Bismuth-Driven Flow-Through pH Swing Architecture for Membrane-Free Electrochemical Marine Carbon Removal

Abstract

Marine carbon dioxide removal (CDR) through electrochemical pH swings offers an energy-efficient pathway for scalable CO2 removal, utilizing the natural properties of seawater as a major carbon sink and conductive electrolyte. However, the implementation of such technologies has largely been limited by costly selective membrane materials. The recently established membrane-free pH swing via bismuth−bismuth oxychloride electrodes presents a promising energy-efficient approach, although demonstration has been thus far limited by modest reaction rates. Here, we advance this approach to industrially relevant current densities by developing a flow-through pH swing cell designed for elevated reaction rates. Porous flow-through electrodes are fabricated by coating active materials onto high-surface-area porous felt substrates, providing both large electroactive area and electrolyte permeability. These porous bismuth electrodes demonstrate high reaction activity and material utilization at elevated current densities. Integrated cell testing demonstrates an order-of-magnitude increase in current density over prior work with minimal associated energy losses, operating at 0.7 V at 20 mA/cm2 compared to 0.6 V at 1 mA/cm2 in previous work. This flow-through architecture maintains performance upon scale-up and shows consistent performance with both simulated seawater and real seawater after nanofiltration. CO2 extraction is demonstrated with up to 80 mL/min of simulated seawater, with an electrochemical energy consumption of 136 kJ/mol at 20 mA/cm2. Technoeconomic analysis translates these performance advancements to a 14% reduction in projected system cost from the flow-by cell of this work and provides guidance for future improvements, including the replacement of silver−silver chloride counter electrodes. Together, this work illustrates the potential of bismuth-based flow cells for cost-effective marine carbon removal while identifying key directions for future development.