Soft-template hydrothermal synthesis of N and B co-doped walnut-shaped porous carbon spheres with hydrophilic surfaces for supercapacitors

Biomass-derived carbon electrodes have attracted attention for alternative energy storage devices. However, it remains a challenge to obtain a hydrophilic surface while preserving the structure, stability, and reactivity. N and B co-doped walnut-shaped porous carbon spheres with hydrophilic surfaces were prepared by hydrothermal carbonization of glucosamine, boric acid, and triblock copolymers, followed by chemical vapor phase oxidation. Walnut-shaped structure can shorten the electron and ion transport pathways and improve diffusion efficiency. N and B atoms doped in the forms of pyridine N (N-6), pyrrole N (N-5), graphite N (N-Q), N oxide (N-Ox), or N-B, as well as CB3, CB2O, CBO2, or B-N can improve the stability, conductivity, and reactivity. Hydrophilic surface can promote the wetting of the electrolyte on carbon electrodes. Therefore, N and B co-doped walnut-shaped porous carbon spheres exhibit useful electrochemical performance. The specific capacitance reaches 281F·g−1 at a current density of 0.2 A·g−1, and the rate performance reaches 75% at 5 A·g−1. The assembled symmetrical supercapacitor shows a high specific capacitance of 189 F·g−1 at 0.1 A·g−1 and an energy density of 6.56 W·h·kg−1. Furthermore, it exhibits a high cycle stability after 5000 cycles at 2 A·g−1, maintaining 95% of initial specific capacitance.