Ab initio molecular dynamics study of proton transport in imidazolium-based ionic liquids with added imidazole.

Abstract

Development of efficient anhydrous proton-conducting materials would expand the operational temperature ranges of hydrogen fuels cells (HFCs) and eliminate their dependence on maintaining sufficient hydration levels to function efficiently. Protic ionic liquids (PILs), which have high ionic densities and low vapor pressures, have emerged as a potential material for proton conducting layers in HFCs. In this work, we investigate proton transport via the Grotthuss mechanism in 1-ethylimidazolium bis-(trifluoromethanesulfonyl)imide ([C2HIm][TFSI]) protic ionic liquids with added imidazole (Im0) using ab initio molecular dynamics. In particular, we vary the composition of the systems studied from pure [C2HIm][TFSI] to those where the mole fraction of Im0 is 0.67. Given the large difference in pKa between C2HIm+ and HTFSI, TFSI- does not accept acidic protons from C2HIm+; conversely, imidazolium (HIm+) and C2HIm+ have very similar pKa values, and thus Im0 can readily accept protons. We find that the unprotonated nitrogen on Im0 dominates solvation of the labile protons on C2HIm+ and other Im0 species, resulting in formation of robust imidazole wires. Given the amphoteric nature of Im0, i.e. its ability to accept and donate protons, these wires provide conduits along which protons can rapidly traverse via the Grotthuss mechanism, thereby greatly increasing the proton coefficient of self-diffusion. We find that the average length of the wires increases with added Im0, and thus as the mole fraction of Im0 increases so too does the proton diffusion constant. Lastly, we analyze our trajectories to determine the energy and time scales associated with proton transfer.