Electrochemistry of anodic F2 evolution at carbon electrodes: Bubble adherence effects in the kinetics at rotating cone electrodes

Abstract

Fluorine-evolving carbon anodes exhibit unusually high overvoltages characterized also by remarkably large Tafel slopes having values 0.4–0.8 V per decade of current density change. Also, at high current densities, a so-called ‘anode effect’ associated with a type of passivation sets in. Experiments are described which aim to distinguish high polarization arising from an intrinsically large Tafel slope, generated by a non-ohmic charge transfer barrier layer effect due to ‘CF’ film formation, from effects due to difficulties of F2 bubble detechment and F2 gas film formation at the ‘CF’ film. Steady state polarization measurements have been made at a rotating carbon cone electrode from which F2 bubbles, which otherwise remain attached to the electrode and block access to the electrolyte, can be spun away. At the rotated electrode, at low and intermediate current densities, linear Tafel behaviour is still observed but with high slopes associated with the barrier layer film effect. At higher current densities an ‘anode effect’, associated with the F2 gas film, is developed, leading to a type of passivation of the electrode. The two sources of unusually high polarization in the F2 evolution reaction at carbon are not independent as it is also the formation of the ‘CF’ film that causes difficulties in gas bubble detachment owing to the lyophobic properties of the fluorinated C/F2/KF·2HF interface. Polishing effects confirm this conclusion.