Artificial Leaf Brews Liquid Fuel From Carbon Dioxide

The idea of artificial photosynthesis is an enticing one: Devices that could soak up sunlight and carbon dioxide, and then, in the presence of water, produce fuels. The designs have evolved from systems in 2011 that split water to produce hydrogen fuels to more recent, more complex ones that aimed to reduce carbon dioxide by using it to create carbon-based fuels.

But it's been slow going. Researchers have, over the years, succeeded in making systems that produce hydrogen efficiently and have more recently devised a system that can make syngas, which is a mix of carbon monoxide and hydrogen that's used to make other products like methanol. But a device that can directly brew useful liquid fuels has eluded researchers, until now.

Researchers at the University of Cambridge have now made the first artificial leaf that can convert carbon dioxide into the liquid fuels propanol and ethanol. While others have demonstrated the electricity-driven conversion of carbon dioxide to fuels before, the new work reported in the journal Nature Energy is a key advance in using sunlight to directly produce clean, useful fuels from carbon dioxide in one step.

Scientists all over the world are trying to make solar fuels as a way of bottling the sun's energy for use later, and of removing carbon dioxide emissions from the atmosphere. Artificial photosynthesis falls into that broad category and can be achieved using several approaches. One is photocatalysis, in which sunlight shines directly on a light-driven catalyst like titanium dioxide that triggers the chemical reactions to reduce carbon dioxide and split water.

The Cambridge team instead adopted a photoelectrochemical approach, explains Motiar Rahaman, a member of the team and a chemistry researcher at Cambridge. This approach involves a cell with semiconducting photoelectrodes that absorb sunlight and produce electricity, which powers a catalyst-driven chemical reaction. In 2019, Cambridge chemistry professor Erwin Reisner and colleagues made the first such artificial leaf device, which produced syngas, followed in 2022 with a lightweight floating version of such a device.

Each of these devices has a cathode composed of a photovoltaic perovskite and a cobalt catalyst, and an anode made of the photocatalyst bismuth vanadate. When the device is immersed in water, bismuth vanadate absorbs sunlight and triggers the process of splitting the water at the anode. Meanwhile, at the cathode, the perovskite generates electricity, which drives the cobalt catalyst to reduce carbon dioxide and produce syngas.

Rahaman, Reisner, and the team have now upgraded the device with a special catalyst they formulated, which allows the device to produce multicarbon alcohols instead of syngas. Copper is the only known metal that can form multicarbon products from carbon dioxide, Rahaman says, but the process needs a lot of energy. So the researchers doped it with palladium to make a bimetallic copper-palladium catalyst that "does this job at low potential, needing low energy."

An artificial leaf is tested by being subjected to solar irradiation inside of a photoreactor.Motiar Rahaman

The device activates and starts producing the alcohols—with a one-to-one ratio of propanol to ethanol—almost immediately when immersed in water under sunlight. The researchers let the reaction run for 20 hours in the laboratory, and then separated the alcohol from the reactor.

It's in the early stage still, Rahaman says, and the device is tiny—only 5 millimeters to a side. It produces just microliters of alcohol per square-centimeter area. But the team are working on improving the device efficiency by optimizing the light-absorber materials to harvest more sunlight and tweaking the catalyst to convert more carbon dioxide to fuel. They also plan to scale up the device so that it can produce larger volumes of fuel.

"The device is still small because we just invented the technology," he says. "We are now getting alcohols in microliter amounts. But we have invented the science. Now it will be a technical engineering effort to get to bigger scale. If we increase the surface area, then the amount of product will increase."

This story was updated on 5 June 2023.