New hydroxylammonium-based protic ionic liquids: Influence of cation and anion structure on thermal, viscosity and conductive properties

In the research work, new protic ionic liquids (Pr-ILs) were synthesized by an acid-base neutralization reaction. The influence of IL chemical structure (i.e. anion and cation nature) on the thermal behavior, viscosity and ionic conductivity was evaluated. To confirm the chemical structure of synthesized Pr-ILs, nuclear magnetic resonance (1H and 19F) and Fourier transform infrared spectroscopy analyses were performed. Thermal analysis demonstrated that the influence of anion nature on the IL thermal stability was more pronounced than that of cation. The ionic liquids with trifluoromethanesulfonate anion showed the highest thermal stability (Tdeg ∼ 410–420 °C) as compared with other studied anions (i.e. phosphate and acetate anions). The trifluoromethanesulfonate-based Pr-ILs also demonstrated high ionic conductivity – between ∼58 and ∼64 mS·cm−1 at 150 °C and non-humid conditions. Despite the fact that phosphate-based Pr-ILs were more thermally stable (Tdeg ∼ 250–280 °C) than acetate-based ones (Tdeg ∼ 215 °C), acetate-based Pr-ILs revealed a higher ionic conductivity (∼15–19 mS·cm−1 at 80 °C) than phosphate-based Pr-ILs (∼0.2–1 and ∼0.4–2 mS·cm−1 at 80 and 120 °C, respectively). The obtained results confirm that the synthesized Pr-ILs are promising candidates for using them in electrochemical applications, namely, electrodialysis and fuel cells.