Influence of CaF₂ and AlF₃ on the kinetics and mechanism of the Al electrode reaction in cryolite melts with various alumina contents

Abstract

Electrochemical techniques were used to study the kinetics and mechanism of the aluminium electrode reaction in two cryolite-based melts containing cryolite with either 11 wt % AlF₃ or 5 wt % CaF₂ additions and variable alumina contents at 1000 °C. A three step electrode process was observed in both melts, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current density of the cathodic reaction was found to be dependent on the concentration of aluminium fluoride. By a combination of electrochemical impedance spectroscopy (EIS) and galvanostatic relaxation methods (GRM), the exchange current density of the first (slower) charge transfer step, the Warburg diffusion impedance, the double layer capacitance of the aluminium electrode and the rate of the preceding chemical step, were evaluated in the range of 2–8 wt % alumina. The role of the two additives, AlF₃ and CaF₂, was evaluated.