An Electrochemistry-Assisted Carbon Capture Process

Abstract

The goal of the Paris Climate Agreement is to limit the increase in the global average temperature to 2 °C above pre-industrial levels in this century. [ 1 ] To meet this requirement, capturing CO2 is becoming essential for (1) point sources, including fossil fuel power generation plants, oil refineries, cement and steel manufacturers, and (2) from an open source like air. A common CO2 capture practice employs a two-step process, absorption-desorption, where organic amines capture CO2 in an absorber followed by the liberation of the captured CO2 and regeneration of the capture solvent at an elevated temperature in a stripper. While such methodology is adequate to capture CO2 for point sources, (e.g., 3.5-15 vol% CO2 from fossil-fuel power coal-fired plants and 20-30 vol% from cement and steel production[ 2 , 3 ]), the low concentration of CO2 for other sources (e.g., 0.04 vol% CO2 in air and 1-2% from femetation or brewing[ 3 , 4 ]) stresses the capture process with the implication that new approaches are required to capture CO2 at its low level. In this work, UK CAER explores an alternative approach assisted mainly by an electrochemical flow cell, which leverages pH swings resulting from water electrolysis to recondition hydroxide-based facile carbon capture solvents in a membrane-electrochemical carbon capture system for low concentration CO2.As sketched in Figure 1 of the membrane-electrochemical carbon capture system, CO2 from air reacts with KOH in the membrane contractor, forming CO3 2- and HCO3 - via CO2 + 2OH- → CO3 2- + H2O and CO2 + OH- → HCO3 -. Meanwhile, CO3 2- and HCO3 - are employed for solvent regeneration in the electrochemical flow cell. Such a process is predominantly driven by water electrolysis, as depicted in Figures 1(b) and (c). Due to producing O2 by consuming OH-, 4OH- → O2↑ + 2H2O + 4e-, in the anode chamber, CO3 2- is transformed to CO2 via CO3 2- + H2O → HCO3 - + OH- followed by HCO3 - → CO2↑ + OH-. Concurrently, to balance the negative OH- ionic charge coming from 2H2O + 2e- → H2↑ + 2OH-, K+ migrates through the cation-exchange membrane to the cathode chamber, thereafter producing KOH that will be used to capture CO2 in the membrane contactor.Unlike thermal solvent regeneration, the electrochemical method releases CO2 while producing O2 at the anode. Under such a competing scenario, a high yield of CO2 will be preferred if a purified CO2 is needed for beneficial uses. Thus, a major focus of this work is to promote CO2 liberation from solvents of CO3 2- and/or HCO3 - by investigating the operating parameters of an electrochemical flow cell such as the charging current, volume of the anode chamber, volumetric flow rate of the anode, and solvent concentration. Experimental results will be compared with model predictions from a CO3 2- and HCO3 - system, providing insights for the first time on both the electrochemical flow cell design and operation for carbon capture. Figure 1. (a) A process sketch of a membrane-electrochemical carbon capture system featuring a membrane contactor for CO2 capture and an electrochemical flow cell for solvent regeneration. (b) and (c) Demonstration of reactions in the electrochemical flow cell when the solvent is regenerated.