Mechanism and kinetics of chromium electrochemical nucleation and growth from a choline chloride/ethylene glycol deep eutectic solvent

The nucleation and growth of chromium onto glassy carbon electrode from Cr(III) ions dissolved in a deep eutectic solvent, composed by a mixture of choline chloride and ethylene glycol were studied. From both potentiostatic and potentiodynamic methods, it was evidenced that Cr electrodeposition, from Cr(III) ions, takes place through two stages: firstly, from Cr(III) to Cr(II), forming soluble species, followed by reduction from Cr(II) to Cr(0). It was found that the Cr electrodeposition kinetics and mechanism are different depending on whether it is occurring from Cr(III) or from Cr(II) ions. From analysis of the potentiostatic experimental current density transients, new models were proposed to explain the chromium nucleation and growth in these initial conditions. These models involve contributions to the overall current due to: an adsorption process, diffusion of Cr(III) ions to the electrode surfaces to form Cr(II) ions, Cr 3D nucleation and diffusion-controlled growth and residual water reaction over the growing surfaces of the Cr nuclei. The proposed models help determining the charge percentage due to each individual contribution to the total process. Scanning electron microscopy and energy dispersive spectroscopy were used to determine the morphology and the elemental composition of the deposit respectively.