ZnxCo(1−x) coatings from choline chloride-ethylene glycol deep eutectic solvent as electrocatalysts for hydrogen evolution reaction

Abstract

Hydrogen has emerged as a clean and renewable energy and its production by water splitting is a promising production route. However, to meet the demand on a commercial scale, research focusing on more efficient electrocatalysts is necessary. In this work, new findings on Zn, Co and Zn-Co coatings produced in deep eutectic solvent based on choline chloride (ChCl) and ethylene glycol (EG) are reported. Varying the concentrations of Zn2+ and Co2+ ions in 1ChCl:2EG, crystalline electrodeposits with fine control of composition and morphology were obtained, and which present different reactivity to electrocatalyze the hydrogen evolution reaction (HER) in alkaline medium. The performance of metallic coatings is influenced by temperature, due to changes in viscosity, ionic diffusion coefficient and charge transport in the electrolyte. The results also revealed that increasing the Co content in the coatings, changes occur in the morphological organization, stability, and electrode area, which positively influence the hydrogen production. Among the different coatings tested (Zn, Co, Zn96-Co4 and Zn3-Co97), Zn3-Co97 was the most promising in terms of Tafel coefficient (108 mV dec–1), exchange current density (8.57 × 10−6 A cm−2) and overpotential estimated for HER (333 mV at 10 mA cm−2) in 1 mol L–1 KOH at 298.15 K, although the other materials also showed electrochemical advantages over the unmodified Cu substrate. The reported data also reiterate the great electrochemical potential of metallic coatings for water splitting and complement the growing energy demand for hydrogen gas.