Powered only by sunlight, a new device converts carbon dioxide and water to liquid fuels that could be directly used in car engines.
The device, presented in the journal Nature Energy, is the latest evolution of an artificial leaf. Researchers have been working for years on such systems that mimic photosynthesis to sustainably produce fuels at low cost. Previous artificial leaves produced simple products like carbon monoxide and hydrogen. But the new one takes an important step forward by making energy-dense ethanol and propanol fuels.
“This is the very first standalone artificial leaf that can convert carbon dioxide and water directly into liquid multi-carbon fuel using sunlight as the only energy source,” says Motiar Rahaman, a chemistry researcher at the University of Cambridge, and co-author on the new paper.
Plants use sunlight to make sugars from carbon dioxide and water. To emulate that process, researchers have made artificial leaves based on light-absorbing solar cells and chemical catalysts. The Cambridge group have made several artificial leaves before. One such device, made with a class of materials called perovskites, which are excellent at converting sunlight to electrical energy, could float on water and produce a mix of carbon monoxide and hydrogen. Another, made using a low-cost and water-resistant light absorber called bismuth oxyiodide produced hydrogen fuel for weeks.
The team has now made the technology more practical by making complex liquid fuels in a single step.
Like their previous devices, this one has two light-absorbing electrodes. One is composed of a perovskite while the other is made of bismuth vanadate, a promising photocatalyst. Photocatalysts are materials that absorbs sunlight and produces energy to power chemical reactions. Here, the bismuth vanadate triggers the solar-driven splitting of water into hydrogen and oxygen.
The key advance in the new work is the team’s development of a new catalyst made of copper and palladium. The researchers treat it with a special three-step activation process, which allows it to turn carbon dioxide and water into liquid fuels. “Development and optimization of catalysts and light-absorbers and their proper assembly made this possible,” Rahaman says.
For now, the small proof-of-concept device has low efficiency and does not produce large amounts of fuel. The team is now optimizing the light absorbers and the catalyst so it can convert more sunlight into fuel.
The next step will be scaling up the device so it can produce large volumes of fuel, he says. “It is definitely possible to develop it in a large scale and we have already started working on that. However, this technology is still in its initial stage, and it will take quite some time to scale up.”
If successful, ethanol produced using this technology would be a more sustainable alternative for transport than the bioethanol mixed into gasoline today, the researchers said. Biofuels, made today by fermenting sugars, takes up land that could be used for agriculture, and researchers have been searching for better options. Going from real plants to artificial leaves might just be the answer.
Source: Rahaman, M., Andrei, V., Wright, D. et al. Solar-driven liquid multi-carbon fuel production using a standalone perovskite–BiVO4 artificial leaf. Nat Energy, 2023.
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