Influence of anodic treatment of a copper-nickel alloy in a eutectic mixture of choline chloride and urea on the surface morphology and electrocatalytic behavior in the hydrogen evolution reaction

Abstract

For the first time, we investigated the process of potentiostatic anodic treatment of the surface of a copper (≈55%)-nickel alloy in a eutectic mixture of urea and choline chloride (reline), which is a typical representative of a new generation of ionic liquids, deep eutectic solvents. The anodic behavior of the alloy in the used solvent was characterized by cyclic voltammetry, and the nature of the electrochemical dissolution reactions of individual components of the alloy corresponding to several anodic current waves registered in voltammograms was determined. It was established that the anodic dissolution of the alloy occurs under conditions of salt surface passivation due to the formation of a layer of poorly soluble products of the electrode reaction. It was shown that under conditions of prolonged (150 min) potentiostatic polarization of the alloy in reline for various values of the electrode potential (in the range from 0.1 to 1.7 V relative to the Ag reference electrode), the chemical composition of the surface remained unchanged (i.e., there was no selective etching of individual components of the alloy), but an evolution of surface morphology patterns was observed, the specific type of which depended on the value of the applied potential. Anodic treatment of the Cu-Ni alloy in the reline solvent at any of the investigated anodic potentials led to an increase in the surface roughness coefficient, and electrochemical polishing did not occur. Analysis of kinetic data related to the hydrogen evolution reaction on the surfaces of reline-treated copper-nickel alloys in a 1 M NaOH aqueous solution showed a significant increase in exchange current density. This indicates enhancement of electrocatalytic activity compared to the untreated surface. The observed effect is likely associated with an increase in the true surface area of the alloy available for electrochemical reaction and an increase in the surface concentration of electrocatalytic sites resulting from the anodic dissolution of the alloy. The obtained results can be used in the development of highly efficient and relatively inexpensive electrocatalysts for hydrogen energy.