Hierarchical porous carbon biochar nanotubes encapsulated metal nanocrystals with a strong metal-carbon interaction for high-performance supercapacitors

Biomass-based engineered hierarchical porous carbons derived from biomass waste are deemed cost-effective and sustainable materials for supercapacitors, thanks to their tunable properties, e.g., high conductivity, and stability; however, their multiple preparation steps and low capacitance are deemed significant challenges. These issues were addressed herein through the rational design of hierarchical porous carbon nanotubes (p-CNTs) enriched with metal nanocrystals (M/p-CNTs) (M=Cu, Co, W, Bi, and Mo) via the ultrasonic impregnation of coconut silks in metal precursor solutions and pyrolysis under nitrogen. This approach promotes a green, one-pot method that eliminates the need for activation steps or hazardous chemicals, and endows the formation of nanotubes through a strong-metal carbon interaction, resulting in metal electron-deficiency, with a BET surface area of 288.6 m²/g, along with interconnected tri-modal porosity, which accelerates charge mobility and facilitates ion transport. These advantages significantly improved the performance of supercapacitors, which was fine-tuned by the intermolecular electron transfer between metals and p-CNTs, which reached the optimum specific capacitance of 558.1F/g at 0.5 A/g, energy density of 27.9 Wh/kg, and power density of 150/3000 W/kg at 1/10 A/g on Cu/p-CNTs, which was amongst highest reported for active supercapacitors. These findings pave the way for the simple and sustainable synthesis of active materials from biomass for energy storage devices.