High current density electrodeposition of silver from silver-containing liquid metal salts with pyridine-N-oxide ligands

New cationic silver-containing ionic liquids were synthesized and used as non-aqueous electrolytes for the electrodeposition of silver layers. In the liquid state of these ionic liquids, a silver (I) cation is coordinated by pyridine-N-oxide (py-O) ligands in a 1 : 3 metal-to-ligand ratio, although in some cases a different stoichiometry of the silver center crystallized out. As anions, bis(trifluoromethanesulfonyl)imide (Tf₂N), trifluoromethanesulfonate (OTf), methanesulfonate (OMs) and nitrate were used, yielding compounds with the formulae [Ag(py-O)₃][Tf₂N], [Ag(py-O)₃][OTf], [Ag(py-O)₃][OMs] and [Ag(py-O)₃][NO₃], respectively. The compounds were characterized by CHN analysis, FTIR, NMR, DSC, TGA and the electrodeposition of silver was investigated by cyclic voltammetry, linear potential scans, scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). With the exception of [Ag(py-O)₃][Tf₂N], which melts at 108 °C, all the silver(I) compounds have a melting point below 80 °C and were tested as electrolytes for silver electrodeposition. Interestingly, very high current densities were observed at a potential of −0.5 V vs. Ag/Ag⁺ for the compounds with fluorine-free anions, i.e. [Ag(py-O)₂][NO₃] (current density of −10 A dm¯²) and [Ag(py-O)₃][OMs] (−6.5 A dm¯²). The maximum current density of the compound with the fluorinated anion trifluoromethanesulfonate, [Ag(py-O)₃][OTf], was much lower: −2.5 A dm¯² at −0.5 V vs. Ag/Ag⁺. Addition of an excess of ligand to [Ag(py-O)₃][OTf] resulted in the formation of the room-temperature ionic liquid [Ag(py-O)₆][OTf]. A current density of −5 A dm¯² was observed at −0.5 V vs. Ag/Ag⁺ for this low viscous silver salt. The crystal structures of several silver complexes could be determined by X-ray diffraction, and it was found that several of them had a stoichiometry different from the 1 : 3 metal-to-ligand ratio used in their synthesis. This indicates that the compounds form crystals with a composition different from that of the molten state. The electrochemical properties were measured in the liquid state, where the metal-to-ligand ratio was 1 : 3. Single crystal X-ray diffraction measurements showed that silver(I) is six coordinate in [Ag(py-O)₃][Tf₂N] and [Ag(py-O)₃][OTf], while it is five coordinate in the other complexes. In [Ag₃(py-O)₈][OTf]₃, there are two different coordination environments for silver ions: six coordinate central silver ions and five coordinate for the outer silver ions. In some of the silver(I) complexes, silver–silver interactions were observed in the solid state.

Publication year:2014

Keywords:Analytical, inorganic & nuclear chemistry

BOF-keylabel:yes

IOF-keylabel:yes

BOF-publication weight:3

CSS-citation score:2

Authors from:Higher Education

Accessibility:Open