19
VOL. 2.2
The promise of
arti ficial
photosynthesis
Solar fuel with no net
consumption of water
By Peter Jaret
Trees do it.
Weeds do it.
Even algae in the seas do it.
And now scientists are
trying to do it – to turn
sunlight into fuel through
artificial photosynthesis.
BIOENERGY CONNECTION
20
Record droughts in the U.S. have sparked new interest in using this
potentially renewable resource to power the nation’s transportation.
But creating what is, in effect, a bionic leaf poses formidable challenges.
After all, when it comes to photosynthesis, plants have had eons to perfect their technique. They’ve evolved to gather energy from the sun and
used it to reshuffle the molecules in water and carbon dioxide to create
fuel in the form of a sugar molecule, or carbohydrate. The holy grail of
artificial photosynthesis is to turn the same three ingredients that plants
use – sunlight, water, and carbon dioxide – directly into a cost-effective
transportation fuel.
But though natural photosynthesis serves as the original inspiration, researchers working on an artificial version can’t simply mimic plants to
create a prototype.
“It’s the difference between birds and airplanes,” says Tanja Cuk, Ph.D., an
assistant professor of chemistry at the University of California at Berkeley, who is conducting basic research into artificial photosynthesis. “They
both fly. But airplanes don’t work simply by mimicking the movements
of birds.”
For all its challenges, the benefits of artificial photosynthesis, if realized,
would be enormous.
As a renewable energy, artificial photosynthesis could offer several key
advantages over other technologies. Unlike biofuels, artificial photosynthesis doesn’t require arable land, so it wouldn’t compete with food crops – a crucial consideration as
the world’s population grows and the pressure
on water resources intensifies.
Even an artificial photosynthesis system with relatively modest solar
conversion efficiency could provide energy for all the nation’s
transportation needs using
an area of non-arable land
roughly equal to that currently used by the country’s
interstate highway system,
according to Heinz Frei,
Ph.D., a senior scientist at
Lawrence Berkeley National
Labs. Although the process
requires water, it gives off equal
amounts of water, so it is entirely renewable.
“While water is an essential reactant,
it is not needed at high concentrations,”
says Frei, who has been involved in solar
photochemistry and artificial photosynthesis
research for several decades.“It could be in the form
of water vapor no higher in concentration than in typical
air. Also, water is regenerated when the fuel is consumed, so there is no net
consumption of water.”
Solar fuel vs electric cars
Photovoltaic cells can already convert sunlight into electricity, but using
electricity to run vehicles requires a new infrastructure of vehicles and
charging stations. Also, batteries as a source of stored electricity are not
suitable for powering airplanes, ships, or heavy trucks. Artificial photosynthesis, in contrast, can produce a storable and stable fuel that could
theoretically be used for transportation using the existing infrastructure
of airplanes, cars, trucks, and filling stations.
The problem is producing it in a way that is scalable, with components
that can be manufactured using affordable and widely available materials.
In addition, the devices have to be capable of generating fuel on the scale
needed for transportation.
Experts in the field acknowledge that we are still years, even decades away
from filling our gas tanks with solar fuel. The LBNL’s Frei also directs the
Joint Center for Artificial Photosynthesis’s (JCAP) science-based scale-up
efforts. The goal of JCAP is to have a scalable working prototype within
five to 10 years, but developing systems to produce the most
desirable solar fuel for pipe distribution will require
more time, he says. According to some skeptics,
cost-effective solar fuels may not be ready
for several decades.
To be economically viable, artificial
photosynthesis must be far more
efficient at using sunlight to create fuel than plants. (This may
not be that hard a step, since
some artificial photosynthesis projects can already produce fuel from sunlight up
to ten times more efficiently
than plants.) But the technology, composed of nonbiological
materials,
must also be durable
enough to work
Drs.Tanja Cuk
for years under
and Heinz Frei
the glaring sun
with minimal
Photo Credit:
m a i nt e n a n c e .
Saul Bromberger
And it must be
and
Sandra Hoover
cost-effective to
manufacture on a
large scale.
21
Nanotechnology front and center
Scientists have been pursuing the dream of artificial photosynthesis since
the 1960s. The first proof of concept came in the late 1990s, when researchers at the National Renewable Energy Laboratory demonstrated the world’s first integrated device that converted sunlight into fuel.
That breakthrough demonstrated that artificial photosynthesis
was possible – but not yet practical. The first prototype used
rare earth materials and methods for manufacturing on the
computer chip scale, far from the scale needed to produce
transportation fuel on a national or even global scale. What’s
more, its components disintegrated within hours.
