Electrochemical behaviour of LaCl3 at tungsten and aluminium cathodes in LiCl-KCl eutectic melt

The electrochemical behaviour of lanthanum was studied at inert tungsten electrode and reactive aluminium electrode in LiCl-KCl eutectic melt in the temperature range 698-798 K using transient electrochemical techniques. Reduction of La(III) to La(0) at the tungsten electrode takes place in a single step. The reduction shows quasi-reversible behaviour for polarization rates, 25 less than or equal to nu less than or equal to 150 mV s-1 and is predominantly controlled by charge transfer of La(III) ions for scan rates higher than 75 mV s-1. The heterogenous rate constant of the process was estimated from impedance spectroscopy and from the semi-integrals of the cyclic voltammograms. The redox potential of the La(III)/La couple at the Al electrode was observed to be more positive than that at the inert electrode. This potential shift is due to the lowering of the activity of la in the metal phase caused by the formation of the intermetallic compound Al11La3. Thermodynamic properties such as Gibbs energy of formation of Al11La3, excess Gibbs energy and the activity coefficient of la in Al were calculated from the open circuit potential measurement.