September 27, 2023

Novelties: The Answer Is (Artificially) Blowing in the Wind

ONLY God can make a tree, the poem says. But scientists are working on making artificial leaves that can produce fuels directly from sunlight, water and carbon dioxide, just as real leaves do. One day, the new leaves could help people heat their homes and drive their cars.

“If nature can do it, so can we,” said Gary Brudvig, a Yale chemistry professor who studies photosynthesis, the process by which plants convert and store energy from the sun. “We want to use the principles from nature to design an artificial leaf,” he said, adding that research groups around the world are working on the idea.

The artificial leaves won’t rustle soothingly in a summer breeze, or change from green to red and fall to the ground in autumn. Instead, they will probably be thin sheets of plastic embedded with light-absorbing materials, or sheets of bubble-wrap-like material spread out over a field that take in sunlight and water vapor and emit, for example, hydrogen or methanol.

“Artificial leaves are inspired by leaves, but they won’t look like them,” said Nathan S. Lewis, a chemistry professor at the California Institute of Technology in Pasadena. He is the principal investigator for a five-year artificial-photosynthesis project that was awarded a grant of up to $122 million by the federal Department of Energy. “We will be leading a national and international effort to produce fuels directly from sunlight,” said Dr. Lewis, whose Joint Center for Artificial Photosynthesis is one of three Energy Innovations Hubs set up by the Energy Department.

The center is now hiring chemists, software engineers, theorists and others to work both on the development of artificial photosynthesis, and on the prototypes that will perform the conversion and storage, Dr. Lewis said.

Artificial leaves are hardly new in laboratories, Dr. Brudvig of Yale said, but they have been too expensive, too fragile or too inefficient to compete commercially with fossil-fuel systems.

Dr. Lewis agreed. “For example, a system may be cheap and efficient, but not enduring,” he said. During the next five years, however, he and his group will be building prototypes designed to overcome these problems. “We are going to take the best efforts from around the world,” he said, “and bring them together in a system to absorb sunlight at the right energy to make fuel.”

One creator of artificial leaves, Daniel Nocera, the Henry Dreyfus Professor of Energy at the Massachusetts Institute of Technology, wants to start by putting them on the rooftops of homes in developing countries. He demonstrated the latest version of his artificial leaf at the national meeting of the American Chemical Society in March.

The demonstration is detailed in two papers that are about to be published, he said — one in the Proceedings of the National Academy of Sciences — as well as in one more paper he is still working on. “Our goal is to make each home its own power station,” he said.

The leaf is made of inexpensive materials and works with ordinary water, he said. For the demonstration, he used a slim piece of silicon about the size of a playing card. The silicon was coated with catalysts, created by him and his group, that speed the breakdown of water into hydrogen and oxygen.

“On one side of the silicon, hydrogen starts bubbling up, and oxygen bubbles up on the other side,” Dr. Nocera said.

The catalysts are placed directly on the silicon, so no extensive wiring is needed, as in standard photovoltaic cells, to convert sunlight into current and break down the water into hydrogen and oxygen. No extensive membrane is required, either. He plans to collect and use the hydrogen as a fuel.

His system is designed not for the relatively high energy use of a typical American home, but for homes with much more modest needs. One of its advantages is that the water it splits into hydrogen and oxygen need not be pure — Dr. Nocera sometimes uses ordinary water from the Charles River in Cambridge, Mass., for his demonstrations. Typically for electrolysis, microorganisms in the water, for example, would need to be removed to avoid competitive reactions, said Daniel R. Gamelin, a chemistry professor at the University of Washington in Seattle who also works on artificial photosynthesis.

ANOTHER advantage of Dr. Nocera’s system, Dr. Gamelin said, is that the hydrogen need not be delivered to people’s homes, but could instead be created on the spot. “The technology is important,” he said, “because people in third-world countries may generate the hydrogen in the place they want to use it.”

In 2008, Dr. Nocera formed a company, Sun Catalytix of Cambridge, Mass., with financing from the Tata Group of India and Polaris Ventures Partners, among others. He said his partners at the company were working on commercial development of the technology.

“They are well beyond the science and into engineering and reliability,” he said. “They are building and assessing prototypes for systems for the poor.”

Harry B. Gray, the Caltech chemist in whose lab both Dr. Nocera and Dr. Lewis once apprenticed, said he was looking forward to Dr. Nocera’s publication that will provide detailed information about the catalysts on the silicon leaf.

“We need robust catalysts made from earth-abundant materials to help us reach the goal of scalable systems for the production of solar fuels,” he said.

Such systems could be transformative, Dr. Brudvig of Yale said.

“We don’t have a system right now that can be used commercially for photosynthesis that competes with fossil fuels,” he said. “But developing one is of central importance if we are to move from a fossil-fuel energy economy to a renewable-energy economy.”


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