Water as a gas separation membrane

Domain: biology
Published: 2026-03-23
Editorial outcome: minor_revision

Abstract
Efficient gas separation membranes are essential for carbon capture, biogas upgrading, and hydrogen purification. Inspired by how plants absorb CO2 through water, we present a membrane platform that uses liquid water as the selective layer. Hydrophilic sub-100-nm pores stabilize water through strong capillary forces, enabling operation at feed pressures above 72 bar under dry and humid conditions. Selectivity is governed by gas solubility in water, while permeance is tuned by adjusting the water layer thickness. Reducing this thickness below 200 nm yields CO2 permeances up to 11,600 gas permeation units with CO2:N2, CO2:CH4, and CO2:H2 selectivities of 40, 26, and 31, surpassing the performance of state-of-the-art membranes. Operation is sustained for over a week without water loss, and performance scales using commercially available porous polymer supports under mixed-gas crossflow conditions. Water’s dissolution-based transport avoids saturation and reaction-rate limits, enabling a robust, high-performance, and environmentally benign gas separation platform.

Introduction
Gas separations are crucial to industrial and environmental processes such as carbon capture, biomethane purification, and syngas upgrading, where efficient separation of CO2 from gases like N2, CH4, and H2 is necessary1–3. Membrane-based gas separations have the potential to be up to ten-times more energy efficient than conventional technologies, such as amine scrubbing and cryogenic separation, while also offering simpler operation and modular design4–8. However, current membranes are fundamentally limited by the trade-off between permeance and selectivity, where most membranes exhibit moderate selectivity but their throughput is constrained by low CO2 permeance9,10. Furthermore, many widely considered membrane materials are susceptible to plasticization or physical degradation when exposed to high-pressure and humid conditions, limiting their use in real-world applications11–16.

Supported liquid membranes, which consist of a porous support impregnated with a gas-selective liquid phase, have shown promise in addressing some limitations of current materials. The liquid used, commonly an ionic liquid or amine, can be tailored to selectively interact with CO2 via favorable physical and chemical interactions, allowing for exceptionally high CO2 selectivities. However, the gas permeance of supported liquid membranes is often limited by low gas diffusivity, difficulty in making thin liquid layers15,17, and slow reaction kinetics in certain liquids that rely on chemical reactions with CO218. In addition, many supported liquid membranes exhibit performance losses as CO2 partial pressure increases due to saturation of chemical sorption sites, and experience liquid displacement or blowout at relatively low pressures (less than 5 bar)19,20. Thus, there is a need to explore material systems that are more permeable, selective, and robust under pressure.

In nature, water plays a central role in gas separations: the two largest carbon sinks on Earth, oceans and plants, both rely on liquid water as a medium to absorb CO2 21,22. Plants, in particular, have evolved to uptake CO2 by dissolving it in water-filled nanochannels lining the walls of leaf mesophyll cells (Figure 1a)23–25. The gas–liquid interfaces in these channels both serve as a platform to absorb CO2 for photosynthesis and sustain the large negative pressures (up to 150 bar) needed to drive water up from the roots using strong capillary forces26,27. Nature therefore leverages two unique properties of water in its CO2 uptake mechanism: (i) water has high CO2 solubility through physical dissolution and (ii) water has a high surface tension that allows it to remain stable in nanoscale capillaries under high pressure differences25. Despite these promising properties of water for gas separations, relatively few studies have explored water in an engineered gas separation membrane. Prior work has incorporated carbonic anhydrase, ionic liquids, or other reactive chemicals into an aqueous layer to enable facilitated transport3,28,29, but membranes that mimic the high-pressure tolerance of capillary-stabilized water observed in leaves to improve CO2 separations remain unexplored.

Inspired by the mechanisms of carbon capture in nature, we explore membranes that use water as a selective layer for gas separations. These membranes employ hydrophilic nanopores to stabilize a thin, pressure-stable water layer (Figure 1b). To transport through the membrane, gases must dissolve in the water, diffuse through the liquid layer, and desorb on the permeate side of the membrane. We use experiments and theory to probe gas transport behavior in these membranes, finding that selectivity is primarily governed by gas solubility in water which results in high CO2 permeance relative to other gases. By decreasing the thickness of the water layer, we show that gas permeance can be increased without sacrificing selectivity, allowing the membrane to exceed the perm-selectivity limits of state-of-the-art materials in key separations. Furthermore, we investigate the ability of nanoscale pores to stabilize the water layer under high applied pressure and enable days-long operation of the process without changes in performance. We also explore scale up with commercial, large-scale porous polymer membranes and demonstrate consistent performance under mixed-gas conditions relevant to practical applications. Overall, this work provides a proof-of-concept demonstration for high performance gas separation membranes leveraging liquid water.

Results
Principles of liquid water membranes

Gas transport through liquid water membranes proceeds through a solution-diffusion mechanism, in which gases first dissolve at the feed-side gas–liquid interface, diffuse through the water layer, and then desorb into the gas phase on the permeate side30. The dissolved gas concentration at the gas–liquid interface is defined by Henry’s law:

Cg = KH ⋅ Pg (1)

where Cg is the dissolved gas concentration, KH is the Henry’s constant, and Pg is the gas partial pressure. A difference in the gas partial pressure across the membrane, ∆Pg, leads to a concentration gradient of dissolved gas across the membrane that drives gas flux, J, as described by Fick’s first law of diffusion:

J = −D (dCg / dx) (2)

where D is the diffusion coefficient of the respective gas in water and dCg/dx is the concentration gradient of dissolved gas over length x. Combining equations (1) and (2) yields a flux expression in terms of known parameters:

J = (D KH ∆Pg) / l (3)

where l is the effective water layer thickness. Together, these relations demonstrate that transport is dictated by gas solubility, diffusivity in water, partial pressure gradients, and membrane thickness.

