The ultimate goal of an MIT professor, who has organized two teams to pioneer an artificial photosynthesis devices, is to produce an “artificial leaf” so simple and so inexpensive that it could be made widely available to the billions of people in the world who lack access to adequate, reliable sources of electricity. |
An
important step toward realizing the dream of an inexpensive and simple
“artificial leaf,” a device to harness solar energy by splitting water
molecules, has been accomplished by two separate teams of researchers at
MIT. Both teams produced devices that combine a standard silicon solar
cell with a catalyst developed three years ago by professor Daniel
Nocera. When submerged in water and exposed to sunlight, the devices
cause bubbles of oxygen to separate out of the water.
The
next step to producing a full, usable artificial leaf, explains Nocera,
the Henry Dreyfus Professor of Energy and professor of chemistry, will
be to integrate the final ingredient: an additional catalyst to bubble
out the water’s hydrogen atoms. In the current devices, hydrogen atoms
are simply dissociated into the solution as loose protons and electrons.
If a catalyst could produce fully formed hydrogen molecules (H2), the
molecules could be used to generate electricity or to make fuel for
vehicles. Realization of that step, Nocera says, will be the subject of a
forthcoming paper.
The
reports by the two teams were published in the journals Energy &
Environmental Science on May 12, and the Proceedings of the National
Academy of Sciences on June 6. Nocera encouraged two different teams to
work on the project so that each could bring their special expertise to
addressing the problem, and says the fact that both succeeded “speaks to
the versatility of the catalyst system.”
Ultimately,
Nocera wants to produce a low-cost device that could be used where
electricity is unavailable or unreliable. It would consist of a glass
container full of water, with a solar cell with the catalysts on its two
sides attached to a divider separating the container into two sections.
When exposed to the sun, the electrified catalysts would produce two
streams of bubbles — hydrogen on one side, oxygen on the other — which
could be collected in two tanks, and later recombined through a fuel
cell or other device to generate electricity when needed.
“These
papers are really important, to show that the catalyst works” when
bonded to silicon to make a single device, Nocera says, thus enabling a
unit that combines the functions of collecting sunlight and converting
it to storable fuel. Silicon is an Earth-abundant and relatively
inexpensive material that is widely used and well understood, and the
materials used for the catalyst — cobalt and phosphorus — are also
abundant and inexpensive.
Putting it together
Marrying
the technologies of silicon solar cells with the catalyst material —
dubbed Co-Pi for cobalt phosphate — was no trivial matter, explains
Tonio Buonassisi, the SMA Assistant Professor of Mechanical Engineering
and Manufacturing, who was a co-author of the PNAS paper.
That’s because the splitting of water by the catalyst creates a “very
aggressive” chemical environment that would tend to rapidly degrade the
silicon, destroying the device as it operates, he says.
In
order to overcome this, both teams had to find ways to protect the
silicon surface, while at the same time allowing it to receive the
incoming sunlight and to interact with the catalyst.
Professor
of Electrical Engineering Vladimir Bulovi?, who led the other team,
says his team’s approach was to form the Co-Pi material on the surface
of the silicon cell, by first evaporating a layer of pure cobalt metal
onto the cell electrode, and then exposing it to a phosphate buffer
solution under an electrical charge to transform it into the Co-Pi
catalyst. By using the layer of Co-Pi, now firmly bonded to the surface,
“we were able to passivate the surface,” says Elizabeth Young, a
postdoc who was the lead author of the E&ES paper — in other words, it acts as a protective barrier that keeps the silicon from degrading in water.
“Most
people have been staying away from silicon for water oxidation, because
it forms silicon dioxide” when exposed to water, which is an insulator
that would hinder the electrical conductivity of the material, says
Ronny Costi, a postdoc on Bulovi?’s team. “We had to find a way of
solving that problem,” which they did by using the cobalt coating.
Buonassisi’s
team used a different approach, coating the silicon with a protective
layer. “We did it by putting a thin film of indium tin oxide on top,”
explains Joep Pijpers, a postdoc who was the lead author of the PNAS
paper. Using its expertise in the design of silicon devices, that team
then concentrated on matching the current output of the solar cell as
closely as possible to the current consumption by the (catalyzed)
water-splitting reaction. The system still needs to be optimized,
Pijpers says, to improve the efficiency by a factor of 10 to bring it to
a range comparable to conventional solar cells.
“It’s
really not trivial, integrating a low-cost, high-performance silicon
device with the Co-Pi,” Buonassisi says. “There’s a substantial amount
of innovation in both device processing and architecture.”
Both
teams had to add an extra power source to the system, because the
voltage produced by a single-junction silicon cell is not high enough to
use for powering the water-splitting catalyst. In later versions, two
or three silicon solar cells will be used in series to provide the
needed voltage without the need for any extra power source, the
researchers say.
One
interesting aspect of these collaborations, says postdoc Mark Winkler,
who worked with Buonassisi’s team, was that “materials scientists and
chemists had to learn to talk to each other.” That’s trickier than it
may sound, he explains, because the two disciplines, even when talking
about the same phenomena, tend to use different terminology and even
different ways of measuring and displaying certain characteristics.
Portable power?
Nocera’s
ultimate goal is to produce an “artificial leaf” so simple and so
inexpensive that it could be made widely available to the billions of
people in the world who lack access to adequate, reliable sources of
electricity. What’s needed to accomplish that, in addition to stepping
up the voltage, is the addition of a second catalyst material to the
other side of the silicon cell, Nocera says.
Although
the two approaches to bonding the catalyst with a silicon cell appear
to produce functioning, stable devices, so far they have only been
tested over periods of a few days. The expectation is that they will be
stable for long periods, but accelerated aging tests will need to be
performed to confirm this.
Rajeshwar
Krishnan, Distinguished University Professor of Chemistry and
Biochemistry at the University of Texas at Arlington, says it remains to
be seen “whether this ‘self-healing’ catalyst would hold up to several
hours of current flow … under rather harsh oxidative conditions.” But he
adds that these papers “certainly move the science forward. The state
of the science in water photo-oxidation uses rather expensive noble
metal oxides,” whereas this work uses Earth-abundant, low-cost
materials. He adds that while there is still no good storage or
distribution system in place for hydrogen, “it is likely that the solar
photon-to-hydrogen technology will ultimately see the light of day — for
transportation applications — with the hydrogen internal combustion
engine.”
Meanwhile,
Nocera has founded a company called Sun Catalytix, which will initially
be producing a first-generation system based on the Co-Pi catalyst
material, connected by wires to conventional, separate solar cells.
The
“leaf” system, by contrast, is “still a science project,” Nocera says.
“We haven’t even gotten to what I would call an engineering design.” He
hopes, however, that the artificial leaf could become a reality within
three years.
Bulovi?’s
team was funded partly by the Chesonis Family Foundation and the
National Science Foundation. Buonassisi’s team had support from the
Netherlands Organization for Scientific Research (NOW-FOM), the National
Science Foundation and the Chesonis Family Foundation. Nocera’s work
was funded by the Chesonis Family Foundation, the Air Force Office of
Scientific Research and the National Science Foundation.
