Green synthesis of some selected transition metal oxide nanoparticles for energy storage applications in supercapacitors

The increasing demand for viable and proficient energy storage systems has driven extensive research into environmentally friendly electrode materials. Transition metal oxide nanoparticles (TMO NPs) synthesized through green plant-mediated (GPM) approaches, have arose as auspicious candidates for supercapacitor applications owing to their high specific capacitance, tunable electrochemical properties, and eco-friendly synthesis routes. Traditional chemical and physical synthesis techniques often involve toxic reagents, high energy consumption, and complex procedures, making the transition toward biological methods an attractive alternative. GPM synthesis offers an environmentally benign, cost-effective and scalable approach by exploiting plant extracts as natural reducing and stabilizing agents. The review systematically examines the recent improvements in the GPM synthesis of TMO NPs, electrochemical properties, synthesis mechanisms, and performance of various TMO NPs, including nickel oxide (NiO), ruthenium oxide (RuO₂), zinc oxide (ZnO), copper oxides (CuO), manganese dioxide (MnO₂) and titanium dioxide (TiO₂), for supercapacitor applications. These materials exhibit remarkable charge storage capabilities owing to their high surface area, tunable redox properties, and enhanced ion diffusion. The discussion also highlights the advantages of green-synthesized TMO NPs, such as improved biocompatibility, sustainability, and reduced environmental footprint, compared to conventional synthesis methods. Despite significant progress, challenges such as scalability, nanoparticle stability, and optimization of reaction conditions persist. This review provides insights into overcoming these challenges and outlines future directions for developing cost-effective, high-performance, and sustainable energy storage solutions using plant-mediated transition metal oxide nanoparticles.