Klaus Lackner in his lab at Arizona State University. Photo: Ira A. Fulton Schools of Engineering.
Klaus Lackner in his lab at Arizona State University. Photo: Ira A. Fulton Schools of Engineering.

A key component of ambient direct air capture (DAC) systems that remove carbon dioxide from the air is the sorbent material used to capture the carbon and then to release it. Certain sorbent materials can pull carbon dioxide from the air as it flows over the material, and then release the carbon when water is applied. As the material dries, it starts to absorb carbon dioxide again.

This elegant function of specific materials has been observed for several years by those working on DAC systems, like Klaus Lackner, a professor in the School of Sustainable Engineering and the Built Environment at Arizona State University. Lackner has developed a system called MechanicalTree that uses sorbent materials to remove carbon dioxide from air.

Unlike other carbon-capture technologies, the MechanicalTree can remove carbon dioxide from the atmosphere without needing to draw air through the system mechanically using energy intensive devices. Instead, the technology uses the wind to blow air through the system.

Each ‘Tree’ contains a stack of sorbent-filled disks. When the tree-like column is fully extended and the disks spread apart, air flow makes contact with the disk surfaces and the carbon dioxide gets bound up. For regeneration, the disks are lowered inside the bottom container, where the carbon dioxide is released from the sorbent. This released gas is then collected, purified, processed and put to other uses, such as in synthetic fuels, enhanced oil recovery or the food, beverage and agriculture industries, while the disks are redeployed to capture more carbon dioxide.

Now, in a paper in Joule, Lackner and his colleagues lay out exactly how some of these sorbent materials capture and release carbon, a finding that could lead to the smarter design of the sorbent materials at the heart of all carbon-removal systems.

"We developed a better understanding of the moisture swing mechanism of these sorbents by demonstrating it in various materials and by developing computational tools and models that explain the concept," Lackner said. "We now understand the effect that drives the moisture swing, and this insight increases the range of materials that can do that."

The paper describes in detail and at microscopic scale what is happening with the sorbent material when it is dry and binding carbon from the air, and when it is wet and desorbing the carbon. Lange and his colleague examined the system with quantum mechanics simulations and verified their findings with experiments.

"This concept is not surprising to me because I've been playing with this stuff for a decade, but the moisture swing concept is still very novel and very different from other ways of loading and unloading a sorbent," Lackner explained. "We discovered this phenomenon 14 years ago, and for a long time it was a mystery on how it worked. Now it seems pretty obvious.

"This advance opens the door for more candidate materials and rational design. Many of those materials are far cheaper than what is often used as sorbents."

This story is adapted from material from Arizona State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.