HMN 2026: How Plant-inspired water membrane filters CO? with constant selectivity and adjustable permeance

Gas separation membranes are vital for carbon capture, biogas upgrading, and hydrogen purification, all of which require the separation of carbon dioxide from gases like nitrogen, methane and hydrogen. However, the membranes currently in use for these applications suffer from limitations like low throughput or performance under high pressure and humidity, low gas flow, instability, and reaction rate limits.

Plants may have inspired a solution to many of these issues with the way their leaves absorb CO₂. In a new study, published in Nature Communications, a team of researchers tests out a plant-inspired, water-based membrane that offers highly selective and permeable gas separation that outperforms many other materials, while also providing a greener, safer, and potentially cheaper way to capture CO₂ and purify gases.

The selectivity vs. permeance trade-off

Technologies like amine scrubbing and cryogenic separation are typical in many industrial applications for CO₂ separation, but these methods require a significant amount of energy and the use of hazardous chemicals. Membranes offer better efficiency, but often have low throughput or lose performance under high pressure and humidity. In particular, they suffer from a trade-off between gas selectivity and permeance. So, as the membrane’s ability to filter out only CO₂ improves, less of the gas is let through. Under high-pressure or humid conditions, these materials are also susceptible to plasticization and physical degradation.

The study authors say that some liquid membranes, which consist of a liquid sandwiched between porous materials, have shown promise. “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 layers, and slow reaction kinetics in certain liquids that rely on chemical reactions with CO₂,” they write.

Filtering CO₂ like a plant

Plants take in CO₂ by dissolving it in water-filled nanochannels in the cell walls of their leaves. The gas-liquid interfaces in these channels absorb CO₂ for photosynthesis, while sustaining large negative pressures in order to drive water up from the roots using strong capillary forces.

The team involved in the new study saw this mechanism as a potential framework for creating a better membrane. They note two useful properties in this CO₂ uptake mechanism that are also needed in industrial CO₂ membranes: the high CO₂ solubility of water through physical dissolution and water’s high surface tension, which allows it to remain stable in tiny capillaries under high pressure differences.

By mimicking the mechanisms used in plant leaves, the researchers were able to create a highly selective and permeable gas separation membrane from liquid water stabilized in between hydrophilic nanopores. Relying on the inherent solubility of CO₂ in water allowed the membrane to maintain a constant selectivity, even when permeance is altered.

The study authors write, “By fabricating membranes with hydrophilic sub-100-nm pores, we show that a stable water layer can be maintained at pressures exceeding 72 bar. Selectivity in the liquid water membrane is based primarily on solubility, where CO₂ is up to 40 times more permeable than other gases, such as N₂, due to its uniquely high solubility in water.”

Performance and scalability testing

The team measured gas transport ability under various pressures and humidity levels and also tested different water layer thicknesses. They found that the gas permeance can be increased by decreasing the water layer thickness without compromising selectivity. Membranes with 190-nm thick water layers showed gas permeances over 11,000 gas permeation units (GPU), while maintaining selectivities of 40 for CO₂:N₂, 26 for CO₂:CH₄, and 31 for CO₂:H₂. They also found that the membrane remains stable for over a week, even under dry and high-pressure conditions.

“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 GPU. 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,” the study authors write.

The researchers also tested scalability of the membranes using commercially available large-area membranes made of hydrophilic polyvinylidene fluoride (PVDF) and polyethersulfone (PES). The membranes maintained constant CO₂:N₂ selectivities around 40 regardless of the substrate used, but permeance was low.

“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 compared to 190 nm for the fabricated membranes,” the team explains.

Overall, these water-based membranes have a lot of potential for real-world applications, like carbon capture at power plants and industrial sites, hydrogen purification, syngas processing and removing CO₂ from biogas. However, further optimization is still needed to address low permeance in scaled models. The team also notes that the long-term stability under very dry or contaminated conditions will require further study.

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