# Electrospinning-Based Carbon Nanofibers for Energy and Sensor Applications
## 4 Application of Carbon Nanofibers
### Energy Storage Applications
#### 4.1.1 Supercapacitor  
[PERSON] et al. demonstrated porous, hollow CNFs that were applied to a supercapacitor (Figure 12a) [90]. Using a dual nozzle with poly(methyl methacrylate) (PMMA) solution for the inner nozzle and PAN/PVP solution for the outer nozzle. The dual-nozzle configuration formed a three-layer structure containing PMMA/PAN/PVP, respectively, and minimized the thickness of the PAN layer. Moreover, the defects of the interface between PAN and PVP created pores in the carbon structure, which further improves the volume ratio of carbon material. In addition, the authors also proved that the use of PMMA for the inner nozzle can make the PAN fibers thinner compared to the fibers that used mineral oil for the inner nozzle. With a large pore volume, the porous, hollow CNF electrode showed higher response frequencies, lower resistance with the electrolyte, and higher capacitance.  
Besides increasing the surface ratio of the CNFs, one of the other common directions for enhancing the working capacity of the supercapacitor is metal-oxide decoration [91]. [PERSON] et al. investigated CNFs containing cyclodextrin (CD) surfactant, MnO\({}_{2}\), and graphene (Figure 12b) [92]. Surfactants such as \(\alpha\)-CD, \(\beta\)-CD, and \(\gamma\)-CD can bind to MnCl\({}_{2}\) and absorb on the graphene sheets, creating the MnCl\({}_{2}\) shell structure cover around the graphene materials. The graphene material in this study improves the electrical conductivity of CNFs, as well as electrochemical performance. Though the percentage of MnO\({}_{2}\) is very small, the capacitance of the material can be as high as 250 F g\({}^{-1}\). The effects of different surfactants were also observed, and the results showed that \(\alpha\)-CD can provide the largest pore volume. The surface resistance and capacity of CNFs with \(\alpha\)-CD also have the best value.  
[PERSON] et al. introduced flexible carbon fibers using the PAN precursor for the core material (Figure 12c) [93]. The carbonization process was carried out at a low temperature of 600 \({}^{\circ}\)C so that the fiber could retain its flexibility. During the 600 \({}^{\circ}\)C carbonizations, the PAN structure was not completely carbonized, hence its electrical conductivity was low. It was noted that the diameter of the PAN fiber was only 350 nm. Then, the fibers were covered with a layer of reduced graphene oxide (rGO) using tin chloride (SnCl\({}_{2}\)) and a low concentration of hydrochloric acid [94]. The low conductivity of the carbon/PAN fiber was improved due to the high conductivity of rGO. Finally, the fibers were coated with polypyrrole (PPy) to stabilize the rGO layer as well as enhance the flexibility of the fibers. Not only did the authors operate the synthesized material as the electrodes for a coupling capacitor, but they also analyzed the contribution of diffusion and capacitive in the capacitance of the electrode. Another flexible supercapacitor based on the Sn/SnOanchored by a carbon skeleton was made by [PERSON] et al. (Figure 12d) [95]. The authors prepared two different electrospinning solutions for the fabrication of the two electrodes. The cathode material was made from the precursor solution containing SnCl\({}_{2}\), graphene quantum dots (GQD), and PAN. After carbonization at 500 \({}^{\circ}\)C in the N\({}_{2}\) environment, only some of the SnO\({}_{2}\) particles were decomposed into Sn particles. The resulting carbon fibers were completely covered with GQDs, Sn, and SnO\({}_{2}\) nanoparticles, which ensured the CNFs' good electrical conductivity. By using the CNFs as the base for the particles, the capacity was greatly enhanced along with the stability over many cycles. The anode electrode material was made from the solution containing only PAN and GQD. Finally, a thin flexible capacitor can be fabricated with the two electrodes.