Artificial leaf paves way to sustainable fuel

12 November 2019

POLLUTION FROM INDUSTRY

Polluted sky, silhouettes of trees and electricity pylons

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Sunlight can be harnessed with a new catalyst to turn CO2 back into a carbon neutral fuel, scientists revealed this month.

Though artificial leaves have been built before, this new contraption makes the technology greener and produces crucial components for fuel.

As the decade comes to a close, climate change is looming like a dark cloud over public consciousness. An excess of carbon dioxide in our atmosphere, produced over years of combustion, is catching up with us, and scientists are working to find new ways to slow this pollution.

Syngas, or synthetic natural gas. This oxymoronic portmanteau may seem alien, but it forms the basis of several products we all use on a daily basis, from plastics to pharmaceuticals. In chemical terms, syngas is a mixture of carbon monoxide (CO) and hydrogen (H₂) which can be used to make industrial products or be converted into fuel.

Industrial production of syngas involves reforming methane from farmed natural gas, but this produces too much carbon dioxide for long-term use. The greener alternative, inspired by photosynthesis, harvests energy from sunlight and uses it to react carbon dioxide and water to produce the syngas mix.

Photoelectrochemical synthesis in an artificial leaf was first shown in 2011, so this technique isn’t new. But, the precious metal catalysts used weren’t ideal: the cost was high and the efficiency was low. In this paper, published in Nature Materials, a previously unexplored cobalt catalyst was tested.

Cobalt is cheaper and enables production of the CO component of syngas. This is in contrast to the majority of previous leaf models, which produced only hydrogen. It uses two light absorbers immersed in water; the first makes oxygen, while the second reduces the carbon dioxide and water into syngas.

The researchers found that, with this set-up, the chemical process of reducing carbon dioxide can occur at just 0.1 sun - around the same level of daylight as occurs on a rainy day. According to PhD student Virgil Andrei, first author of the paper, “you could use it from dawn until dusk, anywhere in the world.”

How much can be produced at lower light, and whether energy could be stored remains to be seen, but this does mark an improvement on previous technology.

The end goal is “closing the global carbon cycle,” according to senior author Erwin Reisner, meaning liquid fuel could be made from the carbon dioxide produced by burning fuel. Reusing carbon dioxide helps avoid the pitfalls of fossil gasoline, which net contributes CO2 into the atmosphere, promoting global warming.

Sustainable syngas production represents a significant step towards this goal, improving photoelectrochemical technology to take production of green fuels like ethanol to the next level. While renewable energy has taken great strides in recent years, as Reisner points out, electricity can currently meet only a quarter of our current global energy demand, so his research offers one solution to help fill the gap.

Mitigating damage to our planet is crucial for our future and the success of this experiment is promising. In switching out inefficient components, scientists get closer to making this technology streamlined and one day introduced on a larger scale.

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