Electrochemistry of fluorine production

Abstract

The electronic properties of carbon–fluorine films (denoted C–F) formed on carbon electrodes in KF–2HF during fluorine evolution reaction were investigated in aqueous solution containing a redox couple and in mercury. It was shown that the passivating C–F films behave as electronic conductors. STM measurements have shown composition heterogeneities at the surface of fluorinated HOPG (conducting ans insulating areas). The influence of the amount of insulating graphite fluorides on the surface of the electrodes was demonstrated. Thus, the high anodic overvoltage observed during fluorine evolution on C/C–F anodes in KF–2HF is mainly attributed to the poor wettability of the electrodes by the melt, which results in a small electroactive area. A new model was proposed for representing the electrode/electrolyte interface; it includes the presence of a “fluidized” layer between the surface C–F film and the fluorine gas film. The “fluidized” layer is composed of liquid KF–2HF melt and dissolved fluorine gas. The influence of the mass transfer phenomenon occurring in that layer was pointed out mainly by impedance measurements. Finally, the contributions of the C–F film, η C–F, and of the “fluidized” layer, η fluid, to the total anodic overvoltage, η T, were studied using a numerical calculation method. Both contributions must be taken into account for a global understanding of the fluorine evolution process.