Negative dielectric constant of water confined in nanosheets

Akira Sugahara,Masashi Okubo,Atsuo Yamada,Minoru Otani,Koji Yazawa,Yasunobu Ando,Satoshi Kajiyama,Kazuma Gotoh

doi:10.1038/s41467-019-08789-8

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.

Highlights

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems
Our aim was to quantify the contribution of the confinement effect to capacitance in electric double-layer capacitor (EDLC) electrodes consisting of twodimensional materials, theoretically modeled as a slit capacitor (Fig. 1a)[9,13]
As pioneered by research groups led by Gogotsi and colleagues[5,14,15], MXene is a class of two-dimensional materials with the following chemical formula: Mn+1XnTx and gives a large capacitance that is associated with ion intercalation[16]

Summary

Introduction

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems. Our aim was to quantify the contribution of the confinement effect to capacitance in EDLC electrodes consisting of twodimensional materials, theoretically modeled as a slit capacitor (Fig. 1a)[9,13]. This conceptually straightforward but experimentally difficult study was performed using MXene EDLC electrodes. MXene maintains its stacked structure with a short interlayer distance during ion (de)intercalation, owing to strong interactions among the surface termination groups, intercalated ion species, and embedded solvents[17] Such an anomalous interlayer interaction has led us to expect that MXene nanosheets strongly confine the intercalated ions and MXene is an ideal platform to study the guest confinement effect in two-dimensional materials

Objectives

Methods

Results

Conclusion