Solar-powered device turns captured carbon dioxide and plastic waste back into useful products

Researchers have made a solar-powered device which can capture carbon dioxide from the air and combine it with plastic waste to make into syngas.

“This solar-powered system takes two harmful waste products – plastic and carbon emissions – and converts them into something truly useful,” says Dr Sayan Kar, a researcher at the University of Cambridge, UK, and co-first author on a paper describing the device, published in Joule.

The researchers had previously shown that they could turn pure CO₂ and water into syngas: a mixture of carbon monoxide and hydrogen, or CO and H₂, can be used as a fuel or to make feedstocks like ammonia and methanol.

But the CO₂ that comes from industrial exhaust is less concentrated, and CO₂ in the the air is less concentrated again, than the lab-grade sources they’d been using.

The researchers have now shown they can selectively remove carbon dioxide from the air or flue gas.

The device starts with a solution of amines and hydroxides. When air is bubbled through the solution, these chemicals bind specifically to CO₂ and let the other components of air leave.

Then, the concentrated CO₂ is pumped into a “reduction-oxidation” reaction: a system where one side is reduced and the other is oxidised simultaneously.

The carbon dioxide is reduced at a “photocathode” made from crystals called perovskites.

The photocathode uses photons from sunlight and a cobalt-based catalyst to turn the CO₂ into syngas.

Meanwhile, at an anode on the other side of the reaction, a plastic-derived substance called ethylene glycol is oxidised into glycolic acid, which is used in cosmetics, dyeing and tanning. The anode uses a copper and palladium-containing catalyst to do this.

“The plastic component is an important trick to this system,” said co-first author Dr Motiar Rahaman, also a chemist at the University of Cambridge.

“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.”

Syngas can be turned back into carbon-based fuel, or used as a fuel itself, which would return CO₂ to the atmosphere when burned – making it a neutral, rather than carbon-negative, fuel.

It can also be used to make methanol and other useful industrial chemicals.

“Instead of storing CO₂ underground, like in CCS, we can capture it from the air and make clean fuel from it,” says Rahaman.

“This way, we can cut out the fossil fuel industry from the process of fuel production, which can hopefully help us avoid climate destruction.”

The researchers are now working on improving the efficiency and practicality of their device.

“The fact that we can effectively take CO₂ from air and make something useful from it is special,” says Kar.

“It’s satisfying to see that we can actually do it using only sunlight.”