Electrochemical stripping of CO2 from potassium-based salts to facilitate direct air capture

Abstract

This work presents an experimental study of an electrochemical regenerator for stripping CO₂ and recovering potassium hydroxide from potassium carbonate/bicarbonate streams. This regenerator is applicable in the regenerative step of an aqueous Direct Air Capture (DAC) process during which CO₂ is captured from air using potassium hydroxide. Experimental tests were conducted in a near zero-gap cell designed with nickel foam electrodes to show CO₂ stripping from carbon-loaded streams, investigating the effects of current, flowrate, and concentration of anolyte and catholyte, on the performance of the electrochemical regenerator in terms of the CO₂ stripping efficiency (CSE) and energy consumption (EC). Results from the tests show that about 90 percent of the CSE is achievable in this process with a mixture of 0.15 M potassium carbonate and 0.15 M potassium bicarbonate as the feed stream at potassium factor, $k_f,$ of ∼1, whereby decreasing the alkalinity to carbon ratio can further boost the CSE under the same current density. The study shows that increasing the potassium factor favors the CSE, while increasing the buffering factor has a negative impact on the CSE. Furthermore, for potassium factors below the “buffering threshold”, the CSE is negligible. Results also show that the CSE is constant for varying applied current at constant potassium factor. In addition, the influence of supporting electrolyte (SEL) in the cathode chamber of the regenerator, on both the CSE and the voltage penalty is studied. The use of concentrated KOH in the cathode helps to reduce voltage penalty. However, it shows a detrimental effect on the CSE due to the uphill difference in K⁺ concentration between the two chambers, compared to deionized water. Experimental results show energy consumption in the range of 250–500 kJ/mol-CO₂ as a lower range of energy penalty for the regenerator, with CO₂ purity more than 70 percent for a DAC process.