Electronic properties of passivating compounds: application to the fluorine evolution reaction

Abstract

The electronic properties of carbon–fluorine films (denoted C–F) formed on carbon electrodes in KF–2HF during fluorine evolution reaction were investigated. Impedance spectra obtained with C/C–F/Hg, C/PbO 2/Hg and Ta/Ta 2O 5/Hg structures were compared. The latter exhibits a capacitive loop whereas the former does not. However, if a breakdown of the Ta 2O 5 oxide occurred, the capacitive loop is not observed, as in the case of C/PbO 2/Hg structures which are characterised by the electronic conductivity of PbO 2. The passivating C–F films formed on carbon anodes in KF–2HF behave as electronic conductors. This result was confirmed by studying the kinetics of the electron transfer of the Fe III/II redox couple in aqueous solution, using a C/C–F electrode. These electrodes behave like metallic platinum electrodes. 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. STM measurements have shown composition heterogeneities at the surface of fluorinated HOPG. Moreover, hexagonal symmetry was pointed out; the electron density map is interpreted on the basis of bonding of HF 2 − species to carbon atoms.