An investigation of fluorine overvoltage at carbon anodes

Abstract

The high anode overvoltage exhibited by fluorine cells has been variously attributed to bubble overvoltage and to inhibition of the electron transfer process by a layer of insulating carbon fluoride on the surface of the electrode. This thesis presents the results of various experiments which attempt to assess the contribution of the carbon fluoride film to the anode overvoltage in the absence of bubble overvoltage. Fluorine evolution from the molten salt KF.2HF at 85°C was studied at vertical carbon anodes. The techniques employed to study the kinetics and mechanism of this reaction included cyclic voltammetry, steady state potentiostatic polarisation, A.C. impedance, potential pulse and constant current electrolysis. Such measurements indicated that several types of carbon fluoride can be produced on the anode evolving fluorine, depending on the potential of electrolysis. These measurements also show the difficulty in obtaining kinetic parameters for the fluorine evolution reaction. The nature of the fluoride film produced on the carbon anode was investigated using XPS and SIMS analysis of electrodes polarised in the KF.2HF melt at 5V and 9V. These measurements indicated that the film is not one single carbon fluoride but more of a graded nature becoming more heavily fluorinated at higher potentials. The inhibiting effect of the solid film was studied by transfering carbon electrodes, following fluorine evolution in the KF.2HF melt under well' defined conditions, to other electrochemical systems using aqueous or organic electrolytes at room temperature, where electrode kinetic studies were made of redox reactions. Kinetic data were compared with those obtained with unfluorinated carbon surfaces. Observations obtained by the above methods have been related to estimates of the thickness of the carbon fluoride film as determined by measurements of the double layer capacitance in the KF.2HF melt and in other solvent systems. Both capacitance measurements and kinetic studies of redox reactions indicate that the film became progressively thicker as the potential of the fluorine anode was raised. Rates of redox reactions fell by at least two orders of magnitude in the presence of a film formed by polarising the carbon at 6.0V in the KF.2HF melt. Attempts were made to overcome the inhibiting effect of the fluoride films by incorporating transition metals in the carbon anodes. The last section of this thesis explains the manufacture of doped carbon samples and presents results obtained in the KF.2HF melt using such anode materials.