Current, single electrode reaction

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

It should be emphasized that the physical meaning of the analysis in Fig. 2.71 corresponds to the comparison of the net peak crrrrents of various electrode reactions characterized by different electrode kinetics. For a single electrode reaction, the electrochemical reversibihty will vary by changing the frequency. The net peak current is expected to be a complex function of the frequency, since the latter affects simtrltaneously all three parameters m, p and 7. Simrrlations have shown that the A fp is a non-hnear function of / without reaching a maximum. However, the dependence of the ratio vs. log(/) exhibits a maximum, the position of... 

Metallic corrosion can be characterized by two electrochemical quantities, current and potential. The current associated with a single electrode reaction on a metal surface is related to the potential of the metal by ... 

Anodic partial current (density) — For a single - electrode reaction, the total current is composed of an anodic and a cathodic - partial currents. 

When single -> electrode reaction occurs, the current can be split into two parts... 

Current-potential curve for a single electrode reaction with single electron transfer description by standard rate constant... 

However, the active dissolution of titanium depends markedly on temperature in acid solution. At lower temperatures, the picture is not so clear. It is necessary to have a quantitative measure of the rate of the hydrogen reaction and the titanium dissolution reaction. The complete set of current-potential and impedance-potential data has been tested against the theory given above. The best strategy seems to be to fit to a single electrode reaction and then to look for deviations from the expected behaviour for a perfect redox reaction. A convenient way of doing this is to represent the electrochemical data as a standard rate constant-potential curve in conjunction with a double layer capacity-potential curve [21]. 

Mass transport in an electrochemical reactor occurs by three mechanisms migration in the electrical field, film diffusion, and convection. The first of these is a special feature of electrochemical reactions, whereas the other two are common to all reactions that have a solid phase. However, where an inertsupporting electrolyte is used, the effect of migration can be neglected. With this assumption, let us consider a single electrode reaction given by reaction 21.3. When a finite current is passed through the cell and conditions are perfectly reversible, the concentration overpotential can be expressed as (Pickett, 1979)... 

According to Erdey-Griiz and Vol-mer [15], the supersaturation c/cqo, where c is the concentration of ions in solution in equilibrium with a surface of radius of curvature r, and Cqo is the corresponding concentration of ions in equilibrium with a planar surface, is directly determined by the overpotential, r] = E — Erev. where E is the electrode potential and iirev is the equilibrium potential, in the absence of net current flow for a single electrode reaction. 

In these researches, a mainly purpose is to acquire EPHs of cell or half-cell reactions. The EPH could be considered as a basic issue of TEC. Before the identification of this problem there had been two puzzled questions, one is that the heat effects for a reversible reaction, Q can be calculated by the formula Q = TAS where AS is the entropy change of this reaction and T temperature in Kelvin. However, this formula that is valid for most reactions is not viable at least for a reversible single electrode reaction in aqueous solution. For a reversible single electrode reaction, the experimental value of the heat effect is not in agreement with that calculated on the current thermodynamic databank of ions, that is, with which, the product of the calculated entropy change and the temperature of the electrode reaction always differs from the experimental measurements [2]. For example, for the electrode reaction at the standard state ... 

In Eq. (30), Q is a product of temperature T and the entropy change derived from the current thermodynamic databank including the ion data which is constructed on the conventional scale, and can be named as "the traditional heat effect" while in Eq. (15), 77is the heat effect identified by the experiments, called as "the measured heat effect". The difference between them is z TAS (H+/H2). Consequently, the first problem mentioned above, why is this formula, Q = TAS, unsuitable for a reversible single electrode reaction, is answered. 

Fig. 10.8 Steady-state log (current)-electrode potential diagram for a metal M corroding via hydrogen evolution. Both electrode processes are under activation control. The diagram shows the definition of corrosion current i corr the corrosion potential Ecokr The reversible potential and corresponding to the exchange currents i and io for the single electrode reactions are also shown together with the cathodic polarization rj = corr E and the anodic polarization = corr E. ...

The potential of a mixed electrode at which a coupled reaction of charge transfer proceeds is called the mixed electrode potential , this mixed electrode potential is obviously different from the single electrode potential at which a single reaction of charge transfer is at equilibrium. For corroding metal electrodes, as shown in Fig. 11—2, the mixed potential is often called the corrosion potential, E . At this corrosion potential Eemt the anodic transfer current of metallic ions i, which corresponds to the corrosion rate (the corrosion current ), is exactly balanced with the cathodic transfer current of electrons for reduction of oxidants (e.g. hydrogen ions) i as shown in Eqn. 11-4 ... 

The concept of mixed potential can be extended to multi-electrode reactions on one electrode. If N electrode reactions happen on a single electrode simultaneously, the outside electrode current is up to zero. When N >2, all these electrode reactions constitute the multi-electrode reaction coupled system, in which a part of the electrode reactions belong to the anodic reaction and the others are cathodic reaction. Given that the current of the anodic reaction is positive value and that of the cathodic reaction is negative value, the following formula can be given. 

Consider a system in which a potential difference AV, in general different from the equilibrium potential between the two phases A 0, is applied from an external source to the phase boundary between two immiscible electrolyte solutions. Then an electric current is passed, which in the simplest case corresponds to the transfer of a single kind of ion across the phase boundary. Assume that the Butler-Volmer equation for the rate of an electrode reaction (see p. 255 of [18]) can also be used for charge transfer across the phase boundary between two electrolytes (cf. [16, 19]). It is mostly assumed (in the framework of the Frumkin correction) that only the potential difference in the compact part of the double layer affects the actual charge transfer, so that it follows for the current density in our system that... 

---