Water Interplays during Dysprosium Electrodeposition in Pyrrolidinium Ionic Liquid: Deconvoluting the Pros and Cons for Rare Earth Metallization

Abstract

The electrochemical production of rare earth metals (REMs) in ionic liquids (ILs) has received much attention as a promising, sustainable replacement to molten salt electrolysis. Water additives have been suggested as a promoting strategy for the ionic liquid process; however, the fundamental understanding of the interfacial processes required to assess the overall viability for REM production is lacking. In this regard, a full investigation of the impact of water on dysprosium (Dy) electrodeposition in pyrrolidinium triflate (BMPyOTf) ionic liquid was carried out. Water introduction was revealed to involve an interplay of implications on the electrodeposition process, including coordination, speciation, reduction pathways, interfacial dynamics, nucleation, and metal stability and purity. Under highly dry conditions, the reduction occurs at a very negative potential (-3.3 V) in a consecutive pathway, resulting in negligible metal electrodeposition (low rate and efficiency) at the electrode surface. Small water concentrations (<500 ppm) lead to partitioning of the Dy complex between water and IL-coordinated speciation, giving rise to an additional wave at a more positive potential (-2.4 V). Probing the heterogeneous Dy speciation by spectroscopic analyses enabled uncovering of the reduction mechanism and evaluation of the mass transport properties. In addition to lowering the reduction thermodynamics, water introduction also improved the nucleation, deposition rate, and faradic efficiency. Despite these benefits, stripping voltammetric analysis predicts substantial chemical reactivity of the deposited Dy metal with water additives and/or electrolyte components, under long timescales. Surface characterization of the obtained product confirmed the instability of Dy metal as an oxidized/fluorinated material and its limited purity (∼60%). Moreover, high water introduction triggered a fast hydrogen evolution reaction (HER), downgrading the robustness of system efficiency. The overall impact of water additives seems to engender both promoting and mitigating effects on electrochemical REM production in IL, requiring a specific technoeconomic assessment and/or more innovative strategies to be sought.