IEEE Spectrum November, 2011 - 15

Prospects for
an Artificial Leaf
Are Growing
scientists design photosynthesis devices
that could make hydrogen or other fuels

I
Juan estey/Istockphoto

f every leaf on the planet
can do it, maybe we can too.
Scientists have long tried
to mimic photosynthesis as
a way to harness the energy
in sunlight and turn it into
a usable fuel, just as plants do.
There have been big technical
challenges for just as long,
and though researchers are
far from the ultimate goal,
last month two groups of
scientists described some
ways to hurdle those obstacles.
One of the groups, led by
spectrum.ieee.org

MIT chemistry professor
Daniel Nocera, found a new
way to reproduce part of
the photosynthesis process,
using light to split water
molecules into oxygen and
hydrogen. The gases can then
be stored and used as a fuel.
Other groups have had
some success with this process before, but there were
always stumbling blocks
that would make it hard to
scale up or commercialize,
such as extremely acidic or

basic conditions, expensive
catalytic materials, or both.
However, Nocera's group
managed to get artificial
photosynthesis to work
using benign conditions
and cheap, abundant
materials as catalysts.
Specifically, the team
joined a commercially
available triple-junction
silicon solar cell to two
catalysts: cobalt borate for
splitting the water molecule
and a nickel molybdenum
zinc alloy to form the
hydrogen gas. The watersplitting reaction achieved a
sunlight-to-fuel conversion of
4.7 percent in one incarnation
of the device and 2.5 percent
in another. The difference
between the two was that the
more efficient device housed
the hydrogen-generating
alloy on a mesh wired to the
solar cell. The less efficient
version needed no wires,
and the alloy was instead
deposited onto the stainlesssteel back of the solar cell.
It is the wireless possibility,
where the entire device is selfcontained, that researchers
say is most exciting. "Because
there are no wires, we are
not limited by the size that
the light-absorbing material
has to be," says Steven Reece,
a research scientist with
Sun Catalytix (a company
cofounded by Nocera) who
worked on the discovery.
"We can operate on the
micro- or even nanoscale...
so you can imagine microor nanoparticles, similar to
the cells we've worked with
here, dispersed in a solution."
The final product could be
much larger, too-a leaf-size
stand-alone system, for
instance. Whatever the
size, the researchers believe
such devices could help

provide power in poor
areas that lack consistent
sources of electricity.
Sun Catalytix expects to
be able to bring the device to
the point where a kilogram of
hydrogen could be produced
for about US $3, according
to its chief technology officer,
Thomas Jarvi. Given that
about 3.75 liters (1 gallon) of
gasoline contains about the
same amount of energy as
1 kilogram of hydrogen, the
cost would compare favorably
to gasoline, which is
currently higher than $3 per
gallon in the United States.
Daniel Gamelin, a
professor of chemistry at the
University of Washington,
who works on related topics
but was not involved with
the new research, says the
MIT and Sun Catalytix work
represents an "impressive
accomplishment." However,
he says, it remains to be seen
whether silicon is really the
most desirable material to use.
Something less susceptible
to degrading by oxygen may
be a better option, he says.
"For these specific
devices, there remain open
questions about their longterm stability," Gamelin
says. "And their efficiencies
would still need to be
increased substantially to
be commercially viable.
But there is obviously
potential for improvement
on both fronts. In the bigger
scheme, [this research]
marks important progress
toward the development
of truly practical solar
hydrogen technologies."
Separately, researchers
in Illinois demonstrated a
different part of the photosynthesis process-a step
toward using sunlight to
recycle carbon dioxide. In

november 2011 * Ieee Spectrum * nA

15
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Table of Contents for the Digital Edition of IEEE Spectrum November, 2011
