# Electrospinning-Based Carbon Nanofibers for Energy and Sensor Applications
## 4 Application of Carbon Nanofibers
### Energy Storage Applications
#### 4.1.5 Catalyst  
Recharge energy processes such as metal-air batteries, fuel cells, and water spitting require electrocatalysis, such as the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), or hydrogen evolution reaction (HER) [115; 116; 117]  
[PERSON] et al. used \(\beta\)-CD and PAN to make precursors for the fabrication of the porous N-doped CNFs (Figure 16a) [118]. Although the \(\beta\)-CD surfactant had been used in numerous studies as mentioned in the previous sections, the nitrogen function group present in the structure after carbonization had not been thoroughly analyzed. The authors suggested that although \(\beta\)-CD could attach to the PAN chain via hydrogen bonding, it would be partially decomposed when calcined at high-temperature calcination and form the functional groups. The XPS analysis illustrated that the CNFs with \(\beta\)-CD have a higher percentage of graphitic-N and pyridinic-N in their structures compared to normal CNFs. The nitrogen functional groups play the role as the catalyst for the OER and ORR. In the case of the ORR, the electron-donating of graphitic is the active side, and the electron-withdrawing of pyridinic-N is the active side of the OER [119; 120]. In addition, the \(\beta\)-CD-containing CNFs also have a large number of oxygen functional groups that will react to operate the system. The authors also presented the theory of operational mechanisms. Though the surfactant did not show any effects on the BET, the working performance was improved. In applications, the rechargeable zinc-air batteries that used the developed material also have outstanding stability over time.  
[PERSON] et al. tried to decorate cobalt nanoparticles on CNFs made from a PAN precursor (Figure 16b) [121]. After fabrication, the activities of the cobalt nanoparticles were investigated throughout. Although not an ideal material to be used in the ORR system, it can act as an OER catalyst better than the traditional Pt/C electrode. Later, [PERSON] et al. fabricated CNFs containing metal alloy particles of nickel, cobalt, and iron (Figure 16c) [122]. The carbon source used in the work is PVP; after a longtime stabilization, PVP can turn into carbon at temperatures up to 800 \({}^{\circ}\)C [123; 124]. PVP requires a lower carbonization temperature and the carbon structure can be naturally doped with nitrogen. On the other hand, the mechanical properties of the produced CNFs are not good, and the fibers can be easily broken during the carbonization process. The combination of the three metals shows further improvements in the catalyst activities. It is noted that when activated as an ORR catalyst, the current density and halfwave potential of the synthesized material can be compared with the counter Pt/C electrode. Yet, when activated as a catalyst for an OER, the decorated material can work much better compared to Pt/C or CNFs embedded with other combinations of the used ingredients. Hence, it can be said that carbon-based materials have the potential to replace Pt materials, which are known for their expensive costs. Furthermore, the authors investigated the effects of different ratios of the ingredients on the working performance of the catalyst. It seems that the alloy that has Ni, Co, and Fe with a ratio of 1:1:1 has the greatest catalytic properties.  
Inspired by the rich-in-nitrogen functional groups of fibroin fibers, [PERSON] et al. used the processed fibroin for electrospinning to make CNFs (Figure 16d) [125]. After a complicated process to remove the impurities, the regenerated silk was dissolved in a solution containing potassium chloride (KCl). It is said that the KCl not only reduces the diameter of the electrospun nanofiber but also affects the crystalline structure of the polymer composite. Although KCl can produce a porous structure for the CNFs, the high concentration of KCl (higher than 5%) cannot create plentiful pores. Among the samples, the one containing 4% of KCl exhibits the highest catalytic activity in HER application in both acidic and alkaline environments.