Researchers from the Israel Institute of Technology are designing a photocatalyst capable of breaking down water into hydrogen fuel. The group reports record efficiency for solar-to-fuel conversion and intends to incorporate the mechanics of photosynthesis to push that efficiency further. “When we place our rod-shaped nanoparticles in water and shine light on them, they generate positive and negative electric charges,” said Lilac Amirav, principal investigator on the project. “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.” If the negative and positive charges manage to recombine, they cancel each other and the energy is lost. To make sure the charges are far enough apart, the team has built unique heterostructures composed of a combination of different semiconductors, together with metal and metal oxide catalysts. Using a model system, the researchers studied the reduction and oxidation reactions separately and altered the heterostructure to optimize fuel production. The team designed a heterostructure with a spherical cadmium-selenide quantum dot embedded within a rod-shaped piece of cadmium sulfide, with a platinum metallic particle 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 noted. In those experiments, however, they studied only half 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 multistep process, and as a result remains a significant challenge. In addition, its byproducts seem to compromise the stability of the semiconductor. Seeking different compounds that could be oxidized in lieu of water, the group looked 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 was able to perform 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 world record,” she said. “The U.S. Department of Energy defined 5% to 10% as the ‘practical feasibility threshold’ for generating hydrogen through photocatalysis. Hence, we are on the doorstep of economically viable solar-to-hydrogen conversion.” The researchers are now seeking 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. Additionally, they are investigating ways to improve their photosystem. For that, the researchers are looking to nature. A protein complex in plant cell membranes that comprises the electrical circuitry of photosynthesis was successfully combined with nanoparticles. According to Amirav, that artificial system has thus far proven fruitful, supporting water oxidation while providing photocurrent 100× greater than that produced by similar systems.