# Electrospinning-Based Carbon Nanofibers for Energy and Sensor Applications
## 3 Different Carbon Nanofibers Obtained by Electrospinning
### Carbon Nanofibers from Modified Single-Nozzle Electrospinning  
#### 3.2.1 Porous Carbon Nanofibers  
To enhance the material's work efficiency, researchers tried to make some modifications to CNFs during or after the carbonization process. One of the most common ways is to increase the surface area of the CNFs by combining more than one type of polymer in one syringe and electrospinning using one single nozzle. Although the carbon precursor polymer turns to carbon material at high temperatures, the other polymers decompose and evaporate. Finally, the resulting fibers will contain holes and defects [68]. The physical interaction between two polymers can create many shapes that can significantly change the surface area and this, in turn, can contribute to better uses in applications [69]. However, the structure of the precursor electrospun nanofiber may become unstable during the carbonization process due to the decomposition of the material's components. This limitation means that the fabrication of CNFs requires numerous optimizations in terms of stabilization, carbonization time, and temperature.  
[PERSON] et al. used a mixed solution of PAN and polystyrene (PS) to fabricate multiscale porous CNFs for transistor biosensors (Figure 8a) [70]. After the fabrication of the multichannel CNFs, the multichannel CNFs were steam-treated in the carbonization process to create pores on the wall [71]. The structure of multichannel CNFs helps to increase the surface area to a very impressive number of 1350 m\({}^{2}\) g\({}^{-1}\). Later, the multichannel CNFs were attached to the surface of a transistor as a channel for Nesfatin detection. The porous structure makes the device capable of detection even for a very low target concentration and short response time. Moreover, the device showed excellent selectivity toward other biomaterials. Other sponge-like carbon fibers were created by [PERSON] et al. using the macro-micro dual-phase separation method (Figure 8b) [72]. The authors used poly (tetrafluoroethylene) (PTFE) nanoparticles as pore inducers, PVA as a carbon precursor, and boric acid as a crosslinker. The generated CNFs exhibited extremely high porosity, up to 80%, and a large surface area of 750 m\({}^{2}\) g\({}^{-1}\). Although the large surface area increases the electrode's ability to function as a supercapacitor, the B-F-N functional group doped material provides the carbon film with high conductivity as well as a rapid transfer of matter such as CO\({}_{2}\) or methylene blue. In addition, the solvent used for this work was water, which is an environmentally friendly solvent.  
It is noticeable that surfactants can work as a separating agent that can form the shape of the precursor polymer and the carbon material [73]. The surfactant cyclodextrin (CD) is known to be good material for carbon material since it can be decomposed into carbon layers with an sp\({}^{2}\) network. [PERSON] et al. used \(\beta\)-CD and PAN as porous CNF precursors (Figure 8c) [74]. In optimized conditions, the surface area can be increased to 156 m\({}^{2}\) g\({}^{-1}\). The supercapacitor not only operates at a smaller potential window compared to the device fabricated for a pure PAN precursor but also the IR drop value appears to be smaller.  
Electrospinning using a combination of the precursor of carbon and secondary materials can significantly alter the physical structure of CNFs, which leads to improvements in the working capacitance of the carbon material. However, the surface tension between the ingredients can result in many difficulties for the spinning process.