Transformations de l’interface d’une anode de carbone dans l’électrolyte 2HF-KF

Laboratoire d’Electrochimie, E.R.A. 310, Université Pierre et Marie Curie, 4 place Jussieu 75230 Paris Cedex 05, France.

Résumé

Le comportement des anodes de carbone dans l’électrolyte fondu 2HF-KF s’interprète par la formation d’une couche passivante de composés carbone-fluor solides.

La voltammétrie cyclique fait apparaître un pic de passivation à un potentiel inférieur à celui de la formation du fluor. La chronoampérométrie montre que la cinétique de croissance de la couche est conforme à la théorie de Mott-Cabrera avec un coefficient d’interdiffusion : D ≈ 4,5 × 10 _–14cm².s^–1.

Lors des mesures d’impédance, la forte diminution du terme capacitif est liée à la croissance de l’épaisseur du film de diélectrique.

Abstract

The behaviour of carbon anodes in molten 2HF-KF has been studied by three conventional electrochemical methods and has been proved to be related to the growth of an insulating layer of solid carbon-fluorine compounds.

An anodic peak, appearing in cyclic voltammetry at a potential below the fluorine evolution, is observed only once, during the first sweep on a new electrode. This corresponds to the formation of a stable passivating layer.

The kinetics of the anodic film growth has been investigated by chronoamperometry, which shows that, in definite conditions of applied potential, the mechanism fits the theory of Mott and Cabrera. Then, according to the variation of current density vs time, the interdiffusion coefficient of carbon and fluorine is evaluated to: D ≈ 4,5 × 10^–14 cm².s^–1 in the solid CF compound.

The impedance measurements show that, as the potential of the anode is made more positive, the capacitance term decreases very sharply, as a consequence of the increase of the thickness of the insulating layer.