Interfaces metal electrode/aqueous electrolyte

The oxygen reduction reaction (ORR) at the cathode of an electrochemical cell involves the transfer of four electrons from the cathode to an O2 molecule followed by removal of the reduced product from the interface. Reduction of an O2 molecule to water or to hydroxyl anions at an electrode/aqueous electrolyte interface is to be distinguished from reduction of O2 to 202- at the surface of an oxide or at an electrode/solid electrolyte interface in solid-state electrochemistry. In the latter case, the ORR is generally different at the surface of a noble metal like Pt from that at the surface of a metallic oxide. 

Although one of the more complex electrochemical techniques [1], cyclic voltammetry is very frequently used because it offers a wealth of experimental information and insights into both the kinetic and thermodynamic details of many chemical systems [2], Excellent review articles [3] and textbooks partially [4] or entirely [2, 5] dedicated to the fundamental aspects and apphcations of cyclic voltammetry have appeared. Because of significant advances in the theoretical understanding of the technique today, even complex chemical systems such as electrodes modified with film or particulate deposits may be studied quantitatively by cyclic voltammetry. In early electrochemical work, measurements were usually undertaken under equilibrium conditions (potentiometry) [6] where extremely accurate measurements of thermodynamic properties are possible. However, it was soon realised that the time dependence of signals can provide useful kinetic data [7]. Many early voltammet-ric studies were conducted on solid electrodes made from metals such as gold or platinum. However, the complexity of the chemical processes at the interface between solid metals and aqueous electrolytes inhibited the rapid development of novel transient methods. 

The calomel electrode Hg/HgjClj, KCl approximates to an ideal non-polarisable electrode, whilst the Hg/aqueous electrolyte solution electrode approximates to an ideal polarisable electrode. The electrical behaviour of a metal/solution interface may be regarded as a capacitor and resistor in parallel (Fig. 20.23), and on the basis of this analogy it is possible to distinguish between a completely polarisable and completely non-polarisable... 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

The basic components of an electrolytic cell for electrodeposition of metals from an aqueous solution are, as shown in Figure 2.1, power supply, two metal electrodes (Mj and M2), water containing the dissolved ions, and two metal-solution interfaces Mj-solution and M2-solution. An electrolytic cell for electroless deposition is shown in Figure 8.1. A comparison of Figures 2.1 and 8.1 shows that in electroless deposition there is no power supply and the system has only one electrode. However, the solution is more complex. It contains water, a metal salt MA A ), and a reduc-... 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

Baier, Giesen et al. have described the dynamics of step and island on Ag(lll) and Cu(lOO) [31], as well as on Au(lOO) [32] electrodes in the electrolyte. Baier and Giesen [33] have determined the activation energies of mass transport processes on Ag(lll) electrodes in the aqueous electrolyte. Haftel and Einstein [34] have studied the influence of the electrochemical potential on energy landscapes close to step-and island edges Ag(lll) and Ag(lOO). A model of the metal/solution interface involving hydrophilicity of Ag(lll) and based on the capacitance analysis, has been published by Emets et al. [35] Electron... 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

Most researchers have studied photocatalytic reactions in an aqueous environment. It is therefore essential to understand the semiconductor-electrolyte interface, preferably from an energetic point of view. To illustrate this, the semiconductor-electrolyte interface in the absence of redox species is depicted in Fig. 16.1 (Memming 2001). Compared with metal electrodes, the charge-carrier density... 

In the first part of this century, electrochemical research was mainly devoted to the mercury electrode in an aqueous electrolyte solution. A mercury electrode has a number of advantageous properties for electrochemical research its surface can be kept clean, it has a large overpotential for hydrogen evolution and both the interfacial tension and capacitance can be measured. In his famous review [1], D. C. Grahame made the firm statement that Nearly everything one desires to know about the electrical double layer is ascertainable with mercury surfaces if it is ascertainable at all. At that time, electrochemistry was a self-contained field with a natural basis in thermodynamics and chemical kinetics. Meanwhile, the development of quantum mechanics led to considerable progress in solid-state physics and, later, to the understanding of electrostatic and electrodynamic phenomena at metal and semiconductor interfaces. 

