Transport electron migration

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Electrons liberated at the anode (negative pole of the cell) by the electro-oxidation of the fuel pass through the external circuit (producing electric energy equal to —AG) and arrive at the cathode (positive pole), tvhere they reduce oxygen (from air). Inside the fuel cell, the electric current is transported by migration and diffusion of the electrolyte ions (H, OH, CO ), for example, in a PEMFC. 

Charge transfer comprises also particle transfer, especially when a proton is present. Therefore, even protonic acidity itself can not be completely distinguished from the capacity to transport electrons and holes, i.e. oxidation/reduction, if for instance, hole migration to the surface can provoke proton dissociation. Protonic and aprotic acidities have long been known to be interlinked. In some, but not all, cases, Lewis acids can be converted to Bronsted ones by addition of water (132). 

For strongly ionic solids where cation and anion sizes are comparable, e.g. NaCf, KBr, etc., Schottky defects will predominate and both transport numbers and are greater than zero t = 0, no current is carried by electron migration). When the size of the cation is considerably smaller than the anion, eg. AgBr, AgCf, etc., Frenkel defects occur and the interstitial cations are the dominant current carriers 1). 

Figure 7. Schematic of transport processes through an oxide layer growing on a metal. Two limiting cases may be distinguished. First, metal ions and electrons may migrate from the metal toward the oxide gas interface and second, oxygen ions may migrate toward the metal-oxide interface with electrons migrating in the opposite direction. In any volume element of the oxide, electrical neutrality is required. The chemical potential of oxygen is fixed at both the metalr-oxide and-the oxide-gas interface. The former is fixed by the dissociation pressure of the oxide, po/, and the latter by the ambient oxygen partial presure, po"-...

In principle, the formation of halide scales on metals exposed to halide atmospheres will follow the same mechanisms found to be valid for both oxide and sulphide formation. A comprehensive review of the reactions of metals with halogens has been given by Daniel and Rapp. In fact, many metal halides are almost pure ionic conductors so that the growth of halide scales is expected to be controlled by electron migration rather than by ionic transport. In tbis case, the halide scale will grow laterally over the metal surface more readily than it will thicken. 

Assuming that only ions and electrons migrate in the scale and not neutral atoms, Wagner (1933, 1936) established a formula whereby the parabolic scale constant of a pure metal can be calculated from the free formation enthalpy of the corrosion product, the electrical conductivity of the protective layer, and the transport nnmbers of cations, anions, and electrons in the film ... 

In the previous chapter we have introduced the case of multiple-electron transfers (multi-E mechanisms). As discussed then, depending on the formal potentials of the different electrochemical steps comproportiona-tion/disproportionation reactions may be thermodynamically favourable and may affect the voltammetry if the electron transfers are not reversible, the diffusion coefficients of the species are different, there is mass transport by migration or other chemical reactions take place. For example, let us consider the case of two consecutive reduction processes (the EE mechanism) where the formal potential of the second step is much more negative than the first one ... 

If a potentiostat is used in electrochemical corrosion studies, it will consume these electrons and there will be no need for a redox system and electric conductivity of the passive layer. But if cations of lower valency are formed within the film first and will be oxidized later, electronic conduction of the layer is necessary to transport electrons to the metal surface. Time-resolved XPS studies have shown an initial formation of lower valency cations which are oxidized at a later stage of the development of the passive layer. Similarily, Fe(II) will be oxidized to Fe(III) when the potential is increased. These details have been examined, e.g., on iron in alkaline solutions, as already discussed in Sec. 1.4.2.1. These layers are only several nanometers thick, and the migration of cations follows an exponential cur-rent/potential relationship due to the pres-... 

The electrochemical reaction, the transfer step, can only take place where electrons can be supplied or removed, which means that this conversion is not possible on the surface of the lead sulfate, as lead sulfate does not conduct electric current. For this reason, the Pb " ions must be dissolved and transported by migration or diffusion to the conductive electrode surface (lead or lead dioxide). 

Now we should keep in mind that no net current flows through the growing scale, once the stationary state has been reached. Accordingly, equivalent amounts of oppositely charged particles are transported across the scale. Cations and electrons migrate in the same direction opposite to that of the anions. Therefore, we can write ... 

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