The approach belongs to a family of technologies known as electrochemically mediated carbon capture, or EMCC. Rather than relying on heat to strip carbon dioxide from a solvent, as conventional amine scrubbing plants do, EMCC uses molecular sorbents whose affinity for CO2 can be switched on and off simply by adding or removing electrons. In the capturing state, the sorbent molecule is reduced at an electrode and becomes a strong Lewis base that eagerly binds carbon dioxide. When the bound gas needs to be released, a small reverse voltage oxidizes the adduct, liberating a concentrated stream of CO2 and regenerating the sorbent for another round. Because the process is driven by electricity, it pairs naturally with renewable power and avoids the enormous thermal energy penalty that has historically made post-combustion capture so expensive.

In most demonstrations of this concept, however, the electrochemical cell has contained a critical complication: an ion-exchange membrane that physically separates the sorbent compartment from the counter-electrode compartment. The membrane’s job is to shuttle charge-balancing ions between the two sides while keeping the redox-active sorbent molecules away from the counter-electrode, where they would otherwise react indiscriminately. But membranes bring a long list of problems. They add resistance, which raises the voltage and therefore the energy cost of every cycle. They degrade in the organic solvents often used in these systems. They are expensive to manufacture in large areas, and their permselectivity is rarely perfect, allowing some sorbent to leak across and undermining efficiency over time. For a technology that aspires to gigatonne-scale deployment, the membrane has become a liability.

The Johns Hopkins team, led by corresponding author Yayuan Liu, set out to eliminate the membrane entirely by substituting a solid-state counter-electrode that can host the charge-balancing ions within its own crystal lattice. The idea sounds simple, but it collides with a fundamental obstacle that has deterred researchers for years: self-discharge. In a membraneless cell, the reduced sorbent molecules inevitably encounter the counter-electrode surface. If the counter-electrode is thermodynamically poised to accept their electrons, the sorbent will be re-oxidized there unintentionally, quietly undoing the capture chemistry and wasting the electrical energy that was invested in the first place. It is analogous to a battery that drains itself even when nothing is connected.

The pivotal insight of the new study is that this self-discharge is not governed by the thermodynamic driving force, as conventional wisdom assumed, but rather by kinetics, and specifically by the rate at which ions diffuse within the solid counter-electrode material. The team demonstrated that even when the thermodynamic potential difference between the sorbent and the counter-electrode would predict rapid parasitic reaction, a counter-electrode with sluggish solid-state ion transport can suppress the process to negligible levels. The practical consequence is profound: the design criterion for a membraneless capture cell shifts from hunting for counter-electrode materials with precisely matched redox potentials, a nearly impossible constraint, to selecting materials whose ionic diffusion is kinetically slow on the timescale of a capture cycle. That reframing opens a vastly larger palette of candidate materials.

Guided by this kinetic design rule, the researchers screened sodium intercalation compounds and identified sodium iron phosphate, NaFePO4, as an ideal partner for an azopyridine molecular sorbent dissolved in a DMSO electrolyte. Azopyridine is a nitrogen-rich organic molecule that reversibly binds CO2 in its reduced state, and sodium iron phosphate provides a lattice that accommodates sodium ions during charging but conducts them so sluggishly that self-discharge is effectively throttled. The system captures CO2 when the azopyridine is reduced at the working electrode while sodium ions insert into the phosphate counter-electrode, and releases the gas when the current is reversed and the sodium ions return to solution. The electrochemistry is elegant in its symmetry: the same ion traffic that stores charge in a sodium-ion battery underpins the capture and release of a greenhouse gas.

The experimental results are striking for their durability. The membraneless cell operated through 75 consecutive capture and release cycles spanning 350 hours, maintaining consistent CO2 capacity utilization and high Coulombic efficiency throughout. Equally important, the performance held up under conditions that matter in the real world rather than only in idealized laboratory settings. The system continued to function at high current densities, which determines how compact and productive a commercial module could be. It captured CO2 efficiently from dilute feed gases, the regime relevant to direct air capture, where the target gas is present at roughly 420 parts per million. And it tolerated aerobic environments, a notorious Achilles heel for redox-active capture chemistries, since oxygen competes for the electrons intended for the sorbent and can degrade it irreversibly.

To assess what these performance figures would mean commercially, the team built a techno-economic model comparing the membraneless architecture with its membrane-based counterpart. The analysis indicated a potential 28.9 percent reduction in the cost per tonne of captured CO2, driven by the elimination of membrane capital costs, reduced ohmic losses and the simplified engineering of a single-compartment cell. While the authors are careful to frame this as a projected figure rather than a demonstrated one, the magnitude of the savings is significant in a field where every dollar per tonne matters for adoption, particularly for direct air capture, where costs remain the central barrier to scale.

Beyond the specific materials pairing, the study establishes what the authors describe as a scalable and generalizable framework for next-generation electrochemical carbon capture. Because the key design principle is kinetic rather than thermodynamic, other sorbent chemistries, including quinones, alkoxides and redox-tunable Lewis bases explored by this and other groups, could in principle be matched with kinetically suppressive solid counter-electrodes without membranes. The conceptual bridge to battery science is also notable: the same intercalation compounds engineered for sodium-ion energy storage become enabling components of climate infrastructure, and the self-discharge problem they were designed to mitigate turns out to be the very lever that makes membraneless operation viable.

The work arrives at a moment when the urgency of carbon removal has never been clearer. Direct air capture and point-source capture both need processes that run on clean electricity, tolerate real-world gas mixtures and cost little enough to deploy by the thousands of tonnes. By eliminating the membrane and rewriting the design logic that had constrained the field, the Johns Hopkins team has taken a concrete step toward electrochemical capture systems that could eventually be manufactured as simply as batteries. Much work remains, from long-term degradation studies to fully continuous flow operation and pilot-scale demonstrations, but the 350 hours of stable membraneless cycling and the projected cost reduction suggest that the field’s most persistent architectural assumption was one it could finally afford to abandon.

Subject of Research: A membraneless electrochemically mediated carbon capture architecture using solid-state counter-electrodes to suppress self-discharge and reduce capture costs

Article Title: Electrochemically mediated carbon capture using a membraneless architecture

Article References: Liu, A., Mathur, A., Jayarapu, K. N., Li, Z., Li, T., McDaniel, G., & Liu, Y. (2026). Electrochemically mediated carbon capture using a membraneless architecture. Nature Chemical Engineering. https://doi.org/10.1038/s44286-026-00438-4

Image Credits: AI Generated

DOI: 10.1038/s44286-026-00438-4

Keywords: carbon capture, electrochemistry, membraneless architecture, sodium iron phosphate, azopyridine sorbent, self-discharge, direct air capture, CO2 removal, redox-active sorbents, techno-economic analysis, chemical engineering, climate technology

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