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 have presented their results on August 17, 2020 at the American
Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo. ACS is holding the
meeting through Thursday. It features more than 6,000 presentations on a wide
range of science topics.
“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.â€
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.â€
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.