Electrodeposition of nickel–cobalt alloys from metal chloride-l-serine deep eutectic solvent for the hydrogen evolution reaction

Abstract

Deep eutectic solvents provide a new platform for metal electrodeposition as the intermolecular forces of the solvent can be tuned. Here, metal chloride (CoCl₂ and/or NiCl₂) and L-serine deep eutectic solvents are, for the first time, utilized as electrolytes for the electrodeposition of metallic films (Ni, Co, and Ni–Co alloy). The electrochemical properties of these deep eutectic solvents are investigated, and at low overpotential, the electrodeposition follows the three-dimensional instantaneous nucleation and diffusion-controlled growth mechanism. The deposited Ni–Co film exhibits the highest activity towards the hydrogen evolution reaction (HER), with an overpotential of 62 mV in 1 M KOH to reach 10 mA cm⁻² HER current densities. The larger number of electrochemically active sites exposed and the electron interaction between Ni and Co promote the HER kinetics, as evidenced by the lower Tafel slope value, decreased charge transfer resistance, and lower apparent activation energy. The Ni–Co thin film sustains a long-term galvanostatic HER in 1 M KOH. Characterization shows that crystalline metal hydroxides and oxides are formed after the long-term HER test.