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would work this well. I couldn't have predicted it. The stu that was
falling out of the compounds turned out to be what we needed.
"Now we want to understand it," he continues. "I want to know
why the hell cobalt in this thin film is so active. I may be able to
improve it or use a di erent metal that's better." At the same time,
he wants to start working with engineers to optimize the process
and make an e cient water-splitting cell, one that incorporates
catalysts for generating both oxygen and hydrogen. "We were really
interested in the basic science. Can we make a catalyst that works
e ciently under the conditions of photosynthesis?" he says. "The
answer now is yes, we can do that. Now we've really got to get to
the technology of designing a cell."
CATALYZING A DEBATE
Nocera's discovery has garnered a lot of attention, and not all of
it has been flattering. Many chemists find his claims overstated;
they don't dispute his findings, but they doubt that they will have
the consequences he imagines. "The claim that this is the answer
for artificial photosynthesis is crazy," says Thomas Meyer, who
has been a mentor to Nocera. He says that while Nocera's cata-
lysts "could prove technologically important," the advance is "a
research finding," and there's "no guarantee that it can be scaled
up or even made practical."
Many critics' objections revolve around the inability of Nocera's
lab setup to split water nearly as rapidly as commercial electrolyz-
ers do. The faster the system, the smaller a commercial unit that
produced a given amount of hydrogen and oxygen would be. And
smaller systems, in general, are cheaper.
The way to compare di erent catalysts is to look at their "cur-
rent density"---that is, electrical current per square centimeter---
when they're at their most e cient. The higher the current, the
faster the catalyst can produce oxygen. Nocera reported results of
1 milliamp per square centimeter, although he says he's achieved
10 milliamps since then. Commercial electrolyzers typically run at
about 1,000 milliamps per square centimeter. "At least what he's
published so far would never work for a commercial electrolyzer,
where the current density is 800 times to 2,000 times greater,"
says John Turner, a research fellow at the National Renewable
Energy Laboratory in Golden, CO.
Other experts question the whole principle of converting sun-
light into electricity, then into a chemical fuel, and then back into
electricity again. They suggest that while batteries store far less
energy than chemical fuels, they are nevertheless far more e -
cient, because using electricity to make fuels and then using the
fuels to generate electricity wastes energy at every step. It would
be better, they say, to focus on improving battery technology or
other similar forms of electrical storage, rather than on developing
water splitters and fuel cells. As Ryan Wiser puts it, "Electrolysis
is [currently] ine cient, so why would you do it?"
THE ARTIFICIAL LEAF
Michael Grätzel, however, may have a clever way to turn Nocera's
discovery to practical use. A professor of chemistry and chemi-
cal 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 o a water-
splitting reaction. Instead, they're collected by a film of titanium
dioxide and directed through an external circuit, generating elec-
tricity. 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 e cient 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 pos-
sibilities 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 e ort 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.
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