The O2 Reduction on Carbon

Carbon can be used [7,11,62,70—73] as an electrocatalyst for the O2 reduction in alkaline electrolytes (compare also section 5 in chapter VIII). The performance which is not so good as that of silver (see Fig. 79) appears adequate for certain purposes, for instance, in small zinc-air cells. Activation procedures [72,73] which are not of an electrochemical nature improve the performance of carbon oxygen electrodes. The performance rapidly becomes poor with decreasing pH below pH 14. In acid solution, the impregnation of carbon with platinum metals or other electrocatalysts is required. The data [73] in Table 8 

Ringsdorff-Kohle Aktiv-Kohle Elorit DA Elorit TD 

Carbon electrodes of the type Elorit TD are desirable for practical oxygen electrodes. 

The results in Table 6 were obtained on carbon electrodes for which the efficiency of H2O2 production is smaller than 100%. Experiments which allow to decide whether the main path of the O2 reduction is the sequence of reactions 13 and 15 or reaction 4 have not yet been carried out on carbon electrodes. As demonstrated by Frumkin and coworkers [74—77], the adsorption of oxygen on carbon occurs in two different forms. The formation of the first type requires the dissociation of the O2 molecules into O atoms. The second type is formed without dissociation. It was suggested [36] that the ratio /13//4 of the rates of reactions 13 and 4 depends upon the ratio between the rate constant of the electrochemical production of H2O2 and the rate constant for the formation of the first type of adsorbed oxygen. Thus the mechanism of the O2 reduction on carbon will vary with the type of carbon and the pretreatment of the surface. 

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