Electrode mechanism

Elving, P. J. Enyo, M. Critical Observations on the Measurement of Adsorption at Electrodes Mechanism of the Hydrogen Electrode Reaction as Studied by Means of Deuterium as a Tracer 7... 

In the membrane-electrode assembly (MEA a membrane squeezed in between two electrodes), mechanical forces are applied. 

Also in these cases, mathematical treatments of Fick s second law, under proper boundary conditions, afford diagnostic criteria to identify the possible electrode mechanisms. 

The typical scheme describing the electrode mechanism complicated by adsorption processes is the following ... 

Table 3 provides more detailed information on those cases in which the number of experimentally measured electrons ( app) equals, or is different from, the theoretical number (n) involved in a particular electrode mechanism. 

At times it can be useful to carry out a sort of reversal electrolysis, i.e. reoxidize the reduced species (or reduce that oxidized) immediately after generating it to re-obtain the initial species. As shown in Table 3, the comparison between the amount of electricity (in C) consumed in the forward process, Qf, and that consumed in the return process, Qr, can be useful in the identification of the electrode mechanism involved in the process under study (even if in general it is less simple than appears). 

Chronoamperometry is a useful technique in those cases where cyclic voltammetry does not succeed in identifying the electrode mechanisms underlying certain redox changes. It is important to state that chronoamperometric measurements can be performed using the same equipment of cyclic voltammetry. 

It undergoes either two separated reductions or one oxidation, all having features of chemical reversibility in the cyclic voltammetric time scale. Since no detailed studies have been carried out on the underlying electrode mechanism, it is noted that these redox transformations could formally correspond to the sequence V(III)/V(II)/V(I), but, as far as the reduction processes are concerned, it is also likely that they are centred on the polypyridine ligand. 

It is hence evident that the following electrode mechanism holds ... 

For example, conversion of the hexacoordinate (octahedral) derivatives [MnHCl(PP3)] to the corresponding deprotonated pentacoordinate (trigonal-bipyramidal or square pyramidal) derivatives [MnCl(PP3)]+ (M = Fe, Ru, Os), Scheme 6, follows electrode mechanisms that are more or less complicated depending upon the nature of the central metal.16... 

Based on this result, the following electrode mechanism has been proposed ... 

Upon bubbling dinitrogen, two new peak-systems add to such a reduction step a reduction process at the peak-system E/F E° = -2.40 V) and an oxidation process at the peak-system I/G ( °/ = —1.84 V), respectively, Figure 41b. These results have been interpreted as due to the following electrode mechanism ... 

Fig. 11-4. Catalytic redox reactions through mixed electrode mechanism (a) on bio-membranes and (b) on enzymes.

In the CE electrode mechanism the electroactive reactant is produced by means of a preceding homogeneous chemical reaction [15,55,60], Assuming an oxidative mechanism, the simplest form of the CE scheme is as follows ... 

Although it is difficult to generalize the dependence of the peak potential on e, in general, for an oxidative electrode mechanism, the position of the peak shifts to positive potentials by increasing the rate of the preceding chemical reaction. At the same time, the half-peak width is largely insensitive to the chemical reaction. If log( ) < -2, Ap vs. log(e) is a linear function with a slope of about 30 mV. 

Note that the chemical step (2.75) is totally irreversible, attributed with a pseudo first-order rate constant (s ) defined as Atc =, rCx, where cx has the same meaning as for the CE and EC mechanisms (Sect. 2.4.1). Although this is the simplest case of an electrode mechanism involving chemical reaction, it has particular analytical utihty [53]. The mass transport of the redox species is described by the following model ... 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

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