Converting solar energy into hydrogen, using photosynthesis

With global economic growth comes growing demand for energy, scaling up energy production can be difficult. Recently, scientists have achieved record efficiency in converting solar to fuel, 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 build a photocatalytic system that uses sunlight to drive chemical reactions of environmental significance,” says Lilac Amirav, Ph.D., principal investigator of the project.

Specifically, his group at the Israel Institute of Technology is designing a photocatalyst capable of breaking down water into hydrogen. “When we place our rod-shaped nanoparticles in water and light them up, they generate positive and negative electrical charges,” Amirav explains. “Water molecules break down; negative charges produce hydrogen (reduction), and positive charges produce oxygen (oxidation). The two reactions, involving positive and negative charges, must take place simultaneously. Without taking advantage of the positive charges, the negative charges cannot be channeled to produce the desired hydrogen. “

If the positive and negative charges, which are attracted to each other, manage to recombine, they cancel each other out and energy is lost. So, to ensure that the charges are far enough apart, the team built unique heterostructures made up of a combination of different semiconductors, as well as metal catalysts and metal oxides. Using a model system, they separately studied the reduction and oxidation reactions and modified the heterostructure to optimize fuel production.

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

The group published its results in the journal ACS Nano Letters. But in these experiments, they only studied half of the reaction (the reduction). For proper operation, the photocatalytic system must withstand both reduction and oxidation reactions. “We are not yet converting solar energy into fuel,” Amirav says. “We still needed an oxidation reaction that would continuously supply electrons at the quantum dot.” The oxidation reaction of water occurs in a multistep process, and therefore remains a significant challenge. In addition, its by-products appear 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 place of water – that led them to benzylamine. The researchers found that they could produce hydrogen from water, while simultaneously converting benzylamine into benzaldehyde. “With this research, we have transformed the process from photocatalysis to photosynthesis, that is to say a real conversion of solar energy into fuel,” explains Amirav. The photocatalytic system performs a true conversion of solar energy into storable chemical bonds, with a maximum of 4.2% solar-chemical energy conversion efficiency. “This figure sets a new world record in the field of photocatalysis, and doubles the previous record,” she notes. “The US Department of Energy has defined 5-10% as the ‘practical feasibility threshold’ for the production of hydrogen by photocatalysis. Therefore, we are on the verge of economically viable solar-to-hydrogen conversion.”

These impressive results motivated the researchers to see if there are other compounds with high solar-chemical conversions. To do this, the team uses artificial intelligence. Through collaboration, researchers are developing an algorithm to search for chemical structures for a compound ideal for fuel production. Also, they are looking for ways to improve their photo system, and one of them might be to take inspiration from nature. A protein complex in plant cell membranes that includes the electrical circuits of photosynthesis has been successfully combined with nanoparticles. Amirav says this man-made system has so far proven successful, supporting the oxidation of water while providing 100 times more photocurrent than that produced by other similar systems.