A recent collaborative study led by researchers at the Renewable and Sustainable Energy Institute (RASEI), including Professors Wilson Smith and Bri-Mathias Hodge, presents an incisive techno-economic comparison of two frontier methods for atmospheric carbon removal: direct air capture (DAC) and direct ocean capture (DOC). This work, published in the journal Joule, leverages integrated modeling frameworks to assess both technologies under an innovative regeneration strategy powered by bipolar membrane electrodialysis (BPMED), a promising electricity-driven process.

Direct air capture, the more mature of the two approaches, employs liquid solvents to scrub CO₂ directly from ambient air. Facilities like the under-construction plant in Texas, capable of capturing half a million tonnes of CO₂ annually, showcase the scalability potentials of DAC technology. In contrast, direct ocean capture capitalizes on the ocean’s natural propensity to absorb a substantial fraction of anthropogenic CO₂ emissions—roughly 30% per year. By extracting dissolved inorganic carbon from seawater, DOC circumvents the energy-intensive need to process vast quantities of dilute atmospheric air, leveraging the ocean’s carbon reservoir as a more concentrated carbon source.

A critical obstacle shared by both techniques is the regeneration of the sorbent medium, which conventionally requires thermal input near 900°C to release concentrated CO₂. This step not only demands significant energy, often sourced from fossil fuels, but also emits greenhouse gases that compromise the net efficacy of CO₂ removal. Recognizing this challenge, the RASEI team simulated replacing thermal regeneration with BPMED, wherein electrical currents drive chemical shifts to release CO₂ under ambient temperature conditions, potentially reducing energy consumption and emissions.

The study’s integrated techno-economic analysis (TEA) bridges physical capture mechanisms, energy expenses, and full cost implications, enabling a holistic understanding of scale-up feasibility. Lead author Dr. Hussain Almajed emphasizes the study’s goal to elucidate trade-offs rather than declare a definitive winner, contextualizing the comparison within varying energy grid scenarios, including current and projected decarbonized states of the California electricity grid as well as off-grid renewable power supplies.

Fundamental disparities in carbon concentration between air and seawater define the operational and economic characteristics of DAC versus DOC. While atmospheric CO₂ is exceedingly dilute—approximately 120 times less concentrated than dissolved carbon in seawater—once captured, the typical DAC solvent solution exhibits carbon concentrations 160 to 320 times higher than that of seawater. This means DAC systems process smaller liquid volumes but operate BPMED under high electrical currents, resulting in high energy consumption despite a more compact equipment footprint.

Conversely, DOC systems must handle vast volumes of seawater with low carbon content, necessitating membrane areas roughly 20 times larger than DAC facilities. Although this significantly elevates capital costs, the BPMED process for DOC runs at lower current densities, translating to decreased energy per tonne of CO₂ captured. In modeled scenarios for a plant capturing 100,000 tonnes of CO₂ annually, DAC-BPMED’s cost approximated $470 per tonne under California’s existing grid, while DOC-BPMED was near $1,500 per tonne, predominantly due to capital expenditure rather than operational energy use.

An unexpected insight emerged regarding the economic role of sodium hydroxide (NaOH), a co-product generated during BPMED regeneration. NaOH is a globally traded industrial chemical, valued at around $450 per tonne, serving industries from paper manufacturing to water treatment. The DOC process, by processing expansive seawater volumes, produces surplus NaOH beyond its operational needs. Modeling suggests that in a decarbonized energy future circa 2050, revenue from NaOH sales could wholly offset the CO₂ capture costs, potentially resulting in net profitability for DOC-BPMED.

Despite these promising indications, the researchers caution about market scale limitations. The global NaOH market’s size constrains how much of the carbon capture industry’s output it can absorb without saturation effects. Even if DOC-BPMED supplied 20% of 2050 NaOH demand, it would offset less than 0.1% of today’s global energy emissions. Nonetheless, this finding highlights the broader strategic potential of integrating carbon capture with valuable commodity production, a synergy already pursued by companies like Travertine Tech, which simultaneously captures CO₂ and manufactures commercially valuable phosphoric acid and cementitious materials.

