Nitrogen Self-Doped Biochar Sustainably Self-Activated from Cactus Solidified with Freeze-Drying Strategy for Lightweight Supercapacitor

To assemble lightweight devices for portable electronics, an electrode material with matched surface area and density is essential to balance the mass and volume specific capacitance. Herein, the biochar is directly self-activated from cactus pretreated by freeze-drying. The solidified pore structure of the pretreated cactus is easy to maintain during the subsequent carbonization process. As an interesting result, the typical biochar from freeze-dried cactus BFC-700 shows the highest specific surface area (904.4 m² g^–1) and micropore volume (0.36 cm³ g^–1) among the samples, which are quadruple those of the control sample BC-700. It shows well-balanced mass specific capacitance and volume specific capacitance (528.3 F g^–1, 464.9 F cm^–3). In a two-electrode system with 1.0 M H₂SO₄ as the electrolyte, an energy density of 24.3 Wh kg^–1 is achieved at a power density of 137.7 W kg^–1. The capacitance retention is calculated to be 95.5% after 10,000 cycles of charge/discharge at a current density of 5.0 A g^–1. It can be assembled into a coin-type (CR2025) symmetrical supercapacitor and can power 2.0 V bulbs. The results demonstrate that the freeze-drying treatment is suitable for biomass precursors with water-rich and mucilaginous structure to be converted into biochars. This work provides a green and sustainable method for preparing biochar electrodes with balanced capacitance properties.