Moisture Sorption by Low-Cost Pyridinium-Based Protic Ionic Liquids: Kinetics and Physico-Electrochemical Properties

Abstract

We report the synthesis of two pyridinium-based room temperature protic ionic liquids (PILs), pyridinium bisulfate, [HPyr][HSO₄] and pyridinium sulphate, [HPyr]₂[SO₄] and investigation of the kinetics of their water sorption behaviour and its influence on their density, ionic conductivity, and potential windows. The PILs were synthesized by the reaction of pyridine base with an acid, H₂SO₄, under solventless conditions, and confirmed by FTIR spectroscopy and 1H NMR spectra. The appearance vibration bands in the 3095–3252 cm⁻¹ range for −NH⁺ stretching in the FTIR spectra and a peak at a chemical shift of 8.439 ppm in the 1H-NMR of the liquids confirm their synthesis as no such bands/peaks can be seen in that of the pure pyridine spectra. The PILs’ hygroscopic nature was examined by exposing them (5 mL each sample with exposed surface area 3.143 cm²) to air for varied time intervals at a relative humidity, RH=58±5 % and T=20±5 °C. Coulometric Karl-Fischer (KF) titration was used to determine how much moisture each PIL sample absorbed at each time interval. The findings reveal that when the PIL was exposed to air for longer periods of time, more moisture was absorbed, and the results correspond well with the pseudo first-order kinetic model. The densities and conductivities of several samples of the two PILs were examined, and it was discovered that as the percentage water content of the PILs grew, density decreased but conductivities increased. Furthermore, it was discovered that when temperature rose, the conductivity of each of the PILs increased, and the results fit well to the Arrhenius linear equation since the regression coefficient, R², for each of the samples approached the perfect fit value of one. The electrochemical window (EW) data, the mechanism of moisture oxidation within the EWs of each PIL at Pt and Au electrodes, and the electrocatalytic role played by the Pt and Au surface oxides during ethanol oxidation are evaluated and discussed in light of their future sustainable energy applications.