The solution-diffusion model reveals important features of water membrane behavior and design. Since gas diffusivities in liquid water are relatively similar across species, selectivity arises mainly from differences in solubility; these solubility differences are captured by the gas’s Henry’s constant which describes the equilibrium partitioning of gas molecules into the water layer. Notably, CO2 exhibits approximately 40 times higher solubility in water than other gases such as nitrogen, hydrogen, and methane, leading to high selectivity for CO2. Moreover, since selectivity primarily depends on the Henry’s constant, decreasing the thickness of the water layer can increase gas flux without sacrificing selectivity.

To operate effectively, the liquid water layer must remain stably confined within the membrane pores under an applied transmembrane pressure. The maximum pressure the membrane can withstand before gas displaces the water layer, Pmax, is governed by capillary forces and described by the Young–Laplace equation:

Pmax = (2γ cosθ) / r (4)

where γ is the surface tension of the liquid-gas interface, θ is the intrinsic water contact angle on the membrane surface which serves as an approximation for the receding contact angle, and r is the pore diameter.

In addition to pressure-driven displacement, the stability of water within the pores is also influenced by ambient humidity. At low relative humidities, the confined water phase will evaporate if liquid–vapor equilibrium is not sustained. Capillary forces of water in the nanopore inhibit evaporation by decreasing the equilibrium partial vapor pressure of water, as described by the Kelvin equation:

Pv = Pv,0 exp(− (2γ Vw) / (rc R T)) (5)

where Pv is the vapor pressure at the gas-water interface, Pv,0 is the equilibrium vapor pressure as a function of temperature, Vw is the water molar volume in the pore, R is the universal gas constant, rc is the radius of curvature of the liquid-vapor interface, and T is absolute temperature. As ambient humidity decreases, the radius of curvature at the interface must also decrease for vapor–liquid equilibrium to be maintained. The minimum radius of curvature possible in a capillary is a function of the pore radius, r, and the intrinsic water contact angle, θ:

rc,min = r / cosθ (6)

If the ambient water vapor pressure drops below the equilibrium pressure corresponding to this minimum curvature, capillary drying occurs, and the water is removed from the pore. Thus, maintaining stable operation of liquid water membranes requires both pressure resistance and stability under low relative humidity, both of which depend on small, hydrophilic pores to retain the water layer within the membrane.

Experimental gas transport behavior and membrane robustness

To experimentally demonstrate water membranes and validate theory, we tested gas transport performance of water-filled porous membranes using a pressurized gas feed (Figure 2a). Our initial testing used custom-fabricated anodic aluminum oxide membranes with controlled pore diameters (40, 80, 120, 160, and 200 nm) that were impregnated with water simply by pipetting a small droplet on the surface. The thickness of the water layer was varied from 100 µm to 190 nm either by using membranes of different thicknesses (Figure 2a, top inset) or by selectively functionalizing the surface to have a hydrophilic layer of controlled thickness (Figure 2a, bottom inset; Supplementary Note 1; Supplementary Figures 1–3; and Supplementary Table 1).

The aluminum oxide membranes provided an ideal platform to explore the fundamental behavior and performance limits of water membranes because (i) they have uniform isopores of known pore size and porosity (Figure 2b,c and Supplementary Figure 4), (ii) they experience minimal deformation under pressure, allowing for reliable measurements, and (iii) they can be functionalized to trap water on the top surface using available surface science techniques (Figure 2d). Although aluminum oxide membranes were advantageous for proof-of-concept testing and model validation, lower cost and more readily scalable materials will likely be used in practice, as we demonstrate later. Characterization of the fabricated membranes is detailed in Supplementary Note 2 and Supplementary Figures 4–10.

Experimental evaluation of the liquid water membranes confirmed their ability to resist water displacement under high applied gas pressures. Young–Laplace theory (equation (4)) predicts that displacement pressure scales inversely with pore diameter, and experimentally measured displacement pressures of membranes with different pore diameters from 40 to 200 nm demonstrated quantitative agreement with values modeled using Young–Laplace (Figure 2e). The highest measured water displacement pressure was 72 bar in the membrane with 40-nm diameter pores. This pressure exceeds the requirements for most applications, including natural gas sweetening and syngas upgrading, where the feed gas pressure can be 20–70 bar31,32. The maximum measured displacement pressure is also at least three times higher than values for supported ionic liquid membranes in the literature, which range from 0.1–20 bar33,20. We note that it is possible to achieve even higher displacement pressures using membranes with sub-40-nm pore diameters, but safety limitations in our current experimental setup prevented testing beyond 72 bar.

Gas flux measurements through the liquid water membranes agree with predictions based on gas dissolution and diffusion theory. The measured gas flux increased linearly with feed gas pressure for CO2 and N2 pure gases (Figure 2f). The CO2 flux was approximately 40-times greater than the N2 flux, consistent with the difference in aqueous solubility between the two gases. Gas fluxes modeled using solution-diffusion theory with Henry’s law for gas solubility and Fick’s first law for diffusion (equations (1–3)) showed strong agreement with experimental values using zero fitting parameters.

