ANAHEIM, CALIFORNIA—Nearly all the energy we use on this planet starts out as sunlight that plants use to knit chemical bonds. Now, for the first time, researchers at the Massachusetts Institute of Technology (MIT) in Cambridge have created a potentially cheap, practical artificial leaf that does much the same thing—providing a potentially limitless source of energy that’s easy to tap.
The new device is a silicon wafer about the shape and size of a playing card coated on either side with two different catalysts. The silicon absorbs sunlight and passes that energy to the catalysts to split water into molecules of hydrogen and oxygen. Hydrogen is a fuel that can be either burned or used in a fuel cell to create electricity, reforming water in either case. This means that in theory, anyone with access to water can use it to create a cheap, clean, and available source of fuel.
"It's spectacular," says Robert Grubbs, a chemist at the California Institute of Technology in Pasadena, who saw the presentation here yesterday at the biannual meeting of the American Chemical Society. "There's still obviously a long way to go" to make the new device into a rugged, real-world technology, Grubbs says. But the approach is important because its potential low cost could make it widely available. It “has a chance of being scalable," Grubbs says.
The new device isn't the first semiconductor capable of splitting water. Over a decade ago, a team led by John Turner of the National Renewable Energy Laboratory in Golden, Colorado, created a gallium arsenide chip capable of splitting water, ultimately storing 12% of the energy in sunlight in hydrogen. But gallium arsenide is expensive, and the device quickly corroded in water, making it unusable.
Splitting water into its components of hydrogen and oxygen requires orchestrating two chemical reactions at the same time. Electrons must be stripped from hydrogen atoms in water, which causes water molecules to fracture into positively charged hydrogen ions, or protons, and negatively charged oxygen atoms. One catalyst then must knit together two oxygen atoms to form O2, while a second catalyst welds two hydrogen atoms with two electrons to make H2.
Three years ago, a team led by chemist Daniel Nocera of MIT solved half the problem with a special cobalt and phosphorus-based catalyst that knit O2 molecules. This catalyst was unique in that it dissolves and reforms as part of its catalytic cycle. That behavior turned out to be a huge advantage, Nocera says. Although the catalyst corrodes during use, each time it starts over it’s working with a pristine, noncorroded surface to continue the reaction.
Stitching together hydrogen atoms to create H2 had its own roadblocks. Platinum works well, but it’s rare and expensive. Yesterday, Nocera reported devising a cheap catalyst that uses three different metals to form H2, getting around the platinum problem.
Nocera didn't reveal the makeup of the new catalyst, as the work is not yet published, and he is in the process of patenting it.
But Nocera did say that the three metals each have different roles. The first, like an active ingredient in a medicine, welds H2 molecules together. The second initially helps lock the other two metals into an alloy that can be manipulated. Once the alloy is coated on a surface and exposed to water, this second metal dissolves, leaving behind the other two metals in a highly porous material. That creates extra surface area for the H2 reaction to take place.
Normally, this reaction would be quickly doused thanks to phosphate ions that are present. So Nocera's team added a third metal to drive phosphate away from the material's surface and allow the device to function continuously.
To make its artificial leaf, the MIT team spread its catalysts on opposite sides of a silicon wafer. The silicon absorbs sunlight and passes energetic, negatively charged electrons and positively charged electron vacancies to the catalysts on opposite sides that use them to make H2 and O2.
The solar collector is actually slightly more complex than a uniform slab of silicon. That was necessary because splitting water requires at least 1.23 volts, but a single silicon cell provides only about 0.5 volts. So the MIT team used a commercially available material with three silicon cells layered atop one another, giving them enough voltage to drive the water-splitting reaction.
When the device is placed in a clear jar and exposed to sunlight, it produces a steady stream of oxygen and hydrogen bubbling up to the surface. According to Nocera, the setup converts 5.5% of the energy in sunlight into hydrogen fuel. "You literally walk outside, hold it up, and it works," Nocera says.
The new catalyst also appears highly stable. Nocera says his team has been operating the device for a week, using water from the nearby Charles River in Cambridge, without any drop in efficiency. The next step is to find out whether the device works equally well in seawater. If so, it could dramatically lower the cost of producing hydrogen fuel.