The ‘artificial leaf,’ a device that can harness sunlight to split water into hydrogen and oxygen without needing any external connections, is seen with some real leaves, which also convert the energy of sunlight directly into storable chemical form. Photo: Dominick Reuter |
Researchers
led by Massachusetts Institute of Technology (MIT) professor Daniel Nocera have
produced something they’re calling an “artificial leaf”. Like living leaves,
the device can turn the energy of sunlight directly into a chemical fuel that
can be stored and used later as an energy source.
The
artificial leaf—a silicon solar cell with different catalytic materials bonded
onto its two sides—needs no external wires or control circuits to operate.
Simply placed in a container of water and exposed to sunlight, it quickly
begins to generate streams of bubbles: oxygen bubbles from one side and
hydrogen bubbles from the other. If placed in a container that has a barrier to
separate the two sides, the two streams of bubbles can be collected and stored,
and used later to deliver power: for example, by feeding them into a fuel cell
that combines them once again into water while delivering an electric current.
The
creation of the device is described
in a paper published in Science. Nocera, the Henry Dreyfus Professor
of Energy and professor of chemistry at MIT, is the senior author; the paper
was coauthored by his former student Steven Reece PhD ’07 (who now works at Sun
Catalytix, a company started by Nocera to commercialize his solar energy
inventions), along with five other researchers from Sun Catalytix and MIT.
The
device, Nocera explains, is made entirely of earth-abundant, inexpensive
materials—mostly silicon, cobalt, and nickel—and works in ordinary water. Other
attempts to produce devices that could use sunlight to split water have relied
on corrosive solutions or on relatively rare and expensive materials such as
platinum.
The
artificial leaf is a thin sheet of semiconducting silicon which turns the
energy of sunlight into a flow of wireless electricity within the sheet. Bound
onto the silicon is a layer of a cobalt-based catalyst, which releases oxygen,
a material whose potential for generating fuel from sunlight was discovered by
Nocera and his coauthors in 2008. The other side of the silicon sheet is coated
with a layer of a nickel-molybdenum-zinc alloy, which releases hydrogen from
the water molecules.
“I
think there’s going to be real opportunities for this idea,” Nocera says.
“You
can’t get more portable—you don’t need wires, it’s lightweight, and it doesn’t
require much in the way of additional equipment, other than a way of catching
and storing the gases that bubble off. You just drop it in a glass of water,
and it starts splitting it,” he says.
Now
that the “leaf” has been demonstrated, Nocera suggests one possible further
development: tiny particles made of these materials that can split water molecules
when placed in sunlight—making them more like photosynthetic algae than leaves.
The advantage of that, he says, is that the small particles would have much
more surface area exposed to sunlight and the water, allowing them to harness
the sun’s energy more efficiently.
The
new device is not yet ready for commercial production, since systems to
collect, store and use the gases remain to be developed. “It’s a step,” Nocera
says. “It’s heading in the right direction.”
Ultimately,
he sees a future in which individual homes could be equipped with
solar-collection systems based on this principle: Panels on the roof could use
sunlight to produce hydrogen and oxygen that would be stored in tanks, and then
fed to a fuel cell whenever electricity is needed. Such systems, Nocera hopes,
could be made simple and inexpensive enough so that they could be widely
adopted throughout the world, including many areas that do not presently have
access to reliable sources of electricity.
At
present, the leaf can redirect about 2.5% of the energy of sunlight into
hydrogen production in its wireless form; a variation using wires to connect
the catalysts to the solar cell rather than bonding them together has attained
4.7% efficiency. Typical commercial solar cells today have efficiencies of more
than 10%. One question Nocera and his colleagues will be addressing is which of
these configurations will be more efficient and cost-effective in the long run.
Another
line of research is to explore the use of photovoltaic (solar cell) materials
other than silicon—such as iron oxide, which might be even cheaper to produce. “It’s all about providing options for how you go about this,” Nocera says.
