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During the fluorine evolution reaction occurring in the electrolysis of molten KF-2HF, a thin, solid fluorocarbon layer is formed on the carbon anode. The high anodic overvoltage which characterized this process is mainly due to the presence of such a film which drastically changes the wettability of the electrode by the melt. Activation of carbon at 40 V in KF-2HF modifies strongly the surface morphology ͑scanning electron microscopy͒ and composition ͑energy-dispersive X-ray and X-ray photoelectron spectroscopy͒ of the surface fluorocarbon film. In addition, it enhances the electrochemical performances of the carbon electrodes vs fluorine evolution. Local microconductivity measurements carried out by atomic force microscopy have revealed heterogeneities of composition of the carbon-fluorine film due to the presence of conducting fluorine-graphite intercalation compounds and insulating graphite fluorides ͑CF x ͒ within the film. Finally, the peculiar shape of the fluorine bubbles observed during electrolysis of molten KF-2HF was studied, taking into account the presence of these two fluorocarbon compounds on the surface.

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