(Invited) Toward Understanding Work Inputs for pH Swing-Based Electrochemical CO2 Separation

Abstract

Large-scale CO2 separation and concentration from point sources, seawater and the atmosphere can play key roles in mitigating climate change and promoting the conversion of CO2 into chemical fuels and high-value products. Electrochemical CO2 separation processes have the potential to be more cost-effective than incumbent thermal (e.g. amine-based) analogues if they use inexpensive active materials and are sufficiently energy-efficient. Understanding the various thermodynamic and kinetic factors that contribute to the net electrical work consumed per unit of concentrated CO2 is therefore a critical step in developing practical systems.We discuss a method for electrochemical CO2 separation based on a pH swing cycle driven by redox reactions involving species capable of proton-coupled electron transfer (PCET). Electrochemical reduction of these species results in the formation of alkaline solution, into which may be CO2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in acidification of the solution, triggering the release of pure CO2 gas. This method is attractive for its operational simplicity, and because it can be driven by a wide range of aqueous-soluble or electrode-conjugated PCET-active reactants. Thermodynamic analysis and an experimental implementation of this idea in an electrochemical flow cell demonstrate that the net electrical work input per unit of separated CO2 can be low (< 100 kJ/molCO2). However, this work input comprises coupled kinetic and exergy losses that are highly sensitive to the cell’s electrochemical properties and its manner of operation. We describe how zero-dimensional modeling can play a role in rationalizing the various contributions to the work input, and thus promoting the development of energy-efficient systems which are capable of high throughput CO2 separation.