Investigation of the Protolytic Equilibrium of a Highly Brønsted Acidic Ionic Liquid and Residual Water using Raman Spectroscopy

Abstract Protic ionic liquids (PILs) are promising candidates as non-aqueous proton-conducting electrolytes for use in polymer electrolyte membrane fuel cells with operating temperatures over 100 °C. 2-sulfoethylammonium triflate [2-Sea][TfO] is one such PIL electrolyte, in which the highly Bronsted acidic sulfoalkylammonium cations act as mobile protonic charge carriers and proton donors. In order to gain a molecular-level understanding of proton transfer in a PIL electrolyte containing a small amount of residual water from fuel cell operation, the protolytic equilibrium of the highly acidic cation was investigated by means of Raman spectroscopy. Density functional theory (DFT) calculations were conducted to identify the vibration modes sensitive to protonation and to gain information on the possible conformation of the cation. The deprotonation of the 2-sulfoethylammonium cation resulted in a characteristic upward frequency shift in the ν(SC) stretching vibration. An equilibrium constant of 0.23 ± 0.09 was calculated for the protolytic reaction, indicating [2-Sea][TfO] as a promising proton donor for the fuel cell application.