Permeation experiments with different gases demonstrated that selectivity in the water membrane is primarily governed by the aqueous solubility of each gas (Figure 2g). Experimentally measured gas permeances increased with aqueous gas solubility for N2, H2, CH4, O2, and CO2 (Supplementary Table 2). Since most gases have similar diffusion coefficients in water, differences in experimental gas permeances were well-described by the Henry’s law constant (Supplementary Table 3 and Supplementary Figure 11). CO2 exhibited a 24–40 times higher permeance (average of 11,600 gas permeation units, GPU; 1 GPU = 10−6 cm3 [STP] cm−2 s−1 cmHg−1) than other gases due to its uniquely high solubility arising from strong chemical interactions with water and polarizability.

Decreasing the water layer thickness led to a proportional increase in gas permeance without altering selectivity (Figure 2h). To probe the impact of water layer thickness on transport rates, we fabricated membranes with water layer thicknesses spanning three orders-of-magnitude: 20, 50, and 100 µm thick alumina membranes were tested with water filling their entire thicknesses (Supplementary Figure 12) in addition to membranes with 190 nm, 457 nm, and 1.7 µm thick water layers fabricated using controlled surface modification (Supplementary Notes 1 and 2). Since transport resistances in the membrane are dominated by aqueous-phase diffusion in the liquid water layer, the observed CO2 permeance of the water layer was inversely proportional to the membrane thickness, increasing from 46 to 11,600 GPU as the thickness decreased from 50 µm to 190 nm. In contrast, prior supported liquid membranes typically employ much thicker active layers on the order of tens of microns, constraining their achievable CO2 permeances to below 1000 GPU17,35. The selectivity between CO2 and N2, which is primarily dependent on gas solubility, maintained between 31 and 40 for all membranes regardless of their thickness, showing that permeance can be increased by nearly three orders-of-magnitude by decreasing thickness without compromising selectivity.

Long-term testing showed that the water membranes remained stable over at least eight days of continuous operation with a dry gas feed (Figure 2i). Continuous gas permeation tests were conducted using feed gases with a relative humidity below 1% and an applied pressure of 6.89 bar (Supplementary Figures 13 and 14). The permeate side of the membrane was exposed to water vapor from a condensed water droplet. During the eight-day test, CO2 permeance showed low variation, and CO2:N2 selectivity remained consistent between the first and eighth day of testing. This stability with dry feed conditions can be explained by the vapor pressure depression of water in nanoscale capillaries described by equation (5), with corresponding values given in Supplementary Table 4. By reducing the equilibrium vapor pressure of water in the pores, nanoscale confinement of water both increases the humidity threshold required for evaporation to occur and decreases the driving force when evaporation does take place. Under the tested conditions, these factors slow water loss from the dry feed side and promote net condensation from the permeate side, allowing the water layer to be sustained with only minimal moisture present. The humidity needed to maintain the water layer is governed by the pore-size-dependent equilibrium vapor pressure described by the Kelvin equation (Supplementary Figure 15).

Permeance and selectivity performance in gas separations

Conventional gas separation membranes are constrained by a trade-off where increases in gas throughput come at the expense of selectivity. In water-based membranes, a key advantage is the ability to reduce the thickness of the water layer without introducing defects, which are a major limitation in polymeric and supported liquid membranes. Since selectivity in the solution-diffusion framework depends only on differences in solubility and diffusivity between species (equation (3)), decreasing the water layer thickness enhances the gas permeance without affecting selectivity, allowing for operation with both high throughput and suitable discrimination between gases.

Gas separation performance for CO2:N2 separations, which are critical in carbon capture, demonstrates the ability of liquid water membranes to increase permeance without sacrificing selectivity (Figure 3a). Perm-selectivity trade-offs have historically limited membrane performance for CO2:N2 separations, where conventional membranes exhibit an inverse relationship between permeance and selectivity, captured by the Robeson upper bound36–38. In contrast, the liquid water membranes show increasing CO2 permeance with decreasing water layer thickness, reaching values as high as 11,600 GPU with a water layer thickness of 190 nm while maintaining a consistent CO2:N2 selectivity of 40:1. This performance exceeds the Robeson upper bound and highlights the potential of liquid water membranes to outperform state-of-the-art materials reported in the literature (Supplementary Table 5). We note that any membrane governed by solution-diffusion should, in theory, be able to similarly increase permeance without sacrificing selectivity since selectivity arises from either solubility or diffusivity differences (equation (3)); in practice, however, fabricating conventional polymer and ionic liquid membranes that are thin and defect-free has been challenging, leading to the trade-off observed in Figure 3a10.

In addition to CO2:N2 separation, other gas separations such as CO2:CH4 and CO2:H2 are critical for applications including natural gas upgrading and syngas conditioning50–52. Such separations benefit from membranes that maintain suitable selectivity while also achieving high CO2 permeance. Our membranes demonstrate selectivities of 26 for CO2:CH4 and 31 for CO2:H2, values that are competitive with state-of-the-art materials. More importantly, these selectivities are sustained at CO2 permeances exceeding 11,000 GPU, surpassing the performance of conventional membranes that typically operate in the range of 100–5,000 GPU (Figure 3b and 3c). In addition, hydrogen sulfide (H2S) and ammonia (NH3), common contaminants in natural gas, biogas, and syngas, are predicted to permeate even faster than CO2 through liquid water membranes, with theoretical selectivities of up to 3.4 and 1,800 for H2S:CO2 and NH3:CO2, respectively (Supplementary Figure 16). Our results demonstrate that the high solubility-based selectivity and low resistance to transport provided by the sub-micron thick water layer enables strong CO2 separation performance across multiple gas pairs.

