Ionic liquids (ILs) are widespread candidates to replace organic solvents in electrochemical applications due to their interesting characteristics such as low flammability, low vapor pressure, high ionic conductivity, and high thermal stability [1]. Within the ever-growing family of ILs, protic ionic liquids (PILs), which are simply prepared through a neutralization reaction of certain Brønsted acids and Brønsted bases, became promising candidates as electrolytes components [2]. A drawback still hampering the applications of PILs is the reduced numbers of dynamic species contributing to the conduction properties, frequently termed as ionicity. To establish the features governing the transport properties of PILs, the present work provides a deeper insight into the charge transport and ionicity of the selected PILs.
The PILs studied were based on the super-strong base 1,8-diazabicyclo- [5,4,0]-undec-7-ene (DBU) and three very strong acids trifluoromethylsulfonic (HTFO), bis(trifluoromethylsulfonyl)imide (HTFSI) and (trifluoromethylsulfonyl-nonafluorobutylsulfonyl)imide (HIM14). The diffusion coefficients of the molecular constituents were determined by Pulsed-Field-Gradient (PFG) NMR. Then, the ionicity of the systems was quantified by the reciprocal Haven ratio comparing the conductivity estimated by the Nernst−Einstein equation with that accessed from the ionic conductivity measured using impedance spectroscopy (EIS).
All the systems showed a well-defined vehicular mechanism of charge transport and lower molar conductivity in respect to that estimated by the diffusion measurements. Additionally, while the ionicity of the DBUH-TFO remains almost constant with the temperature increasing, for the DBUH-PILs based on the IM14- and TFSI- anions a linear decreasing of their ionicity was detected. Further investigations are currently performed to provide a deep understanding of the role of the anion on this linear trend. Also, we aimed to clarify the contribution of structure and acidity of the PIL constituent in governing their transport features.