Fluorine-free ionic liquid electrolytes for sustainable neodymium recovery using an electrochemical approach

Abstract

Rare earth metals (REMs) are considered critical materials due to their extensive demand for use in essential technologies that enable the transition to a greener energy technology and economy. However, the significant environmental and health impact caused by the current primary sourcing of REMs, i.e. mining, urgently demand more sustainable and environmentally friendly alternatives. Herein we report a cleaner approach to recover Nd via electrochemical deposition using low cost and non-fluorinated ionic liquid (IL). In contrast to most typically studied ILs utilising the bis(trifluoromethanesulfonyl)imide anion, here we show successful electrodeposition of Nd using 0.5 mol kg⁻¹ neodymium nitrate (Nd(NO₃)·6H₂O) in N-butyl-N-methylpyrrolidinium dicyanamide ([C₄mpyr][DCA]) IL electrolyte while reporting an eight times higher current density (−38 mA cm⁻²) at a lower temperature (halved to 50 °C) and less controlled environment (0.15–4.6 wt% H₂O) compared to parameters previously reported in the literature. Further, the effect of Nd salts and their concentration on the electrolyte physical properties, Nd³⁺ electrochemical behaviour, electrodeposit composition and Nd recovery efficiency were investigated using Nd(OTf)₃ (trifluoromethanesulfonate [OTf]⁻) and Nd(NO₃)·6H₂O in [C₄mpyr][DCA] IL electrolytes. The XPS analysis confirmed the presence of a higher Nd metal content in the electrodeposit resulting from the nitrate system.