Anodic surface treatment of nickel in eutectic ionic liquids based on choline chloride for electrochemical polishing and enhancement of electrocatalytic activity in hydrogen evolution reaction

Abstract

The paper reports the impact of anodic potentiostatic treatment of nickel in two representatives of a new type of eutectic ionic liquids (deep eutectic solvents), ethaline and reline, which are eutectic mixtures of choline chloride with ethylene glycol and urea, respectively. The influence of anodic treatment on surface morphology, roughness coefficients, and electrocatalytic activity towards the hydrogen evolution reaction is characterized. It is demonstrated that the current densities of nickel anodic dissolution in reline are approximately an order of magnitude lower than in ethaline under all other identical conditions. Significant differences in the kinetics of nickel anodic dissolution and passivation during anodic polarization in ethaline and reline have been established, which may be attributed to both a substantial difference in the viscosity of these solvents and differences in the chemical nature and composition of the ions present in them. It is found that anodic treatment in ethaline, at certain potentials, results in electrochemical polishing of the surface, confirmed by a decrease in measured roughness coefficients, while anodic treatment in reline does not allow effective electropolishing and only surface etching (increase in roughness coefficients) is observed. Anodic potentiostatic treatment of nickel in both investigated deep eutectic solvents at specific electrode potential values significantly enhances the electrocatalytic activity of the surface towards the hydrogen evolution reaction in an alkaline environment. This finding can be utilized in the development of electrocatalytic materials for the electrolytic synthesis of green hydrogen.