Selenium electrochemistry in choline chloride–urea deep eutectic electrolyte

The electrochemical behaviour of selenium in the deep eutectic solvent made of a 1:2 molar ratio of choline chloride and urea (ChCl–U) has been investigated at a polycrystalline gold electrode by voltammetry and chronoamperometry. In order to favour the deposition of grey selenium, selenium oxide was chosen as the solution precursor and a temperature range from 70 to 110 °C was selected. Cyclic voltammograms recorded in the 1:2 choline chloride–urea liquid containing 10 mM SeO₂ are strongly affected by the temperature. At 110 °C, three main cathodic responses are evidenced around ?0.075, ?0.2 and ?0.7 V. These cathodic peaks have been attributed respectively to the underpotential deposition (upd) of Se, the bulk deposition of Se and the cathodic stripping of selenium associated to the formation of Se(?II). Potentiostatic current transients obtained at 110 °C are indicative of a nucleation with diffusion-controlled growth mechanism for the selenium electrodeposition and support the formation of a upd layer preceding the bulk deposition. The dissolution transients triggered by double potential step perturbations could however not be interpreted on the basis of a similar formalism.