Israel Institute of Technology is designing a photocatalyst that can break down water into hydrogen fuel. Recently the scientists achieved record efficiency for solar-to-fuel conversion, and now they want to incorporate the machinery of photosynthesis to push it further. Global economic growth comes with increasing demand for energy, but stepping up energy production can be challenging.
The researchers presented their results at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.
Lilac Amirav, Ph.D., the project’s principal investigator said, “We want to fabricate a photocatalytic system that uses sunlight to drive chemical reactions of environmental importance.”
“When we place our rod-shaped nanoparticles in water and shine light on them, they generate positive and negative electric charges,” Amirav said. “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,” she explained.
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.
She noted hat 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 said. A single photocatalyst nanoparticle can produce 360,000 molecules of hydrogen per hour, she said.
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 said. “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 said. 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 noted. “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 said 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.
With a doubling of the energy conversion efficiency the team is within striking range of the minimum target. Doubled again they would be past half way ro the max target. Better watch this team, there is a good likelihood they’ll break through again.
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