Nitrogen-doped biomass-derived porous carbon spheres for supercapacitors with ultrafast charge/discharge rate up to 2.5 V s−1

Biomass-derived carbon materials are often applied in green energy storage systems like supercapacitors due to the abundance, facile access, low cost and nontoxicity. In this work, biomass-derived porous carbon spheres (PCSs) have been prepared by the carbonization and etching of sodium alginate (SA). Using polyacrylic acid (PAA) as a template, N-doped graphene quantum dots (N-GQDs) are grown evenly on the surface of the PCSs, which drastically improves the pseudocapacitive activity and electrical conductivity. The prepared N-GQD/PCS/PAA electrode has a remarkable specific capacitance of 419.3 F g−1 at 1 A g−1. The symmetric supercapacitor assembled with N-GQD/PCS/PAA electrode exhibits a great specific capacitance of 337.5 F g−1 at 0.5 A g−1. Particularly, the device displays an ultrafast charge/discharge rate of 2.5 V s−1, demonstrating excellent rate capability and structural stability. The supercapacitor based on N-GQD/PCS/PAA possesses high energy density of 11.72 W h kg−1 and excellent cycle stability. A high mass loading (9.18 mg cm−2) with the energy density of 8.54 W h kg−1 is also investigated to realize the practical application for the device. In general, a novel biomass-derived carbon electrode modified with highly electrochemical active N-doped graphene quantum dots has been constructed to provide an effective way for high performance energy storage.