▎ 摘　　要

Doping heteroatoms especially nitrogen is widely applied for improving the capacitive energy storage performance of carbon materials. However, preserving nitrogen in carbon skeleton is still challenging because nitrogen species are unstable at high temperature. Herein, we develop a simple boric acid-assisted method for trapping nitrogen into carbon skeleton. Using the graphene quantum dots with an initial nitrogen content of 5.0 at.% as precursors, the B, N co-doped carbon nanosheets with an increased nitrogen content of 7.2 at.%, a boron content of 4.4 at.%, and a surface area of 817 m(2) g(-1) are prepared. Benefiting from the high heteroatom content, the nanosheets show a significantly improved capacitance of 257 F g(-1) at 1 A g(-1) and a good rate performance of 150 F g(-1) at 50 A g(-1). The pseudocapacitance contribution at 10 mV s(-1) is as high as 48%. The assembled symmetric capacitor shows a superior cycle stability with no obvious fading after 10,000 cycles at 10 A g(-1). Our work can help to understand the synergistic effect of heteroatoms for co-doping of carbon and broaden the avenue for designing functional carbon materials.