Study of electrode processes and deposition of cobalt thin films from ionic liquid analogues based on choline chloride

The cathodic processes of cobalt ion in ionic liquids consisting in binary mixtures of choline chloride (ChCl) with urea, ethylene glycol, malonic acid or oxalic acid were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The as-received hexahydrate cobalt chloride has been the precursor of cobalt ionic species. It was found that the reduction mechanism of Co2+ in ChCl-urea and ChCl-ethylene glycol is a quasi-reversible (or with high degree of irreversibility at high scan rate) and diffusion controlled process, whereas in ChCl-carboxilic acid solvents the cathodic process of Co2+ is overlapped with reduction of protons. By using Pt and vitreous carbon electrodes, one has computed the value for the Co2+ diffusion coefficient in ChCl-urea at 80 °C; in comparison with values obtained in aqueous solutions, this is up to two orders of magnitude lower. The paper also describes preliminary experiments concerning electrodeposition of cobalt thin films on copper substrate from the four ionic liquid analogue solvents containing choline chloride. The crystalline morphology and structural constitution of Co deposits were examined by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Good results regarding the quality and cobalt content in films were obtained for Co deposits using ChCl-urea or ChCl-ethylene glycol mixtures as solvents. The behaviour of Co films grown from these media was typical for a ferromagnetic material.