INFLUENCE OF BUBBLE ADHERENCE ON THE KINETICS OF THE FLUORINE EVOLUTION PROCESS

This paper is devoted to the mechanism of fluorine evolution reaction on carbon anodes during electrolysis of molten KF-2HF. Usually, the an odic transfer coefficient, alpha, is deduced from Tafel plots and the partial coverage of the electrode by bubbles is neglected. Low values such as alpha less than or equal to 0.15 are usually reported. A new a pproach is proposed, taking into account the low surface energy of the graphite fluoride surface film generated on the anodes during the ele ctrochemical reaction: the formation of fluorine gas occurs only on th e part of the carbon electrode which is not covered by the gas bubbles . In this work it is shown that, when allowance is made for partial bl ockage of the electrode, the calculated value of the transfer coeffici ent alpha for the anodic fluorine evolution reaction is increased (alp ha approximate to 0.28 for an activated carbon electrode and alpha app roximate to 0.23 for a non-activated carbon electrode). The exchange c urrent density, J(0), is estimated to be approximate to 9.2 x 10(-5) A m(-2) and 5.5 x 10(-5) A m(-2), respectively, in the case of activate d and non-activated carbon electrodes. In the studied potential range, the contribution of the fluorine bubbles to the anodic overvoltage is about 0.35 V.