Recently, Cambridge was witness to leaves floating on River Cam, near iconic Cambridge sites, including the Bridge of Sighs, the Wren Library, and King’s College Chapel. Pretty normal, you might think. But these leaves were 'artificial,' converting sunlight into fuels as efficiently as plant leaves.

Researchers from the University of Cambridge developed these floating 'artificial leaves,' inspired by photosynthesis to generate clean fuels from sunlight and water. The ultra-thin, flexible, autonomous devices are light enough to float and could be a great, sustainable alternative to petrol without taking up space on land.

This is the first time that clean fuel has been generated on water. If scaled up, the artificial leaves could be used on polluted waterways, in ports, or even at sea and could help reduce the global shipping industry’s reliance on fossil fuels.

The results are reported in the journal Nature.

For several years, Professor Erwin Reisner's research group in Cambridge has been working to develop sustainable solutions to petrol based on the principle of photosynthesis. But existing technology has been either inefficient or so heavy that it had to be confined to land, where space was an issue.

To illustrate, in 2019, researchers developed an artificial leaf that makes syngas from sunlight, carbon dioxide, and water. Though it generated fuel by combining two light absorbers with suitable catalysts, it incorporated thick glass substrates and moisture-protective coatings, resulting in a bulky device.

"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," Virgil Andrei from Cambridge’s Yusuf Hamied Department of Chemistry, the paper’s co-lead author, said in a statement.

"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."

The experts sought inspiration from the electronics industry as the field is known for its miniaturization techniques.

But, the challenge was to deposit light absorbers onto lightweight substrates and protect them against water infiltration. To overcome these, the team used thin-film metal oxides and materials known as perovskites, which can be coated onto flexible plastic and metal foils. The devices were covered with micrometer-thin, water-repellent carbon-based layers that prevented moisture degradation.

"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," said Andrei. "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."

"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."

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability1, whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H2 g−1 h−1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attained a high CO: H2 selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm2 stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm2 counterparts. Bubbles formed under operation further enabled 30–100 mg cm−2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use.