Gas sorption capacity and kinetics

In addition to showing high CO2 permeance, our experiments observed that the permeability of CO2 in water membranes is maintained even at high CO2 partial pressures up to 27 bar (Figure 4a). This behavior contrasts with that of ionic liquids and some polymeric membranes, in which CO2 permeability decreases at elevated partial pressures. The loss of CO2 permeability in current materials limits CO2 throughput and causes a loss of selectivity. Mechanistically, ionic liquids experience a loss in CO2 permeability at high partial pressures because CO2 sorption decreases as chemical sorption sites become saturated. Sorption of CO2 in water relies on physical dissolution of gas molecules in water, rather than the formation of carbamate or other chemically bound species and is weakly dependent on CO2 partial pressure for all practical gas separation scenarios. As a result, CO2 permeability in liquid water membranes is maintained even at elevated CO2 partial pressures and, at CO2 partial pressures above 1 bar, water surpasses the CO2 permeability of many high-performing ionic liquids. This stability under high CO2 loadings is particularly advantageous for treating flue gas or natural gas streams, which contain high concentrations of CO2 53,54.

The physical dissolution mechanism of gases in liquid water membranes allows them to avoid the reaction-rate limitations encountered in membranes that rely on reaction-facilitated transport of a carrier species. Transport limitations in these systems can be described by the Damköhler number (Da) which quantifies the ratio of reactive transport to diffusive transport and can be calculated by

Da = (kr l^2) / D (7)

where kr is the rate constant for the gas interacting with the liquid (s−1), l is the liquid film thickness (m), and D is the diffusivity of a gas through the liquid (m2 s−1). When the Damköhler number is much greater than unity, transport is diffusion limited. For a Damköhler number less than unity, the system is reaction rate limited, a regime that results in decreased selectivity for CO2 over non-reactive species. In systems governed by amine- or carbonate-based chemistry, transport becomes limited by CO2 reaction rates even at high thicknesses of 10–100 µm due to the relatively slow reaction rates (Figure 4b). The carbonic anhydrase enzyme enables more rapid carbonate reaction kinetics in liquid membranes, but the system will still become reaction-rate limited at low thicknesses. In contrast, transport in liquid water membranes relies on physical dissolution of CO2 which has a rate constant on the order of 1010 s−1 so the system does not become dissolution rate limited at thicknesses above 1 Å59. Thus, water membranes are unique from other materials in that they can be fabricated as ultrathin layers, such as the 190-nm thick membranes demonstrated in this work, to increase permeance without encountering reaction rate limitations that limit permeance and reduce selectivity.

Scalability and mixed gas testing of liquid water membranes

Liquid water membranes can be implemented using any hydrophilic porous substrate with sufficiently small pores to maintain the water layer, making the approach inherently scalable. We demonstrated this principle using commercially available large-area membranes. Commercially produced hydrophilic polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes were tested in gas separations after filling the pores with liquid water. PVDF and PES membranes were chosen because of the following: (1) they are commercially produced in large sheets (Figure 5a) by a scalable phase inversion process, (2) they are modified to be hydrophilic by the commercial supplier (Figure 5a, inset), and (3) they have sub-micron pore sizes (Figure 5b,c) which allow for stabilization of the liquid water layer.

Testing with commercially available hydrophilic membranes showed that liquid water membranes demonstrate constant CO2:N2 selectivities around 40 regardless of the substrate used (Figure 5d, left panel), in agreement with membrane transport theory and our results with ceramic membranes. For the unoptimized microns-thick commercial membranes used in this work, the CO2 permeance was relatively low (5.1 GPU for PVDF and 6.1 GPU for PES). We attribute this low permeance to the large thickness of the water layer, which likely filled the entire thickness of the hydrophilic membrane and resulted in a long diffusion pathway for gases. The thickness of the water layer in the commercial membranes was approximately 100 µm (Supplementary Table 6), compared to 190 nm for the fabricated membranes.

To further assess performance in realistic conditions, multicomponent gas testing under crossflow was conducted on a commercial PES membrane using a feed gas mixture of 90% N2 and 10% CO2 at a total pressure of 3.1 bar (Figure 5d, right panel). The mixed-gas CO2/N2 selectivity was approximately 40, consistent with single-component measurements, indicating minimal interaction between the two gases in the aqueous phase.

Pressure stability measurements showed water displacement pressures of 1.5 bar for PVDF and 6.6 bar for PES. These pressures are sufficiently high for low-pressure gas separation membrane applications but lower than those observed with alumina membranes due to the larger, polydisperse pore sizes that are vulnerable to gas breakthrough at lower pressures. Based on the wetting pressure, the largest pores in the PVDF and PES membranes are approximately 960 nm and 218 nm, respectively, far larger than the 40 nm pores used in fabricated membranes (Supplementary Table 6). Overall, results with commercial membranes show that large-scale porous substrates can be used as liquid water membranes, but further optimization to decrease the water layer thickness and membrane pore size is necessary to enable improved permeance and pressure tolerance.

