Application of a deep eutectic solvent to prepare nanocrystalline Ni and Ni/TiO2 coatings as electrocatalysts for the hydrogen evolution reaction

The paper reports the electrochemical deposition of nanocrystalline nickel and composite nickel-titania films as effective electrocatalysts for the hydrogen evolution reaction. To produce the composite Ni/TiO₂ electrodeposits, a plating bath based on a deep eutectic solvent, a novel kind of ionic liquids, was used for the first time. The electrolyte contained ethaline (a eutectic mixture of choline chloride and ethylene glycol), 1 M NiCl₂⋅6H₂O and the addition of extra water (3, 6, 9 mol dm⁻³). Titania dispersed phase was introduced into the electrolyte as nanopowder Degussa P 25 (0–10 g dm⁻³). It was shown that the introduction of extra water to the plating bath allowed appreciably increasing the content of TiO₂ phase in the coating (from ca. 2 to 10 wt%). The effects of electrolysis conditions on the TiO₂ content in the coatings, surface morphology and microstructure were determined. The results of voltammetry measurements showed that the Ni and composite Ni/TiO₂ coatings electrodeposited from the plating electrolyte based on a deep eutectic solvent exhibit improved electrocatalytic properties towards the hydrogen evolution reaction as compared with deposits obtained from commonly used aqueous electrolytes. The mechanism of the hydrogen evolution reaction on the Ni and composite Ni/TiO₂ coatings is a combination of Volmer-Heyrovsky reactions. The introduction of TiO₂ particles into the nickel matrix results in the acceleration of the hydrogen evolution reaction. An improved catalytic activity of Ni/TiO₂ composites towards the hydrogen evolution reaction can be associated with the presence of titanium-containing redox couples on the surface.