## 3 Results and Discussions
However, with the increase of carbonization temperature and the reduction of other components except carbon in the nanofibers, the nanofiber structure would become fragile, resulting in poor flexibility (the flexibility of the nanofiber mats is determined by bending and folding the mats for several times), especially in the case of 1000 \({}^{\circ}\)C carbonization. Figure 3a-f display the SEM images of samples C-1, C-2, C-3, C-4 C-5 and C-6, respectively, demonstrating that the morphologies and average diameters of the obtained carbon nanofiber mats were distinctly influenced by the thermal stabilization and carbonization temperature. The fiber average diameters are determined by measuring the diameters of 30-50 fibers in the SEM images. Figure 3a shows that sample C-1 is composed of cross-linked nanofibers forming a three-dimensional porous carbon structure with pore size ranging from tens of nanometers to several micrometers. A magnified SEM image of sample C-1 is provided in Figure S1 contained in the supplementary. As the thermal stabilization temperature increased, the crosslinking phenomenon gradually disappeared. The average diameter of the carbon nanofibers is around 200 nm for samples C-2 and C-3, but 300 nm for sample C-4, as shown in Figure 3b-d. The magnified SEM images of samples C-2, C-3 and C-4 are displayed in Figure S2a-c contained in the supplementary. It is supposed that further reaction happens with a thermal stabilization temperature of 300 \({}^{\circ}\)C, leading to a decrease of structure stability, and the nanofibers might melt during the carbonization process. Thus, the average diameter of the carbon nanofibers increases from 200 to 300 nm. Figure 3e,f shows that even if the carbonization temperature is elevated to 900 and 1000 \({}^{\circ}\)C, the average diameters of the carbon nanofibers for the samples C-5 and C-6 remain approximately 200 nm (higher magnification of SEM images are presented in Figure S3a,b contained in the supplementary), demonstrating that the nanofiber diameter is mainly dependent on the thermal stabilization temperature. However, a higher carbonization temperature makes the nanofibers crack, as shown in Figure 3f. Using the above analysis, with a thermal stabilization temperature of 270 \({}^{\circ}\)C and a carbonization temperature of 900 \({}^{\circ}\)C, the nanofibers show a uniform average diameter of around 200 nm, low resistance of 154 \(\Omega\)/sq and good flexibility. Thus, these temperatures are regarded as the optimized experimental factors in the following investigation. XRD patterns of the carbon nanofiber mats at various thermal stabilization and carbonization temperatures were obtained to elucidate the structures. Figure 4 suggests that there are two diffraction peaks located at 20 = 16.2\({}^{\circ}\) and 22.6\({}^{\circ}\) in all patterns of the six samples, which respectively refers to the lateral packing of crystalline regions in PAN linear macromolecules and the conjugated ladder polymer structure along the chain axis [27; 28; 29]. The relative intensity ratios of 20 = 16.2\({}^{\circ}\) to 20 = 22.6\({}^{\circ}\) for the samples C-1, C-2, C-3, C-4, C-5 and C-6 are correspondingly 1.19, 1.06, 1.03, 1.09, 0.98 and 1.12, demonstrating that sample C-5 has the largest relative intensity ratio. When the nanofibers were \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Thermal Stabilization** & **Carbonization** & **Sheet Resistance** & **Average Diameter** \\  & **Temperature (\({}^{\circ}\)C)** & **Temperature (\({}^{\circ}\)C)** & **(f)/sq)** & **(nm)** \\ \hline
**C-1** & 220 & 800 & 665 & Interconnected networks \\
**C-2** & 250 & 800 & 661 & 200 \\
**C-3** & 270 & 800 & 651 & 200 \\
**C-4** & 300 & 800 & 517 & 300 \\
**C-5** & 270 & 900 & 154 & 200 \\
**C-6** & 270 & 1000 & 116 & 200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sheet resistance and average diameter of the PAN-derived carbon nanofibers with 12 wt% PAN treated at various thermal stabilization and carbonization temperatures.