Electroreduction of Holmium Chloride in the Equimolar NaCl–KCl Melt

Abstract

Rare-earth metals and their alloys are widely used in various engineering fields due to their unique physical and chemical properties. To prepare rare-earth-containing coatings, data on the kinetics of electrode processes with the participation of the rare-earth metals in salt molten media should be available. These problems are poorly understood for rare-earth metals. In the present study, the kinetic parameters of the cathode reduction (transfer coefficient, heterogeneous charge transfer rate constants) of holmium ions in the equimolar NaCl–KCl melt are determined in a temperature range of 1073–1173 K in a holmium chloride concentration range of 1–10 wt % using chronopotentiometry and chronovoltammetry. The dependences of the kinetic parameters on the temperature and concentration of rare-earth metal chloride are determined. The following reduction mechanism of complex ion in the aforementioned melts is suggested: $${\text{HoCl}}_{6}^{{3 - }}$$ = $${\text{HoCl}}_{4}^{ - }$$ + 2Cl–, $${\text{HoCl}}_{4}^{ - }$$ + e → $${\text{HoCl}}_{4}^{{2 - }}.$$