The escalating concentration of atmospheric CO2 necessitates innovative strategies for carbon capture and conversion. While capturing CO2 from point sources like power plants has been a primary focus, achieving long-term negative emissions requires direct capture from either air or ocean water. The world's oceans serve as a significant carbon sink, absorbing about 40% of anthropogenic CO2 since the industrial era. Ocean water possesses a considerably higher CO2 concentration than the atmosphere, making it an attractive alternative to direct air capture (DAC). However, challenges exist, such as the high cost of ocean water intake, pretreatment, and outfall in land-based systems. Co-locating such a system with a desalination plant might reduce costs, but the scale of CO2 removal would remain limited. The development of offshore, stand-alone systems powered by renewables offers advantages like reduced land use and access to offshore CO2 storage sites. Existing electrodialysis designs for CO2 capture from ocean water suffer from energy-intensive water-splitting reactions at the electrodes. This paper introduces a novel bipolar membrane electrodialysis (BPMED) cell design aiming to overcome this limitation.

Previous research has explored two types of electrodialysis designs for CO2 capture from ocean water. These methods typically acidify ocean water to shift the CO2/bicarbonate equilibrium toward dissolved CO2, subsequently captured using liquid-gas membrane contactors. However, these earlier designs incurred significant voltage losses due to the unavoidable water-splitting reaction (hydrogen evolution reaction at the cathode and oxygen evolution reaction at the anode). This study builds upon this existing knowledge by proposing a BPMED cell that replaces the water-splitting reactions with reversible redox couples, thereby aiming to minimize energy consumption.

This research involved the design, fabrication, and evaluation of a novel BPMED cell for CO2 capture from ocean water. The cell comprises two ocean water compartments separated by a bipolar membrane (BPM), two reversible redox-couple compartments separated from the ocean water compartments by cation exchange membranes (CEMs), and two electrodes. The electrodes utilize a reversible redox couple (potassium ferro/ferricyanide) to replace the energy-intensive water-splitting reactions. The BPM generates proton (H+) and hydroxide ion (OH-) fluxes via water dissociation, converting the input ocean water into acidified and basified streams. CEMs maintain charge balance by selectively transporting cations. A multi-physics model was used to simulate the voltage-current density characteristics, electrochemical potentials, and partial current densities. The experimental setup included a vacuum stripping stage to remove dissolved gases (O2 and N2) from the input ocean water before the acidification compartment and another stripping stage for CO2 removal from the acidified stream. The acidified ocean water was then fed to the base compartment to restore its pH. The electrochemical performance of the BPMED cell was assessed by measuring the voltage-current density characteristics, pH changes, CO2 capture efficiency, and electrochemical energy consumption. A vapor-fed CO2R cell was directly coupled to the BPMED system for converting the captured CO2 into CO using Ag catalysts or a wider range of products using Cu catalysts. The performance of the CO2R cell was evaluated using gas and liquid product analyses, calculating Faradaic efficiency (FE). A techno-economic analysis (TEA) was also conducted to assess the viability of the system, comparing it to other CO2 capture methods like direct air capture.

The BPMED cell achieved a record low electrochemical energy consumption of 155.4 kJ mol−1 or 0.98 kWh kg−1 of CO2 at an operating current density of 3.3 mA cm−2 and an ocean water flow rate of 37 ml min−1. The CO2 capture efficiency reached 71% of the total dissolved inorganic carbon (DIC). The direct coupling with the vapor-fed CO2R cell demonstrated a total Faradaic efficiency of up to 95% for CO production using an Ag catalyst and up to 73% for various multicarbon products using a Cu catalyst. The multi-physics model accurately predicted the voltage-current density characteristics of the BPMED cell. Analysis showed that minimizing polarization losses through optimized redox couple concentration and flow rate was crucial for efficient operation. The electrochemical energy consumption was optimized by selecting an appropriate operating current density that balanced sufficient acidification of the ocean water with minimized voltage losses across the BPMED cell. The TEA indicated that co-locating the CO2 capture plant with a desalination plant could significantly reduce the levelized costs (between $0.5 and $0.54 kg−1 CO2), making it economically competitive with other CO2 capture technologies. The study also showed that the output gas stream, initially containing lower CO2 concentration due to residual air, gradually stabilized to a concentration of 93% CO2 after an hour of operation. The discrepancy between the CO2 capture efficiency (71%) and membrane contactor efficiency (76%) suggested that most of the DIC was converted into dissolved CO2 under the optimized operating conditions. However, the presence of Mg2+ and Ca2+ ions in the ocean water led to a lower than expected pH increase in the basified stream due to the formation of precipitates.

This study successfully demonstrates a direct-coupled electrochemical system for capturing and converting CO2 from ocean water. The innovative use of a BPMED cell with reversible redox couples significantly reduces electrochemical energy consumption compared to previous methods. The high CO2 capture efficiency and Faradaic efficiency for CO2R showcase the potential of this technology for achieving net-negative emissions and producing valuable chemicals and fuels. The techno-economic analysis suggests that the system, especially when co-located with a desalination plant, could become economically viable. The findings address the limitations of existing electrodialysis designs by significantly reducing energy consumption and enhancing capture efficiency. The results have broad implications for the development of sustainable carbon management technologies and contribute to the growing research on ocean-based carbon removal.

This research presents a proof-of-concept electrochemical system for efficient CO2 capture and conversion from ocean water. The integration of a BPMED cell and a vapor-fed CO2R cell achieves low energy consumption and high efficiency. Future work should focus on scaling up the system, optimizing the membrane contactors or exploring direct conversion of dissolved CO2, and mitigating the impact of divalent cations in the ocean water on the basified stream. Addressing these aspects will further improve the economic and environmental feasibility of this technology for large-scale carbon removal.

The current study utilized a laboratory-scale system, which limits the generalizability to larger-scale industrial operations. The impact of divalent ions (Mg2+ and Ca2+) in ocean water on alkalinity restoration needs further investigation and potential mitigation strategies. Long-term stability and durability of the BPM and membranes under continuous operation need to be assessed. The techno-economic analysis relied on certain assumptions and estimations; further detailed cost analysis and modelling of industrial-scale deployment would strengthen the results.