# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 7 Capacitive Deionization of Water  
The increase in the world population is reflected in a growing demand for fresh water, further boosted by the increasing level of environmental contamination of the commonly exploited aquifer sources. This situation has encouraged the development of the water desalination technologies, and particularly of those addressed to cost-effectiveness and energy-efficiency. Among them, the capacitive deionization (CDI) is one of the most attractive for its simplicity and eco-friendliness, and it has recently received great attention [189; 190; 191]. CDI, also known as electrosorptive desalination, is based on the formation of an EDL between the parallel electrodes forming the CDI cell, upon application of a voltage between them [192; 193; 194; 195]. In the presence of the EDL, the Na\({}^{+}\) and Cl\({}^{-}\) ions, present in the salty water inlet into the cell, are reversibly adsorbed onto the porous surface of the negatively and positively charged electrodes, respectively (Figure 24), and the so-obtained desalted water goes out from the cell. During this process, no secondary contaminant is released. Once their surface is saturated by salt ions, the electrodes can be regenerated by reducing/reversing/shorting the cell voltage. This causes the stored ions to be desorbed and released into a wastewater solution. Figure 23: Fabrication process of (**a–d**) fiber materials and (**e**) fiber-shaped all-solid-state supercapacitor. (**a**) Schematic illustration of the PAN nanofibers yarn fabrication; sketch of the (**b**) carbonized carbon nanofibers yarn (CNY), (**c**) polypyrrole (PPy)-coated CNY (CNY@PPy), and (**d**) reduced graphene oxide (rGO)-coated CNY@PPy electrode (CNY@PPy@rGO) with core-shell fiber. (**e**) The digital photograph of a fiber-shaped all-solid-state supercapacitor (reproduced from reference [186] with permission. Copyright 2018, Elsevier). The low operating voltage of CDI (0.8-1.6 V) can be of great help in remote areas. In addition, the energy required to desalinate one cubic meter of brackish waters by this method is lower than for the reverse osmosis (only 0.1 against 0.2 kWh) [194]. The capacitance of the porous electrode material and, hence, its desalination performance depends on many parameters, including its specific surface area and hydrophilicity, the size and distribution of its pores, as well as its ionic conduction properties [189; 191; 192; 193; 194; 195]. Based on the electrosorption mechanism and kinetics, porous carbons with good electrical conductivity, hierarchical pore distribution, and high specific surface area are commonly regarded as the optimal materials for the achievement of high CDI performance. A great variety of carbon-based electrode materials, including carbon aerogel [196; 197], activated carbons [198], carbon nanotubes [199; 200], graphene [201], and graphene-enriched carbonaceous nanostructures [202], have been developed in order to reach this goal. However, some of these materials (e.g., nanotubes) are produced through time-expensive and/or sophisticated processes and have high costs, while others (e.g., activated carbons) involve the use of harsh chemical oxidants, such as KOH or piranha-solution, for activation before their use as CDI electrodes. Electrospinning is a simple and cost-effective technique; moreover, electrospun carbon nanofibers (ECNFs) do not necessarily require activation treatments to exhibit good electrosorptive performance [55]. In the last decade, electrosorption capacities ranging between 1.9 and 6.4 mg/g have been reported for ECNFs and ECNF-based composite materials [203; 204; 205; 206; 207; 208; 209]. In particular, [PERSON] et al. [206] have utilized electrospinning followed by CO\({}_{2}\) activation to prepare a hybrid nanomaterial (Figure 25a-c) consisting of CNTs embedded in activated PAN-based carbon fibers (ACF).