Anodic Evolution of Chlorine at a Graphite Electrode

Anodic evolution of chlorine at graphite has been investigated many times mainly for applied value (see, for example, the review articles[308-311]). Ksenzhek and Stender[312-315] were the first to analyze the effect of porosity of a graphite anode on the kinetic parameters of the process. Proceeding from the idea formulated by Ioffe[316] about the relation between the IR drop in electrolyte in the pores of the anode and a decrease in the true current density the authors analyzed the shape of the polarization curve and showed, in particular, that in the Tafel region the true current density on the outer surface of the electrode is proportional to the square of the apparent current density, which results in the doubling of the slope b. This result is valid for any true value of the Tafel slope[317]. 

The sharp kink of the curve cannot be explained either by experimental errors or by the effect of some impurities. The latter assumption is rejected by the fact that multiple electrolytic purification of the solution does not change the value of overpotential. Moreover, a vigorous stirring does not change the value of overpotential, though in this case there would be a much greater supply of hypothetical impurities to the surface. The latter circumstance also points to the absence of any appreciable concentration polarization. 

The decomposition of surface complexes, if a first order reaction with respect to adsorbed chlorine, results (when 6 1) in a 

The value of b = 59 mV is also typical of activationless electrochemical desorption, following an almost equilibrium discharge. 

In this case, however, the overpotential should depend considerably on the solution composition (3n/31og C = -59 mV, 3r)/3i( ] -1). 

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