Electrode processes involving multiple electron transfer

for example, CGC = CH, nrc t = 0.5 and z = 1 we obtain ac = —0.25. As can be seen, the apparent value of ac can be positive or negative. This example shows the extreme importance in correcting values of ac and aa for double layer effects. 

We can ask how effects of the double layer on electrode kinetics can be minimized and if the necessity of correcting values of a and of rate constants can be avoided In order for this to be possible, we have to arrange for f)t — / s, that is all the potential drop between electrode surface and bulk solution is confined to within the compact layer, for any value of applied potential. This can be achieved by addition of a large quantity of inert electrolyte (—1.0 m), the concentration of electroactive species being much lower ( 5mM). As stated elsewhere, other advantages of inert electrolyte addition are reduction of solution resistance and minimization of migration effects given that the inert electrolyte conducts almost all the current. In the case of microelectrodes (Section 5.6) the addition of inert electrolyte is not necessary for many types of experiment as the currents are so small. 

10 Electrode processes Involving multiple electron transfer 

In many reduction or oxidation half-reactions, the oxidation state changes by a value greater than 1. Examples for metallic cations are Tl(III) — T1(I), Cu(II) — Cu(0), and examples of other species 02— H202 (2e ) or 02— H20 (4e ), these also involving other species in the half-reactions. In this section we consider metal ions given that, at least apparently, there are no other species involved, except for molecules of solvation etc. If the reactions are irreversible we can investigate their kinetics. 

Second step occurs at a more negative potential than the first  

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Electrode processes involving multiple electron transfer... 

Electrode processes involving multiple electron transfer 119 are two extreme cases ... 

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

In the two-state approximation (TSA), ET kinetics for the DBA —> D" BA process may be modeled in terms of initial- ( ,) and final-state (T /) wave-functions, in which the transferring charge is localized primarily on the D and A sites, respectively. In the case of electrodes (e.g., metal or semiconductor), where multiple electronic states are involved, the D and A sites may still be taken to be localized and to involve atomic sites of the electrode near the site/or sites of attachment or contact with the bridge) SAM = self assembled monolayer STM = scanning tunneling microsccopy. 

Electrochromism. Electrochromic materials have the property of a reversible and visible change in transmittance and/or reflectance associated with an electrochemically induced redox process involving electroactive species typically deposited onto an electrode surface as a thin film. The redox state of the material may be switched by an electron transfer reaction at an electrode and the observed colour change results from the generation of different electronic absorption bands according to the redox state. Such a colour change is commonly reported between a transparent ( bleached ) state and a coloured state, or between two coloured states. or even between multiple coloured states (multichromic). ... 

Interestingly, the anodic dark current at n-Ge electrodes increases considerably upon addition of the oxidized species of a redox system, for instance Ce" ", to the electrolyte, as shown in Fig. 8.4 [7]. The cathodic current is due to the reduction of Ce. The latter process occurs also via the valence band (see Chapter 7), i.e. since electrons are transferred from the valence band to Ce", holes are injected into the Ge electrode. Under cathodic polarization these holes drift into the bulk of the semiconductor where they recombine with the electrons (majority carriers) and the latter finally carry the cathodic current. In the case of anodic polarization, however, the injected holes remain at the interface and are consumed for the anodic decomposition of germanium, as illustrated in the insert of Fig. 8.4. Accordingly, the cathodic and anodic current should be compensated to zero. Since, however, the anodic current is increased upon addition of the redox system there is obviously a current multiplication involved, similarly to the case of two-step redox processes (see Section 7.6). Thus, in step (e) (Fig. 8.1) electrons are injected into the conduction band. This experimental result is a very nice proof of the analytical result presented by Brattain and Garrett [3]. 

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