## 3 Results and Discussion
### Electrochemical Performance of the Multiporous Activated Carbon Nanofiber-Based Electrodes
In the plot, the x-intercept starting with the semicircle and the diameter of the semicircle represent the equivalent series resistance (ESR) and the charge transfer resistance at the electrode-electrolyte interface (R\({}_{\text{ct}}\)), respectively. The ERS value, i.e., the charge transfer resistance in the electrode structure, does not differ significantly depending on the material (0.82 \(\Omega\) for MPCNFs, 0.33 \(\Omega\) for O-MPCNFs, 0.45 \(\Omega\) for N-MPCNFs and 0.58 \(\Omega\) for F-MPCNFs). On the other hand, the R\({}_{\text{ct}}\) value, which can be calculated using the diameter of the semicircle, tends to decrease with increasing surface area of the carbon structure (0.95 \(\Omega\  
for O-MPCNFs, 1.18 \(\Omega\) for N-MPCNFs, 1.54 \(\Omega\) for F-MPCNFs and 2.14 \(\Omega\) for MPCNFs). This is because the contact area between the electrode and the electrolyte is increased due to the formation of micropores in the carbon by the introduction of heterogeneous elements, thereby facilitating charge transport. Electrochemical Performance of the Multiporous Activated Carbon Nanofiber in a Symmetric Supercapacitor  
To further confirm the electrochemical performance of the multiporous carbon nanofibers, they were applied as electrodes in a symmetrical supercapacitor device. The structure of the symmetrical supercapacitor consists of a separator with a 1 M KOH aqueous solution between two carbon fiber-based working electrodes placed onto a basic frame designed using acrylic (Figure S6). Figure 7 shows the electrochemical performance of each carbon electrode in a symmetrical supercapacitor device. The CV curve of each device was measured from 0 V to 1 V. The current of the symmetrical supercapacitor device varied with increasing voltage scan rate, while all four types of carbon electrode maintained a similar rectangular shape (Figure 7a and Figure S7). However, the plasma-treated carbon electrode shows a wider curved area than the non-plasma-treated carbon electrode, and the oxygen-plasma-treated carbon electrode has the largest curved area. Comparing the galvanostatic curve of each supercapacitor device depending on the change in current density, the change in the graph shape according to the increase in current density is not shown, and the shortening of the discharge time is also maintained in a constant ratio (Figure 7b and Figure S8). The specific capacitance of each device at 1 A g\({}^{-1}\) is as follows: 96 F g\({}^{-1}\) (MPCNFs); 173.28 F g\({}^{-1}\) (O-MPCNFs); 138.12 F g\({}^{-1}\) (N-MPCNFs); 128.2 F g\({}^{-1}\) (F-MPCNFs) (Figure S9). In addition, the specific capacitance change in each device with increasing current density (from 1 A g\({}^{-1}\) to 5 A g\({}^{-1}\)) is 73.4% (93.44 F g\({}^{-1}\)) for MPCNFs, 84.2% (145.91 F g\({}^{-1}\)) for O-MPCNFs, 81.7% (112.74 F g\({}^{-1}\)) for N-MPCNFs and 80.1% (102.68 F g\({}^{-1}\)) for F-MPCNFs. The EIS method was applied to confirm the charge transfer of each storage device (Figure 7c). The ESR and R\({}_{\text{ct}}\) values of each storage device show similar patterns to those of the three electrodes. However, the slope value of the straight line at low frequencies varies depending on the material. The devices based on non-plasma-treated carbon electrodes (MPCNFs) exhibit a straight-line slope of about 45\({}^{\circ}\) at low frequency. However, when the carbon electrodes exposed to plasma are treated in sequence with C\({}_{4}\)F\({}_{8}\), NH\({}_{3}\) and O\({}_{2}\) plasma, the slope of the straight line approaches 90\({}^{\circ}\). This is because the supercapacitor device behaves like an ideal capacitor as the movement of charges becomes smooth due to the micropores generated by the plasma treatment. The verification of the repeatable energy storage performance of a supercapacitor device is an essential factor for the actual application of the device. Figure 7d shows the specific capacity changes in the energy systems according to 5000 repeated charging/discharging experiments with constant current (1 A g\({}^{-1}\)).