Simultaneously obtaining fluorescent carbon dots and porous active carbon for supercapacitors from biomass

We present a facile and green approach to simultaneously synthesize fluorescent carbon dots and porous active carbon for supercapacitors via a two-step carbonization process from a widely available protein-rich biomass precursor – soybeans. Fluorescent soybean carbon dots (SCDs) with inherent nitrogen-doping were obtained in the first low-temperature carbonization step, while hierarchical porous carbon with an interconnected microstructure was obtained by follow-up high-temperature carbonization with the insoluble residues. The effect of the activating agent (KOH) on the microstructure, conductivity, and nitrogen-doping degree of the porous carbon were explored. Significantly, we demonstrate that much less corrosive KOH (weight ratio: KOH/residue = 0.5/1) is needed to achieve a high specific surface area (1663.1 m2 g−1) and remarkable capacitive performance with high specific capacitance (337.3 F g−1 at 1 A g−1). This work provides a completely distinctive way for full utilization of biomass.