Thermophysical properties and ionic conductivity of new imidazolium-based protic ionic liquids

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This research aimed to develop new low-cost proton-conducting electrolytes for fuel cells operating above 100 °C. Thus, two room temperature protic ionic liquids (PILs), imidazolium dibutyl phosphate (IM-DBP) and imidazolium bis(2-ethylhexyl)phosphate) (IM-BEHP) were prepared by neutralization of corresponding dialkyl phosphate with imidazole. The cation–anion ion pairs of the synthesized PILs were optimized within the density functional theory (DFT) approach utilizing the Becke three-parameter exchange–correlation-functional (B3LYP). It was found that hydrogen bonds are formed mainly between the N–H and C2-H hydrogen atoms of the imidazolium cation and the oxygen atoms of the dialkyl phosphate anion. According to thermal characterization, imidazolium dialkyl phosphates have glass transition temperatures around −60 °C and thermal degradation points (i.e. 5% weight loss) of 190 °C (IM-DBP) and 229 °C (IM-BEHP). The rheological studies indicate that the viscosity of IM-BEHP at room temperature is four times higher compared to IM-DBP. A significant decrease in the viscosity was observed for both PILs while increasing the temperature from 22 to 60 °C. The ionic conductivity of PILs was studied by electrochemical impedance spectroscopy in wide frequency (10−1 – 106 Hz) and temperature (25 – 180 °C) ranges. At room temperature, the conductivity of IM-DBP is much higher than that of IM-BEHP (4.5·10−3 S/cm versus 9.3·10−5 S/cm). However, above 140 °C the conductivity of both PILs reached the level of 10−2 S/cm. The activation energy Ea calculated from the slope of the Arrhenius plot of ionic conductivity is 19 kJ/mol for IM-DBP and 39 kJ/mol for IM-BEHP. The relationship between the molar conductivity and the fluidity was described using the Walden rule. The Walden lines for both PILs are close to the “ideal” KCl line that indicates the presence of almost completely dissociated ions. Overall, the obtained results show the availability of imidazolium dialkyl phosphates as proton conducting electrolytes for medium temperature fuel cell applications.