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.

Efficient Room Temperature Neodymium Electrodeposition in a Calcium-Based Electrolyte

“…In contrast to previously studied IL-based systems, the highly anhydrous and reducing environment of borohydride salts excluded the water-driven alteration of reduced metal . While such prevention could be evident from the efficient Ca metal stripping, voltammetric Nd reduction was still irreversible.…”

“…In contrast to previously studied IL-based systems, the highly anhydrous and reducing environment of borohydride salts excluded the water-driven alteration of reduced metal. 23 While such prevention could be evident from the efficient Ca metal stripping, voltammetric Nd reduction was still irreversible. While this latter aspect has been observed in most of the reported works at room temperature conditions, the voltammetric irreversibility of RE reduction has not been fully investigated.…”

“…Room temperature electrodeposition of rare-earth (RE) elements has been suggested as a sustainable replacement to the energy-intensive and nonenvironmental MS process. − Due to their unique properties, including intrinsic conductivity, large electrochemical window, and low vapor pressure, the use of ionic liquids (IL) as electrolyte media has dominated the room temperature RE electrodeposition studies. − As of today, development of the IL process has faced major challenges, mostly due to high viscosity and complex RE chemistry occurring at the electrode–IL interface . Several chemical engineering strategies have been proposed to promote electrochemical REM production in these electrolytes, including increased operating temperature, the use of organic cosolvents, and the use of neutral ligands and additives. − In particular, water has been suggested as a promoting additive for the electrochemical production of REMs in ILs. ,, However, REM instability and limited purity ultimately remained the major challenge for the efficiency of the water strategy . Based on several reported electrolyte systems, water inclusion and high RE salt hygroscopicity seem to be common issues.…”

Fluorine-free organic electrolytes for the stable electrodeposition of neodymium metal

“…not rigorously dried) environment, these wet electrolytes also provide an inherently unstable environment for the deposits, leading to contamination of the metallic REEs with oxides and hydroxides, and a decreased current efficiency. 23…”

“…not rigorously dried) environment, these wet electrolytes also provide an inherently unstable environment for the deposits, leading to contamination of the metallic REEs with oxides and hydroxides, and a decreased current efficiency. 23 Organic electrolytes are composed of salts which are dissolved in an organic solvent and they could be a less expensive and sustainable alternative to ILs, provided that they are sufficiently electrochemically stable to support REE electrodeposition. This category of electrolytes has been much less explored than ILs.…”