Methods
Liquid water membrane preparation. Membranes were prepared by pipetting 20 µL of deionized water onto a hydrophilic porous support and allowing the water to wick into the pores. Commercially available flat-sheet anodic alumina (InRedox, CO, USA), hydrophilic polyethersulfone (Sterlitech PES00347100), and polyvinylidene fluoride (Millipore HVLP02500) membranes were tested without modification. Membranes with a thin (less than 1 μm) hydrophilic layer were fabricated using anodic alumina substrates with varying pore diameters (20, 40, 80, and 120 nm) and a thickness of 50 μm (InRedox, CO, USA). The substrates were rinsed with acetone, ethanol, and isopropyl alcohol before being dried with nitrogen gas. The substrates were then hydroxylated using a UV ozone cleaner (OssSila, Sheffield, UK) for 5 mins and placed in a 10% H2O2 solution at 40 °C for 15 minutes before being exposed to 1H,1H,2H,2H-perfluorodecyltriethoxysilane at a temperature of 130 °C and a vacuum pressure of 50 mTorr for 8 hours to create a hydrophobic layer of fluorocarbons on the anodic alumina membrane. All silane modifications were performed in the vapor phase using a chemical vapor deposition chamber (Supplementary Figure 17). After hydrophobic modification, platinum was then deposited on the membranes using a magnetron sputter coater (Leica ACE600, Germany) with varying incident angles of 65° to 85° with a target distance of 50 mm, a current of 35 mA, and argon as the sputtering gas. The angle of sputtering determined the depth of the platinum layer using geometric calculations confirmed with experimental measurements and imaging. Platinum sputtered membranes were then exposed to 10 mM thioglycolic acid in isopropyl alcohol for 3 hours to selectively modify the platinum layer to become hydrophilic. The membranes were then rinsed with isopropyl alcohol, dried with nitrogen gas, and placed on a hot plate set at 40 °C for a minimum of 3 hours before performance testing.

Displacement pressure measurements. The pressure at which the water layer is displaced from the pores of the membrane was determined by the measurement of abrupt changes in gas flow, up to 7 x 105 GPU for dry anodic alumina membranes (Supplementary Figure 18), from the permeate side of the membrane under applied pressure. The high-pressure (up to 75 bar) membrane holder is shown in Supplementary Figure 19. Membrane samples with a 13 mm diameter were loaded in the test cell, and pressure was applied using compressed nitrogen gas and held for at least 5 minutes at each pressure increment. Pressure was increased in increments of 0.7 bar until membrane failure was observed via rapid gas transport through the pores.

Gas permeance testing. Gas transport across the membrane was measured using a membrane flow cell (In Redox, 304 stainless membrane holder) immersed in a DI water bath to maintain a temperature of 22 ± 2 °C (Supplementary Figure 14). Single gas permeation testing was performed using H2, CO2, CH4, N2, and O2 gases (Airgas Part HY UHP300, CD FG20, ME CP80, NI UHP300, and OX USP200 cylinders, respectively). Fabricated thin membranes were oriented in the membrane cell with the thin, hydrophilic layer facing the permeate side to prevent condensation and accumulation of water on the feed side. Depending on the gas permeation rate, gas flux was measured using an electronic gas flow meter (Aalborg Digital Mass Flowmeter Part 503924), a bubble gas flow meter (Manual Bubble Flowmeter Part 20433-U), or by tracking the displacement of a water column on the permeate side. Transmembrane pressure was controlled by adjusting the feed-side pressure from 1 to 72 bar, while the permeate side was maintained at atmospheric pressure. Relative humidity was measured on the feed side using a high-pressure humidity sensor (Roscid Technologies HDR200). Humidity was controlled by bubbling the feed gas through 15 ml of water at temperatures of 25–50 °C. Gas flux was quantified in units of cm³ (STP)/(cm²·s), and permeance is shown in gas permeation units (GPU), where 1 GPU = 10⁻⁶ cm³ (STP)/(cm²·s·cmHg). Multicomponent gas testing under crossflow conditions was conducted using CO2 and N2 mass flow controllers on the feed side, with helium used as a sweep gas on the permeate side (Maxwell Robotics, Mixed Gas Permeation Analyzer, Austin, Texas, USA). Humidity was introduced to both the feed and permeate streams using water vaporizers and was maintained above 90% relative humidity, as measured by hygrometers installed on both sides of the membrane. Gas permeability and selectivity were determined by gas chromatography analysis of the permeate stream.

