Researchers Use Photosynthesis to Achieve Record Solar-to-Fuel Conversion Efficiency

While the growing demand for energy promotes global economic growth, it can be quite difficult to step up energy production.

By harnessing the machinery of photosynthesis, researchers could improve the conversion of solar energy to hydrogen fuel. Image Credit: Quality Stock Arts/Shutterstock.com.

Scientists have recently achieved a record efficiency for converting solar energy into fuel, and now they have planned to integrate the machinery of photosynthesis to increase this efficiency further.

The team will present the study results at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo. The meeting will be held through Thursday, August 20th, 2020. It features over 6000 presentations on a broad range of topics related to science.

We want to fabricate a photocatalytic system that uses sunlight to drive chemical reactions of environmental importance.

Lilac Amirav, PhD, Study Principal Investigator, Israel Institute of Technology

In particular, Amirav’s team at the Israel Institute of Technology is currently developing a new photocatalyst that can convert 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,” added Amirav. “The water molecules break; the negative charges produce hydrogen (reduction), and the positive charges produce oxygen (oxidation).”

Amirav further added, “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.”

Negative and positive charges are attracted to each other. When these charges manage to combine again, they annul one another, leading to energy loss. Therefore, to ensure that the charges are sufficiently far apart, the researchers have constructed special heterostructures that include a mix of different types of semiconductors, along with both metal and metal oxide catalysts.

With the help of a model system, the team analyzed the oxidation and reduction reactions individually and then modified the heterostructure to improve the production of fuel.

The researchers had earlier developed a heterostructure in 2016. This heterostructure includes a spherical cadmium-selenide quantum dot integrated inside a rod-shaped bit of cadmium sulfide. The tip features a platinum metallic particle. Positive charges were attracted by the cadmium-selenide particle, while negative charges collected 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.

Lilac Amirav, PhD, Study Principal Investigator, Israel Institute of Technology

She noted that a single photocatalyst nanoparticle is capable of producing as many as 360,000 molecules of hydrogen every hour.

The researchers have published the results of the study in Nano Letters—a journal from ACS.

However, in such experiments, the team had analyzed only 50% of the reaction (that is, the reduction). The photocatalytic system should support both oxidation and reduction reactions for accurate function.

We were not converting solar energy into fuel yet. We still needed an oxidation reaction that would continually provide electrons to the quantum dot,” added Amirav.

A multi-step process is involved in the water oxidation reaction; therefore, this reaction continues to pose a major challenge. Moreover, the byproducts of this reaction appear to affect the stability of the semiconductor.

The researchers and their colleagues then investigated a novel method—searching for different types of compounds that can possibly be oxidized instead of water—which eventually led them to benzylamine. They observed that hydrogen could be produced from water, and at the same time, benzylamine could be converted to benzaldehyde.

With this research, we have transformed the process from photocatalysis to photosynthesis, that is, genuine conversion of solar energy into fuel,” added Amirav.

The new photocatalytic system indeed converts solar energy into storable chemical bonds, with a record 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. 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.

Lilac Amirav, PhD, Study Principal Investigator, Israel Institute of Technology

Such remarkable results have encouraged the team to look for other compounds that have a high solar-to-chemical conversion efficiency. The researchers are now using artificial intelligence to achieve this feat. Through a collaboration, the team is also creating an algorithm to look for chemical structures for a perfect fuel-producing compound.

They are also looking for ways to enhance their new photosystem, and one way to do this would be to take a cue from nature. A protein complex present in the cell membranes of plants contains the electrical circuitry of photosynthesis. This electrical circuitry was effectively combined with nanoparticles.

According to Amirav, this kind of artificial system demonstrated to be useful, to date, supporting the oxidation of water while offering photocurrent that is 100 times more than that created by other analogous systems.

The team acknowledged funding from the Horizon 2020 program of the European Commission and the Israeli Ministry of National Infrastructures, Energy and Water Resources.

Source: https://www.acs.org/

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