Protic Ionic Liquids in Contrast to Aprotic Analogs: Transport Properties and Electrochemical Reactivity

Since ionic liquids (ILs) are fluids composed entirely of ions, their transport properties differ fundamentally from those of conventional electrolyte solutions containing nonionic solvents. Ionicity serves as a useful metric for characterizing IL transport behavior, defined as the ratio between the experimentally measured conductivity and that calculated from the pulsed-field gradient (PFG) NMR diffusivities of cations and anions using the Nernst-Einstein equation. In this account, we review the ionicity of aprotic ILs and discuss how its magnitude depends on the chemical structures of constituent ions, considering directionally correlated ionic motion. We then contrast the ionicity and thermal stability of protic ILs, prepared via proton-transfer reactions between Brønsted acids and bases, with those of aprotic counterparts. Protic ILs are characterized by strong hydrogen-bonding interactions between cations and anions and by relatively low potential barriers for back proton transfer. Finally, the reactivity of labile protons in protic ILs is exploited in the context of their function as proton-conducting, nonaqueous electrolytes, and their potential application as fuel cell electrolytes is discussed.