Waste Biomass Based-Activated Carbons Derived from Soybean Pods as Electrode Materials for High-Performance Supercapacitors

In this paper, a simple yet effective chemical activation/carbonization strategy is presented to prepare a hierarchically porous activated carbon with multi-heteroatom (O, N, and P) self-doping using waste soybean pods as carbon source. It is found that the resulting carbon activated by KOH possesses richer textural properties than the one activated by ZnCl₂. With abundant micro-, mesopores and interconnected macropores, the KOH-activating carbon possesses a specific surface area up to 2245 m² g⁻¹ and a large pore volume of 1.35 cm³ g⁻¹. In a three-electrode system, the KOH-activating carbon can deliver a high specific capacitance of 321.1 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH aqueous electrolyte. It still can maintain 215.7 F g⁻¹ at a current density of 10 A g⁻¹ with a good rate capacity (67.2% retention). When used as symmetric electrode in 1 M Na₂SO₄ neutral aqueous solution, the SPAC−K//SPAC−K supercapacitor can provide a high energy density up to 22.28 Wh kg⁻¹ at a power density of 450 W kg⁻¹ in a wide voltage window of 0–2.0 V, surpassing most of the previously reported waste biomass-based carbon materials. The impressive electrochemical performances as well as the low-cost scalable production manifest that the SPAC−K derived from waste soybean pods can be a promising electrode candidate for supercapacitor application.