Biomass-derived porous carbon has garnered extensive attention as a high-performance electrode material for supercapacitors, yet its practical energy density (below 10.0 Wh/kg) is severely limited by low effective microporosity. Herein, mugwort stem (MWS) herbal residue was employed as a sustainable carbon precursor to fabricate highly microporous carbon via a facile ball milling-assisted K₂CO₃ activation strategy. By optimizing the precursor-to-activator ratio, the resultant carbon material (KQA-1-1) exhibits a microporosity ratio exceeding 90%, a large specific surface area of 897 m2/g, and abundant oxygen-containing functional groups. When evaluated in a three-electrode system, KQA-1-1 delivers a high specific capacitance of 338 F/g at 1.0 A/g with outstanding rate capability. More importantly, the assembled symmetric supercapacitor achieves an ultrahigh energy density of 30.9 Wh/kg at a power density of 1000 W/kg, and retains 20.9 Wh/kg even at an ultrahigh power density of 20 kW/kg, significantly outperforming commercial activated carbon (YP-80F). This work not only develops a mild, scalable, and low-cost approach to prepare high-microporosity carbon but also provides a promising route for the high-value resource utilization of biomass waste in advanced carbonaceous materials.
