An Ammonium Solvate Ionic Liquid

• Published in: Journal of the Electrochemical Society 2021-02, Vol.168 (2), p.26515
• Main Authors: Kawata, Kio, Kitada, Atsushi, Fukami, Kazuhiro, Saimura, Masayuki, Nagata, Takashi, Katahira, Masato, Murase, Kuniaki
• Format: Article
• Language: English
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1945-7111/abe512

The first example of ammonium (NH4+) solvate ionic liquids (ILs) is reported. The compound is ammonium bis(trifluoromethylsulfonyl)amide-18-crown-6 (1/1), i.e. [NH4+][Tf2N-]−18C6 (1/1), where Tf represents SO2CF3. Raman spectra, NMR spectra, and DFT calculations support the conclusion that the compound can be described as an ammonium solvate IL [NH4+ 18C6][Tf2N-], which consists of 18C6-coordinated NH4+ cations and Tf2N- anions. The conductivity of the ammonium solvate IL reaches as high as 10 mS cm-1 at 150 °C. The negligible volatility below 200 °C is confirmed by thermogravimetry. Compared with a hydronium (H3O+) solvate IL [H3O+ 18C6][Tf2N-], the ammonium solvate IL shows better thermal stability, which strongly suggests long residence time of 18C6 with NH4+ cation. The stability may lead to the vehicular-type translational motions of NH4+ cations with 18C6 solvents as proved by their self-diffusion coefficients. The findings regarding this ammonium solvate IL can provide the guidelines to design new NH4+ or proton conductors for ammonium ion batteries and fuel cells, which work at medium-low temperatures of 150 °C-200 °C.