Effects of water content on metallic Co electrodeposition process for preparing nanocrystalline Co coatings in ethylene glycol solution

The hygroscopic nature of ethylene glycol (EG) can induce spontaneous fluctuations in water content throughout anhydrous EG media. However, the residual water content may affect metal electrodeposition process in the EG media. Therefore, this study elucidates water content effect on Co(II) reduction process and cobalt electrocrystallization mechanism. Compared with those in anhydrous EG, the Co(II) electrochemical reduction mechanism and the metallic cobalt electrocrystallization mechanism in EG containing various water contents remain unchanged. Co(II) reduction follows one-step irreversible reaction. However, varying water content can modify some reduction characteristics such as nucleation overpotential, peak potential, peak current density, and kinetic irreversibility in the EG-H 2 O system. Cobalt nucleation follows a typical diffusion-controlled mechanism and demonstrates the three-dimensional instantaneous growth behavior. Additionally, adding water to anhydrous EG can alter the Co(II) transport mechanism and interface reaction kinetics, and lead to the occurrence of adsorption phenomena that do not occur in anhydrous EG. Importantly, with increasing water content, the charge transfer resistance of Co(II) decreases gradually. The kinetic control can transition from electrochemical polarization at low overpotentials to mixed diffusion/charge-transfer control at high overpotentials. Water content exhibits a negligible effect on the composition of cobalt deposits, but a significant effect on their microstructure. These findings collectively demonstrate that the role of water in the cobalt electrodeposition process cannot be ignored in EG-based electrolytes, and controlling the water content can precisely regulate the cobalt coatings microstructure. This work provides a simple and efficient method for precisely controlling material microstructure. Section Graphical The alternative text for this image may have been generated using AI.