Developing a New Tool for Direct Air Capture

Researchers at Newcastle University have created a novel ambient-energy-driven membrane that extracts carbon dioxide from the atmosphere. The study was published in Nature Energy.

Direct Air Capture

Among the "Seven Chemical Separations to Change the World" was direct air capture. Although carbon dioxide is the primary cause of climate change (approximately 40 billion tons of it are released into the atmosphere annually), it is extremely difficult to separate it from the air because of its low concentration (about 0.04 %).

Dilute separation processes are the most challenging separations to perform for two key reasons. First, due to the low concentration, the kinetics (speed) of chemical reactions targeting the removal of the dilute component are very slow. Second, concentrating the dilute component requires a lot of energy.

Ian Metcalfe, Study Lead Investigator, Professor and Royal Academy of Engineering Chair, Emerging Technologies, School of Engineering, Newcastle University

The Newcastle researchers set out to address these two issues with their new membrane process, working with colleagues at Victoria University of Wellington, New Zealand; Imperial College London, UK; Oxford University, UK; Strathclyde University, UK; and UCL, UK. The team overcame the energy challenge by pumping carbon dioxide out of the air using naturally occurring humidity differences as a driving force. Water also solved the kinetic problem by hastening the passage of carbon dioxide through the membrane.

Direct air capture will be a key component of the energy system of the future. It will be needed to capture the emissions from mobile, distributed sources of carbon dioxide that cannot easily be decarbonized in other ways.

Dr. Greg A. Mutch, Royal Academy of Engineering Fellow, School of Engineering, Newcastle University

Mutch said, “In our work, we demonstrate the first synthetic membrane capable of capturing carbon dioxide from air and increasing its concentration without a traditional energy input like heat or pressure. I think a helpful analogy might be a water wheel 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.”

Separation Processes

Most aspects of modern life are based on processes of separation. Most goods, including food, medications, car fuels, and batteries, have undergone multiple separation processes. Separation procedures are also necessary to reduce waste and the need for environmental remediation, such as carbon dioxide capture by direct air.

As the world gets closer to a circular economy, separation procedures will be even more important. In this case, direct air capture could produce carbon dioxide in a carbon-neutral or even carbon-negative cycle, serving as a feedstock for many daily hydrocarbon products.

The Paris Agreement's 1.5 °C target and other climate targets can only be achieved by utilizing direct air capture in conjunction with the traditional carbon capture from point sources such as power plants and the shift to renewable energy.

The Humidity-Driven Membrane

In a departure from typical membrane operation, and as described in the research paper, the team tested a new carbon dioxide-permeable membrane with a variety of humidity differences applied across it. When the humidity was higher on the output side of the membrane, the membrane spontaneously pumped carbon dioxide into that output stream.

Dr. Evangelos Papaioannou, Senior Lecturer, School of Engineering, Newcastle University

Together with partners at the University of Oxford and UCL, the team used X-ray micro-computed tomography to characterize the membrane's structure precisely. As a result, they could present reliable performance comparisons with other cutting-edge membranes.

Modeling the molecular processes in the membrane was a crucial component of the work. Together with a collaborator from Victoria University of Wellington and Imperial College London, the team used density-functional-theory calculations to identify "carriers" within the membrane. The carrier uniquely transports both carbon dioxide and water but nothing else. Carbon dioxide must be released from the membrane by water, and water must be released by carbon dioxide. This means that carbon dioxide can be driven through the membrane from a low concentration to a higher concentration using the energy from a difference in humidity.

Prof. Metcalfe added, “This was a real team effort over several years. We are very grateful for the contributions from our collaborators, and for the support from the Royal Academy of Engineering and the Engineering & Physical Sciences Research Council.”

Journal Reference:

Metcalfe, S. I., et al. (2024) Separation and concentration of CO2 from air using a humidity-driven molten-carbonate membrane. Nature Energy. doi.org/10.1038/s41560-024-01588-6