Molten KF-2HF is used for the electrolytic preparation of elemental fluorine at 85-100°C. The operating voltage in industrial cells lies in the range 8-10 V to obtain the current density I = 10 -12 A/dm2. The high anodic overvoltage (≈ 3 V) is commonly assigned to the formation of a solid film of carbon-fluorine compounds which lowers the wettability of the electrode by KF-2HF and induces a decrease of the electroactive surface area of the electrode since bubbles are large, lenticular and adherent to the anode surface. In this paper, the kinetics of the fluorine evolution reaction on crude carbon, studied by I-E curves and impedance measurements, is reviewed. The coverage of the electrodes by fluorine bubbles and its effect on the anodic overvoltage is studied. When doping metallic particles are inserted into the carbon lattice before electrolysis, the fluorine evolution reaction is enhanced owing to the presence of Graphite Intercalation Compounds randomly scattered on the electrode surface and within the film. The presence of these particles induces an increase of i) the electron transfer rate from the electrolyte to the substrate; ii) the wettability of the electrode by KF- 2HF; iii) the fluorine bubbles detachment from the surface of the electrode. Surface analysis of the electrodes by Scanning Electron Microscopy (SEM) (coupled with an X-ray detection) and X-ray Photoelectron Spectroscopy (XPS) permits the determination of the elemental composition of the surface of the electrodes, and thus allows a better understanding of the charge transfer mechanism.
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