Artificial leaf helps researchers make headway turning photons to fuel
On a bright spring morning in Pasadena, California, the air is rich with
the smells of cut grass and flowers. Photosynthesis seems effortless
here: the fronds and blooms that line the walkways of the California
Institute of Technology (Caltech) bask in the sunlight, quietly using
its energy to store sugars, stretch their leaves, deepen their roots and
tend to their cellular processes.
Inside Caltech's Jorgensen Laboratory, however, more than 80
researchers are putting a lot of effort into doing the leaf's job using
silicon, nickel, iron and any number of other materials that would be
more at home inside a cell phone than a plant cell. Their gleaming new
labs are the headquarters of the Joint Center for Artificial
Photosynthesis (JCAP), a 190-person research programme funded by the US
Department of Energy (DOE) with US$116 million over five years. The
centre's goal is to use sunlight to make hydrogen and other fuels much
more efficiently than real leaves ever made biomass.
The
researchers are pursuing this goal with a certain urgency. Roughly 13%
of greenhouse-gas emissions worldwide come from transportation, so
phasing out polluting fuels is a key environmental target. One approach
is to replace cars and light trucks with electric vehicles charged by
solar cells or wind — but that cannot tackle the whole problem. Nathan
Lewis, an inorganic chemist at Caltech and JCAP's scientific director,
says that some 40% of current global transportation cannot be
electrified. For example, barring a major breakthrough, there will never
be a plug-in hybrid plane: no craft could hold enough batteries. Liquid
fuels are unbeatable when it comes to convenience combined with compact
energy storage.
That is why funding agencies
around the world — and at least a few private companies — are putting
unprecedented resources into making fuels using power from the Sun,
which is not only carbon-free but effectively inexhaustible. JCAP stands
out not only for its scale, but also for its ambition. It is one of
five Energy Innovation Hubs created by the DOE beginning in 2010 to
focus on specific problems using basic research, applied research and
engineering. JCAP has promised to deliver a working prototype of an
artificial leaf by the time its initial grant runs out in 2015.
Although the centre has taken some important steps in that direction — including one reported just last week
1
— it is still a long way from delivering on that promise, “This is a
really, really difficult, challenging problem,” says electrochemist John
Turner of the US National Renewable Energy Laboratory in Golden,
Colorado. “The payback would be huge, but it's not as simple as everyone
wanted it to be when we started playing in this area 40 years ago.”
Still,
the surge of funding and attention has given many researchers reason to
hope for long-term success. “If you could sustain this type of effort
for the next ten years,” says Michael Wasielewski, a chemist at
Northwestern University in Evanston, Illinois, “it's conceivable you
could have a practical solution.”
Catching rays
The
concept of artificial photosynthesis goes back to 1912, but the push to
achieve it did not start until 1972, when Japanese researchers outlined
what a device would need to take in sunlight and use it to split water
into oxygen and hydrogen fuel
2. Progress was slow. In 1998, Turner reported
3
a complete system that showed a major advance — it stored 12% of the
incoming solar energy as fuel, compared with 1% of energy stored as
biomass in real leaves. But it cost more than 25 times too much to be
competitive, and its performance dropped off after 20 hours of sunshine.
Source: JCAP; Art: Nik Spencer/Nature
There are three things you want from an
artificial leaf, says Lewis: “You want it to be efficient, cheap and
robust. I can give you any two today, but not the third at the same
time.”
JCAP's mission is to fix that problem —
and in the process, to create a system that is much cheaper than just
splitting water with electricity from a solar panel. At the heart of
JCAP's artificial-leaf design are two electrodes immersed in an aqueous
solution. Typically, each electrode is made of a semiconductor material
chosen to capture light energy from a particular part of the solar
spectrum, and coated with a catalyst that will help to generate hydrogen
or oxygen at useful speeds (see 'Splitting water').
Like many other artificial-photosynthesis devices, JCAP's system is
divided by a membrane to keep the resulting gases apart and reduce the
risk of an explosive reaction.
