Synergistic integration of graphene and ZnMn₃O₇ via green hydrothermal route for high-capacitance electrodes

The development of sustainable, high-performance electrode materials is critical for next-generation supercapacitors. Herein, we report a green and eco-friendly hydrothermal synthesis of ZnMn₃O₇ and Graphene@ZnMn₃O₇ nanocomposites using neem (Azadirachta indica) leaf extract as a natural reductant and stabilizer. X-ray Diffraction (XRD) analysis confirms successful phase integration with an optimized crystallite size of ~20.9 nm and reduced dislocation line density (2.29 × 1015 m−2) for the composite. Fourier Transform Infrared Spectroscopy (FTIR) reveals strong Mn–O–Zn bonding and effective graphene coupling through CC vibrations. Scanning Electron Microscopy (SEM) images show a porous, flake-like interconnected morphology that suppresses agglomeration and improves electrolyte accessibility. Cyclic Voltammetry (CV) measurements show enlarged enclosed areas with mixed capacitive–diffusion-controlled charge storage behavior. Galvanostatic Charge–Discharge (GCD) results reveal that the Graphene@ZnMn₃O₇ composite delivers a high specific capacitance of 306 F g−1 at 0.8 A g−1 with excellent rate capability, significantly outperforming the pristine electrodes. Electrochemical Impedance Spectroscopy (EIS) analysis yields a low charge-transfer resistance of 1.19 Ω and a high ion diffusion coefficient of 7.96 × 10−9 cm2 s−1, confirming rapid charge transport. Overall, the synergistic graphene–ZnMn₃O₇ architecture offers strong potential for scalable, high-energy, and sustainable supercapacitor applications.