Quantum Mechanical Investigation of Proton Transport in Imidazolium Methanesulfonate Ionic liquid

Abstract

Protic ionic liquids (PIL) are promising anhydrous proton conductors for fuel cell applications especially at higher temperature. In this study, quantum chemistry calculations using density functional theory (DFT) were performed to investigate proton conduction in imidazolium methanesulfonate (IMMSA) PIL on interaction with varying imidazole (IM) concentration. These investigations include the extent of proton transfer (PT) from methanesulfonic acid to the imidazole base and calculation of various energy barriers of PT. The results show that the difference in gas phase proton affinity (ΔPA) of the anion and base is a reliable predictor of the extent of proton transfer from acid to base, and ΔPA values <90 kcal/mol indicates facile PT. These gas phase DFT calculations show that, on addition of at least one IM to IMMSA, proton dissociates from MSA to IM leading to the generation of charged species, cations and anions, essential for proton conduction. The PT barrier from IMH+ to IM reduces with IM molecules added to IMMSA. The calculated rotational barrier associated with the molecular reorganization increases with number of IM molecules added to IMMSA, and this can be attributed to the more number of hydrogen bonds broken during this process. In the case of IMMSA with two IM molecules, a different pathway was also explored where a barrierless rotation of IM molecules was observed. The results from this study can assist in understanding the proton transport mechanism in the IM based ILs under base rich conditions.