Rational design of graphene-metal oxide nanocomposites for advanced supercapacitors: synergistic mechanisms, sustainable synthesis, and scalability challenges

• Rational design of graphene-metal oxide nanocomposites for supercapacitors • Compares synthesis methods for morphology and electrochemical control • Highlights sustainable synthesis using biochar and green precursors • Analyzes synergistic mechanisms boosting energy and power density • Discusses scalability challenges and future research directions The escalating global energy demand necessitates the development of advanced, sustainable energy storage devices. Supercapacitors, renowned for their high-power density and long cycle life, play pivotal roles in this landscape, yet their limited energy density remains a key challenge. This review comprehensively examines the rational design of graphene metal oxide nanocomposites as next-generation electrode materials to overcome this challenge. We delve into the fundamental principles behind electric double layer capacitance (EDLC) enabled by graphene and pseudocapacitance contributed by various metal oxides (e.g., MnO 2 , RuO 2 , Fe 3 O 4 , NiO), highlighting enhanced synergistic effects upon their integration. The core analysis contrasts synthesis methodologies, including chemical vapor deposition, hydrothermal/ solvothermal processes, sol-gel techniques, and electrochemical deposition, evaluating their impact on key parameters such as morphology, porosity, electrical conductivity, and ultimately, electrochemical performance. Furthermore, we emphasize sustainable innovation by reviewing renewable precursors (e.g., biochar) for graphene derivation and green nanoparticle synthesis. The discussion additionally addresses the environmental implications and end-of-life considerations of these nanomaterials. By linking synthesis strategies to material properties and electrode performance, our analysis provides actionable insights for designing efficient, durable, and scalable graphene metal oxide nanocomposites, accelerating next-generation supercapacitors.