Scheme 1. Schematic process for the preparation of NPC-X samples.

Fig. 1. SEM images of NPC-600 (a, d), NPC-700 (b, e), and NPC-800 (c, f); TEM images of NPC-700 (g-i) and EDS elemental mapping images for C, N, O, and P elements (j).

Fig. 2. XRD patterns (a) and Raman spectra (b) of all samples.

Fig. 3. (a-c) N2 adsorption/desorption isotherms of NPC-X samples (insets are the plots of pore size distributions calculated from NLDFT model), and (d) comparison of SSA for all samples.

Fig. 4. (a) XPS survey of NPC-600, NPC-700, and NPC-800, (b-e) high-resolution XPS spectra of the C1s, O1s, N1s, and P2p of NPC-700, and (f) schematic of different N functionalities and P-containing functional groups in the carbon matrix.

Fig. 5. Illustration (a) and electrostatic potential plots (b) of GR-N4, GR-N4P and GR-N5P, where carbon, hydrogen, oxygen, nitrogen and phosphorus atoms are colored in gray, white, red, blue and orange, respectively.

Fig. 6. Electrochemical performance of all samples in a three-electrode system (6 M KOH): (a) CV curves of all samples at a scan rate of 20 mV s-1; (b) GCD curves of all samples at a current density of 1 A g-1; (c) CV curves of NPC-700 at different scan rates (5-100 mV s-1); (d) GCD curves of NPC-700 at different current densities (0.5-20 A g-1); (e) Specific capacitance of all samples calculated from GCD curves at various current densities; (f) The correlation among specific capacitance, specific surface area, and heteroatom contents of as-prepared samples; (g) Nyquist plots of all samples (inset is the magnified plots); (h) Bode plots for all samples; (i) Cycle life and coulombic efficiency of NPC-700 electrode (the inset presents the GCD curves for the first five cycles and the last five cycles).

Fig. 7. (a) Schematic diagram of the assembled NPC-700//NPC-700 symmetric supercapacitor; (b) CV curves at different voltages; (c) CV curves at various scan rates; (d) GCD curves at various current densities; (e) specific capacitance of the supercapacitor calculated at various current densities; (f) Nyquist plots of the supercapacitor before and after 10,000 cycles (the inset is the amplified high-frequency region); (g) Ragone plots of the NPC-700//NPC-700 symmetric supercapacitor in comparison with some previously reported results; (h) Cycling stability after 10,000 cycles at a current density of 5 A g-1 (the inset shows a red LED light powered by three symmetric supercapacitors in series connection).