Green Synthesis and Characterisation of Reduced Graphene Oxide Nanosheets (2D) for High Performance Supercapacitor Electrode Materials

Supercapacitors have emerged as promising energy storage devices due to their high-power density, long cycle life, and rapid charge–discharge capabilities; however, their relatively low energy density limits broader applications in modern portable technologies. Therefore, the development of high-performance and sustainable electrode materials, particularly through green synthesis of reduced graphene oxide using plant-based extracts, has become essential to enhance their electrochemical performance. The study aims to synthesize reduced graphene oxides (RGO) with physicochemical and electrochemical properties that are promising for energy storage. Graphene oxide was reduced using plant extracts from Garcinia kola Heckel (GK) and Piper nigrum L. (PN), resulting in the materials GO-GK and GO-PN respectively. Structural analyses such as XRD, Raman spectroscopy and SEM were carried out. Electrochemical performances were investigated using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD). The results showed that the GO-GK and GO-PN materials exhibited a capacitive behaviour, typical characteristic of EDLC-type supercapacitors. GO-PN exhibited the highest specific capacitance, reaching 148.57 F·g⁻¹ in CV mode at 5 mV·s⁻¹ and 139. 67 F·g⁻¹ in GCD at 0.5 A·g⁻¹, as well as excellent cycling stability (91.25% capacitance retention after 2000 cycles). It also achieved the highest energy and power densities (19.42 Wh·kg⁻¹ and 3453.48 W·kg⁻¹), respectively.