Researchers efficiently convert solar energy into hydrogen fuel

Researchers could make the conversion from solar energy to hydrogen fuel more efficient by using the machinery of photosynthesis. (Image Credit: Clay Banks, Unsplash)

Scientists from the Israel Institute of Technology have achieved record efficiency for solar-to-fuel conversion. Now, the team wants to integrate a photosynthesis device into the process to produce more efficient results. The team presented their findings 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.

The scientists are developing a photocatalyst capable of splitting water into hydrogen fuel. With the help of light, they used rod-shaped nanoparticles to generate positive and negative electrical charges. During the process, water molecules break down; negative charges produce hydrogen (reduction), and positive charges produce oxygen (oxidation). Both of the reactions, which involve positive and negative charges, must occur simultaneously. Without leveraging the positive charges, the negative charges can’t be channeled to produce the desired hydrogen.

If the positive and negative charges that are attracted to each other manage to rejoin, they cancel each other out, causing energy loss. To ensure the charges aren’t close together, the team developed special heterostructures made up of a combination of different semiconductors. The team used a model system to study the reduction and oxidation reactions and modified the heterostructure to optimize fuel production.

The researchers only observed half of the reaction (reduction) in their experiments. To operate correctly, the photocatalytic system needs to support the 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 takes place in a multi-step process, making it very challenging. Additionally, its byproducts jeopardize the stability of the semiconductor.

With the help of collaborators, the team explored a new method using benzylamine. They discovered that hydrogen could be produced from water, while simultaneously converting 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.

This photocatalytic system is able to convert solar power into storable chemical bonds with a solar-to-chemical energy conversion efficiency of 4.2%.  “This figure establishes a new world record in the field of photocatalysis, and doubles the previous record,” Amirav 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 results motivated the team to use AI to look for other components that contain high solar-to-chemical conversions. The team is developing an algorithm to find chemical structures for a fuel-producing compound. Additionally, they are exploring ways to upgrade the photosystem, one of which involves using a nature-inspired approach. A protein complex in plant cell membranes that contain the electrical circuitry was combined with nanoparticles. So far, this new system is beneficial since it can support water oxidization. At the same time, it provides photocurrent 100 times larger than other systems that produce it.

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