Kinetics of fluorine evolution reaction on carbon anodes: influence of the surface CF films

During the fluorine evolution reaction by electrolysis of molten KF–2HF, a thin solid fluoro-carbon layer is formed on carbon anodes. This film is mainly composed of fluorine–graphite intercalation compound and a small amount of insulating graphite fluorides. Impedance measurements performed in mercury and cyclic voltammetry studies in aqueous solution containing a redox couple have shown that the surface film behaves like an electronic conductor and cannot constitute a high energy barrier for the electron transfer in electrochemical reactions. However, the kinetics of the fluorine evolution reaction is strongly dependent on the water content in KF–2HF, indeed, water contributes to the formation of graphite fluorides which limit the fluorine evolution reaction. STM measurements performed on crude and passivated highly oriented pyrolitic graphite (HOPG) samples have pointed out the heterogeneities of composition of this carbon–fluorine film and the influence of water. It has been shown that, if the passivation of the carbon electrodes was performed in molten KF–2HF containing a high amount of water, the hexagonal symmetry of the images obtained with HOPG is lost.