# Porous Doped Carbons from Anthracite for High-Performance Supercapacitors
## 3 Results and Discussion
### Effect of Nitrogen Content on Supercapacitor Performance  
To eliminate the nitrogen elements in KAC-800, the KAC-800 sample was subjected to a thermal annealing in N\({}_{2}\) at 900 \({}^{\circ}\)C for another 2 h. The textural properties of as-obtained KAC-800-900 were identified by N\({}_{2}\) adsorption-desorption measurement. The type I adsorption isotherm of KAC-800-900 (Figure S2) reflects its micropore-predominant structure, which is reminiscent of all above-mentioned samples. The corresponding PSD of KAC-800-900 (Figure S3) also falls into a narrow size range of 1-4 nm. Details regarding physical characteristics of KAC-800-900 are summarized in Table S2. It is noted that there emerges no obvious difference of SSA and PSD between KAC-800 and KAC-800-900. Plus, the XRD pattern of the KAC-800-900 (Figure S4) likewise resembles the counterpart of KAC-800. These results unambiguously expound that the further high-temperature treatment at 900 \({}^{\circ}\)C under N\({}_{2}\) for 2 h is non-destructive for textural properties of KAC-800. Nevertheless, taking into account the fact that their electrochemical properties are divergent, the reason behind this disparity must correlate with the influence from the other factor. In the three-electrode test, KAC-800-900 shows a more rectangular curve with smaller area than KAC-800 (Figure 6a), uncovering its lower capacitance (Figure 6b) which can primarily emanate from the absence of the N-containing functional groups [32]. This conclusion can be confirmed by analysis of the elemental compositions and chemical states of the samples from XPS study. As shown in XPS survey spectra (Figure 7a), only the N signals are only spotted in KAC-800, revealing the existence of nitrogen-containing functional groups in KAC-800. As generalized in Table S3, the nitrogen contents are calculated to be approximately 2.77 at% and 0.08 at% for KAC-800 and KAC-800-900, explicitly highlighting that the high-temperature treatment can diminish the nitrogen species in KAC-800 to an extremely limited level which can impose the negligible impingement upon the pseudocapacitance contribution. C1s core-shell emission spectrum of KAC-800 (Figure 7b) can be compartmentalized into four envelops, corresponding to C-C (284.6 eV), C=N/C-O (285.7 eV), C-N/C=O (286.7 eV), and O-C=O (289.8 eV) functional groups. Furthermore, the XPS N1s spectrum of KAC-800 (Figure 7c) could be deconvolved into three kinds of nitrogen functional groups, including pyridinic (N-6, 398.1 eV), pyrrolic/pyridonic (N-5, 399.6 eV), and quaternary nitrogen (N-Q, 401.5 eV), validating that the nitrogen atoms inserted into KAC-800 are in different binding states [33]. Moreover, three types of the oxygen species are discerned for KAC-800 (Figure 7d). The peaks at roughly 531.1, 532.9 and 536.2 eV can appertain to C=O, C-O, and O-C=O groups, respectively [34]. These surface N-/O-containing groups will substantially ameliorate wettability and capacitive properties of porous carbon electrode materials for supercapacitors. Figure 5: XRD patterns of samples. Figure 6: (**a**) Comparison of CV curves of KAC-800 and KAC-800-900 at the scan rate of 50 mV s\({}^{-1}\); (**b**) specific capacitances of KAC-800 and KAC-800-900 as a function of the current densities. Figure 7: (**a**) XPS survey spectra of KAC-800 and KAC-800-900; (**b**) C1s spectrum of KAC-800; (**c**) N1s spectrum of KAC-800; (**d**) O1s spectrum of KAC-800. The nitrogen content variation across samples was confirmed by Raman spectra (Figure 8a). Two characteristic peaks centered at approximately 1354 and 1596 cm\({}^{-1}\) sequentially belong to the disordered and graphitized carbon [35]. The peak intensity ratios of the D and G bands (I\({}_{\text{D}}\)/I\({}_{\text{G}}\)) of KAC-800 and KAC-800-900 are 1.40 and 1.30 (Table S3), respectively. The position, intensity (relative to the G-band) and broadening of the D band depend on the nature of impurities and functional groups, and the type of disorder.