Effects of Temperature on Amine-Mediated CO 2 Capture and Conversion in Li Cells

Abstract

Copyright © 2020 American Chemical Society. Integrated CO2 capture-conversion, which directly employs postcombustion CO2 in the chemisorbed state for subsequent transformations, is becoming an interesting avenue to facilitate CO2 utilization and storage. Such a process has potential to eliminate the conventional sorbent regeneration step normally required between capture and utilization, which is highly energy-intensive. We previously reported the scientific feasibility of such an integrated process, which was studied as a first exploration-of-concept in a nonaqueous, Li-based cell containing 2-ethoxyethylamine, LiClO4 salt, and dimethyl sulfoxide solvent. The amine-modified electrolyte activated otherwise-inactive CO2 for electrochemical reduction at voltages up to â2.9 V versus Li/Li+ at room temperature, and kinetically facilitated conversion of CO2 to lithium carbonate, indicating that amines can successfully act as electrochemical mediators. However, much remained to be understood about the functionality and compatibility of amine capture chemistry in nonaqueous electrochemical environments containing alkali salts, as well as the kinetics of conversion, particularly at temperatures where thermal desorption via N-C bond cleavage can become a competing issue. Here, we investigated the conversion (discharge) reaction in an elevated temperature range (40 °C < T < 70 °C) to evaluate these points. We find that CO2-amine chemistry is chemically and electrochemically stable in nonaqueous electrolytes (containing both amine and inorganic salt) at these higher temperatures, and that electrochemical conversion kinetics of CO2-loaded amines are competitive and enhanced at higher temperature, especially in the low-current regime. However, new issues arise from the Li anode as temperature increases. These issues can be directly addressed by identifying new amine-solvent combinations, such as diispropylamine in a glyme-based electrolyte (tetraethylene glycol dimethyl ether (TEGDME)) reported herein. These results indicate feasibility to pursue amine-facilitated conversion of CO2 over flexible temperature conditions, while also reporting for the first time that additional amine structures are active for integrated capture-conversion processes, broadening the parameter space for further research.