Membrane characterization. Membrane hydrophobicity was characterized with an optical tensiometer (Biolin Scientific, AZ, USA) using the sessile drop method with one water droplet of 10 μL. The top surface of the membrane was imaged using field-emission scanning electron microscopy (SEM) from a JEOL JSM-7401F (Tokyo, Japan). The membrane cross section and thickness of the platinum layer was imaged using scanning transmission electron microscopy (STEM) (FEI Co. Talos F200X 200keV). X-ray photoelectron spectroscopy (XPS) (Kratos Supra, Kratos Analytical, UK) was performed by irradiating samples with a monochromatic Al K beam with a source energy of 1486.69 eV, and an X-ray beam power and resolution of 225 W.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "• 1) My overall impression is that the analogy to water uptake and transport in trees is over hyped. Water is transported in\ntrees through combined transpiration (evaporation) and capillary stabilization in small pores like aquaporin channels. Here\nevaporation is to be avoided to maintain the water in the membrane pore channels.\n• 2) The novelty is limited in that there are review articles concerning supported liquid membranes for gas separation, e.g.,\nref.35, where it is stated, “capillary forces and are primarily responsible for solvent persistence; therefore, they contributes\nsignificantly to SLM stability by preventing the solvent from being escaped especially under high differential gas\npressures.129−134 The capillary forces and hence the SLMs stability can be significantly increased by immobilizing the\nsolvent in micropores or nanopores rather than mesopores or macropores, which prevents the solvent from escaping.\n• 3) In Fig. 3, the authors selected only a limited set of membranes to which to compare their membranes and claim that they\nexceed the existing Robeson upper bound. Other supported liquid membranes like those described in ref. 28 along with\ncarbon molecular sieves, e.g. Rahimalimamaghani et al. (Industrial & Engineering Chemistry Research 2023, 62, 45, 19116-\n19132) have CO2/N2 selectivities in excess of 150 and should be plotted in Fig. 3.\n• 4) I maintain that to be published in a top tear journal like Nature Comm, a membrane article should report mixed gas\npermeabilities. That could be done using simply GC analysis of the permeate and retentate streams compared to the feed\nstream.\n• .", "reviewer_2": "This reviewer (#2) was not part of (and did not see) the first round of reviews. The second round reviews by the other\nreviewers is quite extensive and covers topics not addressed by me. I have decided to stay out of those discussions.\nThe author’s response to my comment 6 raises a new question: All permeation experiments were carried out with the\nhydrophilic side facing the permeate? I must have missed it if there is any mention of this in the manuscript, but it certainly\nhas to be disclosed. Furthermore, this membrane orientation works for single component permeation experiments but not for\nmixture experiments: concentration polarization in the pore section above the water phase will reduce selectivity and\npermeance. Something to be aware of when mixture experiments are carried out (which as other reviewers have mentioned\nis a required next step).\nIt will take a minor revision to address my comment above, after which the manuscript can be published as far as this\nreviewer is concerned.", "reviewer_3": "The authors have addressed all my comments in the previous round. I now support the publication of this work in Nat.\nComm.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_5": "I am satisfied with the revisions that the authors have made in response to reviewer comments."}, "author_response": "Response to Reviewers\nWe thank the five reviewers for their comments in both the first and second rounds of review.\nThe feedback provided in the first round led to substantial improvements in the manuscript, and\nwe have made additional revisions in response to the suggestions offered in this round. Below,\nwe provide a point-by-point response to each comment and describe the revisions made in the\nmanuscript. Reviewer comments are shown in black italic text, newly added material appears in\nblue, and deleted text is indicated with red strikethrough. All line numbers refer to the revised\nmanuscript.\nREVIEWER COMMENTS\nReviewer #1\nComment 1: My overall impression is that the analogy to water uptake and transport in trees is\nover hyped. Water is transported in trees through combined transpiration (evaporation) and\ncapillary stabilization in small pores like aquaporin channels. Here evaporation is to be avoided\nto maintain the water in the membrane pore channels.\nOur Response: We thank the reviewer for this comment and agree that the analogy to\nnatural systems must be framed properly. Our intention introducing this reference was to\nhighlight that water is frequently used as a medium for carbon dioxide uptake in nature. Plants in\nparticular leverage two unique properties of water in their carbon dioxide uptake: (i) water has a\nhigh CO solubility relative to other gases, and (ii) water has a high surface tension that allows\n2\nfor air-water interfaces to remain stable in nanoscale, hydrophilic pores, even under very high\npressure differentials.\nWe agree with the reviewer that the analogy to water transport in trees should not be interpreted\nas a direct mechanistic comparison. In plants, water transport is driven by transpiration, whereas\nin our membrane system, evaporation is minimized to maintain water-filled pores. Our reference\nto plant systems was intended only to highlight capillary stabilization of liquid water under large\npressure differentials and the use of water as a medium for CO dissolution, rather than\n2\nevaporation-driven transport.\nWe have revised the manuscript to clarify this distinction and avoid overinterpretation of the\nbiological analogy.