Cobalt nanostructures electrochemical nucleation from a type-IV deep eutectic solvent based on CoCl 2 ·6H 2 O and ethylene glycol

Abstract In the present work, electrodeposition and electronucleation of cobalt on a glassy carbon electrode from a type-IV deep eutectic solvent, mixture of CoCl 2 ·6H 2 O and ethylene glycol, were studied. It was found, by cyclic voltammetry, that cathodic efficiencies of Co electrodeposition were high for both cases without and with supporting electrolyte, yielding their values of 95 % and 96 %, respectively, which helped to consider negligible contribution of side reaction during the electrodeposition process. The selection of used supporting electrolyte concentration was based on Gibbs free energy for nucleation criterion. The presence of supporting electrolyte improved the voltammogram’s shape and reduction current density by controlling the system under diffusional regime. From chronoamperometry, it was established that the instantaneous nucleation was confirmed to be predominant at lower deposition potentials (i.e., E < -0.90 V), while progressive nucleation was governed at higher potentials (i.e., E > -0.92 V) during the Co electrodeposition. Current density transients of the Co(II) system could be analyzed by the three-dimensional nucleation and diffusion-controlled growth model of Co metallic nuclei, to derive kinetic parameters such as frequency of nucleation, A , number of active sites, N 0 , and diffusion coefficient, D, of the Co (II) in type-IV DES system. Moreover, excellent quality of Co electrodeposits with different morphological features was obtained from the type-IV eutectic mixture and it was even better without the use of the supporting electrolyte, for the same applied potential and electrodeposition time. Surface characterization methods, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy, have revealed the presence of high purity electrodeposited Co nanoparticles on the electrode surfaces. Both cases, with and without supporting electrolyte, produced cobalt electrodeposited nanostructures with the hexagon-close-packed crystal lattice, which was verified by X-ray measurements.