Fabrication of Hierarchical Porous Carbon Frameworks from Metal-Ion-Assisted Step-Activation of Biomass for Supercapacitors with Ultrahigh Capacitance

Advancement in the application of biomass-derived carbon is viewed as one of the most important drivers of sustainable and renewable technologies for energy storage. High-performance biochar with a well-defined structure is extremely attractive as a supercapacitor electrode material. Herein, carbon with an ultrahigh capacitance of 682 F/g at 0.2 A/g for biochar materials and remarkable cycling stability was synthesized from Chinese parasol fluff (CPF). Three-dimensional interconnected hierarchical porous carbon was fabricated via a feasible metal-ion-assisted step-activation method from the biomass precursor. CO₂ was used to treat the carbonaceous material from CPF after molten KOH activation for pore structure tuning, while Co²⁺ was selected as the metal ion additive for directing graphitization. In addition to the extremely high capacitance, the as-synthesized carbon also delivered an energy density of 46.38 W h/kg at a power density of 300 W/kg in 1-butyl-3-methyl imidazolium tetrafluoroborate/acetonitrile (BMIMBF₄/AN) electrolyte, which is much better than for most reported biochar materials and about 10 times better than for commercial supercapacitors. The current work points out an excellent sustainable source for fabrication of high-performance electrode materials and, more importantly, a feasible design strategy route for improvement of the electrochemical performance of energy storage devices.