## 3 Results and Discussion  
Figure 1 shows the synthetic process and photographic image of the PCNF samples. Firstly, a nanofibers thin film structure was prepared by a simple electrospinninging technique following a stabilization process which made the thin film more stable due to cyclization and cross-linking reactions of PVP. Then the as-prepared PVP nanofibers films was heated under 600 \({}^{\circ}\)C for 2 h in argon atmosphere to produce CNF materials. At last, the PCNF sample was prepared by heating the CNF samples in air at 280 \({}^{\circ}\)C for 2 h. It is found that the obtained PCNF samples possess a great deal of mesopores which were irregularly scattered among every fiber and also exhibited good flexibility. Figure 1: (**a**) Diagrammatization of synthesis process of the polyporous carbon nanofibers films; (**b**) Photographic images of the PVP fiber, pretreated PVP fiber, CNF and PCNF. Figure 2a exhibits the XRD pattern of CNF (black) and PCNF (red) samples. It is found that almost the same diffraction pattern of CNF and PCNF samples has been obtained. In Figure 2a, the broad diffraction peaks and the inferior peak at 2\(\mathrm{\SIUnitSymbolDegree}\) of 25\(\mathrm{\SIUnitSymbolDegree}\) and 43\(\mathrm{\SIUnitSymbolDegree}\) which corresponded to the (002) graphitic layers and the (100) disordered carbon flat [42; 43], respectively. It shows that the crystalline structure of PCNF sample didn't make a difference during the air etching process, compared with that of CNF sample. In the Raman spectrum (Figure 2b), both of the CNF and PCNF sample had a couple of peaks at 1343 \(\mathrm{cm}^{-1}\) and 1588 \(\mathrm{cm}^{-1}\), which could be ascribed to D and G-band of a typical carbonaceous material. In order to characterize the microstructure of the PVP nanofibers, CNF and PCNF samples, SEM and TEM tests were carried out. Figure 3 exhibited the SEM and TEM micrographs of PVP nanofibers (a, d, g), CNF nanofibers (b, e, h) and PCNF nanofibers (e, f, i). It is found from the images that similar interconnected nanofibers were interwoven into these thin film samples. The original PVP nanofiber thin film (Figure 3a,c) was glossy on the outside and the diameter of each of the nanofibers was about 300-400 nm. So to improve the electrical property of PVP nanofibers, the samples were carbonized under Ar atmosphere. The obtained CNF thin film still maintained the original PVP nanofiber structure, but the diameter decreased to 200-300 nm and the surface became rough (Figure 3b,e). The constriction which gave rise to PVP nanofibers was mainly brought about by the evaporation of solvent and the decomposition of PVP under a high temperature. After undergoing the air etching process, the PCNF still kept a uniform cylindrical fiber structure. However, the diameter and appearance changed obviously, the carbon nanofibers occurred severely to the sink mark and the surface became more porous and rough due to the carbon reacting with oxygen (Figure 3c,f). TEM test were performed to further confirm the porosity characteristic of PCNF samples. As Figure 3g-i showed that the carbon nanofibers shrunk from 300-400 nm to 100-150 nm and the surface became obviously porous after a carbonization and an air etching process. To acquire specific information of the specific surface area and hole sizes for CNF and PCNF samples, nitrogen adsorption-desorption tests were carried out. Figure 4 exhibits shows the N\({}_{2}\) adsorption-desorption isotherms and hole sizes distribution of CNF and PCNF samples. Compared with the CNF samples (Figure 4a), PCNF nanofibers showed a type-IV isotherm with H3-type hysteresis loop and demonstrated a high specific surface area of 288.50 \(\mathrm{m}^{2}\) g\({}^{-1}\) (Figure 4c) which was twice that of the CNF (117.84 \(\mathrm{m}^{2}\) g\({}^{-1}\), Figure 4a), implying a more porous structure which could provide more active sites for lithium storage. This is attributed to oxygen reacting with carbon nanofibers during the air etching process.