Converting solar energy to hydrogen fuel, using photosynthesis

Global economic growth comes with increasing demand for energy, but supporting energy production can be challenging. Recently, scientists have reached record efficiency for conversion from solar to fuel, and now they want to incorporate photosynthesis machines 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 fabricate a photocatalytic system that uses sunlight to drive chemical reactions of environmental importance,” said Lilac Amirav, Ph.D., the project’s lead researcher.

Specifically, their group at the Israel Institute of Technology is designing a photocatalyst that can decompose water into hydrogen fuel. “When we place our rod-shaped nanoparticles in water and shine light on them, they generate positive and negative electrical charges,” says Amirav. “The water molecules break; the negative charges produce hydrogen (reduction), and the positive charges produce oxygen (oxidation). The two reactions, with the positive and negative charges, must take place simultaneously. Without taking advantage of the positive charges the negative charges must not be passed on to produce the desired hydrogen. “

When the positive and negative charges, which attract each other, manage to recombine, they cancel each other out, and the energy is lost. That is, to ensure that the charges are far enough apart, the team has built unique heterostructures consisting of a combination of several semiconductors, together with catalysts of metal and metal oxide. Using a model system, they studied the reduction and oxidation reactions separately and modified the heterostructure to optimize fuel production.

In 2016, the team designed a heterostructure with a spherical cadmium-selenide quantum dot embedded in a rod-shaped piece of cadmium sulfide. A platinum-metal particle lay at the tip. The cadmium selenide particle attracts positive charges while collecting negative charges on the tip. “By adjusting the size of the quantum dot and the length of the stick, as well as other parameters, we have obtained 100% conversion of sunlight to hydrogen from water reduction,” says Amirav. A single photocatalyst nanoparticle can produce 360,000 molecules of hydrogen per hour, she notes.

The group published its results in the ACS journal Nano letters. But in these experiments, they study only half of the reaction (the reduction). For proper functioning, the photocatalytic system must support both reduction and oxidation reactions. “We have not yet converted solar energy into fuel,” says Amirav. “We still needed an oxidation reaction that would continuously supply electrons to the quantum dot.” The reaction of oxidation of water occurs in a multi-step process, and as a result remains a major challenge. In addition, their by-products seem to compromise the stability of the semiconductor.

Together with collaborators, the group explored a new approach – looking for various compounds that could be oxidized instead of water – that led to benzylamine. The researchers found that they could produce hydrogen from water while simultaneously converting benzylamine to benzaldehyde. “With this research, we have transformed the process from photocatalysis to photosynthesis, which is real conversion of solar energy into fuel,” says Amirav. The photocatalytic system performs true conversion of solar energy into storable chemical compounds, with a maximum of 4.2% efficiency from solar to chemical energy. “This figure sets a new world record in photocatalysis, doubling the previous record,” she said. “The U.S. Department of Energy defines 5-10% as the ‘practical feasibility threshold’ for generating hydrogen through photocatalysis. This puts us on the doorstep of economically viable conversion from solar to hydrogen.”

These impressive results have motivated researchers to see if there are other compounds with high solar-to-chemical conversations. To use this, the team uses artificial intelligence. Through a collaboration, the researchers develop an algorithm to search for chemical structures for an ideal fuel-producing compound. In addition, they are exploring ways to improve their photosystem, and one way may be to draw inspiration from nature. A protein complex in plant cell membranes that encompassed the electrical circuit of photosynthesis was successfully combined with nanoparticles. Amirav says that this artificial system has been proven fruitful so far, supporting oxidation of water while delivering photocurrent is then 100 times greater than that produced by other similar systems.