Bringing the concept of an "artificial
leaf" closer to reality, a team of researchers at MIT has published a
detailed analysis of all the factors that could limit the efficiency of such a
system. The new analysis lays out a roadmap for a research program to improve
the efficiency of these systems, and could quickly lead to the production of a
practical, inexpensive and commercially viable prototype.
Such a system would use
sunlight to produce a storable fuel, such as hydrogen, instead of electricity
for immediate use. This fuel could then be used on demand to generate
electricity through a fuel cell or other device. This process would liberate
solar energy for use when the sun isn't shining, and open up a host of
potential new applications.
The new work is described
in a paper this week in the Proceedings of the National Academy of
Sciences by associate professor of mechanical engineering Tonio
Buonassisi, former MIT professor Daniel Nocera (now at Harvard University), MIT
postdoc Mark Winkler (now at IBM) and former MIT graduate student Casandra Cox
(now at Harvard). It follows up on 2011 research that produced a "proof of
concept" of an artificial leaf -- a small device that, when placed in a
container of water and exposed to sunlight, would produce bubbles of hydrogen
and oxygen.
The device combines two
technologies: a standard silicon solar cell, which converts sunlight into
electricity, and chemical catalysts applied to each side of the cell. Together,
these would create an electrochemical device that uses an electric current to
split atoms of hydrogen and oxygen from the water molecules surrounding them.
The goal is to produce an
inexpensive, self-contained system that could be built from abundant materials.
Nocera has long advocated such devices as a means of bringing electricity to
billions of people, mostly in the developing world, who now have little or no
access to it.
"What's significant is
that this paper really describes all this technology that is known, and what to
expect if we put it all together," Cox says. "It points out all the
challenges, and then you can experimentally address each challenge
separately."
Winkler adds that this is a
"pretty robust analysis that looked at what's the best you could do with
market-ready technology."
The original demonstration
leaf, in 2011, had low efficiencies, converting less than 4.7 percent of
sunlight into fuel, Buonassisi says. But the team's new analysis shows that
efficiencies of 16 percent or more should now be possible using single-bandgap
semiconductors, such as crystalline silicon.
"We were surprised,
actually," Winkler says: Conventional wisdom held that the characteristics
of silicon solar cells would severely limit their effectiveness in splitting
water, but that turned out not to be the case. "You've just got to
question the conventional wisdom sometimes," he says.
The key to obtaining high
solar-to-fuel efficiencies is to combine the right solar cells and catalyst --
a matchmaking activity best guided by a roadmap. The approach presented by the
team allows for each component of the artificial leaf to be tested
individually, then combined.
The voltage produced by a
standard silicon solar cell, about 0.7 volts, is insufficient to power the
water-splitting reaction, which needs more than 1.2 volts. One solution is to
pair multiple solar cells in series. While this leads to some losses at the
interface between the cells, it is a promising direction for the research,
Buonassisi says.
An additional source of
inefficiency is the water itself -- the pathway that the electrons must
traverse to complete the electrical circuit -- which has resistance to the
electrons, Buonassisi says. So another way to improve efficiency would be to
lower that resistance, perhaps by reducing the distance that ions must travel
through the liquid.
"The solution
resistance is challenging," Cox says. But, she adds, there are "some
tricks" that might help to reduce that resistance, such as reducing the
distance between the two sides of the reaction by using interleaved plates.
"In our simulations,
we have a framework to determine the limits of efficiency" that are
possible with such a system, Buonassisi says. For a system based on
conventional silicon solar cells, he says, that limit is about 16 percent; for
gallium arsenide cells, a widely touted alternative, the limit rises to 18
percent.
Models to determine the
theoretical limits of a given system often lead researchers to pursue the
development of new systems that approach those limits, Buonassisi says.
"It's usually from these kinds of models that someone gets the courage to
go ahead and make the improvements," he says.
"Some of the most
impactful papers are ones that identify a performance limit," Buonassisi
says. But, he adds, there's a "dose of humility" in looking back at
some earlier projections for the limits of solar-cell efficiency: Some of those
predicted "limits" have already been exceeded, he says.
"We don't always get
it right," Buonassisi says, but such an analysis "lays a roadmap for
development and identifies a few 'levers' that can be worked on."
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