Brønsted Acid−Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes

Abstract

A new series of Brønsted acid−base ionic liquids were derived from the controlled combination of a monoprotonic acid with an organic base under solvent-free conditions. Appropriate amounts of solid bis(trifluoromethanesulfonyl)amide (HTFSI) and solid imidazole (Im) were mixed at various molar ratios to have compositions varying from an equimolar salt to HTFSI- or Im-rich conditions. The mixture at equivalent molar ratio formed a protic neutral salt with a melting point of 73 °C, which was thermally stable at temperatures even above 300 °C. The melting points of other compositions were lower than those of the equimolar salt and Im or HTFSI, giving eutectics between the equimolar salt and HTFSI or Im. Some of the compositions with certain molar ratios of Im and HTFSI were liquid at room temperature. For Im excess compositions, the conductivity was found to increase with increasing Im mole fraction, and the 1 H NMR chemical shift of the proton attached to the nitrogen atom of Im was shifted to a lower magnetic field. On the contrary, the conductivity decreased with increasing HTFSI mole fraction, and the 1 H NMR chemical shift of the proton attached to the TFSI imide anion also shifted to a higher magnetic field. Self-diffusion coefficients, measured by pulsed-gradient spin−echo NMR (PGSE-NMR) methods in Im- or HTFSI-rich compositions, indicated that fast proton exchange reactions between the protonated Im cation and Im take place in excess Im. The proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms. Direct current polarization measurements were used for the confirmation of proton conduction in Im-rich compositions. Furthermore, reduction of molecular oxygen could be observed at the interface between a Pt electrode and these ionic liquids. This introduces the Brønsted acid−base ionic liquid system as a new candidate for proton conductor such as a fuel cell electrolyte to operate under anhydrous conditions and at elevated temperature.