Once the water
has been split, the hydrogen is harvested. It can be used as a fuel by
itself — perhaps in hydrogen-powered cars such as those already making
their way into showrooms in California — or be reacted with carbon
monoxide to make liquid-hydrocarbon fuels.
Making
any one of the artificial leaf's components work well is a challenge;
combining all of them into a complete system is even harder. “This is
exactly like building a plane,” says Lewis. “You've got to not just have
an engine, you have to have a design with wings and the fuselage and
the engine and the avionics — and the plane, in the end, has to fly.”
Much
of the difficulty comes down to finding the right materials. Silicon,
for instance, makes a good photocathode — the electrode that produces
hydrogen gas — but is stable only when the solution around it is acidic.
Unfortunately, the situation is reversed with photoanodes, which
produce oxygen: the good ones are stable only when the solution is
basic, not acidic. And the best catalyst for the oxygen-producing
electrode, iridium, is both rare and expensive, which makes it
unsuitable for commercial-scale devices.
JCAP's High Throughput Experimentation lab is tackling the materials
problem with inkjet printers modified to churn out spots of alloys on
glass plates for testing as catalysts and photoabsorbers. Together, the
printers are able to produce up to one million spots of varying
composition per day.
In one experiment to find the best proportions of nickel, iron, cobalt and cerium oxides
to generate oxygen from water, the team screened nearly 5,500
combinations for stability and function using a miniaturized chemical
lab that glided over the glass plates tirelessly. The best-performing
combination is not the most effective catalyst ever found for this
reaction, but it is transparent, allowing light to pass through to the
photoabsorber, and it has good chemical compatibility with that
material.
One of the toughest challenges for
artificial photosynthesis has always been getting a good material for
the photoanode, says Carl Koval, an electrochemist and JCAP's director.
“Those things were always horribly unstable, often not even stable for
minutes.” Many researchers have focused their search on materials known
to be cheap and stable — certain metal oxides, for example — and tried
to make them into good light absorbers. Others feel that it is better to
start with materials that are known to be efficient light harvesters,
and to work at making them stable and cheap.
Just last week, a JCAP team reported success with the latter approach. By putting a protective coating of
titanium dioxide on high-performing photoabsorbers such as silicon, the
researchers achieved big gains in stability. “That's basically the last
piece of the puzzle to create the first-generation prototype,” says
Koval, who predicts that JCAP will have an artificial leaf running in
the next few months.
Publication of a
preliminary system including the titanium dioxide coating is in the
works, says Lewis. “That's going to be a double-digit-efficiency, stable
system.” The threshold for commercial viability is thought to be in the
10–20% range. The photoabsorbers will not be cheap enough to bring to
market, concedes Lewis, because their cores are made from expensive
single-crystal silicon. But if subsequent research shows that cheaper
fabrication methods work, the system could be cost-effective.
Spectrum of ideas
JCAP
will soon complete its fourth year of operations. It got off to a slow
start as new labs were built, but researchers both in and outside the
centre praise its systematic focus on producing a practical system, and
its progress so far. Even Turner, whose lab bid to become the
solar-fuels hub but lost out to Lewis's team, is encouraged by JCAP's
latest direction.
Still, says Koval, the
centre has its critics. Some take issue with its emphasis on engineering
and prototype development. But if JCAP were to focus on basic science,
he says, it would not be “doing what the DOE created the hub for in the
first place”. Other critics object to how JCAP concentrates on just one
of several possible ways to tackle artificial photosynthesis. “A lot of
people would have been happier if the DOE had spread the funding around
all these different ways of doing this,” says Koval. But that kind of
dilution of effort would be risky in its own way, he argues: “Then you'd
have progress on none of them.”
“The payback would be huge, but it's not as simple as everyone wanted it to be.”
Besides, many of the alternative approaches
are being pursued elsewhere. Up the coast in Santa Barbara, California,
for example, a start-up firm called HyperSolar is testing a system in
which coated nano- or micro-particles combining a photoabsorber and a
catalyst are placed in a transparent, water-filled plastic bag. The bag
will inflate as it is exposed to sunlight, and hydrogen and oxygen gas
form inside. Such units could be deployed in sunny regions such as
deserts. A 2009 DOE report estimated that, if it uses cheap materials, this 'baggie' approach
could produce hydrogen economically with 10% efficiency, stable for 10
years.
