Effect of chemical fluorination of carbon anodes on their electrochemical properties in KF-2HF

Two kinds of carbon-fluorine surface layers on carbon electrodes were prepared and the electrochemical properties of these electrodes were studied in the KF-2HF melt. The first film was prepared in situ by electrochemical oxidation of carbon in KF-2HF during the fluorine evolution reaction. The surface layer is mainly composed of conducting graphite intercalation compounds, C-x,(HF)(y)F, usually denoted C-x,F. The second film was prepared ex situ by chemical fluorination of carbon with fluorine gas at 275 degrees C, leading to a mixture of C-x,F and graphite fluorides, CFx. The fluorine evolution reaction is thus completely inhibited in KF-2HF. XPS investigations have shown that ionic and covalent carbon-fluorine bonds are present in the C-x,F and CFx compounds, respectively. When doping aluminum particles were inserted into carbon before chemical fluorination, the fluorine evolution reaction occurred due to the presence of C-x,F-AlF3 phases randomly scattered on the electrode surface and within the film. It is supposed that the electron transfer occurs either directly from the electrolyte to the substrate or by tunnel effect via intermediate-states, These particles also facilitate fluorine bubble detachment from the surface of the electrode.