Passivation Phenomenon and Variable Properties of Ytterbium in Different Lewis Acid AlCl3-NaCl Melts

[Display omitted] •The Yb metal reacts spontaneously in AlCl3-NaCl melt to obtain YbCl2.•In-situ absorption spectroscopy studied the species of Yb(II) in AlCl3-NaCl melt.•The diffusion ability of Yb(II) ions varies with the Lewis acidity of the melt.•The electrooxidation of Yb(II) causes passivation of the inert electrode.•Soaking the passivated electrode in the melt can alleviate the passivation. To further explore the application of the low-temperature molten salt in processing heavy rare earths, the properties of ytterbium in AlCl3-NaCl melts have been investigated. In this melt, YbCl2 powder products are prepared by electrolysis based on the dissolution reaction of Yb metal. The properties of Yb(II) in AlCl3-NaCl melts have also been studied in detail. In-situ UV-VIS absorption spectroscopy demonstrates that the coordination of Yb(II) changes in AlCl3-NaCl melts with different Lewis acidity, leading to variations in its diffusion ability. In addition, the conversion of Yb(II)/Yb(III) shows typical soluble-soluble reversible electrochemical behavior in strongly acidic AlCl3-NaCl. However, the electrochemical behavior of Yb(II) on inert electrodes becomes complicated when the amount of AlCl3 in AlCl3-NaCl melts less than 52 mol%. This change is attributed to the formation of a YbCl3 passivation film on the inert electrode during the electro-oxidation of Yb(II). Soaking the passivated electrode in the melt can alleviate the passivation. Understanding the electrochemical and spectroscopic properties of ytterbium in AlCl3-NaCl melts paves the way for its subsequent applications.