Proton transport properties in zwitterion blends with brønsted acids

We describe zwitterion, 3-(1-butyl-1<i>H</i>-imidazol-3-ium-3-yl)propane-1-sulfonate (Bimps), mixtures with 1,1,1-trifluoro-<i>N</i>-(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf)₂) as new proton transport electrolytes. We report proton transport mechanisms in the mixtures based on results from several methods including thermal analyses, the complex-impedance method, and the pulsed field gradient spin echo NMR (pfg-NMR) method. The glass transition temperature (Tg) of the mixtures decreased with increasing HN(Tf)₂ concentration up to 50 mol %. The Tg remained constant at −55 °C with further acid doping. The ionic conductivity of HN(Tf)₂ mixtures increased with the HN(Tf)₂ content up to 50 mol %. Beyond that ratio, the mixtures showed no increase in ionic conductivity (10⁻⁴ S cm⁻¹ at room temperature). This tendency agrees well with that of Tg. However, the self-diffusion coefficients obtained from the pfg-NMR method increased with HN(Tf)₂ content even above 50 mol % for all component ions. At HN(Tf)₂ 50 mol %, the proton diffusion of HN(Tf)₂ was the fastest in the mixture. These results suggest that Bimps cannot dissociate excess HN(Tf)₂, that is, the excess HN(Tf)₂ exists as molecular HN(Tf)₂ in the mixtures. The zwitterion, Bimps, forms a 1:1 complex with HN(Tf)₂ and the proton transport property in this mixture is superior to those of other mixing ratios. Furthermore, CH₃SO₃H and CF₃SO₃H were mixed with Bimps for comparison. Both systems showed a similar tendency, which differed from that of the HN(Tf)₂ system. The Tg decreased linearly with increasing acid content for every mixing ratio, while the ionic conductivity increased linearly. Proton transport properties in zwitterion/acid mixtures were strongly affected by the acid species added.<br />