# Porous Doped Carbons from Anthracite for High-Performance Supercapacitors
## 3 Results and Discussion
### Supercapacitor Performance of KAC-800 Symmetric Device  
In order to further demonstrate the actual electrochemical behavior and potential applications of our materials, we assembled two electrode symmetric cell devices based on the KAC-800 electrode in 6 M KOH and 1 M Na\({}_{2}\)SO\({}_{4}\), and its schematic structure is illustrated in Scheme 2. Electrochemical performances of the KAC-800 symmetrical two-electrode system were first studied in the 6 M KOH electrolyte. As offered in Figure 10a, the cyclic voltammetry of the KAC-800 symmetric cell is characterized by a rectangular shape between 0 and 1.0 V in 6 mol L\({}^{-1}\) KOH solution, which exemplifies the ideal electrochemical double-layer capacitive behavior [41]. Furthermore, KAC-800 still assumes a typical rectangular CV curve even at a scan rate as high as 300 mV s\({}^{-1}\), which sets forth the efficient charge transfer and electrolyte diffusion inside porous carbon material [32]. The nearly symmetrical triangular form of the GCD curves (Figure 10b) accounts for the impressive charge propagation behavior of the electrolyte ions [42]. Specific capacitances calculated from GCD curves at various current densities of the cell are presented in Figure 10c. The calculated largest specific capacitance of the cell is 47 F g\({}^{-1}\) at the current density of 0.5 A g\({}^{-1}\). The Ragone plot for the KAC-800 symmetric cell in 6 mol L\({}^{-1}\) KOH electrolyte can be found in Figure 10d. The specific energy density of the two-electrode cell approaches 6.5 Wh kg\({}^{-1}\) with a power density of 250 W kg\({}^{-1}\) under a current density of 0.5 A g\({}^{-1}\), which is superior to commercial devices (<3 Wh kg\({}^{-1}\)) [43; 44]. Figure 9: Impedance Nyquist plots of KAC-800 and KAC-800-900 (inset enlarged high-frequency region of Nyquist plots). KAC-800 also bestows the fascinating supercapacitor performance in 1 mol L\({}^{-1}\) Na\({}_{2}\)SO\({}_{4}\) electrolyte. The neutral Na\({}_{2}\)SO\({}_{4}\) electrolyte has been extensively scrutinized for electrical double-layer capacitors by virtues of its environmental friendliness, cost-effectiveness, and great ease to construct relative to the organic solution [45]. On the other hand, it is endowed with a higher operating voltage than those of acid and alkali solutions, and thus the high energy density can come true for supercapacitors [46]. Therefore, the KAC-800 symmetric supercapacitors were further canvassed in 1 M Na\({}_{2}\)SO\({}_{4}\) electrolyte. Figure 11a pictures CV curves of KAC-800 symmetric supercapacitor at different potential windows at the scan rate of 50 mV s\({}^{-1}\). The CV curves of the supercapacitor still sustain a rectangular-like feature when the operational voltage extends to a very high value of 1.8 V, explaining the ideal capacitive behavior and striking reversibility. However, the anodic current undergoes a dramatic increment at the voltage of 2 V due to hydrogen and oxygen evolution reactions [46]. Accordingly, the KAC-800 symmetric supercapacitor experiences a detail examination in the voltage range of 0-1.8 V. The CV curves of the symmetric cell monitored at the different scan rates from 10 to 300 mV s\({}^{-1}\) are shown in Figure 11b. The rectangular CV curve of the as-assembled KAC-800 supercapacitor can be maintained even at a high scan rate of 300 mV s\({}^{-1}\), emphasizing an ideal electrochemical capacitive behavior with the rapid ion diffusion [47]. Figure 11c displays GCD curves of the symmetric cell at the varying current densities from 0.5 to 10 A g\({}^{-1}\). The linear and symmetrical GCD profiles substantiate that the as-fabricated KAC-800 symmetric supercapacitor features the excellent electrochemical reversibility. The symmetric supercapacitor achieves a maximum capacitance of 52 F g\({}^{-1}\) at 0.5 A g\({}^{-1}\) (Figure 11d). The inset in Figure 11d shows a Ragone plot.