Catalysis at Multilayer Coated Electrodes

In zone R, all three phenomena that take place in the film are fast compared to the diffusion of the substrate from the bulk of the solution to the film-solution interface. The concentrations of both Q and A are constant through the film. The RDEV response is similar to that of a monolayer coating (Section 4.3.2), except that more catalytic material is present on the surface of the electrode (it is multiplied by the number of layers in the multilayered coating). A linear Koutecky-Levich plot is obtained from the intercept, from which the kinetics of the catalytic reaction can be characterized. 

If electron transport is fast, the system passes from zone R to zone S+R and then to zone SR. In the latter case there is a mutual compensation of diffusion and chemical reaction, making the substrate concentration profile decrease within a thin reaction layer adjacent to the film-solution interface. This situation is similar to what we have termed pure kinetic conditions in the analysis of an EC reaction scheme adjacent to the electrode solution interface developed in Section 2.2.1. From there, if electron transport starts to interfere, one passes from zone SR to zone SR+E and ultimately to zone E, where the response is controlled entirely by electron transport. 

In the converse case where substrate diffusion in the film is fast, the system passes from zone R to zone E+R and then to zone ER. This is again a purely kinetic situation, but it now involves the catalytic reaction and electron diffusion rather than substrate diffusion in the preceding case. At this point, interference of substrate diffusion triggers the passage from zone ER to zone ER+S and ultimately to zone S, where the response is controlled entirely by substrate diffusion. 

It is interesting to note that there is no complete symmetry between the role of substrate diffusion and electron transport in their combination with the catalytic reaction, as can be seen in the structures compared in the equations and also in the fact that linear Koutecky-Levich plots are not obtained in all cases, as noted in Table 4.1. 

What benefits and drawbacks to these problems can one expect from the use of cyclic voltammetry instead of RDEV They are related. In a general case, the application of cyclic voltammetry will be more complicated, because playing with the scan rate, one can make the diffusion layer penetrate the film or remain outside, as is the case with RDEV. We have already seen a fruitful application of the first of these possibilities in the use of cyclic voltammetry to the characterization of electron hopping transport within the redox films (Section 4.3.4). In the second situation, cyclic voltammetry may replace RDEV in a manner similar to what has been seen in Section 4.3.2 Each time a term (1 — ///a) is encountered in the analysis, it suffices to replace it by 

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