Suppression of Fast Proton Conduction by Dilution of a Hydronium Solvate Ionic Liquid: Localization of Ligand Exchange

Abstract

A dilution effect on the proton conduction of a hydronium solvate ionic liquid [H 3 O + •18C6]Tf 2 N, which consists of hydronium ion (H 3 O +), 18-crown-6-ether ligand (18C6), and bis[(trifluoromethyl)sulfonyl]amide anion (Tf 2 N-; Tf = CF 3 SO 2), has been studied. When [H 3 O + •18C6]Tf 2 N was diluted using equimolar 18C6 solvent, the distinctive fast proton conduction in [H 3 O + •18C6]Tf 2 N was suppressed in stark contrast to the case of common protic ionic liquids. Nuclear magnetic resonance spectroscopy showed that the fast exchange between free 18C6 molecules and coordinated ones, suggesting that the added solvent had induced a local proton exchange rather than a cooperative proton relay.

An Ammonium Solvate Ionic Liquid

“…They are described as [H 3 O + •oligoether][Tf 2 N -] (oligoether = 18-crown-6 (18C6), dicyclohexano-18-crown-6 (Dh18C6), benzo-18-crown-6 (B18C6), and pentaethyleneglycol dimethylether (G5); Tf 2 N = bis (trifluoromethylsulfonyl)amide (Tf = SO 2 CF 3 )). [30][31][32][33] Here, each H 3 O + ion (solute) is solvated by oligoether ligands (solvent) to form a [H 3 O + •oligoether] complex cation (solvate), and the counter anion is the Tf 2 Nanion. These hydronium solvate ILs show interesting proton conduction behaviors.…”

“…31 Although the ligand exchange conduction may be related to the strong acidity of [H 3 O + •18C6] cations -as indicated for some superacid-weak base mixtures 51 it is still unclear if the vehicle conduction can be attributed to the weak acidity of the [NH 4 + •18C6] cations. This is because ligand exchange conduction is absent in another analogous compound 32 which is a little more acidic than + and 0.44 for the H 3 O + compound). 30 These results can be explained by focusing on the ionicity, the value of Λ imp /Λ NMR .…”

“…supplementary information). However, the amount of H 3 O + can be determined by Karl-Fischer titration as proved in hydronium solvate ILs 30,32,33. Therefore, in the [NH 4+ •18C6][Tf 2 N -] sample with 0.012 wt% water could contain at most 0.04 mol% H 3 O + given all the water were protonated.…”

A Concentrated AlCl3–Diglyme Electrolyte for Hard and Corrosion-Resistant Aluminum Electrodeposits

“…The linear relationship between ln η or ln σ c and T –1 is provided in Figure S2c,d, obeying the Arrhenius law. The Walden plots, i.e., the plots of molar conductivity ( Λ imp ) versus fluidity ( η –1 ), are frequently used to evaluate how electrolytes are relatively conductive compared to ideal KCl aqueous solution. − Figure shows the Walden plots of the AlCl 3 –G2 electrolytes together with the ideal KCl aqueous solution. The ratio of Λ imp / Λ ideal can be obtained from the difference in the vertical axis, i.e., log­( Λ imp / Λ ideal ), giving Λ imp / Λ ideal = 0.40 at all temperatures for x = 0.4.…”

Proton conduction in hydronium solvate ionic liquids affected by ligand shape

“…Further, unlike common solvate ILs and protic ILs, this cooperative proton relay is suppressed and H3O + come to move as slow as 18C6 ligands when diluted using equimolar 18C6 solvent. 26 The proton conduction of the hydronium-based electrolytes would be interesting from the viewpoint of recently-proposed hydronium ion batteries 27,28 as well as fuel cells. 6,[13][14][15] In these studies, however, a key factor of the fast proton conduction has not been revealed.…”