Electrodeposition from Liquid Metal Salts

Ionic liquids are ideal for performing electrochemical reactions which cannot be carried out in aqueous solutions, due to their wide electrochemical window, intrinsic electrical conductivity and large liquidus range. One drawback is the limited solubility of simple inorganic metal salts, such as chlorides and sulfates. The lack of solubility leads to a correspondingly low limiting current density. This disadvantage can be circumvented by making the metal ion a part of the ionic liquid in a so-called Liquid Metal Salt. A general formula is e.g. [M(L)_x][A], with M a metal ion, L a neutral ligand and A a counter anion.

By making the metal ion a structural part of the ionic liquid, the concentration of metal ions can be as high as 3 mol dm^-3 without the need to add a salt as metal source. Hence, even at 25 A dm^-2 which is a large current in unstirred solutions, no sign of the decomposition of the ligand L or anion A could be detected in the metal deposits. These high current densities are explained by the fact that the electrochemical experiments are performed in a pure ionic liquid that acts as both the supporting electrolyte and the electrochemically active species, which is therefore present in a high concentration. Additionally, these Liquid Metal Salts do not have a cathodic decomposition potential since their cathodic limit is the metal reduction. The use of solution additives, such as thiourea or benzotriazole, was investigated by Raman spectroscopy for their influence on the deposit morphology.

 So far we have synthesized and tested Liquid Metal Salts based on copper, silver, lithium, zinc, cobalt, nickel and palladium. Investigated ligands are (o.a.) acetonitrile, alkylimidazoles, py-N-O and amines. These cations were combined with anions such as bistriflimide, triflate or nitrate.

These liquid metal salts show great promise in a wide variety of electrochemical applications.