News from Nanotechjapan

物性・原理

Negative Dielectric Constant of Water Confined in Nanosheets---Novel Guideline for High Energy Density Capacitor---

 On February 21, 2019, the University of Tokyo, and National Institute of Advanced Industrial Science and Technology (AIST) announced that the research group led by Professor Atsuo Yamada at the University, in collaboration with the group led by Minoru Otani at AIST, showed enhanced capacitance in electric double-layer capacitors (EDLC) with a characteristically negative dielectric constant of water confined in nanosheets.Details were published in Nature Communications*.

 EDLCs are expected to enable load-leveling of intermittent power from renewable energy sources because of the capability of operation at high charge/discharge rates without degradation over millions of repeated cycles. Although two-dimensional material MXene has recently been proposed as electrode materials in EDLCs to increase the accessible surface area and reduce the interlayer separation, research on confinement effect of water molecules as electrolyte has been limited.

 The research group prepared and characterized EDLCs with MXene (composition: Ti2C(OH)0.3O0.7F0.6Cl0.4) electrodes and aqueous solution of electrolyte including alkali ions, Rb+, K+,Na+, and Li+. Capacitance was found to increase from Rb+ electrolyte to Li+ electrolyte by 1.7 against the prediction expected from ion size. This anomaly was explained by first principle theoretical calculation with negative dielectric constant of water confined in nanosheets. 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.

*Akira Sugahara, Yasunobu Ando, Satoshi Kajiyama, Kazuma Gotoh, Koji Yazawa, Minoru Otani, Masashi Okubo, Atsuo Yamada, "Negative dielectric constant of water confined in nanosheets", Nature Communications, Vol. 10, Article number: 850 (2019), doi: 10.1038/s41467-019-08789-8; Published 20 February 2019