Influence of bubble adherence on the kinetics of the fluorine evolution process

Abstract

This paper is devoted to the mechanism of fluorine evolution reaction on carbon anodes during electrolysis of molten KF-2HF. Usually, the anodic transfer coefficient, α, is deduced from Tafel plots and the partial coverage of the electrode by bubbles is neglected. Low values such as α ⩽ 0.15 are usually reported. A new approach is proposed, taking into account the low surface energy of the graphite fluoride surface film generated on the anodes during the electrochemical reaction : the formation of fluorine gas occurs only on the 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 blockage of the electrode, the calculated value of the transfer coefficient a for the anodic fluorine evolution reaction is increased (α ≈ 0.28 for an activated carbon electrode and α ≈ 0.23 for a non-activated carbon electrode). The exchange current density, J 0, is estimated to be ≈ 9.2 × 10 −5 Am −2 and 5.5 × 10 −5Am −2, respectively, in the case of activated 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.