Turning sunlight into syngas


British researchers report a new benchmark in the field of solar fuels.


Nature could eventually provide a way to develop a sustainable liquid fuel alternative to petrol, researchers suggest.

VIRGIL ANDREI

This “artificial leaf” uses water, sunlight and carbon dioxide rather than fossil fuels to produce the widely used gas known as syngas.

It was inspired, say researchers from the University of Cambridge, UK, by the natural process by which plants use the energy from sunlight to turn carbon dioxide into food – and as such sets a new benchmark in the field of solar fuels.

It even works efficiently on cloudy and overcast days, says Erwin Reisner from Cambridge’s Department of Chemistry, and unlike industrial processes does not release additional carbon dioxide into the atmosphere.

Syngas is currently made from a mixture of hydrogen and carbon monoxide, and is used to produce a range of commodities, such as fuels, pharmaceuticals, plastics and fertilisers.

"Being able to produce it sustainably would be a critical step in closing the global carbon cycle and establishing a sustainable chemical and fuel industry," says Reisner, senior author of a paper in the journal Nature Materials.

Reisner and colleagues have taken many years to finetune what appears a novel and rather elegant approach.

On the artificial leaf, two light absorbers, similar to the molecules in plants that harvest sunlight, are combined with a catalyst made from the naturally abundant element cobalt.

When the device is immersed in water, one light absorber uses the catalyst to produce oxygen, while the other carries out the chemical reaction that reduces carbon dioxide and water into carbon monoxide and hydrogen, forming the syngas mixture.

As an added bonus, the light absorbers work even under the low levels of sunlight on a rainy or overcast day.

"This means you are not limited to using this technology just in warm countries, or only operating the process during the summer months," says first author Virgil Andrei. "You could use it from dawn until dusk, anywhere in the world."

Other “artificial leaf” devices have been developed, the researchers say, but these usually only produce hydrogen. Theirs does more thanks to the combination of materials and catalysts used.

The light absorbers are made from perovskite rather than silicon or dye-sensitised materials, which provides a high photovoltage and electrical current to power the chemical reaction by which carbon dioxide is reduced to carbon monoxide, and the molecular catalyst is made from cobalt rather than platinum or silver. Cobalt is not only cheaper, the researchers say, but better at producing carbon monoxide.

They are now looking at ways to use the technology to produce a sustainable liquid fuel alternative to petrol.

"What we'd like to do next, instead of first making syngas and then converting it into liquid fuel, is to make the liquid fuel in one step from carbon dioxide and water," says Reisner.

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  1. https://dx.doi.org/10.1038/s41563-019-0501-6
  2. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/perovskites
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