A new system developed by MIT chemical engineers may provide a way to continuously remove carbon dioxide from waste gas streams, or even from air. An important component is the electrochemically assisted membrane, which has no moving parts and relatively low energy, allowing the gas permeability to be turned on and off at will.
The membrane itself, made of anodized aluminum, has a honeycomb-like structure consisting of hexagonal openings that allow gas molecules to enter and exit in the open state. However, if a thin layer of metal is electrically deposited over the pores of the membrane, it can impede the passage of gas.The work is listed in the journal today Science Advances, T. Papers by Professor Alan Hutton, postdoctoral fellow Yayuan Liu, and four others.
The team says this new “gas-gating” mechanism can be applied to continuously remove carbon dioxide from various industrial exhaust streams and ambient air. They built a proof-of-concept device to show that this process is actually working.
The device uses a redox activated carbon absorbent material sandwiched between two switchable gas gate membranes. The adsorbent and the gate film are in close contact with each other and provide a medium in which zinc ions reciprocate back and forth when immersed in an organic electrolyte. These two gate films can be electrically opened and closed by switching the polarity of the voltage between them, shuttleting zinc ions from one side to the other. The ions simultaneously block one side, form a metal film on it, and open the other side to dissolve the film.
If the adsorbent layer is open on the side of the waste gas flow, the material will easily absorb carbon dioxide until it reaches its capacity. Then switch the voltage to shut off the supply side and open the other side, releasing a concentrated stream of nearly pure carbon dioxide.
By building a system with alternating sections of membranes that operate in opposite phase, the system allows continuous operation in settings such as industrial scrubbers. At any given time, half of the section absorbs gas and the other half releases gas.
“That is, one end of the feed stream goes into the system, the product stream comes out of the other, and on the surface it’s a continuous operation,” says Hatton. “This approach avoids many process problems with traditional multi-column systems that require the adsorption beds to be alternately shut down, purged, and regenerated before being exposed to the supply gas again to initiate the next adsorption cycle. To do”. In the new system, no purging steps are required and all steps are done cleanly within the unit itself.
An important innovation for researchers was the use of electroplating as a way to open and close the pores of a material. The team has tried various other approaches to reversibly close the pores of the membrane material, such as using small magnetic spheres that can be placed to close the funnel-shaped opening, but these other methods It has not proved to be efficient enough. Metal thin films are particularly effective as gas barriers, and the ultra-thin layers used in new systems require abundant, inexpensive and minimal zinc material.
“We create a very uniform coating layer with a minimum of material,” says Liu. One of the key advantages of electroplating is that it does not require an energy input to maintain its condition when conditions change, whether in the open or closed position. Energy is only needed to switch again.
Potentially, such a system could make an important contribution to limiting the emission of greenhouse gases into the atmosphere and even capturing the already emitted carbon dioxide directly into the air.
Although the team’s initial focus was on separating carbon dioxide from the gas stream, Hatton says the system was actually able to adapt to a variety of chemical separation and purification processes.
“We’re pretty excited about the gating mechanism, and I think it can be used in different applications and in different configurations,” he says. “Maybe in a microfluidic device, or you can use it to control the gas composition for a chemical reaction. There are various possibilities.”
The research team included graduate students Chun-Man Chow, postdoctoral fellow Katherine Phillips, and recent graduates Miao Wang PhD’20 and Sahag Voskian PhD’19. This work was supported by ExxonMobil through the MIT Energy Initiative.
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