Metal-electrolyte interface diffusion

The model more generally accepted for metal/electrolyte interfaces envisages the electrical double layer as split into two parts the inner layer and the diffuse layer, which can be represented by two capacitances in series.1,3-7,10,15,32 Thus, the total differential capacitance C is equal to... 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

Another consideration in the use of hydride materials in Ni/MH batteries is related to the electrochemical kinetics and transport processes. The power output of the battery depends critically on these processes. During discharge, hydrogen stored in the bulk metal must be brought to the electrode surface by diffusion. The hydrogen then must react with hydroxyl ions at the metal electrolyte interface. As a consequence, surface properties such as oxide thickness, electrical conductivity, surface area, porosity and the degree of catalytic activity... 

There is no doubt that the variants described above cannot comprehend aU the possible ways of the reaction zone extension, even for the relatively simple electrode system. It is possible that some of the electrode processes can take place simultaneously on the gas-electrolyte, gas-metal, and metal-electrolyte interfaces. The removal of oxygen in the second variant, for instance, can be represented by the following reactions diffusion of subions along the metal-electrolyte interface, and diffusion of oxygen atoms on the gas-metal interface. Prior to this, the oxidation reaction of subion to atom O should take place with the transfer of electrons into the metal. 

If the density of the electrical current i corresponds to the diffusive flow on the metal-electrolyte interface j, then the electrochemical balance equation can be presented as follows ... 

The diffusive purification flow from the liquid-metal flat layer with thickness 5 will follow the jc coordinate toward the metal-electrolyte interface, where the condition Cl = const should be fulfilled. Coordinate x = 0 will be combined with the free surface of the liquid-metal layer. Then, the relative change of the oxygen concentration can be found from the solution of the following nonstationary diffusion equation ... 

In cell in there is an Fe/FeO electrode on one side and a metal containing dissolved oxygen on the other side. The emf of the cell before beginning the experiment is a measure of the initial activity or concentration of the dissolved oxygen. At a certain time an emf is applied to the cell to make the oxygen activity at the metal/electrolyte interface very small. Then oxygen diffuses out of the metal and is carried as an electrical current through the electrolyte to the other side of the cell. In this way the diffusion current is transformed into an electrical current and can be measured. 

Oxygen reduction following equation (8.36) can take place either at the metal-electrolyte interface situated at the base of a pore or at the surface of the magnetite present in the rust layer. From a diffusion point of view, the magnetite at the pore walls of the rust layer is more accessible than the metal surface at the base of the pore and oxygen reacts preferentially there. As a consequence, an electrochemical cell is established in which the base of the pore becomes the anode and the magnetite the cathode (Figure 8.19(b)). 

At ambient temperature, the rate of charge-transfer at the metal-electrolyte interface often limits the corrosion rate (Chapter 4). Because transfer reactions generally exhibit higher activation energy than diffusion phenomena, their rate... 

The double-layer structure at ITIES shows different features than that formed at the metal/electrolyte interface. The charge distribution in the both phases preserves its diffuse property even in rather concentrated solutions [7]. The potential difference in the compact double-layer is much smaller than the potential differences in the adjacent diffuse layers. Thus, practically, the overall potential difference only consists of the potential difference in the diffuse layers 02(w) and (f) o)... 

When a metal is in contact with an electrolyte solution, a dc potential occurs which is the result of two processes. These are (1) the passage of metallic ions into solution from the metal, and (2) the recombination of metal ions in the solution with free electrons in the metal to form metal atoms. After a metal electrode is introduced into an electrolyte, equilibrium is eventually established and a constant electrode potential is established (for constant environmental conditions). At equilibrium, a dipole layer of charge (electrical double layer) exists at the metal-electrolyte interface. There is a surface layer of charge near the metal electrode and a layer of charge of opposite sign associated with the surrounding solution. Although diffuse, this dipole layer produces an effective electrical capacitance (Cp) which accounts for the low-frequency behavior of the electrode polarization impedance as discussed in Chapters 2, 3, and 4. 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

Consider a cathodic site where oxygen is diffusing to the metal/electrolyte interface. If an inhibitor, like zinc and magnesium, is added to the metal/electrolyte system, it would react with the hydroxyl ion and precipitate insoluble compounds which would, in turn, stifle the cathodic sites on the metal. In oxygen-induced corrosion. 

An uncommon, rather new technique where temperature affects electrochemical processes is DBMS (differential electrochemical mass spectrometry) [100, 177-180]. In this method, tiny amounts of organic substances, e.g., in adsorption films, are desorbed and/or oxidised and the volatile reaction products are determined by mass spectrometry. This technique is useful to detect organic substances adsorbed at the metal-electrolyte interface. In a thin-layer cell, adsorbed molecules are desorbed by potential variation. The desorbed material diffuses through a porous PTFE membrane and is detected by mass spectrometry. Besides potential, also temperature variation influences the desorption process. This way, temperature dependence of adsorption at single crystal platinum surfaces has been studied [178]. Other DBMS experiments have been done imder pressure in autoclave cells [179, 180]. 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

The basic difference between metal-electrolyte and semiconductor-electrolyte interfaces lies primarily in the fact that the concentration of charge carriers is very low in semiconductors (see Section 2.4.1). For this reason and also because the permittivity of a semiconductor is limited, the semiconductor part of the electrical double layer at the semiconductor-electrolyte interface has a marked diffuse character with Debye lengths of the order of 10 4-10 6cm. This layer is termed the space charge region in solid-state physics. 

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

A Schottky diode is always operated under depletion conditions flat-band condition would involve giant currents. A Schottky diode, therefore, models the silicon electrolyte interface only accurately as long as the charge transfer is limited by the electrode. If the charge transfer becomes reaction-limited or diffusion-limited, the electrode may as well be under accumulation or inversion. The solid-state equivalent would now be a metal-insulator-semiconductor (MIS) structure. However, the I-V characteristic of a real silicon-electrolyte interface may exhibit features unlike any solid-state device, as... 

---