The source and nature of electricity powering BPMED regeneration is a paramount factor influencing the sustainability and cost profile of these capture systems. Through four electricity scenarios—California’s current grid, a highly decarbonized 2050 projection, and dedicated off-grid wind and solar installations—the study elucidates that grid-connected systems currently outperform standalone renewables on cost efficiency. The continuous operation enabled by grid reliability dilutes capital costs compared to intermittent renewables, which lack integrated energy storage optimizations in the model, elevating capture costs per tonne.

These findings underscore a vital policy message: achieving effective carbon removal at scale is intricately linked to grid decarbonization. Clean, reliable electricity supply is not ancillary but foundational to deploying next-generation carbon capture technologies sustainably and economically.

While the study offers rich insights, the authors acknowledge areas for refinement. Advanced membrane material characterization, updated equipment cost data, and integration of hybrid energy systems with storage promise to sharpen future model fidelity. These enhancements yield not only more precise cost predictions but also strategic direction on research investments—such as efforts to increase seawater carbon concentration for DOC, which the study’s sensitivity analysis indicates could slash capture costs by up to 50%.

Ultimately, removing atmospheric carbon on a scale commensurate with global emissions reduction targets demands interdisciplinary approaches spanning chemistry, engineering, economics, and policy. This study’s comprehensive techno-economic framework demystifies the complex trade-offs that define carbon removal technologies, presenting an informed roadmap for optimizing research and deployment strategies. Recognizing bottlenecks, evaluating synergies with commodity markets, and embedding the carbon capture systems in the context of a clean energy grid are pivotal steps en route to meaningful climate mitigation.

Subject of Research: Carbon dioxide removal technologies; direct air capture and direct ocean capture using bipolar membrane electrodialysis.

Article Title: Comparative Techno-Economic Analysis of Electrically Regenerated Direct Air and Ocean Carbon Capture Systems.

News Publication Date: 10-Apr-2026

Web References:

• https://climate.copernicus.eu/copernicus-2024-first-year-exceed-15degc-above-pre-industrial-level
• https://www.iea.org/reports/net-zero-by-2050
• https://www.colorado.edu/rasei/wilson-smith
• https://www.colorado.edu/rasei/bri-mathias-hodge
• https://doi.org/10.1016/j.joule.2026.102424
• https://doi.org/10.1038/s41467-020-18232-y
• https://travertinetech.com

References:
Almajed, H., Smith, W., Hodge, B.-M., et al. (2026). Comparative Techno-Economic Analysis of Electrically Regenerated Direct Air and Ocean Carbon Capture Systems. Joule. DOI: 10.1016/j.joule.2026.102424.

Keywords:
Carbon capture, Direct air capture, Direct ocean capture, Bipolar membrane electrodialysis, Carbon dioxide removal, Techno-economic analysis, Climate change mitigation, Renewable energy integration, Sodium hydroxide co-production, Grid decarbonization.

The direct air capture of CO₂ from ambient air constitutes one of the most challenging yet vital endeavors for mitigating climate change. Unlike point-source carbon capture, DAC must function efficiently at the extraordinarily low concentrations of CO₂ present in the atmosphere—roughly 420 parts per million—while coping with the presence of oxygen, nitrogen, moisture, and other components of ambient air. Traditional electrochemical approaches have struggled with oxygen sensitivity in redox-active species, leading to undesirable side reactions that degrade performance, diminish capture capacity, and inflate energy consumption. Overcoming this challenge was a key motivator behind the new system described by Jin et al. in their pivotal 2025 Nature Energy publication.

Central to this innovation is the conceptual and practical separation of oxygen-sensitive reduced phenazine species from the CO₂-containing gas stream, achieved by fabricating redox-active cyclic poly(phenazine sulfide) as solid electrodes within a hybrid phenazine flow cell. This architectural choice ensures that the susceptible reduced organic molecules do not directly interact with atmospheric oxygen, thereby preserving their redox integrity and maintaining high coulombic efficiency. The protective spatial isolation achieved here not only mitigates side reactions but also preserves the reversible proton-coupled electron transfer cycles critical for driving a pH swing that captures and releases CO₂.

The principle underlying the pH swing approach hinges on an electrochemically induced transformation of solution acidity that modulates CO₂ solubility and binding affinity. Through this reversible chemical process, the system can capture CO₂ from the air by acidifying the electrolyte to promote bicarbonate or carbonate formation and subsequently release the CO₂ upon basification. The reversible redox behavior of phenazine derivatives plays an integral role in toggling the electrolyte’s pH, facilitated by the electron-transfer reactions coordinated with proton exchanges. The solid-state cyclic poly(phenazine sulfide) electrodes act as stable, reusable active sites that effectuate this pH modulation with remarkable efficiency.

