More than 35 billion tonnes of carbon dioxide (CO₂), the main contributor to climate change, are released into the atmosphere each year. Scientists investigating ways to “capture” some of this gas have landed on a new method that they believe could see CO₂ extracted from the air and potentially reused in carbon-neutral manufacturing processes.

Professor Patricia Hunt, from the School of Chemical and Physical Sciences and an associate investigator at the MacDiarmid Institute, says extracting CO₂ from the air is a “challenging” puzzle to solve.

“Some CO₂ can be captured at source—for example, from the smokestacks of factories and power plants that burn fossil fuels. However, once CO₂ is in the atmosphere it’s significantly harder to extract.

“The first problem is that air is mostly nitrogen and oxygen, and we need to select and then capture CO₂, which is only present in tiny amounts—about 0.04 percent. The next step is concentrating the CO₂, which requires significant energy,” she says.

Professor Hunt and researchers from the UK hit upon an approach that doesn’t require huge amounts of energy.

They developed and tested a CO₂-permeable synthetic membrane, a bit like a high-tech filter. The membrane system was designed to “hijack” the energy generated by differences in humidity between dry air on one side of the membrane and wet (humid) air—made wet by the introduction of water—on the other side, she explains.

In the lab, the researchers were able to exploit this energy to pump CO₂ out of the air, avoiding the need for an external energy source.

“If we think of a hydropower station, we know that water flowing downhill produces energy. In the membrane system we developed, water flowing downwards was used to power the capture of CO₂, allowing it to be concentrated so it could be reused or stored.

“For each water molecule going downhill inside the membrane, one CO₂ molecule was stored. This gave us a hint that the two processes were connected. Using computer modelling, we were then able to look at things on a molecular level to pinpoint what was happening.”

Co-researcher Dr Greg Mutch, from the UK’s Newcastle University, compared the process to a waterwheel on a flour mill: “Whereas a mill uses the downhill transport of water to drive milling, we use it to pump carbon dioxide out of the air.”

Luckily, the researchers didn’t have to construct a flour mill. Their experiments were done in a membrane reactor—a device used to separate different chemicals. Outside the lab, membrane reactors are a common way of treating wastewater by filtering out contaminants.

Professor Hunt says the membrane they used looks a bit like a pencil-sized water filter cartridge—one where the CO₂ needs a water molecule and an energy “push” to help it pass through.

“Our research has shown that a membrane system can remove CO₂ from the air, without the need for a large external energy source. This technology could play a part in the huge task of tackling carbon emissions, but more work will be needed to develop and test its application beyond the lab,” she said.

Results of the research, led by Professor Ian Metcalfe from Newcastle University, are published in Nature Energy.

Researchers from the University of Strathclyde, University College London, University of Oxford, and Imperial College London were also involved in the work.