New Catalyst Marks Major Step in The March Toward Hydrogen Fuel

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 watersplitting
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
f irst, 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 lowcost
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 cobaltbased
catalyst that did the reverse reaction,
catalyzing the production of water from O2,
protons, and electrons. “That told us cobalt
could manage multielectron and protoncoupled
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.