Capacitive deionization (CDI) has newly emerged as a desalination technology because of its energy and costeffectiveness. In particular, CDI using a flow electrode (FCDI) significantly increased salt removal by continuous desalination even at salt concentrations of both brackish and seawater. Since CDI mainly uses the electrosorption of salt ions onto an electric double layer on electrodes, carbon materials and their derivatives have been widely used. Even with various approaches, including modification and synthesis of new carbon-based electrode materials, the salt-removal capacity of CDI and FCDI is still limited by the accessible surface area and charge-transfer properties of carbon electrodes. Hydroquinone (HQ) is a redox-active organic molecule with highly reversible properties in electrochemical reactions and is a low-cost and non-toxic material. In this study, we introduced quinone in the activated carbon (AC) electrodes for CDI and FCDI desalination systems. It exhibited a significant increase in salt-removal performance (for CDI, more than a 65% increase for 300 mM HQ, and for FCDI, more than a 19% increase for 500 mM HQ). Even though the specific surface area of an HQ-added AC electrode is reduced because of the penetration of small HQ molecules into pores in the AC surface, the pseudocapacitive contribution from HQ and benzoquinone (BQ) increased the saltremoval performance. We analyzed such electrochemical properties by using cyclic voltammetry and electrochemical impedance spectroscopy. We expect that such an approach will open a new door for realizing excellent deionization of saline water, even of seawater, and will have a strong potential for large-scale desalination and energy-storage systems.

Perfluorinated polymers are widely used in polymer electrolyte membranes because of their excellent ion conductivity, which are attributed to the well-defined morphologies resulting from their extremely hydrophobic main-chains and flexible hydrophilic side-chains. Perfluorinated polymers containing quaternary ammonium groups were prepared from Nafion- and Aquivion-based sulfonyl fluoride precursors by the Menshutkin reaction to give anion exchange membranes. Perfluorinated polymers tend to exhibit poor solubility in organic solvents; however, clear polymer dispersions and transparent membranes were successfully prepared using N-methyl-2-pyrrolidone at high temperatures and pressures. Both perfluorinated polymer-based membranes exhibited distinct hydrophilic-hydrophobic phase-separated morphologies, resulting in high ion conductivity despite their low ion exchange capacities and limited water uptake properties. Moreover, it was found that the capacitive deionization performances and stabilities of the perfluorinated polymer membranes were superior to those of the commercial Fumatech membrane.

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Flexible supercapacitors based on electrostatic double-layer capacitors as next-generation energy storage systems hold great promise for wearable and flexible electronic devices owing to their high flexibility and electrochemical cyclability. However, the challenge of low charge-storing ability still remains, as electronic devices require a high charge storage capability. As one of the advanced strategies for enhancing charge-storing ability in flexible supercapacitors, the introduction of a redox mediator (RM) into a gel electrolyte has recently attracted great interest. Here, we present a fiber-based flexible energy storage system (f-FESS) integrated with iodine-based chemical species as a novel RM, carbon fiber-based electrodes, and a solid-state polymer-gel electrolyte so as to enhance the charge storage capability of flexible supercapacitors. The as-prepared f-FESS-RM exhibits enhanced charge storage capabilities of up to 461.8 F L −1 and 64.14 mWh L −1 , which are 3.6 times higher than those of f-FESS without RM. The enhanced capacitive properties of f-FESS-RM are attributed to their additional Faradic redox reaction of iodine-based chemical species in the electrolyte as well as electrical double-layer capacitive behavior. Also, the f-FESS-RM shows remarkably superior mechanical robustness under various bending, winding, knotting, and weaving conditions. Furthermore, we have demonstrated that the f-FESS-RM is stably workable under deionized water and base electrolytes. In combination, these outcomes provide a novel advanced strategy to enhance the chargestoring ability with strong mechanical robustness in flexible supercapacitors.

Capacitive deionization (CDI) based on ion electrosorption has recently emerged as a promising desalination technology due to its low energy consumption and environmental friendliness compared to conventional purification technologies. Carbon-based materials, including activated carbon (AC), carbon aerogel, carbon cloth, and carbon fiber, have been mostly used in CDI electrodes due their high surface area, electrochemical stability, and abundance. However, the low electrical conductivity and non-regular pore shape and size distribution of carbon-based electrodes limits the maximization of the salt removal performance of a CDI desalination system using such electrodes. Metal-organic frameworks (MOFs) are novel porous materials with periodic three-dimensional structures consisting of metal center and organic ligands. MOFs have received substantial attention due to their high surface area, adjustable pore size, periodical unsaturated pores of metal center, and high thermal and chemical stabilities. In this study, we have synthesized ZIF-67 using CNTs as a substrate to fully utilize the unique advantages of both MOF and nanocarbon materials. Such synthesis of ZIF-67 carbon nanostructures was confirmed by TEM, SEM, and XRD. The results showed that the 3D-connected ZIF-67 nanostructures bridging by CNTs were successfully prepared. We applied this nanostructured ZIF-67@CNT to CDI electrodes for desalination. We found that the salt removal performance was significantly enhanced by 88% for 30% ZIF-67@CNTs-included electrodes as compared with pristine AC electrodes. This increase in salt removal behavior was analyzed by electrochemical analysis such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, and the results indicate reduced electrical impedance and enhanced electrode capacitance in the presence of ZIF-67@CNTs.