Assessment of The Hygroscopic Behaviour of Proton-Conducting Ionic Liquids: Kinetics, Conductivity and Electrochemical Analysis

Abstract

We report the synthesis of five proton-conducting ionic liquids (PCILs) by a neutralization reaction. The proton transfer from the corresponding Brønsted acid to the respective base was confirmed by FT-IR spectroscopy. The Karl-Fischer titration was used to measure the water contents of the PCILs exposed to air for different time intervals at a relative humidity of 36 ± 5% and 31 ± 5 °C. The rate of moisture uptake by the PCILs increased with the increase of their time of exposure to air and follows pseudo-first-order kinetics. The conductivity analysis reveals that the PCILs’ ionic conductivities further rise with increasing water contents and temperature. The conductivity data were fitted with the Arrhenius equation and activation energies (Ea) were deduced. The electrochemical analysis of the triethylammonium dihydrogen phosphate, TEADHP shows that the liquid has a wide (2.8 V) electrochemical window (EW) at the Pt electrode at 60 °C and 5% water. The EW becomes narrower as the temperature and water content of the TEADHP increase due to the increase in the rate of the water electrolysis. The data showed that during positive-going scans, Pt-oxides were formed due to the oxidation of water in the PCIL which plays an electrocatalytic role during formic acid oxidation in the medium.