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.

Alkylimidazolium Protic Ionic Liquids: Structural Features and Physicochemical Properties

“…The calculated threshold ΔPA value is slightly lower than those obtained in Ref. [21] in the investigation of the Im interaction with TFSI, MsO, TfO, Ac and TFA, and this is quite expectable if we take into account the big number of acids and bases investigated in this work.…”

“…The same conclusion about the non-equivalence of the NH…O and C(2)H…O bonds was made by the authors of refs. [21,24,44] when they studied the structures of ion pairs of different protic imidazole and alkylimidazole ILs.…”

“…Computation chemistry methods are frequently attracted to investigate the acid-to-base proton transfer extent. [21][22][23] The authors of Ref. [21] using electronic structure calculations of imidazolium-based PILs with various anions concluded that the difference in the anion and base proton affinity in the gas phase (ΔPA) may serve as an alternative to ΔpK a , i. e. when the ΔPA values are lower than 90 kcal/mol, a proton transfer from the acid to the base takes place.…”

“…[21][22][23] The authors of Ref. [21] using electronic structure calculations of imidazolium-based PILs with various anions concluded that the difference in the anion and base proton affinity in the gas phase (ΔPA) may serve as an alternative to ΔpK a , i. e. when the ΔPA values are lower than 90 kcal/mol, a proton transfer from the acid to the base takes place. A specific feature of the PILs is their ability to form hydrogen bonds between the cation and anion, also known as "doubly ionic H-bonds" [24] or "Z-bonds".…”

Proton Propensity and Orientation of Imidazolium Cation at Liquid Imidazole–Vacuum Interface: A Molecular Dynamics Simulation

“…Compared with water, the hydrogen bonds between imidazole molecules are relatively weak, 18 and proton transportation can occur along consecutive hydrogen-bonded chains with the imidazole ring reorientation via Grotthuss mechanism. This process has been extensively studied in bulk imidazole, 18−21 imidazole-based protic ionic liquids, 22,23 polymers, 24,25 and porous materials containing imidazole molecules. 10,26 In power generating devices, such as PEMFCs, the electrochemical reactions are controlled by the characteristics of the liquid/solid interface, or the electrolyte/electrode interface, and thus a detailed understanding of proton transportation in imidazole and imidazole-based protonconductive materials should include the proton solvation and diffusion behavior of the imidazolium cation in the interfacial region.…”