The promoting effect of water on the electrodeposition of Eu in a dicyanamide ionic liquid

Abstract

Rare-earth elements (REEs) are one of the most important raw materials and are of vast significance for the modern hi-tech industry. Electrochemical deposition of REE-containing materials can provide a real alternative to currently used high-temperature processes of molten salt electrolysis. Here we study the process of Eu electrodeposition from 1‑butyl‑1-methylpyrrolidinium dicyanamide IL, [BMP][DCA], containing controlled amount of water. The Eu deposition is performed on Pt(111) single crystal as well as on Au(111) and glassy carbon electrodes. We show that addition of water (up to 3.092 M) to a [BMP][DCA] solution promotes the electroreduction of Eu ions to Eu(0) shifting the potential of Eu deposition in the positive direction. The potential shift is already pronounced after addition of a moderate water amount and is equal to ~0.28 V upon increase in cH2O from 0.012 to 0.167 M. Microscopic and X-ray spectroscopic analyses of the electrodes after potentiostatic polarization confirm the formation of Eu deposit, however Eu in the deposit is mainly in the oxidized form. The oxidation of electrochemically deposited Eu(0) can occur simultaneously with the electrodeposition process in the chemical reaction with residual or added water. Furthermore, the electrode material is found to affect the overpotential of Eu deposition, and the deposition accelerates in the order of GC < Au(111) < Pt(111), which can be related to different adsorption behavior of solution components on these substrates.