Outdoor test of a floating artificial leaf, which generates clean fuel from sunlight and water, on the River Cam near Cambridge, UK. [Virgil Andrei, PhD]

Artificial leaves that mimic photosynthesis to generate clean fuels from sunlight and water have been developed. The latest iteration is lightweight and floatable, much like a lily pad, with the potential to operate on a large scale in open water.

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 even at sea, and could help reduce the global shipping industry’s reliance on fossil fuels.

Researchers say that although improvements need to be made before the new leaves are ready for commercial applications, their development—reported in Nature—opens new avenues in their work. (See “Floating perovskite-BiVO4 devices for scalable solar fuel production.”)

For several years, the research group run by Erwin Reisner, PhD, a professor at the University of Cambridge, has been working to develop sustainable solutions to petrol that are based on the principles of photosynthesis. A few years ago, they developed an artificial leaf that makes syngas—a key intermediate in the production of many chemicals and pharmaceuticals—from sunlight, carbon dioxide, and water.

However, the device was bulky, with thick glass substrates and moisture-protective coatings in addition to the necessary light absorbers and catalysts.

“We wanted to see how far we can trim down the materials these devices use, while not affecting their performance,” Reisner said. “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 challenge for the Cambridge researchers was how to deposit light absorbers onto lightweight substrates and protect them against water infiltration. To overcome these challenges, 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.

They ended up with a device that not only works, but also looks like a real leaf. “These artificial leaves attain a high activity per gram, comparable to that of plant leaves, by replacing conventional glass substrates and bulk encapsulants with flexible, thin-film alternatives,” the researchers wrote in their paper.

Importantly, the artificial leaves float. “Bubbles formed under operation further enabled [the] devices to float, and lightweight reactors facilitated gas collection during outdoor testing on a river,” they continued. This means that the leaves could be used to generate a sustainable alternative to petrol without using land space.

“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,” Reisner said. “In theory, you could roll up these devices and put them almost anywhere, in almost any country, which would also help with energy security.”

Before production of the devices can be scaled up, further progress needs to be made in the areas of stability, gas collection, and pressurization. But these proof-of-concept devices help envision a future in which sustainable fuel production occurs on a large scale in open water.

“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,” s ummarized Virgil Andrei, PhD, a co-lead author on the paper.

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