## 3 Results and Discussions
carbonized at a certain temperature, the chemical structure was transformed, ascribed to chemical reactions such as cyclization, dehydrogenation, aromatization and oxidation, which could result in the formation of the conjugated ladder structure [27]. Thus, sample C-5 exhibits the most stable structure among the six samples. Furthermore, a weak and wide peak can be observed at 42.7\({}^{\circ}\), corresponding to (100) reflections of carbon materials, which generally appears in the carbon nanofibers with a carbonization temperature over 1000 \({}^{\circ}\)C [29]. Thus, the result indicates that the spun nanofibers are more likely to be completely carbonized at a relatively low carbonization temperature as undergoing a thermal stabilization process. Electrochemical performances of the PAN-derived carbon nanofiber mats were also investigated. Figure 5a displays CV curves of sample C-5 with the scan rate of 5-600 mV/s and a potential window of \(-\)0.8-0 V. There is no redox peak in the curves under various scanning speeds, revealing the absence of defects or impurities in the carbon nanofibers. The shape of the curve is closer to the rectangle at a low  
Figure 4: X-ray diffraction (XRD) patterns of the PAN-derived carbon nanofiber mats treated at various thermal stabilization and carbonization temperatures. Figure 3: Scanning electron microscopy (SEM) images of PAN-derived carbon nanofiber mats of (**a**) C-1, (b) C-2, (**c**) C-3, and (**d**) C-4; SEM images of PAN-derived carbon nanofibers of (**e**) C-5 and (**f**) C-6. scan rate. With the scan rate enhanced, the deviation in shape is gradually increased. The GCD curve of sample C-5 at the current density of 0.1 mA/cm\({}^{2}\) is plotted in Figure 5b. The IR drop (where I and R represents the current and resistance, respectively) of the carbon nanofiber is 0.22 V. Furthermore, the specific capacitance of the carbon nanofiber mat is calculated to be 25.0 F/g from the GCD curve. To understand the diffusion effect on the electrochemical performances of the carbon nanofiber mat, the EIS spectrum was carried out in a frequency range from 1 MHz to 0.01 Hz, as shown in Figure 5c. The impedance spectrum consists of two parts with a semicircle in the high frequency region and a straight slope in the low frequency region. The internal resistance value of the carbon nanofiber mat is around 100 \(\Omega\), and the large radius of the semicircle is mainly attributed to the relatively low conductivity of nanofiber mats. Based on the above analysis, the electrochemical performance of the PAN-derived carbon nanofiber mats should be further improved in the application of energy storage devices. In order to avoid any side reaction or introducing other elements, we added MWCNTs into PAN precursor to co-spin the nanofibers due to the high intensity and good conductivity of MWCNTs. The effects of MWCNT addition on the average diameter and sheet resistance of the carbon nanofiber mats are listed in Table 2. The added quantity of MWCNTs is 0.05, 0.10 and 0.15 g, corresponding to the samples PMC-1, PMC-2 and PMC-3, respectively. The resultant mass fraction of MWCNTs is 0.7 wt%, 1.5 wt% and 2.4 wt%. The co-spun nanofibers were treated at a thermal stabilization temperature of 270 \({}^{\circ}\)C and carbonization temperature of 900 \({}^{\circ}\)C. By comparison, the conductivity of the carbon nanofiber mats was greatly modified by adding MWCNTs. The sheet resistances are decreased to 66, 59 and 60 \(\Omega\)/sq for the three samples. Figure 6a-c display SEM images of the samples PMC-1, PMC-2 and PMC-3 after carbonization. The average diameter of the carbon nanofibers for three samples is increased from 140 to 240 nm, as the mass fraction of MWCNTs enhanced from 0.7 to 2.4 wt%. Moreover, the surface of the carbon nanofibers would be rougher, and there are some bulges on the nanofiber surface (Figure 6c). It might be ascribed to the exposure of several MWCNTs to the nanofiber surface as the MWCNT addition increased.