Correlation of the Alkyl Chain Length and Electrochemistry of Neodymium in a Phosphonium Ionic Liquid with Water as Additive

Neodymium is considered a critical metal widely used in important sectors including mobility, clean energies, and telecommunications. Current processes to recycle neodymium from waste have major drawbacks, such as emissions of greenhouse gases or high energy consumption. Recently, electrodeposition of Nd at low temperatures using ionic liquids (ILs) (e.g., phosphonium, pyrrolidinium) has been reported as a safer and low-cost process. Herein, we show that modifications on the alkyl chain length of a phosphonium IL can affect Nd electrodeposition behavior. Electrochemical, spectroscopy, and classical molecular dynamics (MD) simulation studies were carried out on neodymium bis(trifluoromethylsulfonyl)imide in tributylmethylphosphonium bis(trifluoromethylsulfonyl)imide ([P1444][TFSI]) with various water concentrations, and the results were compared with those previously obtained with the long alkyl chain phosphonium trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P666,14][TFSI]). It was found that water was beneficial for Nd electrodeposition from [P1444][TFSI], less water than the optimum amount for [P666,14][TFSI] was needed (0.1 wt % H2O), and purer deposits than those with [P666,14][TFSI] were obtained with this water content. Additionally, reduction current densities up to 5 times greater were achieved with the shorter alkyl chain phosphonium ([P1444][TFSI]). Upon water addition, FTIR and MD studies suggested different effects on TFSI conformers and structuring of the IL cation in the first coordination shell, which could be the origin of the different electrodeposition behavior observed in the two ILs. These findings highlight the importance of a careful design of the electrolyte composition and understanding the speciation for an efficient Nd recycling process.