Cathodic overvoltage and the contents of sodium and lithium in molten aluminium during electrolysis of cryolite-based melts

The cathodic concentration overvoltage in Na 3AlF 6–AlF 3–Al 2O 3(sat.) melts was investigated at the temperature of 1000°C. The melts contained 10 and 20 wt.% of AlF 3. Experimental data on the cathodic overvoltage agreed well with values calculated from the content of sodium in aluminium. The dependence of the cathodic overvoltage on temperature was studied as well. It was shown that the addition of 5 wt.% CaF 2 or 5 wt.% MgF 2 does not influence the overvoltage significantly, while the addition of 2 wt.% LiF decreases the overvoltage by 20 mV at a cathodic current density of 0.25 A cm −2 and by 50 mV at 0.75 A cm −2. These results are consistent with the influence of these additions on the content of sodium in aluminium.