High-throughput measurements of CO2 permeance and solubility in ionic liquid reveal a synergistic role of ionic interactions and void fractions

The factors that govern CO2 solubility in ionic liquids (ILs) are of great interest for the development of new materials for CO2 capture and utilization. The cationic functional group (i.e., imidazolium, pyrrolidinium, pyridinium, etc.), alkyl chain length of cation, degree of fluorination of anion, anion size, and the void fraction in IL are known to influence CO2 solubility. However, a comprehensive explanation of how these factors collectively affect CO2 solubility has not been developed yet. This knowledge gap is largely attributed to the lack of CO2 solubility data for IL structures other than imidazolium based ILs. We report here an automated high-throughput (HT) setup for the measurement of CO2 solubility in room-temperature ILs (RTILs) combining six different anions and nine different cations for a total of 19 different specific ranges of RTILs. The HT setup first dispenses up to 200 µL of RTILs in a 96-well microtiter plate and then utilizes a robotic arm to measure cyclic voltammogram (CV) in each well using maneuverable Ag electrodes. The Cottrell analysis of the CO2 reduction CV peak provides a direct measurement of CO2 permeance in RTILs, which yields Henry's constant from the estimated diffusion coefficient of CO2. Henry's constants thus obtained are in very good agreement with those reported earlier. The measured CO2 permeance and Henry's constant of all RTILs seem to follow a first-order dependence on void fraction and a second-order dependence on electrostatic interaction between anion and cation of IL, with some synergistic dependence on the product of a void fraction and electrostatic interaction, making them two important descriptors for the design of novel ILs.