Binary Protic Ionic Liquid Mixtures as a Proton Conductor: High Fuel Cell Reaction Activity and Facile Proton Transport

Binary mixtures of two protic ionic liquids (PILs), namely, diethylmethylammonium hydrogensulfate ([dema]HSO4) and diethylmethylammonium bis(trifluoromethanesulfonyl)amide ([dema][NTf2]), were prepared by mixing in various weight ratios for prospective use as fuel cell electrolytes. The binary mixtures showed significantly higher electrochemical activity compared with the constituent pure PILs, and the activity changed depending on the composition of the mixtures. Specifically, the open circuit potential (OCP) for a H-2 | O-2 cell using a binary electrolyte consisting of 56 wt % [dema][NTf2] was 1.03 V vs a reversible hydrogen electrode (RHE), whereas the values were 0.90 and 0.77 V for pure [dema]HSO4 and [dema][NTf2], respectively, under similar conditions. The electrochemical activity of the binary mixtures was interpreted by comparing their molecular characteristics inferred from Fourier transform infrared (FT-IR) and H-1 NMR spectroscopy with those of the constituent PILs. The binary systems showed enhanced electrochemical activity, possibly due to anion/proton exchange through the formation of hydrogen bonds of varying strengths via the NH bond. The anion/proton exchange appears to average the NH bond strength to render it suitable for fuel cell reactions. Bulk physicochemical properties such as thermal properties, viscosity, ionic conductivity, and ionicity were also measured precisely. The results of the pulsed gradient spin echo (PGSE) NMR and Walden plot collectively suggest that the Grotthuss mechanism in addition to the vehicle mechanism contributes to proton transport in the binary systems, possibly due to the coexistence of [dema] cation and HSO4 anion, whereas the vehicle mechanism is dominant for pure [dema][NTf2].