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Science 1 August 2008:
Vol. 321. no. 5889, p. 620

CHEMISTRY:
New Catalyst Marks Major Step in the March Toward Hydrogen Fuel

Robert F. Service

Climate change concerns, high gas prices, and a good deal of 
international friction would fade if scientists could learn a trick 
every houseplant knows: how to absorb sunlight and store its energy 
in chemical bonds. What's needed are catalysts capable of taking 
electricity and using it to split water to generate hydrogen gas, a 
clean fuel. Unfortunately, the catalysts discovered so far work under 
harsh chemical conditions, and the best ones are made from platinum, 
a rare and expensive metal.

No more. This week, researchers at the Massachusetts Institute of 
Technology (MIT) in Cambridge led by chemist Daniel Nocera report 
online in Science a new water-splitting catalyst that works under 
environmentally friendly conditions

(www.sciencemag.org/cgi/content/abstract/1162018).

More important, it's made from cobalt and phosphorus, fairly cheap 
and abundant elements. The new catalyst needs improvements before it 
can solve the world's energy problems, but several outside 
researchers say it's a crucial development.

"This is a great result," says John Turner, an electrochemist and 
water-splitting expert at the National Renewable Energy Laboratory in 
Golden, Colorado. Thomas Moore, a chemist at Arizona State University 
in Tempe, goes further. "It's a big-to-giant step" in the direction 
of powering industrial societies with renewable fuels, he says. "I'd 
say it's a breakthrough." Meanwhile, on pages 671 and 676, other 
groups report related advances--a cheap plastic fuel cell catalyst 
that converts hydrogen to electricity, and a solid oxide fuel cell 
catalyst that operates at lower temperatures--that affect another 
vital component of any future solar hydrogen system.

English chemists first used electricity to split water more than 200 
years ago. The reaction requires two separate catalytic steps. The 
first, the positively charged electrode, or anode, swipes electrons 
from hydrogen atoms in water molecules. The result is that protons 
(hydrogen atoms minus their electrons) break away from their oxygen 
atoms. The anode catalyst then grabs two oxygen atoms and welds them 
together to make O2. Meanwhile, the free protons drift through the 
solution to the negatively charged electrode, or cathode, where they 
hook up with electrons to make molecular hydrogen (H2).

The hard part is finding catalysts that can orchestrate this dance of 
electrons and protons. The anode, which links oxygens together, has 
been a particularly difficult challenge. Platinum works but is too 
expensive and rare to be viable on an industrial scale. "If we are 
going to use solar energy in a direct conversion process, we need to 
cover large areas," Turner says. "That makes a low-cost catalyst a 
must." Other metals and metal oxides can do the job but not at a 
neutral pH--another key to keeping costs down. In 2004, Nocera's team 
reported in the Journal of the American Chemical Society a 
cobalt-based catalyst that did the reverse reaction, catalyzing the 
production of water from O2, protons, and electrons. "That told us 
cobalt could manage multielectron and proton-coupled reactions," 
Nocera says.

Unfortunately, cobalt is useless as a standalone water-splitting 
anode because it dissolves in water. Nocera and his Ph.D. student 
Matthew Kanan knew they couldn't get over this hurdle. So they went 
around it instead. For their anode, they started with a stable 
electrode material known as indium tin oxide (ITO). They then placed 
their anode in a beaker of water, which they spiked with cobalt 
(Co2+) and potassium phosphate. When they flipped on the current, 
this created a positive charge in the ITO. Kanan and Nocera believe 
this initially pulls electrons from the Co2+, turning it first to 
Co3+, which pairs up with negatively charged phosphate ions and 
precipitates out of solution, forming a film of rocklike cobalt 
phosphate atop the ITO. Another electron is yanked from the Co3+ in 
the film to make Co4+, although the mechanism has not yet been nailed 
down. The film forms the critical water-splitting catalyst. As it 
does so, it swipes electrons from hydrogen atoms in water and then 
grabs hold of lone oxygen atoms and welds them together. In the 
process, the Co4+ returns to Co2+ and again dissolves into the water, 
and the cycle is repeated.

The catalyst isn't perfect. It still requires excess electricity to 
start the water-splitting reaction, energy that isn't recovered and 
stored in the fuel. And for now, the catalyst can accept only low 
levels of electrical current. Nocera says he's hopeful that both 
problems can be solved, and because the catalysts are so easy to 
make, he expects progress will be swift. Further work is also needed 
to reduce the cost of cathodes and to link the electrodes to solar 
cells to provide clean electricity. A final big push will be to see 
if the catalyst or others like it can operate in seawater. If so, 
future societies could use sunlight to generate hydrogen from 
seawater and then pipe it to large banks of fuel cells on shore that 
could convert it into electricity and fresh water, thereby using the 
sun and oceans to fill two of the world's greatest needs.


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All the best....

Mike