Kinetics of fluorine evolution reaction on carbon anodes: influence of thesurface 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 mai nly composed of fluorine-graphite intercalation compound and a small amount of insulating graphite fluorides. Impedance measurements performed in merc ury and cyclic voltammetry studies in aqueous solution containing a redox c ouple have shown that the surface him behaves like an electronic conductor and cannot constitute a high energy barrier for the electron transfer in el ectrochemical reactions. However, the kinetics of the fluorine evolution re action 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 hig hly oriented pyrolitic graphite (HOPG) samples have pointed out the heterog eneities of composition of this carbon-fluorine him and the influence of wa ter. It has been shown that, if the passivation of the carbon electrodes wa s performed in molten KF-2HF containing a high amount of water, the hexagon al symmetry of the images obtained with HOPG is lost. (C) 2001 Elsevier Sci ence B.V. All rights reserved.