Mass transport as a variable in the study of electrode processes

1 MASS TRANSPORT AS A VARIABLE IN THE STUDY OF ELECTRODE PROCESSES 

Let us first consider briefly how the use of mass transport as a variable can provide a guide to the reaction mechanism and give quantitative kinetic detail. As an illustration, we consider the behaviour of CE and EC processes (where E signifies electron transfer and C represents a chemical step) at a rotating disc electrode (RDE). This hydrodynamic system has already been discussed by Albery et al. and the reader is referred to Chap. 4 for details. CE and EC processes represent the simplest conceivable electrode reactions involving coupled homogeneous kinetics mechanistic examples of both types are shown in Table 1. In the discussion which follows, the electron-transfer reaction in the two mechanisms is considered to be a cathodic process the extension to the anodic case is trivial. 

At the RDE, mass transport to the electrode is varied by altering the disc rotation speed (W/Hz). The consequences of this on the current-voltage relationship, for the two reactions, are shown schematically in Fig. 1. If the E step is considered to be electrochemically reversible, then the voltam-metric wave is defined by the two parameters Elj2 (the half-wave potential) and Jim (the transport-limited current), as depicted. It is the dependence of these two quantities on the disc rotation speed which allows the deduction of the mechanism, as shown in Fig. 1. For a kinetically uncomplicated reversible electrode reaction, JLIM varies as W112 [2] and EV2 is independent of W [3]. For a CE process, at fast rotation speeds the limiting current is 

Turning to the EC process, it is evident from Fig. 1 that the chemical step 

Another consideration of practical significance might be the ease with which the electrode and associated flow system (where applicable) can be constructed, particularly since the transit time of material to channel, wall-jet, and microelectrodes is decreased as the electrode is made smaller. This has a direct bearing on condition (c). 

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