Energy consumption is a paramount metric in evaluating DAC technologies, often dictating the viability of large-scale deployment. Impressively, the hybrid phenazine flow cell system achieves a coulombic efficiency as high as 99%, indicating near-complete utilization of electrical charges in driving the redox processes without significant losses. This efficient charge management translates to an energy requirement of just 73 kJ per mole of CO₂ captured when tested with simulated flue gas containing typical levels of O₂, and only modestly higher at 104 kJ per mole during direct air capture from ambient air. These figures not only underscore the electrochemical system’s energy efficiency but position it favorably against other DAC technologies, some of which are hampered by energy-intensive thermal regenerations or absorption-desorption cycles.

A critical strength of this hybrid flow cell lies in its ability to decouple the delicate reduced organic species from direct oxygen exposure. Oxygen is notoriously reactive and can irreversibly oxidize reduced phenazine molecules, severely limiting the operational lifetime and capacity of many electrochemical carbon capture strategies. By spatially isolating the phenazine-based electrodes within a flow cell design, the system confines the reduced species away from the gas-liquid interface where oxygen is prevalent, preserving functionality over extended cycling. This design innovation also stabilizes the chemical environment, reducing degradation rates and enhancing the potential for long-term deployment.

Moreover, the precise engineering of cyclic poly(phenazine sulfide) polymers as electrode materials extends beyond passive retention of active species. These redox polymers exhibit robust electron-conducting properties, chemical resilience, and operational stability under the variable pH and redox conditions of DAC cycles. Their cyclic structure endows them with enhanced solubility tuning and electrochemical accessibility compared to linear analogs, contributing to their superior performance in the flow cell. This synthesis approach points toward a class of organic materials tailored for reversible proton-coupled electron transfer reactions, opening pathways to design tailored redox polymers for various emerging electrochemical technologies.

From a system engineering perspective, the flow cell configuration exploits fluid mechanics and electrochemical control to create spatial gradients in chemical composition and charge density. By circulating electrolyte through the phenazine electrodes, the system dynamically modulates pH levels in the reactor volume to effect sequential capture and release of CO₂. This continuous flow operation contrasts with batch systems common in many sorbent-based DAC designs, offering prospects for enhanced scalability, integration with renewable electricity sources, and coupling with downstream CO₂ utilization or sequestration processes.

Beyond the core technical achievements, the strategy epitomizes an overarching approach of spatially isolating vulnerable chemical species to preclude problematic side reactions. While demonstrated here for phenazine-based DAC systems vulnerable to oxygen attack, the principle holds broad applicability across electrochemical devices challenged by parasitic reactions. For instance, organic redox flow batteries, electrocatalytic reactors, and bioelectrochemical systems may benefit from analogous design philosophies that segregate reactive intermediates to optimize performance and durability.

The implications of this work ripple beyond immediate technical gains, offering a glimpse into a new paradigm of clean energy-driven carbon management. By leveraging electrochemical pH swings powered entirely by renewable electricity, this system embodies the vision of decarbonization pathways uncoupled from fossil-fuel-derived thermal inputs. This alignment with sustainable energy vectors is crucial for DAC technologies aspiring to operate at global scales without contributing counterproductively to carbon emissions.

Furthermore, the modularity and flexibility inherent in flow cell architectures suggest ease of integration within bespoke industrial environments and existing infrastructure. The adaptability to different gas feed streams—whether simulated flue gases rich in CO₂ or ambient atmospheric air—underscores the versatility of the approach and its potential to address diverse carbon capture challenges across sectors.

While further work remains to optimize long-term stability, electrode fabrication scalability, and system economics, the presented advances mark a decisive step forward. The experimentally demonstrated near-ideal coulombic efficiency and low energy consumption metrics establish a compelling benchmark competing with incumbent DAC methods. Ongoing research focused on tuning phenazine derivatives, enhancing polymer electrode design, and integrating advanced membrane technologies could further realize the commercial potential of this platform.

