The global push for sustainable energy and efficient storage solutions has made the development of next-generation energy storage devices a critical challenge for materials scientists. Supercapacitors, known for their ability to combine the high energy storage capacity of batteries with the ultra-fast charging capabilities of traditional capacitors, are at the forefront of this research. However, a major drawback remains their relatively low energy storage capacity compared to conventional batteries. In a recent research article in the journal Next Materials, Bruna Andressa Bregadiolli, Glauco Meireles Mascarenhas Morandi Lustosa, João Vitor Paulin, Waldir Antonio Bizzo, Lauro Tatsuo Kubota, Shuguang Deng, and Talita Mazon explored a sustainable approach to enhance these devices by fabricating electrodes from biomass-derived biochar and its metal oxide composites.

The material chosen for this study was sugarcane bagasse, an abundant agricultural waste, reflecting a trend toward using sustainable carbon sources for electrodes. The biochar powders were synthesized using a multi-step process involving acid pretreatment, hydrothermal synthesis, and thermal graphitization at 750°C in a nitrogen atmosphere. A crucial aspect of this preparation was engineering the material’s porous structure, a key factor for maximizing energy storage, which relies on the adsorption and desorption of ions on the electrode surface.

The pure biochar successfully obtained in this study exhibited remarkable structural characteristics. Morphological characterization revealed powders with submicrometer-diameter pores. Critically, the pure biochar was found to have a high surface area and a pore volume. To investigate a strategy for potentially boosting performance, composites were created by growing zinc oxide (ZnO) or copper oxide (CuO) nanostructures directly onto the biochar surface using a hydrothermal technique. This method resulted in ZnO nanorods and CuO nanosheets covering the carbon surface.

However, contrary to expectations that combining carbon with metal oxides might create a synergistic effect, the structural and electrochemical results demonstrated that the composites actually underperformed compared to the pure biochar. The growth of the metal oxide nanostructures physically filled some of the pores, leading to a significant drop in surface area: ZnO@biochar dropped to 163.69 m²⋅g⁻¹ and CuO@biochar dropped to 152.48 m²⋅g⁻¹. This reduction in accessible surface area had a direct negative impact on the device’s electrical performance.

Electrochemical characterization was performed in a three-electrode system using an aqueous electrolyte. The pure biochar electrode achieved a high specific capacitance of 446 F⋅g⁻¹, a significantly better performance than the metal oxide composites. This performance hierarchy was also reflected in the Ragone plot, which compares power density and energy density, where the pure biochar delivered a power density of 46.2 W⋅kg⁻¹ and an energy density of 1.8 W⋅h⋅kg⁻¹, outperforming both metal oxide composites.

The strong performance of the pure biochar material is directly attributed to its high surface area and porous structure, which allows for better electrolyte penetration and offers more electrochemical active sites for ion adsorption and desorption—the fundamental mechanism of charge storage in these devices. While the composites showed improved capacitive behavior compared to the pure biochar, they were limited by their reduced surface area and increased resistance values. These quantitative results lead to a strong conclusion: for this material and application, developing a synthesis route that maximizes the surface area and promotes porous structures in the graphitized biochar is more important than incorporating metal oxides. This sustainable, cost-effective approach using biomass waste offers exciting possibilities for the future development of advanced carbon-based energy storage materials.

Source: Bregadiolli, B. A., Lustosa, G. M. M. M., Paulin, J. V., Bizzo, W. A., Kubota, L. T., Deng, S., & Mazon, T. (2025). Synthesis of biochar and its metal oxide composites and application on next sustainable electrodes for energy storage devices. Next Materials, 7, 100444.