On kinetics of leaching process and metals (Ni, Co) recovery from Ni-MH spent batteries using a deep eutectic solvent based on choline chloride

Deep eutectic solvents (DESs) based on choline chloride have been demonstrated to be effective as a leaching agent by their capability of dissolving metal oxides. However, the low solubility and slow kinetics of the DESs hinder their application in the extraction of metals from different sources. In this paper, two approaches for the recovery of cobalt (Co) and nickel-cobalt (Ni-Co) alloys from Nickel–Metal Hydride (Ni-MH) spent batteries using the DES as a leaching agent have been studied. From Ultraviolet–visible spectroscopy (UV–Vis), Cyclic Voltammetry (CV), and Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES) measurements, a faster approach to selectively extract cobalt from the eutectic mixture has been proven. Electrochemical behavior and electrodeposition of Ni and Co metals from the cathodic powder of spent Ni-MH batteries dissolved in the DES solution have also been performed by electrochemical techniques such as CV and chronoamperometry (CA). Results from electrochemical techniques have verified the extractability of Ni and Co from DES liquor. Voltametric traces also offered a simple way to determine qualitatively and quantitatively the concentration of metallic ions dissolved in the DES. By analyzing the potentiostatic current density transients recorded from the DES liquor, it was possible to explain the mechanisms and kinetics of formation of the recovered materials on the glassy carbon electrode surface using a model composed of a 3D nucleation and diffusion-controlled growth of bimetallic nuclei on the growing surfaces. Scanning Electron Microscopy (SEM) images of the glassy carbon electrode (GCE), after the potentiostatic electrodeposition, showed evidence of the formation of Ni and Co nuclei distributed all over the substrate surface. Meanwhile, Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Diffraction (XRD) analyses confirmed the successful recovery of Co and Ni-Co alloys separately using electrochemical means.