# Electrospun Nanomaterials for Energy Applications: Recent Advances
## 7 Capacitive Deionization of Water
For this purpose, CNTs have been dispersed in a PAN/DMF solution, and the resulting homogeneous CNT/PAN suspension has been electrospun by the use of the spinning set-up schematized in Figure 25d. After stabilization in air at 280 \({}^{\circ}\)C for 2 h, the as-obtained web-like CNT/PAN fibers, have been carbonized in N\({}_{2}\) at 800 \({}^{\circ}\)C for 2 h and finally activated for 1 h in CO\({}_{2}\) at the same temperature, obtaining a composite material (CNT/ACF) with high specific surface area (651 m\({}^{2}\)/g) and mesopore ratio (64%). The test of the composite nanostructures by means of the setup schematically depicted in Figure 25e has led to a desalination capacity of 6.4 mg/g at an initial NaCl concentration of 400 mg/L and a working potential of 1.2 V. The great improvement (39%) with respect to the CNT-free nanomaterial, prepared and tested under the same conditions, has been ascribed to the increase of conductivity brought about by the incorporation of the high aspect-ratio morphology conductive additive coupled with the high porosity due to the CO\({}_{2}\)-mediated activation [206]. Indeed, activation by means of different agents is a strategy followed by various groups to enhance the specific surface area of the ECNFs and improve their NaCl removal capacity [203; 207; 210]. [PERSON] et al. [210] have reported an electrosorption capacity of 10.52 mg/g for PAN-based ECNFs obtained by stabilization in air at 250 \({}^{\circ}\)C for 1.5 h, followed by carbonization in N\({}_{2}\) at 800 \({}^{\circ}\)C for 1 h and activation by means of ZnCl\({}_{2}\). Activation consists of impregnation of ECNFs by immersion in a ZnCl\({}_{2}\) solution,  
Figure 24: Schematic illustration of the capacitive deionization (CDI) of water (inspired by reference [189]). Salty water is inlet into the CDI cell, Na\({}^{+}\) (Cl\({}^{-}\)) ions are adsorbed on the negative (positive) porous carbon electrode, and the as-obtained desalted water goes out. subsequent heat-treatment in N\({}_{2}\) atmosphere at 500 \({}^{\circ}\)C, immersion in 1 mol/L hydrochloric acid, and removal of the residual zinc compounds through thorough washing. The consequent increase in specific surface area, micro- and meso-pore volume, and electrode surface wettability by water is thought of as responsible for the relevant improvement in the CDI performance. Very recently, remarkably higher electrosorption capacities (>15 mg/g at initial NaCl concentrations of 500-600 mg/L) have been reached by the use of graphene-enriched nanohybrids as CDI electrodes [55; 211]. Graphene oxide (GO), incorporated at any stage during their preparation and transformed to thermally reduced graphene oxide (TRGO) during carbonization, acts as a bi-dimensional (2D) conductive additive, thus improving the conduction properties of the TRGO-containing hybrid. [PERSON] et al. [211] have produced a carbon aerogel with a structure consisting of 1D and 2D carbon nanomaterials, namely ECNFs coupled to graphene (Figure 26a,b), following the three-step procedure sketched in Figure 26c. Briefly, the membrane consisting of PAN-based ECNFs stabilized in air at 250 \({}^{\circ}\)C for 2 h (s-PAN), is cut into pieces and, after homogenization in a mixed solvent, is filtered. The resulting short fibers are redispersed, mixed with a GO suspension and homogenized at a very fast rotating speed to assure the sufficient contact and interactions between s-PAN fibers and GO sheets. s-PAN/GO composite aerogel is obtained via freeze-drying of the resulting material, whereas subsequent carbonization at 800 \({}^{\circ}\)C in N\({}_{2}\) for 2 h of the s-PAN/GO composite aerogel leads to PG carbon aerogel. With respect to the pristine s-PAN/GO composite aerogel, PG carbon aerogel is endowed with higher specific capacitance and remarkably improved electrosorption capacity (Figure 26d). The enhanced electrochemical performance is due to the strong interconnection between the 1D and 2D aerogel components, which favors the electron transfer within the porous structure.