## 3 Results and Discussions  
The overall fabrication process for PAN-derived electrospun carbon nanofiber mats is briefly illustrated in Figure 1. PAN was selected as the spinning precursor due to its high carbon content and stable structure during the carbonization process. Generally, as a kind of current collecting material, a carbon nanofiber mat requires both self-supporting flexibility and excellent electrical conductivity. However, there is always a trade-off between flexibility and conductivity during the carbonization process. In order to improve the toughness of carbon nanofibers while maintaining its excellent conductivity, MWCNTs (XFNANO, Nanjing, China) were added into PAN precursor as supporting material. Furthermore, the electrochemical performances of the carbon nanofibers can be further improved by the activation of nitric acid and sulfuric acid. Their strong oxidation abilities not only produce more active groups on the surface but also acquire a rough nanofiber surface, which is beneficial for further deposition of other active materials. Figure 2 displays optical micrographs of the electrospun PAN nanofibers with PAN mass fraction of 8, 10, 12 and 15 wt%, respectively. It is difficult to form nanofibers with 8 wt% PAN due to a low polymer viscosity. As the mass fraction of PAN increases to 12 wt%, the continuous nanofibers with uniform diameters can be collected. In this work, the fiber yield is determined by the number ratio of fiber to particles in the same field of view. Compared to 15 wt% PAN, a higher yield of nanofibers is gained in the case of 12 wt% PAN. A higher concentration of PAN would cause polymer aggregation in some areas. Hence, the precursor concentration is one of the key factors in spinning continuous and uniform nanofibers. In the following investigation, we chose 12 wt% PAN as the spinning precursor. It is well known that after carbonization of electrospun PAN nanofibers in a nitrogen atmosphere, elements such as hydrogen (H), oxygen (O) and nitrogen (N) in the fibers can be removed, excluding the element carbon (C). However, the carbonization process is prone to destroying the internal structure of the nanofibers and making the nanofibers fragile. Therefore, it is necessary to conduct a thermal stabilization process to ensure the structure stability before the nanofiber carbonization. In this work, the influence of thermal stabilization and carbonization temperatures on the morphology, conductivity and flexibility of carbon nanofiber mats were explored. The thermal stabilization and carbonization temperature rise rates were controlled in the range of 1.5-2 \({}^{\circ}\)C/min and 4-6 \({}^{\circ}\)C/min, respectively. The nanofiber mats with 12 wt% PAN were heat-treated with a thermal stabilization temperature of 220, 250, 270 and 300 \({}^{\circ}\)C, and a carbonization temperature of 800 \({}^{\circ}\)C. The carbonized nanofiber mats were correspondingly named as samples C-1, C-2, C-3 and C-4, respectively. Their sheet resistances and average diameters are listed in Table 1. Samples C-1 and C-2 present shrinking and a corrugated surface, indicating that the nanofibers are not stable enough to adapt to the stress release during the carbonization process at a low thermal stabilization temperature. With the thermal stabilization temperature elevated to 270 and 300 \({}^{\circ}\)C, the samples C-3 and C-4 show a deep black color and large,  
Figure 2: Optical micrographs of PAN nanofiber films electrospun with PAN mass fraction of (**a**) 8 wt%; (**b**) 10 wt%; (**c**) 12 wt% and (**d**) 15 wt%, respectively. The scale bar is 100 \(\upmu\)m.  
smooth surface. The carbonization temperature of the samples C-5 and C-6 is further increased to 900 and 1000 \({}^{\circ}\)C, while the thermal stabilization temperature is maintained at 270 \({}^{\circ}\)C. The sheet resistance of six samples is decreased from 665 to 116 \(\Omega\)/sq. The conductivity enhancement is mainly attributed to the increase of carbonization temperature, leading to the relatively higher carbonization degree.