The Exchange-Current Density

The linear E versus log i curve, reflecting Tafcl-typc behavior, is referred to as charge-transfer polarization because it is associated with the actual separation of charge at the electrode interface. In the case of a metal, charge transfer involves either transfer of a metal ion into the solution and an electron(s) into the metal (oxidation or corrosion) or the combination of a metal ion in solution with an electron(s) to form an ef- 

Theoretical electrochemistry is concerned with developing models for these charge transfer processes and with deriving mathematical expressions based on these models from which values of the exchange current density may be calculated. It is sufficient for present purposes to examine one particularly simple model and derive, semiquantitatively, expressions for the exchange current density. Details of the model and the derivations are open to argument, but the result is of a mathematical form that is observed experimentally for a number of half-cell reactions. 

The chemical and electrical components of the electrochemical free energy (Gel = G + mF( ) as discussed in Chapter 2) also are represented in Fig. 3.6. The shape of the electrical-component curve is defined by a, the fractional change in the potential as a function of position a, the transfer coefficient, is the fractional change at the maximum of the Gel curve. G and G el are the GFE and the electrochemical free energy, respectively, of the ion at the position of the maximum of these energies on traversing the interface. Ions in this state are frequently referred to as 

Gibb s Free Energy of Gibb s Free Energy of 

Representation of the environment of metal ions in the metal and aqueous phases at the interface. Based on Ref 2 

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For a more general reaction of the fonn Ox + ne Red, with differing concentrations of Ox and Red, the exchange current density is given by... 

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

The exchange current density, depends on temperature, the composition of the electrolyte adjacent to the electrode, and the electrode material. The exchange current density is a measure of the kinetic resistance. High values of correspond to fast or reversible kinetics. The three parameters, a, a. ... 

The slope of the Tafel curve drj/d log / is only one of the criteria that are required to determine the mechanism of the h.e.r., since different mechanisms, involving different r.d.s. often have the same Tafel slope. Parameters that are diagnostic of mechanism are the transfer coefficient, the reaction order, the stoichiometric number, the hydrogen coverage, the exchange current density, the heat adsorption, etc. 

Equilibrium Potential ( o) the electrode potential of an unpolarised electrode at equilibrium. At the equilibrium potential there is no net reaction. The potential is controlled by the same electrode reaction occurring anodically and cathodically at an equal rate, called the exchange current density. 

Exchange Current Density (/ o) the rate of exchange of electrons (expressed as a current per unit area) between the two components of a single electrochemical reaction when the reaction is in equilibrium. The exchange current density flows only at the equilibrium potential. 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

The exchange current density for common redox couples (at room temperature) can range from 10-6 pAcm-2 to A cm"2. Equation (1-24) can be written in terms of the exchange current to give the Butler-Volmer equation ... 

Again the extent to which such parallel reactions contribute to the measured current is not very easy to quantify. However, fortunately, such a quantification is not necessary for the description of NEMCA. What is needed is only a measure of the overall electrocatalytic activity of the metal-solid electrolyte interface or, equivalently, of the tpb, and this can be obtained by determining the value of a single electrochemical parameter, the exchange current I0, which is related to the exchange current density i0 via ... 

In addition to the exchange current density the transfer coefficient a is needed to describe the relationship between the electrode potential and the current flowing across the electrode/solution interface. From a formal point of view a can be obtained by calculating the partial current densities with respect to the electrode potential for the anodic reaction ... 

With the electrochemical rate constant a relationship between the exchange current density, the electrochemical rate constant and the concentration of the reacting species can be derived ... 

Because of the outlined difficulties in a simple transformation of the exchange current density jo into the standard exchange current density Joo or the rate constant k all values arc listed as reported in the original literature. The exchange current density is usually reported in units of A cm, the rate constant in units of cm s Sometimes the exchange current density or the standard exchange current density are reported in units of A cm mol All values reported below refer to the units outlined initially referring to... 

Presumably the most important kinetie parameter used in the deseription of the kineties of an eleetrode is the exchange current density or the almost equivalent rate constant. It indicates the speed of the heterogeneous process of charging or discharging species at the phase boundary, i.e. the charge transfer process. Its value is influenced by numerous factors of the investigated system. For both applied and fundamental aspects of electrochemical research a list of reported values should be helpful. It concludes this volume. 

The exchange current density for methanol oxidation depends on the methanol concentration, i.e. ... 

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