Competition between redox reaction and anodic dissolution

Competition between Redox Reaction and Anodic Dissolution 

The competition between redox reaction and anodic dissolution became very important in the development of stable regenerative solar cells on the basis of semiconductor-liquid junctions. As shown in the previous section, it is determined by the thermodynamic and kinetic properties of the processes involved. Information on the competitions between these reactions cannot be obtained entirely from current-potential curves, because in many cases they do not look very different upon addition of a redox system, especially if the current is controlled by the light intensity. Therefore, a rotating ring disk electrode (RRDE) assembly consisting of a semiconductor disk and a Pt ring is usually applied, that is, a technique which makes it possible to determine separately the current corresponding to the oxidation of a redox system [63, 64]. 

In this case, the oxidized species of a redox couple produced at the semiconductor disc, can be collected at the Pt ring, provided that the ring is polarized negatively with respect to the standard potential of the redox system. (For details of the RRDE technique see Section 4.2.3). Usually, the competition between the redox process and the dissolution is quantitatively described by the stability factor, as defined by [65] 

The lower part of the reaction scheme describes three possible dissolution pathways (Eqs. (8.25) and (8.26)). Only the first step in each sequence is essential. It is important to note that in one pathway (case A) the first step is a pure chemical reaction, whereas in the others (case B) a hole transfer Is involved. According to the kinetics of this reaction scheme, an intensity dependence of the stability factor s or the ratio y jpj occurs only for case B [73]. Most experiments were performed 

From the thermodynamic point of view, a redox process would preferably proceed if the redox potential Fredox is located above Fdecomp as illustrated in Fig. 8.16a and, conversely, the decomposition reaction should dominate if Fredox occurs below Fdecomp (Fig. 8.16b). Many experimental investigations have shown, however, that such a picture is far too simple because the kinetics of both processes play a dominant role. Accordingly, it is very difficult to predict whether corrosion or the redox process will dominate under given circumstances. 

A particular class of electrode materials is represented by the transition metal chal-cogenides, such as n-WSe2, n-MoSc2 and others, which form layer crystals. As already 

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S.5 Competition between Redox Reaction and Anodic Dissolution... 

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