A new kind of ionic liquid (IL) with strong Brønsted acidity, i.e., a hydronium (H 3 O + ) solvate ionic liquid, is reported. The IL can be described as [H 3 When two or more kinds of substances are mixed, the properties of the mixture can differ from those of the pure substances. Depending on the interactions between the components, some components undergo ionization, e.g. dissociation, protonation, solvation and complexation, due to the neutralization of Brønsted or Lewis acids and bases. Sometimes, the acid-base mixtures can be categorized as ionic liquids (ILs) with melting points below 100• C, which consist only of cations and anions. ILs are regarded as "the third liquid" and some ILs have fascinating physicochemical properties, such as low volatility, low flammability, high chemical, and thermal stability. Furthermore, ILs have received growing interest due to their various uses as media in chemical, biochemical, and/or electrochemical systems. [1][2][3][4][5][6] Common anions of ILs such as BF 4 -, PF 6 -, and AlCl 4 -are produced by neutralization, i.e. the adducts of a Lewis acid (BF 3 , PF 5 , or AlCl 3 ) and a Lewis base (F -or Cl -). Examples of ILs prepared through neutralization include solvate ILs.7-13 Solvate ILs often consist of equimolar molten mixtures of polyethers (linear or cyclic ones, i.e. glymes or crown ethers) and certain metal salts, where the Lewis acidic metal cations are solvated by equimolar amounts of Lewis basic glymes to give complex cations. Besides, neutralization of Brønsted acids and bases produces so-called protic ILs, which are preferred as acid-catalyzed reaction media due to the presence of an acidic proton.3-6 Since proton transfer is a common chemical transformation, Brønsted acidic ILs receive research attention especially for acid catalysts, fuel cells, biomass processing, polymerization, CO 2 fixation, etc.6,14-19 The design of stronger Brønsted acidic ILs and evaluation of the acidity in ILs are important for exploring their possible applications as media for acid-base reactions.Although 21-24 However, there is an exception. In solid, anhydrous HTf 2 N, the proton is bonded to the anion nitrogen and two oxygens of a neighboring HTf 2 N molecule. 25 Likewise, in the case of monohydrate, the imide superacid HTf 2 N remains associated and does not protonate water, as suggested experimentally and theoretically. In the infrared measurements for an H 2 O and HTf 2 N mixture in CCl 4 , strong Fermi resonance for N-H vibrations serves as evidence that HTf 2 N remains * Electrochemical Society Member.z E-mail: kitada.atsushi.3r@kyoto-u.ac.jp intact, and from the ab initio calculations, spontaneous proton dissociation was not observed for the Fig. 1a

We observed a new class of proton conduction in a hydronium (H 3 O + ) solvate ionic liquid (IL). The IL is described as [H 3 O + · 18C6]Tf 2 N, composed of 18-crown-6-ether (18C6)-coordinated H 3 O + cation (solvate) and bis(trifluoromethylsulfonyl)amide (Tf 2 N -; Tf = CF 3 SO 2 ) anion. Self-diffusion coefficient measurements revealed that protons of H 3 O + (solute ion) move faster than 18C6 ligand (solvent), marking the first observation of ligand exchange conduction not only among solvate ILs but also among protic ILs without free neutral molecules. The presence of ligand exchange conduction without inducing external electric field indicates that solvation of H 3 O + by 18C6 is kinetically labile, i.e. ligand exchange is very fast for the reaction H 2 O + 18C6 + HTf 2 N [H 3 O + · 18C6]Tf 2 N, while the equilibrium shifts to the right. The fundamental findings regarding the hydronium solvate IL will help in the design of proton conductors.