Joint Electroreduction of La3+ and Ni2+ Ions in the Equimolar KCl-NaCl Melt at 973 K

Cyclic chronovoltammetry, chronopotentiometry, open-circuit chronopotentiometry (turn on/off curves) are used to study the joint electroreduction of lanthanum and nickel ions at a tungsten electrode in the equimolar potassium and sodium chloride melt at 973 K. Metallic lanthanum is shown to deposit at an inert tungsten electrode from the KCl-NaCl-LaCl3 melt at potentials of -2.1 to -2.2 V relative to a silver/silver chloride reference electrode in a single three-electron stage, which is controlled by a mass-transfer stage at polarization rates of less than 0.1 V/s. At higher polarization rates, the charge-transfer stage is slower. In the case of joint presence of lanthanum and nickel ions in the KCl-NaCl equimolar melt at 973 K, the current-voltage dependences exhibit a wave related to the reduction of nickel and lanthanum ions at potentials of 0.0 to -0.1 and -1.85 to -1.9 V, respectively. Along with the waves, the current-voltage dependences exhibit two reduction waves at potentials of -1.5 to -1.6 and -1.75 to -1.8 V. The appearance of these two waves is related to the reduction of lanthanum ions with depolarization at metallic nickel, which was preliminary deposited at a tungsten electrode, with the formation of lanthanum and nickel LaxNiy intermetallics differing in composition by the reaction xLaCl (3- )(6)+yNi -> LaxNiy 6 xCl(-) . The wave observed at potentials of -1.85 to -1.9 V corresponds to the electroreduction of LaCl63- chloride complexes at the LaxNiy intermetallic with the formation of an intermetallic compound with a higher lanthanum content at the expense of the reaction of the LaxNiy intermetallic with metallic lanthanum, LaxNiy + zLa -> Lax +zNiy. The (E-t) open circuit potentiometry dependences (turn on/off curves) are found to exhibit potential-delay plateaus, which correspond to the dissolution of individual lanthanum and nickel intermetallic phases. Based on an analysis of cyclic current-voltage dependences for the KCl-NaCl-LaCl3-NiCl(2 )melt and the (E-t) dependences obtained by direct chronopotentiometry and open-circuit chronopotentiometry, we conclude that the electrochemical synthesis of lanthanum-nickel intermetallics can occur only in a kinetic mode, namely, via the electroreduction of ions on metallic nickel preliminary deposited at the tungsten electrode with the LaCl63- formation of LaxNiy intermetallics due to reaction diffusion.