Superionicity by design: high proton conductivity in a fluorine-free protic ionic liquid

In this study, the protic ionic liquid N,N-diethyl-3-sulfopropan-1-ammonium hydrogen methanedisulfonate is synthesized and characterized with the aim of elucidating its transport properties. A unique feature of this ionic liquid are the exchangeable acidic protons present on the anion and the cation, both of which contain low proton affinity sulfonic groups. The pronounced superionicity of this liquid, highlighted in the Walden-plot (Delta W > 0.8), indicates the decoupling of the conductivity from ionic diffusion. To explicate this enhanced proton mobility, the diffusion coefficients of the protons were measured by Pulsed-Field Gradient Spin-Echo NMR experiments at different temperatures. Below 373 K the diffusion coefficients of the cationic and anionic exchangeable protons are comparable with the ones of the other protons of the respective ions, indicating a vehicular transport mechanism. However, in the temperature range 383-413 K the diffusion coefficient of one of the exchangeable protons exceeds the diffusion of both ions by an unprecedented factor between 2 and 3. This temperature-induced switch in the proton transport mechanism is the result of a mixed-ion proton-transfer chain, as also supported by computational simulations. Both experimental and theoretical findings suggest the existence of a Grotthuss(-like) proton transport mechanism at high temperature in this system.