Researchers in Japan reported a 100-fold improvement in their solar energy conversion method
By Maria Gallucci, IEEE Spectrum
April
9, 2021 -- Converting sunlight into hydrogen is a seemingly ideal way to
address the world’s energy challenges. The process doesn’t directly involve
fossil fuels or create any greenhouse gas emissions. The resulting hydrogen can
power fuel-cell systems in vehicles, ships, and trains; it can feed into the
electrical grid or be used to make chemicals and steel. For now, though,
that clean energy vision mainly exists in the lab.
Recently, Japanese researchers said
they’ve made an important step toward making vast amounts of hydrogen using
solar energy. The team from Shinshu University in Nagano studies
light-absorbing materials to split the hydrogen and oxygen molecules in water.
Now they’ve developed a two-step method that is dramatically more efficient at
generating hydrogen from a photocatalytic reaction.
The researchers began with barium
tantalum oxynitride (BaTaO2N), a semiconductor material that can
absorb light at up to 650 nanometers (a visible wavelength at the orange end of
red). The powdery substance serves as the photocatalyst, harnessing solar
energy needed to drive the reaction. They also used an aqueous methanol
solution instead of water, which allowed them to focus only on the hydrogen
component and reduce the complexity of the reaction.
By itself, BaTaO2N can
hardly “evolve” hydrogen gas from the solution. So, using their new method, the
Shinshu team “loaded” the powder granules with a platinum-based co-catalyst to
improve the chemical activity.
As a result, the materials evolved
hydrogen much more efficiently—about 100 times more efficiently—than BaTaO2N
that’s been loaded with platinum using conventional
methods, according to their paper in the journal Nature
Communications.
Takashi Hisatomi, a co-author of the
study, said the results are a “remarkable finding” in this research field.
Hisatomi is a professor with Shinshu’s Research Initiative for Supra-Materials
in Nagano, Japan, and he has studied BaTaO2N for nearly a decade.
“This is personally very exciting for me,” he said of the 100-fold improvement.
Solar energy experts have called efforts
to make hydrogen more easily or efficiently a “Holy Grail quest.” When used in fuel-cell-powered vehicles or
buildings, the odorless gas doesn’t produce emissions or air pollution—just a
little heat and water. However, nearly all hydrogen today is made using an
industrial process that involves natural gas, which ultimately pumps more
emissions into the atmosphere. A handful of facilities can make “green”
hydrogen using renewable electricity to split water molecules, but the process
itself is energy-intensive. If scientists can directly make hydrogen from the
sun’s energy, they could bypass this expensive step.
In Belgium, a team at Katholieke
Universiteit Leuven is developing solar panels that collect moisture in the
air, then use chemical and biological components to split water directly on the
surface. The researchers envision putting these panels on top of houses,
allowing people to heat their homes with hydrogen gas made on site. Separately,
Israeli and Italian scientists are advancing methods to extract as much
hydrogen as possible from solar-to-chemical energy conversion. The
international group has developed rod-shaped nanoparticles, tipped with
platinum spheres, that prevent hydrogen and oxygen from recombining after the
molecules are separated.
At Shinshu, the researchers sought to
improve the efficiency of the BaTaO2N photocatalyst by
depositing the platinum-based co-catalyst. But the conventional methods
for doing so weren’t initially effective, Hisatomi said.
For instance, in the
impregnation-reduction process, a surface is filled with a solution containing
metal precursors and then subjected to elevated temperatures, which evaporates
the solvent and leaves behind the metal catalysts. When the Shinshu team
applied fine particles of platinum to the BaTaO2N granules, the particles
tended to aggregate, restricting the electronic interaction between the
materials. Another method, called photodeposition, resulted in weak contact
between the BaTaO2N and co-catalyst, in turn weakening the
interaction.
So the researchers combined these two
methods. First, they deposited only a small amount of the co-catalyst using the
impregntation-reduction process, and that prevented the particles from
aggregating. Then they applied a second layer using photodeposition; this time,
the fine particles grew on the widely dispersed “seeds” planted in the first
step. Zheng Wang and Ying Luo conducted the investigation, under the
supervision of Kazunari Domen and Katsuya Teshima, respectively.
Although the study involved an
aqueous methanol solution, not water, the team confirmed that the newly
developed platinum-loaded BaTaO2N can split the hydrogen and
oxygen molecules in water more efficiently than earlier BaTaO2N
versions, when combined with another photocatalyst that drives the oxygen
evolution process.
Hisatomi said the team is considering
printing the powdery photocatalyst on a panel-type reactor. He and his
colleagues have already built such a device using another material,
aluminum-doped strontium titanate (SrTiO3), which has the same
crystal structure as BaTaO2N but absorbs lights at different
wavelengths. The 1-square-meter panel reactor is filled with a
1-millimeter-deep layer of water. When exposed to sunlight, the chemical
reaction rapidly releases gas bubbles. A related research effort aims to
develop membranes that can keep the hydrogen and oxygen bubbles separated.
Still, despite the 100-fold efficiency
jump, BaTaO2N isn’t quite ready for prime-time hydrogen production.
“We still need a similar jump in
improvement of the efficiency to make this technology practically useful,”
Hisatomi said. As researchers continue improving the photocatalyst, they’ll
also begin applying the two-step approach to other types of materials. “We
don’t know which material will become the best in the end,” he added.
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