Sustainable fuels made out of 'thin air' and plastic waste

Researchers from the University of Cambridge have demonstrated how carbon dioxide can be captured from industrial processes — or even directly from the air — and transformed into clean, sustainable fuels using just the energy from the sun. Their results have been reported in the journal Joule.

For several years, Professor Erwin Reisner’s research group has been developing sustainable, net-zero carbon fuels inspired by photosynthesis — the process by which plants convert sunlight into food — using artificial leaves. These artificial leaves convert CO₂ and water into fuels using just the power of the sun.

To date, their solar-driven experiments have used pure, concentrated CO₂ from a cylinder, but for the technology to be of practical use, it needs to be able to actively capture CO₂ from industrial processes, or directly from the air. However, since CO₂ is just one of many types of molecules in the air we breathe, making this technology selective enough to convert highly diluted CO₂ is a huge technical challenge.

“We’re not just interested in decarbonisation, but de-fossilisation — we need to completely eliminate fossil fuels in order to create a truly circular economy,” Reisner said. “In the medium term, this technology could help reduce carbon emissions by capturing them from industry and turning them into something useful, but ultimately, we need to cut fossil fuels out of the equation entirely and capture CO₂ from the air.”

The researchers took their inspiration from carbon capture and storage (CCS), where CO₂ is captured and then pumped and stored underground. Reisner noted, “CCS is a technology that’s popular with the fossil fuel industry as a way to reduce carbon emissions while continuing oil and gas exploration. But if instead of carbon capture and storage, we had carbon capture and utilisation, we could make something useful from CO₂ instead of burying it underground, with unknown long-term consequences, and eliminate the use of fossil fuels.”

The researchers adapted their solar-powered reactor so that it works with flue gas or directly from the air, converting CO₂ and plastic waste into fuel and chemicals using only the power of the sun. By bubbling air through the system containing an alkaline solution, the CO₂ selectively gets trapped, and the other gases present in air, such as nitrogen and oxygen, harmlessly bubble out. This bubbling process allows the researchers to concentrate the CO₂ from air in solution, making it easier to work with.

The integrated system contains a photocathode and an anode. The system has two compartments: on one side is captured CO₂ solution that gets converted into syngas, a simple fuel. On the other, plastics are converted into useful chemicals using only sunlight.

“The plastic component is an important trick to this system,” said co-first author Dr Motiar Rahaman. “Capturing and using CO₂ from the air makes the chemistry more difficult. But if we add plastic waste to the system, the plastic donates electrons to the CO₂. The plastic breaks down to glycolic acid, which is widely used in the cosmetics industry, and the CO₂ is converted into syngas, which is a simple fuel.”

The scientists are now working on a benchtop demonstrator device with improved efficiency and practicality.

This is a modified version of a news item published by the University of Cambridge under CC BY-NC-SA 4.0. This version is similarly licensed under CC BY-NC-SA 4.0.

Image caption: Carbon capture from air and its photoelectrochemical conversion into fuel with simultaneous waste plastic conversion into chemicals. Image credit: Ariffin Mohamad Annuar.