\nLines 74 – 87: “In nature, water plays a central role in gas separations: the two largest carbon\nsinks on Earth, oceans and plants, both rely on liquid water as a medium to absorb CO 21,22. Plants,\n2\nin particular, have evolved to uptake CO by dissolving it in water-filled nanochannels lining the\n2\nwalls of leaf mesophyll cells (Figure 1a)23–25. The gas–liquid interfaces in these channels both\nserve as a platform to absorb CO for photosynthesis and sustain the large negative pressures (up\n2\n1\nto 150 bar) needed to drive water up from the roots using strong capillary forces26,27. Nature\ntherefore leverages two unique properties of water in its CO uptake mechanism: (i) water has high\n2\nto remain stable in nanoscale capillaries under high pressure differences25. Despite these promising\nproperties of water for gas separations, relatively few studies have explored water in an engineered\ngas separation membrane. Prior work has incorporated carbonic anhydrase, ionic liquids, or other\nreactive chemicals into an aqueous layer to enable facilitated transport3,28,29, and none have\ndemonstrated it’s unique advantages in CO transport and pressure tolerance but membranes that\n2\nmimic the high-pressure tolerance of capillary-stabilized water observed in leaves to improve CO\n2\nseparations remain unexplored.”\nComment 2: The novelty is limited in that there are review articles concerning supported liquid\nmembranes for gas separation, e.g., ref.35, where it is stated, “capillary forces and are\nprimarily responsible for solvent persistence; therefore, they contributes significantly to SLM\nstability by preventing the solvent from being escaped especially under high differential gas\npressures.129−134 The capillary forces and hence the SLMs stability can be significantly\nincreased by immobilizing the solvent in micropores or nanopores rather than mesopores or\nmacropores, which prevents the solvent from escaping.\nOur Response: We thank the reviewer for this comment and agree that capillary\nstabilization of liquid phases in porous supports is well established in the supported liquid\nmembrane literature, including the cited review. Our work does not claim novelty in the use of\ncapillary forces to stabilize selective liquids, but in how they are applied and demonstrated in this\nsystem.\nThe key novel contribution of this study is that we demonstrate high permeance and pressure-\ntolerant membranes using ultrathin layers of liquid water. Specifically, this study demonstrates:\n(i) stable sub-micron water selective layers (down to 190 nm), which enable CO permeances\n2\nexceeding 11,000 GPU, an order of magnitude higher than typical SLMs that rely on 10–100 µm\nthick liquid layers; (ii) operation with pure liquid water as the selective phase, avoiding sorption\nsaturation and reaction-rate limitations common to ionic liquid and facilitated-transport\nmembranes, and maintaining permeability at CO partial pressures up to 27 bar; and (iii)\n2\nexceptional pressure tolerance, with experimentally measured water retention above 72 bar using\nnanoscale hydrophilic pores, exceeding reported stability ranges of most SLMs.\nExamples of these discussions on these contributions are included below:\nLine 61 – 71: “Supported liquid membranes, which consist of a porous support impregnated\nwith a gas-selective liquid phase, have shown promise in addressing some limitations of current\n2\nmaterials. The liquid used, commonly an ionic liquid or amine, can be tailored to selectively\ninteract with CO via favorable physical and chemical interactions, allowing for exceptionally high\n2\nlow gas diffusivity, difficulty in making thin liquid layers15,17, and slow reaction kinetics in certain\nliquids that rely on chemical reactions with CO 18,19. In addition, many supported liquid\n2\nmembranes exhibit performance losses as CO partial pressure increases due to saturation of\n2\nchemical sorption sites, and experience liquid displacement or blowout at relatively low pressures\n(less than 5 bar)20,21. Thus, there is a need to explore material systems that are more permeable,\nselective, and robust under pressure.”\nLine 192 – 194: “The maximum measured displacement pressure is also at least three times\nhigher than values for supported ionic liquid membranes in the literature, which range from 0.1–\n20 bar32,21.”\nLine 252 – 257: “Since transport resistances in the membrane are dominated by aqueous-phase\ndiffusion in the liquid water layer, the observed CO permeance of the water layer was inversely\n2\nproportional to the membrane thickness, increasing from 46 to 11,600 GPU as the thickness\ndecreased from 50 µm to 190 nm. In contrast, prior supported liquid membranes typically feature\nmuch thicker active layers in the order of tens of microns, constraining their achievable CO\n2\npermeances to below 1000 GPU17,34.”\nComment 3: In Fig. 3, the authors selected only a limited set of membranes to which to compare\ntheir membranes and claim that they exceed the existing Robeson upper bound. Other supported\nliquid membranes like those described in ref. 28 along with carbon molecular sieves, e.g.\nRahimalimamaghani et al. (Industrial & Engineering Chemistry Research 2023, 62, 45, 19116-\n19132) have CO2/N2 selectivities in excess of 150 and should be plotted in Fig. 3.\nOur Response: We thank the reviewer for this suggestion. In Figure 3, we selected\nCO /N permeability-selectivity data from a range of membrane materials, including polymeric\n2 2\nmembranes, mixed matrix membranes, supported ionic liquid membranes, and metal organic\nframework containing membranes. These examples were chosen to provide a relevant\nbenchmark of materials commonly discussed in relation to the Robeson upper bound.\nIn response to the reviewer’s comment, we have now added the high-selectivity CO /N data\n2 2\nreported by Rahimalimamaghani et al. to Figure 3a to broaden the comparison and include\ncarbon molecular sieve membranes with high selectivity. The updated figure is shown below.\nmembrane article should report mixed gas permeabilities. That could be done using simply GC\nanalysis of the permeate and retentate streams compared to the feed stream.\nOur Response: We thank the reviewer for this comment and have completed mixed gas\npermeation tests to address this point. To do this, we collaborated with the National Lab of the\nRockies to use their mixed gas filtration apparatus equipped with a GC as well as humidity,\ncrossflow, and volumetric flow controls. The mixed-gas CO /N selectivity was approximately\n2 2\n40 with a feed gas mixture of 90% N and 10% CO , consistent with the single gas\n2 2\nmeasurements we had taken in our lab previously. We have revised the manuscript to\nincorporate this data as follows:\nLines 411 – 414 (Main text): “Multicomponent gas testing was conducted on a commercial\nPES membrane using a feed gas mixture of 90% N₂ and 10% CO at a total pressure of 3.1 bar\n2\n(Figure 5d). The mixed-gas CO /N selectivity was approximately 40, consistent with single-\n2 2\ncomponent measurements, indicating minimal interaction between the two gases. Overall, results\nwith commercial membranes show that large-scale porous substrates can be used as liquid water\nmembranes, but further optimization to decrease the water layer thickness and membrane pore size\nis necessary to enable improved permeance and pressure tolerance.”