A three-dimensional hierarchical porous carbon is synthesized via a facile chemical activation route with garlic skin as the precursor and KOH as the activating agent. The as-obtained carbon presents a high specific surface area of 2818 m² g⁻¹ and a hierarchical porous architecture containing macroporous frameworks, mesopores (2–4 nm), and micropores (0.6–1.0 nm). As the electrode material for a supercapacitor, due to its unique interconnected porous structure, this garlic skin-derived carbon exhibits excellent electrochemical performance and cycling stability. At a current density of 0.5 A g⁻¹, the capacitance is up to 427 F g⁻¹ (162 F cm⁻³). Even at a high current density of 50 A g⁻¹, the capacitance can be maintained to a high value of 315 F g⁻¹ (120 F cm⁻³). After charging–discharging at a current density of 4.5 A g⁻¹ for 5000 cycles, the capacitance retention is as high as 94%. The results suggest that this garlic skin-derived 3D hierarchical porous carbon is a promising electrode material for high-performance supercapacitors.

Ultrasonic-assisted impregnation is used to synthesize physically modified garlic peel-based 3D hierarchical porous carbons (PCs), and the effect on PCs is investigated by changing ultrasonic time. The results show that ultrasonic waves can effectively peel off surface attachments of the carbonized product, so that activator has a better mass transfer process and create more active sites. The connectivity of 3D pore network is enhanced as well, so the structure and electrochemical properties of garlic peel-based porous carbon (GBPC) are improved. The ultrasonic disperser is used as an ultrasonic generator, specific conditions are as follows: ultrasonic frequency is 40 kHz, ultrasonic power is 500 W, and ultrasonic time is 0, 3, 6, and 9 min, respectively. With the increase of ultrasonic time, impurities again block the pore structure during dynamic movement, resulting in a decrease in electrochemical performance. Specifically, the performance of GBPC-6 is the most excellent, the specific surface area (SSA) increases from 2548 m2 g-1 to 3887 m2 g-1, the specific capacitance increases from 304 F g-1 to 426 F g-1 at a current density of 1 A g-1 in a two-electrode test system. Energy density and cycle performance are also improved at the same time, which are attributed to rational structure. In addition, the effectiveness of the strategy of ultrasonic-assisted synthesis has been confirmed on another precursor material-scallion, meaning that this work proposes a new and simple modification method for improving the performance of biomass-based PCs.