Ag Electrodeposition from Silver Solvate Ionic Liquid
Authors: Mori Takahiro , Tsuchida Naoki , Kitada Atsushi , Fukami Kazuhiro , Murase Kuniaki 
Published: December 2020

Ag electrodeposition technics have been used in many fields, such as jewelry, tableware, lead frame of semiconductors, and aerospace materials. The practical Ag plating aqueous baths contain highly toxic cyanides to suppress dendritic growth. Toward the realization of green industry, cyanide-free alternative baths are needed. Ionic liquids (ILs) attract attention as green solvents with less volatility and less flammability and also as functional liquids which enable non-dendritic Ag electrodeposition without cyanides [1,2]. However, there have still been few studies on “silver solvate IL baths”, composed of Ag(I)-oligoether cationic complex and counter anion like other solvate ILs [3-5]. Silver solvate ILs are expected to offer an advantage in mass transport due to high Ag(I) concentration. In this study, we synthesized glyme-based silver solvate IL and investigated Ag electrodeposition.
An equimolar mixture of glyme (G2, G3, G4, or G5) and silver bis(trifluoromethylsulfonyl)amide (AgTf2N) resulted in a liquid-state only for G5 at room temperature, which became a colorless and transparent liquid using activated carbon. Electrochemical properties of Ag(G5)Tf2N were examined using Pt plate as working electrode (WE) and Ag wires as reference electrode (RE) and counter electrode (CE). Cyclic voltammogram demonstrates distinct redox waves of Ag plating and stripping, evidencing that Ag(I)-G5 complexes are an electroactive species and this solvate IL can work as an Ag-plating bath. Reductive wave does not show a peak owing to diffusion limitation even though cathodic potential limit is extended up to –3.0 V vs. Ag. The cathodic current density reaches –7.0 mA cm–2 at –3.0 V. For the previously-reported Ag deposition from conventional ILs, poor solubility of salts caused the low current density of deposition. Thus, the large reductive current density from Ag(G5)Tf2N is quite advantageous.
A galvanostatic electrodeposition at 0.08 mA cm–2 was performed to investigate the morphology of Ag deposits. Figs. 1c,d demonstrates that the obtained electrodeposits were quite dense and smooth. For comparison, we plated Ag from a diluted AgTf2N-G5 solution (Figs. 1e,f). Unlike the case of Ag(G5)Tf2N, the deposits were coarse and faceted. It is also notable that Tf2N ions have a certain leveling effect in Ag electrodeposition [6]. Therefore, the extremely high concentration of Ag(I) ion and Tf2N– ion is an important factor to form the dense and smooth Ag deposits. Suppose the high concentration is the only determinant, other electrolytes with a high AgTf2N concentration such as aqueous solutions could give dense and smooth Ag deposits. We prepared diluted and saturated AgTf2N aqueous solutions and conducted Ag electrodeposition. From the saturated aqueous solution, however, the shape of Ag has no difference, although from the diluted aqueous solution the morphology of Ag is quite similar to that from the diluted G5 solution. Consequently, not only a high AgTf2N concentration but also solvent is the factor for realizing highly smooth Ag deposits (e.g. difference of Ag(I)-solvent complex, physicochemical property of electrolyte, or electrode/electrolyte interface). This work offers a guideline to design Ag plating baths that have extremely high Ag(I) concentration and can electrodeposit dense and smooth Ag.
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Figure 1