A new Massachusetts Institute of Technology system could provide a way of continuously removing carbon dioxide from a stream of waste gases, or even from the air. The key component is an electrochemically assisted membrane whose permeability to gas can be switched on and off at will, using no moving parts and relatively little energy.

The membranes themselves, made of anodized aluminum oxide, have a honeycomb-like structure made up of hexagonal openings that allow gas molecules to flow in and out when in the open state. However, gas passage can be blocked when a thin layer of metal is electrically deposited to cover the pores of the membrane.

The work is described in the journal Science Advances, in a paper by Professor T. Alan Hatton, postdoc Yayuan Liu, and four others.

This new “gas gating” mechanism could be applied to the continuous removal of carbon dioxide from a range of industrial exhaust streams and from ambient air, the team says. They have built a proof-of-concept device to show this process in action.

Schematics showing the working principles of an electrochemically mediated gas gating membrane. (A) Gating membrane in its open state, where the AAO membrane with ordered one-directional pores allows rapid mass transfer to the gas-liquid interface. The top surface of the membrane is rendered electrically conductive via physical deposition of a gold thin film. (B) Gating membrane in its closed state. Electrodeposition of a dense metallic nanocrystal layer on the conductive membrane surface can afford an effective gas barrier to reduce the membrane permeability. (C and D) Operational modes of the electrochemically mediated CO2 concentrator demonstrated in this work, which is constructed by integrating an electrochemical CO2 capture cell with two gating membranes. Quinone molecules serve as the redox-active sorbents for CO2 capture/release, which can form stable CO2 adducts when being reduced and release CO2 when being oxidized. M denotes an electrochemically deposited metal coating, which is Zn in this study. Mn+ denotes the corresponding cation of M, which is Zn2+ in this study. (C) For CO2 capture, the gate is open toward a dilute feed stream, and the quinone electrode is being reduced to absorb CO2. (D) For CO2 release, the gate polarity switches to open toward the product stream, and the quinone electrode is electrochemically oxidized to release pure CO2. Image Credit: MIT. Click Image for the largest view. Click the Study Paper link for more information and graphics.

The device uses a redox-active carbon-absorbing material, sandwiched between two switchable gas gating membranes. The sorbent and the gating membranes are in close contact with each other and are immersed in an organic electrolyte to provide a medium for zinc ions to shuttle back and forth. These two gating membranes can be opened or closed electrically by switching the polarity of a voltage between them, causing ions of zinc to shuttle from one side to the other. The ions simultaneously block one side, by forming a metallic film over it, while opening the other, by dissolving its film away.

When the sorbent layer is open to the side where the waste gases are flowing by, the material readily soaks up carbon dioxide until it reaches its capacity. The voltage can then be switched to block off the feed side and open up the other side, where a concentrated stream of nearly pure carbon dioxide is released, available for CO2 recycling.

By building a system with alternating sections of membrane that operate in opposite phases, the system would allow for continuous operation in a setting such as an industrial gas scrubber. At any one time, half of the sections would be absorbing the gas while the other half would be releasing it.

Hatton explained, “That means that you have a feed stream coming into the system at one end and the product stream leaving from the other in an ostensibly continuous operation. This approach avoids many process issues” that would be involved in a traditional multicolumn system, in which adsorption beds alternately need to be shut down, purged, and then regenerated, before being exposed again to the feed gas to begin the next adsorption cycle. In the new system, the purging steps are not required, and the steps all occur cleanly within the unit itself.

The researchers’ key innovation was using electroplating as a way to open and close the pores in a material. Along the way the team had tried a variety of other approaches to reversibly close pores in a membrane material, such as using tiny magnetic spheres that could be positioned to block funnel-shaped openings, but these other methods didn’t prove to be efficient enough. Metal thin films can be particularly effective as gas barriers, and the ultrathin layer used in the new system requires a minimal amount of the zinc material, which is abundant and inexpensive.

Liu said, “It makes a very uniform coating layer with a minimum amount of materials.” One significant advantage of the electroplating method is that once the condition is changed, whether in the open or closed position, it requires no energy input to maintain that state. Energy is only required to switch back again.

Potentially, such a system could make an important contribution toward limiting emissions of greenhouse gases into the atmosphere, and even direct-air capture of carbon dioxide that has already been emitted.

Hatton noted that while the team’s initial focus was on the challenge of separating carbon dioxide from a stream of gases, the system could actually be adapted to a wide variety of chemical separation and purification processes.

“We’re pretty excited about the gating mechanism. I think we can use it in a variety of applications, in different configurations,” Hatton said. “Maybe in microfluidic devices, or maybe we could use it to control the gas composition for a chemical reaction. There are many different possibilities.”

This work looks to be quite an improvement across a wide swath of chemical activities. If it scales up, and it does look like it will, we may look back and think “innovation” was way to light of a descriptive term.


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