Ni–La electrocatalytic coatings were electrodeposited from electrolytes based on a eutectic mixture of choline chloride and ethylene glycol (the so-called deep eutectic solvent "ethaline") containing dissolved NiCl2 and LaCl3 salts. It was shown that in this case, nickel alloys containing up to approximately 1.75 wt.% lanthanum were formed. An increase in the cathodic current density and the content of La(III) salt in the solution contributed to a higher content of lanthanum in the electrodeposits. The presence of a lanthanum(III) salt in the electrolyte led to a noticeable leveling of the surface microprofile. The electrocatalytic activity of the deposited coatings towards the hydrogen evolution reaction was evaluated by linear voltammetry in an aqueous solution of 1 M NaOH at a temperature of 298 K. It was found that the polarization of hydrogen evolution decreased, and the exchange current density increased with an increase in the lanthanum content in the coating. For example, the calculated hydrogen evolution exchange current density is 4.2610–5 A cm–2 and 1.0310–3 A cm–2 for a lanthanum-free nickel deposit and a nickel-based coating containing 1.75 wt.% La, respectively. The increased electrocatalytic activity observed when lanthanum was introduced into the nickel matrix can be attributed to both the synergistic interaction of the nickel and lanthanum components of the alloy (as previously described, the catalytic effect resulting from the hypo-hyper-d-electron interaction of transition metals) and the presence of surface active sites with lanthanum in different oxidation states (La(III)/La(II)), which can serve as electron carriers. The significant electrocatalytic effect observed when nickel is doped with lanthanum during deposition from an electrolyte based on DES allows us to consider such electrode materials as very promising for use in the electrolytic synthesis of "green" hydrogen.

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This paper concerns electrodeposition of coatings from electrolytes containing NiCl2 (0.1 mol dm–3) and CeCl3 (0.1–0.4 mol dm–3) salts dissolved in a deep eutectic solvent, ethaline (ethylene glycol+choline chloride). The electrochemical deposition was carried out at cathodic current densities from 1 to 5 mA cm–2 and temperatures from 50 to 900C. It was shown that micromodification of the chemical composition of coatings with cerium occurred (no more than ~1–2.65 wt.% Ce), and the content of cerium in the coating was increased with an increase in the concentration of the Ce(III) salt in the plating electrolyte, an increase in the current density, and a decrease in the temperature. It was shown for the first time that the Ni-based electrodeposits micromodified with cerium exhibited an enhanced electrocatalytic activity in the reaction of hydrogen evolution in an alkaline medium (a decrease in the cathodic polarization by ~200–250 mV). Electrocatalytic activity correlated with the content of cerium in the coating. The enhancement of the efficiency of electrocatalysis resulting from micromodification of the chemical composition of the coatings with cerium was explained within the framework of the well-known concept of the synergistic effect of hypo-hyper-d-electronic combinations of transition metals. The use of electrolytes based on deep eutectic solvents provides additional opportunities for flexible control of the composition of deposited coatings and their electrocatalytic behavior via changes in the concentration of electrolyte components and electrolysis conditions in relatively wide intervals.

The paper reports the main features of electrochemical deposition of nickel-iron alloy from electrolyte based on the eutectic mixture of choline chloride and ethylene glycol, which is a typical representative of a new type of ionic liquids, deep eutectic solvents (DES). It is found that the iron content in the deposited alloy increases with both increasing the applied cathode current density and increasing the concentration of iron ions in the electrolyte and the introduction of water additives. Thus, variation in the current density and the concentration of water additive in electrolytes based on DES is the factor of influence on the kinetics of partial electrode reactions, and hence on the composition and properties of the coating. It is shown that it is possible to deposit uniform coatings with iron content up to 10–13% from the investigated electrolyte containing water additive (up to 10 wt.%) at the deposition current density not exceeding 1–1.2 A/dm2. The current efficiency of the alloy deposition is close to the theoretical value (97–99%), i.e. the electrodeposition is practically not complicated by electrochemical processes involving components of a deep eutectic solvent. The surface of pure nickel deposited from an electrolyte based on DES without additional water is quite uniform with a small number of defects, pitting and small pores, while coatings deposited from the electrolyte containing water additives are characterized by granular surface morphology with many asymmetric spheroidal crystallites. The electrodeposition of a nickel-iron alloy yields the surface built of irregular spheroids that overlap and form a scaly-like type of surface morphology. Nickel-iron electrolytic coatings containing up to ~7% Fe, formed from the ethaline-based electrolyte, are nanocrystalline solutions of iron in nickel with a face-centered cubic nickel lattice and an average nanocrystallite size of about 6–15 nm. Nickel-iron alloy coatings electrochemically deposited under the conditions established in this work may be considered as promising electrode materials for the creation of new cheap and highly efficient electrocatalysts for water electrolysis in hydrogen energy.