Coral-Shaped NiO@Graphene Composite Derived from Spent Zinc–Carbon Batteries: Geometric Structure and Electrochemical Studies of Supercapacitors

An easy, low-cost, and environmentally safe method for preparing graphene nanosheets (GNs) from carbon electrodes in waste zinc–carbon batteries is presented, which employs an electrochemical exfoliation process using table salt (also known as natural sea salt) as an electrolyte under 5 V from a recycled cell phone charger. The GNs are then creatively used as a framework for a two-dimensional nanostructure and conductive backbone, while nickel acetate tetrahydrate is used as a precursor for shrinking and bundling to create the coral flower structure of the graphene/NiO (GN/NiO) composite by a simple calcination process. The coral GN/NiO exhibits a beautiful nanoflower structure, which is clearly confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, the coral GN-NiO composite shows potential as a candidate for supercapacitor electrodes. For a three-electrode measurement cell in 3 M KOH electrolyte, the GN-NiO material exhibits a maximum specific capacitance of 140.7 F g–1, which is higher than that of GNs (97.5 F g–1) at the same current density of 0.5 A g–1. Additionally, density functional theory (DFT) is employed to investigate the structure and electronic properties of the coral flower structure of the graphene/NiO (GN/NiO) structure. Using DFT, the electronic properties of the structure, including the band structure, density of states, and charge transfer between the graphene and NiO, as well as the stability of the structure, are calculated.