In this proof-of-concept study, we introduce and demonstrate MXene as a novel type of intercalation electrode for desalination via capacitive deionization (CDI).

Most efforts to improve the energy density of supercapacitors are currently dedicated to optimized porosity or hybrid devices employing pseudocapacitive elements. Little attention has been given to the effects of the low charge carrier density of carbon on the total material capacitance. To study the effect of graphitization on the differential capacitance, carbon onion (also known as onion‐like carbon) supercapacitors are chosen. The increase in density of states (DOS) related to the low density of charge carriers in carbon materials is an important effect that leads to a substantial increase in capacitance as the electrode potential is increased. Using carbon onions as a model, it is shown that this phenomenon cannot be related only to geometric aspects but must be the result of varying graphitization. This provides a new tool to significantly improve carbon supercapacitor performance, in addition to having significant consequences for the modeling community where carbons usually are approximated to be ideal metallic conductors. Data on the structure, composition, and phase content of carbon onions are presented and the correlation between electrochemical performance and electrical resistance and graphitization is shown. Highly graphitic carbons show a stronger degree of electrochemical doping, making them very attractive for enhancing the capacitance.

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This work establishes molybdenum disulfide/carbon nanotube electrodes for the electrochemical desalination of high molar saline water.

The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons. ■ INTRODUCTIONNanoporous carbons are an important class of materials used in a range of applications including capacitive energy storage, gas storage, water treatment, and catalysis. 1−3 In each case, the nanoporosity and high specific surface areas (typically >1500 m 2 g −1 ), achieved by activating carbonaceous precursors, are exploited to store molecules or ions. In principle, carbon structures can be engineered for a given application, though characterization of the highly disordered structures poses a significant challenge. The challenges in determining local-and long-range structure make it extremely difficult to establish structure−function correlations beyond those simply derived from surface area and pore-size distributions.The structures of carbon materials 4,5 have been actively researched since the pioneering X-ray diffraction studies of Franklin. 6,7 She distinguished between graphitizing and nongraphitizing carbons, the former transforming into graphite upon heating to high temperature, and the latter showing no such transformation at temperatures as high as 3000°C. 7 For nanoporous carbons, analysis of the broad Bragg peaks is generally of limited use due to the long-range disordered structures of these materials. However, inclusion of the diffuse scattering in the analysis allows the extraction of a pair distribution function (PDF), which is a weighted histogram of atom-to-atom distances showing the likelihood of finding an atom pair separated by a certain distance. 8 PDF studies show that porous carbons often exhibit a high degree of local ordering, with a propensity for hexagonal carbon rings in which the carbon atoms are sp 2 hybridized. 9−11 Correlations in the PDFs typical...

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This work introduces for the first time titanium disulfide (TiS 2 )/carbon nanotube (CNT) electrodes for desalination of high molarity saline water. Capitalizing on the two-dimensional layered structure of TiS 2 , cations can be effectively removed from a feedwater stream by intercalation. The TiS 2 −CNT hybrid electrode is paired in an asymmetric cell with microporous activated carbon cloth without an ion exchange membrane. By electrochemical analysis, the correlation between the state of charge and the stability of TiS 2 was investigated. By using post-mortem X-ray diffraction, the sodium-ion intercalation mechanism gives an insight into how the state of charge affects the structure and cyclic stability. Our system showed stable desalination performance over 70 cycles at high molar concentration (600 mM), with a cell salt removal capacity of 14 mg/g (equivalent to a sodium removal capacity of 35.8 mg/g normalized to the mass of TiS 2 −CNT). This novel approach of membrane-free hybrid Faradaic capacitive deionization paves the way toward energy-efficient desalination of seawater.

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