Palladium electrochemistry in the choline chloride-urea deep eutectic solvent at gold and glassy carbon electrodes

The electrochemical behaviour of Pd(II) in choline chloride-urea eutectic (ChCl-U) has been investigated at polycrystalline gold and glassy carbon electrodes at 60 degrees C using voltammetry (cyclic and hydrodynamic) and chronoamperometry. The voltammograms recorded at Au in the palladium solution show three reduction signals, assigned to Pd bulk deposition (around -0.2 V vs. Ag QRE), the formation of palladium hydrides PdHx (around -0.4 V vs. Ag QRE) and an underpotential deposition (upd) of Pd (around 0 V vs. Ag QRE). Two anodic peaks present in the cyclic voltammograms correspond to the oxidation of Pd hydrides and Pd deposits. In contrast to the results obtained at Au, no upd response is observed at a GC electrode and a crossover between the forward and backward scan, characteristic of a kinetic control by nucleation and growth, is clearly evidenced. Typical peaked transients are observed whose analysis unambiguously demonstrates that the deposition occurs through a 3D progressive nucleation mechanism. (C) 2020 Elsevier Ltd. All rights reserved.