Study of the mass transfer at fluorine-evolution carbon electrodes

The kinetics rate of the fluorine evolution reaction (FER) was studied in molten KF-2HF mainly by impedance spectroscopy with a rotating disk electrode in large potential and rotation speed ranges. A new model for the schematic representation of the electrode/electrolyte interface has been proposed including the presence of an intermediate layer between the electrode surface and the fluorine gas film. This layer is composed of a solid C-F compound (generated on carbon anodes surface during the electrolysis), liquid KF-2HF melt, and fluorine gas. The influence of the mass-transfer phenomenon occurring in that layer was pointed out in the low frequency part of the impedance diagrams. This intermediate layer gives also rise to a capacitive loop 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 by the mass transfer and the charge transfer. Finally, the influence of the C-F on the anodic overvoltage was studied. (C) 2000 The Electrochemical Society. S0013-4651 (00)04-082-9. All rights reserved.