Fluorine evolution at carbon/KF–2HF interface

Abstract

A solid carbon–fluorine film (C–F) is formed on carbon electrodes during fluorine evolution reaction in KF–2HF. Impedance spectra performed with C/CF/Hg structures do not exhibit any capacitive loop whereas the Ta/Ta2O5/Hg structures do, owing to the insulating properties of the tantalum oxide film. It was concluded that the passivating C–F films behave as electronic conductors. Same conclusions were deduced from the study of the kinetics of the electron transfer of the FeIII/II redox couple in aqueous solution, using a C/C–F electrode. One can deduce that the electrons may be transferred easily through the C–F film from the carbon substrate to the electrolyte during the fluorine evolution reaction in KF–2HF. Therefore, the high anodic overvoltage observed during fluorine evolution reaction is mainly attributed to the poor wettability of the electrodes by the melt, which results in a small electroactive area. Surface analyses by STM measurements have pointed out heterogeneities of composition at the surface of fluorinated highly oriented pyrolitic graphite. Hexagonal symmetry was observed as well as an overlap of the electronic densities between two neighbouring atoms which was interpreted by the existence of chemical bonds between one fluorine species with one or two carbon atoms.