## 3 Results and Discussion
### Electrochemical Performance of the Multiporous Activated Carbon Nanofiber-Based Electrodes  
A three-electrode system was used to compare the energy storage performance of porous carbon nanofibers according to the plasma process. The three-electrode system consisted of the multiporous carbon fibers as the working electrode, Hg/HgO as the reference electrode, and Pt as the counter electrode in the 1 M KOH electrolyte. Figure 6a shows the cyclic voltammetric (CV) curve according to the constant voltage rate (5 mV s\({}^{-1}\)) of each carbon fiber electrode (\(-1\) V to 0 V vs. Hg/HgO). Each CV curve is similar in shape, but its area increases in the order of C\({}_{4}\)F\({}_{8}\), NH\({}_{3}\) and O\({}_{2}\) plasma. This is because the introduction of O atoms into the carbon structure via O\({}_{2}\) gas plasma is smoother than in the other cases (N atoms via NH\({}_{3}\) and F atoms via C\({}_{4}\)F\({}_{8}\)). A galvanostatic charge/discharge measurement was also conducted to confirm the charge storage performance of each carbon electrode (Figure 6b). The voltage range for measurement is from \(-1\) to 0 V for a current density of 1.0 A g\({}^{-1}\). All charge/discharge curves have linear symmetrical shapes with materials that exhibit excellent capacitive behavior. The specific capacitance of each carbon electrode calculated using the discharge time from the curve is as follows:  
Figure 4: C 1s X-ray photoelectron spectroscopy (XPS) of different multiporous carbon nanofibers: (**a**) without plasma (MPCNFs); (**b**) after O\({}_{2}\) plasma (O-MPCNFs); (**c**) after NH\({}_{3}\) plasma (N-MPCNFs); (**d**) after C\({}_{4}\)F\({}_{8}\) plasma (F-MPCNFs). 202.4 F g\({}^{-1}\) for MPCNFs, 358.2 F g\({}^{-1}\) for O-MPCNFs, 286.1 F g\({}^{-1}\) for N-MPCNFs and 268.2 F g\({}^{-1}\) for F-MPCNFs. For a better comparison, the energy storage performances of the O-MPCNF-based electrode and other carbon fiber-based electrodes are listed in Table 1.  
To confirm the rate capability of each carbon electrode, CV curves were suggested according to the voltage scan rate (from 5 mV s\({}^{-1}\) to 100 mV s\({}^{-1}\)). Figure 6c shows the CV curve of the O-MPCNF electrode, and as the voltage scan rate increases, the intensity of the responding current steadily increases, and thus the area increases. Other porous carbon nanofiber electrodes show the same phenomenon, but the intensity of the corresponding current is lower than that of oxygen plasma (Figure S4). Even on the galvanostatic charge/discharge curve of each carbon electrode, the reduction in the discharge time according to the increase in current density does not decrease sharply and remains constant (Figures 6d and S5). As the current density of each carbon electrode increases (from 1 A g\({}^{-1}\) to 5 A g\({}^{-1}\)), the calculated specific capacitance based on the discharge time changes as follows: 202.41 F g\({}^{-1}\) to 154.87 F g\({}^{-1}\) for MPCNFs; 358.21 F g\({}^{-1}\) to 299.72 F g\({}^{-1}\) for O-MPCNFs; 286.13 F g\({}^{-1}\) to 234.76 F g\({}^{-1}\) for N-MPCNFs; 268.2 F g\({}^{-1}\) to 219.24 F g\({}^{-1}\) for F-MPCNFs (Figure 6e). Comparing the reduction ratio of each carbon electrode, the oxygen-plasma-treated electrode and non-plasma-treated electrode show the minimum and maximum values, respectively (83.4% for O-MPCNFs, 81.8% for N-MPCNFs, 80.5% for F-MPCNFs and 76.5% for MPCNFs). Figure 5: Nitrogen adsorption/desorption isotherms and pore-size distribution curves (inset) of different MPCNFs: (**a**) without plasma, (**b**) after O\({}_{2}\) plasma, (**c**) after NH\({}_{3}\) plasma and (**d**) after C\({}_{4}\)F\({}_{8}\) plasma. To investigate the electrochemical and structural performance of the electrode materials, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100 kHz to 10 mHz. Figure 6f shows the Nyquist plots of the carbon nanofiber-based electrodes that suggest the semicircle is amplified in the high-frequency impedance region, a linear shape at low frequencies and the x-intercept.