A Study on Electrochemical pH swing Conditions for Direct Ocean Capture （DOC） using Bipolar Membrane Electrodialysis （BPMED） Hoyoung Jeong College of Engineering

Abstract

Direct Ocean Capture （DOC） is an emerging carbon capture technology, since the ocean absorbs over 25% of the total carbon emission and the CO2 concentration in seawater is 150 times higher than that of atmosphere. Particularly electrochemical DOC （eDOC） uses an electrochemical cell to adjust the pH of seawater. However, eDOC technology has critical drawbacks, such as chlorine gas generation at the anode and low thermodynamic efficiency due to its single compartment structure. To overcome the drawbacks, this study introduced a Bipolar Membrane Electrodialysis （BPMED） system utilizing the Water Dissociation Reaction （WDR）. For experimental verification, three configurations — a 2-compartment cell for acidic decarbonization, a 2-compartment cell for alkaline demineralization, and a 3-compartment cell for simultaneous decarbonization and demineralization - were systematically compared and analyzed. The process optimization was carried out by investigating the influence of applied voltage and flow rate on system resistance and ion transport with various flow rates. An optimal condition in terms of energy efficiency and capture capacity was found. Experimental results showed that the 3-compartment cell suppressed extreme pH swings through an internal pH neutralization, achieving a high carbon removal efficiency （RE） of approximately 100% and a lower energy consumption of 7.7 kWh·kg-1. During long-term operations, calcite and brucite-based mineral fouling occurred on the anion exchange membrane （AEM） of the alkaline demineralization cell which caused irreversible physical damage to the membrane surface. In contrast, the 3-compartment cell maintained excellent physical reversibility under the same conditions, demonstrating its potential for long-term operation. The results of the Technoeconomic Evaluation （TEA） confirmed that the 3-compartment cell is an engineering solution for commercialization, exhibiting the most competitive total process cost based on high processing capacity and low operating costs （OpEX）, despite the occurrence of initial investment cost. The findings of this study are significant in that they go beyond laboratory-scale observations and provide a feasible route for establishing a sustainable marine carbon capture system.