Oyster-shell-assisted layer-loading activation of Leucaena leucocephala biochar into transport-accessible micro/mesoporous carbons for high-rate organic-electrolyte supercapacitors

Sustainable preparation of porous carbons with ion-accessible pore networks remains an important challenge for high-performance organic-electrolyte supercapacitors. Herein, a strong-alkali-free layer-loading activation strategy is developed using invasive Leucaena leucocephala biochar as the carbon precursor and waste oyster shell powder (OSP) as the CO 2 -releasing activation source. In this layer-stacked configuration, oyster shell powder is placed beneath the biochar and thermally decomposes to release CO 2 , which progressively etches the upper carbon framework while minimizing direct solid mixing. This design reduces cross-contamination, simplifies post-activation washing, and enables the recovery of CaO-rich mineral residue. By regulating the activation cycle number, the pore architecture evolves from initial micropore formation to a more open and balanced micro/mesoporous network. Among the conditions examined, the carbon product obtained at a C:OSP ratio of 1:6 after three activation cycles exhibits a high BET surface area of 2005 m 2 g −1 , a total pore volume of 1.262 cm 3 g −1 , and an average pore size of 3.07 nm. In symmetric coin cells using 1.0 M TEABF 4 /PC organic electrolyte, this carbon delivers a specific capacitance of 129 F g −1 at 2 mV s −1 , 79% capacitance retention at 200 mV s −1 , a low IR drop of 0.19 V, and an energy density of 21.4 Wh kg −1 at 11.1 kW kg −1 . It also retains 77.8% of its capacitance after 10,000 cycles. Compared with commercial micropore- and mesopore-dominant carbons, this three-cycle carbon product shows better rate capability, lower interfacial resistance, higher energy retention at high power, and improved cycling stability. Structure–performance analysis further confirms that the high-rate response is governed not only by surface area but also by pore accessibility, pore volume, and a balanced micro/mesoporous architecture. This work provides a waste-derived activation strategy and a pore-design principle for practical organic-electrolyte supercapacitor carbons.