Study of the mass transfer at fluorine-evolution carbon electrodes

The kinetics rate of the fluorine evolution reaction (FER) was studied in m olten KF-2HF mainly by impedance spectroscopy with a rotating disk electrod e in large potential and rotation speed ranges. A new model for the schemat ic representation of the electrode/electrolyte interface has been proposed including the presence of an intermediate layer between the electrode surfa ce and the fluorine gas film. This layer is composed of a solid C-F compoun d (generated on carbon anodes surface during the electrolysis), liquid KF-2 HF melt, and fluorine gas. The influence of the mass-transfer phenomenon oc curring in that layer was pointed out in the low frequency part of the impe dance diagrams. This intermediate layer gives also rise to a capacitive loo p at very high frequency which does not vary with the applied potential and the rotating speed values. The kinetics rate of the FER is governed both b y the mass transfer and the charge transfer. Finally, the influence of the C-F on the anodic overvoltage was studied. (C) 2000 The Electrochemical Soc iety. S0013-4651 (00)04-082-9. All rights reserved.