Electrochemical Chloride-Mediated Marine Carbon Dioxide Removal

Negative Emissions Technologies (NETs) are crucial to avert catastrophic disruption of global climate patterns caused by the continuing atmospheric accumulation of CO₂ due to industrial emissions. The recent surge of interest in NETs in which CO₂ (currently at an atmospheric concentration of ~420 ppm) is removed from the ambient environment itself, by, e.g., direct air capture (DAC), has not yet been matched by a similar drive to reduce CO₂ in oceans, where increasing acidification has led to destruction of coral reefs, and reduced carbonate ion concentrations harm shellfish and other marine life. The total CO₂ accumulation rates by oceans rivals that in the atmosphere, and thus effective means for CO₂removal could augment the other NETs to reduce the environmental burden imposed by this greenhouse gas. The concentrations in water (on a volumetric basis) are much higher at 100 mg/L than that in the ambient air (0.77 mg/L), and thus smaller volumes will need to be treated than in DAC, which could provide a processing advantage. <br/><br/>In current approaches for the removal of CO₂ from oceanwaters bipolar membrane catalysis water splitting coupled with electrodialysis is used to modulate the pH and thereby release the CO₂ as a gas (low pH) or carbonate salt (high pH). We have proposed an alternative approach to marine carbon dioxide removal (mCDR) that does not require expensive membranes or addition of chemicals, is easy to deploy, and does not lead to formation of byproducts or secondary streams. In this approach, the pH is regulated through a chloride-mediated reaction with electrodes in asymmetric electrochemical cells through which the seawater flows. In the acidification cell the DIC speciation shifts from bicarbonate and carbonate to CO₂ which can then be removed in a membrane contactor. The now-decarbonized water is introduced to a second electrochemical cell where the reverse reaction is promoted, the electrodes are regenerated, and the pH increases before the water is discharged back to the ocean. The CO₂ removal approach has perceived advantages in that it does not require expensive membranes or addition of chemicals, is easy to deploy, and does not lead to formation of byproducts or secondary streams. <br/><br/>We will discuss the overall electrochemical swing process, which can be enhanced through flexible electrode configurations to reduce transport and electrical resistances while enabling treatment of large quantities of water. Novel methods for the recovery of molecular CO₂ without the need for high vacuum desorption, and of calcium carbonate precipitates without fouling of the electrodes, will be highlighted.