Electrochemical performances of NOCs are first investigated in a three-electrode configuration. Compared with the nearly rectangular cyclic voltammetry (CV) curves and isosceles-triangular shaped galvanostatic charging/discharging (GCD) profiles in the KOH electrolyte (Figs. S6a and b in Supporting information), the extra redox peaks illustrated in
Fig. 2a and slight distortions in
Fig. 2b verify the occurrence of faradaic reactions by N, O-containing functionalities (N-6, N-5 and O-I) gaining/losing protons in an acidic solution (Fig. S6c in Supporting information)[
39-
41]. The gravimetric capacitances calculated by the GCD discharging branches are 232/251 (NOC
700, 1:3), 290/311 (NOC
700, 1:1), 212/232 (NOC
700, 3:1), 251/272 (NOC
600, 1:1), 209/ 237 F/g (NOC
800, 1:1) in KOH/H
2SO
4 electrolytes, respectively. The optimal NOC
700, 1:1 electrode also gives strong rate capability (Figs. S6d and e in Supporting information) with capacitance retention of 66% in KOH and 69% in H
2SO
4 at 10 A/g. It can be found that as the pyrolysis temperature rises from 600 ℃ to 700 ℃, the capacitances exhibit an obvious improvement (
Fig. 2b). This is mainly because sufficient pyrolysis at 700 ℃ guarantees a developed porous architecture equipped with better conductivity and lower contact resistance, which can be validated by electrochemical impedance spectrogram (EIS) plots (Fig. S6f in Supporting information). The ohmic resistance (
Rs) deduced by the first
Z' intersection is only 0.25 Ω for NOC
700, 1:1, suggesting that the electrode possesses a low internal/contact resistance [
42]. Moreover, the small charge-transfer resistance (Rct) value (0.84 Ω) and the Warburg impedance (
W, 0.45 Ω) can guarantee the superb propagation capability for ions within NOC
700, 1:1 [
43]. Further increase of pyrolysis temperature to 800 ℃ brings about decayed utilization of adsorbing surface, which explains the unsatisfactory capacitance of NOC
800, 1:1 in
Fig. 2b. Meanwhile, faradaic-active species regulated by
o-tolidine/
p-benzoquinone molar ratios endow the electron-donor features and supply abundant electrochemically active sites. To better confirm the pseudocapacitive contributions, the relationship between specific capacitance and square root of charge/discharge time is plotted in Fig. S6g (Supporting information). The Y-intercepts account for the rate-independent parts, while the remainders of total capacitances are pseudocapacitances [
44]. The correlation between capacitances, surface areas and faradaic-active heteroatom species is given in
Fig. 2c. With the increasing faradaic-active heteroatom species, the pseudocapacitive contribution is evidenced by a scrambling tendency from 35% to 39%. However, excessive introduction of N-containing dopants leads to a decayed surface area of the flimsy carbon matrix for physical adsorption, and renders NOC
700, 3:1 with inferior capacitive performance. Therefore, NOC
700, 1:1 takes full use of the integrated superiorities from adsorbing surface and faradaic-active sites, obtaining a maximized capacitance.