Photosynthesis, nature’s way of converting sunlight to fuel, happens all around us, from leaves on a tree to the smallest blade of grass. But finding a way to mimic the ability cheaply and efficiently has confounded engineers for decades.
Now researchers have taken a step toward this elusive feat, with a device that is even more efficient than natural photosynthesis and relies on low-cost, abundant materials.
Conventional solar cells produce electricity when a photovoltaic material is exposed to light. The new device goes a step further, using the resulting electricity to split water into hydrogen and oxygen, which can be stored and used to generate electricity via a fuel cell.
The new device is still in early laboratory development, and significant challenges remain before it can be commercialized.
Daniel Nocera, a professor at MIT, revealed preliminary details of the device, which he calls the first practical “artificial leaf,” at the national meeting of the American Chemical Society in California on March 27. The device combines a commercially available solar cell with a pair of inexpensive catalysts made of cobalt and nickel that split water into oxygen and hydrogen. Using this approach, a solar panel roughly one square meter bathed in water could produce enough hydrogen to supply a house in a developing country with electricity for both day and night, Nocera says.
Using a thin-film silicon solar cell that converts the energy in light with 7 percent efficiency, Nocera says his group achieved 5 percent efficiency for the conversion of sunlight to hydrogen. Natural photosynthesis is less than 1 percent efficient at converting sunlight to energy.
The device is not the first to attempt to improve upon natural photosynthesis. It does, however, offer distinct advantages over previous devices, which either used costly precious-metal catalysts to split water into hydrogen and oxygen, or performed the water splitting indirectly with a separate device, which is less efficient and more expensive.
Nocera’s device is the first to use inexpensive and abundant catalyst materials that are incorporated into the solar cell. “You just have a piece of silicon coated with catalysts that you can put in a glass of water, and it starts splitting the water into hydrogen and oxygen,” he says.
The device is made possible by several recent advances. Nocera first developed a cobalt catalyst capable of splitting oxygen from water in 2008, but the catalyst couldn’t be applied directly to silicon because it would block incoming sunlight. For his new device, Nocera applied a thin film of cobalt to the silicon that blocks only 2 to 3 percent of incoming light. Prior to applying the catalyst, he coated the silicon with a thin membrane that protects it from oxidization but allows electrical current to pass through.