Physicochemical and electrochemical properties of a new series of protic ionic liquids with N-chloroalkyl functionalized cations

Abstract

Six new protic ionic liquids (PILs) based on N-chloroalkyl functionalized morpholinium, piperidinium, pyrrolidinium and alkylammonium cations, with bis[(trifluoromethyl)sulfonyl]imide as counter-ion, were synthesized by a metathesis reaction. To understand the differences of structure, charge distribution and volume between the various investigated N-chloroalkyl functionalized cations, as well as between their non-chloro analogues, computational methods were used to generate the COSMO volume and the sigma profile of each ion. Physicochemical investigations showed lowered melting point of these PILs (−8.5 °C < T_m < 34.1 °C) as compared to their non-functionalized analogues and a high thermal stability with T_5%onset in the range 280–337 °C. The alkylammonium and pyrrolidinium-based PILs display reasonable conductivity (1.23 mS cm⁻¹ < σ < 1.71 mS cm⁻¹), although their viscosity values are relatively high (0.0665 Pa s < η < 0.1093 Pa s). The effect of temperature on the transport properties of each PIL has then investigated by fitting the experimental data with the Arrhenius law and the Vogel–Tamman–Fulcher (VTF) equations, revealing the convergence of viscosity with the former model and conductivity with the latter one. The electrochemical stability of cations towards reduction is discussed in light of the frontier orbital theory. All N-chloroalkyl functionalized PILs display a wide electrochemical stability window (4.1–5.1 V), in the same range as the representative of non-chloro functionalized analogues, [HN₂₂₂][TFSI], (4.3 V).