# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 6 Supercapacitors  
A wide branch of research on energy storage is focused on the electrochemical capacitors [174; 175], better known as supercapacitors and also including pseudocapacitors. Supercapacitors and pseudocapacitors differ for the mechanism through which the charge is stored. In the former, the charge storage is based on the fast electrostatic interactions at the electrode surface in contact with the electrolyte (Figure 19)--during the charging process, the ions from the electrolyte are reversibly adsorbed on the high-surface-area electrodes, finally leading to an electrical double layer (EDL). In the latter, fast and reversible redox (Faradaic) processes mainly take place at the surface of the electrodes. Graphene and other carbonaceous materials show EDL behavior and are featured by high power densities [175]. Transition metal oxides (such as RuO\({}_{2}\), MnO\({}_{2}\) Fe\({}_{3}\)O\({}_{4}\), V\({}_{2}\)O\({}_{5}\), and Fe\({}_{3}\)O\({}_{4}\)[48; 176; 177]) show pseudocapacitive behavior and possess high energy density [143]. Hybrid capacitors, consisting of a pseudocapacitive electrode and an EDL electrode, try to efficiently combine the two characteristics. In spite of the higher specific capacitance generally delivered by materials with pseudocapacitive behavior [48; 176; 177; 178], in recent years, the research efforts are being increasingly focused on \(sp^{2}\)-carbon-based materials, which, with their tunable pore structure, abundant surface functionalities, and active defective sites, offer appealing advantages for all applications concerning energy conversion and storage [179] and, in the cases of supercapacitors, also the possibility of fabricating stretchable devices [180]. Since good performance of capacitors with EDL behavior requires appropriate pore size distribution (micro-, meso-, and macro-pores), high electrochemically accessible surface area, and high electrical conductivity [175], porous carbon materials have received great attention. Among them, electrospun carbon fibers, which couple short diffusional paths and flexibility to high conductivity, large aspect ratio, and surface area, represent the most promising candidates. Table S5 reports some examples of electrode ENMs and their preparation conditions; the main parameters describing their performance are reported in Table S6. PAN [181; 182; 183; 184; 185; 186; 187] or a blend of PAN and PVP [52; 53; 54; 59] is the most frequently utilized polymeric component of the spinnable solution, whereas \(N\),\(N\)-dimethylformamide (DMF) commonly acts as a solvent [52; 53; 54; 59; 181; 182; 183; 184; 185; 186; 187]. A variety of different additives is employed to generate carbon fibers with specific textural, compositional, or conductive properties [46; 47; 59; 182; 183; 184; 185; 186; 187]. By the use of a blend of PAN and PVP dissolved in DMF, [PERSON] et al. [54] have obtained non-woven membranes consisting of porous nitrogen-doped carbon fibers, whose porosity and nitrogen-content can be easily controlled by adjusting the relative-to-PAN PVP content (150-240/100 wt%/wt%) of the solution. As binder-free electrodes, the membranes are able to deliver specific capacitance of 148-198 F/g and exhibit excellent cyclic stability with a retention rate of 104% after 5000 cycles at a rate of 1 A/g [54]. Figure 19: Schematic illustration of a supercapacitor in (**a**) discharged and (**b**) charged states. [PERSON] et al. [59] have produced binder-free flexible electrodes for supercapacitors consisting of nitrogen-enriched carbon fiber networks, by adding silica (SiO\({}_{2}\)) NPs to the spinnable PAN+PVP/DMF solution (Figure 20a-c). SiO\({}_{2}\) NPs act as a as a sacrificial templating agent and, after carbonization (Figure 20d-f), are removed by chemical NaOH etching (Figure 20g), finally giving rise to the formation of meso-macroporous fibers with a pretty morphology featured by conjugated surfaces (Figure 20h,i). The specific capacitance delivered by these electrodes is 242 F/g at a rate of 0.2 A/g.