Electrode-solution interface electrochemical processes

In general, the physical state of the electrodes used in electrochemical processes is the solid state (monolithic or particulate). The material of which the electrode is composed may actually participate in the electrochemical reactions, being consumed by or deposited from the solution, or it may be inert and merely provide an interface at which the reactions may occur. There are three properties which all types of electrodes must possess if the power requirements of the process are to be minimized (i) the electrodes should be able to conduct electricity well, i.e., they should be made of good conductors (ii) the overpotentials at the electrodes should be low and (iii) the electrodes should not become passivated, by which it is meant that they should not react to form on their surfaces any compound that inhibits the desired electrochemical reaction. Some additional desirable requirements for a satisfactory performance of the cell are that the electrodes should be amenable to being manufactured or prepared easily that they should be resistant to corrosion by the elements within the cell that they should be mechanically strong and that they should be of low cost. Electrodes are usually mounted vertically, and in some cases horizontally only in some rare special cases are they mounted in an inclined manner. 

Since the electrochemical reduction or oxidation of a molecule occurs at the electrode-solution interface, molecules dissolved in solution in an electrochemical cell must be transported to the electrode for this process to occur. Consequently, the transport of molecules from the bulk liquid phase of the cell to the electrode surface is a key aspect of electrochemical techniques. This movement of material in an electrochemical cell is called mass transport. Three modes of mass transport are important in electrochemical techniques hydrodynamics, migration, and diffusion. 

The important concept in these dynamic electrochemical methods is diffusion-controlled oxidation or reduction. Consider a planar electrode that is immersed in a quiescent solution containing O as the only electroactive species. This situation is illustrated in Figure 3.1 A, where the vertical axis represents concentration and the horizontal axis represents distance from the electrodesolution interface. This interface or boundary between electrode and solution is indicated by the vertical line. The dashed line is the initial concentration of O, which is homogeneous in the solution the initial concentration of R is zero. The excitation function that is impressed across the electrode-solution interface consists of a potential step from an initial value E , at which there is no current due to a redox process, to a second potential Es, as shown in Figure 3.2. The value of this second potential is such that essentially all of O at the electrode surface is instantly reduced to R as in the generalized system of Reaction 3.1 ... 

The objective of most electrochemical experiments is to allow the experimenter to investigate one or more of three types of parameters (1) the concentration and identity of one or more solution components, (2) the kinetics of chemical, charge transfer, or adsorption processes, and (3) the nature of the double-layer capacitance associated with the electrode-solution interface. Historically, most small-amplitude techniques have been developed in an attempt to allow an easier separation of the contributions of these basic parameters. 

Marken, F., Tsai, Y.-C., Coles, B.A., Matthews, S.L. and Compton, R.G., Microwave activation of electrochemical processes convection, thermal gradients and hot spot formation at the electrode-solution interface, New. Chem., 2000, 24, 653. 

The membrane system considered here is composed of two aqueous solutions wd and w2, separated by a liquid membrane M, and it involves two aqueous solution/ membrane interfaces WifM (outer interface) and M/w2 (inner interface). If the different ohmic drops (and the potentials caused by mass transfers within w1 M, and w2) can be neglected, the membrane potential, EM, defined as the potential difference between wd and w2, is caused by ion transfers taking place at both L/L interfaces. The current associated with the ion transfer across the L/L interfaces is governed by the same mass transport limitations as redox processes on a metal electrode/solution interface. Provided that the ion transport is fast, it can be considered that it is governed by the same diffusion equations, and the electrochemical methodology can be transposed en bloc [18, 24]. With respect to the experimental cell used for electrochemical studies with these systems, it is necessary to consider three sources of resistance, i.e., both the two aqueous and the nonaqueous solutions, with both ITIES sandwiched between them. Therefore, a potentiostat with two reference electrodes is usually used. 

In contrast to many chemical measurements, which involve homogeneous bulk solutions, electrochemical processes take place at the electrode-solution interface. The distinction between various electroanalytical techniques reflects the type of electrical signal used for the quantitation. The two principal types... 

The electrical behavior of the electrode-solution interface and the processes which can take place at it, due to an electrochemical reaction, can be treated in terms of an electrical equivalent circuit. Such an equivalent circuit must represent the time-dependent behavior of the mechanism of the reaction but usually it is possible that more than one equivalent circuit can model the reaction behavior. The simplest equivalent circuit is (Cl) for a charge-transfer process not involving the production of an adsorbed intermediate, for example, for the case of an ionic redox reaction such as Fe(CN)e3- +e-- Fe(CN)6 - ... 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

The potential difference across the electrode/solution interface is dropped by the accumulation of ions of opposite charge in the solution immediately adjacent to the electrode surface in the electrochemical double layer. The spatial distribution of ions gives a potential profile across the double layer into the solution over a distance that is dependent upon the electrolyte concentration. Given this position-dependent potential profile, it is possible that species undergoing electrochemical reaction, which are assumed to reside in the outer Helmholtz plane of the electrical double layer adjacent to the substrate electrode (otherwise known as the plane of closest approach of nonspecifically adsorbed ions), may not actually be at ([is and hence would not experience the full electrical field corresponding to the electrode/solution potential difference. The result of this is that only a part of the measurable applied r] affects the Gibbs energy of activation of the process. The potential at the OHP with respect to solution, (t)s, is denoted t /i and is known as the potential of the (inner limit... 

Basics of Cyclic Voltammetry. Electrochemical techniques such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are most appropriate to the study of electronic processes and redox reactions. These techniques are conceptually elegant and experimentally simple thus they are popular for studying redox reactions at the electrode-solution interfaces and have been increasingly employed by electrochemists (2, 7). Several remarks regarding the cyclic voltammograms of electron-conducting BLM should be made. 

Application of the foregoing relations to the study of adsorption at electrode surfaces requires an understanding of the electrochemical processes at electrode-solution interfaces. Consider an electrode in contact with a solution containing electroactive species along with supporting electrolytes. Two important processes occur at the electrode surface a faradaic process in which electrons are transferred across the electrodesolution interface (oxidation-reduction reaction). As a result of these reactions current flows through the medium. Adsorption-desorption is... 

The combination of ultra-high vacuum (UHV) surface science techniques with electrochemical methods of electrode surface characterization (voltammetry, chrono-coulometry) resulted in a spectacular progress in the investigation and molecular level understanding of some processes occurring at electrode/solution interfaces. Evidently the experimental approach strongly depends on the aims of the investigation and the systems to be studied. 

With modern computerized frequency-analysis instrumentation and software, it is possible to acquire impedance data on cells and extract the values for all components of the circuit models of Figure 2.>7, This type of analysis, w hich is called electrochemical impedance spectroscopy, reveals the nature t>f the faradaic processes and often aids in the investigation of the mechanisms of electron-transfer reactions. In the section that follows, we explore the processes at the electrode-solution interface that give rise to the faradaic impedance. 

The electrical behavior of an electrode-solution interface and the processes that take place due to an electrochemical reaction can be treated in terms of an equivalent circuit [1,65,66]. It has been shown that when a Faradaic reaction is occurring at low overpotentials, without the involvement of adsorbed intermediates, the equivalent circuit may be expressed as shown in Fig. 4.3.19A. 

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