Physico-chemical characterization of choline chloride based deep eutectic solvents used in CO₂ absorption and electrochemical conversion process

Abstract

Increasing CO₂ concentration in the atmosphere has caused significant concern, paving the way to research and develop technologies like capture carbon and storage (CCS) and carbon capture and utilization (CCU). This thesis focuses on extracting CO₂  from Chimney stacks and regenerating the solvent using electrochemistry. The main aim of this thesis is to identify a solvent that is capable of being used as a good CO₂ capture medium and at the same time as an electrolyte for CO₂ reduction.
Deep eutectic solvents (DES) have been gaining much attention due to their desirable properties such as biodegradability, low vapour pressure, and high tunability for the required purpose. Research has shown promising results in the application of DES in the field of CO₂ capture at a lower price with more eco-friendly solvent. Choline chloride is the most widely used quaternary amine salt which has all the desirable properties; when combined with a CO₂ philic hydrogen bond donor group such as amines, a novel solvent could be formed. The low/negligible vapour pressure of DES makes it suitable for CO₂ absorption in industrial applications. Based on the literature, three different solvents were selected, Choline chloride and Ethylene glycol (ChCl:EG), Choline chloride and Monoethanolamine (ChCl:MEA) and Choline chloride with Aminomethyl propanol (ChCl:AMP).
Various experiments were conducted on different molar ratios of selected solvents to determine the physico-chemical properties. Viscosity was measured as it affects the CO₂ absorption capacity due to limiting the mass transfer and has a significant impact on ion mobility resulting in high ohmic drops and reduced efficiency of the electrochemical extraction process. Conductivity was also measured as having higher conductivity will reduce the ohmic drop and improve the CO₂ removal process; conductivity is inversely related to viscosity. Effect of varying temperature, water and CO₂ loading was observed on these physical properties.
Based on the experiments conducted, it was found that ChCl:EG solutions have the highest conductivity among the pure solvents, with some of the lowest viscosities. In the case of ChCl:MEA, the viscosity reduced with the increase in temperature and increased drastically on the absorption of CO₂. This is because of the formation of carbamates as confirmed by FTIR. ChCl:AMP is a unique solvent as absorption of CO₂ results in the formation of a bicarbonate precipitate, as shown in FTIR.
Addition of EG to ChCl:MEA solution improves the performance of the DES significantly. At similar CO₂ loading, the viscosity of ChCl:EG:MEA (1:4:3) is almost 63.88% less and the electrical conductivity 134.45 % more than ChCl:MEA(1:6)