But the system is risky, says Turner,
because it produces oxygen along with the hydrogen. “If you're talking
about 100 square miles of baggies in the desert with this explosive
mixture,” he says, “one lightning bolt and you have a disaster.”
HyperSolar researchers are exploring several ways to eliminate that
danger. One is to use a system that separates the gases into two bags,
says Syed Mubeen, a postdoc at the University of California, Santa
Barbara, and lead scientist at the company. Another is to run the system
using waste water rather than pure water, so that the oxygen reacts
with organic impurities and degrades them into valuable chemicals. This
approach “completely removes oxygen out of the equation”, says Mubeen.
As with JCAP's stable photoanode, HyperSolar's photoabsorber is
protected by a coating.
Light industry
Another
entrant in the artificial-photosynthesis field is the Japan
Technological Research Association of Artificial Photosynthetic Chemical
Process (ARPChem), a consortium of universities and companies that has
government funding comparable to JCAP's grant — although over ten years
rather than five — to develop a bag-based approach. Kazunari Domen, a
chemist at the University of Tokyo and leader of ARPChem's
water-splitting group, says that one of the companies in the consortium
has been working on a membrane to separate the hydrogen and oxygen
products.
Other projects are making
photoabsorbers from organic molecules, rather than semiconductors. Some
are building molecular assemblies inspired directly by the
photosynthetic apparatus of plants. And in the past few years, a class
of materials called perovskites has drawn the attention of the
solar-photovoltaic community for its high energy-conversion efficiency;
some researchers think that the materials also have potential in
artificial photosynthesis.
Daniel Nocera, a
chemist at Harvard University in Cambridge, Massachusetts, launched Sun
Catalytix to develop his work on a low-cost catalyst. But the company
announced last year that it has put that research on hold to pursue a
less challenging product with prospects of turning a profit for
investors sooner. The decision underscores the challenges of bringing a
commercially viable artificial-photosynthesis system to market.
Berkeley bubbles
On
a spring day in the arty–industrial district of Berkeley, California,
researchers demonstrate a prototype system inside the temporary lab
space that houses JCAP's northern site. As a sunlamp shines on a
CD-sized plastic box, fine streams of hydrogen bubbles rise between blue
strips of catalyst-coated silicon and exit through tubes in the box's
top. This prototype system is not the team's best: it won't last and it
is not very efficient. But it is still encouraging to see champagne-like
bubbles triggered simply by light.
Then Karl
Walczak, a postdoc in JCAP's prototyping group, slides a second plastic
box in front of the lamp. Inside is a small black square: a new titanium
dioxide-coated photocathode. This second system immediately begins to
generate bubbles much faster than the first. “This is where the field is
going,” says Walczak.
JCAP researchers hope
that such prototypes will ultimately lead to industrial
hydrogen-production plants. They predict arrays of cells kilometres
long, with a tower supplying water and pipes drawing the hydrogen to a
storage tank. Some researchers propose that domestic units may also be
part of the future, but Lewis warns that the small amount of sunlight
that falls on a rooftop cannot make enough hydrogen to supply a family's
energy needs. Others say that the technology could be useful in areas
of the developing world that lack an energy infrastructure, offering
distributed fuel generation where it is needed.
In
the meantime, researchers at JCAP and elsewhere are moving forward on
all fronts. Devens Gust, a chemist at Arizona State University in Tempe,
echoes a near-universal sentiment. “The bottom line,” he says, “is that
nobody really knows yet what's going to win out, what's going to be
practical.”
But whatever technology prevails,
says Lewis, the logic behind artificial photosynthesis is inexorable.
“The biggest energy source we have by far is the Sun,” he says. “The
best way to store energy other than in the nucleus of an atom is in
chemical fuels. It's inevitable someone is going to take the biggest
source and store it in the most dense way.”