Figure 12.13 illustrates a versatile experimental set-up for microwave conductivity measurements with the microwave source (8 0 GHz), a circulator and a detector, which monitors the microwave energy reflected from the electrochemical or photovoltaic cell. The cell and electrode geometries are designed in such a way that the microwave power can reach the energy-converting interface (losses in metal contacts or aqueous electrolyte should be minimised). Depending on the experimental conditions, time-resolved, space-resolved or potential-dependent measurements are possible as well as combinations (for further details, see Schlichthbrl and Tributsch, 1992 Wiinsch et al., 1996 Chaparro and Tributsch, 1997 Tributsch, 1999). 

Fig. 10.6 Point of zero charge (PZC) for the Hg aqueous solution interface for several alkali metal halides as electrolytes at 25°C using a calomel reference electrode (1 M KCl) plotted against electrolyte concentration.

For a limited number of metal surfaces, adsorption of a molecular species in a thin (monomolecular layer) film results in a huge increase in the effective vibrational Raman scattering cross-section (again, as with RR scattering, up to ca. 106 times) of the adsorbate species. The SERS effect was discovered more than ten years ago for pyridine adsorbed at a silver electrode surface in contact with an aqueous electrolyte [1, 2]. In the intervening period, many hundreds of papers devoted to SERS phenomena have been published, extending the studies to other metals than silver, to non-aqueous as well as aqueous electrolytes, to colloidal dispersions of metals as well as metal electrodes, and even to vacuum-deposited thin film systems under UHV conditions. This review will concentrate on studies of metal-electrolyte interfaces. 

Actually, interfacial potential differences can develop without an excess charge on either phase. Consider an aqueous electrolyte in contact with an electrode. Since the electrolyte interacts with the metal surface (e.g., wetting it), the water dipoles in contact with the metal generally have some preferential orientation. From a coulombic standpoint, this situation is equivalent to charge separation across the interface, because the dipoles are... 

In the case where the ionic species in the aqueous electrolyte are fairly hydrophilic and the organic phase features hydrophobic ions, the liquid]liquid junction behaves similarly to an ideally polarizable metal electrode. Under this condition, the Galvani potential difference can be effectively controlled by a four-electrode potentiostat [4,5]. A schematic representation of a typical electrochemical cell is shown in Fig. 1 [6]. Cyclic voltammo-grams illustrating the potential window for the water] 1,2-dichloroethane (DCE) interface for various electrolytes are also shown in Fig. 1. In the presence of bis(triphenylpho-sphoranylidene)ammonium hexafluorophosphate (BTPPA PFe) the supporting electrolyte in DCE, the potential window is limited to less than 200 mV due to the hydrophilicity of the anion. Wider polarizable potential ranges are obtained on replacing... 

Brief consideration is now given to the solvent structure at metal/aqueous electrolyte interfaces.Several molecular models have been proposed which treat a single layer of water molecules at the metal surface. Within the layer, the individual water molecules (or clusters of molecules) are allowed to have certain orientations. In the earliest and simplest molecular model, an inner-layer water molecule is oriented as a result of its dipole interaction with the charge on the metal electrode. Orientation is limited to either of the two positions in which the molecular dipole is perpendicular to the electrode surface. More realistic treatments have since been described which variously... 

Interestingly, when the potential is made more positive so that the film becomes oxidized, the observed force-distance data remain the same. Similar effects were observed with polypyrrole films. This suggests that the positive charge in the film is fully compensated by negatively charged solution ions that can penetrate the polymer matrix either within the polymer strands or within pores in the polymer, as depicted in Fig. 11. Thus, at the conductive polymer-solution interface, no DL exists. This is in stark contrast to the behavior of metal electrodes in contact with aqueous electrolytes. 

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