Liquid metal salts for the electrodeposition of metal layers

Ionic liquids are interesting electrolytes for electrochemical applications due to their wide liquidus range and wide electrochemical window, allowing metals to be electrodeposited from them that cannot be deposited from aqueous solutions. However, solubilities for many metals are low in ionic liquids, especially ionic liquids with weakly coordinating anions. Moreover, viscosities of ionic liquids are significantly higher than those of aqueous solutions. Therefore, liquid metal salts were developed. Liquid metal salts are ionic liquids in which a redox-active metal is incorporated into the cation of the ionic liquid. This way, very high metal concentrations are obtained. Liquid metal salts are excellent electrolytes for the electrodeposition of metals. Because the metal is
incorporated into the cation, it will electromigrate towards the cathode during deposition, allowing a better mass transport to counteract the higher viscosity of the ionic liquid. Since the cathodic reaction is the deposition of metal, there are no issues with the cathodic decomposition of the ionic liquid components. In this work, the concept of liquid metal salts was further expanded to
silver(I), nickel(II), manganese(II) and cobalt(II) compounds. Except for silver(I), which is two-coordinate, all compounds consist of a metal(II) cation that is octahedrally coordinated by six ligands. A range of different ligands is used in order to lower the melting point of the compounds. Several different N-alkylimidazoles were considered as ligands, but also common organic solvents such as dimethylsulfoxide, diglyme and triglyme and pyridine-N-oxide were used. The alkyl side chain on the N-alkylimidazole ligands was varied in length from methyl to dodecyl. The different counter anions that were used are bis(trifluoromethanesulfonyl)imide (bistriflimide), trifluoromethanesulfonate (triflate), methanesulfonate (mesylate) and nitrate. All newly synthesized liquid metal salts were characterized by CHN analysis, FTIR, DSC, TGA, viscosity measurements and, if good quality single crystals were obtained, single crystal X-ray diffraction was performed. Not only the chemical synthesis and characterization was performed, the compounds with the lowest melting points were tested for the electrodeposition of silver, nickel, manganese and cobalt. In all cases, reduction of the metal cations occurred without the presence of a limiting current during cyclic voltammetry experiments. This means that the system is never diffusion controlled, indicating a high mobility of the metal cation during electrodeposition. The original goal of the research was to build a library of several liquid metal salts using different metals, ligands and anions, and to test the compounds with the lowest melting points for electrodeposition. It was found however that for certain six-coordinate metal cations, no deposits were obtained but electrochemical metal nanoparticle synthesis was observed, as was evidenced by transmission electron microscopy measurements. This interesting observation was not observed for every liquid metal salt, but only occured when a certain ligand-anion combination was used. This was further investigated and a mechanism for the nanoparticle formation from nickel(II), manganese(II) and cobalt(II) liquid metal salts was formulated.