Direct air capture is one of the ‘Seven chemical separations to change the world’. Although carbon dioxide is a major contributor to climate change (we release ~40 billion tons into the atmosphere every year), separating it from air is difficult due to its low concentration (~0.04%).
Prof Ian Metcalfe, Royal Academy of Engineering Chair in Emerging Technologies at Newcastle University, UK, and lead investigator states, “Dilute separation processes are the most challenging for two reasons. First, the low concentration means the chemical reactions targeting the removal of the dilute component are very slow. Second, concentrating the dilute component requires a lot of energy.”
These are the two challenges that Newcastle researchers (with colleagues at Victoria University of Wellington, New Zealand, Imperial College London, UK, Oxford University, UK, Strathclyde University, UK, and UCL, UK) aimed to address with their new membrane process. By using natural humidity differences to pump carbon dioxide out of the air, the team overcame the energy challenge. The presence of water also sped up the transport of carbon dioxide through the membrane, addressing the kinetic challenge.
The work is published in Nature Energy and Dr Greg A. Mutch, Royal Academy of Engineering Fellow at Newcastle University, UK explains, “Direct air capture will be a key component of the future energy system. It will be needed to capture emissions from mobile, distributed sources of carbon dioxide that cannot be easily decarbonized in other ways.”
“In our work, we demonstrate the first synthetic membrane capable of capturing carbon dioxide from air and increasing its concentration without traditional energy inputs like heat or pressure. 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
Separation processes are essential in modern life. From the food we eat to the medicines we take and the fuels or batteries in our cars, most products go through several separation processes. These processes also help reduce waste and the need for environmental cleanup, such as capturing carbon dioxide from the air.
In a circular economy, separation processes will become even more important. Direct air capture could provide carbon dioxide to make many hydrocarbon products we use today, but in a carbon-neutral or even carbon-negative way.
Transitioning to renewable energy, traditional carbon capture from sources like power plants, and direct air capture are crucial for meeting climate targets, such as the 1.5 °C goal of the Paris Agreement.
The humidity-driven membrane
Dr Evangelos Papaioannou, Senior Lecturer in the School of Engineering, Newcastle University, UK explains, “The team tested a new carbon dioxide-permeable membrane with different humidity levels across it. When the humidity was higher on the output side, the membrane pumped carbon dioxide into that stream.”
Using X-ray micro-computed tomography with collaborators at UCL and the University of Oxford, the team precisely characterized the membrane’s structure. This allowed them to compare its performance with other advanced membranes.
A key part of the work was modeling the processes in the membrane at the molecular scale. Using density-functional-theory calculations with a collaborator from Victoria University of Wellington and Imperial College London, the team identified ‘carriers’ within the membrane. These carriers transport both carbon dioxide and water but nothing else. Water releases carbon dioxide from the membrane, and carbon dioxide releases water. Because of this, the energy from a humidity difference can move carbon dioxide through the membrane from a low concentration to a higher concentration.
Prof Metcalfe adds, “This was a real team effort over several years. We are very grateful for our collaborators’ contributions and the support from the Royal Academy of Engineering and the Engineering & Physical Sciences Research Council.”
