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Issue 4, 2014
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High current density electrodeposition of silver from silver-containing liquid metal salts with pyridine-N-oxide ligands

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Abstract

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)3][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.

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

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Publication details

The article was received on 02 Sep 2013, accepted on 21 Oct 2013 and first published on 24 Oct 2013


Article type: Paper
DOI: 10.1039/C3DT52416G
Citation: Dalton Trans., 2014,43, 1589-1598
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    High current density electrodeposition of silver from silver-containing liquid metal salts with pyridine-N-oxide ligands

    J. Sniekers, N. R. Brooks, S. Schaltin, L. Van Meervelt, J. Fransaer and K. Binnemans, Dalton Trans., 2014, 43, 1589
    DOI: 10.1039/C3DT52416G

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