A KINETIC-STUDY OF CARBON ELECTRODE-REACTIONS IN VARIOUS MOLTEN FLUORIDES

The electrochemical behavior of several kinds of carbon have been studied in various molten fluorides. The temperature range of the electrolyte employed in this experiment is related to the melting point of each melt, e.g., 50-degrees-120-degrees-C for NH4F . 2 HF, 80-degrees-150-degrees-C for KF . 2 HF, 260-degrees-C for KF . HF, and 470-degrees-600-degrees-C for LiF-NaF-KF. The values of the peak current density (i(p)) at ca. 6V vs. Pt and the minimum current density (i(a)-) at the potentials over 6V vs. Pt were measured from the anodic polarization curves obtained by cyclic voltammetry with a sweep rate of 200 mV . s-1, and then they were interpreted kinetically. The value of i(a) was theoretically proved to be proportional to the rate of decomposition of (CF)n film formed on the surface of carbon anode. From the plots of log i(a) against the reciprocal of absolute temperature (1/T), the activation energy of the disproportionation of (CF)n film obtained in the molten LiF-NaF-KF system was 22.0-25.9 kcal . mol-1 In contrast, that in each case of the molten KF . HF, KF . 2 HF, and NH4F . 2 HF was only 5.5-9.4 kcal . mol-1. However, the carbon anode was often broken in the presence of HF. The disproportionation of the (CF)n film is catalyzed by HF and the physical etching of the carbon anode could be explained in terms of the formation of an intercalation compound containing HF.