In summary, Jin and colleagues’ development of an electrochemical hybrid flow cell utilizing spatially isolated phenazine electrodes represents a milestone in direct air capture technology. It merges sophisticated molecular design with clever system engineering to solve the persistent oxygen sensitivity dilemma, achieving stable and energy-efficient CO₂ capture directly from air. Embodying the promise of organic redox flow chemistry harnessed for climate solutions, this strategy illuminates a viable pathway toward scalable, sustainable, and impactful carbon dioxide removal.

As the climate crisis deepens and the hunt for effective carbon management intensifies, innovations of this caliber instill hope for technologically realistic interventions. By marrying fundamental electrochemistry with practical materials science, this work sets the stage for a new generation of DAC technologies—capable, efficient, and attuned to the energy transition imperatives of the 21st century.

Subject of Research:
Electrochemical direct air capture of CO₂ using pH swing driven by reversible proton-coupled electron transfer of organic molecules.

Article Title:
Direct air capture of CO₂ in an electrochemical hybrid flow cell with a spatially isolated phenazine electrode.

Article References:
Jin, X., Jin, S., Li, L. et al. Direct air capture of CO₂ in an electrochemical hybrid flow cell with a spatially isolated phenazine electrode. Nat Energy (2025). https://doi.org/10.1038/s41560-025-01836-3

Image Credits: AI Generated

Direct air capture has emerged as one of the most promising climate interventions—extracting CO₂ directly from ambient air and subsequently storing it to prevent further atmospheric accumulation. However, the physical and chemical properties of captured CO₂, especially its purity, have long been suspected to significantly influence the viability and cost-effectiveness of transportation and storage. This new study provides rigorous quantitative analysis demonstrating that only when CO₂ is concentrated beyond the critical threshold of 70% does geological storage become economically tenable. Below this level, the costs associated with compression, transportation, and injection escalate steeply, thereby undermining the practical deployment of such interventions on a global scale.

The researchers meticulously evaluated multiple facets of CO₂ capture and storage operations, integrating system-level modeling with real-world geophysical parameters. Their approach included simulations of geological formations typically targeted for CO₂ injection, such as deep saline aquifers, depleted oil fields, and basalt formations. These reservoirs exhibit varying trapped capacities, permeability ranges, and mineralogical compositions that interact complexly with the injected CO₂. The study’s critical finding hinges on the recognition that dilute CO₂ streams—those below the 70% purity mark—require disproportionately higher energy inputs for processing and pressurization to achieve supercritical states necessary for efficient, long-term subsurface storage.

Furthermore, the analysis addressed how CO₂ purity corresponds with associated impurity gases such as nitrogen and oxygen, which remain after air capture and influence storage dynamics. These inert and reactive gases can alter the phase behavior of the CO₂ stream and affect caprock integrity as well as the geochemical reactions occurring within storage reservoirs. The economic models factored these complexities, revealing that when impurity levels are high (thus reducing CO₂ concentration below the threshold), the likelihood of leakage and increased monitoring costs rises, further detracting from the method’s cost-benefit balance.

In addition to geological and chemical considerations, the study sheds light on how feedstock purity directly impacts the engineering requirements of the capture-to-storage chain. Technologies commonly employed for DAC, such as solid sorbents, liquid solvents, or membrane separation, vary in their ability to concentrate CO₂. Achieving the 70% concentration threshold may necessitate integrating multiple purification stages, which in turn drives capital and operational expenditures. The researchers argue convincingly that advancing innovative purification methods—which can efficiently reach or exceed this benchmark—is essential to unlocking economies of scale in CO₂ storage infrastructures.

Perhaps most compelling is the study’s implication for policy and investment frameworks governing carbon capture and storage (CCS). Existing subsidies and carbon pricing mechanisms often overlook the nuanced relationship between CO₂ purity and storage costs. By introducing a scientifically grounded concentration threshold, the research provides policymakers with a concrete metric to steer funding towards capture technologies and workflows that meet or surpass this critical point. This alignment could ensure that public and private investments maximize returns on climate impact without incurring unsustainable economic burdens.

Industries poised to leverage DAC technologies, ranging from synthetic fuel producers to large-scale industrial emitters, stand to benefit from this threshold insight. For example, capturing CO₂ from distributed sources or low-concentration industrial flue gases may require strategic upgrading or integration with purification facilities to achieve the economically optimal CO₂ purity. This research may thus catalyze a revision of current operational blueprints, prompting a shift toward centralized purification hubs or hybrid approaches that balance capture efficiency with storage economics.

