Carbonate/Hydroxide Separation Boosts CO₂ Absorption Rate and Electrochemical Release Efficiency

Electrochemical CO2 capture systems using hydroxide solutions face stiff performance trade-offs, as the hydroxide ions necessary for rapid CO2 absorption reduce the current efficiency of subsequent electrochemical CO2 release. In this work, we propose a carbonate/hydroxide separation step between CO2 absorption and release to provide a concentrated carbonate stream for efficient electrochemical release and a separate hydroxide stream for rapid absorption. We combine experiments on CO2 absorption, nanofiltration separation, and electrochemical release to build a comprehensive model that illustrates system performance trade-offs. We find that employing commercial nanofiltration membranes for separation increases the electrochemical current efficiency by as much as six-fold without sacrificing absorption rate. In the case of Direct Air Capture, the nanofiltration approach reduces costs by 20-30% and significantly increases the operational flexibility of the system. Such carbonate/hydroxide separations may also find use in other systems such as point source capture and integrated CO2 capture and conversion to valuable products.