(390u) Modeling and Optimization of Direct Ocean Capture (DOC) Using Electrochemical Hydrogen-Looping

As the urgency to mitigate atmospheric CO₂ levels increases, direct ocean capture (DOC) has emerged as a potential alternative to direct air capture (DAC) that avoids some of its key challenges. The inorganic carbon concentration in seawater is ~150x higher than in air, which could lead to capture processes with significantly better scalability and energy efficiency than existing DAC processes. Among the two primary DOC pathways—acidification to release gaseous CO₂ and basification to precipitate carbonate—we focus on acidification due to its ability to directly generate high-purity CO₂ gas and its relatively simpler downstream handling. In this route, seawater is locally acidified to shift the carbonate equilibrium toward dissolved CO₂, which is then released as a gas and captured, avoiding the need for solid precipitate management or additional ion balancing steps required in basification-based approaches. We specifically investigated a recently proposed variant of this route that utilizes electrochemical hydrogen-looping, which replaces the high-overpotential oxygen evolution reaction with a more efficient hydrogen oxidation reaction at the anode. This design eliminates the need for expensive bipolar membranes and significantly lowers energy consumption. Despite its promise, no prior modeling has been conducted for this process—not even at the electrochemical cell level—and no optimization or cost estimation studies exist. In this work, we develop a process-level model of the hydrogen-looping DOC system and apply optimization methods to assess whether the process is viable in terms of both economic and operational performance. Economic feasibility serves as a central design objective, guiding the search for configurations that meet the target CO₂ purity and productivity while reducing costs toward the target range of $100–250 per ton CO₂.

We propose a novel process model for direct ocean capture (DOC) using hydrogen-looping electrochemical systems. This model integrates mass and energy balances, detailed acid–base equilibria, carbonate speciation, and vapor–liquid equilibrium (VLE) to capture the complex behavior of seawater chemistry that governs pH and CO₂ release. Carbon speciation is determined based on pH and salinity and incorporated into steady-state mass and energy balances throughout the system. The electrochemical cell acidifies seawater to shift carbonate speciation toward dissolved CO₂, but due to the low Henry’s constant of CO₂ in seawater, effective gas-phase separation remains challenging. VLE calculations were used to evaluate the conditions under which CO₂ can be effectively separated from solution as a gas in each relevant process step. Electrochemical kinetics are also incorporated to represent cell behavior under relevant operating conditions, and the cell geometry was tailored to accommodate the required seawater flow rate efficiently. This design choice helped reduce acidification costs by avoiding unnecessary oversizing of the system.

Our process design includes a new gas–liquid separation system, which we developed by evaluating and selecting among multiple configurations. This separation system was key to achieving high CO₂ purity and productivity and highlights a major contribution of this work. Optimization efforts focused on minimizing cost drivers such as the high operational and capital costs associated with large-volume seawater handling. To achieve the desired CO₂ productivity, the process requires substantial seawater throughput, leading to significant pumping energy and equipment costs—roughly three times the cost of seawater acidification alone. We addressed this by optimizing pump configuration and compression ratios. Temperature swings due to unavoidable compression and expansion in the separation process posed additional energy burdens. To mitigate this, we designed an efficient heat exchange network that minimizes thermal losses and ensures that seawater is discharged close to its original inlet temperature, reducing non-carbon environmental impacts. These results provide design insights into the coupling of electrochemical systems with seawater chemistry, revealing trade-offs between energy input, capital cost, and separation performance. The proposed separation unit was central to achieving the target performance metrics and may have broader applicability to other CO₂ capture processes, including direct air capture (DAC) systems.