Displacement of the Electrochemical Interface

When an oxidizable metal is coated by a conducting-polymer film in its oxidized state the situation might seem to be similar to that described above. Indeed, the initial electrochemical interface moves from metal-solution to polymer-solution and, as long as the 

Another illustration of the displacement of the electrochemical interface can be found in a recent study reported by Michalik and Rohwerder [42], They were able to locate the oxygen reduction process on a metal-polymer system using an isotopic marker ( 02). The reduction of heavy oxygen will create anions which can be recognized by mass 

It can thus be stated that if such electrochemical interface displacement were permanent in a defect-free coating, and if no scratches were made during the lifetime of the eoating, a steady state would be obtained in which the conductive polymer protects the underlying metal permanently, since no or minimal electrical potential drop arises at the metal-polymer interface. 

Finally, O2 reduction on the polymer is crucial since it can help to maintain the polymer in an oxidized state as long as the O2/H2O oxidation potential is above that of the polymer 

For instance, with a PPy film deposited on a zinc electrode in aqueous salicylate solution, the reduction potential of the doped polymer is around 0.9 V/SCE [43], whereas the thermodynamic reduction potential of O2 varies considerably with the pH, and can shift from 1.2 V/NHE (at pH 0, when the reduction involves four electrons) to 0.4 V in basic medium at pH 12 [44]. 

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In conclusion, the displacement of the electrochemical interface can slow down corrosion for a time, but not indefinitely, and the ideal and permanent steady-state picture described above is rarely achieved. 

As shown previously, anticorrosion properties are the result of a complex steady-state situation in which several phenomena are in action (i) displacement of the electrochemical interface, (ii) ennobling and self-healing effects, (iii) barrier effect offering nonspecific protection. 

It is, of course, usual in discussing the electrochemical interface to use a dielectric constant, which is the ratio of the electric displacement to the electric field. By Fourier transforming the dielectric function e(k), one would obtain an effective dielectric constant, which would, however, depend on position. In fact,48 the screening... 

We measure the current through the interface of the working electrode as a function of the potential difference at it. This current is either a displacement current or a real current. The displacement current, which is an undesirable effect in nearly all electroanalytical work, can be described as a charging of a capacitor, located at the interface, and one speaks about the capacitive current. The other, more important, part is due to electrochemical processes, in which ions or electrons are transferred from the electrode to the solution or vice versa. As these processes are governed by Faraday s law, one speaks of faradaic currents. Faraday s law states that the electrochemical conversion of m moles yields an amount of electricity of mnP coulombs, where n is the number of electrons released or taken up in the reaction and F the Faraday constant, with a value of about 10 coulombs/mole. This high value of the electrochemical equivalent is, of course, very attractive from the analytical point of view. The measurement of picocoulombs of electricity is extremely simple nowadays and detection limits of 10 mole could be expected from this simple calculation. 

Although there is no external current, anodic and cathodic processes can still occur at sites on the interface between solid and aqueous solution because of the electrolytic conductance of the corrosive medium. At electrochemical equilibrium, this leads to a definite jump in the electrical potential at the phase boundary. Kinetic barriers to certain partial reaction steps of the electrochemical process can cause the potential to be displaced from its equilibrium value. Thus, for example, instead of a dissolution of metal ... 

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

To be able to investigate the hydrodynamics of the system, the device was additionally equipped with a video camera (SONY, Japan) for observations of the displacement of tracer particles located at the gas-liquid interface. The experimental system could be also adapted for direct measuring of the mass transfer rate across the interface in the presence of the active phospholipid monolayer. For that purpose, the electrochemical system was developed [1], where the oxygen flux across the interface could be determined by the measurement of the electric current intensity. The results of experimental investigations will be presented in the further part of the paper. 

The Stern model provides a reasonable description of the electric behavior of the metal-electrode interface for electrochemical systems, but it does not allow us to explain all experimental observations. In particular, it does not offer a satisfactory explanation for the influence of crystal orientation on the capacity of the double layer, nor for the effect of the chemical nature of anions. Figure 3.49 [17] shows the capacity of the double layer of a monocrystalline silver electrode, with three different orientations. The resulting curves are similar, but they are displaced along the axis of the potential. 

The H entry into a metal fiom an aqueous electrolyte is believed to involve the same surface-bulk transfer step as in the gas phase, but the preliminary adsorption step is a more complex process because more H sources are involved in aqueous solution, allowing more possible H surface reactions, and also because of the specificity of the electrolyte-metal interface. Whereas H adsorption in the gas phase occurs by dissociative adsorption of gaseous H2 on the free sites of a bare metallic surface, H adsorption in aqueous solution may occur either chemically by dissociation of dissolved H2 or electrochemically from solvated (hydrated) protons or water molecules it takes place on a hydrated surface and thus implies the displacement of adsorbed water molecules or specifically adsorbed ions and local reorganization of the double layer [20] competition with the adsorption of oxygen species formed from the dissociation of water may also occur [21-23], The adsorbed H layer is also in interaction with surrounding water molecules, i.e., it is hydrated [8c,24,25],... 

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