O and N co-doped porous carbon derived from crop waste for a high-stability all-solid-state symmetric supercapacitor

As a green and renewable material, biomass-based carbon is one of the prospective electrode materials for supercapacitors. Limited by their microstructure and composition, the activation of biomass-based materials is challenging for enhancing the performance of supercapacitors. Herein, a series of porous carbon-based supercapacitor electrode materials have been synthesized via pyrolysis combined with KOH activation using the biomass waste of soybean pods as the carbon source. Theoretical calculation results proved that doping with N can improve the conductivity and K⁺-adsorption capacity or carbon materials, which can further improve their capacitive performance. O and N co-doped porous carbon with an optimal KOH content and operation temperature showed a large specific surface area (SSA) of 1807.56 m² g⁻¹, with a large capacitance of 366.1 F g⁻¹ at 1 A g⁻¹ and a great rate capability of 65.6% capacitance retention at 100 A g⁻¹ (240 F g⁻¹). In addition, the all-solid-state symmetric supercapacitor assembled using O and N co-doped porous carbon exhibits an energy density of 8.34 W h kg⁻¹ at 247.95 W kg⁻¹ with an excellent cycling stability of 96.03% capacity retention over 50 000 cycles. This work provides a new approach for the synthesis of biomass-based carbon materials, which can be used for energy-storage and -conversion applications.