Nitrogen Self-Doped Biochar Sustainably Self-Activated\nfrom Cactus Solidified with Freeze-Drying Strategy for Lightweight\nSupercapacitor

To\nassemble lightweight devices for portable electronics, an electrode\nmaterial with matched surface area and density is essential to balance\nthe mass and volume specific capacitance. Herein, the biochar is directly\nself-activated from cactus pretreated by freeze-drying. The solidified\npore structure of the pretreated cactus is easy to maintain during\nthe subsequent carbonization process. As an interesting result, the\ntypical biochar from freeze-dried cactus BFC-700 shows the highest\nspecific surface area (904.4 m<sup>2</sup> g<sup>–1</sup>)\nand micropore volume (0.36 cm<sup>3</sup> g<sup>–1</sup>) among\nthe samples, which are quadruple those of the control sample BC-700.\nIt shows well-balanced mass specific capacitance and volume specific\ncapacitance (528.3 F g<sup>–1</sup>, 464.9 F cm<sup>–3</sup>). In a two-electrode system with 1.0 M H<sub>2</sub>SO<sub>4</sub> as the electrolyte, an energy density of 24.3 Wh kg<sup>–1</sup> is achieved at a power density of 137.7 W kg<sup>–1</sup>. The capacitance retention is calculated to be 95.5% after 10,000\ncycles of charge/discharge at a current density of 5.0 A g<sup>–1</sup>. It can be assembled into a coin-type (CR2025) symmetrical supercapacitor\nand can power 2.0 V bulbs. The results demonstrate that the freeze-drying\ntreatment is suitable for biomass precursors with water-rich and mucilaginous\nstructure to be converted into biochars. This work provides a green\nand sustainable method for preparing biochar electrodes with balanced\ncapacitance properties.