Electrochemical and Spectroscopic Studies of Zinc Oxide in an Eco-Friendly Deep Eutectic Solvent for Zn Electrodeposition.

Zinc oxide (ZnO) is a waste produced from end-of-life Zinc-air batteries. Recycling of these spent batteries is important to reduce the pressure on primary zinc sources and to mitigate environmental pollution. To address the recyclability of waste ZnO from different sources, here we have studied ZnO dissolution and Zn electrodeposition in deep eutectic solvents (DES) of betaine hydrochloride (Betaine-HCl) and formic acid (FA) using spectroscopic and electrochemical techniques. Additionally, the effect of water in the DES on the dissolution of ZnO and its electrochemical and spectroscopic behaviour was also studied. Solubility of ZnO in DES was found to be 95 g l^-1 which decreased slightly to 85 g l^-1 in the DES containing 10% water. Fourier transform infrared and Raman studies revealed that ZnO dissolved in DES by forming a [ZnCl₃FA]^- complex. Electrochemical studies showed that Zn deposit morphology and structure varied with water content in the DES. A uniform Zn deposit was achieved in ZnO-DES mixture whereas the presence of water gave a porous morphology. Thus, this study revealed an easy and eco-friendly route to recycle ZnO.