Palladium Electrodeposition from Deep Eutectic Solvents

Abstract

Palladium is a platinum group metal that has been reported to be useful as electrocatalyst for several important reactions such as formic acid oxidation, alcohol oxidation and oxygen reduction. In this context the electrodeposition of Pd is of particular interest. Palladium electrodeposition in aqueous media requires potentials close to the decomposition of water, leading to low faradaic yields and embrittlement of electrodeposited palladium. The use of unconventional solvents such as deep eutectic solvents (DESs) is expending as a promising alternative for the electrochemical deposition of metals. These are obtained by mixing a quaternary ammonium salt and a hydrogen-bond donor, leading to a melting point much lower than the pure components. These eutectics are easy to prepare, inexpensive and offer a high electrical conductivity. Owing to their thermal stability and wide electrochemical window, these media become more widespread in the field of metal electrodeposition, though fundamental studies are relatively scarce. The present work focuses on the electrodeposition of palladium from deep eutectic solvents based on choline chloride. The electrochemical behaviour of Pd(II) has been investigated at 60°C in choline chloride–urea (ChCl-U) and choline chloride–oxalic acid (ChCl-Ox). Cyclic voltammetry (CV) and potentiostatic measurements performed in the presence of Pd(II) at a glassy carbon electrode (GC) are compared with data obtained at a gold electrode. Peaks in the CV curves are assigned to the Pd deposition and its dissolution but also to the formation of palladium hydride (PdHx). The Pd(II) electrochemistry is significantly different in the two DESs, owing to their differing proton donating properties and Pd speciation. By selecting appropriate potentials, Pd electrodeposits were formed and analysed. The kinetics of Pd electrodeposition was followed by chronoamperometry. The i-t curves obtained at a glassy carbon electrode clearly revealed that the metal deposition is controlled by a nucleation and growth process. The current transients, discussed on the basis of existing models, indicate that the deposition occurs via 3D nucleation with diffusion controlled growth.