Abstract
This study reports the patterns of electroplating a composite coating containing phases of nanocrystalline metallic nickel and cerium dioxide from an electrolyte based on a new generation of ionic liquids, deep eutectic solvents (DESs). Currently, the use of DESs is an extremely promising direction for the electrochemical deposition of electrocatalytic coatings for electrochemical energy, particularly for "green" hydrogen energy. The electrolyte for deposition contained a liquid eutectic mixture of choline chloride and urea (the so-called "reline", a typical representative of DESs), in which nickel (II) chloride (0.1 mol/dm³) and cerium (III) chloride (0.2–0.4 mol/dm3) were dissolved. According to the results of energy-dispersive X-ray analysis and XRD investigations, the composite coating contains approximately 12–16 at.% cerium, presumably in the form of cerium dioxide. A reaction scheme for the formation of the composite nanocoating, which includes a combination of chemical and electrochemical stages, is proposed. It was found that the incorporation of cerium oxide leads to a significant increase in electrocatalytic activity (compared to nickel coating) in the cathodic hydrogen evolution reaction and anodic oxygen evolution and urea oxidation reactions. It is hypothesized that the observed enhancement in electrocatalytic effect is due to the formation of additional active catalytic sites on the surface containing cerium in different oxidation states (e.g., Ce(+4)/Ce(+3)), which can act as electron carriers in both cathodic and anodic reactions. A significant advantage of this composite is its bifunctionality as an electrocatalyst, meaning it can be used for both cathodic and anodic processes. The obtained results can be used in the development of new high-efficiency processes for the electrochemical synthesis of "green" hydrogen.
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