Electrode vicinity

Angelucci CA, Herrero E, Feliu JM. 2007b. Bulk CO oxidation on platinum electrodes vicinal to the Pt(lll) surface. J Solid State Electrochem 11 1532-1539. 

This reaction resembles decarboxylation of carboxylates during electrode one-electron oxidation (Kolbe reaction). Kolbe reaction also consists of one-electron oxidation, decarboxylation, and culminates in dimerization of alkyl radicals just after their formation at the electrode surface. When the sulfate radical acts as a one-electron oxidant, the caboradical dimerization is hampered. The radicals can be used in preparative procedures. One typical example is alkylation of heterocyclic nitrogen bases (Minisci et al. 1983). This difference between Kolbe reaction and the reaction with the help of a dissolved electrode (the sulfate radical) deserves some explanation. The concentration of the one-electron oxidation products in the electrode vicinity is significantly higher than that in the bulk of the solution. Therefore, in the case of anode-impelled reactions, the dimerization of radicals produced from carboxylates proceeds easily. Noticeably, 864 secures the single electron nature of oxidation more strictly than an anode. In electrode reactions, radical intermediates can... 

The histamine [131], adenine [132] and dopamine [133] amines are electroactive in the positive potential range, in which the thiophene is electropolymerized. Therefore, these amines could be oxidized at the electrode surface in the course of deposition of the MIP film. That way, products of these oxidations might be available in the electrode vicinity for imprinting rather than the desired pristine... 

The direct reduction of an aryl radical Ar formed in the electrode vicinity may be advantageously avoided using an electron carrier [184]. The advantage of such an indirect initiation process is apparent [185] in the synthesis of diphenyls substituted by both electron-donating and electron-withdrawing groups. However, one should take care that the AE between the ArX compound and the mediator does not exceed 0.5 V, or induction could become too slow, especially toward side reactions like termination reactions. 

The goal of this proposition is to present the electrochemical engineering study and modelling of two-phase electrolysis properties at electrode vicinity. This work is due to the necessity for a better knowledge of the actual interface condition during electrolysis, for example to have a better process optimisation or electrode consumption prevention. 

If the voltage scan rate is fast then we speak of fast linear sweep voltammetry. This technique gives rise to peaks rather than waves. As the potential rises at first no current will flow until the minimum deposition potential is reached. The current will then rise rapidly but the resultant electrolysis causes major depletion of the analyte in the electrode vicinity. Thus the current will decrease giving the peak shown in Fig. 2.7a. 

Electrochemical polymerization is a radical combination reaction and is diffusion controlled. The radicals generated thus diffuse together and react faster than they can diffuse away from the electrode vicinity. Hence at lower potentials the generation of radicals can be controlled in such way that diffusion away from the... 

These two equations allow to define a relationship between the process rate i and the electrode potential p. However, one needs to know the concentration distribution in the electrode vicinity. This can be found from the Pick s second law. For linear diffusion ... 

Mass transfer and electron-exchange processes In considering electron-exchange reactions at electrodes we are concerned essentially with the layer of solution very close to the electrode surface. It must be borne in mind, however, that an oxidant or reductant in solution has to have some means of reaching the electrode vicinity. There are a number of ways in which this can occur and these are included under the general heading of mass transfer processes. They are ... 

The quartz crystal microbalance (QCM) consists of a quartz crystal that is electrically driven into oscillation. The resonance frequency of the crystal is monitored. This frequency is highly dependent on any mass added to the crystal surface. Hence the mass dependence of the QCM resonance frequency can be, in air, used to weigh minute amounts of material with a sensitivity of the order of 1 ng/cm. QCM can also be coupled with electrochemistry here, the quartz crystal surface is coated with an appropriate electrode material, for example, thin film gold. This electrochemical QCM (EQCM) configuration can be used to monitor electrochemically triggered surface processes associated with the deposition (or loss) of material at the working electrode surface. However, in liquid medium the frequency shift of the QCM crystal is not solely sensitive to added mass but is also influenced by changes in the local property of the medium associated with the surface electrochemical process of interest. For example, density or viscosity variation of the medium in the electrode vicinity, in addition to variation in the viscoelastic properties of the deposited layer, can cause shifts in the resonant frequency of QCM. 

That is, water is electrolyzed. The hydrogen gas produced at the cathode can be hazardous, especially because it is in the vicinity of an electrode that is also producing heat. For this reason, electrode chambers are usually open to the atmosphere so that gases can vent. 

By using only a single reference electrode in the object to be protected, the potential can be determined only in the vicinity of this electrode and not in more remote areas. Section 3.3.1 together with Eq. (3-27) provides further explanation of this. To improve the current and potential distribution, the number and location of the anodes must suit the geometry of the object to be protected. Occasionally, additional reference electrodes are required for potential control [2]. The optimum nominal potential for potential control can be found by this method by considering remote IR errors. 

If ignition is assumed to occur in a hotspot formed at the electrode, the local release of potential energy W = QV/2 is directly proportional to charge while independent of electric field except in the immediate vicinity of the electrode, since this determines the local change in potential. The electric field near the electrode becomes increasingly uniform as electrode radius increases and eventually, uniform field breakdown conditions are approached (C-2.5.3). These concepts allow first approximations for effective energy to be made. First it is assumed that air breakdown occurs at 30 kV/cm and that the electric field is approximately uniform between about 2-5 mm from the electrode (C-2.5.2). Second, it is assumed that minimal ignition occurs in a 3 mm diameter hotspot formed sufficiently far from the electrode to minimize heat losses. This distance is assumed to be about 2 mm. 

Fig. 8 (left) shows the ion density distributions of Na and CP in the vicinity of the metal electrode. Near the uncharged electrode there are no pronounced adsorption maxima. The density of Na (full line) is slightly increased in the range around z = 4.3 A between the first and second density... 

The change in concentration, or the concentration gradient, existing in the immediate vicinity of the electrodes as this increases, the overpotential rises. 

One-layer systems. One-layer systems might easily overcome most of the above-mentioned problems. Such materials show predominantly ionic conduction in the as-prepared state but behave as electrodes in that the concentration of the mobile component is increased and decreased by the charging process in the vicinity of the two electronic leads. 

Figure 5.45 shows a Pt electrode (light) deposited on YSZ (dark). There are three circular areas of bare YSZ connected via very narrow bare YSZ channels. The rest of the surface is Pt. Note that, as will be discussed in Chapter 7, the Fermi levels of the Pt film and of the YSZ solid electrolyte in the vicinity of the Pt film are equal. The YSZ, however, appears in the PEEM images much darker than the Pt film since YSZ has a negligible density of states at its Fermi level in comparison to a metal like Pt. 

The significant point is that PEEM, as clearly presented in Figures 5.45 to 5.47, has shown conclusively that follows reversibly the applied potential and has provided the basis for space-and time-resolved ion spillover studies of electrochemical promotion. It has also shown that the Fermi level and work function of the solid electrolyte in the vicinity of the metal electrode follows the Fermi level and work function of the metal electrode, which is an important point as analyzed in Chapter 7. 

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