In this work, we report a facile method for preparing the B/N co-doped 2D porous carbon nanosheets (BNHCs) with high contents of boron (3.41-3.96 at.%) and nitrogen (5.1-6.09 at.%) via annealing the mixtures of ammonium humate and H3BO3. The ammonium humate is used as nitrogen-containing carbon precursor, and H3BO3 acts as boron source and pore-forming agent. Moreover, we demonstrate firstly that the existence of H3BO3 can enhance the graphitization degree of BNHCs by inducing the conversion of BCO2 to BC3 with increasing the H3BO3/ammonium humate mass ratio to 2. BNHC-1.5 exhibits the highest specific surface area of 592m(2) g(-1) among all the samples, and BNHC-2 shows the enhanced graphitization degree and highest mesopore ratio (60.5%). In a three-electrode system, the BNHC-1, BNHC-1.5 and BNHC-2 exhibit high specific capacitances of 305 F g(-1), 311 F g(-1) and 225 F g(-1) at 0.2 A g(-1), and 64 F g(-1), 125 F g(-1) and 120 F g(-1) at ultrahigh current density of 100 A g(-1), respectively. Moreover, as assembled symmetric supercapacitor electrodes with high active material mass loading (similar to 180 mu m, similar to 13 mg cm(-2)), BNHCs deliver high specific capacitances of 170-210 F g(-1) at 0.05 A g(-1), and BNHC-2 presents an outstanding cyclic stability (94.8% capacitance retention after 10,000 cycles), making them promising candidates for practical supercapacitor application. (C) 2018 Elsevier Ltd. All rights reserved.