Moreover, the identified 70% CO₂ concentration standard hints at the importance of adaptive storage site characterization. Not all geological formations respond identically to CO₂ of varying purity, and the interplay between impurity gases and reservoir parameters will demand site-specific assessments. Future exploration and reservoir appraisal protocols might incorporate these purity thresholds to refine estimates of storage capacity, injection feasibility, and long-term containment security. This evolution in geologic appraisal techniques translates into better risk management and optimized deployment pathways for CCS projects.

Another dimension underscored by this study involves the lifecycle emissions and energy footprint associated with DAC and storage workflows. The extra energy needed for compressing and transporting less concentrated CO₂ effectively offsets some of the environmental gains. By defining the concentration benchmark, the research contributes invaluable guidance for system designers aiming to minimize net emissions. A holistic approach, encompassing capture technology choice, purification process, transport logistics, and storage geology, is imperative to ensure the true sustainability of DAC-enabled CCS systems.

Intriguingly, the researchers project that surpassing the 70% concentration mark could unlock significant cost reductions, opening pathways for larger-scale commercial applications. This revelation carries enormous weight as DAC matures from pilot projects into industrial-scale operations. With the climate urgency intensifying, economies must accelerate the adoption of CCS technologies capable of handling millions of tons of CO₂ annually. Establishing concentration thresholds essentially sets the stage for standardized operational criteria, accelerating cross-sector collaboration and technology integration.

Despite these advancements, the authors caution that navigating the complexities of DAC-related storage is far from trivial. Challenges remain in guaranteeing reservoir integrity under prolonged injection of high-purity, sometimes supercritical, CO₂. Monitoring and verification technologies must continue evolving to detect even minute migration pathways or leaks, preserving public trust and regulatory compliance. Additionally, the economics favoring high-purity CO₂ storage may stimulate innovation in capture technology design toward achieving these purity levels more effectively, presenting a fertile ground for interdisciplinary research and industrial partnerships.

The implications of this discovery extend beyond mere technical feasibility and cost optimization. It compels a reevaluation of how climate mitigation strategies prioritize technologies for large-scale deployment. As governments and corporations draft roadmaps toward net-zero emissions, anchoring decisions in solid empirical thresholds addressing economic and environmental performance is crucial. This study provides a cornerstone for such grounding, blending engineering rigor with applied geoscience and techno-economic analysis.

On a broader horizon, the threshold concept may inspire new avenues in carbon management paradigms. For instance, integration of DAC with renewable energy sources for purification, compression, and transport could synergistically advance both clean energy transition and greenhouse gas reduction goals. It also highlights potential for regional hubs specializing in high-purity CO₂ production and centralized geological storage, fostering economic clusters and enabling more coordinated climate action.

In conclusion, this pioneering research boldly defines a quantitative boundary shaping the future of economically viable geological CO₂ storage from direct air capture. By pinpointing the 70% CO₂ concentration as a critical threshold, it bridges fundamental science with real-world application, catalyzing informed investment and innovation. The path toward scalable, effective, and sustainable CO₂ sequestration may now be clearer, provided the global scientific and industrial community heed this call to optimize purity alongside capture volumes.

The unveiling of this concentration criterion marks a monumental leap in understanding the intricate economics of DAC-CCS systems, potentially accelerating their maturation into indispensable tools in the climate mitigation arsenal. With carbon removal technologies advancing rapidly, such insights are invaluable beacons guiding the collective endeavor to safeguard the planet’s future. As the climate crisis deepens, harnessing the synergy of science, engineering, and policy becomes imperative—and this study stands as a testament to that transformative power.

Subject of Research: Economically viable geological CO₂ storage from direct air capture and the critical threshold of CO₂ concentration required for cost-effective sequestration.

Article Title: Economically viable geological CO₂ storage from direct air capture has critical threshold of 70% CO₂ concentration.

Article References:
Zhang, L., Liang, Y., Kioka, A. et al. Economically viable geological CO₂ storage from direct air capture has critical threshold of 70% CO₂ concentration. Commun Eng 4, 127 (2025). https://doi.org/10.1038/s44172-025-00468-5

Image Credits: AI Generated