\nLine 492 – 496 (Methods): “Multicomponent gas testing under crossflow conditions was\nconducted using CO and N mass flow controllers on the feed side, with helium used as a sweep\n2 2\ngas on the permeate side. Humidity was introduced to both the feed and permeate streams using\nwater vaporizers and was maintained above 90% relative humidity, as measured by hygrometers\ninstalled on both sides of the membrane. Gas permeability and selectivity were determined by gas\nchromatography analysis of the permeate stream.”\nof the hydrophilic polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membrane samples.\nb,c, Scanning electron micrograph of the top surface of the hydrophilic PVDF and PES membranes,\nrespectively. d, Measured CO permeance and CO :N selectivity of PVDF membranes using\n2 2 2\nsingle-component gas feeds and PES membranes using both single-component and\nmulticomponent gas feeds with trapped water layers. Error bars denote the mean ±1 s.d. for\nmeasurements from at least three distinct membrane samples. e, Measured displacement pressures\nof water for PVDF and PES membranes.\nReviewer #2\nsecond round reviews by the other reviewers is quite extensive and covers topics not addressed\nby me. I have decided to stay out of those discussions.\nOur Response: We thank the reviewer for their time and for their comments during the\nprevious round of review, which contributed to improving the quality of the manuscript.\nComment 2: The author’s response to my comment 6 raises a new question: All permeation\nexperiments were carried out with the hydrophilic side facing the permeate? I must have missed\nit if there is any mention of this in the manuscript, but it certainly has to be disclosed.\nFurthermore, this membrane orientation works for single component permeation experiments\nbut not for mixture experiments: concentration polarization in the pore section above the water\nphase will reduce selectivity and permeance. Something to be aware of when mixture\nexperiments are carried out (which as other reviewers have mentioned is a required next step).\nIt will take a minor revision to address my comment above, after which the manuscript can be\npublished as far as this reviewer is concerned.\nOur Response: We thank the reviewer for raising this point. In permeation experiments\nwith the ultrathin water layer, the membranes were oriented with the hydrophilic water-filled\nside of the membrane facing the permeate, while the feed gas contacted the dry portion of the\nporous support. The membrane was oriented this direction to ensure that water drained out from\nthe pores, and the selective layer was a consistent thickness. The membrane can also operate\nwith the opposite orientation, but we wanted precise thickness control since we were reporting\npermeance as a function of the thickness in the manuscript.\nWe agree that future mixed gas testing should orient the water layer facing the feed to minimize\nconcentration polarization. Based on our work so far, this orientation should operate without\nissue.\nWe have clarified the membrane orientation in the Methods section of the revised manuscript:\nLine 486 – 487: “Fabricated thin membranes were oriented in the membrane cell with the thin,\nhydrophilic layer facing the permeate side to prevent condensation and accumulation of water on\nthe feed side.”\nBased on this comment, we estimated the impact concentration polarization would have, in\ntheory, if the membrane were oriented as we tested it with mixed gas steams. Concentration\npolarization (CP) can be estimated by modeling the dry region of the pore as a 50 µm thick\nboundary layer:\n6\n𝐽 δ\n%&\nexp ( )\n𝐶 𝐷\n!,# = !\n𝐶 𝐽 δ\n!,$ 1+𝐸 /exp0 %& 1−13\n# 𝐷\n!\nwhere 𝐶 and 𝐶 are the concentrations of CO at the feed-side interface and in the bulk,\n!,# !,$ 2\nrespectively, 𝐽 is the velocity of transport through the membrane active layer (m/s), δ is the\n%&\nboundary layer thickness (50×10'( m), 𝐷 is gas diffusivity within the boundary layer, and 𝐸\n! #\nis selectivity.\nThe most permeable membrane in this work had a permeance of 11,600 GPU with a pore\ndiameter of 120 nm. With a pressure differential of 0.2 bar CO , similar to what was used in\n2\nmixed-gas testing in the revised work, 11,600 GPU corresponds to a velocity of 𝐽 = 0.0017\n%&\nm/s. The Knudsen diffusivity of CO in a cylindrical pore with 120 nm diameter is estimated\n2\n𝐷 = 1.51×10') m2/s. Using these values, we estimate a CP factor of 0.82. Increasing the pore\n!\nsize or decreasing the CO partial pressure difference would increase the CP factor. For example,\n2\na CO differential of 0.1 bar would result in a CP factor of 0.9.\nso no concentration polarization occurred. However, even with mixed gases, the approximate CP\nfactors are relatively low. As we suggest above, future work with multicomponent gas testing\nand crossflow should orient the water layer towards the feed side and control operating\nconditions to prevent water buildup on the active layer.\nReviewer #3\nComment: The authors have addressed all my comments in the previous round. I now support\nthe publication of this work in Nat. Comm.\nOur Response: We appreciate the reviewer’s careful evaluation in the previous round of\nreview and their time in assessing the revised manuscript. Their support for publication is\ngratefully acknowledged.\nReviewer #4\nComment: I co-reviewed this manuscript with one of the reviewers who provided the listed\nreports. This is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\ntheir time spent evaluating the manuscript.\nReviewer #5\nComment: I am satisfied with the revisions that the authors have made in response to reviewer\ncomments.\nOur Response: We thank the reviewer for their comments during the previous round and\ntheir review of the revised manuscript. We are grateful for their assessment of the revisions.\n8"}]

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