Fuel production takes to water with ‘artificial leaves’

This is the claim of researchers at Cambridge University who have designed ultra-thin, flexible devices that split water into hydrogen and oxygen, or reduce CO2 to syngas, which is used to produce commodities including fuels, pharmaceuticals, plastics and fertilisers. Since the low-cost, autonomous devices are light enough to float, they could be used to generate a sustainable alternative to petrol without taking up space on land, the researchers said.

Outdoor tests of the lightweight leaves on the River Cam showed that they can convert sunlight into fuels as efficiently as plant leaves, which us photosynthesis to convert sunlight into chemical energy. This is the first time that clean fuel has been generated on water, and if scaled up, the artificial leaves could be used on polluted waterways, in ports or at sea. The results are reported in Nature.

Professor Erwin Reisner’s research group in Cambridge has been working for several years on developing sustainable solutions to petrol which are based on the principles of photosynthesis. In 2019, they developed an artificial leaf, which makes syngas from sunlight, carbon dioxide and water. The earlier prototype generated fuel by combining two light absorbers with suitable catalysts, but its thick glass substrates and moisture protective coatings made the device bulky.

“Artificial leaves could substantially lower the cost of sustainable fuel production, but since they’re both heavy and fragile, they’re difficult to produce at scale and transport,” said Dr Virgil Andrei from Cambridge’s Yusuf Hamied Department of Chemistry, the paper’s co-lead author.

“We wanted to see how far we can trim down the materials these devices use, while not affecting their performance,” said Reisner, who led the research. “If we can trim the materials down far enough that they’re light enough to float, then it opens up whole new ways that these artificial leaves could be used.”

For the new version of the artificial leaf, the researchers took their inspiration from miniaturisation in the electronics industry. The Cambridge researchers needed to find a way to deposit light absorbers onto lightweight substrates and protect them against water ingress. To overcome these challenges, the team used thin-film metal oxides and perovskites, which can be coated onto flexible plastic and metal foils. The devices were covered with micrometre thin, water-repellent carbon-based layers that prevented moisture degradation and ended up with a device that works and looks like a real leaf.

“This study demonstrates that artificial leaves are compatible with modern fabrication techniques, representing an early step towards the automation and up-scaling of solar fuel production,” Andrei said in a statement. “These leaves combine the advantages of most solar fuel technologies, as they achieve the low weight of powder suspensions and the high performance of wired systems.”

Tests of the new artificial leaves showed that they can split water into hydrogen and oxygen, or reduce CO2 to syngas. While additional improvements will need to be made before they are ready for commercial applications, the researchers said this development opens whole new avenues in their work. “Solar farms have become popular for electricity production; we envision similar farms for fuel synthesis,” said Andrei. “These could supply coastal settlements, remote islands, cover industrial ponds, or avoid water evaporation from irrigation canals.”

“Many renewable energy technologies, including solar fuel technologies, can take up large amounts of space on land, so moving production to open water would mean that clean energy and land use aren’t competing with one another,” said Reisner. “In theory, you could roll up these devices and put them almost anywhere, in almost any country, which would also help with energy security.”

The research was supported in part by the European Research Council, the Cambridge Trust, the Winton Programme for the Physics of Sustainability, the RAEng, and EPSRC.

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