NMR determination of the diffusion mechanisms in triethylamine-based protic ionic liquids

Protic ionic liquids (PILs) are highly conductive fluids that have emerged as alternative solvents for electrochemical applications including fuel cells. In the present contribution, the transport properties for a set of triethylamine (TEA)-based PILs were explored with pulsed field gradient stimulated echo (PFG-STE) NMR spectroscopy to obtain the self-diffusion coefficient (D) of the individual molecular species that comprise each system. We report self-diffusion coefficients from measurements conducted in two different laboratories for PILs that were produced by two distinct synthetic routes, a conventional route and a strictly anhydrous route. The PILs of interest are comprised of TEA cations and different anions, methane sulfonate (MS), bis(trifluoromethylsulfonyl)imide (TFSI), tetrachloroaluminate (AlCl₄^-), and bis(perfluoroethylsulfonyl)imide (BETI). In contrast with previous reports, the PFG-STE diffusion measurements presented here show no proton decoupled (Grotthuss mechanism) mobility in these PILs. Additionally, it is shown that the presence of moderate amounts of water (∼1000 ppm) can have a marked influence on D of the acidic proton.