Metal-electrolyte interface electrical fields

The case of polarized interfaces is usually described within the context of the metal-electrolyte interface where the metal charge dependence of the SH intensity is dramatic because of the strong interfacial electric field present at the interface [16]. It has long been a real challenge at the polarized liquid-liquid interface but has, however, been observed at charged air-water interfaces [48]. 

Useful atomic and subatomic scale information on hydroxylated oxide surfaces and their interaction with aggressive ions (e.g., Cl ) can be provided by theoretical chemistry, whose application to corrosion-related issues has been developed in the context of the metal/liquid interfaces [34 9]. The application of ah initio density functional theory (DFT) and other atomistic methods to the problem of passivity breakdown is, however, limited by the complexity of the systems that must include three phases, metal(alloy)/oxide/electrolyte, then-interfaces, electric field, and temperature effects for a realistic description. Besides, the description of the oxide layer must take into account its orientation, the presence of surface defects and bulk point defects, and that of nanostructural defects that are key actors for the reactivity. Nevertheless, these methods can be applied to test mechanistic hypotheses. 

Future work should address more realistic conditions for the passivated surfaces, including the complete system metal(alloy)/oxide/electrolyte, its interfaces, electric field, and temperature effects. The experimental knowledge of the atomic structure of passivated surfaces, their defects, and their nanostructural features needs to be faithfully input when available. Potential-driven atomic transport and pH should he implemented. Testing of the existing models of passivity breakdown also requires a realistic implementation of their characteristic features. It is foreseen that DFT will be applied to test mcffe accurately specific steps of the complex pathways leading to passivity breakdown, while MD simulations will be developed to test the complete reaction pathways. 

According to (6.24), the ionic current density in the film varies exponentially with the electric field. Even though this relation has been derived here from a rather simple model, it holds true quite generally. We therefore can also look at equation (6.24) as an empirical equation that describes the relation between the ionic current, the potential and the thickness of solid oxide films, in a similar way as the Butler-Volmer equation describes the relation between current and potential for a metal-electrolyte interface. 

The quantum mechanical treatment of the metallic surface has been developed to a large extent for metal-vacuum interfaces. Similar techniques should be applicable to the metal electrolyte interface. The main difference is that the fluid contributes not only to the electric fields in the metal surface, but also to specific chemical bonding interactions. 

Most corrosion processes, e.g., metal dissolution, hydrogen or oxygen evolution, and passive film formation, involve at least one adsorption step as a part of the overall reaction. This step can be significantly affected by the presence on the metal surface of a monolayer of nonmetal species. As evidenced by studies described in this chapter, adsorbed species may act by loosening the metal-metal bond or changing the electric field at the metal-electrolyte interface. They can also favor or inhibit the adsorption or the recombination of adsorbed atoms normally involved in the anodic or cathodic reactions. 

In a PEC, light is incident on the n-type semiconductor/electrolyte junction (photoanode), where light absorption occurs and an electron-hole pair is formed. The pair is separated by the strong electric field found just beneath the semiconductor surface, and the hole is driven towards the interface between semiconductor and electrolyte. Charge transfer to the redox species A contained in the electrolyte results in the oxidation to Conversely, the electron is driven to the metal/electrolyte interface (counter electrode), where the redox species is reduced. No net chenaical work is done and we can extract the energy as a current from the cell. 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

Quantum mechanical calculations are appropriate for the electrons in a metal, and, for the electrolyte, modern statistical mechanical theories may be used instead of the traditional Gouy-Chapman plus orienting dipoles description. The potential and electric field at any point in the interface can then be calculated, and all measurable electrical properties can be evaluated for comparison with experiment. 

Electrons, generated near the semiconductor-electrolyte interface are unable to stay in this region because of the electric field there which drives them into the bulk of the TiOz crystal, out through the metallic contact, the external circuit (where the photo-current may be measured) and into the catalytically active metal. At the interface of this metal with the electrolyte solution, reaction occurs ... 

When charges are separated, a potential difference develops across the interface. The electrical forces that operate between the metal and the solution constitute the electrical field across the electrode/electrolyte phase boundary. It will be seen that although the potential differences across the interface are not large ( 1 V), the dimensions of the interphase region are very small (—0.1) and thus the field strength (gradient of potential) is enormous—it is on the order of 10 V cm. The effect of this enormous field at the electrode/electrolyte interface is, in a sense, the essence of electrochemistry. 

Things are simple at the instant of immersion of a metal in an electrolytic solution. There is no field and no potential difference across the interface. Reactions (e.g., M+ + e — M) run for a very short while chemically. However, the very occurrence of a charge-transfer reaction across the interface in one direction creates an electric field, a fraction of which puts a brake on the reaction M+ + e — M. The same field, however, has an accelerating effect on the charge-transfer reaction in the opposite direction, M — M+ + e. 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

---