# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 6 Supercapacitors
They are able to retain 99% of the initial capacitance after 5000 cycles [59]. The size of the electrodes tested in the research works is generally small (their surface area does not usually exceed 1 cm\({}^{2}\)[54; 59; 183; 184]). [PERSON] et al. [182] have built supercapacitor cells by assembling two 2.25 cm\({}^{2}\) electrodes and, more remarkably, have demonstrated the possibility of depositing an electrospun thin A3-sized (297 mm \(\times\) 420 mm) fibrous membrane. The addition of (1\(-\)5 wt%) zinc chloride to the PAN/DMF solution, enhancing its conductivity, brings about a fiber diameter shrinking during the spinning stage (from 350 to 250 nm). During the subsequent heat treatments (stabilization and carbonization), producing a further fiber thinning, ZnCl\({}_{2}\) exerts a catalytic function, accelerating the oxidative stabilization rate and creating micropores on the outer surface of the fibers by etching C atoms [182]. The specific capacitance of the electrodes based on electrospun carbon fibers (Table S6) usually does not exceed 250 F/g [52; 53; 54; 59; 181; 182; 183; 184; 188]. Various additives, such as graphene nanoplatelets [184], biochar [183], and carbon nanotubes (CNTs) [46; 47; 185], have been used to improve their performance. By adding 12 wt% CNTs to the polyaniline/polyethylene oxide (PANI/PEO) spinnable solution, [PERSON] et al. [46] have fabricated free-standing, binder-free supercapacitor electrodes by a single-step process requiring no heat treatment. The PANI/CNTs-based electrodes are able to deliver a capacitance of 385 F/g at a current density of 0.5 A/g with a retention capability of 81.4% after 1000 cycles. The three-dimensional electrode structure characteristic of the non-woven interconnected fiber network is responsible for their good electrochemical performance since it facilitates efficient  
Figure 20: (**a**,**d**,**g**) Fabrication schematic of nitrogen-enriched carbon fiber (NCF) mat and related (**b**,**e**,**h**) SEM and (**c**,**f**,**i**) TEM images. (**a**–**c**) Polyacrylonitrile (PAN)/polyvinyl pyrrolidone (PVP)/SiO\({}_{2}\) polymer fibers; (**d**–**f**) carbon/SiO\({}_{2}\) carbonized fibers; (**g**–**i**) NCF fibers (reproduced from reference [59] with permission. Copyright 2016, Elsevier). electron conduction, while the inter- and intra-fiber porosity enables excellent electrolyte penetration within the polymer matrix, allowing fast ion transport to the active sites [46]. [PERSON] et al. [47] have used a similar single-step process to fabricate self-supporting supercapacitor electrodes by electrospinningning a CNT-enriched polyindole-based polymeric solution directly on stainless steel current collectors (Figure 18a,b). The flexible symmetric all-solid-state supercapacitor fabricated by interposing PVA/H2 SO4 gel electrolyte between the two so-prepared electrodes (Figure 21c,d) is able to deliver 476 F/g at a current density of 1 A/g, with an excellent retention capability (95% after 2000 cycles) [47]. At the same rate, much higher specific capacitance (1119 F/g) has been obtained by [PERSON] et al. [185] by coating CNT-enriched carbon fibers (produced by stabilization and carbonization of as-spun PAN/CNTs fibers under conditions reported in Table S5) with aniline monomer, through in situ chemical polymerization, to form PANI-coated C/CNTs fibers (Figure 22). At very high current density (10 A/g), these composite fibers are still able to deliver 748 F/g. Their high specific capacitance is attributed to the synergic effects of the combination of the high surface area resulting from CNT incorporation, the pseudocapacitive property of PANI, which increases the electrical conductivity, the interaction between the CNT-enriched C fibers and PANI, and the uniform distribution of CNTs within the fibers [185]. PANI-coated C/CNTs fibers are able to retain 98% of the initial capacitance even after 2000 cycles at a current density of 10 A/g [185]. Figure 21: Fabrication of all-solid-state flexible supercapacitor.