|Leaves that are not green: Silicon coated with inexpensive catalysts splits water into hydrogen and oxygen inside an illuminated container of water. |
Credit: Daniel Nocera, MIT
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.
A novel nickel-based catalyst also developed recently by Nocera is applied to the other side of the silicon to split hydrogen from water. The nickel catalysts already used in other water splitting devices known as electrolyzers would quickly be rendered useless by phosphate and borate present in the water. The results of initial tests of the device have been submitted for publication. They show that it can operate for at least six days without a drop in efficiency, Nocera says.
John Turner, a research fellow at the National Renewable Energy Laboratory in Golden, Colorado, says the ability to use a virtually transparent cobalt catalyst is a key advance, and the reported efficiency is promising. "He is getting most of the efficiency out of the cell," Turner says. "If he [starts with] an 11 or 12 percent cell, which is commercially available, he should be able to do much better. But we would need to see what he can do once he gets a better cell."
However, Turner says, Nocera will have to demonstrate significantly longer run times—tens of thousands of hours. "They may have the durability, but they need to continue to show it," he says.
Sun Catalytix, a company founded by Nocera, will now work with Indian industrial giant Tata to commercialize the technology for residential use in developing countries. The companies are already working together to develop another artificial photosynthesis device developed previously by Nocera. This initial device will be based on a 100-watt solar panel and will require a separate electrolyzer connected by wires to the panel. It should sell for around $100.
The device would also have to be paired with a fuel cell to convert stored hydrogen to electricity. Nocera says he hopes to deliver this initial solar-powered hydrogen-producing device to Tata by the end of 2011. The new artificial leaf should be less expensive, but it could be another two and a half years before a commercial prototype would be ready, Nocera says.
James Stevens, a research fellow at Dow Chemical, says the technology still has a long way to go. "There is a lot that has to be done before this could be practical," he says. "The efficiency is low and the capital costs of these things are very high."
Other practical issues, such as safely storing hydrogen gas and preventing the system from freezing in subzero temperatures, are also significant challenges, Stevens says. "We're not really interested in the state of the art as it now stands," he says.