Frumkin effects

One should note that the field in the outer part of the double layer should repel OH ions and attract hydrogen ions. This, however, is not expected to have a major effect on the kinetics (the Frumkin effect ) since the main reactant is likely to be water. 

Finally, it must be taken into account that the use of large concentrations of supporting electrolyte minimizes the Frumkin effects. This is important in that we can now realize that high concentrations of supporting electrolyte not only minimize either migration or the capacitive currents, but also allow us to adopt the simple electrode kinetics discussed in Section 4. 

The addition of the inert electrolyte affords other advantages. The most important point is that the conductivity of the solution increases (and thus the ohmic drop decreases through a decrease of the resistance of the cell, Rccw see Sect. 1.9). Moreover, the diffuse double layer narrows, being formed mainly by the ions of the inert electrolyte (with a sharp potential drop over a very short distance from the electrode surface). This makes the capacitance more reproducible and the Frumkin effects less obtrusive. Activity coefficients of the electroactive species are also less variable (and, therefore, quantities like formal potentials and rate constants), since... 

When reactants or intermediates are adsorbed, the rate of the reaction may no longer be related to the concentration by a simple law. This situation is best understood where a reactant is nonspecifically adsorbed in the outer -> Helmholtz plane. The effect of such adsorption on the electrode kinetics is usually termed the -> Frumkin effect. Physical and chemical adsorption on the electrode surface is usually described by means of an -> adsorption isotherm and kinetic equations compatible with various isotherms such as the - Langmuir, -> Temkin, -> Frumkin isotherms are known. 

There is another type of oscillation which is a consequence of the - Frumkin effect. A typical example is the reduction of S20g at Cu or Ag electrodes [xv.xvi]. 

Frumkin effect - double layer, effect on charge transfer rate... 

Girault (lb) pointed out that the apparent potential dependence of the ET rate may be attributed to the change in concentration of the reactants near the interface rather than to activation control. This model, further developed by Schmickler (9), postulates that the rate constant is essentially potential-independent because the potential drop across the compact part of the double-layer at the ITIES is small. In this model, the ET rate dependence on the interfacial potential drop is only due to the diffuse layer effect similar to Frumkin effect at metal electrodes. 

These effects can be understood and interpreted in terms of the variation of potential in the double-layer region, as discussed in Section 13.3. The basic concepts were described by Frumkin (7), and this effect is sometimes called the Frumkin effect. 

Because of the Frumkin effect, Tafel plots are not linear and have the following varying slopes in the cathodic region ... 

Specifically adsorbed anions can also significantly affect the stability and electrochemical reactivity of metal electrodes. Examples are the alteration of the potential (charge) distribution in the double layer known as Frumkin effect [248], the lifting of the surface reconstruction of Au(hkl) electrodes [249], the electrochemical deposition and dissolution of metals in the presence of anionic ligands [250], and the role of halides and sulfate on the oxygen reduction on Pt and Au [251]. 

It should also be noted that the actual charge on the metal is of importance reduction of anions may occur at potentials higher or lower than fipzc. In some cases (e.g., reduction of S20g ), nonlinear Tafel plots have been observed as a consequence of the Frumkin effect [1,4, 15]. 

Finally, it should be noted that non-Butler-Volmer behaviour may be observed in the analysis of cyclic voltammetric data. For example, particularly in the presence of low concentrations of supporting electrolyte, electron transfer kinetics of charged species may be significantly modified due to the double layer or Frumkin effects [79]. Under these conditions, (i) the potential experienced by the reactant at the point of closest approach to the electrode can be different from the applied potential, and (ii) an additional energy barrier for the approach of charged reactants to the electrode may exist. Corrections to account for Frumkin effects have been proposed. Deviations from Butler-Volmer behaviour may also be interpreted in terms of the Marcus theory [80]. A further interesting case of non-Butler-Volmer voltammetric characteristics is observed with semiconducting elecfrode materials [81]. 

Finally, it should be noted that non-Butler-Volmer behaviour may be observed in the analysis of cyclic voltammetric data. For example, particularly in the presence of low concentrations of supporting electrolyte, electron transfer kinetics of charged species may be significantly modified due to the double layer or Frumkin effects... 

On the other hand, electrostatic attraction between carboxyl groups and positively charged redox species [Ru(NH3)e]2+ 3+ should increase with increasing pH. In fact, the redox behavior for these at GC depends on pH, and this effect is an example of the Frumkin effect [10]. However, the rate constants for redox reactions at oxygenated diamond electrodes do not obey the Frumkin effect (Fig. 

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