VOL. 2.2
gets are basic fuels such as methane or methanol, which can be
used to replace fossil fuels.
Achieving all that is currently possible – using layered photovoltaics and expensive, rare materials such as iridium
or platinum as catalysts. “Now the challenge is to create little photovoltaics connected to little catalysts on
a nanoscale, and to use materials that are abundant
and scalable,” says Cuk.
In the laboratory where Cuk works at the University of California, behind drapes of heavy black curtains used to protect
scientists’ eyes, lasers shoot high-energy beams of light through
Advances on several fronts – a better understanding of nature’s design
principles and the explosive growth of nanotechnologies – have moved the
dream of artificial photosynthesis closer to reality.
Since then, advances on several fronts have moved the dream of artificial
photosynthesis closer to reality, says LBNL’s Frei.
“First, we’ve seen tremendous progress in understanding natural photosynthesis. Not to mimic Nature, but to take advantage of its design principles. At the same time, the explosive growth of nanotechnologies starting
in the mid-1990s has provided essential new tools. The natural process of
photosynthesis is controlled on a nanometer scale,” explains Frei. “For the
first time, nanotechnology allows us to engineer, control and manipulate
the process of artificial photosynthesis at this critical length scale.”
Recognizing that the time is ripe for progress in the field, the U.S. Department of Energy established the Joint Center for Artificial Photosynthesis in
2010. The Center, an energy innovation hub, brings together scientists from
California Institute of Technology and Lawrence Berkeley National Laboratory. Its mission: to develop working prototypes that use widely available
materials that can be scaled up to generate large amounts of fuel from sunlight efficiently and cost-effectively. In October 2012, LBNL broke ground
for a new bricks-and-mortar home for Joint Center’s work, the Solar Energy
Research Center building, which is scheduled to be completed in late 2014.
High hopes, enormous challenges
In one of eight scientific and engineering projects now underway, JCAP
researchers are currently testing a prototype device, about the size of a
laptop computer, which represents a crucial leap over the first proof of
concept. “If you look under the hood, the control is entirely on the molecular and nanoscale,” says Frei. But while the device efficiently converts
sunlight and water into the components necessary for making fuel, it’s still
far from economically viable.
The components of an artificial photosynthesis system are fairly basic.
The process requires a photovoltaic material that absorbs light energy
from the sun. This energy must then be directed to two separate catalysts – one that splits water into protons and oxygen and another that
converts carbon dioxide and protons into hydrocarbons. Because the
two catalytic processes compete for electrical charges, the system requires a membrane to separate the two chemical reactions. Initial tar-
intricate mazes of instruments. These detect how fast excitations of the photovoltaic material get directed to the surfaces of catalysts where chemical
bonds are rearranged. The more difficult task will be to map out, using some
of the same techniques, how these excitations rearrange chemical bonds in
water on the surfaces of different catalysts. Insights from this kind of basic
research, Cuk believes, will be crucial to solving these last remaining challenges.
“I like to use the analogy of the transistor,” she explains. “The scientific
community was able to go from the original vacuum tubes to millions of
tiny transistors in a single integrated circuit due to a good understanding of the principles at work in creating an on/off switch from solid-state
materials. We may not need as new a principle as the p-n junction was
to the vacuum tube. But the insights we can get from basic research into
how highly active catalysts work will help lead us to better nanostructured
solutions for photosynthesis.”
At the Caltech site of the hub, for example, scientists have come up with
a process to develop millions of different variations of possible catalysts
almost simultaneously – each sample of which is as tiny as a pixel on a
screen. Rather than a few discoveries of new catalysts a year, researchers
can now have new candidates every few milliseconds.
Cuk, who received her Ph.D. in physics, shifted her focus to artificial
photosynthesis because of its enormous promise as a renewable energy
source. “I wanted to be involved in research that could make a real difference in the world,” she says. As a Miller Research Fellow at Berkeley, she
worked closely with Frei, who she regards as a mentor.
Today, young scientists like Cuk inspire Frei to hope that, after decades of
slow but steady progress, the development of artificial photosynthesis is
poised to shift into high gear. “That’s what makes this field so exciting,” he
says. “The Joint Center for Artificial Photosynthesis comes at a moment
when we can start putting things together to see how poorly they work.
That’s the way you improve and get to a viable technology – by seeing
what you need to solve. Fortunately, we have young scientists who are
looking for those solutions, exploring fresh ideas that will hopefully lead
to new designs that we haven’t even thought about.” n