2D/2D Nanocomposite of Graphene-Supported Cu-Doped Co-MOF for High-Performance Supercapacitor Applications

Carbon nanomaterials (CNMs) and metal–organic frameworks (MOFs) with high specific surface areas and various functional groups are promising electrode materials for supercapacitor applications. Here, different structural models of simple and functionalized CNMs, including graphene, carbon nanotubes, and graphene quantum dots, and plain and doped zeolitic imidazolate frameworks (ZIF-9), were computationally designed. The electronic properties of the optimized structures were further studied. Among the investigated categories, graphene and ZIF-9 (CoCu) showed the shortest bandgap, and their composite was chosen for charge storage demands. Therefore, the optimum structure of ZIF-9 (CoCu) was synthesized by the hydrothermal method and characterized by different techniques, such as Fourier transform infrared and energy-dispersive X-ray spectroscopies. The ratio of Co/Cu was found to be 7:1 by inductively coupled plasma-optical emission spectroscopy. Field-emission scanning electron microscopy and transmission electron microscopy determined a two-dimensional (2D) morphology for the as-synthesized MOF structure. According to the X-ray diffraction patterns, the doping of copper ions has shrunk the crystal structure of ZIF-9 (Co), and the crystallite size was reduced to half in ZIF-9 (CoCu). X-ray photoelectron spectroscopy has reported the coexistence of Co2+/3+ and Cu0/2+ species in the sample, as well as their bonding with the heteroatoms of benzimidazole ligand. The specific surface area of the nanocomposite was calculated to be 134.39 m2 g–1 using the Brunauer–Emmett–Teller (BET) method. A cyclic voltammetry study at the nickel-foam-supported 2D/2D nanocomposite electrode, i.e., NF/Graphene/ZIF-9 (CoCu), revealed a battery-type charge storage mechanism in a 3 M KOH electrolyte. A negligible charge-transfer resistance of 2 Ω was obtained by electrochemical impedance spectroscopy. The prepared supercapacitor electrode provided a specific capacitance of 2668 F g–1 at the current density of 1 A g–1 and 100% capacitance retention after 1200 consecutive galvanostatic charge–discharge cycles.