Nonstoichiometric Protic Ionic Liquids: The Role of Excess Acid in Charge Transport Mechanisms.

A study of charge transport mechanisms in an electric field was conducted on nonstoichiometric protic ionic liquids (PIL) based on triethylamine (TEA), in combination with an excess of either trifluoroacetic acid (TFA) or trifluoromethanesulfonic acid (TfO). The addition of excess precursor acid adds proton-donor sites to the system to support potential structural proton transport, which could, for example, enable the use in fuel cells. Transport measurements by pulsed field gradient (PFG) NMR diffusion and, in particular, electrophoretic NMR (eNMR) are supported by NMR chemical shifts and Raman spectroscopy, where the latter techniques elucidate the local solvation structures. Migration of the acidic proton of the excess acid in the electric field occurs toward the cathode with a velocity larger than that of the anions. This intriguing feature of a rapid drift of a neutral molecule is explained by the interplay of strong correlations between anion and cation as well as between anion and acid. The neutral acid is subject to vehicular transport with the anion, while the anion is partitioning between anion-acid and anion-cation clusters, resulting in a lower average drift velocity. The negative drift direction of the neutral acid and its proton is superimposed to and thus counteracts the vehicular transport of protons with the cation. The study sheds light on the role of excess acid in PIL and reveals the versatile interactions between anion, cation, and excess acid within a PIL determining its charge transport properties.