Converting solar energy to hydrogen fuel, with help from photosynthesis


Date:
August 17, 2020
Source:
American Chemical Society
Summary:
Global economic growth comes with increasing demand for energy,
but stepping up energy production can be challenging. Recently,
scientists have achieved record efficiency for solar-to-fuel
conversion, and now they want to incorporate the machinery of
photosynthesis to push it further.



FULL STORY ========================================================================== Global economic growth comes with increasing demand for energy, but
stepping up energy production can be challenging. Recently, scientists
have achieved record efficiency for solar-to-fuel conversion, and now they
want to incorporate the machinery of photosynthesis to push it further.


==========================================================================
The researchers will present their results today at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.

"We want to fabricate a photocatalytic system that uses sunlight to
drive chemical reactions of environmental importance," says Lilac Amirav, Ph.D., the project's principal investigator.

Specifically, her group at the Israel Institute of Technology is designing
a photocatalyst that can break down water into hydrogen fuel. "When
we place our rod-shaped nanoparticles in water and shine light on
them, they generate positive and negative electric charges," Amirav
says. "The water molecules break; the negative charges produce hydrogen (reduction), and the positive charges produce oxygen (oxidation). The
two reactions, involving the positive and negative charges, must take
place simultaneously. Without taking advantage of the positive charges,
the negative charges cannot be routed to produce the desired hydrogen."
If the positive and negative charges, which are attracted to one another, manage to recombine, they cancel each other, and the energy is lost. So,
to make sure the charges are far enough apart, the team has built unique heterostructures comprised of a combination of different semiconductors, together with metal and metal oxide catalysts. Using a model system,
they studied the reduction and oxidation reactions separately and altered
the heterostructure to optimize fuel production.

In 2016, the team designed a heterostructure with a spherical
cadmium-selenide quantum dot embedded within a rod-shaped piece
of cadmium sulfide. A platinum metallic particle was located at the
tip. The cadmium-selenide particle attracted positive charges, while
negative charges accumulated on the tip. "By adjusting the size of the
quantum dot and the length of the rod, as well as other parameters, we
achieved 100% conversion of sunlight to hydrogen from water reduction,"
Amirav says. A single photocatalyst nanoparticle can produce 360,000
molecules of hydrogen per hour, she notes.

The group published their results in the ACS journal Nano Letters. But
in these experiments, they studied only half of the reaction (the
reduction). For proper function, the photocatalytic system must support
both reduction and oxidation reactions. "We were not converting solar
energy into fuel yet," Amirav says.

"We still needed an oxidation reaction that would continually provide
electrons to the quantum dot." The water oxidation reaction occurs in a multi-step process, and as a result remains a significant challenge. In addition, its byproducts seem to compromise the stability of the
semiconductor.

Together with collaborators, the group explored a new approach -- looking
for different compounds that could be oxidized in lieu of water -- which
led them to benzylamine. The researchers found that they could produce
hydrogen from water, while simultaneously transforming benzylamine to benzaldehyde. "With this research, we have transformed the process from photocatalysis to photosynthesis, that is, genuine conversion of solar
energy into fuel," Amirav says. The photocatalytic system performs
true conversion of solar power into storable chemical bonds, with a
maximum of 4.2% solar-to-chemical energy conversion efficiency. "This
figure establishes a new world record in the field of photocatalysis,
and doubles the previous record," she notes. "The U.S.

Department of Energy defined 5-10% as the 'practical feasibility
threshold' for generating hydrogen through photocatalysis. Hence, we are
on the doorstep of economically viable solar-to-hydrogen conversion."
These impressive results have motivated the researchers to see if there
are other compounds with high solar-to-chemical conversions. To do so,
the team is using artificial intelligence. Through a collaboration,
the researchers are developing an algorithm to search chemical
structures for an ideal fuel- producing compound. In addition, they
are investigating ways to improve their photosystem, and one way might
be to draw inspiration from nature. A protein complex in plant cell
membranes that comprises the electrical circuitry of photosynthesis
was successfully combined with nanoparticles. Amirav says that this
artificial system so far has proven fruitful, supporting water oxidation
while providing photocurrent than is 100 times larger than that produced
by other similar systems.


========================================================================== Story Source: Materials provided by American_Chemical_Society. Note:
Content may be edited for style and length.


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Link to news story: https://www.sciencedaily.com/releases/2020/08/200817104305.htm

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