A review on advances in green-synthesized graphene–metal nanohybrids for symmetrical supercapacitor applications

Abstract The present review critically explores the green synthesis of graphene nanosheets (GNs)/metal nanohybrids as advanced electrode materials for symmetrical supercapacitors. With the growing demand for sustainable energy storage technologies, the development of eco-friendly synthesis routes has become a necessity of the day. This review consolidates and analyzes recent progress in eco-friendly fabrication strategies, including plant extract-mediated reduction, microwave-assisted synthesis, and other low-toxicity approaches that minimize hazardous reagents and energy consumption. Special emphasis is placed on the synergistic combination of GNs exceptional surface area, conductivity, and mechanical strength with metal and metal oxide nanostructures (e.g., ZnO, TiO₂, CuO, and activated carbon, etc.). These hybrid materials exhibit remarkable enhancements in electrochemical performance, including high specific capacitance, rapid charge-discharge capability, and superior cycling stability, which are critical for practical supercapacitors applications. The review also provides a comparative evaluation of various synthesis methods, highlighting their scalability, reproducibility, and impact on material performance. Additionally, insights into the challenges associated with achieving uniform dispersion, optimizing electrode architecture, and balancing cost-effectiveness with high energy and power densities are presented. By integrating findings from a wide range of studies, this work underscores the immense potential of green-synthesized GNs-based NCs as sustainable solutions for next-generation energy storage devices. Future perspectives are outlined to address current limitations and accelerate the transition toward scalable, green energy storage technologies that align with environmental and economic sustainability goals.