Cobalt(ii) liquid metal salts for high current density electrodeposition of cobalt

Abstract

Cobalt(II)-containing ionic liquids were synthesized using N,N-dimethylformamide, N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAc), pyridine-N-oxide (py-O), 1,10-phenanthroline (phen), ethylenediamine (en) and dimethylimidazolidinone (DMI) as ligands. The weakly coordinating bis(trifluoromethylsulfonyl)imide (Tf₂N⁻) was used as a counter anion. Several compounds had a melting temperature below 100 °C, and the compound [Co(DMAc)₆][Tf₂N]₂ was liquid at room temperature, with a viscosity of only 18 mPa s at 80 °C. Several compounds were recrystallized to give high quality single crystals and their crystal structures were determined. EXAFS measurements were performed on [Co(DMAc)₆][Tf₂N]₂ at different temperatures and it was observed that the [Co(DMAc)₆]²⁺ ion partially dissociated at higher temperatures, which explains the temperature-dependent color change (thermochromism). The electrochemical properties of the compounds with the lowest melting points were also investigated. Adherent, crack-free metallic cobalt layers could be electrodeposited from [Co(DMAc)₆][Tf₂N]₂, [Co(DMI)₆][Tf₂N]₂ and [Co(NMP)₆][Tf₂N]₂. From the first two, black deposits consisting of micrometer-sized needles were obtained, whereas the latter resulted in a dull grey cobalt layer consisting of micrometer-sized cobalt spheres. The Co(III)/Co(II) redox couple was not found to occur in any compound with an O-donor ligand, but the Co(III)/Co(II) redox couple was found to be quasi-reversible for [Co(phen)₃][Tf₂N]₂ dissolved in [BMP][Tf₂N].