Electrochemical formation of holmium-copper alloys on copper cathode in molten KCl-HoCl3

Cyclic voltammetry, open circuit potential—time curve after potentiostatic electrolysis and potential step chronoamperometry were used to investigate the electrochemical formation processes of holmium-copper alloys on copper cathode in molten HoCl3-KCl. Intermetallic compounds HoCu5, HoCu4, HoCu2 and HoCu are formed in sequence and then the metallic Ho is deposited when Ho3+ is reduced on copper electrode in molten KCl-HoCl3 at 1 066 K. The first charge-transfer reaction is reversible. The structure of holmium-copper alloy film deposited on copper electrode by potentiostatic electrolysis was characterized by X-ray diffraction. The standard free energies of formation for the intermetallic compounds HoCu5, HoCu4, HoCu2 and HoCu are −95.5, −92.6, −73.8 and −44.0 kJ/mol, respectively. The diffusion coefficient and diffusion activation energy of Ho atom in the alloy are estimated to be 10−10–10−11 cm2/s and 75.35 kJ/mol, respectively, from the chronoamperometry data.