Negative dielectric constant of water confined in nanosheets

Abstract

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems. Earlier attempts to improve an unsatisfactory capacitance of electric double-layer capacitors have focused on meso- or nanostructuring to increase the accessible surface area and minimize the distance between the adsorbed ions and the electrode. However, the dielectric constant of the electrolyte solvent embedded between adsorbed ions and the electrode surface, which also governs the capacitance, has not been previously exploited to manipulate the capacitance. Here we show that the capacitance of electric double-layer capacitor electrodes can be enlarged when the water molecules are strongly confined into the two-dimensional slits of titanium carbide MXene nanosheets. Using electrochemical methods and theoretical modeling, we find that dipolar polarization of strongly confined water resonantly overscreens an external electric field and enhances capacitance with a characteristically negative dielectric constant of a water molecule.

Specific ion effects at graphitic interfaces

“…This behavior, which is further validated by our experimental data, is the result of an interplay between these various factors, which together form the basis of the relationship between electrolyte composition, confinement, and the measured capacitance. Although our studies were carried out for carbon materials and aqueous solutions, the findings presented here, particularly on specific ion effects, charge transfer, and their interplay with confinement, are likely to hold for other systems with different electrolytes, such as ionic liquids and organic electrolytes, and non-graphitic electrodes, such as low-dimensional systems, e.g., MXene and MoS 38–40 . Finally, we note that the computational strategy based on the DFT-RISM approach presented in this work is general and applicable to other electrochemical interfaces.…”

A Facile, High‐Yield, and Freeze‐and‐Thaw‐Assisted Approach to Fabricate MXene with Plentiful Wrinkles and Its Application in On‐Chip Micro‐Supercapacitors

“…The FAT method is commonly utilized in the exfoliation of several 2D nanomaterials, including metal–organic nanosheets, boron nitride nanosheets (BNNSs), graphene oxide (GO), and antimony triselenide (Sb 2 Se 3 ) In contrast, the FAT method might be more suitable for MXene. Due to the selective etch of Al atom layer of MAX, the obtained m‐MXene is accordion‐like, and thus exists wider interstices between interlayer, which may be helpful to the intercalation of ample water molecules to swell . Through several cycle times of FAT, the yield of large delaminated‐MXene (FAT‐MXene) flakes with special wrinkles can reach 39.0% without sonication.…”

Solvation‐Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

“…Yamada and co‐workers demonstrated that solvent molecules such as water may resonantly overscreen the external electric field and enhance ion storage capacity (Figure c,d) . In this study, water cointercalated with cations into the MXene electrodes resulted in an increased value of capacitance.…”

“…Copyright 2019, Springer Nature. c,d) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/) . Copyright 2019, The Authors, published by Springer Nature.…”