Features of kinetics and mechanism of Ni(II) ion electroreduction in the course of electrodeposition of nickel and Ni–TiO2 composites from a deep eutectic solvent

Abstract

Kinetic characteristics of the nickel ion discharge were investigated for the first time from electrolytes based on a new kind of ionic liquids, deep eutectic solvents. The study was carried out using electrolytes based on ethaline, a liquid eutectic mixture of choline chloride choline and ethylene glycol in a molar ratio of 1:2, respectively, in which 1 М NiCl2·6H2O was dissolved. It was shown that the apparent transfer coefficients for electrochemical process in the system Ni(II)/Ni(0) in ethaline were equal to α=0.309 and β=0.255 for anodic and cathodic reactions, respec-tively. It is difficult to interpret these values in the framework of known criteria of consecutive electrochemical pro-cesses. In order to explain this phenomenon, a theoretical conception was used, that was earlier developed by Gileadi. According to that conception, electric charge during metal electrodeposition is transferred through the electrode interface by metal ions, but not by electrons. Therefore, potential barrier in a double electrical layer be-came very asymmetric (i.e. the apparent transfer coefficient deviates from 0.5). It was found that the introduction of TiO2 particles (0–10 g dm–3) into the electrolyte based on ethaline results in declaration of electrochemical re-duction of nickel (II) ions due to a partial blocking of the electrode surface by adsorbed colloidal particles of titani-um dioxide.

Keywords

• kinetics and mechanism
• electrochemical reaction
• nickel ions
• deep eutectic solvent
• transfer coefficient
• energy barrier
• electrodeposition of composites
• titanium dioxide
• adsorption