# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 4 Secondary Redox Flow Batteries
Just to cite some examples, better performance of non-aqueous vanadium acetylacetonate-based RFBs, in terms of coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE), have been obtained at all of the tested current densities thanks to selective membranes fabricated by depositing Nafion/polyvinyl alcohol  
Figure 16: The schematic illustration of a first-generation vanadium-redox flow battery (RFB) (reproduced from reference [144] with permission. Copyright 2018, Elsevier). (PVA) fibers directly on a rotating drum collector wrapped with a porous Celgard 2400 membrane [151]. Compared to the commercial dispersion-cast Nafion NR 212 membrane, the fibrous Nafion/PVA membrane exhibits lower swelling ratio and lower vanadium acetylacetonate permeability. As for the electrode materials, several interesting results have been achieved by the use of electrospun carbon-based fibers as the negative electrode in RFBs [145; 152; 153]. Generally, the electrode performance of woven electrospun fibrous membranes is limited by low porosity and poor permeability that hampers the mass-transport during the RFB operation. To tackle this problem in vanadium-RFBs, [PERSON] et al. [152] have designed a freestanding carbon electrode consisting of an electrospun fiber web with ultra large pores. The electrode has been fabricated via the horizontally-opposed blending electrospinning method, involving the simultaneous and independent spinning of two separate polymer solutions (PAN/DMF and PVP/deionized water) on a same rolling collector, followed by PVP fiber removal through a selective dissolution approach and standard two-step (stabilization/carbonization) heat treatment. At 60 mA/cm\({}^{2}\) current density, the CE, VE, and EE of the single cell using the so-obtained freestanding electrode reach 95.4%, 81.6%, and 77.9%, respectively. These values are higher than the ones achieved by the use of non-woven fibrous webs. To improve the mass-transport of active electrode species in vanadium-RFBs, [PERSON] et al. [153] have proposed a novel method to fabricate carbon-based electrospun fibrous electrodes endowed with internal channels in each fiber. By a proper tuning of the viscosity of the polyacrylamide/polystyrene (PAN/PS) precursor solution leading to self-assembling of the carbon fibers into large bundles, they have prepared porous carbon fiber bundle mats (Figure 17). The produced bundle structure results in significantly enlarged pore size compared with the traditionally electrospun fiber electrode, whereas the nanochannels formed inside the fibers due to the PS thermal decomposition upon carbonization provide additional specific surface areas. As a result, at current densities of 100 and 200 mA/cm\({}^{2}\), the single cell with the as-prepared bundled electrodes achieves nearly doubled discharge capacity and electrolyte utilization, as well as EE values of \(\sim\)88% and 80%, respectively. As is widely recognized, nitrogen doping of the active material is beneficial for the wettability and electrocatalytic activity of electrodes [6]. He et al. [145] have synthesized PAN-based nitrogen-doped carbon fibers via electrospinning followed by fiber dispersion in the liquid phase (urea solution, with urea acting as an external nitrogen source) and carbonization at different temperatures (800-1000 \({}^{\circ}\)C). Due to the strong electronegativity of nitrogen, doping increases the negative charge density on the carbon fiber surface, facilitating the adsorption of V\({}^{2+}\) and V\({}^{3+}\) ions and providing active  
Figure 17: **(a1–a5)** SEM images of bundled porous carbon fibers after carbonization (polystyrene (PS) content in the bicomponent precursor solution: 0–10 wt%, from left to right). **(b1–b5)** High-resolution SEM images of fiber bundles (PS content: 3–10 wt%, from left to right). **(c1–c5)** Cross-sectional views of bundled fibers obtained with 0–10 wt% PS content (reproduced from reference [153] with permission. Copyright 2018, Elsevier). sites for V\({}^{2+}\)/V\({}^{3+}\) couples.