## 3 Results and Discussion  
### Polymerization and Carbonization  
The SEM images of P-TSA doped PANI depict a predominantly fibrous morphology with diameters varying from 100 to 200 nm (Figure 2c,d). The formation of polyaniline in the chemical oxidation polymerization approach usually consists of three main steps: Chemical reaction, nucleation, and growth [43; 44; 45; 46]. In this case, aniline and oxidant are consumed instantly in the formation of the  
Figure 1: The schematic diagram of the preparation of NCFs. primary fibers, and the secondary growth or overgrowth of primary fibers are, therefore, suppressed during polymerization. However, the rapid mixing polymerization method does not guarantee preparation of the homogeneous polyaniline nanofibers. In this article, the organic sulfonic acid P-TSA, consisting of a hydrophilic SO\({}_{3}\)H group and a hydrophobic C\({}_{6}\)H\({}_{4}\)CH\({}_{3}\) group, acts as a surfactant and protonating agent during the formation of PANI (Figure 3). To understand the effect of P-TSA on the morphology of polymerization products, we conducted a contrast experiment. The rapid mixing polymerization was carried out under the same operating conditions except the use of P-TSA and we obtained a dark green polyaniline product named DW. The SEM image of DW shows micron and submicron lamellar structures with random orientations (Figure 2a). Obviously, P-TSA allows it to shred large agglomerates to form smaller polymer particles, and provides a significant improvement for the easy preparation of PANI nanofibers. Figure 3: The scheme of PANI doped with P-TSA. Figure 2: SEM images of **(a)** DW, **(b)** SC, **(c)** and **(d)** PANI, **(e)** and **(f)** NCFs. The stirring condition also plays a key role in rapid mixing polymerization. In the other control group, the polymerization was performed under the same operating conditions with magnetic stirring. In this process, the solutions were rapidly mixed with vigorous stirring for 12 h. Similarly, the resulting product, named SC, had a dark green color under this operating condition. The SEM image of SC depicts very few nanofibers and a host of irregular particles (Figure 2b). These three products (PANI, DW, and SC) have the same appearance of a dark green color due to the same amounts of aniline and ammonium persulfate in their polymerization process. As shown in Figure 1, NCFs were prepared by one-step carbonization of PANI at 700 \({}^{\circ}\)C for 2 h under N\({}_{2}\) atmosphere. The morphology and structure of NCFs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 2c,d, the as-prepared carbonization product maintains the morphology of primary PANI fibers with some shrinkage, the surface is smoother than PANI fibers, and the length of carbon fibers derived from polyaniline is longer than that of previous reports [31; 34]. To further determine the internal structure and porous texture, TEM images of NCFs are exhibited (Figure 4). Interestingly, both solid and hollow carbon fibers can be observed in the images (Figure 4a,b). As we know, the hollow architecture of the as-prepared NCFs may play an important role in increasing the effective utilization of active material and the cycling stability [16; 31]. In addition, as shown in Figure 4c, the element mappings reveal that a uniform distribution of N and O elements on the surface of carbon fibers exists, which is in agreement with XPS analysis. It is noted that the yield of carbonized product is about 50 wt %. This is the first time that PANI nanofibers doped with P-TSA have been used as a precursor to prepare carbon materials with a high nitrogen content. It also offers the possible development of a template-free, easy, synthetic method capable of producing bulk amounts of pure and uniform PANI nanotubes in the presence of P-TSA.