# Synthesis of Benzoxazine-Based N-Doped Mesoporous Carbons as High-Performance Electrode Materials
## 3 Results and Discussion
### Electrochemical Performance
After 2000 cycles of charge and discharge, the capacitance retention rate of NCM-900 and ACNCM-900 \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Samples/Current Density** & **0.25 A/g** & **0.5 A/g** & **1 A/g** & **2.5 A/g** & **5 A/g** \\ \hline NCM-700 & 236 & 205 & 178 & 139 & 96 \\ NCM-800 & 367 & 303 & 244 & 173 & 129 \\ NCM-900 & 460 & 440 & 400 & 322 & 300 \\ ACNCM-900 & 299 & 265 & 233 & 202 & 185 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The specific capacitance (F/g) of carbon materials at different current densities. Figure 6: (**a**) Cyclic voltammetry (CV) curves of four carbon materials at the scan rate of 5 mV/s, (**b**) galvanic charge-discharge (GCD) curves of four carbon materials at the current density of 1 A/g, (**c**) CV curves of NCM-900, and (**d**) GCD curves of NCM-900. were 96.1% and 94.3%, respectively. This suggested that these electrode materials had a good cycle stability and were suitable for use in supercapacitors. Figure 7c shows the EIS curves of NCM-900 and ACNCM-900 in a 0.5 M H\({}_{2}\)SO\({}_{4}\) electrolyte and the frequency range from 10\({}^{-2}\) to 10\({}^{5}\) Hz. The curve contained a semicircle and a straight line. The diameter of the semicircle at high frequencies represented the charge transfer resistance (\(R_{\rm ct}\)) [43], while the linear slope of the low frequency part of the resistor characterized the diffusion resistance of the electrolyte and protons. The \(R_{\rm ct}\) of NCM-900 and ACNCM-900 were found to be 0.8 \(\Omega\) and 1.1 \(\Omega\), respectively. NCM-900 had a lower resistance, which was attributed to the higher nitrogen content, which caused an increase in carbon surface polarity. This was consistent with the CV and GCD test results. The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) of NCM-900 in a two-electrode system are presented in Figure 8a,b, respectively. The CV curves remained as a nearly rectangular shape, which indicated ideal EDLC behavior and good reversibility for the electrolyte ions diffusing rapidly to the interface of the electrode. The GCD curves all exhibited a triangular-like shape even as the current density increased to 5 A/g, indicating the good coulombic efficiency with superior EDLC performance of NCM-900 [44]. It is confirmed by Figure 8c that NCM-900 had an excellent cycling stability in the two-electrode system and the capacitance retention was 97.5% after 2000 charge-discharge cycles at a current density of 5 A/g. The specific capacitance for NCM-900 calculated from the GCD curve is also exhibited in this graph, where a high capacitance of 317 F/g was observed at 0.25 A/g, and the specific capacitance was 200 F/g was observed when the current density was at 5 A/g. Figure 7: _Cont._  
Figure 8: _Cont._  
Figure 7: **(a)** Specific capacity of NCM-900 and ACNCM-900 at different current densities, **(b)** cycle stability of NCM-900 and ACNCM-900 after 2000 cycles at 5 A/g, and **(c)** electrochemical impedance spectroscopy (EIS) of NCM-900 and ACNCM-900. The results of the energy densities and power densities of the supercapacitors calculated for various current densities are shown as Ragone plots in Figure 8d. There was a decreasing trend in energy density with an increase of power density. The energy density of the NCM-900-based supercapacitor was calculated to be over 11.1 Wh/kg at 0.25 A/g with a power density of 62.5 W/kg. The energy density of the NCM-900-based supercapacitor was 7.1 Wh/kg at a high current density of 5 A/g with a power density of 1250 W/kg, which is highly competitive with most carbon-based materials obtained from other precursors, as shown in Figure 8d [42; 45; 46; 47; 48; 49; 50]. The results confirmed that N-doped carbons with an ordered pore structure and a high surface area could enable higher energy and power densities, and NCM-900 is a promising candidate for capacitive energy storage and conversion.