Driving on sunshine: clean, usable liquid fuels made from solar power

The researchers used artificial photosynthesis to convert CO₂, water and sunlight into ethanol and propanol in a single step. These multi-carbon fuels, which can be used as petrol additives, have a high energy density and can be easily stored or transported.

Unlike fossil fuels, the solar fuels created produce net zero carbon emissions and are completely renewable. And unlike most bioethanol, they do not divert any agricultural land away from food production.

“Biofuels like ethanol are a controversial technology, not least because they take up agricultural land that could be used to grow food instead,” said Professor Erwin Reisner, who led the research. “Our solar fuels do not have this issue.”

Artificial leaves

For several years, the Reisner Lab has been developing sustainable, zero-carbon fuels inspired by photosynthesis – the process by which plants convert sunlight into food – using artificial leaves.

To date, these artificial leaves have only been used to produce simple chemicals, such as syngas, a mixture of hydrogen and carbon monoxide that is used to produce fuels, pharmaceuticals, plastics and fertilisers. But to make the technology more practical, the researchers have been working to produce more complex chemicals directly in a single solar-powered step.

Now, the team has developed a new artificial leaf, which can directly produce clean ethanol and propanol without the need for the intermediary step of producing syngas.

While the technology is still at laboratory scale, the researchers say their artificial leaves are an important step in the transition away from a fossil fuel-based economy. The results are reported in the journal Nature Energy.

A bimetallic catalyst

The researchers developed a copper and palladium-based catalyst. The catalyst was optimised in a way that allowed the artificial leaf to produce more complex chemicals, specifically the multi-carbon alcohols ethanol and n-propanol, which can be easily transported and stored.

Other scientists have been able to produce similar chemicals using electrical power, but this is the first time that such complex chemicals have been produced with an artificial leaf using only the energy from the Sun.

A practical liquid fuel

“Shining sunlight on the artificial leaves and getting liquid fuel from carbon dioxide and water is an amazing bit of chemistry,” said Dr Motiar Rahaman, the paper’s first author. “Normally, when you try to convert CO2 into another chemical product using an artificial leaf device, you almost always get carbon monoxide or syngas, but here, we’ve been able to produce a practical liquid fuel just using the power of the Sun. It’s an exciting advance that opens up whole new avenues in our work.”

Next steps

At present, the device is a proof of concept and shows only modest efficiency. The researchers are working to optimise the leaves so they can better absorb sunlight, and to refine the catalyst so it can convert more sunlight into fuel. Further work will also be required to make the device scalable so that it can produce large volumes of fuel.

“Even though there’s still work to be done, we’ve shown what these artificial leaves are capable of doing,” said Reisner. “It’s important to show that we can go beyond the simplest molecules and make things that are directly useful as we transition away from fossil fuels.”

The research was supported in part by the European Commission Marie Skłodowska-Curie Fellowship, the Cambridge Trust, and the Winton Programme for the Physics of Sustainability. Erwin Reisner is a Fellow and Motiar Rahaman is a Research Associate of St John’s College, Cambridge.

Below: First author Dr Motiar Rahaman (left) with Professor Erwin Reisner, who led the research

Reference

Motiar Rahaman et al., Solar-driven liquid multi-carbon fuel production using a standalone perovskite-BiVO₄ artificial leaf, Nature Energy (2023). DOI: 10.1038/s41560-023-01262-3