Conductivity, spectroscopic, and computational investigation of H3O+ solvation in ionic liquid BMIBF4

The hydrogen ion is one of the most important species in aqueous solutions, as well as in protic ionic liquids (PILs). PILs are important potential alternatives to H2O for swelling the proton exchange membranes (PEMs) and improving the high-temperature performance of fuel cells. The hydrogen ion (H(+)) or hydronium (H3O(+)) solvation mechanism is not only a fundamental principle of acid/base chemistry in ionic liquids but also key to understanding the charge- and proton-transport properties of the PIL solutions. In this paper, a PIL system was prepared by mixing 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIBF4) IL with an aqueous solution of a strong acid, HBF4. Water can be partially evaporated from the solution under a vacuum at room temperature. Conductivity and vibrational spectroscopy (IR and Raman) measurements were used in combination with density functional theory (DFT) calculations to characterize the molecular-level solvation of H(+) and H2O in the IL solution. When water is present at high molar fraction, the cations (BMI(+) and H(+)) and anions (BF4(-)) are both solvated by water and the solutions have high conductivity. After water evaporation, the PIL solution has excess H(+) and reduced conductivity, which is still significantly higher than that of pure BMIBF4. Vibrational spectroscopy suggests that the BMI(+) and BF4(-) ions are desolvated from water during the water evaporation. DFT calculations assist the interpretation of the vibrational spectroscopy and show that the remaining water is in the form of H3O(+) solvated by the IL molecular ions. Hence, the species remaining after evaporation is a ternary PIL consisting of BMI(+) cation, BF4(-) anion, and H3O(+) cation. The H3O(+) may be the principle charge carrier in the PIL solution and responsible for the high solution conductivity.