The big problem with capturing carbon dioxide from air is how little of the greenhouse gas is present in the atmosphere. Its low concentration—only about 0.04% of air—slows down chemical reactions needed for removal. It also requires direct air capture methods to first spend a lot of energy concentrating the carbon dioxide.

A team of researchers in the UK have come up with a solution to these two issues. In a paper published in the journal Nature Energy, they report a novel membrane that, driven by a difference in humidity, can grab carbon dioxide out of air. The researchers say it’s the first synthetic membrane that can capture carbon dioxide from air and also increase the gas’ concentration without requiring energy for operation.

The team took inspiration from nature. Biological membranes transport chemicals passively, relying on a flow from a higher concentration to a lower one. But bio-membranes can also harness the energy released during “downhill” transport of one chemical species to pump another species “uphill” to higher concentration.

With this mechanism in mind, the UK researchers designed and made a special molten-carbonate membrane. They used a laser to drill hundreds of small, conical pores in an alumina tube and fill the material with a molten carbonate salt.

Then, they applied a humidity difference across this membrane so that it has a dry air input stream and a wet air output stream. The membrane uses this humidity difference to spontaneously pump carbon dioxide uphill from the dry input stream to the higher humidity side of the membrane.

“The surprising thing about this membrane is how rapid the permeation process is,” says Ian Metcalfe, a professor of chemical engineering at Newcastle University. “This seems to be due to the ability of water to release carbon dioxide from the strongly-bound carrier in the membrane.”

The membrane materials are very cheap, Metcalfe says, just alumina and carbonates. But there is a catch: to keep the carbonate salts molten, the membrane operates at high temperatures over 400°C. “This means you would also need a large heat exchanger and some heat input to process the air,” he says. “What we would like to do now is drop the temperature of operation, and we have ideas about how we could do this.”

Source: Ian S. Metcalfe. Separation and concentration of CO₂ from air using a humidity-driven molten-carbonate membrane. Nature Energy, 2024.

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