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

Abstract

Electrowinning is regarded as a clean process to recover neodymium metal from secondary sources such as spent Nd–Fe–B permanent magnets, but the current methods are severely limited by a high energy consumption (molten salts), or by the high costs and environmental impact of the electrolyte components (ionic liquids). Therefore, there is a demand for more sustainable electrowinning methods for the recovery of neodymium metal. Inspired by our own previous work and the work of others, we developed new fluorine-free organic electrolytes that enable the electrodeposition of neodymium metal at room temperature. The electrolytes consist of solvated neodymium borohydride, Nd(BH₄)₃, dissolved in the ether solvents tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,2-dimethoxyethane (DME) and diethylene glycol dimethyl ether (diglyme, G2), and these complexes can be prepared entirely from non-fluorinated precursors such as neodymium(III) chloride (NdCl₃) and sodium borohydride (NaBH₄). In contrast to our previous bis(trifluoromethylsulfonyl)imide-containing electrolytes, electrodeposition of neodymium proceeds over time without significant loss of current density, indicating a higher stability against unwanted side-reactions that lead to passivation of the deposit on the electrode. Characterization of the deposits by scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDX), and X-ray photoelectron spectroscopy (XPS) unambiguously indicated the presence of neodymium metal.