double layer on their dipoles. Substances that collect in the interphase as a result of forces other than electrostatic are termed surface-active substances or surfactants. 

As mentioned in Section 5.1, adsorption of components of the electrolysed solution plays an essential role in electrode processes. Adsorption of reagents or products or of the intermediates of the electrode reaction or other components of the solution that do not participate directly in the electrode reaction can sometimes lead to acceleration of the electrode reaction or to a change in its mechanism. This phenomenon is termed electrocatalysis. It is typical of electrocatalytic electrode reactions that they depend strongly on the electrode material, on the composition of the electrode-solution interphase, and, in the case of single-crystal electrodes, on the crystallographic index of the face in contact with the solution. 

Vibrational spectroscopy techniques are quite suitable for in situ characterization of catalysts. Especially infrared spectroscopy has been used extensively for characterization of the electrode/solution interphases, adsorbed species and their dependence on the electrode potential.33,34 Raman spectroscopy has been used to a lesser extent in characterizing non-precious metal ORR catalysts, most of the studies being related to characterization of the carbon structures.35 A review of the challenges and applications associated with in situ Raman Spectroscopy at metal electrodes has been provided by Pettinger.36... 

Before discussing metal-solution interphase, we shall discuss the relevant properties of the individual components of an interphase. These individual components are at the same time also basic components of an electrodep)osition cell (excluding the pKJwer supply). The basic components of an electrodeposition cell are, as shown in Figure 2.1, two metal electrodes (Mj and M2), water containing dissolved ions, and two metal-solution interfaces Mj-solution and M2-solution. 

Figure 4.2. Formation of metal-solution interphase equilibrium state n = %.

The simplest model of the structure of the metal-solution interphase is the Helmholtz compact double-layer model (1879). According to this model, all the excess charge... 

Let us consider the general electrochemical cell shown in Figure 5.2. The potential difference across the electrochemical cell, denoted , is a measurable quantity called the electromotive force (EMF) of the cell. The potential difference in Figure 5.2 is made up of four contributions since there are four phase boundaries in this cell two metal-solution interphases and two metal-metal interfaces. The cell in Figure 5.2 can be represented schematically as Pt/M7S/M/Pt. 

Eor an electrochemical reaction the rate of reaction v and the rate constant k depend on potential E specifically, the potential difference across electrode-solution interphase Acf) through the electrochemical activation energy AGf. Thus, the central problem here is to find the function... 

Equality of i and i on an atomic scale means that a constant exchange of charge carriers (electrons or ions) takes place process the metal-solution interphase. Figure 6.3... 

Figure 6.3. RedOx interphase at equilibrium an equal number of electrons crossing in both directions across the metal-solution interphase.

Thus, the overall reaction [Eq. (8.2)] is the outcome of the combination of two different partial reactions, Eqs. (8.4) and (8.5). As mentioned above, these two partial reactions, however, occur at one electrode, the same metal-solution interphase. The equilibrium (rest) potential of the reducing agent, E eq,Red [Eq. (8.5)] must be more negative than that of the metal electrode, E eq,M [Eq. (8.4)], so that the reducing agent Red can function as an electron donor and as an electron acceptor. This is in accord with the discussion in Section 5.7 on standard electrode potentials. 

One possible, although speculative explanation of the effect of the addition of sulfamic acid or sodium sulfate may be based on Eq. (4.9). According to this equation, the variation in the concentration c of a nonreacting electrolyte changes the thickness of the metal-solution interphase, the double-layer thickness It appears that as the thickness of the double layer, decreases, the coercivity of the Co(P) deposited decreases as well. 

Adsorption of Polymers, The three major characteristics of polymers in the metal-solution interphase of interest in metal deposition are the polydispersity, large number... 

The book is divided into 18 chapters, presented in a logical and practical order as follows. After a brief introduction (Chapter 1) comes the discussion of ionic solutions (Chapter 2), followed by the subjects of metal surfaces (Chapter 3) and metal solution interphases (Chapter 4). Electrode potential, deposition kinetics, and thin-fihn nucleation are the themes of the next three chapters (5-7). Next come electroless and displacement-type depositions (Chapter 8 and 9), followed by the chapters dealing with the effects of additives and the science and technology of alloy deposition... 

By comparing interfacial inactivation rates in a stirred-cell (low and controlled area of exchange) and an emulsion system (high interfacial area), these authors have shown that the use of an emulsion system can be exploited to obtain high solute interphase mass-transfer rates since the rate of specific interfacial inactivation remains low. However, in this system, the presence of an epoxide substrate at high concentration in the organic phase increases the rate of interfacial inactivation. Addition of a sacrificial protein to the system, which can prevent adsorption of the catalytic enzyme at the interface, could provide a method to reduce the rate of interfacial inactivation. 

It is clear that the adsorption of species in the metal-solution interphase region needs a subtle analysis. The unraveling of the complex situation and the building up of a basic picture of the accumulation and depletion of species at an electrified interface is one of the principal achievements of the new electrochemistry and is largely due to the American electrochemist, Grahame. 

Fig. 6.96. Deformation of the adsorbed ion due to the electric field at the metal-solution interphase. (Reprinted from J. O M. Bockris, M. Gamboa-Aldeco, and M. Szklarczyk, J. Electroanal. Chem. 339 355, copyright 1992, Fig. 16, with permission from Elsevier Science.)...

Before considering instrumentation in some detail in later chapters, it will be helpful to outline the kinds of experiments that we wish to implement electronically. It is useful to characterize electroanalytical techniques as either static or dynamic. Static methods are philosophically akin to the passive observation mentioned earlier. They entail measurements of potential difference at zero current such that the system defined by the solid-solution interphase is not disturbed and Nernstian equilibrium is maintained. Although such potentiometric measurements (e.g., pH, pM) are of great practical importance, our focus here will be on the dynamic techniques, in which a system is intentionally disturbed from equilibrium by excitation signals consisting of a wide variety of potential and current programs. 

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