The Artificial Leaf
Michael Grätzel, however, may have a clever way to turn Nocera’s discovery to practical use. A professor of chemistry and chemical engineering at the École Polytechnique Fédérale in Lausanne, Switzerland, he was one of the first people Nocera told about his new catalyst. “He was so excited,” Grätzel says. “He took me to a restaurant and bought a tremendously expensive bottle of wine.”
In 1991, Grätzel invented a promising new type of solar cell. It uses a dye containing ruthenium, which acts much like the chlorophyll in a plant, absorbing light and releasing electrons. In Grätzel’s solar cell, however, the electrons don’t set off a water-splitting reaction. Instead, they’re collected by a film of titanium dioxide and directed through an external circuit, generating electricity. Grätzel now thinks that he can integrate his solar cell and Nocera’s catalyst into a single device that captures the energy from sunlight and uses it to split water.
If he’s right, it would be a significant step toward making a device that, in many ways, truly resembles a leaf. The idea is that Grätzel’s dye would take the place of the electrode on which the catalyst forms in Nocera’s system. The dye itself, when exposed to light, can generate the voltage needed to assemble the catalyst. “The dye acts like a molecular wire that conducts charges away,” Grätzel says. The catalyst then assembles where it’s needed, right on the dye. Once the catalyst is formed, the sunlight absorbed by the dye drives the reactions that split water. Grätzel says that the device could be more efficient and cheaper than using a separate solar panel and electrolyzer.
Another possibility that Nocera is investigating is whether his catalyst can be used to split seawater. In initial tests, it performs well in the presence of salt, and he is now testing it to see how it handles other compounds found in the sea. If it works, Nocera’s system could address more than just the energy crisis; it could help solve the world’s growing shortage of fresh water as well.
Artificial leaves and fuel-producing desalination systems might sound like grandiose promises. But to many scientists, such possibilities seem maddeningly close; chemists seeking new energy technologies have been taunted for decades by the fact that plants easily use sunlight to turn abundant materials into energy-rich molecules. “We see it going on all around us, but it’s something we can’t really do,” says Paul Alivisatos, a professor of chemistry and materials science at the University of California, Berkeley, who is leading an effort at Lawrence Berkeley National Laboratory to imitate photosynthesis by chemical means.
But soon, using nature’s own blueprint, human beings could be using the sun “to make fuels from a glass of water,” as Nocera puts it. That idea has an elegance that any chemist can appreciate–and possibilities that everyone should find hopeful.
Kevin Bullis is Technology Review’s Energy Editor.