Protic Ionic Liquids Based on the Alkyl-Imidazolium Cation: Effect of the Alkyl Chain Length on Structure and Dynamics

Abstract

Protic ionic liquids are known to form extended hydrogen-bonded networks that can lead to properties different from those encountered in the aprotic analogous liquids, in particular with respect to the structure and transport behavior. In this context, the present paper focuses on a wide series of 1-alkyl-imidazolium bis­(trifluoromethylsulfonyl)­imide ionic liquids, [HC n Im]­[TFSI], with the alkyl chain length (n) on the imidazolium cation varying from ethyl (n = 2) to dodecyl (n = 12). A combination of several methods, such as vibrational spectroscopy, wide-angle X-ray scattering (WAXS), broadband dielectric spectroscopy, and 1H NMR spectroscopy, is used to understand the correlation between local cation–anion coordination, nature of nanosegregation, and transport properties. The results indicate the propensity of the −NH site on the cation to form stronger H-bonds with the anion as the alkyl chain length increases. In addition, the position and width of the scattering peak q1 (or the pre-peak), resolved by WAXS and due to the nanosegregation of the polar from the nonpolar domains, are clearly dependent on the alkyl chain length. However, we find no evidence from pulsed-field gradient NMR of a proton motion decoupled from molecular diffusion, hypothesized to be facilitated by the longer N–H bonds localized in the segregated ionic domains. Finally, for all protic ionic liquids investigated, the ionic conductivity displays a Vogel–Fulcher–Tammann dependence on inverse temperature, with an activation energy E a that also depends on the alkyl chain length, although not strictly linearly.

Novel Ionic Conducting Composite Membrane Based on Polymerizable Ionic Liquids

“…The photopolymerization was followed at 25 °C by FTIR-ATR spectroscopy analysis (Figure 3). As one can see, before the polymerization the spectrum of VImTf was characterized by the absorbance at 3100-2940 cm −1 attributed to C-H stretching vibrations of the imidazolium ring, by the bands at 1659 cm −1 and 960 cm −1 attributed to C=C vinyl stretching, by the vibrations at 1573 cm −1 and 1544 cm −1 corresponding to C-C and C=N ring respectively [32,33]. The observed vibrations at 624-514 cm −1 correspond to the vibrations within the imidazole ring.…”

“…Thus, according to the literature [26], the conduction mechanism follows the Grotthuss mechanism (E a ranging from 14 kJ/mol to 40 kJ/mol), also known as proton-jumping through the hydrogen-bond network. Since the polymer electrolyte membrane stays at elevated temperatures for a long time (sometimes up to some months) during the fuel cell's operation, long-term performance is a very important parameter [32,33]. Therefore, the thermal stability of the conductivity of the synthesized PILs was evaluated as a function of time at 130 • C (Figure S1) as well as a function of the temperature in cyclic heating/cooling mode (Figure S2).…”

Ionic liquid‐modified materials as polymer electrolyte membrane and electrocatalyst in fuel cell application: An update

“…1,34 Hydrogen bond formation between this proton and the electronegative atom of anions, such as F, N, and O, is required for the proton hopping process. 48 Examples of common cations in the protic IL group are 1-alkyl imidazolium, 1-alkyl-2-alkyl imidazolium, and ammonium-based cations, including those with primary, secondary, or tertiary structures. 49 In this section, the physicochemical characteristics of ILs are discussed, focusing on the thermal stability behavior, conductivity, and electrochemical window.…”

“…50 From the data on weight changes over temperature, the T onset value can be obtained based on the intercept between the baseline (the line at zero weight loss) and tangent of the weight changes over temperature upon decomposition. 48 However, ILs usually start degrading at a temperature that is lower than T onset.…”

Ion and Proton Transport In Aqueous/Nonaqueous Acidic Ionic Liquids for Fuel-Cell Applications—Insight from High-Pressure Dielectric Studies

“…Consequently, H + hopping is suppressed, and the dc conductivity can be realized only by the mobility of ions. 18 Despite that σ dc (RT) usually takes a satisfactorily high value, such an electrolyte is not suitable for commercial applications due to the poor mechanical properties, being a direct consequence of low T g . As a counterexample, in high T g materials, having a structure of amorphous solid above RT, translational motions of ions are frozen at FC operating conditions, and H + hopping becomes the only source of dc conductivity.…”