Electrodeposition of Dysprosium in pyrrolidinium triflate ionic liquid at ambient temperature: Unraveling system efficiency and impact of solvation interplays on the reduction process

Abstract

Electrochemical reduction of rare earth elements (REEs) in ionic liquids (ILs) has attracted much attention, as a promising low temperature and alternative technology for molten salt electrolysis. While reported studies have established the proof of concept of the ionic liquid technology, efficient recovery and fundamental understanding of the electrodeposition process in these systems are still lacking. In this work, the reduction of dysprosium triflate (Dy(OTf)3) complex is investigated in neat pyrrolidinium triflate (BMPyOTf) ionic liquid, a commercial but poorly explored system in literature. Chronoamperometry and surface characterization (SEM-EDS, XPS and ICP-MS) demonstrated an efficient (~80%) electrochemical recovery of Dy metal, with deposition rates approaching 10 mg.h−1.cm−2 at 0.3 M metal concentration, small applied potentials (~-3 V Fc+/Fc) and ambient temperature (25 °C). Cyclic voltammetry revealed two main electrodeposition waves (Red1 and Red2), suggesting single 3e- reduction of two co-existent metal complexes with different coordination spheres, strongly (Red2) and weakly (Red1) coordinated metal complex with the ionic liquid triflate anions. This suggestion was supported by the vibrational (Mid-IR) analysis, where downshifts of the νas(SO) stretching mode were consistent with an evolution of weaker metal coordination at higher concentrations. Near-IR confirmed the evolution of speciation, from one main complex at low concentrations to a mixture of species at higher concentrations. Estimation of the speciation's distribution from deconvoluted NIR and semi-derivative traces indicated that voltammetric currents are controlled by bulk concentrations. While under coulometric conditions, the metal reduction seems to proceed via the IL-coordinated thermodynamics, due to enhanced anion concentrations at the electrochemical interface. Extended study to bistriflimide analogue systems revealed a distinct infrared signature for the IL-coordination and validated the correlation between the observed reduction processes and the speciation heterogeneity in these systems. The outcomes of this work demonstrate an efficient electrochemical recovery of Dy in triflate-based systems, while shedding the light on the impact of solvation and coordination interplays on the electrodeposition process of REEs in ionic liquids.