# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 2 Principle of Electrospinning, Experimental Setup, and Production of Fibrous Nanomaterials  
The history of the amazing technique that nowadays allows producing a wide variety of functional nanostructured materials endowed with unique physicochemical properties and outstanding characteristics for applications in the most diverse fields starts in 1902 [19; 20] and proceeds through a sequence of patents [21; 22; 23; 24; 25; 26; 27; 28] and related literature \"milestones\" [29; 30; 31; 32; 33]. Nonetheless, it only truly gained attention since the 1990-2000s thanks to the studies conducted to deepen the knowledge of the process [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44]. For the interested readers, a comprehensive illustration of all the advancements in the electrospinning technique, from its invention in 1900 until nowadays, can be found in reference [5]. The electrospinning technology exploits electric forces to draw charged threads of a viscoelastic fluid (polymeric solution). The needed experimental setup is very simple and widely accessible. Basically, it consists of a syringe equipped with a needle and a pump to feed the solution, a collector, and a high-voltage power supply (Figure 2). Due to surface tension, a small amount of the fluid pumped out through the syringe needle tends to form a spherical droplet. If a high voltage is applied between the tip of the needle and the collector, the charged liquid droplet elongates assuming a conical  
Figure 1: Time distribution of papers dealing with energy-related applications of electrospun nanomaterials (ENMs) published in Scopus-indexed scientific journals from 1995 to January 2019.  
shape (Taylor cone) due to electrostatic repulsion between the charges inside it. When the applied voltage is sufficiently high, the electric field force overcomes the surface tension and, if the molecular cohesion of the liquid is sufficiently high, a stable charged liquid jet is ejected from the apex of the cone. The jet continues to decrease in diameter until it starts to bend and enters an instability regime. As a result, it undergoes a continuous elongating-and-whipping process leading to the formation of a long and thin thread that finally deposits onto the collector. Since the solvent almost completely evaporates during the motion of the jet towards the collector, a non-woven mat of solid fibers is obtained by this process [5; 12; 14; 15; 45]. Nowadays, industrial spinners are commercially available and able to produce a continuous spun layer having an effective width of 1.6 m. Depending on the specifics required by the application for which the fibers are designed, spinners with different nozzle number and/or symmetry (single- or multi-nozzle, uni-, multi-, or co-axial nozzle spinners), as well as collectors with different shapes (plate-, disk-, or drum-collectors), can be utilized [15]. Multi-needle and needle-less systems allow for higher productivity with respect to the conventional electrospinning, whereas by the use of co-axial systems, in which precursors are fed through separate co-axial channels, hollow, or composite core-shell nanofibers with additional functionalities can be produced [4]. Mats consisting of uni-axially aligned rather than randomly-oriented fibers can be obtained by utilizing rotating drum-collectors in place of the conventional plate, or by the aid of suitably applied magnetic or electric fields [3]. Besides, through rational design and/or patterning of the collectors, nanofibers with very complex orientation patterns can be prepared [3]. Two separate polymer solutions can simultaneously and independently be spun on a same rolling collector thanks to the horizontally-opposed blending electrospinning method to achieve a bi-component fibrous mat.