ELECTRODEPOSITION OF NANOCRYSTALLINE NICKEL-IRON ALLOY FROM AN ELECTROLYTE BASED ON A NEW TYPE OF IONIC LIQUIDS – DEEP EUTECTIC SOLVENT

Abstract

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.