Accelerating effect of water on electroreduction of lanthanide ions in a dicyanamide ionic liquid: a generic phenomenon

• The electroreduction of Ln(III) to Ln(0) is described for six lanthanides in a dicyanamide IL. • Deposited Ln can be partly oxidized during electrodeposition by water in the solution. • Water accelerates the electroreduction of Ln(III) to Ln(0) in the dicyanamide IL. • The water accelerating effect is a generic phenomenon for the electroreduction of Ln. Electroplating is a relatively cheap and processible technique of metal production. In the case of lanthanides (Ln), however, this technology is inapplicable in water solutions due to their pronounced electropositivity. This work studies the electroreduction of lanthanides (Ln) from a dicyanamide ionic liquid (IL) on a model Pt(111) substrate in dry solutions and upon addition of controlled amounts of water. We demonstrate the possibility of irreversible electrodeposition for six different Ln from the dried IL using a complex set of electrochemical and physical (spectroscopic and microscopic) methods. Addition of water leads to the facilitation of electrodeposition for all the studied Ln, as confirmed by a positive shift in the deposition potentials. We relate this promoting effect to the coadsorption of water on the electrode surface and the relevant changes in the IL|electrode interface structure, e.g., the disordering of the IL multilayer structure at the electrode surface. We believe that this promoting effect of water is a generic phenomenon for Ln electrodeposition in dicyanamide ILs and discuss the possible causes that result in acceleration of deposition. Analysis of the obtained deposits indicates that the deposited Ln are partly oxidized even in the course of the deposition even in dry solutions, probably due to interaction with the residual water. The results of this work may be instrumental in developing lanthanide electrowinning techniques based on mixtures of ionic organic solvents with controlled water additives.