'Artificial leaf' makes fuel from sunlight

Researchers led by MIT professor Daniel Nocera have produced something they're calling an "artificial leaf": Like living leaves, the device can turn the energy of sunlight directly into a chemical fuel that can be stored and used later as an energy source.

The artificial leaf - a silicon solar cell with different catalytic materials bonded onto its two sides - needs no external wires or control circuits to operate. Simply placed in a container of water and exposed to sunlight, it quickly begins to generate streams of bubbles: oxygen bubbles from one side and hydrogen bubbles from the other. If placed in a container that has a barrier to separate the two sides, the two streams of bubbles can be collected and stored, and used later to deliver power: for example, by feeding them into a fuel cell that combines them once again into water while delivering an electric current.

The creation of the device is described in a paper published today in the journal `Science'. Nocera, the Henry Dreyfus Professor of Energy and professor of chemistry at MIT, is the senior author. The paper was co-authored by his former student Steven Reece PhD '07 (who now works at Sun Catalytix, a company started by Nocera to commercialise his solar-energy inventions), along with five other researchers from Sun Catalytix and MIT.

The device, Nocera explains, is made entirely of earth-abundant, inexpensive materials - mostly silicon, cobalt and nickel - and works in ordinary water. Other attempts to produce devices that could use sunlight to split water have relied on corrosive solutions or on relatively rare and expensive materials such as platinum.

The artificial leaf is a thin sheet of semiconducting silicon - the material most solar cells are made of - which turns the energy of sunlight into a flow of wireless electricity within the sheet. Bound onto the silicon is a layer of a cobalt-based catalyst, which releases oxygen, a material whose potential for generating fuel from sunlight was discovered by Nocera and his co-authors in 2008. The other side of the silicon sheet is coated with a layer of a nickel-molybdenum-zinc alloy, which releases hydrogen from the water molecules.

"I think there's going to be real opportunities for this idea," Nocera says. "You can't get more portable - you don't need wires, it's lightweight," and it doesn't require much in the way of additional equipment, other than a way of catching and storing the gases that bubble off. "You just drop it in a glass of water, and it starts splitting it," he says.
Now that the "leaf" has been demonstrated, Nocera suggests one possible further development: tiny particles made of these materials that can split water molecules when placed in sunlight - making them more like photosynthetic algae than leaves. The advantage of that, he says, is that the small particles would have much more surface area exposed to sunlight and the water, allowing them to harness the sun's energy more efficiently. (On the other hand, engineering a system to separate and collect the two gases would be more complicated in such a setup.)