ON THE ORIGINS OF CATHODE HYPERPOLARIZATION EFFECTS IN ELECTROLYTIC FLUORINE PRODUCTION FROM KF.2HF MELTS

The operation of electrolysis cells for fluorine production is accompa nied by appreciable overvoltage effects not only at the anode but also appreciably at the steel cathode. This unexpected large overvoltage i s referred to here as 'hyperpolarization' and sets in under certain co nditions of current density and temperature which can lead to temporar y failure of the cell operation. In a detailed study of hyperpolarizat ion it has been found that its onset is very sensitive to the HF conte nt in the KF.2HF melt electrolyte, as demonstrated by a series of expe riments in which the HF contents of the melt were systematically varie d around the formal 1:2 KF.2HF composition ratio. Experiments at a rot ating cone mild-steel electrode, and one with a solidified melt, as we ll as studies of temperature effects, combined with the phase-diagram, provide evidence that the hyperpolarization arises on account of mass -transport limitation associated with HF consumption. The suddenness a nd 'irreversibility' of the onset of the hyperpolarization effect is d irectly related to HF starvation in the diffusion layer at high curren t-densities which, in turn, leads to a more significant effect, the on set of local solidification of the electrolyte in a thin film at the e lectrode interface. A computer simulation of the diffusion situation p rovides a firm basis for the interpretation of the origin and signific ance of the hyperpolarization effect.