# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 4 Secondary Redox Flow Batteries  
As mentioned above, the predicted severe shortage of the lithium reserves in less than 65 years [105] will result in serious concerns for the future production costs of LIBs, whereas the SIB technology has not reached the needed maturity yet. In this scenario, the secondary redox flow batteries (RFBs) gather great interest among the traditional large-scale EESs. They can convert chemical  
Figure 15: (**a**) Schematic illustration of the preparation process for SnS\({}_{2}\)/NSDC (N\(\mathcal{S}\)-doped dual-doped carbon) fibers. (**b,c**) Different magnification FE-SEM and (**d,e**) TEM images of the fibers. (**f–i**) EDX elemental mapping of the area, marked by the red dotted rectangular in the image (**d**), for (**f**) C, (**g**) N, (**h**) S, and (**i**) Sn components (reproduced from reference [139] with permission. Copyright 2018, Elsevier). energy into electricity by the oxidation-reduction reactions of positive and negative reactants [142]. Thanks to their excellent performance in terms of efficiency, life cycle, environmental friendliness, and low toxicity, particular attention is devoted to vanadium-RFBs, considered prospective future EESs since vanadium is more abundant in nature than lithium and its cost is much lower. In a first-generation vanadium-RFB [143], positive (VO\({}^{2+}\)/VO\({}_{2}\)\({}^{+}\) sulfate) and negative (V\({}^{2+}\)/V\({}^{3+}\) sulfate) electrolytes are stocked in tanks, and a proton-exchange membrane separates the electrodes in the reaction cell (Figure 16). The energy conversion in the cell occurs through the chemical reaction VO\({}^{2+}\)+ H\({}_{2}\)O + V\({}^{3+}\)\(\leftrightarrow\) VO\({}_{2}\)\({}^{+}\) + 2H\({}^{+}\) + V\({}^{2+}\) (\(E^{0}\) = 1.26 V), with VO\({}^{2+}\)+ H\({}_{2}\)O \(\leftrightarrow\) VO\({}_{2}\)\({}^{+}\) + 2H\({}^{+}\) + \(e^{-}\) (\(E^{0}\) = 1 V) and V\({}^{3+}\)\({}^{+}\)\({}^{-}\)\(\leftrightarrow\) V\({}^{2+}\) (\(E^{0}\) = \(-\)0.26 V) reactions taking place at the positive and negative electrode, respectively [142; 144; 145]. Since the mid-1980s, when the research on vanadium-RFBs began [143], new generations of systems have been proposed to improve the stability and the solubility of vanadium ions over a wider range of temperature [142]. Second-generation vanadium-RFBs utilize a mixture of vanadium chloride and vanadium bromide as the electrolyte, whereas third-generation systems operate with mixed H\({}_{2}\)SO\({}_{4}\)/HCl electrolyte solutions in both half-cells [142]. Advances and prospects in the field of RFBs have exhaustively been illustrated in the literature (see f.i. [142; 146]). The strength point of such systems is that the battery capacity can be increased by scaling up the tank volume to contain larger amounts of electrolytes, and the power output can be increased by adding more reaction cells. However, the performance of a battery ultimately depends on its components. Various types of membranes are used in the RFBs. Among them, commercial (cation exchange) Nafion membranes are widely utilized. They exhibit high chemical stability and excellent electrochemical properties, but, conversely, suffer from high permeability and cost [142]. The poor electrochemical activity of graphite felt (GF), commonly employed as an electrode material, limits the practical application of vanadium-RFBs [145; 147]. Although research is still addressed toward carbon-based electrodes due to their low cost, new strategies have been thought up to enhance the electrochemical activity of electrode materials for vanadium-RFBs. Increasing their effective surface area through nanostructuring and accelerating the electrochemical kinetics for redox reactions thanks to the development of novel electrocatalysts represent the usually followed approaches [144]. In all the cases, the use of ENMs appears to be a viable route to improve the RFB performance [148; 149; 150].